National Aeronautics and Space Administration

Glenn Research Center

George Schmidt

The academites were lucky enough to hear a lecture from Dr. George Schmidt on nuclear systems for space propulsion and power. Dr. Schmidt is the Deputy Director of the Research and Technology Directorate at NASA’s Glenn Research Center.

Schmidt began by describing the basics of a nuclear system, the difference between various nuclear systems, and what makes it ideal for power. Radioisotope decay, the least energetic of nuclear reactions, involved the decay of Plutonium 238 to Uranium 234 and an α-particle and does so reliably. This is considered the ideal fuel for space propulsion and is the commonly researched method today, but the small amount of usable Plutonium 238 makes it difficult to broadly utilize this technology. Nuclear fission creates thirty times the energy of radioisotope decay, by splitting a heavy nucleus with a neutron. This causes the heavy nucleus to split into two highly energetic daughter products and neutrons, initiating a chain reaction. Nuclear fusion produces over 150 times the energy of radioisotope decay. Here, two reactants are brought together to produce Helium and a small particle. Deuterium/Helium-3 interactions are the most lucrative, producing no dangerous neutrons, but Helium-3 does not exist on earth and requires collection from the moon and other celestial bodies. Antimatter annihilation is the most energetic reaction, producing about 40,000 times the energy of radioisotope decay by combining protons with antiprotons and electrons with positrons. Unfortunately, antimatter is extremely costly to produce and utilizing this technology for space propulsion or power would require a substantial amount.

A nuclear reaction is one million times more energetic than a chemical reaction and, therefore, more useful for long duration flights or travel though shadowed surface regions when you can’t utilize solar power. Nuclear propulsion is ideal due to the reduction of propellant mass and the high specific impulse.

Dr. Schmidt then continued to discuss the current use of nuclear technology in power and propulsion. Radioisotope power systems have been seen on over 28 successful spacecraft. The heat produced by the plutonium decay was partially converted into electricity. Newer research has been examining ways to make this conversion more efficient, utilizing as much energy produced as possible. Fission based systems are also under development, but no long-term missions have been launched. Systems are being designed for prolonged surface-based power or prolonged space flight. Nuclear thermal propulsion units are also being tested, where a chemical propellant is heated by a nuclear reactor and thermally expanded through a nozzle. This greatly increases the specific impulse of chemical engines and allows much lighter fuels to be utilized (such as O2/H2 exhaust streams).

Dr. Schmidt concluded by examining various advanced power systems for long-duration expeditions. These expeditions require a much higher specific power. Systems under development are attempting to accomplish this through a variety of methods, including fusion generators, vapor core reactors, gas core nuclear thermal rockets, and fission fragment rockets. Nuclear pulse propulsion is also under development, where small nuclear bombs are released by the spacecraft, pushing the rocket along. Antimatter propulsion research lies mostly in trying to reduce the cost of production to make utilization of this technology more practical.