Jet Separation in Conical Nozzles

Abstract

Summary The separation or breakaway of over-expanded gas from a nozzle wall plays an important role in the estimation of the thrust from a rocket engine when run at altitudes considerably less than the design. A number of reports have been written on the subject but the agreement of test results has not appeared to be good. In an effort to clarify the position, the Rocket Department of Bristol Siddeley Engines Ltd. has carried out tests on experimental combustion chambers having static pressure tappings in the nozzle divergent section. These tests have shown that, contrary to some previous suggestions, the pressure at which the gas separates is neither constant nor independent of the nozzle length. The paper, which includes results from other workers, divides the field into separation adjacent to the nozzle exit and also at positions where the area ratio is less than 80 per cent of the exit area ratio. For the most part, consideration is given to conical nozzles having divergence half angles of 15° to 17°, but comment is made on the effects of divergence and other variables. A tentative suggestion as to the mechanism of separation is also made. Finally, the effect of separation on the sea-level thrust of a nozzle designed for high altitude Is shown.