Altitude Dependence of Rocket Motor Performance
United States Air Force Academy USAF Academy United States
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It is well known that rocket motors produce greater thrust at higher altitudes due to the lower ambient pressure. A simple formula predicts that the thrust increase will be equal to the decrease in ambient pressure times the cross sectional area of the throat of the nozzle. This paper provides experimental tests of predicted increases in impulse based on this simple formula. The experimental method employed 20 trials with each of three solid fuel hobbyist rocket motors Estes A10-PT, Estes D5-P and Quest D11-0P. Thrust curves were measured with a force plate for ten trials with each motor at two different altitudes corresponding to ambient pressures of 22.80 in Hg and 29.84 in Hg. Resulting impulses and specific impulses were then computed for each trial and the differences in average impulse and specific impulse were then compared with the predictions using the simple model that predicts thrust dependence on ambient pressure. Although thrust increases were measured in each case of reduced ambient pressure, the gains in impulse were not in agreement with predictions of the simple formula in every case.