Detonation Tube Impulse in Subatmospheric Environments

Thrust from a multicycle pulse detonation engine operating at practical flight altitudes will vary with surrounding environment pressure. We have carried out the first experimental study using a detonation tube hung in a ballistic pendulum arrangement within a large pressure vessel to determine the effect that the environment has on the single-cycle impulse. Air pressure decreased below 100 kPa, whereas initial pressure of the stoichiometric ethylene– oxygen mixture inside the tube varied between 100 and 30 kPa. The original impulse model (Wintenberger et al., Journal of Propulsion and Power, Vol. 19, No. 1, 2002, pp. 22–38) was modified to predict the observed increase in impulse and blowdown time as the environmental pressure decreased below 1 atm. Comparisons between the impulse from detonation tubes and ideal steady-flow rockets indicate incomplete expansion of the detonation tube exhaust, resulting in a 37 % difference in impulse at a pressure ratio (ratio of pressure behind the Taylor wave to environmental pressure) of 100. Nomenclature A = area c = speed of sound F = force FD = force due to diaphrag