Characteristic velocity or , or C-star is a measure of the combustion performance of a rocket engine independent of nozzle performance, and is used to compare different propellants and propulsion systems. c* should not be confused with c, which is the effective exhaust velocity related to the specific impulse by: . Specific impulse and effective exhaust velocity are dependent on the nozzle design unlike the characteristic velocity, explaining why C-star is an important value when comparing different propulsion system efficiencies. c* can be useful when comparing actual combustion performance to theoretical performance in order to determine how completely chemical energy release occurred. This is known as c*-efficiency.
Formula
is the characteristic velocity (e.g. m/s, ft/s) is the chamber pressure (e.g. Pa, psi) is the area of the throat (e.g. m2, in2) is the mass flow rate of the engine (e.g. kg/s, slug/s)
is the specific impulse (e.g. s) is the gravitational acceleration at sea-level (e.g. m/s2) is the thrust coefficient is the effective exhaust velocity (e.g. m/s) is the specific heat ratio for the exhaust gases is the gas constant per unit weight (e.g. J/kg-K) is the chamber temperature (e.g. K)