Numerical Investigation of the Effect of Base Bleed on Thrust of a Truncated Aerospike Nozzle in Off-design Conditions

Aerospike nozzles are known to produce more thrust than conventional convergent-divergent nozzles in off-design conditions because of their capability to adapt their boundaries to ambient conditions. However, the central plug of an ideal aerospike nozzle can be very long and heavy, and its sharp end is susceptible to deformation in high temperatures. Plug truncation is a common measure to reduce an aerospike nozzle\u92s weight and length, and to make it more feasible for application in a rocket propulsion system. While plug truncation makes an aerospike nozzle smaller, lighter and more practical, it causes loss of thrust because of low pressure in the base area of the truncated plug. To reduce this loss of thrust, a technique called base bleed is used, in which a secondary flow is injected at the base of the plug. Success of the base bleed technique depends on several factors, including position, direction, and amount of the injected flow, as well as the nozzles working conditions. In this research, effects of the amount of base bleed and the nozzle\u92s working conditions on total base thrust and base pressure thrust of a truncated aerospike nozzle is numerically analysed, using results of previous research to select bleed position and direction. Numerical solution of the governing equations in axisymmetric coordinates has been carried out over an unstructured grid of triangular cells using a coupled implicit solver. The nozzle\u92s internal and external flow has been analyzed in 6 sets of far-field boundary conditions, corresponding to under-expansion, optimum, and over-expansion working conditions. For each working condition, 5 amounts of base bleed ranging from 0 to 5% of the nozzle\u92s total mass flow have been analyzed. In each case, total thrust and pressure thrust generated by the base of the plug has been determined, and base flow pattern has been studied to understand the mechanism by which base bleed affects base pressure distribution. Results indicate that in under-expansion and optimum working conditions, an aerospike nozzle with base bleed amounting to 2% of the nozzle\u92s total mass flow produces more thrust than nozzles with higher and lower amounts of base bleed, while in over-expansion conditions the maximum achievable amount of base bleed must be used to obtain maximum thrust