Back to the Moon with Nuclear Rockets

higher. Dr. Stanley Borowski from NASA's Lewis ResearchCenter (which was recently renamed for former Senator andastronaut John Glenn), points out that an equivalent amount ofenergy would be released from 13 tons of liquid hydrogen andoxygen, 2.0 grams of uranium, .5 gram of deuterium(as a fusion fuel), and .02 grams of equalparts of hydrogen and antihydrogen (See table, p.59, and Figure 1).In reality, the only advantage to using chemicalpropulsion systems today, is that we know how.With the goal of maximizing the parameters forpropulsion engine efficiency using nuclear energy,teams of scientists and engineers, both in theUnited States and in Russia, have been designingingenious new systems for manned and cargo vehicles.It would be inefficient (and backwards) toplan to move mankind into space using the chemicalpropulsion systems of the Apollo era. We willneed to take many more than three people pertrip, and many tons more of supplies than thosefor a few-day stay.Using nuclear energy technology in space isnot an untested concept. During the space nuclearprogram of the 1960s, NASA designed, built,and tested 20 rocket reactors. The Rover andNERVA (Nuclear Engine for Rocket Vehicle Application)programs demonstrated the feasibility ofspace nuclear power, testing a wide range of enginesizes, using liquid hydrogen as both thecoolant for the reactor, and the propellant, expelledto create thrust.A few years after the start of the nuclear rocketprogram in the United States, a similar effort wasinitiated in the former Soviet Union. Althoughthere were no integrated engine system tests conducted,nuclear and non-nuclear subsystems tests,including fuel element and reactor tests, wereconducted at the Semipalatinsk facility in Kazakhstan.High-temperature fuel elements made ofcarbide composites were developed, capable ofproducing hydrogen exhaust temperatures higherthan 3,000 K, or about 500 degrees more than thebest NERVA fuel elements.Ten years ago, during the celebration for the20th anniversary of the first lunar landing, PresidentGeorge Bush announced that NASA shouldbe looking toward a return to the Moon, and sendingmen on to Mars. NASA's Office of Explorationbegan investigating various technology optionsfor reaching these goals.A New Generation Nuclear Thermal RocketIn 1992, NASA's Nuclear Propulsion Office atthe Lewis Research Center in Ohio funded a jointprogram with the experts in the United States andthe former-Soviet nations to design a small, advancedNuclear Thermal Rocket (NTR) engine (Figure2). The team included experts from Aerojet, theU.S. nuclear supplier Babcock & Wilcox, and aCommonwealth of Independent States (CIS) con-sortium, Energopool. In September of that year, a U.S. team visitedtest facilities in Kazakhstan and met with their CIS counterpartsto conduct detailed studies of the CIS space nuclear reactorconcept.Figure 2SCHEMATIC OF SOLID-CORE NUCLEAR THERMAL ROCKETThe heart of the solid-core nuclear thermal rocket design is shownhere. Cryogenic liquid hydrogen is pumped from storage tanks (right)past the radiator to cool it. The pre-heated hydrogen is then fedthrough the nuclear fission reactor, heated to about 3,000 K, and expelledthrough the nozzle, creating the thrust for the engine.Figure 3MOVING TO A 'BIMODAL' NUCLEARTHERMAL REACTOR STAGEDr. Borowski and his colleagues have proposed that the solid-core reactorused for propulsion also supply the electricity needed on boardthe spacecraft for life support, in the case of manned vehicles, as wellas for powering the communications, electronics, and computer systems.It can also run the refrigeration system that maintains cryogenictemperatures to keep the hydrogen propellant in liquid form.Shown here is a design for a closed Brayton cycle energy conversionsystem that uses the high-temperature gas from the reactor toturn a turbine and produce electric power.58 Summer 1999 21 st CENTURY