Flameout and combustion efficiency of scramjet-powered hypersonic vehicles during ascent

As a hypersonic vehicle travels to high altitudes along an ascent trajectory, the static pressure (p3) in the scramjet combustor will decrease, which can lead to engine flameout and lower combustion efficiency. At low pressures the chemical reactions between the fuel and air become excessively slow. However, during the ascent the flight Mach number is increasing so there is an increase in the stagnation temperature and the static temperature (T3) at the combustor entrance. This tends to prevent flameout and to improve combustion efficiency. To investigate the tradeoff, a reduced-order model called MASIV first was used to compute the static temperature and pressure at the entrance to the combustor (T3, p3) as a function of altitude for a trimmed vehicle. Then the combustion efficiency was computed using the finite-rate chemistry sub-model. To determine if flameout occurs at each altitude, values of (T3, p3) are inserted into an empirical relation that includes the measured value of the critical Damkohler number. Three empirical constants are selected so that the model predictions agree with available measured values combustion efficiencies and flameout limits. The reduced-order model approach provides rapid (but approximate) answers that are useful when thousands of computations are needed for optimization studies or to develop a control algorithm. Results are plotted as three curves on a graph of vehicle altitude versus flight Mach number. The first curve is the vehicle trajectory; the second curve is the combustion efficiency versus altitude and the third curve is the flameout limit. The trajectory curve was optimized to stay near the highest efficiency curves while not intersecting the flameout limit curve. Another optimization study identified the optimum number of wall-port fuel injectors.