Cold gas testing of thrust-optimized parabolic nozzle in a high-altitude test facility

An experimental investigation has been carried out to study the changes in the exhaust flow development of a thrust-optimized parabolic nozzle when it is tested in two different test environments, namely, under sea-level conditions and in a low-pressure environment inside a high-altitude test facility. For tests inside the high-altitude simulation chamber, the incipient separation location is found to be located considerably upstream relative to its location under sea-level tests. Furthermore, it is also observed that the location of normal shock for tests inside the high-altitude chamber is slightly downstream of its position in sea-level tests. These flow modifications shift the momentum imbalance along the overexpansion and reflected shocks in such a way that only free shock separation condition is favored (both during startup and shutdown transients) for all test conditions inside the high-altitude facility. As a result, flow transitions such as free shock separation to restricted-shock separation and vice versa, observed in the present tests under sea-level conditions, are not at all observed for tests inside the high-altitude test facility. The primary cause responsible for such a discrepancy in the observed flow conditions is the significantly reduced mass flow or density of the test gas [due to the lower driving pressure (P₀) required to achieve similar nozzle pressure ratio] inside the high-altitude chamber. The reduced mass flow also prevents the gas tonucleate earlier, and hence delays the onset of nitrogen condensation to a much higher nozzle pressure ratio.