3 .0 ORBITAL TRANSFER MECHANICS (G. Babb, Advanced Mission Design Branch (FM5) 3.1 LOH THRUST ORBITAL OPERATIONS-INTRODUCTION Previous sections indicated that the COTV may be required to utilize thrust/weight levels of 10“^ and below for orbital transfer maneuvers. This is substantially below the accelerations currently considered normal for Earth orbit operations. Orbital operational experience to date has been with thrust levels high enough so that orbital maneuvers could be considered as a perturbation of the impulsive transfer case. At the low thrust levels of the COTV, however, orbital transfers take on an entirely different character. Thrusting now takes place during virtually the entire transfer time and the flight path becomes a tightly coiled spiral connecting the initial and final orbits (Figure VI-D-3-1). At T/W = .001, for instance the transfer from low parking orbit (250 NM) to a geosynchronous orbit (no plane change) takes 33 revolutions of the spacecraft over 5.5 days under continuous thrusting. The delta V requirements, flight times, and guidance and navigation requirements are al! different from the more usual high thrust cases. 3.1.1 DEFINITION OF LOW THRUST The impact of lower thrust levels on delta V requirements and basic operational techniques becomes significant when the thrusting phase becomes equal to or greater than an orbital period. For delta V level on the order of escape, it can be shown that in general the operation is approximately impulsive, that is high thrust, so long as the thrust levels are on the order of the accelerations of the primary (central) body at that point (that is the local g values). When the thrust levels become several orders of magnitude less than this (local g value), the system exhibits all the characteristics of the low thrust systems. Starting in low Earth orbit going to geosynchronous orbit, thrust levels of 10“2 and 10"3 g and below are low thrust in character, that is it requires a number of orbital periods and a number of orbital circuits during the thrust phase. Thrust levels of .1 to 1g and above are high thrust in nature. Between .1g and .01g is a transition region. For thrust levels in this region, operational techniques such as multiple orbit injection (thrusting near perigee on successive orbits) are effective in minimizing the so-called "gravity losses" of low thrust operations. For thrust levels below .01g these techniques lose effectiveness within reasonable time limits.
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