Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Appendix L: Tether Boost Facility Design Final Reporttorques in one direction. Another source of control torques (eg. RCS, or a secondaryset of RWs or CMGs) will be needed to remedy a situation like this.For MMOSTT, the control station mass is roughly 1/5 the mass of Skylab,however the moment of inertia of MMOSTT due to a large amount of solar panelsmay be as much as half that of Skylab. A calculation of inertias for an earlyconfiguration of the MMOSTT control station gives: Ixx = 218,000 kg-m 2 , Iyy =444,000 kg-m 2 , Izz = 647,000 kg-m 2 . With expansion modules the inertia mightreach fairly large values, thus a first approximation for CMG sizing is roughly half thatof Skylab. Once the MMOSTT control station is brought up to maneuver rate aboutthe spin axis, the station itself will have a large momentum vector which will allowattitude control about the two non-spin axes with fairly small torques. This scaled-upversion of gyroscopic control will need to be studied in detail in the next study phase.To solve the problems of desaturation without use of consumables (i.e., an RCSsystem) will require a combination of attitude management and secondary torques.Attitude management techniques may be used to cancel bias torques that areattitude dependent by rotating the vehicle (180 deg) to produce disturbance torquesin the opposite direction. Other techniques for non-maneuvering vehicles that areused are balancing two disturbance torques (e.g., aerodynamic and gravity-gradient)however, for MMOSTT, the large spin maneuver will complicate the use of balancingdisturbance torques. Another possible desaturation technique would be to use acombination of reaction wheels and energy storage transfer techniques with thepower system. Two reaction wheels per axis spinning in opposite directions wouldbe used to add desaturation torque when necessary and each reaction wheel wouldbe chosen depending on required torque direction and whether energy would beadded or captured in the required reaction wheel.Some other possible secondary torque sources are magnetic torquers,aerodynamics, and tether attachment torques (would require tether thrust-line offsetwith respect to control station cg). Magnetic torquers might be very massive but theyare used in many satellites for nutation control, and if sized large enough could beused. Aerodynamic torques would only be present during the perigee part of theorbit and might not be large enough to help. The current mass properties reportincludes the CMGs for attitude control but does not reflect the secondary systemsnecessary for desaturation. Techniques for desaturation will need to be studied indetail in the next study phase.3.1.3 Trades and RecommendationsCurrent and future developments in GNC sensors are expected to drive the sizeand mass of sensor units to extremely small values, thus the major driver in GNCsizing is expected to be the CMGs, RWs, etc. For instance, star trackers used on theClementine vehicle were small and weighed only 3 lbs each including sun shadecover. Current small GPS/IMU systems weigh less than 10 lb.The above discussion can be summarized as:L-57