|Fig. 1: Diagram of a nuclear thermal engine. (Source: Wikimedia Commons)|
In 1969, the world watched in awe as the Saturn V blasted off, carrying Apollo 11 and Neil Armstrong to the moon. The space race of the 60s fueled an era of competition, innovation and public excitement as the United States and the Soviet Union rushed to show their technological dominance. Recent technological developments, however, may be bringing us towards a new space race. On February 6, billionaire technologist Elon Musk and SpaceX made headlines with the successful launch of the Falcon Heavy that sent a Tesla Roadster hurtling towards Mars at a speed of around 3.5 kilometers per second.  The race to put humans on Mars has become the next major milestone in space exploration, but many engineering questions regarding how best to get there. While propulsion options range from the widely prevalent chemical engines to ion thrusters and even electromagnetic propulsion, one potentially affordable way to reduce travel time to Mars drastically is by using nuclear thermal propulsion. This article examines the prospective benefits and potential drawbacks of such a system.
The vast majority of rockets today, especially those for low-earth orbit purposes, are powered by the energy released by exothermic chemical reactions of the propellants. The chemical reactions release heat, causing rapid thermodynamic expansion of the propellant gases. The expulsion of these gases at extremely high velocities generates the momentum that propels a rocket forward. Chemical rocket engines have long been used for their relative design simplicity and reliability. For example, the Rocketdyne F-1, the engine that powered the Saturn V, generated energy from the oxidation of kerosene and liquid oxygen. For chemical rocket engines such as this one, the energy is stored in the propellants themselves. In contrast, the nuclear thermal engine generates heat from a nuclear fission reaction, commonly of U-235. This heat in turn triggers the superheating and thermodynamic expansion of a separate propellant such as liquid hydrogen.  Fig. 1 shows the schematic of a simple nuclear thermal engine.
|Fig. 2: Rendering of NASA's space launch system. (Source: Wikimedia Commons)|
The most significant advantage of nuclear thermal propulsion is that nuclear reactions generate considerably more energy than chemical reactions on a per molecule basis. This translates to a higher specific impulse (Isp), which is essentially a measure of the efficiency of rocket engine. The specific impulses for chemical propulsion commonly ranges from 400-500 seconds whereas the specific impulses for nuclear thermal engines range from 500-1000 seconds.  However, the maximum thrust that a nuclear rocket can generate for a given weight is lower than that of a chemical rocket. In fact, a nuclear engine cannot generate enough thrust to overcome the Earth's gravity at launch, at least given existing technology. Therefore, from a performance perspective, chemical engines are necessary for escaping the Earth's atmosphere, but nuclear engines are optimal once a space vehicle is in low earth orbit.
The ramifications of nuclear thermal engines are considerable. According to one estimate, nuclear propulsion has the potential to reduce the travel time to Mars by 20-25%. Additionally, the higher fuel efficiency by weight gives much more flexibility to any potential Mars missions, including the ability to launch during a broader ranch of times.  Currently, launch windows are currently highly limited by to the relative orbital positions of the Earth and Mars, but a nuclear thermal rocket could carry enough fuel for a longer journey. Finally, the efficiency gains also could reduce mission cost significantly. Any type of rocket, once reaching low earth orbit, will likely need to be refueled by an additional rocket before embarking on the journey to Mars. The cost of refueling would be lower for a nuclear thermal rocket since a much lower amount of fuel is needed.
The technology for nuclear thermal rockets is not as far off as many might imagine. NASA ran the Nuclear Engine for Rocket Vehicle Application (NERVA) project from the 50's to early 70's. It was eventually shut down due in part to a lack of practical applications as chemical rockets functioned effectively for most purposes of the time. However, it did demonstrate the feasibility of nuclear propulsion. Recently, NASA has shown renewed interest in the technology as part of its Space Launch Sysem (shown in Fig. 2), contracting BWXT Nuclear Energy Inc. to develop a nuclear thermal engine. 
While nuclear thermal rockets seem the most immediately feasible nuclear rocket technology currently, there are several other promising nuclear propulsion technologies as well such as pulse fission, fusion rockets and nuclear ramjets.  The technology for nuclear propulsion certainly has a long way to go and is very much unproven in comparison to chemical propulsion. Key challenges include cost of further development and testing as well ensuring a high enough level of reliability given the safety consequences of a nuclear rocket failing.  However, the potential benefits are easy to see. Chemical propulsion is approaching its limitations in traveling to Mars. For travel to Mars to ever become commonplace, short travel time, mission flexibility and cost are enormously important factors, all of which can in theory be improved dramatically by nuclear propulsion.
© Luke Asperger. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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