Saturday 14 April 2018

Permanent and Perfect Stealth in Space

Despite the commonly accepted truth in Hard Science Fiction, spacecraft are able to evade detection in space in many circumstances. The Hydrogen Steamer was a design that used liquid hydrogen evaporative cooling to keep a non-reflective surface practically invisible. 
However, it was vulnerable to RADAR and had extremely poor maneuverability, as it was meant to demonstrate how long it could stay cool. This time, we will design a more advanced, functional and performant stealth spacecraft.

This post builds upon the conclusions drawn from the Stealth in Space is Possible series found here (Part I, II, III and IV). A useful read is the page on the Hydrogen Steamer design.

Detection mechanics

We are considering a large telescope, in space, pointed at a target spacecraft that is very cold, has extremely low reflectivity and is travelling a several kilometers per second. The telescope and the target are separated by a distance of a few dozen to a few million kilometers. To achieve 'stealth', the target must evade detection by the telescope.
The critical question is at what distance the telescope can detect the target.

Previously, we looked solely at the sensitivity of the telescope compared to the intensity of the blackbody emissions from a target at a certain distance. As the distance increases, the inverse square law reduces the intensity of the emissions until they are below the telescope's sensitivity figure. So, for example, a 21 Kelvin target would emit 11 milliwatts per square meter, and a cryogenically-cooled infrared sensor would have a sensitivity of 10^-19W/m^2. By working out the square root of the emissions by the sensitivity, we would get a detection distance - in this case equal to 332 thousand kilometers.

Further research into how telescopes actually work has revealed that this method is unreliable for working out the true detection distances.

The real answer on how far away a stealth spacecraft could be detected actually depends on the relationship between signal strength and noise. Not the usual sense of noise, as in sound you can hear, but noise as all the emissions that a sensor picks up that do not come from a target.

In space, telescopes do not have to deal with atmospheric interference. The sources of noise are instead either internal, such as the thermal photons from hot components, electric resistance in the circuits and quantum inefficiencies in the Charge Coupled Device (CCD), or external, such as sunlight, solar wind, the interstellar medium and the Cosmic Background Radiation. 
Noise threshold can be a million times greater than the actual sensitivity reported.
Many techniques have been developed over the years to improve the performance of telescopes to cut out or minimize the effect of the various sources of noise. These include the use of cryogenic cooling, bandwidth filters, sun-shields, carbon-black coatings, larger collection surfaces, longer observation times and reference sensors to measure deviations. In addition to these physical techniques, digital processing can further improve sensitivity. This is done mainly by subtracting known sources of noise from the final image, to obtain only the difference which can then be attributed to a target's emissions. 
The effect of reducing temperature.

These techniques can be taken to their logical conclusion, resulting in telescopes such as the SPICA-SAFARI proposal. The CCD is cooled to a few milli-Kelvin above absolute zero. The electronics are superconducting, and the optics are also cooled to a handful of Kelvin. This reduces internal sources of noise to near zero. With quantum efficiencies can approach 100%, meaning that every photon collected equals one electron in output, sensitivities on the order of 10^-20W/m^2 and better can be expected. 
However, no amount of cooling can eliminate external sources of noise. Unlike fixed and predictable targets of observation such as a far away star, the background noise cannot be simply be subtracted from the final image, as it might also take with it the emissions of a stealth spacecraft. This creates a 'noise floor' below which a telescope's sensitivity cannot be improved, at least for this task. 
Therefore, we must compare the emissions the telescope receives from the target to the levels of external noise it collects to determine at what distance a stealth spacecraft can be detected.

The noise floor

Background noise is a well-documented aspect of astronomical observation. At different wavelengths, certain sources of noise dominate. The types of target we are interested in detecting have very low temperatures. By using this Spectral Calculator, we can work out that their emissions will have wavelengths in the Mid to Far infrared.
Output from Spectral Calc for a 21 Kelvin object. Peak emissions at 138 microns (Far Infrared).

We can look at this chart to find the dominant source of noise in the Mid to Far Infrared:
The intensity of non-zodiacal light sources, from here.

From CIBR.

We see that the Cosmic Infrared Background Radiation dominates in the Mid to Far Infrared, which ranges from 10 to 100 micrometers in wavelength (or 1 to 10 THz in frequency). Its intensity is roughly 10 nanowatts per square meter per steradian.
A depiction of the Cosmic Background Radiation.

A steradian is a measure of solid angle - it is the projection of a two-dimensional angle onto the surface of a three-dimensional sphere. It is a measure of how large an object appears from an observer's point of view. For example, the apparent size of the Sun or the Moon in the sky. It can be converted into the more common unit that is the degree, and its counterpart the square degree. A steradian is equivalent to about 3282 square degrees. The full spherical sky contains 41253 square degrees, so a single steradian represents 7.958% of the entire sky. 

To work out how this translates into a noise floor, we need to calculate how many watts per square meter the telescope receives. This will depend on its field of view. 

Fields of view are listed in arcminutes or arcseconds, which are 1/60 and 1/3600 of a degree respectively. As CCDs are usually square, this actually means an arcminute x arcminute or arcsecond x arcsecond area

We can then convert this field of view into steradian and then multiply it by the background radiation intensity per steradian to find the actual noise floor.
Typical sensors have very small fields of view. The cancelled NASA WFIRST, or 'Wide' Field Infrared Survey Telescope, only had a 2.5 arcsecond field of view, which works out to about half a millionth of a square degree, or about one billionth of a percent of the entire sky. The delayed James Webb Space Telescope has a field of view of 20 arcseconds, but so far most telescopes only have a single arcsecond field of view or less.

We can use this simple relationship:
  • Noise floor : FoV * BNI
The noise floor will be in W/m^2.
FoV is the field of view in steradian. 
BNI is the background noise intensity in W/m^2/sr.
A field of view of 1 arcsecond converts into 2.351*10^-11 steradian, which receives a background noise level of 2.35*10^-20 W/m^2 in the 100 micrometer wavelength.

Target emissions

A calculation of the emissions of a blackbody from its temperature and its emissivity, using the Stefan-Boltzman equation, actually gives the total emissions across the entire electromagnetic spectrum, from X-rays to Radio waves. 

This is not a useful measure because as shown below, the emissions of a cold object are bunched around a small range of wavelengths, and all infrared sensors can only detect an even smaller range of wavelengths.

Here is the spectral graph calculator's results for the emissions from a perfect blackbody (emissivity = 1) at a temperature of 30K:
Note that the peak spectral radiance is at a wavelength of 96.59 micrometers (Far Infrared) and that the band radiance, which is the total output per square meter per steradian depends on the 'band' of wavelengths the sensor is picking up. 

High bandwidth therefore allows a CCD sensor to sample the photons from a greater number of different wavelengths, allowing more total signal to be picked up from a target. 
The CCD from the JWST.

However, CCDs have a rather narrow range of wavelengths for which they are tuned for in terms of sensitivity, and a greater number of wavelengths also means more noise from those wavelengths.

Typically, a bandwidth of 1 to 10 micrometers is to be expected. We can approximate the target emissions by using the calculator to find the band radiance 5 micrometers above and below the peak. 

The next step is to calculate the portion of the target's emissions that the telescope intercepts. This is done by working out the solid angle the target occupies in the telescope's view in steradians.

A steradian calculator is handy.
From here.

Solid angle: Cross section area / Distance ^2

A 1m^2 cross section area at 1000m has a solid angle of 1/1000^2 : 10^-6 steradians.

If we also take into account the telescope's collector area, which is the effective area of its main mirror, we can produce the following equation:
  • Target emissions received: BR * CSA * TCA / D^2
Target emissions received will be in W/m^2.
BR is the band radiance in W/m^2/sr.
CSA is the Cross Section Area in m^2.
TCA is the telescope collector area in m^2.
D is the distance in m.

Typically, telescopes have many hours to days to repeat their observations of a single spot in the sky, which allows for the collection of a huge number of separate images to be compared for an even greater sensitivity. Data on sensor sensitivity is usually given for 10,000 second observation times for this reason. However, detecting fast spaceships travelling at multiple kilometers per second means that telescopes won't have that luxury - for this reason, we will only consider single-second frames.

Detection equation
From unresolved blob of heat to processed image.

The equations and relationships from above can be combined into a single detection equation that can work out the distance at which a cold stealth spacecraft can be certainly detected. 'Certainly', in this case, is achieved by a signal-to-noise ratio greater than one. In practical terms, this means that the target emissions must exceed the noise floor by a factor of at least 10.
  • D: ((BR * CSA * TCA) / (FoV * BNI * SNR))^0.5
D is the detection distance in m.
BR is the band radiance in W/m^2/sr.
CSA is the Cross Section Area in m^2.
TCA is the telescope collector area in m^2.
FoV is the field of view in steradian. 
BNI is the background noise intensity in W/m^2/sr. 
SNR is the signal to noise ratio, at least 10.
Using this equation, in addition to the calculators and information on the background noise, we can establish the shortest distance a stealth craft can approach a telescope without being detected.  

For example, a human body (310K, 0.68m^2) would be detected in the Near Infrared (10 micrometers) by a 2m wide telescope using an arcminute field of view and a 10 micrometer bandwidth sensor at a distance of approximately 277 thousand kilometers.

That same telescope would pick up the Hydrogen Steamer design (21K, 7200m^2) in the Far Infrared at a distance of only 21 thousand kilometers.

Using this formula and the previous information, we will look at ways to substantially reduce the detection distance of stealth spacecraft.

ATOMSS: the Advanced Triple Observability Mode Stealth Steamer 

The ATOMSS is a stealth spacecraft design that aims to achieve both extreme levels of undetectability, a greatly extended endurance and a much improved propulsive performance compared to the previously described Hydrogen Steamer design.

It achieves these objectives by using three observability modes (helium, hydrogen, warm), a fully insulated hull, anti-radar structures and super-expansion nozzles for its exhaust. 

The three observability modes make the design more or less visible to telescopes and sensors depending on the situation. 
Schematic for a cryogenic cooling system that uses helium evaporation.

The helium mode guarantees perfect stealth: no sensor will be able to detect it under any circumstance. This mode will be used when penetrating deep into enemy defenses, launching an attack or passing through dense sensor networks. 

The hydrogen mode allows the hull to reach a slightly warmer temperature in situations where stealth is unlikely to be so rigorously needed, such as during an approach to a planet or when passing through a lower-quality sensor network. Hydrogen has better heat absorption properties than helium, allowing for more efficient use of the available coolant mass.  

The 'warm' mode extends a huge area of lightweight, low temperature radiators to handle the spacecraft's waste heat during the long months that interplanetary travel takes. This mode can be maintained indefinitely, without consuming any coolant. 

In the original Hydrogen Steamer design, the spacecraft's hull also doubled as propellant tank, so the hull's temperature was that of the propellant. If insulation is used, then the hull's sides can be reduced to extremely low temperatures as they would sit in the shadow cast by the nose. 
The James Webb telescope's sunshield is an example of this use of shadow.

The only detectable cross-section would become the much smaller nose, which is in direct sunlight and so must be actively cooled.

In addition to insulation, the ATOMSS will be equipped with meter-scale structures meant to defeat active detection by high-frequency radar waves. This will give it a bumpy, irregular surface meant to reduce radar returns.
Temperature and pressure drop as the expands.
Finally, the use of nozzles with extreme expansion ratios will allow for high exhaust velocity, high reactor temperature propulsion to be used without leaving a trail of hot or otherwise detectable exhaust. The addition of a pulsed mode with shutters completely eliminates detection by observing the exhaust. The ATOMSS can therefore travel around the solar system just as fast as any other spacecraft.

The details of each of these features will now be described:

-Helium cooling.
Liquid helium cooling is used in specialized electronics.
As made explicit by the detection equation, even the low temperatures achieved by the evaporation of liquid hydrogen are not sufficient to keep a large spacecraft from being detected at tens of thousands of kilometers. 

The only way to achieve even lower temperatures without the use of heavy and energy-expensive heat pumps is to use the evaporation of a fluid that boils at an even lower temperature. In this case, it is liquid helium.

Here is the phase diagram for helium:
From here.
From wikipedia.

The transition from liquid to gas happens at a temperature of 2.17 Kelvin in a vacuum. This is known as the Lambda point of Helium. The phase change from liquid to gas absorbs 20.8 kJ/kg of heat

If a spacecraft's exterior is cooled by liquid helium, it will be practically invisible to any infrared or microwave sensor (peak emissions are in the 1.33 mm wavelength). For example, a 'helium steamer' with 1000m^2 cross-section area and a hull at 2.17 Kelvin, facing a large 5 meter wide microwave telescope with a full 100 micrometer bandwidth, would remain undetectable at a distance of only 43 kilometers!

The main disadvantages of helium are its lower heat of vaporization and heat capacity when compared to hydrogen, so a greater mass of helium is needed to stay cool for the same period. Also, hydrogen can eventually absorb up to 60MJ/kg if heated to a temperature of 3000K, helium will only manage 15.5MJ/kg at that temperature, as its heat capacity is only 5.2kJ/kg/K.

-Vacuum hydrogen
Hydrogen boils at 21K at a pressure of 1 atmosphere. In a vacuum, it instead boils at its triple point of 13.8K. This allows for a substantial reducing in the thermal signature of a stealth spacecraft without having to give up on the incredible heat absorbing properties of hydrogen. 
From this paper.

A 'vacuum hydrogen steamer' of 1000m^2 cross-section area and a hull at 13.8 Kelvin, facing a large 5 meter wide Far Infrared telescope with 10 micrometers bandwidth, would be detected at 6293 kilometers. 

Hydrogen is very interesting when frozen as it can absorb nearly 450kJ/kg when sublimating, and another 14 to 22kJ/kg/K as its temperature increases.

-Warm mode
In this mode, radiators of extremely low mass per area (kg/m^2) would be deployed at a low temperature to remove the few tens of kilowatts that the ATOMSS would generate during the long periods of interplanetary travel. For this mode, we will work backwards from a desired heat rejection performance and a maximum detection distance to find the mass of the radiators dedicated to this mode.

Let us suppose that the ATOMSS needs to get rid of 10 kW of waste heat and that wire radiators are employed. We expect to operate them at low temperatures, so low-density materials and hollow tubing can be used. If the radiator design employed half-empty ultra-high-molecular-weight polyethylene (UHMWPE) tubes a millimeter wide, then it would mass 0.38 grams per meter while having an exposed surface area of 0.00314m^2. A coating of carbon black increases emissivity to near 1. This means that the radiator can dispose of
(5.67*10^-8 * T^4) watts of waste heat per meter of length. 

Only a rectangular cross-section of the wire radiator will be visible to a sensor. This is about 0.001m^2 per meter of wire. 

Together, these factors mean that the radiator will need L: 10000/(5.67*10^-8 * T^4 * 0.00314) meters of wire to dispose of 10kW of waste heat, but it will be detected by a 10m wide telescope at a distance D: ((BR * CSA * 78.5) / 8.46*10^-15))^0.5. We can replace CSA by L*0.001 as it is the exposed cross-section of the wire radiator. This gives a detection distance of D: ((BR * L  * 0.0785) / 8.46*10^-15))^0.5.

At a temperature of 30 Kelvin, the wires will have to be 69,342 km long and will mass 26.35 tons. This gives a detection distance of approximately 25,300 km. 

At a temperature of 50 Kelvin, the wires will be 8,986 km long and mass 3.4 tons. The detection distance increases to 1.03 million km. 

With higher temperatures, such as 100 Kelvin, the radiator mass drops drastically (1478kg) but the detection distance becomes impractical.
A million kilometer detection range might sound very poor, but it is 0.5% of the average distance between Earth and Mars. This means that the ATOMSS can spend up to 99.5% of its travel time in warm mode, without having to expend any coolant. Unless the telescopes are spaced closer than a million km across the Earth-Mars distance (which would result in over a hundred thousand telescopes, more if above-the-plane trajectories need to be covered), then the warm mode is useful as it can increase the spacecraft's endurance significantly. 

-Insulated hull
Vacuum insulation is well-known in the field of cryogenics.
To be used as an open-cycle coolant, the liquid hydrogen or helium must be kept below its boiling point but above its freezing point. While this is a low temperature, it can be at times not low enough to prevent detection.

By using insulation between the propellant tanks and the external hull of a stealth spacecraft, heat transfer is eliminated. This allows the flanks of this design, which are in shadow, the naturally cool down to extremely low temperatures, as low as 2.73 Kelvin, which is indistinguishable from the background temperature of empty space. Insulation can take the form of two or more layers of very reflective and very thin Mylar sheets separated by vacuum that prevent infrared radiation from crossing the gap between them.
Placing propellant tanks in shadow was proposed for the storage of liquid hydrogen.
The nose of a stealth spacecraft is in direct sunlight, and so must be actively cooled. Insulation will not help reduce its temperature, as the main source of heating is external, and the lowest practical temperature is that of the boiling point of whatever coolant (helium or hydrogen) is being used.

An important consequence of this feature is that only the small cross-section area of the nose and perhaps rear will count towards the detection of a stealth spacecraft, since the flanks in the shadow are always too cool to detect.

-Radar countermeasures
Example of RAM applied to a wing's edge.
Radar is a form of active detection, as it relies on a radio signal reaching the target, reflecting off a surface, and then returning to an antenna. Certain techniques and design features can be used to reduce the detectability of the ATOMSS to active detection by radar.

Very short wavelengths, such as millimetric radio or microwaves, can be absorbed by the VANTA-black carbon nanotubes. Longer wavelengths as long as 10 or 100m long can diffract around the spacecraft without interacting with it. The ATOMSS is vulnerable to everything in between: wavelengths of 1cm to 1m (0.3 to 30GHz).

An ideal reflector dish can produce a radio beam up to 70 * Wavelength /Antenna Size degrees wide. 1cm wavelength radio focused by a 10m wide dish would produce a beam width of 0.07 degrees. The beam width can be used to determine the intensity of the radio waves that the target receives, and the return signal spreads again in the other direction. For example, a megawatt radio telescope with a beam width of 0.07 degrees would only produce an intensity of 0.85W/m^2 at a distance of 1000km, which further reduces to 0.72 microwatts per square meter by the time it returns to the antenna. 

This can prevent radio waves from travelling up the exhaust nozzle.

The radio waves then interact with the surface of the stealth craft. Radar-Absorbing Materials (RAM) can be used to absorb between 99.6 and 99.99% of the radio wavelengths between 2.7mm and 10m. 

80MHz is 3.7 meter wavelength. 3GHz is 10cm.

A flat surface reflects 100% of the radio signal back in the direction it came from. A rounded surface spreads the radio waves evenly in all directions. The flanks of a stealth craft are the sides of a cylinder - it causes the sensor signal to be reduced by a factor 1/3.14, compared to a flat surface. The nose of the ATOMSS is rounded, allowing the signal to spread in two dimensions, by a factor 1/6.28.

Using the three effects (beam width, RAM and curved surfaces), the ATOMSS can reliably escape detection by even very powerful radars.

There is more that can be done, such as adding angles to the spacecraft's hull so that the radio waves bounce off in directions that do not return to the sensor platform, but these cannot be relied upon in space because we cannot known where the sensors are, and so attempting to bounce radio waves in one direction might just land them in the antenna of a sensor in an unexpected location. Unlike the ground and sky limiting where stealth aircraft can expect radio waves to appear from, sensor platforms can be placed just about anywhere and can pick up or emit radio waves from any direction.

-Expanded exhaust

Any propellant that is heated by a reactor, such as in a solar or nuclear rocket, needs to be expanded in a nozzle to trade temperature and pressure for exhaust velocity. High expansion ratio nozzle create a flow of exhaust that is at a low temperature, low pressure and near maximal velocity.
An example of a nozzle with a high expansion ratio.
A 'super-expansion' nozzle continues this process to otherwise unreasonable lengths, by creating a cryogenically-cold exhaust at near-vacuum pressure, travelling at the maximum possible velocity. The double benefit to a stealth craft is that they create an undetectable stream of exhaust and increase propulsive performance.

The downside is their size and mass.

From the point of view of a stealth ship, this is an acceptable cost as it would allow them to travel around the Solar System as quickly as any regular military ship. The size and mass penalties are of less consequence to a design that is not supposed to engage in direct combat or in tactical maneuvers in the first place. 

A further improvement to the super-expansion nozzle is the use of shutters. 
Shutters used in pulsejets.

A simple nozzle is open from both ends. It allows observers from a certain angle to look straight up the nozzle to the throat, where hot gases are emitting a clearly detectable heat signature. A shutter can intermittently block this line of sight by staying closed when propellant is being injected into the nozzle and opening just as the cool, high velocity gases reach the end of the nozzle. Of course, this does impose a pulsed mode of operation.

Example design

We will now work out a sample design for an ATOMSS spacecraft.

As for the Hydrogen Steamer, the mission is to depart from Mars, reach Earth orbit and stay on station several months before returning.
To Scale.

The stealth craft will be 10 meters wide, giving it a frontal cross-section of 78.5m^2

The ends of the ATOMSS are hemispherical, and the rest of the body cylindrical. The nose, which is the end facing the Sun, is cooled to either 2.17, 14 or 15 Kelvin by helium evaporation, hydrogen sublimation or a radiator respectively.
The sort of magneto-calorific cooler mentioned below.

The flanks and end of the ATOMSS are in permanent shadow and kept at 2.17K by a closed-cycle loop with liquid helium. A Peltier-effect or magnetocalorific cooler handles the very low amount of energy absorbed from the exterior through the flanks, from interstellar and interplanetary sources of heat.
The missiles can be dozens of nuclear Casaba Howitzer or Explosively Formed Penetrator warheads.

The mission module can be 200 tons of missiles and 2 tons of sensors. The control module contains the 'brains' of the ship, massing 1 ton and consuming 1kW. A reactor power module is needed to power the electronics when the spaceship is drifting through space and mostly inactive. A 1 ton nuclear reactor and generator producing up to 10kW will be used. We include a further ton of avionics and wiring. An additional 4 tons of cryogenic coolers and conductors is needed. These 209 tons can fit inside a tube 17m long at the back of the spacecraft.
The propulsion module is a high temperature nuclear thermal rocket. Operating at higher temperatures, when coupled with a super-expansion nozzle, allows for maximal exhaust velocity and even more heat to be absorbed per kilogram of hydrogen or helium.
Combining the rocket core with a closed-cycle Brayton turbine and generator might save weight and lead to something like the KANUTER.

The nuclear thermal rocket heats the propellant to a 4000K temperature and ejects the resultant gases at velocities up to 6.5km/s (helium) or 14km/s (hydrogen). All of the rocket's heat is absorbed by the propellant flow. It masses 20 tons and can produce up to 2GW of propulsive power.

The propulsion module is placed at the centre of mass of the spacecraft. It can swivel between multiple openings in the hull to apply thrust through the center of mass. This propulsion segment is 5 meters long.

One tank of slush hydrogen at 14 Kelvin containing 400 tons is divided into two segments 30m long.  Another tank containing 888 tons of liquid helium at 2.17 Kelvin is divided into two segments in front and behind the propulsion section, each 39 meters long. Liquid helium can be shifted between these tanks to keep the center of mass in the middle of the propulsion module.

Overall, the ATOMSS is 160m long, giving it a lateral cross-section of 1600m^2.

The hull's insulation, cooling and radar absorbing material adds 6 tons to the craft's mass, while micrometer-thick wire-radiators at 30 Kelvin of the design described above add another 18 tons. 

The total mass is 1548 tons, of which 1288 tons is expendable coolant. A 563 ton drop-tank of liquid hydrogen can be added.

Performance and mission capabilities

We will consider the detection distance of the ATOMSS example design against a small telescope (2m wide collector area), a large telescope (10m wide collector area) and a huge 100MW radio telescope (20m wide dish). 

In helium mode, the entirety of the ATOMSS's hull is at 2.17 Kelvin. Liquid helium flows through a heat exchanger in the nose to absorb solar heat, and the helium gas that is produced is pumped through to the reactor and/or engine nozzles. The peak emissions are in the microwave, at 1335 micrometers. A small telescope detects the ATOMSS from the front at 4.25 km, and from the side at 13.6km. A large telescope only improves these distances to 20.9 km and 67.9km respectively. 

However, in Earth orbit, the ATOMSS can only use the helium mode for 2 days. This is because the nose absorbs 106.8kW of sunlight, which requires 5.14 kg of helium to be vaporized per second.

In hydrogen mode, helium gas is circulated from the nose to the slush hydrogen tanks. This vaporizes the hydrogen and cools down the helium gas, effectively transferring heat from the VANTA-black on the nose to the internal heatsink. The ATOMSS' flanks remain at 2.17 Kelvin but the nose warms to 14 Kelvin as this is the temperature the hydrogen evaporates at. It emits in the 205 micrometer range, which is the Far Infrared. A small telescope finds the nose at 2000km, and a large telescope detects it at 10,001 km.

The hydrogen mode can be maintained for up to 20 days. 

Using the open-cycle cooling reduces the deltaV capacity of the ATOMSS craft, so they are restricted to situations where 'good' or 'perfect' stealth is required.

In the warm mode, the ATOMSS extends 23.16 billion kilometers of micrometer-thick hollow wires, with a total emitting area of 74.4 million square meters. These wires radiate at 15 Kelvin, with peak emissions in the hundreds of micrometers.

The huge radar telescope does not care which mode the ATOMSS is in. The 100MW radar emitter, if producing 1m long radio waves, can be focused into a 0.7 degree wide beam. In the best case scenario, the 100MW radio beam reflects off the nose of the spacecraft. In the worst case, it catches the flat side.

Waves reflected off the spacecraft's hull are weakened by a factor 1/(1.375 * 10^-8 * Distance^4) by propagation, a factor 1/10000 by absorption and by a factor 78.5/6.28 for the nose and 160/3.14 for the flanks. This means that a 100MW signal returns to the 314m^2 dish as 2.58*10^15/D^4 W/m^2 from the nose and 1.16*10^16/D^4 W/m^2 from the flanks. 

If we take into account the 10^-12 W/m^2/sr background noise and want a 10:1 signal to noise ratio, we find out that the nose is detected at a distance of 4007 km and the flanks at 5835 km.
A DeltaV chart that is more useful for spacecraft that stay in space.

The ATOMSS can depart from an orbit near Phobos, which is Mars's largest moon and a likely construction site for space warships, and place itself at the edge of Earth's Sphere of Influence (an altitude of 924,000km) about 8.5 months later with just 4.34km/s of deltaV. The departure and insertion burns consume all of the liquid hydrogen in the drop tank.

Stealth can be maintained in this orbit indefinitely in the warm mode, or for up to 20 days in the hydrogen mode if safety against detection by large telescopes is required. If the stealth ship is detected or needs to approach an enemy craft very closely, it employs the helium mode which guarantees perfect Infrared stealth. In the helium mode, about 5.14kg of liquid helium is vaporized per second. This can be fed to the nuclear thermal rocket to produce a thrust of 35kN, which is enough to accelerate the ship from of 22 to 134 mm/s^2 without any waste. More helium or hydrogen can be consumed to perform escape maneuvers with the full 2 GW output of the main engine, at an acceleration of 185 mm/s^2 to 2.3 m/s^2.  

To perform an attack, it simply releases the weapons it is carrying. They can deorbit themselves and crash down into targets in lower orbits at a relative velocity that can reach 10.7km/s, in addition to whatever propulsion they may have that increases this number.  

After the mission is completed, the ATOMSS can return to Mars by using another 4.3km/s return trajectory and consuming the last 93.5 tons of hydrogen (if the ammunition has been expended). 

Decoys and Laser Jamming

Because of the way the stealth ship is detectable, decoys and laser jamming can be used to great effect.
How an Infrared sensor sees flares.

For example, the front of the ATOMSS spaceship can be reproduced by a 4m wide sphere of graphite cooled by a small tank of liquid hydrogen. To an infrared sensor or a radio telescope, this is indistinguishable from the front of a stealth craft. The decoy cannot be distinguished from the real craft when accelerating to move, because the exhaust is undetectable. 
Radar jamming.

Many of the techniques used to prevent radar from used to locate the ATOMSS become incredibly effective in space, due to the inverse square law allowing a tiny emitter on the stealth ship to overpower or confuse a huge and powerful emitter on the radio telescope. Electronic warfare against radars can be relied upon.

Another method of jamming is to use lasers. Cold objects like a stealth steamer are only detectable by trying to pick up their emissions in the rather narrow back of wavelengths that they emit in. For example, the emissions peak of a 14 Kelvin object is between 140 to 320 micrometers. The intensity of these emissions fall by a factor 100 or more outside of this peak. 

This means that a small number of lasers that can reproduce the emissions peak of a cold object while producing only a few watts can render any sensor looking at these wavelengths useless as the signals from the ATOMSS are drowned out by the signals from these lasers.  


Stealth steamers can stay permanently undetectable at reasonable distances. With the use of solid or slush hydrogen, they can maintain stealth at close ranges for months on end with relatively small supplies of the coolant. The addition of a helium mode makes them perfectly invisible and able to escape tough spots or come within a stone's throw of any target.

With super-expansion nozzles, the stealth ship can be maneuverable and travel great distances at low to moderate accelerations. 

The example worked out in this post uses technologies that can be considered 'near-future'; not much better than what is available today.

'Far-future' technologies can carry this concept to greater heights of performance. Gas-core nuclear rockets, for example, can significantly increase the acceleration under stealth. Superconducting electric motors can allow for lightweight and efficient heat pumps that allow for convenient helium-mode stealth without the need for helium. Extremely long carbon nanotubes that can be layered on top of each other allows the carbon-black coating to start absorbing some of the shorter radio wavelengths and reduce the stealth ship's radar returns...


  1. I will offer a number of challenges. While a spacecraft may incorporate all these features, the opposing side will also be using many different means to detect such a spacecraft.

    1. Optical interferometers. One sensor may have the limitations you describe, but coupling hundreds or potentially thousands of sensors together to create arrays with effective apertures of hundreds of kilometres to a light second would improve resolution. True, you may not be getting full sky coverage, but for practical purposes, you should be looking at threat areas, so scanning the sky around Mars or Jupiter looking for launch signatures etc.

    2. Multi spectral sensors. Missiles today are not just fixated on IR signatures, and future sensors or weapons should not be either. Scanning the same area at multiple frequencies will be able to detect the various differences, especially where things overlap, or more importantly, don't....

    3. Active searches using various methods will also come into play. The most effective of all would be Luke Campbell's RBoD, emitting a powerful Xaser beam. While it is a weapon at a light second, it could be a "searchlight" at beyond a light hour, and the irradiation of a target will upset the various balances needed to maintain a "stealth" profile.

    I suspect that the existence of such spacecraft (or even the hint that someone is forthcoming with that technology) will trigger a sensor race to ensure that you are NOT receiving an existential threat as a surprise, and there may be a multitude of other means of detection that I haven't even considered.

    1. Hi Thucydides!

      Of course, stealth will always be a back an forth between technologies and counters to those technologies.

      Resolution is important in pinpointing the location of a Stealth Steamer, but it will not help distinguishing its emissions from the background radiation in the first place - that is the role of sensitivity. And, as I have shown, the increase in sensitivity will not help reduce the effect of background radiation as technology can only work on internal, not external, sources of noise.

      The emissions curve of a cold object does span every wavelength from the longest radio to the shortest X-ray. However, emissions intensity fall off very quickly outside of the peak in the emissions curve. For example, if you are looking for 30 Kelvin objects (96.6 um peak) using a 200um sensor+filter, then you will be looking at signals that are 2.67x weaker, and this increases the detection distance by a factor 1.63x. If you are looking in the 10um wavelengths, then you will detect the craft 7.6 million times further away than you should have.

      So, it is best to focus on the peak emissions.

      Active detection is a bit of a gamble. Whoever comes up with the X-ray induced fluorescence detection method first, will be able to spot every single stealth steamer of that time. However, they can be defeated immediately after by a jammer that returns an overpowering or fake signal, or by the use of coating and shields that have extremely poor X-ray -to- fluorescent signal conversion efficiency, such as boron or hydrogen.

      A sensor race has already happened in my opion, and telescopes such as SAFIR, that I mentioned, are laready scarping the limits of what is possible to detect. The real war will be in the Electronic Warfare domain, which is much more exciting.

    2. ECM and ECCM are indeed exciting (until you lose and that missile hits your platform), but I tend to think of it as more tactical in nature, since you are now close enough to be actively emitting (you can actively emit whenever you want, of course, but most of the time you don't until you have no choice i.e. the radar warning signal is playing in your headphones).

      Large aperture detectors and interferometers can significantly increase the sensitivity and "magnification" of the sensor. I recall articles back when space telescopes were just coming "on line" predicting mirrors kilometres in diameter with the ability to focus on continents of planets orbiting stars light years away. While I'm not thinking of a massive unitary mirror, interferometers with hundreds to thousands of elements essentially recreate this performance by combining multiple elements.

      I am also thinking that no stealth platform is going to be equally stealthy in all aspects, so large arrays (or better yet, several large arrays) can be looking at areas from different angles, where one edge of an array or one separate array will be seeing the target from a different aspect than the other parts of the array.

      This leads to the question "who is building these things anyway?". Space guard would want something like that to look for asteroids and other space debris which could threaten the Earth (and later on any inhabited structure or body). In my conception of space commerce, where unpowered "pods" make up most of the shipping, the launching and receiving mass drivers will need exceptional optics to track the cargo pods. And if power beaming is the main way to get around in space for military and civilian vehicles (military vehicles to save fuel, they can cast off the beam whenever they need to, while civilian vehicles partially escape the tyranny of the rocket equation by leaving all the heavy power generation stuff behind), then of course high quality optics are part of both the aiming system and the transmission system as well.

      The Solar System will be full of eyes, even without military constellations deploying their armada of sensors. Not to say that people will not try to create stealth spacecraft, but I think it will be exceedingly difficult.

      And WRT illuminating targets. while X-ray fluorescence might be a real threat, just depositing kilowatts of energy on the target will start warming it up, making the spacecraft glow against the thermal backdrop of space, or causing it to ramp up the cryogenic cooling systems until they are producing enough waste heat to register. The Xaser RBoD may be the most threatening, given the fantastic engagement ranges, but even smaller laserstars illuminating targets with optical frequency lasers can still be threats as well.

    3. Edit to add: the biggest threat to stealth spacecraft is given in the opening illustration: if the "steamer" or ATOMSS passes in front of another astronomical body, or even another conventional spacecraft if the geometry is right, then it presents a "hole" in the picture, since it is going to be far colder than the object it is occulting.

  2. I can think of strategic ECM. Imagine a moon base powering a multi-GW laser to blind every sensor looking at the mid to far infrared bands. It could project this (set of) beams across the entire Solar System, and retain effectiveness over millions of kilometers.

    For example, a 10m wide telescope looking at the 188 to 198 micrometer band, sitting at 1000km from an ATOMSS Stealth Steamer in warm mode, is picking up roughly 6.3 nanowatts.

    A laser beam of 10GW emitting in that band can blanket a 1.42 million km wide circle with enough noise to hide that signal. If it is being focused by a 1000m wide dish, then it can do this from a distance of... 19333 AU? If we cut this down to 1 AU, the width of the area being blanketed is increased by a factor x139.

    I still think interferometers won't help. We are comparing the emissions from a spacecraft against the background radiation. The background noise does not get clearer if your 'magnify' it - this is why it is defined in W/m^2 per steradian. A telescope with better resolution can't locate noise!

    I did try to work out how detectable the stealth craft is from the front and the sides.

    Heating by lasers to try to force a detection could be effective. You'd need wave the lasers across a suspect volume of space though, because you can't shine a laser directly at something you haven't spotted yet. Their effectiveness will be over time, however, as the stealth steamer always has the option to boil off more coolant to handle the higher heat load. This reduces their endurance, but doesn't immediately oust them... unless the laser intensity is great enough to overcome their coolant pumps and leave hotspots. The saving grace might come from the fact that 'great enough' is several megawatts, and that if we work out the laser output needed to cover a large volume of space with enough power to pump megawatts into a potential stealth ship, it might necessitate terawatts in total.

    The hole in the picture by occulation is an interesting problem - for now, I am dismissing it as the occulation event happens on the scale of microseconds....

  3. Hi Matterbeam!

    Great article, gives me a lot to think of.
    Had a pair of questions popping up in my mind, thought I could ask you directly.

    First one: in the TV show "the Expanse" we see what they describe as "stealth ship", a strange looking ship that to me resemble a F117 dressed as a spaceship; now, thinking of a scenario where human technology is 150-200 years ahead of us, your specific of a stealth ship could fit in a ship resembling that of the Expanse or we will be forever stuck in the "tube/NASA-build" design?

    Second question: as the trend of stealth technology in aircraft engineering is going from absolute stealth to partial stealth to save on performance, could we see a parallel of that in space too? A ship that is not made to be completely "invisible", but less traceable?

    Thank you in advance, keep up with the awesome work. You are a gold mine of information and ideas for guys like me.

    1. Hi Frank. Welcome to the blog.

      The Expanse is a bit of a special case. While their regular spaceships can be detected from across the Solar System, they are unable to identify spaceships based solely off their exhaust. The 'drive signature' can be modified at will, leaving only the transponder as a method of identification... and that can be switched off entirely.

      This means that the Solar System is full of tens of thousands of mostly anonymous spaceships that can change their trajectory, destination and rate of travel at any time. It is impossible to track everyone's whereabouts in such a setting.

      This means that stealth ships have a very easy time getting somewhat close to their target. They can travel across interplanetary distances unquestion, and just switch off their engines in the last few moments before an intercept. The only people who can catch them are their own targets.

      The stealth ships in The Expanse can therefore use tactics that prevent direction from a single direction, even if it makes them visible from other directions.

      The F117-like angles are there to defeat radio detection. By bouncing off the radio waves in every direction other than straight back at their target, they do not produce any radio returns for a radar to pick and detect.

      The authors also mentioned directional stealth - concentrating your thermal emissions into a narrow cone behind the spaceship. Only sensors that can look into that cone will find an infrared signature. Anyone else sees nothing.

      These tactics make them very good at stealth when facing a single target.

      In reality, it is much more likely that there are enough sensors looking at everything from every direction to prevent these directional tactics from working. If you are bouncing off radio waves at an angle, then a sensor will be sitting at that angle. If you're focusing your heat signature into a narrow cone, then a sensor will be sitting in that cone.

      If we move stealth technology 150-200 years ahead, we might see things such as extremely thin and long hulls (reduces the sunlight being absorbed), efficient heat pumps (don't have to use helium), narrow-band blackbody hulls (only radiate in a narrow section of the EM spectrum), very high temperature reactors (can absorb hundreds of megajoules of heat per kilogram of hydrogen), high efficiency cold engines (like mass drivers), broad band metamaterials (can absorb radio better) and so on...

      The cylindrical shape of the hull is just a compromise - it needs to be equally effective from every direction sensors might be in.

      If you don't care about stealth, warships will look like thin needles to maximize the slope of their armor against a single target. Civilian craft will look like a chain of balloons to minimize the structural mass needed to hold them together.

    2. So that kind of stealth work only in that precise enviroment.
      Ok, thank you a lot!

    3. This all assumes there exhaust to detect. I suspect many ships will just coast if they don't want to be seen. And really, it's a question of cost vs. patience. I can't believe that running an engine is cheap, although many stories seem to think they're free. Serenity for instance. They just go and go and never seem to worry about fuel.

    4. Hi Matterbeam, just re-reading this article and the comment section. Great stuff as always. I'm curious about the scenario you describe involving "tens of thousands of mostly anonymous ships..." A few observations:

      The DIA's Space Threat report states that over 21,000 objects greater than 10 cm in size are currently being tracked (in Earth orbit). It doesn't seem much of a stretch to assume orders of magnitude improvements in the number of objects and the volume in which they are being tracked, especially if you're projecting 150-200 years from now.

      The IMO's vessel tracking system currently monitors over 50,000 ships in real time. By international agreement, all ships must carry transponders. A similar regime will be required in space, and a transponder switching off would trigger immediate special attention, don't you think? You might be able to "disappear", but they'll know when and where you did, and they'll be able to backtrack to your point of origin.

    5. The scenario being described is actually the setting of the Expanse. I don't agree with it, but it is not impossible to end up with a situation as depicted in the book and show.

      I would avoid extrapolating out of current object tracking capabilities. We track objects that are 100 to 1000km over our heads, and we confidently work out current and future positions from a single track.

      If each of those objects are a maneuvering spaceship, then you need a continuously updated database. That is magnitudes harder to achieve. Also, object further away require more powerful RADAR stations, especially in an environment where everybody is pinging away with powerful radio bursts. The truth is, it will be much easier to do than what you would expect by just tracing a line from our current RADAR-based tracking to solar-system-wide tracking.

      We are most likely going to use infrared tracking. We can take high resolution images of the entire solar system, even of room temperature objects, from huge distances, and update them on CCD sensors several times a second. We cannot use this option from the ground, as there is a warm atmosphere in the way... but a space-based IR tracking platform would have its work cut out for it!

    6. I should have stated my points more clearly. My assumptions were that the tracking system is based primarily on spaceborne sensors, and it is widely distributed. Further, that it does not rely solely on the sensor net's ability to detect and track objects, but that this is in combination with a mandated transponder system. I still think there's room for stealth ships to exist, but they would require Trojan horse style subterfuges for deployment.

    7. Those are very reasonable assumptions.

      Transponders are useful because they remove the requirements from every spaceship or station to have a giant cryogenically cooled telescope to sense potential hazards (intentional or accidental) from far away. This is particularly relevant for chemical rockets that have limited deltaV, or electric rockets with low acceleration -- they need that advance warning to dodge in time.

      However, another reasonable assumption is that the task of keeping track of bodies in space is left up to authorities to perform, like modern air traffic control. A civilian spaceship can expect to rely upon updates from the authority, using simple radio communications and without backup from its own sensors.

      This does imply however that transponders are not strictly necessary because of two things:
      -Transponders are only an additional layer of accountability/tracking/safety on top of sensors networks when spacecraft are visible.
      -Transponders won't be used anyway by spacecraft trying to evade sensors in the first place.

      This leaves the narrow opportunity where you spot a ship on the sensor network, and you get a deceptive transponder signal. What consequences does this have?

      I believe that in space there is a natural caution against letting ships get close. A small cargo freighter might have every document double-stamped by the authorities, but it won't mean that it can drift up to a military warship as it wishes. A harmless crew transport in space that has been tracked for the past million km still won't be allowed to cross trajectories with a space station at high velocity, whatever it claims it is doing.

      All in all, I think transponders are a useful but redundant tool. Therefore, Trojan-horse style deceptions need additional contrivances to become relevant.

  4. Great article as usual, MB. Your final paragraph describing far future tech is excellent research material for me. Might I suggest you include such small entries on all write ups in the future?

    1. Thanks John, I will.

      Is there something you want to know specifically about the potential performance of stealth ships with more advanced technology?

    2. Here's one: assuming advances in heat and radiation absorbing materials, combined with ultra efficient heatsinks in a far future setting, could a spacecraft theoretically shut off its fusion engines (bottle up the heat buildup with internal heatsinks and refrain from turning on radiators) and safely hide in an asteroid field for say, a few days?

    3. Well, with perfectly efficient heat pump (superconducting, not hard to achieve with all the cryogenic hydrogen coolant), you are limited only by your thermal coefficient of performance.

      This means you could pump your room temperature heat into hotter and hotter hydrogen.

      Instead of being limited to the evaporation energy of about 450kJ/kg, you could pump up the hydrogen to 6000K before releasing it. This increases the heat you absorb per kilogram up to 18.8MJ/kg.

      So, if you have a crew of 100, and each needs 1kW in life support and electronics, and it is provided by a 50% efficient reactor, you could absorb the heat using only 920kg/day.

      If you use outrageous Magnetohydrodynamic compression heat pumps to reach a 20000K temperature inside a magnetically confined plasma, you could reduce this to maybe 750kg.

    4. If you could do this you wouldn't need an NTR for very long!

      I now wonder about mixing this with a Q-Drive. Your coolant is your propellant for an electric rocket. Your windmill and a very efficient reactor like a PIDEC could ionize the gas if it's not yet hot enough and accelerate it without adding too much heat, but even then it might help.

  5. While the stealth spaceship seems more plausible now, I'm still leaning more towards "Atomic Rockets" view of stealth in space. Like real stealth aircraft, stealth isn't total invisibility, but rather masking or reducing signatures. Some of the comments about "The Expanse" seem more realistic (stealth is only possible from limited angles and wavelengths).

    I am a bit agnostic about long needle shaped spacecraft, since space is a 3D environment, and it is likely that you will be targeted from multiple different directions. The long, thin needle shape makes sense based on having a long mass driver or electron beam accelerator for the FEL as the main weapon, but an equally plausible design might be a "corncob" with rows of VLS cells for missile launchers. And of course the needs of the crew on prolonged cruises go towards spin habs and other more voluminous shapes. Long needle shaped drones under the control of a manned command ship might be the way a constellation might be constituted.

    1. I think stealth will allow combat ranges to shorten and for 'fortifications' that rely on extremely long-ranged weaponry to hold back attackers to fail. Overall, this is a good thing for scifi writers that don't always want to write 'chess in space'.

      The needle-shaped spacecraft is simply a result of there being a single Sun. You want to minimize the ratio of surface area exposed to the Sun, to the volume of space not exposed to the Sun. The result is a long needle.

      You could go for a spherical spaceship, that is equally stealthy from all directions, but the increase in exposed surface area means you absorb more sunlight, which means you need to carry more cryogenic heatsink, which makes you bigger... making the disadvantages relative to a long needle less obvious.

    2. Masking or reducing signatures is already enough to do many things.
      Even Predator can be seen as something transparent that can moves when his cloaking device is on...

  6. Great article, there are interesting concepts there!

    About occultation, I remember a discussion about it before, where it was explained that its range is limited. If I remember correctly, for the same reason for laser decollimation, light will sort of bend around the object to the point that after a few hundred thousand kilometres, it is completely unobservable.
    An article exploring the question, and setting a minimum distance and the kind of occultation detection platforms we could see would be interesting, though!

    With a cylindrical body, some sunlight will hit the sides. How problematic is this? How close to the Sun can it go? Also, why not use a double-cone design to maximise the volume per sunlight received?

    Also, what are the advantages of slush hydrogen? I would have expected small helium tanks (enough for running the coolant loop) and supercold <1K solid hydrogen (possibly as pellets) to maximise endurance, with liquid helium heating the solid hydrogen up to 3K in cold mode.

    Also, I have a hard time understanding the wire radiator part.
    They have to stay in the shadow of the craft (or they would be heated up by the Sun), so this will ultimately limit their surface area. The obvious choice would be to use the hull itself as a radiator, then, but you evoke how wire radiator are transparent of most of their own radiation due to their thinness and letting non-polarised light pass. But then, wouldn't they simply emit polarised light themselves?

    1. Thanks Eth.

      I'll look into whether detection by occultation has a minimum/maximum distance.

      A shadow in space gets wider with distance due to the inverse square law. This means that a shadow created by the nose of a spaceship will protect the flanks from receiving any sunlight, so long as they are as wide as the nose is.

      A double-cone design could add a few tons of cryogenic heatsink, at the risk of greatly decreasing the margin of error allowed in pointing the nose of the spaceship directly at the Sun.

      Slush hydrogen can flow, that is all. It is nearly as dense as solid hydrogen. Completely frozen and sub-cooled hydrogen could work, but I am not sure how you could allow evaporated hydrogen from escaping without creating bubbles...

      The wire radiators absorb roughly 40% of the sunlight that hits them in the direction of the Sun. This means that the ATOMSS's 'mane' of wire radiators around its nose will absorb, in total, about 7.66kW. This represents about 7.17% of the waste heat the the craft is dissipating in the warm mode - I cannot be sure about these numbers right now, so I dismissed them as being insignificant. The same goes for some other very minor things, such as the radiating surface of the hull itself, and the tips of each wire loop radiator.

      You are correct about the polarized light. The radiator forest lets polarized light escape, while light polarized in the other direction gets re-absorbed.

  7. This only works if you're writing about the current epoch, using only Earth technology. RADAR aside, how about mass. Any technology that involves the control of gravity can use it to detect mass as well. True, it's a coarse tool. You aren't going to be able to read their license plate, but you would probably know it was there.

    And think about some of the planetary detection missions. Objects are going to occult stuff no matter how black their surface is. Don't detect, just patiently look.

    Blocking the infrared spectrum isn't going to be that helpful. Satellites, especially passive ones, aren't going to generate that much heat. In Earth's shadow, they won't even reflect it.

    Earth tech will emit radio, which might reflect off of other things. Radio can be pretty sloppy. How about the ionosphere? Look for the reflections. And if you really want to get trippish, perhaps you can detect the receivers! If you could track the changes in the antennas, you could infer the source. How about inductive couplers on the antenna cables with wireless Internet connections sucking the raw signals, before it's filtered. The amassed data could be used build a map of emission sources. The whole planet could be linked into what would amount to a giant eye.

    1. Are you sure about that? I'm considering a much worse situation for the stealth ship than today.

      Today, most infrared sensors are pointed at far away stars or down at the Earth. Instead, I am assuming that the stealth ship must escape thousands of sensors all over the Solar System, looking at it from all directions.

      The positions of all these sensors is why directional stealth doesn't work.

      The stealth ship is trying to not emit infrared radiation at all, not block it.

      The ionosphere reflects some radio frequencies back down to the ground, that is all.

      I'm not sure what you mean about that last part.

  8. Occlusion was considered as a detection method for B-2 stealth bombers. It wasn't considered feasible back then, and it does not seem like a reasonable stealth-defeating method for spacecraft either (sans some sort of continuous whole-sky-scanning device that scans large chunks of sky every second instead of every few days). You could detect lots of exoplanets with that thing.

    It seems like the proposed stealth spacecraft would be significantly limited vs conventional spacecraft optimized for combat; imagine a scenario where such a craft encounters a nuclear pulse vehicle. Are we looking at it as more of a U-2 spy-plane or a general-purpose warship?

    1. A B-2 stealth bomber is a surface area of about 478m^2, flying at an altitude of about 12km. Its solid angle in the sky is therefore 3.3 microsteradians.

      The ATOMSS stealth spacecraft has a surface area of up to 1600m^2. To have the same solid angle as the B-2s, it would have to be as close as 22km.

      Well, that's good news for the stealth spacecraft on the occlusion front!

      Stealth ships, while hiding, can deploy missiles or shoot lasers/coilguns using energy stored in capacitors. A few more shots can be drawn from a reactor if you decided to use up all the cryogenic heatsink. This gives them a very effective first-strike ability. However, if they want to brawl with a dedicated warship, they'll have to extend radiators and lose all stealth advantages. The mass of cryogenic heatsink and the dead weight of all the stealth equipment turns into a big disadvantage in that case.

  9. Reading Mark Bondurant's reply, I believe (and correct me if I'm wrong) he is referring to an idea that a stealth aircraft will leave a "hole" in the pattern of electronic emissions from ground and air based electronics. If you were to "see" the airspace over any first world nation in the EM spectrum, it would be filled with a white noise of emissions from cellular telephones, garage door openers, AM, FM, UHF and other commercial radio broadcasters and the EM "hum" of millions of electric motors. (This incidentally is why lots of computer networks require the use of shielded network cables, cables encased in metal conduits and fibre optic networking. While it is indeed great for high security in your network, it is often more important to shield the cabling from the emissions from the fluorescent lights in the building).

    Stealth aircraft, by deliberately scattering reflections from various wavelengths associated with military radars, will also have an effect on emissions at various harmonic frequencies (partial or multiples of the frequency), creating a "hole" effect if you know what to look for.

    In space, the Sun radiates over a huge electromagnetic spectrum, and it is possible that a grid of wide spectrum receivers could identify "holes" in the EM spectrum created by a stealth spacecraft, although I suspect the issues would be orders of magnitude more difficult due to the overall area of coverage needed.

    As for the military response, if there is an existential danger of "first strike" spacecraft lurking in your Hill Sphere, then part of the Navy would be configured as "ASW" craft, perhaps fleets of Corvettes or Destroyer Escorts, resolutely patrolling the area with high sensitivity scanners, interrogating off board systems (like giant mirror farms used for other purposes) and deploying "active" scanners across multiple wavelengths (much like the often discussed multi frequency "chord" radars often touted as means of deleting stealth aircraft). Since these spacecraft might work in small flotillas, they also serve as distributed receivers (once again like the "Bi Static" receivers also suggested as counters to stealth aircraft i.e. the radar pulse is transmitted from one set, but received by another due to the off axis reflection used by stealth aircraft, with computerized signal processing determining where the aircraft is). Since they are patrolling around the Hill Sphere, they can also look "down" at the planet, where an incautious stealth ship may expose itself much like the picture in the opening of this thread.

    Since we are looking at small, relatively inexpensive spacecraft, these might be the Casaba Howitzer armed "Fire Stars" between "Laser Stars" (with multi gigawatt RBoD lasers) and "Kinetic Stars" with a multitude of high energy rail or coil guns to pelt the target with kinetic energy projectiles. Not needing large generators or radiators because the nuclear "packages" drive the weapons effects, they can be much smaller and cheaper than the capital ships of the constellation.

    1. As an aside, Children of a Dead Earth had a small "Shooting Star" drone as an experimental gun platform. While that particular version was ultimately ineffective in its context, "Shooting Star" sounds better than "Kinetic Star" in my humble opinion

    2. Laser Star and Kinetic Star originated in the "Rocketpunk Manifesto" blog. Although it is inactive, I believe you can still access the posts:

      And Rocketpunk Manifesto is referenced in "Atomic Rockets":, and Rick Robinson also has a true claim to fame as being the namesake of the "Rick", an informal unit of energy which states that any mass moving at 3Km/sec has its kinetic energy equal to it's mass in TNT.

      So I will continue to use Laser Star, Kinetic Star and now Fire Star as homage to their origins. This also means Matter Beam's stealth spaceship is a "Dark Star" (with a nod to Dan O'Bannon).

    3. If you visualize the wings as radiators, and have "Starfury" like RCS thrusters on the ends to allow for rapid changes in roll, pitch and yaw, then yes that would be an excellent vision to start with.

    4. Occlusion against the Sun or the Earth, in any part of the electromagnetic spectrum, would be very problematic if the Stealth Ship was the only spaceship around.

      I imagine you are thinking of a sensor 'looking down' at the space in between the Sun/Earth and the stealth ship and noticing a gap in the emissions. There will be gaps caused by everything solid sitting in that view. Some will cast shadows large enough to hide other ships in, intentionally or not. These shadows will be moving, and most are small enough that you will find multiple shadows in a single pixel of the sensor...

      Anti-stealth ships would definitely be useful. They would have exceedingly power Radars and just enough weaponry and propulsion to dodge warheads from uncovered steamers and take them out. They might need their own stealth tactics to sneak up close before blasting a volume of space with radio waves. Hopefully, it's not a decoy which led them into a NEFP trap.

      The only downside to the prevalence of these miniature nuclear devices is that it lessens the advantage that stealth ships have of being able to close in on an enemy before attacking. Lasers get more intense, kinetics become harder to dodge, but nuclear warheads can just be shot from afar with their own expendable cryogenic heatsink.

      Starfury is a terrible spaceship design. There's no room for any propellant!

    5. I loved the Starfury because it was (to my knowledge) the first spaceship design in mass media to really demonstrate an understanding of the space environment since "2001". Like all media spaceships, it is far too small (a true Starfury would be similar in size to an F-15, and the pilot would be suspended in a very small canopy bubble in the front, not one which dominates the entire front of the spacecraft).

      I als see my reading comprehension is declining with age, the comment about the RQ-3 was meant to say the real life "Dark Star" might make a good mental image of a Fire Star.

      Of course the advent of Dark Stars would lead to changes in fleet tactics, and part of the fun might be pt look at the historical development of fleets. Even in ancient times, Roman galleys became larger and larger, as they evolved into huge troop carrying platforms and needing the stability to carry catapults on deck to cover the marines storming the enemy ship. The largest ship was the Decares ("ten", which seems to indicate the number of oarsmen per level). Once the Mediterranean sea had become a "Roman lake", the sizes of warships abruptly fell and small Liburna became the order of the day. In our day we have seen gun armed ships replaced by aircraft carriers, and now missile armed ships are supplementing aircraft carriers for some roles. And if the issues with electromagnetic railguns get solved, gun armed ships may once again become the order of the day...

      I would suggest a Fire Star would likely devote a large portion of its payload to expendable sensor devices, in order to find and pinpoint targets, both to economize on the number of ships needed, and to minimize the threat of being lured into a trap. There is nothing to stop the Fire Star from sending the coordinates of a bogy to a Laser Star and having it illuminate and destroy a target, rather than expending one of its own nuclear warheads. There are plenty of ways to skin this cat.

    6. If your expensive space warship can be taken out by a single nuclear warhead, then you will make the warships no more expensive than the warhead. Realistically, the warship will be bigger and more expensive than the warhead, because you get to factor in the effectiveness of defense systems, the value of alternative weapons and reusability, as well as the expense of delivering the earhead and penetrating those defenses...

      Expendable sensors could be nuclear warheads too. A 'ping' from a nuclear warhead could send out incredible amounts of radiation, forcing stealth ships in the vicinity to shine like stars. After revealing their position, you shine a bright UV laser on them and keep them tagged until your real warheads deal with them.... although it does introduce the concept of 'breaking lock'.

      Anyhow, the possibilities it opens up for SF warfare is an exciting prospect for any author or worldbuilder who feels trapped by the majestic, pre-determined clashes of laserstars other extreme distances suggested by Rocketpunk Manifesto.

    7. Truthfully, I don't feel "trapped" by the idea of laserstars opening up at 300,000km. What will change is the way combat is depicted, you won't be reading "Master and Commander" or other "Age of Fighting Sail" novels, you will be reading "Red Storm Rising" or other modern combat novels. Many of the sequences will even be similar; commanders determining the correct time to unleash a broadside, damage control parties working frantically in the depths of the ship to plug leaks and get the ship back into action, the ship's surgeon preparing his tools and table for patients with the ship lurching wildly under him....

      So long as the writer respects the "Zeroth law" and remembers fiction is about people, not combat systems, then you can get great stories out of almost any setting; remain consistent [i]in that setting[/i] and avoid deus ex machina (which is more the point of research through Atomic Rockets, Rocketpun Manifesto, Tough SF and a host of other sites).

      My .02 anyway.

    8. The way I see it, laserstars have the effect of greatly simplifying space combat. While there is some complexity to the interactions between laser technologies and armor designs, it still a very clean and clear-cut arena compared to the modern, cluttered environment we have historically experienced and read about. Maneuverability and detection are non-factors, and positioning is as visible and predictable as the movement of pieces on a chess table. It is a game played by Generals, not Pilots or Soldiers.

      Even the great Laser vs Missile debate revolves around economically overwhelming a laser's ability to shoot down a wave of missiles - it is a resource management problem to be optimized, with no decision making to be had on tactical timescales.

      There are always ways to work around the lack of human presence or effect on a conflict, especially with narrative devices and a selection of characters higher up in the command chain, but this is an odd situation where the plot and author must be subservient to the technology depicted in a setting. Authors without the technical knowledge to challenge these conclusions skip straight to soft scifi, where they can have their X-wings swooping over Death Stars.

      Not a great outcome, in my opinion.

  10. Dark Star?.....yes....yessss....goooooooood....I can feel your hatred >:D

  11. S3E03 of the Expanse had stealth spacecraft (in a nuclear first-strike capability) very similar to those proposed on your site, and not depicted in the original novels. The Martians really should have randomized their fleet movements.

    Also, additional proposal:
    "Brilliant" (very smart) conventional forged-fragment projectiles (similar to "Skeet" submunitions) on guided/semi-guided railgun shells.
    While they greatly decrease the mass on target, they increase the area-of-effect of a single projectile and may be more effective than buckshot shells.
    They may be more difficult to deal with, since defensive fire will have to differentiate between missed rounds or sideways-firing forged-fragment projectiles.

    1. The Expanse got a lot of things right with its stealth design (except for the evaporative cooling part) and the entire scene in general was very realistic and well thought out.

      Moving the platforms was likely something they avoided doing to prevent detection because they lit up a fusion drive. A super expansion nozzle would have helped there too.

      Sideways firing projectiles is difficult to pull off in space because the relative velocity of the projectiles and the target is usually multiples of 10s of km/s. The lateral velocity would have to be several times greater than even that to hit a target while passing over to the side.

    2. I realize this is an old comment, but how do you have super expansion nozzles with nuclear drives?! I can't even see how that works with ion drives.

      The only options that come to mind for stealthy rockets that aren't super expansion gas thrusters would be nanoparticle/macron or maybe a neutral particle beam.

    3. Gases cool down when they expand; that's just the PV = nRT relationship. A large nozzle expands exhaust gases more, so the final exhaust stream gets colder. Even the 3000 Kelvin gases from a nuclear thermal rocket can be expanded to the point where they are below 20 Kelvin... you just need a nozzle with an expansion ratio of several tens of thousands (unlike the typical 1:200 of a vacuum nozzle today).

  12. Ah, i see what you mean. The projectile would have to detonate some distance ahead of the vessel targeted. That might still abruptly turn a near miss into a hit.

  13. Hi, really great post! It's the first time I see a serious design to counter the "there is no stealth in space" mantra, and it looks solid, so congratulations.

    A few remarks however:
    - The background noise in your calculation is noted BNI, for Background Non Uniformity. It's noise in the sense that when you take an image of it, you will see structure. However, it is reproducible from one image to the other: if you take a second image, you will see the same structure.
    The difference between the two images will be the quantum noise due to the background, which is equal to the square root of the number of background photons received by the detector. So you have to compute the number of photon first and reinject that in your calculation to get the true detection distances after background subtraction.
    - Regarding radar stealth, you might still want to use "garbage angles" in which you redirect the energy to shorten the detection range. If you have only a few on those over 4pi steradian, the risk of hitting an ennemy sensor is low. Also, you want the beamwidth of the redirected energy to be low, so you need large surface for that. Spheres and cylinders are bad shapes for radar stealth because they always redirect energy in the direction of the emitter (there is always a 90° angle), and because they will create Mie scattering if the wavelength of the radar matches the diameter.
    - You tremendously cut the coolant consumption by putting a mirror in front of your ship: it can reflect at least 95% of sunlight, so extend your endurance by 20x. The downside is you reflect light, but only in a cone that has the same angular diameter as the sun, which is 0.5°. That's a "garbage angle" you have to manage but it is small.

    1. Thanks. We corresponded by twitter DM on this topic, so I don't have much to add on the topic - but do you have numbers for the BNI and how to use the photon count method for detection?

  14. I have a few issues. First is your claim that vantablack will efficiently absorb wavelengths in the millimeter to decimeter range. I have found no data supporting this claim but would love to review your sources.

    Another is the limited aperture size of the detecting telescope. At 10 meters, this is likely to be dramatically cheaper than the spacecraft in question. With this in mind, it seems that you are only out to show that stealth can be maintained against sufficiently impoverished opponents.

    I also have many concerns about your proposed cooling method. Among them:
    -Thermal stress in the reactor's cooling jacket
    -The number of stages the cooling jacket would need to have in order for the outermost layer to be sufficiently cold. How was the mass of this system estimated?
    -The energy cost to pump propellant through such a convoluted system
    -System complexity
    -Any attempt to start up or cool down the reactor will necessitate dumping tremendous amounts of propellant at far from ideal exhaust velocity. While less hydrogen needs to be dumped when the reactor is cold, this hydrogen is completely useless for propulsion. This cuts dramatically into the payload fraction.
    -The mass, size, and radar cross section of the expansion nozzle. I will point out that it is uncomfortably similar in shape to a parabolic antenna.
    -Direct heating of the liquid helium by whatever system pumps it around
    -How liquid-vapor separation is achieved
    -Heat generated by the liquid helium cooler
    -Even if the nozzle is shuttered and propulsion is pulsed (a prospect that creates all manner of headaches for the pumps and reactor), residual heat on the inner surface of the nozzle would still be quite visible when the shutter is open
    -Turbulence in the exhaust causes nozzles with excessively high expansion ratios to perform less effectively than the ideal model would indicate. Pulsed operation will certainly exacerbate this problem, and at the required pulse rate might even completely change the operation of the nozzle. De laval nozzles are designed for continuous operation.
    -The described shutter would have a radar cross section approaching that of a purpose-made retroreflector.
    -Regenerative cooling on nozzles still results in the nozzle being quite hot near the base, so the enormous nozzle will also need to have a cooling jacket like the reactor, though the number of layers could be reduced toward the end of the nozzle.

    Given the numerous concessions of performance and payload, I have a hard time seeing how the stealth features of this spacecraft, even if they perform perfectly as you have predicted, could justify not simply using a larger number of smaller spacecraft, each with the same payload as the stealth spacecraft. I similarly have a hard time seeing how this could be better than a military spacecraft disguised as or carried within a civilian spacecraft.

    A parting countermeasure- fight in a few billion years when the CMB is colder than the boiling point of helium

    1. Vantablack, as presented by the company 'Surrey Nano Systems', is only effective against a narrow range of wavelengths, with the highest emissivity in the near infrared range.

      However, the name 'VANTA' actually designates the entire range of designs possible using nano-technology and nano-manufacturing of carbon nanotubes, and not just specifically that design.

      By stacking layers of Vantablack optimized for each range wavelength, from visual wavelengths up to microwaves, you can get near-perfect absorbance in that band. Above microwaves are the radio frequencies, for which the Radar Absorbing Materials, that coincidentally also use carbon nanomaterials, can be used to create a high absorbing surface. I specifically linked to existing materials that can reduce radio returns by a factor 1 million.

      A single detecting telescope will be cheaper than a single stealth ship. Two, three, ten, twenty of them are still likely to be cheaper. However, covering the avenues of approach possible for a stealth ship will require thousands of satellites within Earth's Hill Sphere, and hundreds of thousands for the rest of interplanetary space. However, cost figures are a very soft science, which I can only give broad comparisons for and not precise numbers.

      Note that even the telescopes need to use large surfaces of Vantablack and cryogenic cooling to stay hidden from sensors onboard the stealth ship. If they allow themselves to become warm and visible, the stealth ship can just weave a path through them and pick opportune moments to enter helium cooling mode to create gaps even in the tightest formation of telescopes. In other words, the number of stealth satellites that matches the cost of a stealth ship isn't too great.

      The reactor's cooling jacket will be fine. Actual 1970s testing of nuclear thermal rockets had high pressure cryogenic liquid hydrogen pumped over 3000K zirconium oxide surfaces and heated to 2800K before it left a reaction chamber a few meters long (Project Rover).

      Thermal insulation will not be a significant hindrance. We already insulate superconducting cryogenic magnets from ambient air using a few centimeters of vacuum-flask-like walls. Nearly 100% of a nuclear thermal rocket's heat is supposed to be absorbed and taken away by the propellant flow, and the remainder is absorbed by active cooling that runs liquid hydrogen over the outside of the reactor.

      While the cooling system will be complex, it only has to handle a few grams per second for stealth cooling. High volumes of cryogenic substances are involved in propulsion, but I'd point towards the turbopumps ejecting hundreds of kilograms per second from the Shuttle External tank into its RS-25 engines as an exmaple of a solved problem.

      System complexity: yes. Worth the cost though.

      Starting up the reactor is easy. You let it heat up to operating temperature with no coolant flow. Stopping it is hard. The heat can only be removed by 'wasting' hydrogen. However, the amount needed to cool a reactor is tiny compared the hundreds of tons a stealth ship normally carries. Also, all endurance calculations for hydrogen cooling mode is done with the assumption that it is vaporized and then thrown overboard. Any additional heating raises the temperature and the energy it can absorb, and thus does not impact the minimum endurance figures calculated and presented in the blog post.

    2. The nozzle will have a narrow opening angle. It will look like a trumpet horn. The end open to space will have a shutter that closes it to external viewers and radar signals. The nozzle walls can also be actively cooled by liquid hydrogen/helium evaporation, to the same temperature as the spaceships' nose.

      Turbulence in the nozzles might be a problem, yes. The solution might be to elongate the nozzle to match the nozzle expansion rate to the gas's expansion rate.

      A shutter can be made of the same materials as the hull exterior, and be hollow to accommodate an active cooling flow.

      A cooling jacket near the nozzle throat is standard design on liquid fuel rockets.

      Stealth craft have the ability to approach an enemy fleet, drop a large number of missiles at close range, slink away and then activate the missiles. This allows it to destroy assets of military value greater than what it cost to build and operate by several hundred fold. It can move into position to sit in front of an advancing enemy fleet halfway into their interplanetary trip, and drop a minefield for them to smack into completely unawares. It forces opponents to launch and maintain a huge number of telescopes, continuously maintain them and dedicate military resources to a stealth ship interceptor fleet that sit twiddling their thumbs, unused, for 99.999% of their time, and so on.

      The disguised-as-civilian ships will only ever work once. The moment an attack is conducted, the game is up and all civilian ships are marked as hostile, shot down, and the rest told not to approach military assets within a million km.

    3. First off, if the spacecraft can be detected in transit then far fewer telescopes are needed and there is no need for the telescopes to be hidden. With better telescopes, detection in transit may well be possible.

      The actual 1970s tests didn't have cooling jackets with exteriors under 20 kelvin. Your proposal is radically different and if the structure isn't enormous then the thermal gradients will be. I'm not sure why you bring insulation of cryogenics when they prevent the movement of heat and the objective here is to facilitate its movement, meaning that vacuum will work against you. The propellant flow will need to go through the large cooling jacket if you want the exterior temperature to be below 20 K.

      The space shuttle's turbopumps significantly heat the propellant. This is no problem for the shuttle but it is a big problem for you.

      If you start up the reactor with no propellant flow then the spacecraft will become quite hot during that time. The same is true of shutdown.

      The interior nozzle walls near the base of the nozzle cannot be cold during propulsion.

      Instantly killing all civilian vessels once a single fake one is detected would be a very serious war crime and has no historical precedent that I'm aware of. There are presumably military assets in mid- to low-orbit around any important planet, and civilian vessels must get much closer than a million km to those in order to do civilian things. It can potentially work many times as evidenced by the fact that it has worked many times before.

      I will also note that if the craft maneuvers stealthily through a net of sensors then its presence can be detected with a mass spectrometer.

      Additionally, the potential trajectories and transit windows of this spacecraft are quite limited and as a result the search requirements for sensors are much reduced.

      As a final point, if this method was effective and didn't dramatically limit performance then it would be an attractive alternative to large radiators and would probably appear with fewer stealth features in quite a few NASA papers regarding nuclear propulsion, since open-cycle cooling is not at all a new idea. Numerous engineering practicalities make this ineffective even before trying to cool an entire spacecraft down to hide against the CMB.

  15. What are the possibilities for electronic warfare and jamming in space? From my knowledge, low wavelength communication is difficult to jam, but requires line of sight, of which there is plenty of in space. How do we get around that issue? Are there other methods to disable enemy autonomous weapons platforms, so as to remove the obvious superiority of unmanned missile and drone swarms, and large unmanned laser cannons and mass drivers? Are the only methods of encouraging human crews light speed lag and damage control? How do we prevent the control ship/drone fighting ship paradigm?

    1. Also, are there any methods to defeat sensor nets and sensor fusion to enable increased stealth for warships without vanta black hulls?

    2. Hi Anonymous.

      The 'how to put humans in spaceships' question is pretty hard to answer. My personal solution is to have AI vs AI tacticians become effective when they are asked to predict each others' moves. The calculations to be made increase exponentially with each move ahead and with increasing numbers of ships and weapons involved... Humans nearby would be needed to 'direct' the AI. There is no better place to put the humans than inside the armored hull of a warship... if armor is expensive and propulsion difficult, then putting all your assets into a few large spaceships is more effective than scattered control ships and drones.

      Jamming is very effective when trying to hide stealth ships because the background noise is very, very weak. Even a modest laser output can blind sensors over huge areas from very far away.

      Vantablack hulls are needed to prevent sunlight reflections giving away the position of a spaceship. They're just the generic name for carbon nanotube arrays that absorb most sunlight wavelengths very well.

    3. Even if armor and propulsion are difficult, why inside the hull of a combat ship? Life support costs mass, which could be devoted to armor and propulsion. The humans could 'direct' the AI from a light second away. Decreased combat efficiency would be compensated by putting humans away from the line of fire, and consolidating life support costs to a few control ships. Keeping humans safe, who would require numerous technical skills and be costly to train, allows generals to be more willing to take losses.

    4. Perhaps it is possible to jam communications between the drones, sensors, and the control ships? Maybe IFF signals can easily be spoofed or drones could be hacked, forcing the operators to include rudimentary crew on such drones to prevent electronic attack (although this especially feels more like pleading)? Frag NEFPs could take out delicate sensors and data transmitters and force the enemy to consolidate with fewer, better armored drones. From there, adding rudimentary crew to stop hacking, make decisions, and perform damage control. But hacking can also be stopped with better software, I don't see how human performed damage control is viable in multi-g accelerations, and NEFPs or Casabas are so powerful that they can oneshot warships and remove the need for damage control (warships would instead have redundant systems or better thrusters).

    5. The reason is two-fold:
      -Life support mass for the crew of a mostly automated space warship is tiny. It is something like <1% of the total dry mass of the ship. In the 'Space Warship Design IV' blog post (, it came out to 0.57% of the dry mass.
      -A light second might sound long distance away from the front-lines, but it isn't really. Lasers would have difficulty firing across that distance, but enemy warships can choose to dodge drone warships and close in directly on the crewed command centers. Alternatively, they can engage the drone warships and shoot missile swarms that ignore the drones and head directly for the command centers. Even worse, they can slip in a handful of stealth cooled missiles past the defenses of drone warships to attack the command centers. It only takes a relative velocity of 40km/s to cross that distance in two hours.

      If your command centers need enough engines and deltaV to keep up with the drone warships, enough anti-missile defenses to defend against direct attacks, enough kinetic armor to fend off the stray missile and enough laser armor to prevent destruction by attackers that are closing in... it would make sense to give it weapons and turn it into a full warship.

      It is certainly possible to jam communications between members of a fleet, but it is hard to do so on practice. They will send coded laser pulses to each other. It is extremely difficult to overpower those signals, to break the code in the heat of combat or to block the lasers.

  16. Stealth on the cheap: the Neon Steamer!

    Just replace (part of) hydrogen with liquid or even solid neon.
    While not as good a heat sink per mass and a mediocre propellant (and random neon is more suspicious than random hydrogen, if detected), it is a much better heat sink per volume, and the Hydrogen Steamer is limited by volume (due to incoming sunlight). In addition, neon is easier to work with than hydrogen. And while not quite as abundant, it is by no means hard to come by.

    Neon boils at about 27 K, which is a bit hotter than the 20 K of hydrogen, but you can keep that temperature for much longer with the same volume. Volumetric specific heat is also much better, but at those temperatures it is quite low anyway, so heat of vaporisation should still be the main factor.

    So if you have a platform that doesn't need to be quite as stealthy and doesn't have to move around too much, but needs higher autonomy (for example if it is closer to the Sun), or if you have budget cuts, you may want to use this.
    Now, if you fear that neon emission will be detected, start by using some hydrogen (for example to put it in place), and then keep the gaseous neon in the now-empty hydrogen tank to account for the volume difference.

    1. I've responded to this comment elsewhere:
      Seeing how the endurance of a stealth steamer depends on how much energy it absorbs from its frontal area, it might be worth exploring very high MJ/m^3 coolants instead of MJ/kg.

      A helium coolant can boil at 4K. Its vapours are compressed in a heat pump to move the heat into neon, then the helium is reused. Your 1.3kW/m^2 of heat absorbed at the spaceship's surface becomes 7.5kW/m^2 after the heat is pumped from 4K to 27K.
      That neon can then run through the ship and cool down habitable spaces, electronics, high powered equipment and then pass through the nuclear reactor at 3600K.
      You can choose to eject it... or heat up Vanadium up to its boiling point. Vanadium will absorb 8.9MJ/kg by vaporizing, 0.45MJ/kg by melting and 1.78MJ/kg from the 20 to 3600K increase in temperature for a total of 11.13MJ/kg. This is 5.39 times worse heat per kg than hydrogen... but it is about 87 times denser.
      If the boiling temp of vanadium is too high to run your reactor at, just lower the pressure and its boiling temp will go down.
      At absolute zero pressure, vanadium boils at something like 1500K.

      The heat absorbed through the skin serves to vaporize a certain quantity of helium. That quantity is then heated further by other steps that put in a lot more heat at higher temperatures. Except, we complicate things by switching out helium for coolants like neon or vanadium.
      What matter is what finally comes out the nozzles of the ship at the end.

  17. Merry Christmas MatterBeam! I think this recent article would be a good idea to have a read of on this topic:

    Looks to be a 48km wide object, visible from five different telescopes on Earth, under the Earth's atmosphere, at a distance of >6 light hours (well beyond Pluto) due to occultation. Perhaps improvements in computer processing and operating out of atmosphere could make this a viable means of detecting stealthed aircraft at long ranges.

    1. Great find!
      I don't think this hurts the possibility of stealth ships. 2014MU69 is about 3.63 times brighter than Halley's comet, and its position was known to a rather good degree of precision after weeks of observation... and they still missed it on the first two tries!

      An interesting fact came up from looking into that topic though: the object's surroundings were only cleared of any other presences up to an apparent magnitude of 29. This is a great reference for how dark an object something like Hubble could pick up, at least in the optical wavelengths.

  18. It is amazing and wonderful to visit your site.Thanks for sharing this information,this is useful to me...
    click here

  19. Sorry to necro this article, but I was wondering of the design could be somewhat miniaturised to get stealthy mines/missiles.

    My idea was that the ATOMSS should optimally be able to do a bombing run without reveling its position. Let's assume we want to attack a fleet of regular warships. We deploy a few dozend Casaba Howitzer warheads with a basic sensor pack and the capability to rotate via a catapult. The weapons themselves have helium tanks and the same stealth technology as the ATOMSS. After they were deployed on a trajectory towards the targets, which they'll hit after several dozend minutes, the spacecraft has already burned to reposition itself.

    Optimally all the missiles/mines go off at the same time. Then we could either attack again or get out of there before they can find us using UV or X-rays.

    This would potentially allow us to use Wolfpack tactics and allows for really devastating and low risk attacks.

    However it depends on how well the stealth tec scales down. We could clearly carry significantly more ordenary missiles. Do you think the stealth missile road is worth travelling?

    1. Hi! Don't worry about returning to these older posts, I still respond to everything.

      Smaller stealth craft like missiles will probably not be able to work like an ATOMSS because it is not useful to give them the machinery (radiators, heat pumps) needed to radiate heat away in a 'warm' mode. They are small and will not spend a lot of time drifting through space before they hit a target. It is best to give them as much helium as they can fit onboard, and send them off under maximal cold stealth until they reach their target.

      The best use of stealth technology in a setting where propulsion technology does not involve torch ships zipping around from planet to planet, is to have one huge mothership with missiles that are shot out of a coilgun. A huge spaceship has a better surface area to volume ratio, so it receives less heat from sunlight relative to the cryogenic coolant it can carry. Shooting off missiles with a coilgun does not release any gases or compromise stealth. They can travel at a decent pace before using their own engines for terminal maneuvers. At 5km/s, they can cross the Earth-Moon distance in just 22.3 hours! That means the mothership can stay very far away and remain even stealthier.

    2. Hi matterbeam to drag you back to this article again. I was just wondering about the last part of this comment. “The best use of stealth technology in a setting without torch ships” obviously raised the question to me, what to do with stealth ships of the steamer lineage in a torch ship setting. In particular stealthy propulsion of that caliber. I was also curious about your comments about more efficient/advanced heat pumps and how they would effect this system. Why would they remove the need for helium but not hydrogen? And your response to the comment about using neon and other elements to add capabilities to this design really intrigues me but I don’t think I understand it enough to ask anything coherent.

    3. Torch ships travel extremely fast, so the period of useful stealth is reduced from months to days. They propulsion systems also remain radioactive for a long time, especially the nozzle which sits right next to the deluge of neutrons from a fusion reaction. So it becomes to very hard to switch from travel mode to stealth mode. And, the gap between the deltaV/acceleration of a stealth ship and a torch ship becomes extremely wide.

      The comment about boiling neon and other cryogenic gases is based on the fact that despite absorbing less Joules per Kilogram when boiling off, they are also much denser. So you might be able to absorb more Joules per Cubic meter! That is important because the major source of heat that forces you to boil off a coolant in a steamer is sunlight touching the exposed front of the spaceship. Denser coolant means smaller spaceship which means less area exposed to sunlight, so you don't have to boil off as much in the first place... making the heat capacity decrease of switching from liquid hydrogen to liquid neon less of an issue...

  20. Hi, Matterbeam!
    I love this interesting ATOMSS idea, but I have question.

    Can I use more 'efficent' engine (NTR-gas, Fusion... etc) for ATOMOSS design?
    More exhaust velocity allows interesting setting (better deltaV for imposing situation!), but it makes open-cycle cooling hard and exhausts warmer gas.

    How do you think 'limit point' for ATOMSS engine?

    1. You can use a more efficient engine! It just becomes much, much more difficult to expand the exhaust enough for it to cool down to invisible temperatures. You also have to absorb and hide every bit of waste heat coming off the engine. That's quite hard when fusion is involved.

    2. Hi MatterBeam,

      Your laser ablative design, with the graphite/carbon shell around a low molar mass gas, is certainly a pulse propulsion design, right? The engine nozzle might be gargantuan depending on power level, but wouldn't that potentially give you some serious legs while staying stealthy?

      I can also seeing this used for beam-power drones, depending on how well lasers scale down and how ridiculous a tiny drone on top of a huge engine nozzle looks.

    3. That's an interesting idea. I'll look into it!

    4. Did anything come of this idea? 👀

  21. Great design !!!

    Can ATOMSS be scale down so il could be carried inside Starship from space X while having a delta V of at least 6km/s ?

    1. Thank you. Yes, that is possible. It would have to be very lightweight though, because it is difficult to store a lot of hydrogen in the volume of the Starship's payload bay.

    2. I was thinking as a initial concept, a combat ready starship going to L2 lagrange point (because they would be very difficult to hit from earth) than dropping a mini-ATOMSS witch go to LEO in stealth mode and once near the target, it drop a few HGV (Hypersonic glider Vehicule) on ground target or some ASAT weapon on ennemy satellites.
      The ennemy woudn't notice the attack probably until all targets are hit.

  22. Hi Matter Beam, thank you for this fascinating article, as always.
    I have a remark regarding the wire radiators, which you may be able to clear up.
    While I like the idea of avoiding reabsorption from nearby radiators by making them thin enough to not absorb perpendicularly polarised light, I am not convinced that this is something that will work:
    The way I see this, is that it could lead to some sort of Maxwell's demon.
    Introduce a single wire of infinite length and surround it by a perfectly reflective, cylindrical mirror (for the sake of argument).
    According to your proposition, the wire will emit in all polarisations, but only reabsorb in the axis of the length of the wire. Here we have a scenario where the optical cavity formed by the mirror has a constantly increasing density of perpendicularly polarised light, as it cannot be absorbed. Hence the energy density of the cavity is increasing to arbitrary levels, without any work being done. This obviously violates thermodynamics: by introducing a small object into the cavity and linking the radiator to a source of heat, we can increase the temperature of the object to a higher temperature than that of the heat reservoir, without doing any work to run this heat pump.
    My best guess as to what is happening, is that if the wire radiators are too thin to absorb in one polarisation, they will also not emit in that polarisation.

    Let me know what you think. Am I wrong in my thinking? It's entirely possible.


    1. I should add, to clarify: the reason why the energy density of the cavity is increasing, is because the perpendicular polarised light is being reflected inside the cavity, by the mirrors, but cannot be reabsorbed. Since it is however still constantly being emitted by the wire radiator, it accumulates to arbitrarily high energy densities.

    2. This does indeed make a lot of sense. I'll look into the physics behind this a bit more.

  23. Your post doesn’t include considerations for quantum radar, laser searching, metamaterial stealth, RAM heating up or fiber radiator efficiency reductions from radiated heat hitting other fibers.

    >Typical sensors have very small fields of view.

    That’s not exactly typical of military sensors.

    >Longer wavelengths as long as 10 or 100m long can diffract around the spacecraft without interacting with it.

    Radar wavelengths are still liable to reflect off larger surfaces and a surface can resonate with wavelengths 1/10 to 1/1 it’s size.

    >the rest of the body cylindrical

    Radar waves tend to bend around circular objects that are 10/1 to 1/1 their size and produce creeping waves that emit circularly in all directions. It’s why stealth aircraft/munitions avoid circular nose cones. Making your stealth ship cylindrical is probably a horrible idea.

    >We will consider the detection distance of the ATOMSS example design against a small telescope (2m wide collector area), a large telescope (10m wide collector area) and a huge 100MW radio telescope (20m wide dish).

    I don’t think those dish sizes seem appropriate. Firstly, there’s no size limits in space, so they seem pretty small. Secondly, the sensors are liable to use high speeds and inverse synthetic aperture techniques to “create larger dishes.” Thirdly, the enemy’s sensors would probably be operate together as a multi-static array and combine their results.

    >the front of the ATOMSS spaceship can be reproduced by a 4m wide sphere of graphite cooled by a small tank of liquid hydrogen. To an infrared sensor or a radio telescope, this is indistinguishable from the front of a stealth craft.

    It’d likely be distinguishable by omni-directional creeping waves and by ISAR.

    >a tiny emitter on the stealth ship to overpower or confuse a huge and powerful emitter on the radio telescope.

    That’s probably a bad idea. Modern radars can utilize narrow beams and frequency hopping. If you try to jam 1 you’re liable to give away your direction. If you try 2, the ship would be at the intersection of the beams. It may be better to disperse a field of decoy repeater jammers in front of the ship at ranges where enemy beams would overlap multiple jammers; however the radio telescopes could angle their beams to overlap behind the grid and only consider signals that returns those 2+ frequencies at the exact same time. Also, SAR/ISAR techniques could help. Standoff jamming from Mars might be more helpful.

    >a small number of lasers...can render any sensor looking at these wavelengths useless as the signals from the ATOMSS are drowned out by the signals from these lasers.

    Why assume a small number of sensors, that you know where they are, that the sensors can’t filter obvious sources of noise and that they can’t just use shades?

  24. How can you be so sure the exhaust would be undetectable? Sure it is extremely cold but it is hardly a thin gas the density and temperature of the interplanetary medium. We cannot assume the engine nozzles function as perfect mathematical models with zero turbulence, and excess cooling would lead to formation of extremely reflective crystalline ices in the exhaust. Having the engine running would be essentially like emitting a constant stream of chaff.
    The exhaust must also be at least war enough to prevent the engine simply filling with ice to begin with. Exhaust moving at km/s striking any irregularity in the nozzle would heat up tremendously.

    1. I don't have enough information to give you a good answer. I know hydrogen doesn't simply freeze solid in the vacuum of space, especially with the Sun present. Also, rocket exhaust expands extremely quickly in vacuum, so even if it is somewhat hot, it becomes so optically thin that it simply doesn't get picked up by cameras.

    2. "Gas-core nuclear rockets, for example, can significantly increase the acceleration under stealth."

      To what degree? What kind of total dv can be had with this? Sure, a big nozzle helps, but that could potentially weigh a lot, and cooling the gas-core could be pretty hard too. And even if the nozzle wasn't heavy, wouldn't the nozzle at the narrow part be difficult to cool?

    3. If a solid-core nuclear rocket can deliver 1000s Isp, then a gas-core nuclear rocket could reach 3000s or beyond.

      With a mass ratio of 10, that's enough for a deltaV of over 67 km/s.
      Stealth ships pay for their ability to hide from sensors with reduced performance - less acceleration, lots of dead weight in the form of massive nozzles and so on.

      The nozzle throat could be hidden if the nozzle was curved in some way. The rest of it would have to be cooled by pumping liquid hydrogen through the walls and keeping the propellant flow rate very low.

    4. If you treat everything as gas, that doesn't seem too bad, granted you somehow get away with a gas core that doesn't need radiators! But the remass coming out of a gas core isn't a gas, it's a plasma, is it not?

      Interesting idea, I just wonder how you'd cool the plasma without losing speed before it became gas again.

    5. Great post. While reading, I admit to being a bit skeptical about being able to deploy 23.16 billion kilometers of wire radiators... how would something like that be possible? Even if you were able to deploy 1000km of wire per second, (or 10 wires at 100 kps, or even 1000 wires at 1 kps), you would still need 8 months to deploy them all. What am I missing?

  25. Great post. While reading, I admit to being a bit skeptical about being able to deploy 23.16 billion kilometers of wire radiators... how would something like that be possible? Even if you were able to deploy 1000km of wire per second, (or 10 wires at 100 kps, or even 1000 wires at 1 kps), you would still need 8 months to deploy them all. What am I missing?

    1. You're not sending out a single 'string' but sprouting billions of 'hairs'. I hope that analogy makes sense.

  26. From the point of view of a stealth ship, this is an acceptable cost as it would allow them to travel around the Solar System as quickly as any regular military ship. The size and mass penalties are of less consequence to a design that is not supposed to engage in direct combat or in tactical maneuvers in the first place.
    For one I slightly disagree we could still have the nozzle built within the ship without it being exposed so that it could still be used in combat and such

  27. I'm having a hard time envisioning the load-bearing structural architecture of this ship, with multi-layered vaccuum-insulated hull, these wire-deploying mechanisms (along with tension-bearing trusses?), and long cryotanks all bent laterally by the engine at COM...

    1. The acceleration is very low, so the stresses on a frame would be modest. Also, the tanks could be supported through internal pressure (balloon tanks).

  28. Would the wire radiators be possible to use in combat? If a normal ship was using wire radiators at, say, 1000K, would they be an effective way to disperse heat? Of course, if hit by a kinetic then it would likely be destroyed, but most kinetics are one-hit-one-kill deals or localized damage or very short range. If it could stand up to lasers, wouldn't it be able to disperse heat without being incredibly fragile or having short combat times so you don't have to dump too much coolant or becoming enormous with the amount of coolant they would have to carry? And it could do this all the way up to mid range, no less! The only ships needing to dump coolant would be short-ranged fighter drones, which are designed to operate for short times anyway!

    1. And, also given the fact it is made to disperse heat and has built in active cooling to distribute the heat around, it should be well suited to standing up to lasers/particle beams