ABSTRACT
Electric propulsion continues its expansion in space. More than 160 spacecraft in the Earth orbit and interplanetary space now use it in some form. Many new programs have begun to develop the next generation of electric propulsion for solar and nuclear powered spacecraft. Although it is another load on the bus, some of its unique features that influence the power system design and operation are discussed in this chapter. In the geosynchronous orbit, the satellite must maintain the assigned
orbit altitude and inclination to stay locked with a fixed ground station. But the orbit slowly degrades due to various drags in the space. The propulsion system maintains the satellite in the desired orbit and controls its attitude towards the Earth. Propulsion is also needed for orbit transfer and to deorbit the satellite for disposal at the end of a mission. The requirements for these functions are listed in Table 23.1. The propulsion thrust in a typical satellite is produced by chemical fuels
carried on board. In a geosynchronous satellite, over 90% of the on-board fuel is used for N-S station keeping, and the rest for E-W station keeping, attitude control, and orbit transfer. The resultant thrust vector of all stationkeeping thrusters must pass through the center of the satellite mass. The thrust vector for the attitude control, on the other hand, must be at a right angle to the center of mass in order to generate the maximum moment. Chemical propulsion typically uses a catalytic monopropellant hydrazine
(N2H4) fuel. No oxidizer is needed. Heat or a catalyst decomposes the fuel, and the chemical reaction produces heat and thrust of the exhaust gas. Some satellites use fuel and oxidizer in separate tanks to achieve higher
thrust levels. Such bi-propellant fuel systems generally uses hydrazine and nitrogen tetroxide (N2O4). This yields specific impulse around 300 s. At present, electric propulsion is used in low-thrust maneuvers.
