A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Thursday, 24 March 2016

Stealth in Space is Possible II

We'll continue looking at the various methods commonly proposed for stealth in space, then start looking into effective solutions.




Cold plate

Insulating your entire spaceship and maintaining the exterior temperature of your hull close to the cosmic background temperature costs mass. 

Today, thermal insulation is achieved through the use of multiple sheets of metal film with low emissivity, such as Aluminized Mylar. A military spaceship would want to maximize insulation, so more layers are used. With 100 layers between a 298K interior and a 3K exterior, thermal conductivity is reduced to 0.05

Our spaceship with 412m^2 to be insulated required 213kg of this insulation. Of the 10kW of internal heat, 500W will reach the external surface of the hull.  

A modern helium-based cryogenic cooler can absorb 1W of excess heat per 20kg mass when operating at 4 Kelvins. This means up to 10 tons of cryogenic equipment will be necessary to maintain a cold exterior. 

Even then, relying on radiators angled edge-on towards the likely positions of enemy sensors is risky. Regular flat-panel radiators emit some of their heat to the sides, and trying to eliminate this leak costs mass, lowers efficiency and increases surface area in a diminishing marginal return loop. 

There is, however, a simpler and more reliable method for stealth.


The James Webb telescope sitting on its 5-layer sunshield.
The solution is place your spaceship behind a large plate with a cool surface. On the cold face, a sensor would only see a disk indistinguishable from the background temperature. On the opposite face, the spaceship hides with its radiators unencumbered and its hull surface insulated but not cooled. 

The advantage is that the 'cold plate' presents a large surface that is easier to cool and handle than the complex shape of a spacecraft with multiple protruding elements. The total surface area is also lower, meaning it can be very lightweight.

Tactically, the spacecraft is less sensible to pointing errors and non-directional leakage from radiators when trying to redirect emissions away from likely positions of enemy sensors.


One possible 'cold plate' configuration.
The simplest configuration is a multi-layer 'cold plate', with the cold face absorbing sunlight and the hot face reflecting radiation from the spacecraft.

A coolant flow is established to move the absorbed sunlight to the spaceship's radiators. 

The occlusion angle is angle formed between the plate's edge and the spaceship's rear-most component. Anything within this cone should not be detectable.

Our example spaceship of 500m^3 can be reconfigured into a cylinder of 8m diameter and 10m length.
A cold plate 10m in diameter placed 1m in front of the long end of the spaceship will cover the spaceship from sensors in 

Disadvantages do exist. The spaceship's own sensors would have to be mounted on periscopes with cooled heads. It is hard to design a spaceship that can change the position of the cold plate without moving the entire spaceship. This can be done with a detached plate, but then it would have to be able to cover the spaceship from off-axis angles, where it may be wider and require a larger plate.
A spaceship designed to hide behind a cold plate would have an optimal 'short and fat' shape, which contradicts with the requirement of reducing exposed area to sunlight ('long and thin' shape) when not using the cold plate.

Finally, the simple cold plate only cover the spaceship from sensors in one hemisphere. The spaceship is completely exposed to detection from the sides and rear.

The solution to that is to extend the edges of the cold plate around the spaceship, increasing the occlusion angle and the volume of space it is undetectable in. However, this reduces the volume of space it can radiate waste heat into proportionally, meaning larger or hotter and heavier radiators pointed directly rear-wards.

Projectile and missile stealth

In some settings, it might not be possible to avoid detection for any practical amount of time. There might be sensors everywhere, or the size of the spaceships and power levels used for travel might be too hard to hide from the prevailing technology used for detection.

Maybe it's even a setting where lasers dominate the battlefield, and have evolved into massive space stations that attack across immense distances at opposing orbital infrastructure, without bothering to deal with transporting weapons in military spaceships across those distances...

In such situations, there is still some merit to stealth projectiles, whether they be nothing more than kinetic impactors with limited maneuvering capability, to self-contained missile-drones.

Stealth projectiles have numerous advantages.
At tactical ranges, they allow the firing spaceship's projectiles to evade detection for longer from the target's defensive fire. This increases average lifetime of the projectiles and therefore the number that survive the trip and reach the target intact.
As mentioned in the Electric Cannons and Kinetic Impactors II post, using hard-to-detect projectiles is necessary if you want to use indirect and alternative modes of fire.

More likely, the missiles will be completely black.

At strategic ranges, stealthed projectiles can be used as a deterrent or last-resort weapon. Streams of missiles sent into heliocentric orbits, accelerating using low-thermal-impact propulsion systems or burning against the backdrop of the sun, then positioning themselves around the target planet would be the equivalent of nuclear weapons today. With a tiny dV maneuver at their apoapsis, they can be sent screaming down onto the target at incredible velocities, instantly destroying orbital installations, low-orbit spaceships and with appropriate shielding, ground targets too.
These projectiles can be positioned around every strategic target, like a nuclear arsenal waiting to drop from the sky. 

The easiest way to cool down a projectile is through open-cycle cooling. They would be too small to carry an onboard cryogenic cooling and waste heat management system. They need to dissipate heat absorbed from sunlight, as they cannot afford to reflect it away.

Here's an example projectile, designed to catch a target accelerating at 0.1m/s^2 from an initial distance of 10000km.

10kg kinetic impactor
Launched at 20km/s at target 
Transit time 500 seconds - deltaV needed 50m/s
Propulsion provided by a cold gas thruster with exhaust velocity 700m/s
Mass ratio 1.074, so total mass is 10.74kg
Average density 8000kg/m^3 (less than iron)
If spherical, surface area exposed to the sun is 0.014m^2
Energy absorbed is 18W at Earth orbit

A liquid hydrogen reserve at 4K could be heated to 20K to achieve about 228 joules of waste heat per gram ejected. At a rate of 87 milligrams per second (43 grams in total), the projectile could be kept extremely cool for the entire trip.

The detection range equation, inputting 4K temperature and 0.0014m^2 surface area, gives us distances of a handful of kilometers. 

At longer distances or with 'hot' propulsion, a missile might not be able to stay entirely cool. However, it can still use the directional tactics discussed before, on a smaller scale. 

This might necessitate that defenders launch sensor drones at the start of every battle to watch for the hot sides and rears of accelerating missiles, and losing those sensors would open up the defenders to attacks from projectiles invisible from the front...

Bright backgrounds

It was noted that not all space combat occurs in 'deep space', where the background is uniformily black and cold. With no terrain, no atmosphere and standing hot against a cold background, it is the worst place to be for a spaceship trying to hide.

However, the situation changes when the spacecraft is in low orbit, hiding its thermal radiation against the brightness of a planet or moon.   


A heat-map of the Earth's top of atmosphere flux
Earth's flux (the proper name for the watts per square meter measurement) is between 66 (cloud cover) and 380 (hot oceans) watts per square meter. This leads to an 'outer space temperature' around Earth of about 10.7 degrees Celcius, or 283.7K

A spacecraft accelerating across the face of the Earth would still stand out, but the 'hot' background it traverses makes distinguishing it harder from long distances.

We substract the Earth's flux from that of the spaceship to determine the new detection range. Let's work with the under possible scenario, with 380W/m^2 behind the spaceship.

1GW spaceship
300MW waste heat
Spaceship flux is 300MW / ( 4π * Distance^2 )
Planet flux is 380W / (( Distance / Planetary Radius ) ^2)

At 100000km, the spaceship's flux is reduced by 99.9999985%
At 100 million km, the spaceship's flux is reduced by 99.99999984%
The reduction increases with distance.
You have to be very close to the planet to distinguish the spaceship from the planetary background. So, your initial burn can be pretty much undetected across interplanetary distances.
The only issue then becomes hiding your spaceship from other detection systems, such as occultation during transits across the planet's face. This is the method used to detect exoplanets that orbit near their star. This would have to be dealt with on a strategic scale.
 
Active defense

Until now, all of our defences against passive thermal detection have been to cool and hide our spaceship in various ways. 

One suggestion is to actively respond to sensors by shooting lasers at them. The idea is that the laser beam gets bounced into the telescope's optics and onto the sensor. The problem is, at ranges where your spaceship is only a few pixels wide on the sensor array, the laser beam will only reach those few pixels.

Overall, it would take a massive coordinated effort from a huge number of angles to burn through a significant number of pixels on a sensor array.

The alternative is to heat up the entire sensor platform so as to increase the operating temperature, lowering the signal-to-noise ratio and decrease sensitivity

The problem is that doing so adds more waste heat to your spaceship than it does to the target. The sensor platform can have cooling systems of its own that could handle the heat load. Also, increasing your waste heat load increases your visibility to other sensor platforms, both visible and invisible to you.

In the end, unless you can deliver a massive amount of energy to all the sensor platforms looking at your spaceship, and do so without increasing your own temperature (through open-cycle cooling, for example), you will become more visible to any sensors you missed or did not detect.

An orbital radio-telescope.
In the next part, we'll look at active detection, maneuvering in space and further concerns.

8 comments:

  1. Note that for the cold plate, it would be very useful to have the layer facing the enemy have as low of an emissivity as possible in the infrared. Highly polished gold or aluminum, for example, might have an emissivity of 0.05 across the relevant infrared wavelengths used for detection. This not only cuts down on your own thermal emissions, but reflects the thermal emissions of space making you look like just empty space.

    It is also useful to note that the largest source of infrared background is not the CMB, but zodiacal dust heated by sunlight. This makes hiding in the infrared zodiacal glow around a star slightly easier (but not too much easier - you're still looking at multi-AU detection distances for realistic scenarios).

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    1. I understand that something like aluminized mylar with emissity of 0.03 would be great at reducing radiations, but won't the same effect be achieved by cooling a super-emissive material like Vantablack to even lower temperatures?

      The second solution requires a larger cryogenic cooling system, but at least you het the side effect of defeating active detection.

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  2. Reducing the temperature a lot will require a lot of work (both in the figurative and physical sense), and in the process generate even more heat that will need to be disposed of - not to mention the extra mass required for the refrigeration and insulation.

    A specularly reflective surface will be hard to detect with active sensors, because the detection and ranging pulse will be reflected specularly - which will usually be in a direction away from the sensor looking for the reflected pulse. So unless you are exactly face-on to the guy looking for you, his pulses just get re-directed out into the void of space someplace where no-one will ever see them again.

    This assumes far infrared lidar, of course. For radar, you need to worry about resonances where bits of your structure act like antennae to enhance the scattering, diffraction from specular scattering, interactions with the edges of the cold plate, and such. Even then, the reflective surface gives something of an advantage by enhancing specular reflection, a principle exploited by current stealth aircraft. A vantablack surface, for example, might just get you diffuse reflection of microwave pulses which are easily seen (has anyone even studied the microwave properties of vantablack?).

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  3. I ve got question, is it possible to make radiator from cold plasma? I don't know if cold plasmas can be cold enough to be used as radiator, but... This could be like improvement over charged dust and droplets, I think.

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  4. I see three issues with 'cold plasma' radiators:

    -cold objects emit less, so you need huge areas to get rid of waste heat. The hotter the radiator, the smaller and lighter it is.

    -plasma would emit heat in all directions. This might not be useful if you are trying to emit away from the positions of likely targets.

    -plasma would have to be controlled through magnetic systems, which increase waste heat heat. Also, it uncertain whether they can be carried along under acceleration...

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  5. So, basically using them doesn't make sense. I really like this cold plate design, spaceship looks like Knight hiding himself behind shield :)
    I wonder if it will be significantly harder to find enemy ships if the space system is full of other ships, like trade, passenger and transport. What do you think about this not really stealth tech, but stealth method of approaching?

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  6. Small correction:
    The image labeled "The Kepler telescope sitting on its 5-layer sunshield" – I believe it is actually the James Webb Space Telescope:

    https://en.wikipedia.org/wiki/James_Webb_Space_Telescope

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    1. Thank you for pointing it out, I'll make the necessary corrections.

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