The propulsive efficiency of a nuclear thermal rocket engine can be expressed in terms of the effective exhaust velocity and the specific impulse. Nuclear thermal rockets use the heat produced by a nuclear reactor to heat a propellant (such as hydrogen) to high temperatures, which is then expelled through a rocket nozzle to generate thrust.
Nuclear thermal rockets can achieve high specific impulses, and their propulsive efficiency is determined by how effectively the heat from the nuclear reactor is transferred to the propellant and converted into kinetic energy of the exhaust gases. The specific design parameters, such as reactor temperature, heat exchanger efficiency, and nozzle design, will influence the performance and propulsive efficiency of a specific nuclear thermal rocket system.
The propulsive efficiency is a crucial parameter in evaluating the performance of a rocket engine, indicating how well it converts propellant energy into useful thrust. Practical rocket engines often have propulsive efficiencies less than 100% due to factors like incomplete combustion, heat losses, and other inefficiencies in the propulsion system.
The propulsive efficiency () is defined by the following formula:
where,
- is the propulsive efficiency,
- is the thrust produced by the rocket engine,
- is the effective exhaust velocity of the rocket,
- ṁ is the mass flow rate of the propellants,
- is the specific impulse of the rocket engine,
- g0 is the acceleration due to gravity (approximately 9.81 m/s²).