Nuclear thermal propulsion (NTP) systems are actually a very old technology. Development of these systems began back in the 60’s, during the space race, but funding to nuclear programs was cut in the late 70’s as the Vietnam War took precedence over space technology. More recently, however, NASA has once again turned to NTP as an efficient new method of rocket propulsion.
There are many advantages to NTP. In some ways, such an engine would work similarly to conventional rocket engines, heating fuel (probably liquid hydrogen) and expelling it out of a nozzle at high velocities. Rather than chemically reacting two propellant components, however, a nuclear reactor heats a single fuel, which is then pushed through a nozzle to accelerate it to high speeds. The greatest draw of this system is that the mass of nuclear propellant needed to generate a certain amount of thrust is, compared to conventional methods, quite small. A rocket with a nuclear propulsion engine will have a launchpad weight half that of the same rocket using chemical fuel. There has also been recent discussion of using low enriched (i.e. less radioactive) uranium in NTP systems, which would make NTP safer for long missions as it is less reactive than its more common counterpart (high enriched uranium). This technology is still in development, as are several technologies that would be needed to keep astronauts safe from radiation generated by the core.
NTP systems could drastically alter the way we conduct space missions. These engines would be able to reduce mission times significantly– one proposed design from the 50’s, Orion, could take us to Mars in just 3 months, rather than the 18 required by conventional methods. Not only would mission times be reduced, NTP offers the option for mission abort far longer into a mission than chemical fuel. Nuclear engines could also serve a dual purpose, powering electrical systems on board the spacecraft and even being converted to stationary reactors once the craft landed (say, on Mars). It should be noted, however, that NTP cannot provide the initial thrust necessary to escape Earth’s gravity. For this reason, a chemical rocket would be used to boost the spaceship into orbit, after which this first stage would be dropped and the nuclear thrusters would take over for the rest of the mission. While details are still in the works, NTP is very likely to be the method by which humans get to Mars and beyond.
On the Horizon 