Characteristic velocity

Characteristic velocity, , 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. 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 (C_F)

References