In the heydays of the early space race, both the Americans and Russians toyed with various concepts involving nuclear propulsion for human space exploration within the solar system.
In the United States, this work focused on three major areas of study: nuclear pulse propulsion (NPP) – literally exploding atomic bombs behind a space vehicle, propelling it forward, as exemplified by Project Orion; Nuclear-Thermal Propulsion (NTP) – the use nuclear motors in place of chemical rockets either from launch or once in orbit as seen with Project NERVA; and Nuclear-Electric Propulsion (NEP) – the use of nuclear energy to power low-thrust ion propulsion motors.
NPP was effectively (and perhaps fortunately) abandoned over both the fear of fallout from the vehicle’s atomic explosions during its ascent through the atmosphere and the signing of the Partial Test Ban Treaty in 1963. NTP, using a nuclear reactor to heat liquid hydrogen (LH2) propellant to create ionized hydrogen gas (plasma) which can be expelled via engine bells, has continued to be researched, although its use from launch was overruled dues to the radioactive exhaust plume (thus requiring liquid-fuelled rocket to lift the propulsion units to orbit where they might be used), and remains a solid concept for propulsion that could help reduce the journey time to Mars by weeks.
Nuclear-Electric Propulsion (NEP), relies on a nuclear reactor to provide electricity to an ion engine using an inert gas (like xenon) to create thrust (rather than spewing a radiative exhaust). The resultant thrust is less than that of either NTP or chemical propulsion, but it has the advantage of being able to be maintained for far longer periods, potentially allowing a crewed vehicle to gently accelerate to the half-way point to Mars before trying around and using that same thrust to decelerate gently and achieve orbit around Mars. This could cut a 6-month journey to Mars in half.
Experiments in NEP have continued through until recent times, including space-based test; NTP, however, only reached the stage of ground-based testing before being curtailed. However, it has remained the preferred approach to crewed deep-space missions, should nuclear propulsion on crewed vehicles again be seriously considered. The interest is now re-awakening in light of Project Artemis and America’s stated desire to both return to the Moon and reach beyond it to Mars, with a focus on new approaches to methods of propulsion.
One of these new approaches is the rather tongue-twisty Bimodal NTP/NEP With A Wave Rotor Topping Cycle. The “bimodal” references combining NTP fission to generate the electricity required to power a NEP ion engine, while the “wave rotor” effectively meaning a “supercharger” which further compresses the reaction mass to deliver greater power to the NEP. Research into the approach suggests a transit time to Mars could be reduced to just 45 days.
Based on conventional propulsion technology, the most fuel-efficient Mars crewed mission profile offering the longest period for surface exploration is the Opposition Mission. This requires crews to spend between 6 an 9 months each way in transit between the two planets, with a surface stay of up to 23 months. However, a bimodal nuclear propulsion system could both reduce the transit time each way to 45-60 days, allowing crews to spend more time on Mars, whilst also potentially releasing a mission for the 26-month launch windows, enabling a crew to make an emergency return to Earth if required.
As well as propulsion, NASA is looking at ideas using nuclear power systems for long-duration surface missions when solar and wind power cannot be used / relied upon, These include KRUSTY, the Kilopower Reactor Using Sterling Technology, a joint venture between the space agency and the US Department of Energy’s National Nuclear Security Administration (NNSA) successfully demonstrated in 2018. Then there is a new take on the hybrid fusion / fission reactor, first selected by NASA for development in 2013 and which has recently seen renewed investigation, and which is now showing promising signs for future use.
Conventional fusion methods generally comprise either inertial or magnetic confinement, using extreme pressure or a powerful magnetic field to compress a fuel such a deuterium (hydrogen-2), forcing fusion to occur. Both require significant energy input and the generation of significant amounts of heat – around 15 million degrees centigrade. As such, both require large, heavy systems and associated cooling – although this hasn’t stopped the likes of Boeing developing concepts for hybrid systems to propel crew-carrying interplanetary spacecraft to rival biomodal NTP / NEP powered craft.
Hybrid fusion / fission utilises high-energy fast neutrons from a fusion reactor to trigger fission in non-fissile fuels. It is still a complex method, but it has the advantage of being capable of of generating multiple fission events from a single neutron, rather than a single reaction per neutron, requiring less fuel feedstock, and as the fuel is non-fissile, output from the reaction is not radioactive. In fact, such a reactor could even use waste from other fission reactions, disposing of it. Even so, the systems required for hybrid fusion / fission reactors have tended to be extensive and mass-heavy, competing directly with bimodal NTP / NEP systems in size, complexity and mass.
Selected for Phase I development by the NASA Innovative Advanced Concepts (NIAC) programme, the team has focused on the development of a special lattice into which deuterium can be packed in densities around a billion times greater than a within the core of a conventional hybrid reactor. This, combined with the ability of the fusion process to generate multiple fission reactions, means that overall, less deuterium fuel needs to be carried for feeding into the reactor, thus also reducing the mass of all the associated tanks, piping, etc., required to handle it. Further, the nature of system means that reactions can occur at far lower temperatures than a standard bimodal system, further reducing mass and complexity by eliminating much of the thermal control mechanisms and radiator surfaces required to remove the heat needed to generate the fusion reaction, and the heat it also generates.
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