Beamed core antimatter propulsion - more efficient, but don't hold your breath!
Antimatter propulsion is the Holy Grail of spaceflight. When matter and antimatter react, the energy produced is several billion times larger than the thermomechanical energy resulting from burning a kilogram of a hydrocarbon fuel. Now a high school student has developed a new magnetic exhaust nozzle that would double the velocity of an antimatter-powered rocket.
Despite the extraordinary difficulty of generating and storing significant amounts of antimatter, the potential it offers as a power source for interplanetary and interstellar voyages is nearly irresistible to spaceflight visionaries. Hearing this call to adventure, Roman Keane, a high school senior at Western Reserve Academy in Hudson, Ohio, and his mentor, Senior Research Fellow Wei-Ming Zhang of Kent State University, decided to optimize an existing model for an antimatter-powered rocket engine.
Matter reacting with antimatter is the ultimate source of energy which might be available to power space travel beyond the Solar System. The energy released by the reaction of a gram each of antimatter and matter is about equal to that of a forty kiloton atomic bomb. As a result, numerous conceptual studies have examined antimatter as a fuel for extrasolar spacecraft. A full design effort is somewhat pointless at this stage, as at present we don't know how to manufacture, store, or manipulate large quantities of antimatter - the current cost of that gram of antimatter is roughly estimated at about a trillion US dollars.
One element of the system that can be studied with existing technology is the design and operation of a magnetic nozzle for a beamed core antimatter rocket engine. By causing moving charged particles to be directed into a beam, the magnetic nozzle generates propulsive thrust from the annihilating antimatter.
The beamed core antimatter rocket depends on a little-known fact about antimatter - the only particle-antiparticle annihilation that immediately converts 100 percent of the particle mass into energy is the reaction of an electron with an positron. In contrast, when protons and antiprotons react, they produce a variety of charged and uncharged pions, which are elementary particles. More interesting phenomena occur when antiprotons annihilate against a compound nucleus, such as copper or lead. Many of the electrically charged reaction products retain their identity as charged particles long enough that they can be focused into a unidirectional beam by a magnetic nozzle.
Past studies of such magnetic nozzles determined that magnet coils providing a magnetic field well in excess of 100 Tesla in strength were required. Such strong magnetic fields can only be produced in extremely short pulses using today's technology, so this early solution was not practical. These studies also suggested that the exhaust velocity of a beamed core antimatter engine would top out at about a third of the speed of light, which is rather marginal for interstellar missions.
To accelerate to a speed equal to that of the engine's exhaust velocity (0.33 c) and then decelerate to a stop at your destination, 86 percent of the initial mass of the spacecraft would have to be fuel - half of that antimatter. One could in principle go faster, but reaching a speed of double the exhaust velocity (0.66 c) and then stopping at your destination would require that 98 percent of the spacecraft initial mass is fuel - a rather difficult build job, although use of multi-stage vehicles could improve the situation in the same way as is seen in chemical rockets.
Keane and Zhang decided to take another go at designing and optimizing a magnetic nozzle for a beamed core antimatter engine. The simulation of antimatter reactions and of the dynamics of charged particles moving in magnetic fields has advanced considerably since the time of the previous studies. This has been the result of three decades of increasingly more subtle simulations to analyze the data from high-energy particle collisions. Keane and Zhang used analysis software from CERN for their study.
Depiction of the simulated trajectories of the products of a proton-antiproton collision. The blue lines represent positive pions, the green lines negative pions, and the brown straight lines are gamma rays mostly generated by the decay of neutral pions. Note that the both positive and negative pions are eventually directed out the nozzle (Image: Keane and Zhang)
They found that a magnetic nozzle about four meters long and 1.5 meters in diameter having a maximum magnetic field of 12 Tesla would represent an optimal configuration for the general design assumed in the study. Such a magnetic nozzle could be made using today's technology, although some rather special engineering would be needed to attain large values of thrust.
Most encouraging is that the effective exhaust velocity of the new nozzle design is about 69 percent of the speed of light. This means that a beamed core antimatter spacecraft with the new nozzle could make a one-way trip at a speed of about two-thirds of the speed of light carrying seven times the payload that could be hauled using the old nozzle.
Clearly, developing a spacecraft to ply the vast expanses between stars is a terribly difficult enterprise, and the methods to be used when our technology has advanced enough will probably not be like anything we can currently conceive. Despite that, it is often inspiring to take a small bite of a nearly impossible project, and discover that we indeed can solve a bit of it in the here and now. That this has been accomplished by a high school student increases one's faith in the future of technology.