Pulsed Injection Flow Control for Throttling in Supersonic Nozzles - A Computational Fluid Dynamics Design Study

Abstract

A vehicle propelled by an engine with a variable geometry nozzle allows the nozzle expansion ratio to vary with altitude and flight condition, thereby optimizing engine performance. Active flow control offers a method of providing the functionality of a variable throat area system without requiring variable geometry. Throttling the mass flow rate through the nozzle throat controls the effective throat area, subsequently controlling the effective expansion ratio of the overall nozzle. This paper presents findings from the Pulsed Injection for Rocket Flow Control Technology (PIRFCT) program, which evaluated potential gains in the overall performance of a rocket using active flow control to optimize nozzle expansion ratio for an Earth to orbit mission. Lockheed Martin Aeronautics Company utilized Computational Fluid Dynamics (CFD) to simulate the rocket nozzle with active flow control. Simulations were performed with steady and pulsed flow control jets which were oriented near the geometric throat and inclined upstream against the primary flow. A low stagnation pressure, steady, tertiary injection stream when combined with a steady, high momentum secondary injector was witnessed to increase throttling performance beyond that of a secondary injector alone. Nozzle discharge coefficient was largely unaffected by changes in pulsation frequency or pulsation duty cycle. Pulsed injection approached, but did not exceed, the throttling performance of a time invariant injector when compared on a equivalent mass flux, momentum flux, and energy flux basis. Simulations incorporating a single injector and large area modulations predicted a 50% area reduction when injecting approximately 18% baseline reference mass flow at Mach 2 conditions. However, the PIRFCT program concluded that secondary injection at the nozzle throat is not a good candidate for this type of throttling/altitude compensation technology for an Earth to orbit mission. This was due to the small portion of its trajectory spent at lower altitudes because of its space access mission. However, potential cadidates for this technology include gas turbines and rockets whose application required the vehicle to stay in the lower atmosphere for a longer duration than a Earth to orbit space access mission.