In the annular aerospike nozzle, flow issues from an annulus at a diameter located some radial distance from the nozzle axis. Flow is directed radially inward toward the nozzle axis. This concept is the opposite of a bell nozzle which expands the flow away from the axis along diverging nozzle walls. In an aerospike, the nozzle expansion process originates at a point on the outer edge of the annulus which is referred to as the "cowl-lip." Because this point is also exposed to ambient pressure, the flow turning or expansion is limited by the influence of the external environment. In a standard bell nozzle, flow expansion continues regardless of what the ambient pressure is, and the flow can con tinue to over-expand until it separates from the nozzle walls.

Another important aspect of the low altitude performance of aerospike nozzles relates to the pressure acting on the nozzle base. When the nozzle pressure ratio is less than roughly one- third of the design pressure ratio, the wake that is present in the base of the plug is said to be "open," meaning that the base is open to the influence of ambient pressure and the base pressure is a strong function of the ambient pressure. This condition is distinctly different from the condition that occurs on the base for high-altitude operation where the wake is "closed," and the base pressure is independent of the ambient pressure.

where a is the cone half-angle. A 15 half-angle yields a geometric nozzle efficiency equal to 0.983 and is typically two-thirds the length of an ideal nozzle. For low-area-ratio nozzles, where simple fabrication methods are desired, 15 cones have become an accepted standard. The length of any nozzle type is commonly referenced to the length of a 15 cone having the same nozzle area ratio.

The overall nozzle efficiency is then given by the combined effects of geometric loss, viscous drag and chemical kinetics:

The two-dimensional kinetics (TDK) computer program, which has been combined with a boundary layer module (BLM), is the industry standard JANNAF (Joint Army-Navy-NASA-Air Force) program which accounts for all three nozzle losses and is used to predict hnoz for conventional bell nozzles.

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