Nuclear Thermal Propulsion Simulator

Nuclear Thermal Engine

Reactor power 70%

Core temp

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Chamber pressure

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Specific impulse (Isp)

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Exhaust velocity

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Isp — NTP vs chemical (LH2/LOX)

Chemical transit–

NTP transit–

How it works

A nuclear thermal engine skips combustion entirely: liquid hydrogen is pumped through channels in a fission reactor core, absorbs the reactor's heat directly, and shoots out a nozzle as hot gas — no oxidizer, no burning, just conduction. This was flight-tested by the NERVA program in the 1960s–70s and is being revived (e.g. DRACO) as a fast-transit option for crewed Mars missions.

v_e = sqrt(2·k·T / m)   (exhaust velocity, ideal)
Isp = v_e / g0
lower propellant molar mass m ⇒ higher v_e ⇒ higher Isp

Reactor power — sets core temperature (real NERVA-class cores ran ~2500–3000 K). Higher T directly raises exhaust velocity and Isp.
Why hydrogen — Isp scales as 1/√(molar mass), so the lightest possible propellant (H₂, M≈2) beats any heavier exhaust gas at the same temperature.
Isp comparison — NTP reaches ≈850–900 s versus ≈450 s for the best hydrogen–oxygen chemical engine: roughly double, for the same propellant mass fraction.
Mars transit time — doubling Isp lets a given propellant load deliver much more delta-v, which is why NTP concepts target roughly half the coast time of an all-chemical trajectory (illustrative estimate scaled from Isp, not a full trajectory solve).

Cold LH₂ feed Heating in core Peak core temp Nozzle exhaust

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