Low Orbit Gravity gradient torque is the predominant attitude disturbance in low orbit. At 500 km altitude, its magnitude is 230 times that in synchronous orbit. Thus, a square array (137.5 km2, 35000 M.T.) would require a maximum RCS thrust of 125,000 N to move from a stable to an unstable attitude. This would be the case, for example, if the array were constructed in a stable attitude (vertically oriented) and rotated to an unstable attitude (horizontal) for transfer to synchronous orbit. The same peak torque would be necessary if a solar orientation were maintained for maximum power output during self-powered transfer. Aerodynamic torque will be important only if the center of mass and center of pressure are not coincident. Even the maximum plausible offset, however, cannot produce torques of the same magnitude as gravity gradient. Since solar radiation pressure is about 5 percent of aerodynamic drag at 500 km, it does not contribute appreciably to the stabilization problem in low orbit. Antenna loads are absent because the antennas are inoperative. Current loop/magnetic field interactions can exist only during self-powered transfer. b. Orientation Considerations As noted in the preceding section, a large, flat, solar- oriented array experiences substantial gravity gradient torques. These can be counteracted by a reaction control system. For the example square array, the propellant penalty is 110,000 kg per year to compensate for the cyclic torque and 68,000 kg per year for the long-term torque, assuming a specific impulse of 98,000 m/s (10,000 lb-s/lb). In addition to the cost of resupplying the propellant, the cloud of material surrounding the SPS will be increased to some extent by the expended propellant. It appears worthwhile, then, to explore ways of reducing propellant requirements. Since the first torque is cyclic with a reasonable period, control moment gyros (CMG) seem worth considering. However, using the same example as before, scaling up the rotor of the Skylab ATM gyro (same material and geometry) results in a rotor mass of 10? kg without considering mounting, actuators, etc. This is three times as great as the propellant supply for 30 years. Aside from the difficulty of achieving a 30-year lifetime, CMG technology would have to advance at least an order of magnitude to be weight-competitive.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==