Linear aerospike engine performance evaluation

After being tested and characterized in the 1960s and early 1970s, the concept of linear aerospike engines was revived in the last few years as a viable propulsion option for potential Single Stage to Orbit (SSTO) vehicles. In contradiction to the common believe in full altitude adaptation and higher vacuum performance, an evaluation of the published test results shows that this type of nozzle behaves just like any nozzle with the flow separating at too high ambient pressures and performance being limited by geometric and gasdynamic constraints. Altitude adaptation results primarily by the opening of the wake at high ambient pressures and the consequent transfer of pressure information between the ambient environment and the base region of the truncated plug. For even higher ambient pressures, the nozzle flow initially overexpands and finally separates from the wall according to one of the wellknown criteria. As for an aerospike engine the aerodynamic area ratio of the expanding core flow is much smaller than the geometric one, vacuum performance will be lower than for a bell nozzle with an identical overall area ratio. With this consideration the projected vacuum performance of the X-33 and Venture Star engines seems to be quite optimistic.