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Antimatter Propulsion

Propulsion using antimatter rockets

Robert Frisbee antimatter ship
Image from Steve Bowers
The Robert Frisbee, an antimatter ship of the middle Federation age. The drive system is located as far as possible from the crew section, because of the dangerous gamma rays emitted by the antimatter reaction; shielding at this time was relatively inefficient.

Antimatter is the most energy-dense fuel available to modosophonts, and antimatter fuelled drives include the most powerful kinds of reaction engine that can be designed and built without transapient assistance.

Antimatter production is a highly energy intensive process, and whilst planetary magnetospheres can be harvested for naturally occurring antiprotons (generated by cosmic ray interactions) the amounts available even in a large planetary system rarely exceed a few milligrams a year. Mid to high-tech societies can use these small quantities of antimatter to trigger fission and fusion reactions, but the actual annihilation of matter in those cases is negligible compared to the energy released by the triggered nuclear reactions. Such drive types will not be considered here, but are described in more detail in the Nuclear Pulse Propulsion article.

dragonfly
Image from Steve Bowers
The Dragonfly Ship, with its large wing-like radiators and rows of massive Penning Traps, demonstrates the difficulties of using Antimatter as a power source.

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When an antinucleon (most commonly an antiproton) collides with a nucleon (which could be a proton or neutron), one of the antinucleon's valence antiquarks may annihilate a valence quark in the nucleon. The remaining quarks and antiquarks form a number of short lived particles.

The principle reaction products of interest are pions. Neutral pions decay almost immediately (within tens of attoseconds) into a pair of high energy gamma rays. Charged pions are slightly longer lived (half lives of tens of nanoseconds) and can be used to heat a working fluid such as hydrogen, or deflected with a magnetic field to generate thrust. The charged pions decay into charged muons and neutrinos. The charged muons may also potentially be used to heat a working fluid, or deflected by a magnetic field to create thrust. Finally, the muons themselves decay (with a half-life of microseconds) into neutrinos and either electrons or positrons.

If an antinucleon hits a heavy nucleus with many nucleons, the energy released may cause that nucleus to undergo fission. This is the mechanism harness by antimatter-initiated nuclear reactions.

Development

The potential of antimatter for rockets was realised even before manned spaceflight became a reality. Difficulties in obtaining and storing useful quantities of antimatter meant that the first spacecraft making use of antimatter did not fly until over a century later. Even then, it took a further two centuries for antimatter synthesis to mature to the point where "true" antimatter drives which derive all of their thrust power from annihilation became practical.

There are five principle types of AM rocket. The first three to be invented, the solid, gas and plasma core closely resemble fission thermal rockets, albeit with somewhat simplified operation and less hazardous resources and waste. Beam core rockets, so called as they were originally thought to be pure photon rockets, annihilate 100% of their fuel and use the resultant subatomic particles as "reaction mass". The final type, photon rockets, emit most of their energy in the form of gamma rays.

Solid Core

Solid core rockets consist of a metal honeycomb reaction chamber, usually fabricated from tungsten or other highly refractory material. Small amounts of antimatter are injected into the chamber, and most of the reaction products (with the exception of neutrinos) simply heat up the reaction chamber. Reaction mass, usually hydrogen, is pumped into the reaction chamber and expelled out of a conventional rocket nozzle.

Reaction mass temperature is strongly limited by the melting point of the reaction chamber, which in the case of tungsten is 3695K, though temperatures are usually kept to 3000K or below to reduce rates of sublimation. This in turn limits exhaust velocity and hence specific impulse will not exceed about 1000 seconds. This is comparable to a solid core nuclear rocket, however there is a much reduced level of radioactivity in the exhaust. An engine powerful enough to lift a fifty tonne ship from an Earth-like gravitational field will use approximately 2 milligrams of antimatter to reach orbit, and can potentially do so with a single stage.

Upwards of 80% of the energy released by annihilation may be used to provide thrust, making this one of the most efficient kinds of antimatter rocket. The very small amount of antimatter required for orbit-surface return trips make potential accidents much less serious than for more energetic designs. The combination of high thrust, simple operation and modest antimatter requirements make this a popular shuttlecraft and taxi engine, even for higher toposophics with limited access to reactionless drives.

Gas Core

This design removes the heating element altogether, and simply injects antimatter directly into the reaction mass stream, and using magnetic fields to confine the charged reaction products and heat the reaction mass. This allows for substantially higher temperatures and hence exhaust velocities, increasing sepcific impulse to 2000-7000 seconds, with a theoretical limit of 10000 seconds. The antimatter use is greater than in a solid core engine, and the process is less efficient with no more than 60% of the annihilation energy contributing thrust. The rocket also emits dangerous levels of gamma rays in use, making it potentially hazardous to use in atmospheres or in proxity to other spacecraft or habitats.

Whilst this design makes for an excellent heavy planetary lift system, the amount of antimatter required to operate it means that it is usually simpler to use fusion powered beam driven thermal rockets instead. Military vessels operating with large budgets and more relaxed safety restrictions may use such engines for short-range interceptors or torpedo terminal manoevering drives. Civilian gas core AM craft are rare, and are generally a sign of a mid to high-tech polity that has been gifted efficient antimatter synthesis mechanisms by a transapient.

Plasma Core

An extension of the gas core system, but with enough antimatter injected to convert the reaction mass to plasma. Conventional combustion chambers and rocket nozzles can no longer be used, and instead magnetic confinement is required. Energy efficiency tends to be much lower, with sometimes as low as 10% of the annihilation energy contributing thrust. Fuel efficiency is much higher though, with specific impulses as high as one million seconds using hydrogen reaction mass, suitable for use in deep space craft or even slow starships.

Unfortunately, the complex reaction chamber design requires much more cooling than an nuclear pulse drive of comparable power, and the antimatter fuel requirement is very much higher than that of AM-initiated fusion. Low thrust, high-Isp plasma core probes are used by some societies with access to large amounts of antimatter but without the sophistication to develop beam core rockets. Everyone else uses fusion.

Beam Core

Also known as a Pion Rocket, beam core drives aim to use equal amounts of matter and antimatter fuel, resulting in complete annihilation of all nucleons and antinucleons in the fuel, generating a pure pion exhaust. Exhaust velocity is extremely high, resulting in specific impulses that can exceed 10 million seconds; a tenfold improvement over fusion. The engineering challenges presented by beam core design are formidable, and it took transapient insight to fully realise the potential of the system. These rockets are highly fuel efficient, with as much as 60% of the annihilation energy contributing useful thrust (with the remainder of the energy largely being lost as gamma rays and neutrinos, and some being lost in the form of the mass energy of deflected pions, depending on drive design). On the down side, the gamma ray flux of such an engine is very high, presenting far greater shielding and cooling challenges than fusion drives.

Most modosophont-originated designs have relatively low thrust, rarely exceeding that of fusion drives: generally centigee or tength-gravity sustained thrust, with the finest examples generating 1g. Transapient beam core rockets can reach vast power levels, often many petawatts and can develop thrusts of 10 gravities or more. Some ultratech societies are capable of copying such designs without assistance.

Beam core interstellar ramjets dispense with the need to carry normal matter reaction mass, thus effectively doubling their delta-V compared to normal beam-core rockets. They are rare at modosophont and low transapient technology levels compared to conversion ramject starships which have no fuel or reaction limitations at all. However, at the Third Singularity and above antimatter-fueled beam core ramjets, which carry their own means of easily creating antimatter using Q-mirrors, become possible. Such systems match the capabilities of the best magmatter based conversion systems with a much lower mass penalty and are generally considered the best reaction drives available for all but the largest vessels.

Antimatter-fueled beam core rockets have become increasingly rare below the S3 level since the development of conversion technology in 1520AT. Given the ease of conversion monopole breeding and the straightforward nature of AM-beam core to conversion-beam core upgrading, and the massive gains in safety associated with not needing to carry or synthesize vast quantities of antimatter, only the most dedicated isolationists, luddites and unfortunately damaged industrial bases need use such drives now.

Photon Rocket

The ne plus ultra of reaction engines, photon rockets use a beam of gamma rays to develop thrust. The exhaust velocity is then the highest possible, giving them a maximum reaction engine specific impulse of ~33 million seconds, impossible to surpass without external propulsion systems (such as beamriders) or reactionless engines.

The design issues of a beam core rocket are increased yet further with a photon rocket. The simplest designs require magmatter gamma ray reflectors which are the product of a third toposophic industrial base, but once obtained may be used even by modosophonts. Such a rocket would be pointless however, as with mastery of magmatter comes the ability to build conversion drives which are far more convenient to fuel and operate than antimatter drives. A few such ships are known to have existed though, with the critical components scavenged from the wreckage of severely damaged conversion drive vessels. All known examples of such ships have since been upgraded to conversion drives, or lost entirely.

A more complex design that can be built using normal matter has been realised by second toposophic minds, however. A carefully controlled magnetic implosion of a hydrogen-antihydrogen ambiplasma can be used to generate a GeV-range gamma ray laser beam, with non-gamma ray annihilation modes largely suppressed. The recoil effect of each emitted gamma ray photon imparts momentum to the rocket. Such a device is vastly more complex to construct and control than a conversion drive, and so are correspondingly extremely rare. Some spacecraft bearing such drives still exist even today and are somewhat faster than conversion ramjets under normal usage, and substantially faster when operated in areas where the interstellar medium is too tenuous to support ramjet operation, such as within the "local bubble" which makes up a substantial volume of the Inner Sphere.

No known examples of a high sustained thrust (>0.01g) conversion-based pure photon rocket have been seen, suggesting that such things are impractical to build. If this is so, then it can be conjectured that a photon rocket bearing its own means of antimatter synthesis and a ramscoop would outperform any existing conversion drive. Presumably there are second and third toposophic minds who do operate such vessels, and whilst deep space observation has tracked what appear to be photonic rockets with implausible delta-Vs for conventionally fuelled vessels no credible reports exist of such ships being seen close up. In this regard, they might be considered the forerunner of the Black Angels used by higher toposophic minds.

 
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Appears in Topics
Interplanetary TransportInterstellar TransportPropulsion Technology
 
Development Notes
Text by M. Alan Kazlev, Mark Mcamuk, Chris Shaeffer, updated by Ithuriel
some comments by Steve Bowers
Initially published on 08 December 2008.

 
 
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