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98 PS | Nuclear-powered spacecraft A fatal explosion involving what the Russian state nuclear corporation Rosatom calls an “isotope power source” but which western analysts believe is associated with a nuclear-propelled cruise missile is an indication that nuclear propulsion for space vehicles is back on the agenda, after decades in the wilderness (writes Peter Donaldson). Another, less immediately threatening one is the US government’s NASA-led revival of nuclear thermal propulsion (NTP) for spacecraft. NTP holds huge promise for exploring the Solar System and beyond in manned as well as unmanned applications. It will enable journeys to Mars in four months instead of six, for example, while reducing the amount of propellant required. NTP’s potential stems from its combination of high thrust capability with around twice the efficiency of the best chemical rockets. The thrust a rocket generates depends on the propellant’s mass flow rate through the engine, the exit velocity of the exhaust and the pressure at the nozzle exit. Rocket engine efficiency is measured in specific impulse (Isp), which is calculated by dividing the engine’s thrust by the propellant mass flow rate. The most common unit used to express Isp is the second, as it can be defined as the number of seconds for which a propellant can accelerate its own initial mass with a force of 1 g . The higher the Isp, the less propellant is needed for a given change in velocity. A high exhaust velocity is crucial to a high Isp, and the lighter the propellant molecules, the higher the exhaust velocity. The Aerojet Rocketdyne RS25 Space Shuttle Main Engine for example achieves an Isp of about 450 s by burning liquid hydrogen in liquid oxygen. A nuclear thermal rocket, which simply heats pure hydrogen to temperatures of 3000 Kelvin or more by passing it through the core of a nuclear reactor, can achieve an Isp of about 900 s. Hydrogen is the lightest element, so it makes an efficient propellant by itself. Electric thrusters, which accelerate charged propellant particles through electric or magnetic fields, can achieve Isp figures of about 1500-20,000 s, thanks to their extremely high exhaust velocities, but they tend to have very small mass flows and therefore low thrust. They can achieve large velocity changes in a very fuel-efficient manner, but it takes them a long time. They can of course use nuclear reactors as sources of electric power, a concept dubbed nuclear electric propulsion. It is NTP though that offers the combination of speed and efficiency that seems more attractive to governments and industry at the moment, particularly if it can be realised with a safer fuel such as low-enriched uranium. This fuel has a concentration of less than 20% U235, which is much less radioactive than its highly enriched counterpart, and unsuitable for bombs. President Trump’s August signing of a presidential memorandum outlining new procedures to launch nuclear power systems into space directs the US Department of Transportation to publish guidelines within a year for companies who want a licence to launch spacecraft with nuclear systems aboard. It has given new impetus to technologies that have been almost dormant since the 1970s, and could help revitalise exploration of the Solar System. However, the accident involving the Russian nuclear-powered cruise missile shows that the technology’s dark side is still alive and kicking. Now, here’s a thing “ ” The President’s memorandum has given new impetus to technologies that have been almost dormant since the 1970s October/November 2019 | Unmanned Systems Technology