Nuclear thermal propulsion

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Nuclear Thermal Propulsion (NTP) uses a nuclear reactor to heat propellant directly and exhaust it through a rocket nozzle. The nuclear reactor core is the hottest element in the engine and limits the effectiveness of the drive. (Tho nuclear reactors could be made to run hotter, removing waste heat is a great concern to prevent the reactor from melting. Radiating heat in space is more difficult than on a planet since there is no mass to dump heat into via convection.)

Liquid hydrogen is usually used as the propellant as it has a higher velocity for the same input power, and therefore produces a faster final velocity according to the rocket equation. An animated illustration of nuclear thermal rockets can be found at [1].

Nuclear thermal engines allow for shorter transit times and/or lower propellant requirements than chemical engines. They are usually planed for orbital operations only, and are in completion with alternative technologies such as Nuclear Electric Propulsion (NEP) and Solar Electric Propulsion (SEP).

In the late 1960's NASA explored nuclear propulsion with the NERVA program. It was expected that such nuclear rockets could achieve efficiencies of 900 seconds ISP. (This is twice the efficiency of chemical rockets where efficiently engineered hydrogen-oxygen engines are around 450 seconds ISP.)

However, a little know technology, the wave rotor, can be used to further compress the hot gas from a nuclear rocket before it enters the rocket nozzle. This can boost the ISP to 1,400 to 2,000 seconds. If such a rocket with an ISP or 1,800 seconds could be created, missions to Mars would need 1/2 the reaction mass of a NERVA rocket, or 1/4 the reaction mass of a chemical rocket. This would allow much more payload to be sent to Mars, and increase the safety, and efficiency of such missions.[2][3]

Wave Rotor Nuclear Thermal Propulsion (WRNTP) is an extremely interesting technology, but as of 2023, no working motors have been demonstrated.

History of nuclear thermal propulsion



Propellant Liquid hydrogen
Thrust (vac.) 246,663 N (55,452 lbf)
Chamber pressure 3,861 kPa (560.0 psi)
Isp (vac.) 841 seconds (8.25 km/s)
Isp (SL) 710 seconds (7.0 km/s)
Burn time 1,680 seconds
Thrust to weigh ratio 1.36
Restarts 24
Length 6.9 meters (23 ft)
Diameter 2.59 meters (8 ft 6 in)
Dry weight 18,144 kilograms (40,001 lb)


Analysis of use


  • Higher ISP than chemical
  • Higher power energy source
  • Shorter travel time
  • Oberth effect
  • Self cooling


  • Cost
  • Cost of development
  • Risk of accident
  • Lower ISP than electric
  • Low public trust
  • Thrust to weight ratio close to 1 (cannot take off from Earth with a significant payload)


  • Solid core[5]
  • Gas core[6]
  • Nuclear light bulb, open and closed[7]
  • Wave Rotor NTP[8]
  • Nuclear salt water rockets[9]