Specific impulse (usually abbreviated
Isp) is a measure of the efficiency of
rocket and
jet engines. By definition, it is the
impulse delivered per unit of
propellant consumed, and is
dimensionally equivalent to the
thrust generated per unit propellant
flow rate. If
mass (kilogram or
slug) is used as the unit of propellant, then specific impulse has units of
velocity. If weight (
newton or
pound) is used instead, then specific impulse has units of time (seconds). The conversion constant between these two versions is the standard gravitational acceleration constant (
g0). The higher the specific impulse, the lower the propellant
flow rate required for a given
thrust, and in the case of a rocket, the less propellant needed for a given
delta-v, per the
Tsiolkovsky rocket equation.