Reaction Control Jet Actuators for Air-Breathing Hypersonic Vehicles

Abstract

Air-breathing hypersonic vehicles have an engine which is tightly integrated with the airframe. This integration leads to complex dynamic characteristics, such as propulsion-pitch coupling. As a result, air-breathing hypersonic vehicles tend to be unstable and high frequency control is required to maintain steady flight. To provide high frequency control, aerodynamic control surfaces may need to be supplemented or replaced by high frequency actuators. Reaction control (RC) jets are an attractive solution, as they are commonly used to supplement aerodynamic control surfaces for exo-atmospheric control of hypersonic vehicles, and can operate at high frequency. If RC jets are operated within the atmosphere, an interaction force is generated in addition to the jet thrust. Understanding this interaction force is important in the design of a control system that implements RC jet actuators for use within the atmosphere. This thesis studies the unsteady interaction between an RC jet, and a hypersonic crossflow. A generic hypersonic vehicle model was used to investigate flight conditions where supplementary control may be required. Due to the instability caused by propulsion-pitch coupling, aerodynamic control surfaces alone are unable to control the vehicle’s rigid-body modes at several flight conditions. To analyse RC jet flow physics, an implicit large-eddy simulation (ILES) methodology was developed, verified and validated using a number of canonical supersonic flow problems, and experimental data for a steady jet in hypersonic crossflow. The steady and pulsed operation of a sonic, round jet issuing from a flat plate into a hypersonic (Mach 5.0) crossflow with a laminar in-flow boundary layer was investigated numerically. The pressure distribution induced on the plate is unsteady, and is influenced by shock and vortex structures that form around the jet. The unsteady and time-averaged behaviour of these structures has been described, leading to an improved understanding of the jet interaction flow physics. When the jet was pulsed, flow structures were influenced by the presence of shocks in the flow, allowing penetration per unit jet mass flow to increase by a maximum of 68% compared with the steady jet. Pulsing also provides a higher jet interaction force per unit jet mass flow compared with a steady jet, with a 52% increase recorded at a 33% duty-cycle. The start-up process for both steady and pulsed jets was also considered. The interaction force during start-up acts as a lightly damped second-order system. An overshoot is observed in the control force, corresponding to expansion of the jet flow behind the initial lead shock, before the flow settles to a quasi-steady state on a timescale related to the time taken for the jet fluid to reach the trailing edge of the flat plate. To assess the effectiveness of RC jets as a supplement to aerodynamic control, jet control was implemented on the generic air-breathing hypersonic vehicle, showing a significant improvement at all supersonic flight conditions. This thesis provides an increased understanding of the unsteady interaction between steady and pulsed sonic jets in hypersonic crossflow, which has applications in reaction jet control of air-breathing hypersonic vehicles.