Characteristic velocity, denoted (pronounced 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. It is independent of the nozzle, making it a useful metric for evaluating propellant combustion alone. 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, or the combustion efficiency. This is known as c*-efficiency, or , and is calculated by dividing with . Standard values for range from 0.85 to 1.03.
Formula
is the characteristic velocity (m/s, ft/s). is the chamber pressure (Pa, psi). is the area of the throat (m2, in2). is the mass flow rate of the engine (kg/s, slug/s).
Alternative Imperial form:
is the specific impulse(s). is the gravitational acceleration at sea-level (m/s2). is the thrust coefficient. is the effective exhaust velocity (m/s). is the specific heat ratio for the exhaust gases. is the gas constant per unit weight (J/kg-K). is the chamber temperature (K).
See also Characteristic length (L*) Thrust coefficient (CF)
References