Recessing shear-coaxial injectors in cryogenic rocket engines: Enhanced performance at the cost of amplifying combustion instabilities

A shear-coaxial injector is a widely used choice in rocket engines using propellant pairs with a high density ratio. Injectors with recessed liquid oxygen tubes (“LOX post”) are known to have improved mixing and thus engine performance, but combustion instabilities were observed with this type of injector in an optically accessible, single-element combustor and a multi-injector, sub-scale thrust chamber. Both experiments showed short-lived events of oscillatory combustion as well as high-amplitude, high-frequency, limit-cycle instabilities within the conducted tests. Analyzing the injection conditions in combination with high-speed imaging showed strong evidence that recessing the injector plays a role in the development of the instabilities. Consolidation of stability measurements obtained with the propellant combinations of liquid-oxygen/hydrogen and liquid-oxygen/natural-gas and with different hardware configurations reinforced this finding.