Multiple on-off valve control for a launch vehicle tank pressurization system

Pressure control inside the propellant tanks of a pump-fed space launch veh icle is almost always a flight-critical task. Pressures are commonly contro lled by admitting helium from high-pressure reservoirs into the propellant tank free volumes, or ullages. Controlled pressures must often satisfy the competing needs of engine net positive suction head, flight dynamic pressur es, tank structural stability, and tank maximum expected operating pressure . Helium flows are usually governed by either regulators, proportional valv es, or on-off solenoid valves implementing relay logic leading to control l imit cycles. A new control scheme using on-off solenoid valves to approxima te the performance of a proportional flow control valve has been synthesize d for both stages of a space launch vehicle. This control system uses the s ame valves required by a competing limit cycle system but significantly imp roves system robustness in the face of system uncertainties, including init ial engine outflow rates, ullage volumes, and helium-to-propellant heat tra nsfer rates. Simulations indicate ullage pressures can be maintained within a 5-psi band at all times, despite the slow 25-Hz data communication rate assumed for this vehicle. Furthermore, the control logic provides a simple way to incorporate additional feedback signals such as engine pressure into the control loop to further improve tracking accuracy.