Apogee manoeuvre with a restartable liquid bipropellant motor

Unified Propulsion Systems present perceptible advantages for geostationary spacecrafts design: mass savings, as the ergols tanks are the same for the apogee motor and for the Attitude and Orbit Control System, higher performance, as specific impulse of bi-ergols motors is higher than the one of solid propellant motors and higher operational flexibility as the fuel amount can be adapted to the real flight conditions and as biliquid motors are restartable. On the other hand, the use of these propulsion systems for geostationary spacecrafts sets quite new mission analysis problems: the “predictability” of each delivered Delta-V is rather coarse (the corresponding uncertainty is about 4% for the existing motors). Also, only midlevel thrusters (about 400N) are available and so the finite burn losses associated with long burns arcs have to be minimized. This paper surveys the problems resulting from these new operational constraints and deals successively with the following items: optimal splitting up of the apogee manoeuvre, taking into account the possible dispersions on each Delta-V and the on-station longitude acquisition; minimization of the finite burn losses; adaptation of the apogee manoeuvre to the initial orbit parameters corresponding to the first North-South station-keeping cycle. The operation procedures derived from this survey will be used for the future launch of the ARABSAT spacecraft and for the following spacecrafts of the SPACEBUS family.