10. What is Isp? (A K2S Question)

From The Space Library

An important measure of rocket performance is specific impulse, (Isp). It is the number of seconds that a thrust of one kilogram of force can be maintained by one kilogram of propellant. It is also the number of seconds that one pound of force can be maintained by one pound of propellant. So, Isp is independent of the system of units: metric or English. Isp can be calculated from the specific energy or from the exhaust velocity. For LOX/ RP1 and for solid rocket motors, the Isp is about 300 seconds. For LOX/ hydrogen the Isp is about 460 seconds. Nuclear thermal rockets have an Isp of about 1,000 seconds and electric thrusters provide an Isp of 1,000 to 10,000 seconds, depending on the design. Additional important measures of performance are the engine thrust-to-weight ratio (measured in "gees") and vehicle specific power (measured in kilowatts per kilogram, or sometimes as the reciprocal kilograms per kilowatt for the engine alone and referred to as "alpha").

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Answer provided by John W. Cole

Question and Answer extracted from the book Kids to Space - by Lonnie Schorer

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