Side Load Reduction of High Area Ratio Nozzles

A laboratory scale evaluation of off-axis side-loads generated by a high area-ratio thrust optimized parabolic contour nozzle during startup is discussed. Nozzle side-loads are generated by well known shock patterns and transition states comprising free-shock separated flow (FSS), restricted-shock separated flow (RSS), and an end-effects regime (EER). Modifications to the test hardware are evaluated in the form of changes to nozzle scale, nozzle wall roughness, as well as the insertion of pressure relief ports near the nozzle lip. The findings demonstrate sensitivities to both FSS to RSS transition, and EER side-loads on account of nozzle wall roughness and the insertion of wall pressure relief ports. For the smooth wall nozzle, fore and aft motions of the separation shock near the nozzle lip during EER generates pressure changes (peak to valley) that are approximately 1.3 atmospheres. For the same nozzle contour with rough wall, the pulsations reduce to 0.50 atmospheres with additional reductions (down to 0.32 atmospheres) being invoked by the insertion of pressure relief ports in the vicinity of the EER shock. EER pressure loads are also delayed to higher nozzle pressures due to wall roughness effects and pressure relief ports. Where off-axis side loads are concerned, the pressure relief ports provide a 35% reduction in peak nozzle side loads during FSS to RSS transition, and a 31% reduction during EER, relative to the rough wall nozzle. It is postulated that the pressure relief ports are effective at reducing nozzle side loads as they provide a pathway for transferring flow mass from within the separation bubble to the low pressure regions of the nozzle located upstream of the wall shock during EER.