A comparison of the three total propulsion system masses indicates that the use of the Boeing MPD thruster results in the least total propulsion system mass required for attitude control and station keeping. Lower mass usually means lower cost; however, the yearly propellant requirement of 800 000 kg for the Boeing MPD system in comparison to 444 464 kg for the Boeing ion system over a period of 30 years could be significant and should be given further consideration. The total amount of power needed to operate the required number of thrusters for each axis was calculated. For the roll control thruster modules, it is assumed that, while thrusting in one direction, the power to the thrusters in the opposite direction is off. However, operating electric thrusters in an on-off manner is questionable (because of lifetime considerations); therefore, power to the proper elements of each thruster may be necessary to keep the operating temperature constant. The propellant flow could, perhaps, be cut off to the thrusters that are not needed at a particular time. Hence, the power required for the roll control axis could be double that shown in Table 7-10. Since the end mounted thruster modules are required to provide control about two axes as well as maintain station keeping, power to the required thrusters in these modules is also never expected to be turned off. Assuming the thrusters can be mounted on the SPS in a square pattern, the approximate minimum thruster planform dimensions and area were calculated for each electric propulsion concept. 7.1. 7 MAINTENANCE AND REPAIR 7.1. 7.1 COLLISION DAMAGE Requirements for maintenance will arise from two primary sources: collision damage and component failure. Satellite population provides the basis for estimating expected collision damage. The increase in satellite population projected for the SPS era (Fig. 7-31) demands close scrutiny to assure that the large areas of SPS at LEO and GEO are safe from potential collision. For the large SPS spacecraft (approximately 150 x 106 m2) and large satellite population, it is expected that during construction in LEO 10 collisions/month will occur, which will decrease to several per year in GEO. The spike of 20 collisions/ month at 1000 km should not significantly increase the total number of hits, since the SPS will quickly pass through this region en route to GEO (Fig. 7-32).
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