NERVA
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The Nuclear Engine for Rocket Vehicle Application was a nuclear thermal rocket engine development program that ran for roughly two decades. Wikipedia
Application: Upper stage engine
Chamber pressure: 3,861 kPa (560 psi)
Diameter: 2.59 m (8 ft 6 in)
Dry weight: 18,144 kg (40,001 lb)
Length: 6.9 m (23 ft)
Power (thermal): 1,137 MW
Propellant: Liquid hydrogen