EXPERIMENTS ON SUPERSONIC COMBUSTION RAMJET PROPULSION IN A SHOCK TUNNEL

Measurements have been made of the propulsive effect of supersonic com bustion ramjets incorporated into a simple axisymmetric model in a fre e piston shock tunnel. The nominal Mach number was 6, and the stagnati on enthalpy varied from 2.8 to 8.5 MJ kg(-1). A mixture of 13% silane and 87% hydrogen was used as fuel, and experiments were conducted at e quivalence ratios up to approximately 0.8. The measurements involved t he axial force on the model, and were made using a stress wave force b alance, which is a recently developed technique for measuring forces i n shock tunnels. A net thrust was experienced up to a stagnation entha lpy of 3.7 MJ kg(-1), but as the stagnation enthalpy increased, an inc reasing net drag was recorded. Pitot and static pressure measurements showed that the combustion was supersonic. The results were found to c ompare satisfactorily with predictions based on established theoretica l models, used with some simplifying approximations. The rapid reducti on of net thrust with increasing stagnation enthalpy was seen to arise from increasing precombustion temperature, showing the need to contro l this variable if thrust performance was to be maintained over a rang e of stagnation enthalpies. Both the inviscid and viscous drag were se en to be relatively insensitive to stagnation enthalpy, with the combu stion chambers making a particularly significant contribution to drag. The maximum fuel specific impulse achieved in the experiments was onl y 175 s, but the theory indicates that there is considerable scope for improvement on this through aerodynamic design.