Thermal rocket

A thermal rocket is a rocket engine that uses a propellant that is externally heated before being passed through a nozzle, as opposed to undergoing a chemical reaction as in a chemical rocket.

Thermal rockets can theoretically give high performance, depending on the fuel used and design specifications, and a great deal of research has gone into a variety of types. However, aside from the simple cold gas thruster and steam rocket, none has yet been put to practical use.

For a rocket engine, the efficiency of propellant use (the amount of impulse produced per mass of propellant) is measured by the specific impulse (${\displaystyle I_{sp}}$), which is proportional to the effective exhaust velocity. For thermal rocket systems, the specific impulse increases as the square root of the temperature, and inversely as the square root of the molecular mass of the exhaust. In the simple case where a thermal source heats an ideal Monatomic gas reaction mass, the maximum theoretical specific impulse is directly proportional to the thermal velocity of the heated gas:

where ${\displaystyle g_{o}}$ is the standard gravity, ${\displaystyle k_{b}}$ is Boltzmann's constant, T the temperature (absolute), and m is the mass of the exhaust (per molecule). For reaction mass which is not monotomic, some of the thermal energy may be retained as internal energy of the exhaust, and this equation will be modified depending on the degree of dissociation in the exhaust, frozen-flow losses, and other internal losses, but the overall square-root proportionality will remain. A more detailed equation for the maximum performance of a thermal rocket can be found in Chung.

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