Antimatter: Beam Drive
Beamed Core Antimatter Drive
The hybrid designs (ACMF and AIM) discussed in previous sections seem quite complicated. They require the juxtaposition of the three highest energy yielding technologies known to humankind just to create a spacecraft that is barely capable of leaving the solar system. What if we just used the highest energy yield technology and forgot about the other two?
Indeed this is possible, but it requires a lot more antimatter than we are currently capable of producing. However, someday we will have actual antimatter factories, rather than relying on retrofitted particle accelerators in scientific laboratories. Dr. Robert Forward has shown, based on his findings in a study of antimatter production, that if a dedicated antimatter factory were built now, it could be approximately 6000 times more efficient than Fermilab's and CERN's antimatter production facilities (bringing it up to a grand 0.01% efficiency).
The theoretical maximum efficiency for antimatter production is 50% (it is not 100% because every production of an antiparticle is accompanied by the production of its normal-matter twin), so there is even more room for improvement.
Once we have ways of producing large amounts of antimatter, we will be able to make pure antimatter/matter reactor drives. What follows is a discussion of the "ultimate" antimatter drive... called the Beamed Core drive.
As discussed in the section on the Plasma Core drive, one way to create an antimatter drive is to use the energy from the annihilation reaction to heat some substance. The substance, in turn, heats the propellant, which is expelled from the drive at high velocity. The reason for doing this is that antimatter is hard to come by, so you can use less if you have a secondary propellant that provides the mass to "throw out" the back of the engine. Of course, some energy is always lost during the transfer, so a drive like the Plasma Core is not getting all of the energy that it could from the antimatter.
A simple way around this problem is to eliminate the secondary propellant all together. Why not simply eject the results from the annihilation out the back of the drive? Indeed, this is the idea behind the Beamed Core drive (called such because its core is almost, but not quite, a beam of light). Figure 10 shows the basic concept of a Beamed Core drive:
Protons and Antiprotons are injected into the magnetic nozzle, where they collide and annihilate producing a collection of pions. The uncharged pions are unaffected by the magnetic field and fly off, almost immediately decaying into gamma rays. These gamma rays would have to be stopped by some sort of high-density shielding. The charged pions shoot down the magnetic nozzle at what is essentially light speed, after traveling a distance of 21 meters in 70 nanoseconds, they decay into muons and neutrinos. The muons are longer lived, they travel 1.85 kilometers in 6.2 microseconds (99.5% light speed) before decaying into electrons and positrons (and more neutrinos). As you may be aware, the performance of a rocket is mainly limited by its exhaust velocity. The Beamed core drive would have an exhaust velocity of near light-speed. Allowing (with sufficient amounts of antimatter) acceleration up to almost any arbitrary percentage of the speed of light.
Two major problems face the Beamed Core drive concept, though. The first is that it requires enormous amounts of antimatter (far more than we could hope to produce currently). The second is the creation of the magnetic nozzle. The point of the Beamed Core is that it can operate at extremely high temperatures (even higher than the Plasma Core), which brings up the question of how to generate the magnetic nozzle. Normally, one would consider using superconducting coils to create the strong magnetic field. However, most superconducting materials do so at low temperatures. Obviously, we need to find a high-temperature superconductor, as well as develop a very extensive cooling system before a Beamed Core drive could be constructed.
Despite its problems, the Beamed Core drive seems a very promising way to get to other star systems, since it would allow accelerations of moderate payloads up to 40% light speed. This is sufficient to make the 40 ly journey to Alpha Centauri in 10 years. Obviously, though, it is a technology still many years in the future.