Wednesday 27 November 2019

Hypervelocity Macron Accelerators

We look at the various ways of accelerating micro-scale projectiles up to hypervelocity (10-10,000 km/s) and their use in space.
Going small to go fast
Macrons or macroscopic particles are tiny projectiles that sit on the border between the complex structures we see under a microscope and the far simpler molecules where we can count individual atoms.
A typical macron is a micrometre in diameter and has a very simple structure. Due to the small size, it exhibits an interesting feature: a very high surface area to mass ratio. A useful number of electrical charges can be placed on a macron’s exterior compared to how much they mass. This feature can be exploited by an electrostatic accelerator.

Tiny particles are too small to survive the heating and friction in a railgun, and cannot support large magnetic fields in a coilgun. However, an electrostatic accelerator can bring particles up to high velocities by using a voltage gradient between an anode and a cathode. Charged particles feel a force when placed in between electrodes, proportional to the voltage gradient multiplied by the particle’s charge. When we divide that force by the mass of the particle, we get force divided by mass, which is an acceleration. A macron, with its high charge to mass ratio, will experience a strong acceleration even under small voltages.
The velocity gained by a non-relativistic charged particle is easy to calculate:

  • Particle Velocity = (2 * Voltage * Charge/Mass)^0.5
The velocity will be in meters per second.
Voltage is in volts.
Charge is in coulombs. Mass is in kg.
Charge to Mass ratio, the critical feature of macrons, is in C/kg.

Electrostatic acceleration is regularly used today to push small things to great speeds. For example, electric rocket engines such as colloid thrusters shoot out tiny liquid droplets at multiple kilometres per second, which is somewhat similar to how we want a macron accelerator to operate. One design accelerates them to 43km/s. We can also find electrostatic accelerators in the medical field. In fact, the majority of them today are used to generate X-rays for therapy.
The most powerful electrostatic accelerators are for nuclear research purposes. They operate at several megavolts and are used to accelerate electrons and ions. 
Van de Graaff accelerators have been used to study the impacts of interplanetary dust grains. The original accelerator facility built by Friichtenicht in 1962 was able to accelerate 0.1 μm iron spheres to 14 km/s using a 2 MV potential. We have also designed Cockroft-Walton, Marx and Pelletron accelerators, each different in their way of creating and holding a large voltage potential.

How much voltage can be obtained in an accelerator?
A voltage ‘V’ between two surfaces separated by a distance ‘m’ will create a voltage gradient of V/m. 1,000 Volts across a 1 centimetre gap will give a gradient of 1,000/0.01 or 100,000 V/m. The gradient creates a force that accelerates charged particles, but can also give electrons within the two surfaces enough energy to jump across the gap. The electrons that escape gain energy and slam into the opposite electrode, damaging it and reducing its ability to maintain a voltage gradient.
Weak voltage gradients are enough to get electrons to jump across a gap filled with a conductor, like salty water. Stronger gradients are needed to cross insulated gaps, like pure vacuum. Charge will accumulate at the tip of any imperfections or contaminants on the surface of an electrode, creating stronger voltage gradients locally and reducing the overall voltage that is possible. Too high a gradient, and enough electrons jump across to create an electric arc. The arcs from too much voltage gradient are lost energy that does not go towards accelerating charged particles. In fact, that energy becomes heat that can burn out anodes and cathodes. A device that operates at the megawatt or gigawatt level will certainly not want electric arcs dissipating that power as heat internally!

Single stage accelerator today manage 10 to 15 MV in total. Getting more than that becomes exceedingly troublesome, as voltage multiplying circuits become larger and larger.
Tandem (or two stage) electrostatic accelerators double their maximum voltage by switching the charge on the particle being accelerated halfway down their length. 
At the upper end, we see Pelletrons with 30 MV. However, the highest voltages are only possible because the electrode gap is filled with a pressurized insulating gas. We cannot use this option inside our macron accelerator as interactions between the charged particle and the gas could create enough friction heating to destroy it. We are therefore forced to rely on simple vacuum. Highly charged macrons cannot be quickly switched from a negative to a positive charge either; the charges that need to move quickly result in destructive currents. Tandem accelerators are not an option either.
A ‘Super Marx Generator’ was proposed for achieving voltages on the order of 1000 MV; it is 1500 meters long and therefore averages 0.6 MV/m. That design stored a gigajoule of energy. Our macrons do not need that much energy but the length requirements are similar; 100 MV would require 167 meters of capacitors in series.
Another option is a pulsed staged accelerator. An electrostatic accelerator can be broken down into a series of stages. Each stage consists of a pair of charged plates with a gap between them. The voltage gradient between each pair of plates, assuming excellent vacuum, no contamination and perfectly smooth surfaces, can be on the order of 1 to 10 MV/m, which is far better than the Super Marx generator design. More realistically, 3 MV/m is achievable.
If an electrical current is supplied to the pairs of plates for just the short period where a macron crosses through them, and then it switched off as the macron exits, we end up accelerator driven by pulses of electricity with multiple stages. This design was suggested and demonstrated (for 5 stages) here. The increased voltage gradient means that 100 MV only needs between 10 and 100 meters of length.

The downside is that switching the plates on and off is not a perfect process. The switches, likely to be solid state transistors, convert some of the electrical energy into heat and can become a major source of inefficiency.

We can also consider a circular accelerator.
Instead of thousands of stages, a single stage is reused multiple times. The macrons will be bent 180 degrees twice by U-shaped magnets to form a looping trajectory. They gain velocity with each revolution, so there is no ‘maximum voltage’. However, we cannot increase the velocity past the point where the magnets cannot bend the macron’s trajectory.

The maximum velocity achievable can be calculated with this equation:

  • Maximum Velocity = Bend Radius * Magnet Strength * Charge/Mass ratio
Maximum Velocity is in m/s.
Bend Radius is in meters.
Magnet strength is in Tesla.
Charge/Mass ratio is in C/kg.

All of these factors affect velocity linearly. You will notice that the voltage gradient does not come into play at all; it simply takes more revolutions to reach the maximum velocity if the voltage is weaker.
Large circles have the lowest magnetic strength requirements to reach a certain velocity. However, spacecraft might have certain constraints on their cross-section and size that prevents them from mounting circular accelerators above a certain radius, so a linear accelerator might be preferred for reaching high velocities. It should be noted that the velocity achieved in a linear accelerator is proportional to the square root of the charge to mass ratio, but it is directly proportional in a circular accelerator. This means that as C/kg values increase, the circular accelerator becomes more attractive.

In spacecraft with both length and radius restrictions, the circular and linear accelerators can work together to maximize the velocity possible.

After exiting an accelerator, macrons can be neutralized by passage through a thin plasma, and at the highest velocities, by a charged particle beam of the opposite charge. The tiny, rapidly cooling particle will become nearly impossible to detect or deflect until it hits a target.

Charge to Mass ratio

To reduce the weight of the accelerator, a lower voltage requirement is needed. To do this, charge to mass ratio must be maximized.

For a spherical macron, surface area to volume ratio increases at the same rate as radius decreases. A sphere with a radius 10 times smaller has a 10 times better surface area to volume ratio. This could mean a 10 times better charge to mass ratio.
A huge amount of charge can be added by various methods. How much charge can a sphere hold?

The total charge is given by:

  • Surface charge = 1.11*10^-10* Voltage Gradient * Radius^2
Surface charge is in Coulombs (C)
The voltage gradient within the projectile is in V/m
Radius is in m
The 1.11*10^-10 coefficient is (4*pi*Permittivity of Vacuum)

The charge divided by mass, or charge to mass ratio, for a sphere is:

  • Charge to mass ratio = 2.655 * 10^-11 * Vg/(Radius * Density)
Charge to mass ratio is in C/kg.
Vg is the voltage gradient within the projectile in V/m
Radius is in meters.
Density is in kg/m^3.

These equations show that want to maximize the charge to mass ratio, the radius has to be very small and the voltage gradient as high as possible. The maximum voltage gradient for a negatively charged particle is about 100 MV/m. For a positively charged particle, this increases to 1,000 MV/m. Other sources mention voltage gradients as high as 50,000 MV/m as being possible, but that is likely to be a theoretical limit. If the particle is charged too much, it will start releasing electrons through field emission and dissipating the excess potential charge. For tiny projectiles, this causes enough heat to destroy them.

We can suppose that any macron that we need to accelerate to very high velocities will be pushed to this limit.
Let’s take the example of a positively charged 1 mm wide iron sphere. Radius is half of the diameter, so 0.5 mm or 5*10^-4 meters. The density is 8600 kg/m^3. The maximum charge to mass ratio will be 0.006 C/kg.

Now let’s work out the C/kg for a positively charged 1 micrometre lithium sphere. Radius is 0.5 micrometres. Density is 534kg/m^3. The maximum charge to mass ratio for this macron is 99 C/kg.

The lithium macron is clearly superior to the iron particle, because it is much smaller and composed of a less dense material.
Another maximum is the strength of the macron’s materials. The voltage gradient creates a force that tensile strength must overcome. The equation for a macron stressed to the limits of its tensile strength is:

  • Strength-limited C/kg = (1.77 * 10^-11 * T)^0.5/(R * Density)
The charge to mass ratio is in C/kg.
T is the tensile strength in Pascals.
R is the radius in meters.
Density is in kg/m^3.

Using the previous examples, a 1mm iron sphere with a strength of 250 MPa could survive a charge to mass ratio of 0.015 C/kg. This is a quarter of the previous limit.

Lithium is rather weak with 15 MPa of tensile strength. A micrometre wide particle of lithium would only survive a charge to mass ratio of 0.99 C/kg, so it is strength limited to a hundredth of the previous value.

To achieve better C/kg, we need stronger materials. The tiny dimensions of macrons bring forward another advantage to help meet this requirement. At a very small scale, we can expect materials to be formed without any defects. This unlocks their full strength potential.

A good example of this small-scale advantage is iron.
Bulk iron has a strength of 250 MPa. However, a micrometre-long monocrystalline whisker of iron displays strengths of 14,000 MPa. The charge to mass ratio allowed by micro-scale iron’s strength is 0.12 C/kg.

This difference in strength between bulk and micro-materials can be demonstrated for graphite, aluminium, silicon and many others.

A silicon nitride whisker has a strength of 13,800 MPa and a density of 3200 kg/m^3. It can have a micro-scale charge to mass ratio of 309 C/kg.

The current champions of strength to weight ratio are carbon fibres. The Toray T1100G fibre is the strongest material commercially available for its weight, at 7,000 MPa for 1,790 kg/m^3. A micrometre-sized sphere of these fibres can support charge to mass ratios up to 393 C/kg.
At the microscopic scale, those same carbon fibres gain the incredible properties of carbon nanotubes. They have shown strength to weight ratios more than ten times better than Toray T1100G fibres (about 63,000 MPa for 1340kg/m^3), which means charge to mass ratios of at least 1,576 C/kg at the same scale.

How do we actually use the full potential of these small-scale materials if by making them stronger, we run again into the field emission limit on charge to mass ratio?

Shaping the macron

The solution to more C/kg is to move past simple spheres.
The spheres can be made hollow. This retains the surface area of a sphere but decreases the mass. We can call W the ratio of wall thickness to radius. W=0.5 means that the walls are half as thick as the sphere’s radius. W=0.01 means that the walls are a hundred times thinner than the sphere’s radius.

The wall thickness ratio can by multiplied against the density in the previous equations to give the field-emission-limited charge to mass ratio of a hollow shell:
  • Hollow C/kg = 0.02655 / (Radius * W * Density)
The charge to mass ratio is in C/kg.
R is the radius in meters.
W is the wall thickness ratio.

Density is in kg/m^3.

This limit improves by a factor 1/W as the wall to thickness ratio decreases. At W:0.1, the field-emission-limited C/kg is increased by a factor 10. At W:0.01, it is a hundred-fold better.

However, a hollow shell has W times less thickness to resist forces, and also has W times less mass to support. The strength-limited charge to mass ratio becomes:
  • Hollow C/kg = (1.77 * 10^-11 * T * W)^0.5/(Radius * Density * W)
The charge to mass ratio is in C/kg.
T is the tensile strength in Pascals.
W is the wall thickness ratio.
R is the radius in meters.

Density is in kg/m^3.

Notice how this limit improves by a factor 1/W^0.5 as the wall to thickness ratio decreases. The benefit from W:0.1 is only 3.3x, and from W:0.01 is 10x better than a full-thickness sphere.

These equations for hollow spheres imply that as the walls get thinner, the strength of the projectiles becomes more important.

There is also the option to shape the macron into a cylinder.
Cylinders can be elongated to large length to width ratios, like in this paper. This gives them a better surface area to volume ratio than spheres.

The ratio between the lateral surface area of a tube of elongation G and a sphere of equal volume is:

  • Surface area ratio for cylinder vs sphere = 0.605*G^0.333
G is cylinder length divided by cylinder radius

A tube that is 1000 times longer than it is wide (G:2000), for example 1 um wide and 1 mm long, would have a surface area that is 7.6 times greater than a sphere of equal volume. Carbon nanotubes that are a few nanometres wide and up to several centimetres long would have G:10,000,000, so they are a 131 times better shape than a sphere.

Cylinders, of course, can be hollowed out to become tubes. The benefits of elongation and wall thickness ratio are multiplied in this case.

Ion beam for macron acceleration

An electrostatic accelerator can be used in an entirely different way to get a macron up to high velocities.
Instead of directly pushing and pulling on a macron using electric fields, it can act on it indirectly with a beam of electrons or protons. This has been called a ‘beam pushrod’ or a ‘beam blowpipe’.

The charged surface of a macron naturally repels objects of similar charge. If it has an internal voltage gradient of 1000 MV/m and a diameter of 1 millimetre, it can repel particles with an energy of up to 500 keV. A thousand times smaller particle can only deflect particles of up to 500 eV, but it will accelerate harder thanks to the square-cube law. A negatively charged macron would only produce internal voltage gradients of 100 MV/m, so it would be deflecting 50 keV beams at 1mm and 50 eV at 1 um.

Less energetic beams can be deflected further away from the particle, giving it a larger effective cross-section. For example, a macron that could deflect a maximum of 50 eV electrons would be able to deflect 25 eV with an effective cross-section twice its actual physical size. We will assume that electric or magnetic fields are used to focus the charged beam onto a spot equal to the size of the macron’s effective cross-section throughout the length of the accelerator, as has been proposed here. Low energy charged beams will tend to expand very rapidly once outside the focusing influence of these lenses, so acceleration past the last focusing element can be ignored.

The maximum acceleration that a macron can survive in these conditions is dependent on tensile strength:

  • Maximum acceleration = (0.75 * T) / (R * Density)
Maximum acceleration is in m/s^2.
T is tensile strength is in Pascals.
R is macron radius in meters.
Density is in kg/m^3.

The value is independent of the wall thickness ratio.

A millimetre-sized projectile made of a material such as aluminium 7075-T651 (570 MPa, 2800 kg/m^3) could be accelerated at up to 1.52*10^8 m/s^2.

Meanwhile, a micrometre-sized sphere of diamond (1600 MPa, 3510 kg/m^3) would accelerate at 3.42*10^11 m/s^2.

To accelerate a positively charged macron, a proton beam would be used. At 500 keV, protons have a velocity of 9,780 km/s. At 500 eV, this falls to 300 km/s.

A negatively charged macron can be pushed by an electron beam. Electrons with an energy of 50 keV travel at 123,000 km/s. At 50 eV, it is 4190 km/s.

These figures do not mean that the macron can only asymptotically approach the beam’s own velocity. They are the maximum relative velocity between the beam and the macron. If the macron is already travelling at 4190 km/s (50 eV electrons), then it can actually deflect 100 eV electrons (5929 km/s). A series of pulses from a particle accelerator, each tuned to have an energy that closely matches that of a macron, can bring that macron up to higher and higher velocities in steps. This is also good for transferring momentum to the macrons efficiently.
Pushing a macron with a charged beam has the advantage that almost any beam intensity can be used. Since the protons or electrons do not touch the macron and are instead deflected electrostatically, none of their energy is converted into heat. Also, the accelerating tube can be equipped with electrostatic or electromagnet lenses that can focus a charged beam and maintain high intensity throughout the duration of the acceleration. The beam energies are relatively low, so the focusing elements can be very lightweight and the acceleration tube extended without great mass penalties.

Other than the strength of the macrons, the limit on ‘pushrod’ acceleration is the beam’s charge density.
Protons or electrons do not like being bunched up behind a macron. They repel each other. If we can only put a certain number of charged particles behind a macron (the current density), we can only deliver so much energy, which limits acceleration.
For a pulse of non-relativistic protons and electrons bouncing off a macron, the maximum current density is given by the Child-Langmuir Law:

  • Maximum Current Density = (7.7*10^6*BE^1.5*R)/(Pulse Duration* BV)
Current density is in Amperes per square meter (A/m^2)
BE is beam energy in electronvolts (eV)
R is macron radius in meters
Pulse Duration is in seconds
BV is Beam Velocity in meters per second.

For a 500 keV beam of protons composed of 1 nanosecond pulses, pushing on a micrometre-sized particle, we have BE: 500,000 eV, radius 0.5*10^-6 meters, pulse duration 10^-9 seconds and BV is 9,782,000 m/s. The maximum current density becomes 1.39*10^12 A/m^2.

For a 50 eV beam of electrons composed of 1 microsecond pulses, pushing on a millimetre-sized particle, we have BE: 50 eV, radius 0.5*10^-3 meters, pulse duration 10^-6 seconds and BV is 4,193,200 m/s. Maximum current density becomes 3.24*10^6 A/m^2.

To maximize intensity, and therefore acceleration, we want the shortest pulses of the highest energy protons.

We can simplify the process for finding out the acceleration of a spherical macron by working with the power delivered by the pulses:

  • PA = ((0.375 * Current Density * BE)/(R * Density * PD * W))^0.5
PA is Pulse Acceleration in m/s^2.
Current density is in A/m^2.
BE is beam energy in eV.
R is macron radius in meters.
Density is in m^2
PD is Pulse duration in seconds.
W is the wall thickness ratio.

Following on from the previous examples:

A 500 keV beam pushing a micrometre-sized particle made of diamond (3510 kg/m^3) with nanosecond pulses of protons would provide an acceleration of 3.85 * 10^15 m/s^2. This is a value greater than the maximum the hollowed-out diamond macron could survive mechanically, as calculated above.

A 50 eV electron beam pushing on a sphere of aluminium a millimetre wide achieves an acceleration of 1.75*10^7 m/s^2. This is lower than the maximum the macron can handle.
The macron’s shape could also be improved for use in this type of accelerator. A flat disk catches a larger beam, and so more energy could be transferred with each pulse. A web of fibres, inspired by the designs for electric sails, could have exceedingly high beam capture areas for their mass.
Producing the beam that pushes the macrons is generally not a challenge. Low energy electron beam specifically can be very lightweight, efficient and small. If we base ourselves on the designs of inductive output tubes, 15 kW/kg at over 80% efficiency is to be expected from today’s technology. Proton beams are trickier to produce, but they will still be small and lightweight in absolute terms. Their energy will come from the same RF generators as mentioned previously. See the Particle Beams in Space post for more details.

Since only one particle can be accelerated in a ‘pushrod’ accelerator at a time, it would make sense to also have those generators feed a multitude of accelerator tubes in sequence. 10 generators, each capable of producing 1 GHz of nanosecond pulses, could feed 10 tubes with a continuous supply of pulses each; if each macron clears a tube in 0.1 milliseconds, then ten tubes would have a maximal firing rate of 100,000 projectiles per second.
This might seem like a lot, but each projectile is expected to carry very little energy. A nanogram at 1000 km/s is still only 0.5 joules. A hundred thousand of them per second is just 50 kW. Accelerators in the megawatt range would end up looking like a volley gun or a ‘Metal Storm’ launcher.

Here is a selection of macrons to represent the different approaches to maximizing their performance

1) A 1 mm diameter sphere of diamond with 1,600 MPa strength and 3510 kg/m^3 density. It is hollowed out to a 1:1000 wall to radius thickness ratio (W:0.001). It masses 1.83*10^-9 kg. When charged negatively, it can support a charge to mass ratio of 1.5 C/kg. Charged positively, it achieves 3 C/kg. 50 keV electrons and 500 keV protons can be deflected. Maximum acceleration is 6.8*10^11 m/s^2.

2) A micrometre-sized macron made up of carbon fibres with 7000 MPa strength and 1790 kg/m^3 density. It is hollowed out to a 1:100 wall to radius thickness ratio (W:0.01). It masses 9.4*10^-16 kg. When charged negatively, it can support a charge to mass ratio of 296 C/kg. Charged positively, it achieves 2960 C/kg. 50 eV electrons and 500 eV protons can be deflected. Maximum acceleration is 5.8*10^12 m/s^2.

3) A micrometre-wide, centimetre-long needle of carbon nanotubes with 63,000 MPa strength and 1000 kg/m^3. Wall to thickness ratio is 1:10 (W:0.01). It masses 7.8*10^-14 kg. When charged negatively, it can support a charge to mass ratio of 356 C/kg. With a positive charge, this becomes 3560 C/kg. 50 eV electrons and 500 eV protons can be deflected. Maximum acceleration (vertical axis) is 9.45*10^13 m/s^2.

4) A 10 nanometre diameter sphere of carbon nanolattice with 200 MPa strength and 300 kg/m^3 density. It masses 1.6*10^-22 kg. When charged negatively, it can support a charge to mass ratio of 8850 C/kg. Charged positively, it achieves 39,665 C/kg. 0.5 eV electrons and 5 eV protons can be deflected. Maximum acceleration is 1*10^11 m/s^2.

We select the maximal value for C/kg depending on the accelerator type. All of these particles are field-emission-limited so they could be further optimized to match their strength-limited C/kg values.

The accelerators we will look at are:

A) A single-stage 10 MV electrostatic accelerator.
B) A 100m long multi-stage electrostatic accelerator with an average acceleration gradient of 3 MV/m, for a final energy of 300 MV.
C) A 100m diameter ring of 10 Tesla field strength.
D) A 100m long electron ‘pushrod’ accelerator.
E) A 100m long proton ‘pushrod’ accelerator.

Their performance is as follows:

A1) 7.7 km/s
A2) 243 km/s
A3) 266 km/s
A4) 890 km/s

B1) 42 km/s
B2) 1,332 km/s
B3) 1,461 km/s
B4) 4,877 km/s

C1) 1.5 km/s
C2) 1,480 km/s
C3) 1,780 km/s
C4) 19,827 km/s

D1) 11,661 km/s
D2) 721 km/s
D3) Cannot.
D4) 35.7 km/s

E1) 11,661 km/s
E2) 10,392 km/s
E3) Cannot.
E4) 913 km/s

Some conclusions can be drawn from these values.

Larger macrons are limited mainly by their strength. The identical D1 and E1 values are because the acceleration reaches the maximum the thin-shelled diamond sphere can handle in the ‘blowpipe’ accelerator.

For the smallest macrons, where C/kg is large, circular accelerators become more interesting than linear accelerators (at least if we are not concerned about the weight of the magnets). We can see the progression of velocities from C1 to C4 being much more pronounced than from B1 to B4.

Also of note is the fact that fibre tubes excel in electrostatic accelerators, but cannot be pushed by a ‘blowpipe’ accelerator as they would bend under those stresses.

Fission Enhancement

Millimetre-sized macrons have an interesting property. They can hold a ‘payload’ of a few milligrams within their hollow core. If they are filled with a fissile material, they can bring it up to sufficient velocities to ignite a nuclear reaction upon impact.  

Studies for ignition of ‘micro-fission’ have been conducted by researchers such as Winterberg. They worked out that a 0.2 milligram projectile of uranium-235, when covered in a shell of deuterium/tritium ice, could ignite when compressed to about 10*10^12 Pa.
In response to those calculations, another study was performed where the critical mass of uranium enclosed deuterium/tritium (DT) ice was calculated. It then goes on to describe methods of igniting micro-fission by the impact of hypervelocity projectiles. At 20 km/s, the critical mass is just 0.04 grams.

This mass of uranium would fit inside a sphere of 1.6 mm in diameter. Surrounded by a shell of DT ice of equal thickness, and then a 10 micrometer thick shell of carbon fibre, it would have a total diameter of 3.18 mm and a mass of 0.0422 grams. Average density is 2502 kg/m^3 and the C/kg value when positively charged is 0.0067 C/kg.

At an average voltage gradient of 3 MV/m, a linear electrostatic accelerator would have to be 9.9 km long to push this macron to the required 20 km/s velocity. 10 tesla strength magnets would give it a bend radius of 298.5 km. These are impractical options.

With the shell of carbon fibre handling mechanical stresses, an acceleration of 8.29*10^6 m/s^2 is possible. This means a velocity of 20 km/s is achievable within an accelerator length of 24 meters. About 8.44kJ of energy is consumed in 2.4 milliseconds.

Upon impact, the uranium releases a large portion of its 80 TJ/kg nuclear energy. With 100% burnup, this amounts to a kinetic-to-nuclear energy multiplier of 379,149! Even at a low 10%, this is 37,915 times the energy invested.

A 1 MW accelerator shooting nearly 120 of these micro-fission macrons per second would produce between 38 and 380 GW of power at the target.

The paper also suggests a scaling law where increasing the impact velocity reduces the critical mass by a factor Velocity^12/5.

An impact at 200 km/s could be enough to reduce the critical mass to 0.16 milligrams. This quantity of uranium fits inside a spherical volume that is 0.252 mm wide. 

Adding the required layer of deuterium/tritium ice brings the diameter to 0.5 mm and the mass to 0.161 milligrams.
This tiny fuel particle can be comfortably help inside the 1mm diameter diamond shell described earlier as projectile 1. The mass of the macron would increase from 0.00183 milligrams empty to 0.162 milligrams loaded. Its C/kg value and maximum acceleration would fall by a factor 89. An electrostatic accelerator with an average voltage gradient of 3 MV/m would have to be 197.8 km long. A 10 Tesla ring of magnets would have to be 1,186 km in diameter. These are again clearly not wise choices.

In a ‘blowpipe’ type accelerator, an acceleration reduced by a factor 89 still permits a velocity of 200km/s to be achieved within a mere 2.2 meters of length.

The advantage of a smaller, faster micro-fission macron is that it can reach targets further away in less time. However, the energy multiplication effect is reduced.

A 200 km/s projectile contains 20 GJ/kg of kinetic energy. Nuclear fission releases 4000x this amount. Even if we factor in incomplete burn-up of the fuel, it is likely that there will be a significant increase in the energy delivered.

Impact Fusion

Greater macron velocities can focus more energy into an impact. These impacts generate incredible temperatures and pressures… conditions under which deuterium-tritium fuel with undergo thermonuclear ignition.
Winterberg once again leads the way in providing the theory and maths behind these exciting high energy physics applications. He states that a macron accelerated to over 100 km/s can generate temperatures of over 300 million K and compress fusion fuel to a density of 1000 kg/m^3 (10x that of DT ice).
Another source mentions 100 km/s ignition being possible only if the fuel is surrounded by a collapsing shell of material, and 50 km/s might only needed if the impactor is shaped into a conical shape.

Igniting fusion using hypervelocity impacts has several major advantages.

There is no minimum ‘critical mass’ of fuel, so the smallest macrons can be used. The confinement and fuel burn time depends on the length of the projectile divided by the velocity. This favours elongated fibres with a tip of fuel; also a great shape for achieving 100 km/s with accelerators of reasonable length.  
DT fuel contains about 330 TJ/kg of energy, meaning that there is an energy multiplication effect of 66,000 at 100 km/s (assuming 100% burnup) and the energy gain can be maintained at up to 25,690 km/s! 
This is an important fact, as many sources mention ignition velocities instead of at least a few thousand km/s.

We could, for example, fill the millimetre-sized diamond shell described above as projectile 1 with DT ice. It would be able to hold 78 micrograms of fuel, which is 43.8 times the shell’s own mass. The macron’s maximum acceleration is reduced by this extra load, but it still manages to reach 100 km/s in a ‘blowpipe’ accelerator of just 31 cm.

A 1 MW accelerator would be firing off 2,500 of these projectiles per second. Up to 64 GW of fusion energy could then be released at the target.

The first obvious application of hypervelocity macrons in space is in propulsion.

The smallest, lightest macrons can be accelerated to velocities exceeding 10,000 km/s. This translates to an Isp of 1,000,000 seconds. Only the most powerful electric engines or advanced propulsion system are capable of this. In the future, fusion energy can exceed this exhaust velocity.

A hypervelocity macron is an improvement over other types of propulsion that achieve those Isps in that it can be shaped out of any sort of common dust, and does not need to emit radioactive material. The energy source could be solar or a closed-cycle nuclear reactor. Kinetic streams of projectiles have been thoroughly discussed as an efficient propulsion system, even up to interstellar velocities.
Fission enhanced macron projectiles are particularly suited for use as a propulsion system. Lower impact velocity requirements means less drive mass, while the ability to greatly reduce the critical amount of uranium also means a great reduction in the minimum pulse energy. Project Orion, for example, assumed a minimum release of 627 gigajoules with each nuclear pulse; any lower and the uranium fuel would be wasted in incomplete burns. Mag-Orion, a Z-pinched variant with a magnetic nozzle, managed 340 GJ thanks to the use of expensive Curium-245. A macron accelerator with 0.04 grams of uranium would only release 3.2 GJ, a hundred times less.

This reduction in pulse energy means smaller suspension, smaller magnetic nozzles or thrust plates, reduced heat loads and smoother accelerations. Less equipment has to be dedicated to recovering and storing energy in between pulses. Maneuvers can be done more precisely. There is much less risk of damage in case of a misfire.
A fusion rocket that uses macron propulsion enjoys many of these advantages too. A kinetic impactor can concentrate the output of an ignition mechanism like an ion beam from a particle accelerator from several milliseconds to less than a nanosecond. The exceedingly difficult peak power needed to ignite fusion is therefore replaced by a thousand to a million times less demanding accelerator. The ignition event can also feasibly be made to take place far away from the engine’s physical structures too. Macrons retain their velocity while drifting through space, so they are just as capable of igniting fusion a hundred meters from a spaceship as a meter away; this is especially important if drive powers on the order of terawatts are needed for a ‘torch drive’.

But these are minor gains compared to the possibility of remotely-accelerated macron-driven propulsion systems.

A stream of macrons can be accelerated a long distance away from a spaceship. They can cross large distances relatively quickly, and then deliver their kinetic energy without any losses. Multiple macrons be fired with a small velocity difference so that over time they bunch together and all simultaneously, providing much higher peak power. All the receiving ship has to do is place an obstacle into the path of the macrons so that the impact creates a plasma explosion. This plasma can be redirected by a magnetic nozzle for thrust. At 100 km/s, the energy needed to vaporize a carbon macron is about 83 times lower than the kinetic energy is contains, so we can expect 1 kg of onboard obstacle material to vaporize 83 kg of incoming carbon macrons.  

This sort of propulsion system is similar to beamed propulsion concepts. Propulsive power is delivered without the need for heavy on-board reactors, so high accelerations are possible. The ratio of obstacle material to macron stream mass means that the effective Isp of propellant onboard the spaceship is greatly multiplied too.

Compared to other mass-stream designs for propulsion, each macron carries a tiny amount of energy. It is unlikely that they will do significant damage if a few of them do not hit the intended target.

There is also the possibility to have fission or fusion-enhanced macrons act as propellant for a spaceship. Now we can have small accelerators delivering huge amount of propulsive energy to lightweight spacecraft, to achieve great accelerations and impressive levels of deltaV.
For example, a fission-enhanced stream of macrons could produce a series of nuclear detonations by impacting an obstacle placed inside a magnetic nozzle. The exhaust would be fission fragments with a velocity of 10,000 km/s. A stream of 10 kg/s would release perhaps 80 TW of power. If it takes 1 kg of obstacle material to receive 100 kg of macrons, then a 1000 ton craft with 200 tons of obstacle material would accelerate on average at 1.81g and achieve a total deltaV of 35.7 thousand km/s.

In other words, a 50 GW accelerator can do the work of an 80 TW drive and reduce trip times from Earth to Jupiter to 9.8-14.5 hours for 1000 ton spacecraft.

A laser or particle beam can be used to vaporize incoming macrons. This removes the need for onboard obstacle material to serve as propellant (so deltaV becomes unlimited), but also destroys the structures needed for fission or fusion enhancement, so the energy multiplication effect is also lost.
Those lasers or particle beams can also be used to ‘guide’ the macrons down a specific path. The beams can effectively create an electrical field gradient that holds the macrons in the beam’s centre, or can even be bent by uneven gradients. More details in the Cold Laser-Coupled Particle Beams post.


Another application for hypervelocity projectiles is as weapons. 

We have discussed the need for faster projectiles to compensate for the combat ranges imposed by powerful lasers. The ‘solution’ to this need was described as a ‘pellet’ or ‘dust’ gun. Its features accurately describe the properties of a macron accelerator.

At first glance, a macron accelerator produces less watts of output for the same mass of equipment when compared to a laser or a particle beam accelerator. It might also be very bulky as it has many hollow spaces. Worst of all, it does not deliver its energy at or near lightspeed.

A deeper look reveals its advantages.

The macrons have a chance to hit determined by:
  • Chance to hit = (TR/(0.5*TA * (Distance/MV)^2))^2
Chance to hit is a fraction.
TR is the target’s radius in metres.
TA is the target’s acceleration in m/s^2
Distance is the distance to cross in meters.
MV is the velocity of the macron in m/s.

The chance to hit equation basically compared the cross-section of the target to the area the target could potentially cover in the time it takes for a projectile to arrive.

We can see that a 1000 km/s macron targeting a 5 meter radius spaceship that accelerates at 0.5 g (4.5 m/s^2) from a distance of 2,000 km will hit about 30% of the time.
The equation can be rearranged to determine the effective range of a macron for a certain hit chance:
  • Effective range = MV * TR^0.5 / (Chance to hit^0.25 * (0.5 * TA)^0.5))
Effective range is in meters.
M is the velocity of the macron in m/s.
TR is the target’s radius in metres.
Chance to hit is a fraction.
TA is the target’s acceleration in m/s^2

If we accept a chance to hit of 10% with a 1000 km/s macron, a 5 meter radius target accelerating at 0.5 g can be engaged at a distance of 2650 km. Note how the range is directly proportional to the macron’s velocity and that increasing acceleration has a much lower effect. Quadrupling the acceleration cuts the target’s deltaV by a factor 4 but only increases the range by a factor 2. This would have to be compensated for by exponentially more propellant.

At the upper end of macron velocities, we can expect effective ranges in the hundreds of thousands of kilometres with good hit chances. 
The projectiles deliver their kinetic energy as small plasma explosions that form craters in a target’s armor. An approximation suggested by Luke Campbell is that a hypervelocity impactor excavate a volume of material equal to the kinetic energy divided by three times the yield strength of that material. For something relatively weak, like graphite, this means that the kinetic energy is divided by 2.4*10^8 J/m^3. Vaporizing graphite requires about 500 times more energy per m^3; in other words, a macron accelerator can be 500 times more efficient at removing armor than a continuous beamed weapon, and even better than a pulsed laser.

Stronger materials like carbon fibres require only 5 times more energy to vaporize than to excavate, but this is a still major boost to the destructive efficiency of kinetic weapons.

We can expect a 100 MW stream of macrons impacting hypervelocity to excavate about 0.41 m^3 of graphite or 0.03 m^3 of carbon fibres per second. Using a 100x nuclear energy multiplier upon impact, these volumes can be multiplied to 40 and 3 m^3/s. The actual penetration rate through armor depends on the spacing of the impacts. Closely space impacts, such as within a spot 1 meter wide, would mean a penetration rate of 50.9 m/s through graphite! A wildly maneuvering target might have this potential damage spread out over their entire cross section and then reduced further by the hit chance. Using previous numbers, a 5 meter radius target with a 10% hit chance would find its exposed surfaces ablated at a rate of 5 cm/s if protected by graphite, and 0.38 cm/s using carbon fibres. 
Whipple shields could be used to defend against kinetic projectiles. As mentioned in the Propulsion section, very little material is required to destroy an incoming macron. However, the gap created in a whipple shield by the impact of one macron can let through thousands more unimpeded. 
It is much harder to add one more layer of shielding material than to fire one more macron to get through it…

The macron accelerator’s offensive performance is also improved by fission or fusion enhancement. It is the only weapon system described so far that output more energy at the target than originally spent. A 1 MW macron accelerator might heavier and bulkier than a 1 MW laser or particle beam, but its actual output can be multiplied a hundred to a thousand times at the target. However much worse the macron accelerator is in a direct comparison, it is more than made up for by the addition of nuclear energy.

To top this all off, macrons are expected to be completely undetectable until they hit their target. Their small size means that they cool down very quickly to temperatures hard to distinguish from background radiation. At the smallest scales (sub-micrometers), even LIDAR cannot interact with them properly, and even if specialized short-wavelength sensors are used, the resolution would be severely limited. If the detection and targeting problems are somehow overcome, the macrons are difficult to destroy en-route. Defensive lasers have very little time to act (seconds at most) and must face the fact that the surface area to mass ratio of the macrons makes them very good at radiating away heat.

For example, a micrometre-sized sphere of carbon fibres could survive a beam intensity of 58 MW/m^2. A micrometre-wide, centimetre-long carbon needle survives 579 GW/m^2. We can therefore expect macrons shaped to handle high laser intensities to defeat even the most power defences (although DT ice is unlikely to remain solid!).  
SDI-era research has been done on macron accelerators for space defense. It is now up to you to decide how to make use of hypervelocity macrons.

Thanks for the help of GerritB, Kerr and other ToughSF members for help with researching this topic.


  1. This update answered a question I always want to ask on server: Can sandblaster fire fusion fuel "sand" and create fusion reaction on impact?

    Seem that the idea can survive.

    1. The answer is a definite yes... at sufficient velocity!

  2. What would the dynamic be between Macron accelerators and pellet guns?

  3. Oh...I'll just...yea

  4. It always amazes me what you come up with, and each post is better than the last. I can't tell you how much I enjoy this website and hope you keep up the fantastic work.
    On a related note, it seems like your macron cannons are the perfect drivers for your gun fusion design; with 200+ Km/s projectile velocities it becomes possible to have pairs of guns firing 3 degrees off axis and still achieve fusion! That I don't expect such an arrangement to produce a particularly useful plasma jet, but the flexibility allowed by milligram-gram sized objects at high velocity is huge.

    1. I am very glad to hear that.

      You can shoot a small piece of metal at 10km/s, and then strike that with a fusion macron at 210km/s. The relative velocity is enough for ignition, but you also get the option to start fusion reactions practically anywhere you want.

    2. Forgive me if I missed the relevant information, but what would the energy spread look like from an impact fusion detonation? What I mean is would it be possible to have an asymmetrical release of energy back towards the launcher?
      Say you use an induction coilgun to fire hemispheres of plastic propellant with a metal backing. With a hollow center axis a fusion macron can be fired right through and detonate on the metal backing. I am imagining a roughly equal amount of energy (50/50) being absorbed by both the propellant and the backing plate.

    3. I see what you are asking. The energy from D-T fusion is mostly released as neutrons in all directions. These neutrons can be absorbed 'upstream' or 'backwards' by the plastic propellant you are describing.

      However, also consider this:
      The launcher, propellant and magnetic nozzle don't all have to aligned. You can have your spaceship accelerate 'sideways' by rotating the nozzle 90 degrees and placing the launchers to either side.

    4. Say one was using a D-He3 fuel mixture and wanted to use as little neutron energy as possible. Would it be possible to have "shaped charges" that direct the majority of fusion energy back towards the nozzle?

    5. I don't think that's possible to achieve.
      What you can do is get the fusion fuel to hit a deep channel inside a propellant ball. That way a large fraction (80%+) gets captured by the propellant, and a small fraction exits through the channel. The propellant then turns into a plasma that a magnetic nozzle can redirect for thrust.

    6. Wait - create a shaped charge by time-on-target-printing!
      Think of that multi-nozzle emitter as a 3D printer nozzle for a nuclear shaped charge.

      This would be similar to the recent advance in pharmaceuticals that allows you to 3D print complex pills

      Based on these responses:
      --Multiple macrons be fired with a small velocity difference so that over time they bunch together--

      This makes is seems like you could "assemble" complex 3d shapes as they move through space by adjusting which nozzles fire, and what velocity they fired at.

      -- It would force the macrons to be bunched up into compact columns of thousands of microprojectiles, all striking within microseconds of each other.--

      Curious whether you'd have a MIRV style problem with fratricide, where the first explosion detonates the second, which sets off a chain reaction and detonates the column before it impacts the surface.

      I guess it depends on distance as to whether the microns would experience "inertial confinement" - e.g. the detonation would propagate so fast that the microns don't move
      or whether the microns would experience acceleration - - e.g. the detonation of one micron pushes the other,
      or whether they would not experience anything?

    7. I like the 3D printing idea, sort of a more advanced version of varying angle and timing to converge fire on a target in artillery doctrine. You do still need to strike a separate target to actually initiate the impact-nuke, but you could do some fancy explosion-shaping with that, I think.

      Re: fratricide, do bear in mind the context here with the dust shield. A single nuclear-capable macron can do a decent bit of damage on its own, enough to seriously damage the target vessel and momentarily scatter the shield. *However*, if they don't strike a dense and thick enough target they don't react at all. Only one needs to get through, but you need to fire thousands for even that one projectile to score a proper hit. So while it might be an issue in theory in practice and doctrine I think you'd just see it as an irrelevant side effect, really. It helps that macrons would likely be tiny and cheap. I don't think sympathetic detonation would be a risk either, macrons need the extreme compression produced by a target impact to react, without it they'll just get vaporised or scattered without reacting in a similar way to getting hit by a casaba-howitzer beam.

      Sympathetic detonation in nuclear weapons in general is quite rare, simply because they need a very precise sequence of events to go off without a hitch in order to fire properly. Even something as user-friendly as macrons is still going to follow this basic rule, if it doesn't hit in the exact way it needs to in order to work, you've just got a dumb grain of sand with some fuel in it. Really, unless your macrons are Fullerened antimatter or something similarly unsafe and deeply insane you're going to be spending all of your time worrying about the exact opposite problem instead.

    8. ---I don't think sympathetic detonation would be a risk either, macrons need the extreme compression produced by a target impact to react, without it they'll just get ... scattered without reacting--

      Well, that is kinda where I was going - a pulsed pellet motor, "hammer and anvil" using "time on target" and velocity difference to create a virtual hard target.

      Shoot the "anvil" (3D time-on-target print) a (relatively speaking) slow volley of macrons which will converge into a dense disk at a suitable distance.
      Shoot the "hammer" (3D time-on-target print) a (relative velocity) high speed inverted cone of macrons that hit the "anvil" from the perimeter in. Desired result is complete detonation, starting at the perimeter so that any "misfires" are scattered inward. Turn the high neutron flux into a benefit, use it to push any leftover macrons into criticality.

      The inverse idea - "Nuclear pellet jellyfish"
      Put an old school Orion-Medusa sail ahead of the craft. Create a "dust disk target". Sail to the stars...

      More insane next-step- "nuclear-saltwater-ink-jet printer" anybody?

    9. Given the weight difference between nuclear and non-nuclear macrons of the same diameter (I'm assuming they're both being fired out of the same accelerators), it would probably make more sense in terms of energy efficiency to use the nuclear macrons as the "slow" anvil

  5. ooooooh. Nice web page renovations. Also. Superb article as always :D

  6. I may not be reading the section on propulsion correctly, but I'm trying to understand how a Macron accelerator shooting a gram of Uranium can detonate and push a pusher plate. And on the topic of pusher plates, wouldn't the the force be despersed in all directions? I'm assuming a Orion pusher plate which would intern mean a casaba howitzer is used, but I don't see that happening with something the sizeof a BB.

    1. When used for propulsion, you have two scenarios: the spacecraft is shooting its own macrons, or it is receiving macrons.

      When it is shooting its own macrons, it will have a magnetic nozzle and a small target launcher near the nozzle. The tube accelerates the macron and it hits the target, creating a plasma explosion that can be redirected by the magnetic nozzle for thrust.

      When receiving macrons, the same magnetic nozzle and target launcher are needed, except the macrons come from any other direction.

      Magnetic nozzles can completely enclose the plasma explosion and redirect it with an efficiency of 80%+.

  7. I first hear of the concept several years ago when reading about the "Sailbeam". The launch velocities were far lower, and in some cases, the macrons were miniature solar sails (postage stamp size, similar to the Starshot, but not made to absorb huge amounts of laser power). In some cases, the Sailbeam could physically strike a pusher plate on the back of a vehicle to provide thrust.

    I'm really interested in working out the effects of using hyper velocity macron beams to propel spacecraft. Changing the flight time to Jupiter from several years to a matter of hours is an massive frame shift, and has political, social and economic effects that can only be guessed at. Tying together the Solar System with flight times similar to transcontinental air transport will be far different than a Solar System settled and connected by "Age of Sail" flight times.

    As always, a great post to spark lots of thinking.

    1. I think it is more like years to days or weeks, not hours.
      Or hours of flight time is not for human-rated ship at least.

    2. Thanks, Thucydides.

      Felix is right, travel time in the hours requires very high accelerations. For example, 100g acceleration for 10 hours is necessary for a transit velocity of 35,316 km/s, which is enough to cross 1 AU every 70 minutes. Total trip time from Earth to Jupiter (5.2 AU average) is 16 days.

      You can get a travel time from Earth to Jupiter with a human crew in 10 days if they accelerate 1 g for 1 day at either end.
      hours if the average velocity is 16,666 km/s. The deltaV requirement is 33,333 km/s.
      1g of acceleration can reach the transit speed in

  8. "In other words, a 50 GW accelerator can do the work of an 80 TW drive and reduce trip times from Earth to Jupiter to 9.8-14.5 hours for 1000 ton spacecraft".

    While humans might not appreciate being sent packed in tubes filled with oxygenated fluid, this sort of performance could be exploited to "FedEx" supplies and equipment across the Solar System. Rather than hardy self reliant pioneers duct taping things together so they can survive another day, they look at the balky item, go to Amazon.sol, order the item on line and then wait for the delivery drone to appear overhead in a blaze of plasma.

    Robert A Heinlein would be appalled.

    1. That's right! Although, it is not hard to imagine mass streams of uranium-filled impactors that continuously accelerate spacecraft over huge distances, thus reducing the acceleration requirements.

  9. Hello! Been following ToughSF for a while thanks to Atomic Rockets, great post as usual.

    Anyway, the impact fusion approach has me curious. The optimistic end of things suggests a fusion gain of 66000x, which compared to most mainstream fusion techniques is, well, not so much breaking even as might-as-well-be-free-energy. And from what I know of fusion tech this would be extremely straightforward to apply as a reactor. My question then is, why isn't it? When even the pessimistic projections suggest a gain any current MIF or MCF design could only dream of, you'd think teams would be all over making an impact fusion reactor, but impact fusion gets even less attention than MIF/MTF. So, what's the catch? Why haven't these things gotten much attention?

    1. Hi ATrifleMiffed. I am very glad to hear that!

      That 66,000x is a theoretical maximum, of course, but even with the 10% or even 1% fuel burnup we are looking at over a 1000x return on energy invested. This seems high, but remember that ICF schemes using laser ablation expects 100x fusion return and that's a 10% efficient process compared to kinetic compression (which is 10x more efficient, so 10x100: 1000).

      The problem with modern fusion attempts is that the full cycle(wallplug to wallplug) efficiency struggles to exceed 1x.

      Also, macron accelerators that were considered realistic were railguns (too slow/inefficient) and electrostatic accelerators (too long). We havent' tried 'blowpipe' accelerators yet!

    2. Cool, thanks for clearing that up.

      Also, regarding weaponising macron accelerators, I should think that the overall approach to defending against one would be roughly similar to defending against more conventional particle beam weapons, using a combination of magnetic fields and counterfiring particle beams? Of course the magnetic fields would have to be considerably stronger to deflect the far more massive macrons, but if we assume the existence of magnetic nozzles that can direct the power of a fusion reaction for thrust or even a straightforward space fountain-style magnetic capture of solid macrons then adapting one to function as a sort of magnetic shield doesn't sound too unreasonable. Likewise sweeping likely approach paths of macron volleys with a charged particle beam or even a defensive casaba howitzer-style nuclear bolt to knock them off course.

      It does also imply a rather funky dynamic for a hypothetical warship with macron accelerators facing off against a similar warship, since both vessels' macron systems might be designed as multi-role devices, able to both fire macrons for an impact fission or fusion drive or using them as an offensive weapon or even to drive missiles.

    3. The macrons will have their charge removed after exiting the accelerator. It is very hard to add that charge back while it is moving at hypervelocity, making it extremely difficult to deflect with a magnetic field!

      The methods to add charge involve things like putting the macron on the tip of a needle and slowly building up the voltage to several kV, or spending quite some time bombarding it with electron beams. These things cannot be done quickly, or at a great distance, without huge difficulty too.

  10. Weapons wise, the Macron beamer (especially the multi barrel "metalstorm" versions) seem like the ideal counter to "Soda Cans of Death" (SCoD) mini warheads. You can "fill the sky" with thousands to overwhelm the Ravening Beam of Death (RBoD) laser, but a close support vessel packing multiple macron launchers can sweep the SCoD's from the sky while the massive RBoD deals death and destruction to high value targets a light second away. The energy release of D2 filled pellets will ensurer the struck SCoDs (or any other sort of physical target) will be rapidly dispersed, so the defended ship or platform won't even need to worry about being struck by debris from incoming missiles or warheads. Using this against "ground" targets like a moon or asteroid would be far different from any other weapon, although the visual might look a bit like a flamethrower as the fusion plasma erupts from the ground and into the buried structures below.

    The taxonomy of warships gets more tangled: Laserstar, Kineticstar, Firestar (armed with third or fourth generation nuclear weapons), Beamstars with hot or cold ion beams and now this (Gunstar?). Finding the ideal "mix" of ships in the constellation will be an interesting problem for naval staffs of the future.

    While it is always possible ships equipped with an impact fusion drive could swing out the projectors from the engine focus to deal with incoming rounds, this is not likely in my opinion since the act of taking the engine off line will create far more problems and vulnerabilities than using dedicated weapons on proper racks, hardpoints or turrets.

    1. On the one hand, yes, absolutely. On the other, having a dual-purpose macron projector does free up room in your mass budget to attach other weapon systems, and as a side-effect produces what game designers might describe as a "versatile verb", that is, a mechanic that can be used for several distinct actions, which I do rather enjoy as a narrative and mechanical conceit.

      As to why you'd do it? Well, the pushrod accelerator type can potentially be made relatively compact and lightweight, and you'd want to have at least one, preferably more redundant projectors anyway (especially on a warship where getting bits of your engine shot off is a genuine risk). From there it's not unreasonable for someone to decide that since you only need one at a time for propulsion but have a spare anyway (yes, I am aware this kind of goes against the mass budget explanation but shut up, it's narrative conceit time) it might not be a bad idea to make them able to multi-task and either fire directly at hostiles or drive missiles using impact fusion. It does make the attendant tracking and fire control hardware for the accelerators considerably more complicated but oh look a distracting handwave. Either that, or a specialised drive-optimised accelerator might be a hypothetical last ditch weapon to use at close range when no other option remains. I do struggle to envision these things being allowed on civilian ships though in spite of their advantages, except for either the "macrons are fired by remote static infrastructure" version or a setting full of crazy space libertarians who see nothing wrong with giving merchantmen access to thermonuclear miniguns. Nobody tell Alistair Young I said that.

      I've also been wondering how you'd actually defend against these. Matterbeam has shot down my CPB idea, but I've been wondering about Whipple shields again. Macrons disintegrate the moment they hit a Whipple shield due to their insane ratio of KE to momentum, but a given patch of shield can only stop a single macron and they can potentially be fired at rates that would make a GAU-8 blush. The issue is that all the follow-up shots can just go through the hole made by the one that was stopped. So we need a shield that can't have holes poked in it. What occurs to me is perhaps modifying something like a charged dust or the Curie point/ferrofluid fountain radiator, except instead of it being a radiator per se you've got colder and denser fluid or dust being circulated around and controlled by the magnetic field to produce a liquid-metal or charged dust umbrella over a section of the ship (mount it at the bow, say, and hide the rest of the ship behind it). The macrons strike the umbrella, disintegrate without encountering enough material to properly compress them to fuse or fission and are instead reduced to a comparatively harmless bolt of plasma that pings off of a layer of armour below the umbrella. The hole the macron made is moved and/or "filled in" by the normal movement of the fluid or dust and the gap in the shield for other macrons to pass through is no longer there. This could still be overwhelmed by a sufficiently high rate of fire and enough projectiles striking the same rough area, so the warship would still need to actively dodge, but it would gain a lot more survivability than from a solid shield.I don't think you'd even need all that much shielding material, since it doesn't take a lot to wreck a macron, just enough to ensure continuous coverage of the area being shielded with a thin layer of shielding.

    2. Thucydides:
      The ion or electron generators used to accelerate macrons down a 'blowpipe' accelerator can be kept in place, and their beams re-routed to empty tubes throughout a spaceship. They are very low energy beams and easy to bend (unlike the relativistic particle accelerators from other posts).

      You call this a 'Sandstar'.

      Using a dust cloud to defend against a macron stream is a brilliant idea. It would work and be quite effective! It would force the macrons to be bunched up into compact columns of thousands of microprojectiles, all striking within microseconds of each other. Only this concentrated assault could get through a dust clouds before it replenishes.

      The amount of shielding material lost would be far less than even the small amounts of ammunition needed by a macron accelerator, so this can be kept up for a long while.

      The dust cloud is vulnerable to other weapons though. It won't protect much against lasers or conventional kinetics. It can be cleared out entirely by a nuclear explosion too, and a charged particle beam can disrupt it quickly. Of course, by using a relatively cheap defensive system to force the opponent to carry an entirely separate weapon to get through it, you maintain the upper hand!

    3. Would bunching up the macrons also make them easier to detect from the target's perspective? The frontal cross-section would be about the same so I don't think it would matter much for direct radar and lidar scans, but perhaps it would make their total thermal signature easier to discriminate from the firing platform. Yes, they would still cool down quickly, but you might be able to get a long enough observation to extrapolate the trajectory of the macron stream.

      As for dust shields being vulnerable to nukes, lasers and CPBs, absolutely, but from a narrative perspective I'm not sure that's really a drawback for this combination of technologies to have. Sci-fi has long had something of an unofficial convention that pew-pew laser guns beat magic energy shields and boring garden-variety bang-bang guns beat boring garden-variety armour and missiles beat everything but you can only ever have less than a dozen of them for balance purposes and they can be shot down or distracted by chaff/flares or evaded (physics says no but trashy space opera says yes), and you can straightforwardly reimplement that basic rock-paper-scissors dynamic with this, only with an extra layer of complexity and fun unexpected interactions from backing it with real science. Macrons and other kinetics get chewed up by dust and Whipple shields, but wreck actively-cooled anti-laser armour, lasers wipe away dust shields and fry sensors and other vulnerable equipment but are made almost useless by cooled armour unless at knife-fighting range, and missiles with nuclear warheads are lethal against both but are big and heavy, so a warship can only carry a limited number of them, and they can be shot down or crippled by countermeasures. Turn a few knobs to adjust precise capabilities of each system, and you get lots of interesting tactical choices for everyone.

    4. Addendum: and of course, you've also got the other weapons systems a ship is armed with that can be used both offensively and defensively. Lasers and particle beams can shoot down larger kinetics and missiles, and disrupt particle beams which can then be spread out to harmlessness by the magnetic field of the dust shield, but are nearly worthless against macrons, macrons can bring down missiles and big kinetics, but also power missiles and casaba-howitzer bolts, big kinetics rip holes in dust and armour, but are countered to a degree by Whipple shields and force enemies to evade or expend power or heat capacity to shoot them down by being big, scary but relatively slow and predictable lumps of lethal KE, and missiles can, depending on design, warhead, whether they've been boosted by their firing platform, etc, fit into basically any role offensively or defensively and be utterly devastating but are expensive and limited in quantity and can also themselves be counteracted by basically all of the above.

  11. Interesting thoughts.

    My main issue with trying to dual purpose the engine accelerators is you will be stuck in a very difficult situation if you suddenly need to realign the accelerators for thrust while engaging a target. Even with multiple accelerators in a ring around the engine, you may have issues with power budget or vibrations (and if you light the engine, the outgoing macrons for defense will be consumed in the fusion plume). There is also the issue you only cover the rear hemisphere of the ship, although you might handwave this the way 1960 engineers thought of an ORION nuclear pulse battleship, just swing the ship around and point the pusher plate at the enemy....

    I think the main issue with trying to mechanically shield a ship (or anything else, for that matter) is nuclear macron rounds with fission or fusion payloads will blast a huge hole in the screen with the nuclear energy, which will take too much time at the scale and speeds being discussed here to fill before the stream of macros sweep in and scour the surface of the target.

    The only thing that comes to mind is "Kirklin mines", essentially portable Whipple shields you fire in the direction of the incoming stream, but if the Gunstar is firing bursts of 10,000 macrons at a time during the gun run, you run out of Kirklin mines pretty quickly. The "combines arms" approach is the constellation uses sensors to identify Gunstars at the maximum feasible range, and then the RBoD Laserstars pick them off at a light second before they get into firing range. Smaller close defense Laserstars can use wide beams to iluminate incoming streams and Firestars then engage using Casaba Howitzer rounds to vapourize the incoming stream. With any luck, you induce detonation and the stream disintegrates in a cloud of plasma.

    1. See, my thinking was that the macrons would need to encounter a certain amount of sufficiently dense material to actually initiate the nuclear reactions. The amount of material needed to shatter the macron without the fuel ever attaining the density needed to react would be a lot less than this - so the "blow holes in your shield" issue is avoided by never giving the macrons the opportunity to explode. So you'd actually want your shield to be as thin as possible while still maintaining coverage, thinner than the macrons that are going to be fired at you (advantage: charged dust, each particle essentially working as a tiny recirculating short-range Kirklin mine). The main issue here is that gosh-darned carbon nanolattice ball because that's going to be directly competing with the minimum size of particle you can realistically use. The other possibility is using a multi-layered shield with a combination of several tiers of the moving umbrella shields and something like graphene or some other monolayer below that to kill the particles too small to intercept with the fluid/dust shield, at the cost of being extremely vulnerable to normal Whipple shield attrition. The other issue is that a casaba-howitzer or a large laser could simply sweep away the shield and let the macrons strike unimpeded, but that's not necessarily a downside if we're thinking in terms of setting or game design.

    2. And of course, I now realise that the "make the shield as thin as possible" thing really only applies to the liquid shield, with charged dust you want to make each individual particle as small as you can manage but the shield they comprise can be thicker, and you in fact want it to be to increase the chance that a given cross-section of the shield the macron is moving through contains at least one particle for it to strike and be shattered by (if anything does get through a monolayer Whipple shield can act as a last line of defence).

  12. Not related to weaponry, here is an article from Crowlspace which talks a bit about using macron beams and Kare sailbeams for propulsion. The interesting thing is the Kare Sailbeam doesn't need lasers to bring them up to high velocity, which has very positive effects on costs and the basic infrastructure (Sailbeams could be created by gently "puffing" the sails out of a space garage and into the sunlight).


    1. I'm actually looking into the concept of micro-electric-sails, something that I do not think has been explored.

  13. Here is an interesting article looking at the effects of high ISP/high thrust drives, from Atomic Rockets. While written in 1965(!), and speaking in terms of gas core fission rockets, many of the assumptions can be transferred to macron drive (or even Epstein Drive) spaceships.

    The economic assumptions are interesting, especially since these sort of high velocity spacecraft are being treated more like jet transports (the example used is the DC-8, which gives you an idea of how long ago this was actually written), rather than the current paradigm of "age of sail" transit times. This goes back to some of the comments I've made on this (and earlier) threads, which is the "dismal science" of economics will have a very profound impact on space operations and space civilization.

    If my guess is correct, most of the future space civilization will be based on fixed infrastructure and energy beaming, since that will allow the cost to be amortized over the greatest number of vessels (and eep the cost of vessels low, creating a virtuous circle where more and more relatively inexpensive space vehicles come into service, creating the conditions to invest in more "base load" infrastructure like fixed macron beam projectors, laser webs, mass drivers etc.).

    If we are going to focus on macron projectors, then there will likely be a divide between Kare sailbeams for low cost, relatively low velocity bulk cargo transport, and hypervelocity macron beams for high priority "packets" carrying passengers, mail and urgent freight. We could do the same thing for virtually every sort of fixed infrastructure system, ligh energy lasers and focusing elements for high priority freight and lower energy lasers for low speed lightsails, short, hyperacceleration mass drivers for bulk freight pods and thousand kilometre long mass drivers for human passengers and so on.

    I also think this is going to be the preferred sort of system since it can be far more easily policed than self propelled spacecraft. The idea of large masses moving at interplanetary velocities with nuclear bomb to dinosaur extinction levels of kinetic energy should scare the crap out of any person in a space civilization, so tight control of trajectories will be a huge factor in allowing space traffic of any sort. Mass Drivers, lasers and hypervelocity macron beams also allow the owning polity to have a means of dealing with errant incoming "packages" before they become an existential danger to the space colony.

  14. Instead of micromachining little balls filled with uranium and deuterium/Tritium ice, use uranium hydride. It's easy to make, can be easily powdered and is ferromagnetic. Hydrate U238 with deuterium and tritium. The U-238 will fission from the neutrons released in the D-T fusion. This lets you store your propellant/ammunition without worrying about criticality.

    1. There has to be a shell of DT ice around the uranium because the DT expands much faster than uranium after being heated by an impact. The quickly expanding DT is used to compress the uranium at a certain velocity. The fact that it absorbs neutrons very well is a bonus!

    2. We aren't interested in compressing the Uranium. We are using the neutrons to trigger fission in the Uranium. It's basically the fusion-fission hybrid model. The D-T fusion only serves as a neutron source.

    3. Thanks again, Matter Beam.

      Re: Gun Stars:
      1) Would there be value in having a G-Star with various types (nuclear/non-nuclear)/sizes (nm/um/mm) of macrons, or would that likely be too expensive/unwieldly?
      Would the various types/sizes of macrons be at ~the same technical level, or would some types be "earlier" and subsequently become obsolescent as technology progresses?

      2) Along these line- is there be any point in "keeping" K-Stars or do macrons obsolesce SCoDs?
      Would there be value in having a combined G-Star/K-Star weapons platform?

      3) Finally, would you be open to creating a "Star Chart"- listing the various "stars" (Beam, Fire, Gun, Kinetic, and Laser) with their ideal uses, ranges, potential countermeasures, technical readiness levels, etc.? (The "rock/paper/scissors" descriptions are hard for me to follow.)



    4. Hi Keith.

      The ammunition load for a macron accelerator (a 'dust gun') is far smaller than that of other gun types. It would be trivial to have a huge variety of macrons, each specialized for different applications.

      K-stars can compete if they get a 'decent' velocity and use their onboard propulsion to maximal effect. A 100 gram projectile is likely to have enough room for sensors, antennae, fuel, engines and more. A macron just travels in a straight path!

      Combining the two is possible... but it would be a bit awkward unless you created a new hybrid-type weapon system: a coilgun accelerator that shoots a large projectile, that is further accelerated by macron impacts, and then guides itself to a target that is very far away. You get the utility of a large gun, with the velocity of a dust gun!

      There isn't really a point to a 'star chart', honestly, because it could only exist if all the different weapon systems were equally well developed and were somehow all being used simultaneously. In the real world, we tend to focus on one great weapon system after another. The evolution of the big cannon to the fighter plane to the missile as the primary naval weapon is an example of a real world scenario where different weapon systems did not co-exist equally.

    5. That was my thinking, regarding combining G- and K-weapons on a single platform. A large coilgun to give the munition an initial boost, then a macron gun fires to accelerate it further, and once they're outside of the macron's range and close to the target they can use their own onboard propellant for final correction before impact. Meanwhile the macron gun can be retasked to work as point defence or drive several missiles at once by switching from one to the other rapidly. It's essentially sort of analogous to dual-purpose DEW emitters that can either fire on a target directly or drive a sail or power an ablative or thermal rocket (I think Mark Kalina's Hegemony uses that with lasers and space fighters).

      You basically get the versatility of a coilgun-and-missile system that can launch basically any payload that will fit inside of a sabot, but you can also get the insane projectile velocities of a much softer SF mass accelerator (*cough*totally not looking at Halo and Mass Effect's mass drivers*cough*) without the ginormous capacitors or coilgun the size of a small planet or the intolerable heat that would realistically involve, since you can power it all with the macron's fusion gain and a magnetic nozzle. Or just make it ride a mass stream of non-explosive macrons if you need a softer touch. The only limit on projectile velocity you have is the relative velocity the macrons need to react (so fission macron-boosted systems would actually be able to achieve higher maximum velocities than fusion ones despite the inferior exhaust velocity of pure fission, since they can afford a much lower relative velocity between the macron and the target so they can boost it closer to the macron's own velocity).

    6. Thank you too, ATM.
      Perhaps instead of 5 "stars), we might have three:
      G- and K-Hybrids
      B- and L-Hybrids

      There's been discussion of at least some types of weapons "buses" as opposed to "stars, e.g., for Casaba-type weapons. We could call these "planets", as they can "wander". Which of the other four types would tend to be too unwieldly to mount on a "planet"?

      MB, how efficient could macrons be for non-propulsive/non-weapon energy transfer, aka:
      "a very long power cord"? Can various other types of energy be more efficiently converted to KE than to various types of DE (laser, charged particle, neutral particle)?



    7. @Keith:
      Macrons would be very efficient at transferring power, but they would be very impractical to use. Slowing down a macron using a direct electrostatic converter would require an incredible length, perhaps several kilometres, so it is unlikely to be used despite 95%+ efficiency.

    8. Thanks. What about the practicality of using fission or fusion macrons for energy generation at the destination. i.e. generating electricity from the small nuclear/thermo nuclear explosions?



    9. I suppose you could magnetohydrodynamically tap the explosions at the destination end for power generation, or use the heat produced to power a generator. It does bring up the "what if you miss" issue though, accidentally setting off nuclear explosions all over your own power infrastructure does not seem particularly healthy even if they are very small nukes. A better approach may be to adapt the kinetic energy exchange system that Zerraspace (I think) described here a while ago, and using the macron beam to throw a big slug of metal at the destination where it's caught by a receiver station (a coilgun run in reverse, pretty much) and the energy captured that way. You again get a very high efficiency and infrastructure that can send both physical cargo and energy with a simple change of payload, or indeed in the same payload. It also makes you less dependent on favourable orbital positions for the KEX system without having to invest enormous amounts of electrical energy up front to launch the payload, thanks to the insane fusion gain potential of macrons.

      On a different note, I've been thinking about how one might apply the macron gun to a Medusa drive, because the two sort of strike me as a match made in heaven, really. The macron gun doesn't lose energy with distance the way a particle beam or laser does (though it will, I suspect, still have a bit of drift which will reduce the total energy that can be put into an area after a few thousand kilometres or so), so it can initiate fusion way more easily. Meanwhile the Medusa's operating mechanism and being based around tensile strength makes it both lightweight and easy to situate far enough away from the payload to minimise radiation exposure without needing heavy shadow shield plates, and deals with heat elegantly without needing enormous radiator wings. You end up with a very lightweight drive (hell, it's not inconceivable you might be able to pack away a spare sail in the payload in case the main one gets damaged, something you can't do with a magnetic nozzle!) with excellent specific impulse. And you get a free atomic minigun in the bargain.

      Very useful things, these macron guns.

      Oh, and lest I forget, happy Christmas and New Year to you folks!

    10. Thanks, ATM. What you describe (for remote power generation) may be different from what MB said above"would be very impractical to use. Slowing down a macron using a direct electrostatic converter would require an incredible length, perhaps several kilometres, so it is unlikely to be used despite 95%+ efficiency."
      Could you elaborate on how your proposal is differs from this?
      Also, " Amacron accelerator with 0.04 grams of uranium would only release 3.2 GJ" this is about .764kg of TNT, which doesn't seem like a big explosion if a macron hits something by mistake (unless it were near an unprotected person, presumably in an atmosphere)...


      I really like your "Macron Medusa" concept!

    11. @Keith:
      My statement was for the 'cold' capture of macrons, where they re slowed down by electrostatic forces gradually and non-destructively.

      A 'hot' capture where the macron smacks into something and creates a plasma explosion is much smaller but it loses a large fraction of the macron energy in various inefficiencies.

      Igniting nuclear detonation at a target would be more energy generation than transmission, but it can overcome the inefficiencies of 'hot' capture several times over.

    12. @Keith
      I was referring to combining Zach Hajj's proposal in this article ( with the macron mass-stream/"macron sailbeam" concept Matterbeam brought up in this article. You take a large payload such as a large (a dozen tonnes or so) solid cylinder of metal or a container full of cargo, and put a magnetic-nozzle-and-obstacle assembly as MB suggested on the end to harness a nuclear macron stream fired from a station. The macrons detonate against the obstacle, the energy is used by the magnetic nozzle to accelerate the payload. So far, so standard "power beam" type macron drive. The payload is then caught at the receiving end by a receiver station that is basically a coilgun running in reverse. Instead of using magnetic fields to accelerate the payload it instead retards it down to a relative halt by drawing kinetic energy from it through magnetic coupling. If you use an induction coilgun (where the gun's fields are coupled to a "sabot"/armature of coils wrapped around the payload) you can use this to simultaneously capture both energy and physical cargo, since the receiver doesn't really care what is inside the sabot as long as you don't pack so much mass into it that you just overload it. Or you could just send huge chunks of kinetic energy by filling it with lead, gold, really anything that's really dense, so you pack as much mass and therefore KE as possible into the payload dimensions the receiver station can accept.

      Compared to the baseline design Zach outlined, you get much greater versatility in when you can launch and your up-front energy cost of launching an outbound payload is reduced thanks to the nuclear power of a macron drive. You can also economically achieve much higher kinetic energies. And unlike a system where you just have macron gun stations at either end of the trip, one to accelerate the cargo with macrons and one to decelerate it, you can actually recover the macron energy used to accelerate the cargo at the destination. This energy can then be used for any number of things - powering a payload going in the opposite direction, powering a local colony, beaming it elsewhere using either a similar macron beam setup or a more conventional microwave-and-rectenna system, anything you like. As a bonus, if you have both the macron gun "transmitter" and the coil receiver emplaced at the destination (or even packaged as a single unit), it also defangs the KEX system's ability to be turned into a weapon of mass destruction with a slight course change of the payload since any rogue payloads can be obliterated by a hail of thermonuclear fire. One macron hit releases a tiny amount of energy. Three million of them release a... less negligible amount.

    13. Oh yeah, previous reply is me, Trifle. Oops.

    14. @ATrifleMiffed:
      Ok, yes, I understand the idea now.
      I guess the comparison of a conventional kinetic energy exchange system and a nuclear macron-driven one is that uranium has to be extremely cheap but big reactors rather expensive.

      If the uranium is expensive, it would more sense to burn it slowly in a reactor and use the resultant electrical energy to accelerate a mass. If it is cheap, throwing it away in a macron stream would be acceptable. This is because the Macron-compressed fission reaction has a relatively poor burnup of the fissile fuel. A reactor can extract 96% of the fission energy, a macron might get 1 to 30%.

    15. Do bear in mind that this is as much a cargo transportation system as a power generation one, the real draw IMO is that it helps you recover part of the energy cost of throwing cargo around a star system and that it is also a very versatile system for general space transportation - there's really no reason I can see why it can't also work in KEX mode, the basic pitcher/catcher tech is going to be present anyway because for local use (say, LEO-Moon launches) a basic coilgun launch may well still win in overall economy, but you also get some bonus features. And unlike the KEX which is constrained by orbital mechanics if it wants to break even on power the macron gun can basically boost cargo from anywhere to anywhere else and give the receiver station energy as a bonus.

      Also, how would the economics of this be affected by using fusion macrons rather than fission? Seeing as fusion fuel is cheap but needs a lot of power to ignite anyway and macrons have a really good theoretical fusion gain, it does seem like the two technologies are quite well suited to being combined. And again, since nuclear macron guns really don't care where the macrons they fire end up reacting, you can simply redirect the gun to pump macrons into a reaction chamber on or near the receiver station when it isn't actively propelling payloads so the downtime between payload boosts isn't wasted and the energy released can be more fully captured.

    16. Thank you both, ATM and MB. I gather macron-compressed fission reaction would be i veryy inefficient as you said: (it) has a relatively poor burnup of the fissile fuel (A reactor can extract 96% of the fission energy, a macron might get 1 to 30%.)
      How does this compare with a macron-compressed fusion reaction?



    17. @ATrifleMiffed:
      A fusion macron requires much higher impact velocities to ignite. I think there would a U-curve: at the low velocity end, it is simpler and cheaper to just use kinetic macrons, but as the velocities exceed 1000km/s, using fusion becomes very economical.

      Fusion fuel is so cheap that we don't usually care about burnup ratios. The numbers I've seen reported range for 3% to 90% depending on the impact velocity and other factors trying to increase or decrease the time the fuel spends compressed.

    18. Thanks for clarifying. So I assume in most cases with either type of nuclear macron (fission or fusion) you'd want to aim for a "sweet spot" where you get a high fuel burnup without it taking too big of a chunk out of your nuclear reaction gain?

      So what's the actual use case for kinetic macrons in a PMF setting here? At low velocities (a few dozen km/s) it seems to me that more conventional railguns and coilguns firing larger payloads have an advantage in terms of bang for your buck, since they convert their input energy into projectile kinetic energy much more efficiently and most of the macron's combat advantages can be replicated by just firing a bag of sand or other loose granular material in a sabot instead of a single solid projectile. At high velocities (several hundred km/s+), I'm not really seeing much of a downside to filling a macron with cheap fusion (or not-so-cheap but easier to ignite fission) fuel in most roles (maybe for boosting really, really radiation-sensitive payloads, but I'm not convinced even that makes much sense considering how easily you can bypass the tyranny of the rocket equation with a macron-gun booster, so you can just pile on the radiation shielding anyway). So I can't really see non-nuclear high-velocity macrons having much of a role.

      Really the only case where I can see a solid kinetic macron accelerator being used is a relativistic macron carrying as much, or more, KE than it would have gained from a nuclear reaction anyway, but those do not seem like the sort of thing a PMF setting could field. I could see a very high-powered, far-future setting (something with the power level of Orion's Arm or thereabouts) having relativistic macron accelerators though.

    19. This comment has been removed by the author.

    20. Thanks again, MB & ATM. I have some vague ideas of a hybrid system of *energy transfer (as opposed to propulsion) combining fusion and/or fission macrons with Inter-Orbital Kinetic Energy Exchanges (differently configured than what ATM mentioned on 5, 7 January), but not being an engineer **I have no idea how it might work.
      Your thoughts...


      * I'm still pondering the "really long power cord" concept...
      **Beyond that it would be a much faster energy transfer (in one direction) than pure IOKEE.

  15. As Matterbeam suggests, not every type of weapons system is going to be developed at the same time, so there will be periods of overlap, or periods when a system will simply disappear into obsolescence.

    My take on this is most space warships will highly specialized (hence *stars) due to the tyranny of the rocket equation. When "every gram counts", then you are not going to sacrifice many tons of your ship to a secondary weapon if it means that the primary weapon will then be far less powerful than it could be. This is true even if the ship is primarily beam driven or towed behind a solar sail - once in action it will need to cast off and become self propelled otherwise it is simply a target. In my conception, warships are actually eggshells with sledgehammers.

    That being said, there are some edge cases, such as using a laser or macron beam to propel missiles from a Kineticstar, or even deliver the nuclear payloads of a Firestar. You could also make the case for cooperative targeting as well, a Laserstar could devote some of its time and output to delivering the first waves of projectiles from Kineticstars before enemy craft come into the one light second range of the RBoD.

    Having this variety as the starting assumption (Laserstars, Kineticstars, Beamstars, Firestars, Gunstars) allows the writer to examine all kinds of different permutations and makes a rich environment to develop stories and ideas (when do Gunstars overtake Firestars in utility? what does a Space Navy do when they realize their constellation full of Kineticstars is going to be neutralized and overmatched by close in defensive Beamstars in the enemy constellation? How do you react to targets hiding in contested and crowded orbital space with your *star or constellation?)

    As you can see, there are a lot of assumptions I am making. You can feel free to disagree and make your own assumptions to come up with ideas and scenarios, just make sure your assumptions are clear so we know where you are coming from.

    1. Those are all valid assumption, Thucydides, and you do nail the intent behind all this on the head: give scifi authors and worldbuilders more options to make their combat exciting.

  16. The link in "A huge amount of charge can be added by /various methods/." seems to be broken.

    1. Thanks! I replaced it with another link to the same paper:

  17. "and cannot support large magnetic fields in a coilgun"

    No. The problem is that the eddy current induced is used to ohmic heat the projectile the smaller it gets. That's why smaller electromagnetic motors are less efficient.

    Drexler and Freitas proposed electrostatic motors for use in nanomachines for the same reason. Current just doesn't flow on the small scale.

    1. Indeed, I was trying to make the point that small particles won't survive in a coilgun, but didn't use enough words to describe exactly why!

  18. Dude, you okay? Been gone for a while.

    1. Yes, I'm alright thank you. I'm busy with new projects but slowly preparing a new post. Follow me on twitter - @toughsf

  19. Really cool stuff. I've been wondering about using macrons in an impact based nuclear propulsion scheme, perhaps in a method similar to your Epstein Drive concept. Lasers may just never cut it, not in a reasonable time-frame at any rate. However, this part is a little off:

    "A 200 km/s projectile contains 20 GJ/kg of kinetic energy. Nuclear fission releases only 4x this amount. If we factor in incomplete burn-up of the fuel, it is likely that there won’t be a significant increase in the energy delivered."

    Fission should release some 80 TJ of energy, around 4 thousand times as much energy. Such an accelerator is still providing net energy through fission, and could enable a fission based torch drive though I suspect the macrons may become more akin to the second stage of a thermonuclear device: still mostly fission but with reasonable amounts of both fuel types and perhaps even the addition of U-238 if the neutron flux is high enough to enable fast fission.

    Still, if the accelerator was remotely delivering macrons to the vehicle, the relative velocity is the key here. The fission energy would be a large increase relative to the vehicle's frame, while most of the kinetic energy would be relative to the accelerator's frame. This means that even if there is more kinetic energy than fission energy relative to the accelerator, the vehicle would still be gaining more energy through fission in its own frame. Of course there is a limit to this, and it depends on the velocity program used by the accelerator. But it is important. A similar concept was developed for use with Kare’s Sailbeam and Mini-Mag Orion – essentially remotely delivering pulse units to push the vehicle to high speeds. If I recall, a maximum speed of around 15% of the speed of light was reported. Of course this still used Z-Pinch to ignite the fission fuel. Using a macron accelerator could enable a similar vehicle without the need for the Z-Pinch system.

    1. Thank you for spotting that mistake! I've corrected the numbers now.

      I agree with your statements regarding sailbeams - ignition upon impact would make for much simpler vehicles. Also, a cooperative target could place specially shaped obstacles into the path of the macron - this would improve compression efficiency significantly and might even reduce the impact velocities required down to a few dozen km/s, even for fusion. This idea was explored in this blog post:

  20. I wonder if you could use a macron accelerator to create tiny black holes and use their evaporation by Hawking radiation as a power source.
    In theory, all you have to do to make a black hole is cram a lot of energy/mass into a small enough space and do it quickly enough to overcome the hawking radiation being emitted. A pair of very small macrons, like nanogram sized, moving at a significant fraction of C and collided would focus a lot of energy into a very tiny point. You could even sharpen the macrons to have very narrow tips, maybe only a few atoms wide to maximize the energy per unit volume. Would that be enough to do it? For it to be worthwhile, the energy emitted by the black hole must be greater than the energy needed to accelerate the macrons but since you get effectively 100% efficient conversion of mass to energy using black holes it may be worth looking at.

    1. I'll refer you to this quote:
      "In familiar three-dimensional gravity, the minimum energy of a microscopic black hole is 10e19 GeV (equivalent to 1.6 GJ or 444 kWh), which would have to be condensed into a region on the order of the Planck length. This is far beyond the limits of any current technology. It is estimated that to collide two particles to within a distance of a Planck length with currently achievable magnetic field strengths would require a ring accelerator about 1,000 light years in diameter to keep the particles on track. Stephen Hawking also said in chapter 6 of his A Brief History of Time that physicist John Archibald Wheeler once calculated that a very powerful hydrogen bomb using all the deuterium in all the water on Earth could also generate such a black hole, but Hawking does not provide this calculation or any reference to it to support this assertion."

      The energy needed to form a black hole is stupendously high. You could collide Macrons to do it, but the accelerators needed to get them up to the necessary energy would circle from the Sun to the closest star a few hundred times.

      Also, a nanogram black hole would evaporate in about 10^-53 seconds. Even at light speed, it wouldn't even travel a single Planck length before disappearing... and when it does, it radiates half of its energy in the wrong direction.

  21. What kind of velocities would one need to use other kinds of fusion with a macron accelerator? I think from a propulsion perspective He3-De might be more interesting, as you avoid having to deal with Tritium breeding and storage. To deliver the Helium, one would have to use graphene structures to deal with its high permeability, but that should not be an issue. However He3-De has a Lawson Criterion of 16,so it is quite a bit harder to ignite.

    So,how fast would the He3-De macron have to travel? And would the slightly better energy/mass unit as well as the fact that the majority of the reactions energy is now thermal and not Neutrons change things?

    1. A rough estimate, you would need 4x higher impact velocities to generate the 16x higher energies needed for DHe3 fusion, and 22x faster for the 500x harder pB11 fusion.

      You only get about 4% more TJ/kg out of DHe3 fusion than with DT fusion, at the cost of much higher macron velocity, so your fusion multiplier falls by a lot. pB11 fusion is massively worse. 500x more kinetic energy cost, but you get 1/5 the energy of DT fusion, so in total, it is 2500x worse!

    2. So a He3-De seems to be doable with reasonably sized pushrod accelerators and properly shaped macron. That's great to hear,thanks for the answer. I really enjoy your blog, keep up the good work.

      Do you think one could set of remotely propelled macrons with the spacecrafts own macron gun? You mentioned that detecting macrons is hard, but that was in the context of a space battle. I'm wondering if this macron on macron strategy could be used to get a higher velocity out of one's interstellar beamed propulsion crafts, as, as the velocity of the vessel increases the relative velocity between it and the home systems macrons decreases to the point where they become useless.

      Or would preseeding the vessels path with kinetic mines, that the macron gun could target be more sensible? Assuming one is up for a few years of relativistic needle threading.

    3. I'm writing something about this, I'll show my calculations soon.

    4. The advantage of p-B for self-powered propulsion is that the products are essentially all alpha particles. That in turn means your torchship's magnetic nozzle doesn't need shielding and cooling against hundreds of gigawatts of neutrons, merely tens to hundreds of megawatts of gamma from bremsstrahlung coming from the macron shell and obstacle material. It also means direct energy conversion (likely using magnetic induction) can power the rest of the ship from the fusion exhaust.

      Decaborane is a dense solid (for a fusion fuel anyway). The ~2200 km/s required should be achievable in mm-scale boron nitride spheres with less than 10-meter blowpipes. Using BN as the shell means ~42% of that mass is available for fusion, although burnup is likely to be low. The target material could be boron, BN and/or decaborane as well.

    5. About 0.2% of the fusion energy from a proton-Boron reaction ends up in the form of neutrons. Much lower than the 80% from a DT reaction, but it cannot be ignored when you have a multi-gigawatt drive. There are other side reactions which produce 2-16 MeV hard gamma rays, on top of the usual X-rays from Bremsstrahlung radiation that comes from any 'exposed' fusion plasma.

      pB fusion is clean, but it is not perfect!

      Decaborane fuel is a good idea, but only for some sort of ignition scheme that doesn't rely on a proton beam. In that case, a pure boron target is better.

  22. Criticism of this scheme:

    You can read it using the google translate service

    (lj user "antihydrogen" it is the same author aka "Полупрозрачный изобретатель" or/aka Владислав Серов (Vladislav Serov) here:

    1. Google Translate of real physicist view (

      "1. The concept of acceleration of millimeter-sized projectiles by an electric field looks dubious (at least because of the self-discharge of the balls charged to a high charge).

      2. The scheme of undermining micro-charges of fissile matter looks dubious (I read the original articles, their author himself writes that he was criticized back in the 70s). And the author of the article from the link still did not understand it correctly - a uranium ball covered with DT ice is not enough, an ablation shell of uranium is supposed around the ball, and around it there is also an outer shell that emits / holds X-rays.

      3. Even if it is possible to compress uranium to critical sizes in this way, only a small percentage of it will have time to share. "100% burnup, this amounts to a kinetic-to-nuclear energy multiplier of 379,149! Even at a low 10%, this is 37,915 times the energy invested." - even 10% here is a monstrous optimism.

      4. Reaching the exhaust speed of 10,000 km / s is impossible, both because of the previous point and because of the emission of the explosion energy in the form of an X-ray, especially for small charges."

    2. 1. Highly charged dust particles are regularly accelerated to 100 km/s today:

      I will think about the rest.

    3. He said about acceleration of _millimeter-sized_ parcticals (as you do in your post), but here is "Particle sizes: 0.2—2.5 µ" accelerator example.

      The other parts are really worse for the whole idea.

  23. I understand detecting a volley of macron projectiles before they impact is almost impossible, and if close enough a ship could blanket an area large enough to ensure it will hit its target regardless of evasive actions. AS someone who enjoys writing space battles I'm having a hard time coming up with passive and active countermeasures. There does not appear to be many options. I was curious if there is any thing more other then, being far away enough to dogde, using a dust shield or having dense meter thick armour?

    1. Individual macron particles would be nearly impossible to detect, but a cloud of them would be visible of radar, similar to how an individual rain droplet is invisible to most sensors but a storm cloud appears on weather radar.

      To defeat macrons, you need to put up a regenerative barrier. For example, a screen of iron particles held by magnetic fields or a bag filled with gas. Each macron penetrates a short distance before its own kinetic energy vaporizes it. The hole it punches needs to be quickly filled, so the next macron cannot dig deeper.

      It is not a very practical defense. It can be eroded over time by a macron gun shooting continuously, or it can easily be swatted away by a beam weapon. But this is where a more detailed analysis is needed: the time that the gas bags or magnetic screens earn you should be used to counter-attack and halt the stream of macrons.

    2. Firstly, the need for protection against such projectiles is "greatly exaggerated", the described idea is fundamentally unworkable.

  24. This article is very informative, thanks for sharing

    1. But unfortunately, it's wrong


  25. Would a foam-like substance (metallic foam, for example) benefit from the increased surface area effect?

  26. Would it be possible to "squeeze" extra negative charge into a particle by charging the walls of the barrel as well?
    if i understand the physics correctly, that would basically lower the effective surface charge (as a function of an angle- straight ahead the reduction would be 0), while keeping the C/m ratio- and if used in a blowgun-style accelerator, would also focus the electron beam some

    1. I can't imagine how that works but this is a generally unexplored space. Lots of potential improvements and very little actual testing. We'll see how this plays in in future decades!