Rockets lifting off from Earth will depend on chemical fuels for the foreseeable future. However, once in orbit, nuclear engines could take over and provide propulsion to accelerate spacecraft through space.
“Crewed interplanetary missions of the future will almost certainly require propulsion systems with performance levels greatly exceeding that of today’s best chemical engines,” said William Emrich, former Lead Project Engineer at NASA, adding that a solid candidate to be used for space travel is nuclear thermal propulsion (NTP).
In NTP, a nuclear fission reactor heats up a liquid propellant, like hydrogen. The heat converts the liquid into a gas, which expands through a nozzle to provide thrust and propel a spacecraft. The advantages of NTP are that space flights would need to lift less fuel into space, and NTP engines would reduce trip times – cutting travel time to Mars by up to 25 per cent compared traditional chemical rockets. Reduced time in space also reduces astronauts’ exposure to cosmic radiation.
Nuclear electric propulsion (NEP), on the other hand, is an option in which the thrust is provided by converting the thermal energy from a nuclear reactor into electrical energy, eliminating the associated NTP needs and limitations of storing propellants onboard. In NEP, the thrust is lower but continuous, and the fuel efficiency far greater, resulting in a higher speed and potentially over 60 per cent reduction in transit time to Mars compared to traditional chemical rockets.
“For space missions that need high electric power output, such as a human Mars mission or space ferries, a fission reactor-based power system can be a very competitive choice,” said Hui Du of the Beijing Institute of Spacecraft System Engineering, citing a China Academy of Space Technology study in 2015, which found that a human Mars mission would not be feasible without space nuclear reactors.
An NEP system being developed by Ad Astra Rocket Company, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), is a plasma rocket in which electric fields heat and accelerate a propellant, forming a plasma, and magnetic fields direct the plasma in the proper direction as it is ejected from the engine, creating thrust for the spacecraft. Unlike traditional NEP, the VASIMR design would enable the processing of large amounts of power while retaining the high fuel efficiency that characterizes electric rockets.
“In the near term, we envision the VASIMR engine supporting a wide array of high-power applications from solar electric in cislunar space, to nuclear-electric in interplanetary space,” said Franklin Chang Díaz, CEO of Ad Astra Rocket Company. “On a longer term, the VASIMR could be a precursor to future fusion rockets still in the conceptual stage,” he added.
Fusion rockets, like the Princeton Field Reversed Configuration reactor concept under development at the Princeton Plasma Physics Laboratory, would have the advantage of producing a direct fusion drive (DFD), directly converting the energy of the charged particles produced in the fusion reactions into propulsion for the spacecraft.
“A DFD can produce specific power several orders of magnitude higher than other systems, reducing trip times and increasing payloads, thus enabling us to reach deep space destinations much faster,” said Stephanie Thomas, Vice President of Princeton Satellite Systems, who discussed possible DFD-powered missions into near-interstellar space, human Mars missions and lunar base surface power. She also explained that a DFD could have the advantages of its small size and the need for very little fuel – a few kilograms could power a spacecraft for ten years.
