An important performance parameter of rocket motors is the specific impulse, Isp, defined by the equation
where go = 9.80665 m/s2 is the internationally agreed standard gravitational acceleration. The higher the specific impulse, the lower the rate of propellant consumption for a given thrust. As a consequence of the lower propellant mass needed for a given space mission, a high specific impulse will therefore result in a higher ratio of payload to propellant mass.
For mainly historical reasons, the specific impulse is defined by the propellant weight flow rate gofh, rather than by the mass flow rate m as would have been more appropriate. Numerically, Jsp is therefore expressed in seconds.
The specific impulse that can be achieved in practice depends in part on the design of the rocket motor, particularly on the expansion ratio of the nozzle. But principally it depends on the chemical reaction energy of the propellant, or—in the case of nuclear-thermal motors — on the reactor temperature. It depends relatively little on the size of the motor. Typical performance figures will be discussed in Chap. 4.
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