To achieve any measure of space travel there is one tool that has always been indispensable, rockets. Rockets have been the primary tool for sending spacecraft into orbit and accelerating them beyond Earth orbit to other planets in the solar system, and for a few craft, on their way out to the rest of the galaxy.
Despite the amazing advances in rocket technology since the days of Apollo, NASA still is forced to rely on chemical combustion to propel vehicles off Earth and to space destinations in a relatively short time period. Chemical rocket engines, while producing a lot of thrust, are highly inefficient and very dangerous as several rocket accidents in the past have proven. Electrical propulsion is a useful alternative for long-term small probes due to its high efficiency, but it produces very low thrust and is not useful for shorter-term manned missions.
Many theoretical concepts for high efficiency and high thrust propulsion offer a tantalizing view for how space travel in the future might work, but for now such concepts are technically unfeasible. There is, however, an option in between the future and now that uses existing technology. The best part is, it is an old idea.
Nuclear thermal rockets, or NTRs for short, are rocket engines that utilize a nuclear fission reactor to heat propellant instead of igniting combustible propellants. The advantages include much higher specific impulses due to a higher range of exhaust velocities that chemical rockets can’t achieve due to limits of the combustible fuels. The idea is surprisingly simple; take a nuclear reactor like the ones used for power generation today, but instead of using it to heat water into steam for power turbines, heat propellant instead and run it out of a rocket nozzle for thrust. This is the simplest form of NTR, which is called a solid core NTR. In fact, it is so simple it has already been done, just not in space.
In 1955, the Atomic Energy Commission started Project Rover, aimed at the development of engines utilizing nuclear technologies, which were in their prime in the 1950s in America. Four basic designs came from this and 20 rockets were tested, but the AEC work was intended to study the reactor design itself for rocket use, rather than actually build a rocket. In 1961, NASA began the Nuclear Engine for Rocket Vehicle Applications program, or NERVA for short, to formalize the entry of nuclear thermal engines into space exploration. In fact, it was President Kennedy’s hope that Project Rover and the NERVA program would be the next step after Apollo, stating such in his famous speech to a joint session of Congress establishing the goal of landing a man on the moon.
Directly comparing the performance of two different rocket systems is not simple however. There are ways in which chemical propulsion is better than nuclear and vice versa. The most basic form of solid core NTR provides much better specific impulse, a measure of how efficient a rocket is (think gas mileage), but doesn’t have comparable thrust. It also takes a lot of time to warm up a nuclear rocket and cool it down between firings, putting stress on the system. The best way around this is what is called a bimodal NTR, which uses the reactor to both provide rocket thrust and supply power to the spacecraft at the same time. The reactor is started up once and when rocket firings are done it is cooled down to regular operating levels and a Brayton power conversion system is used to supply the spacecraft with power. This employs a different working fluid through a turbine and a radiator to cool it. Thus the reactor only needs to be started up and shut off once per mission.
An even better option is the trimodal NTR conceptualized by Pratt & Whitney. This takes the bimodal concept and adds another NTR concept referred to as LANTR, or LOX-augmented NTR, to make the Triton engine. The LANTR mode allows for more thrust by injecting liquid oxygen into the nozzle to act as an afterburner. This design then allows for a ship to have high thrust, high specific impulse, or power generation from one engine depending on the setting.
There are even more ambitious ideas for NTRs including liquid core and gas core engines, but they have never been built beyond the conceptual stage and present several new challenges among which is a high tendency of releasing radioactive elements into the exhaust. Solid core NTRs keep the radioactive elements away from the propellant, thus making them safer. However, all solid core tests such as NERVA resulted in engines with a thrust to weight ratio lower than one, meaning it could not lift a rocket off Earth.
This leads to the obvious fact that despite Kennedy’s high hopes and NASA’s research, nuclear engines never did get used for actual spacecraft. There is a complicated set of reasons for this including cost factor, various issues and most importantly public opinion. The growing public dissatisfaction with nuclear weapons and nuclear power by proxy as a result of the Cold War arms race and later accidents like Chernobyl made it a lot less likely that people would like the idea of a nuclear powered rocket flying, even if it could be safe. Today, nuclear weapon treaties forbid nuclear weapons in space, thus making ideas like Project Orion, which used full nuclear bombs for propulsion infeasible. Such treaties do not disallow nuclear reactors like what NERVA used however.
NASA has always wanted their vehicles to be safe and not cause harm to anyone. As such, the biggest issue with these engines is radiation. Fears of radioactive material dispersed into the atmosphere, or a nuclear explosion happening are common. However, despite the horrible accidents that have plagued nuclear reactors before, they are more safe than many realize and as stated above can be done so that no radioactive material leaves the nozzle. A nuclear explosion is highly unlikely since reactors are not designed to act like nuclear bombs and are more controlled. This aside though, the simplest option is to not use them in the atmosphere at all and make nuclear engines only for use in space, while using chemical engines to get to orbit. The only worry is a sub-orbital structural failure, but designs for the reactors are very robust, leaving it unlikely for radioactive material to be spread. As for fears of the reactor irradiating astronauts, there are ways of shielding them, but studies have shown that the shorter travel times NTRs allow result in less radiation exposure by passengers due to them spending less time in space exposed to cosmic radiation.
Continued research is still being done, in the 1970s a small nuclear engine was designed for possible use with the space shuttle in place of the Space Shuttle Main Engines. The design provided a theoretical specific impulse of 975 seconds, much greater than the 363 – 452 seconds of the SSME for only slightly less of the SSME mass fraction. It was clearly not chosen for the space shuttle however. Continued research under Project Timberwind as part of the Strategic Defense Initiative was done between 1987 and 1991, and in 2012 Icarus Interstellar and General Propulsion Sciences began a development project known as Project Bifrost to develop an NTR system for interplanetary missions.
While it hasn’t been used yet despite all the research behind it, nuclear propulsion represents the next inevitable phase of rocket technology for space exploration and it can help humanity to unlock the solar system. With more research and funding NASA can help to improve this technology and make it safer. If you think NASA should continue to develop new innovative propulsion technologies like this, let Congress know: http://www.penny4nasa.org/take-action/
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