Partially reusable launch vehicle systems, featuring a reusable first stage and expendable upper stage(s), have been investigated as options for the next generation Air Force launch vehicle to meet future spacelift needs. An important area of research for the reusable first stage is the assessment of the aft end thermal environment in order to determine the influence of engine cluster packaging, plume characteristics, and engine protection and packaging approaches, particularly since the engines are to be utilized over again for multiple flights before refurbishment. Lockheed Martin's (LM) objective for the aft-end heating assessment was to generate a broad parametric understanding of engine-plume interactions and engine integration and packaging options to inform successful future configuration definition. Under contract from the Air Force Research Laboratory (AFRL), LM conducted CFD and thermal analyses for a wide baseline of engine cluster arrangements and packaging options for a small-lift 5000 lb payload baseline. The study matrix contains seven unique engine cluster arrangements that utilize from one to five rocket engines which are scaled to provide the same total performance as the baseline vehicle. For each engine arrangement, LM assessed the aft end thermal environment for two different engine packaging concepts. The first concept is an aft end boat tail engine packaging similar to what is seen on many other launch vehicles such as the Atlas V. The second packaging option places the entire engine in its own nacelle up to the engine mount. This second engine packaging concept is the baseline for AFRL's Fully reusable Access to Space Technology (FAST) study and is intended to simplify engine refurbishment and periodic replacement. The assessment of the thermal environment and flow behavior between these nacelles was a principal study goal. The resulting fourteen vehicle engine configurations were modeled in CAD and assessed utilizing Cart3d for CFD analysis. Parametric comparisons of convective heating derived from those solutions were produced in the base region for all cases over a range of Mach number and altitudes.
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