Mathematical modelling of the unified bipropellant propulsion system

Abstract Present day 1.8 tonne class geosynchronous satellites carry on-board the unified bipropellant propulsion system for apogee manoeuvre, attitude and orbit control and station keeping applications. Basically the mass is expelled from the satellite for the control purposes. Hence there is a certain propellant flow rate leading to the ullage volume variations in the propellant tanks as the engines are firing. There are ullage and wall temperature variations as well. There is expansion of the pressurant in the pressurant tank with the resultant changes in the pressurant and the pressurant tank wall temperatures. The mathematical formulation of these thermodynamic processes leads to a twelve parameter initial value problem which is solved using the Runge-Kutta computer code. Real gas effects of the pressurant at high pressures are taken into account by means of the compressibility factor. The change in the temperature of the pressurant due to the throttling process in the pressure regulator is considered by means of the Joule-Kelvin coefficients. The pressure drops in the feed lines and the components are taken into account to compute the propellant supply pressures at the engine valve inlets. In the regulated mode of operation the liquid apogee motor plus a maximum of four reaction control system thrusters firing simultaneously could be handled by the model. The blowdown performance characteristics are also predicted. A computer program in FORTRAN is developed and the results as applicable to the unified bipropellant propulsion system for a 1.8 tonne class geosynchronous communication satellite are presented and discussed.