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Rockets and Missiles

Specific Impulse

The effectiveness of a fuel in propelling a rocket can be measured in a number of ways. For example, the thrust of a rocket is the mass that can be lifted by a particular rocket fuel. The thrust of most rocket propulsion systems is in the range from 500,000 to 14,700,000 newtons (10,000 to 3,300,000 pounds).

The velocity of exhaust gases is also an indication of how effectively the rocket can lift its payload, the cargo being carried by the rocket. One of the most useful measures of a rocket's efficiency, however, is specific impulse. Specific impulse (Isp) is a measure of the mass that can be lifted by a given fuel system for each pound of fuel consumer per second of time. The unit in which Isp is measured is seconds.

For example, suppose that a rocket burns up one pound of fuel for every 400 lb (182 kg) of weight that it lifts from the ground per second. Then its specific impulse is said to be 400 seconds. A typical range of specific impulse values for rocket engines would be between 200 to 400 seconds. Solid rockets tend to have lower specific impulse values than do liquid rockets.


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