>>11679674>H2As everyone already knows, it's got the highest Isp, ~1000 seconds at a temperature of 2800 K. However, it also has the lowest mass flow rate for a given volumetric flow rate, and therefore the lowest thrust to weight ratio. With a density of 70.85 kg/m^3, every 1000 liters per second gives you about 694.33 kN of thrust output. With a mass ratio of 80% due to the low low density, the available delta V from a hydrogen propelled NTR stage is ~15783 m/s.
>HeSuper rare unless we can pull gasses from the giant planets, He offers performance very close to that of hydrogen overall. Isp around 800, with a density of 125 kg/m^3 means with the same mass flow rate as the previous example engine it produces 980 kN of thrust. This higher density and similar insulation requirements means a helium NTR practical mass ratio is probably somewhere around 85%, giving the stage a delta V budget of 14883.5 m/s, only 900 m/s less than the hydrogen stage. This propellant is the most friendly to the components of the engine, being entirely non-corrosive, and possibly allows for higher temperatures in the core without causing problems, which could further improve performance.
>CH4Isp around 606 seconds, density of 422 kg/m^3, and much less cryogenic than the previous two propellants, methane is a very good option for NTR. 1000L/s volumetric flow rate results in 2506.17 kN of thrust, and with a practically attainable mass ratio of 90% provides a delta V of 13683.9 m/s, 86.7% of that of the hydrogen propelled stage. Seems to hit a sweet spot between max efficiency and max thrust.
>H2OWith an Isp of 370, and a density of 1000 kg/m^3, performance is on par with a methalox chemical rocket at least in terms of stage delta V. Engine thrust at 1000L/s is 3626 kN, the highest of the four options here by over 1100 kN. A stage with a 90% mass ratio would provide 8354.8 m/s of delta V. This option offers the best thrust to volumetric flow rate of any common propellant.
