Nuclear Thermal Propulsion (NTP)
High specific impulse (Isp) and high thrust combine to make NTP the choice for cislunar transportation
Unlike conventional chemical propulsion systems, nuclear thermal rockets rely on fission to generate heat. This heat is transferred to a propellant – typically a low-molecular weight fluid such as diatomic hydrogen (H2) and exhausted through a nozzle at high velocities. Since the attainable exhaust velocity is inversely proportional to the propellant’s molecular weight, hydrogen becomes the gold standard – although other alternatives are available (e.g., water, ammonia, and methane).
Nuclear thermal rockets are characterized by very high power densities – in the range of megawatts per liter. As a result, nuclear thermal rockets can achieve thrust levels similar to that of chemical propulsion systems, and several orders of magnitude greater than electric propulsion systems. This comes in handy when you’re trying to reach lunar orbit in a few days, as opposed to months (or even years).
Since a nuclear thermal rocket relies on convective heating of a propellant (vice combustion or electromagnetic acceleration), its performance is limited by its operating temperature. Reactor and engine components need to be maintained at temperatures below thermal limits (less than 3,000 K or ~5000 degrees F).