Dynamic Modeling of Starting Capabilities of Liquid Propellant Rocket Engines

Abstract

An analytical technique is developed for predicting the mass flow rate and heat addition in a liquid propellant rocket engine fuel system during the initial portion of an engine start. The analysis emphasizes nozzle jacket heat exchange; specifically, flow and heat transfer characteristic influence on power availability. The outstanding feature of this model is the accurate representation of fluid properties during phase change, and the subsequent affect on mass flow rates. The model also considers conduction and energy storage within the nozzle walls and makes use of extensive hydrogen heat convection data. The analytical technique is applied to a proposed 20,000-Ib thrust expander engine for the determination of the minimum initial nozzle jacket metal temperature that will promote starting at various operating conditions. The energy content of engine fuel flow during the initial portion of startup is compared to predicted turbomachinery torque requirements to determine start capability. Starting capability is determined for various initial nozzle metal temperatures at fuel inlet pressures of 50 and 70 psi at sea level. The minimum initial jacket metal temperature that will produce enough energy to overcome predicted turbine breakaway torque is determined to be 135°R for 70-psi and 385°R for 50-psi inlet pressures.