High altitude test (HAT) facilities, used to test large area ratio rocket nozzles at ground, are uniquely designed for particular engines. The isolation of nozzle exit from ambient is achieved through oblique shock trains in second throat diffuser which depends upon second throat geometry and hot gas momentum. At off design operation, such as testing of lower thrust engine and its start transients, oblique shock train may not get established. In such situations, ejector recovery pressure will get communicated to nozzle exit and may result in flow separation in nozzle divergent.
This paper presents the CFD study carried out for testing an 8T cryogenic engine in a HAT facility designed for a 20T cryogenic engine. Steady state simulations showed that during steady state operation, ejector is able to maintain required nozzle exit pressure for 8T engine. However, in this condition there is large gap of 275mm between nozzle exit and diffuser inlet, which results in reverse flow of un-burnt propellant into the vacuum chamber. This is prevented by isolating the vacuum chamber from the diffuser. With the vacuum chamber isolated from the diffuser and with continuous ejector operation, the lower thrust engine (8T) was simulated for a short duration test of 6s. The evolution of ejector recovery pressure and pressure load on the nozzle during the test are estimated. It is established through these simulations that 8T engine can be tested in a HAT facility designed for 20T engine with continuous ejector operation and isolating the vacuum chamber from the diffuser.