Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Appendix L: Tether Boost Facility Design Final Report3 CONTROL STATION DESIGN3.1 ADCS/GN&C3.1.1 Design Requirements, Drivers, and AssumptionsAttitude determination and control system requirements are driven mainly by acombination of spin maneuver requirements and solar panel pointing requirements.The following are preliminary requirements based on the current top-level sizing doneto date.• Attitude Knowledge: +/- .05 deg / axis• Attitude Hold Capability (non-maneuvering): +/- 1.0 deg / axis• Attitude Maneuver Axis Accuracy: (spin axis): +/- 1.0 deg (non-spin axis):+/- 1.0 deg• Attitude Rate Accuracy (non-maneuvering): +/- 0.1 deg/sec/axis• Attitude Rate Accuracy (maneuvering): +/- 0.1 deg/sec/axis• Position Knowledge: (GPS Receive)• Velocity Knowledge: (GPS Receive)• Orbit Positioning Accuracy: Inclination, Ascending Node, Arg. of Latitude,etc.: TBDAttitude maneuver capability requirements for spinning the control station whilenot using expendable propellant will require the use of control moment gyros. Thefollowing discussion explains the design drivers for using CMGs and some possibleconcepts for desaturation that also do not use consumables.Reaction wheels provide control torques by changing wheel speed, ie.converting stored momentum into torque (Tj = d (hj)/dt). Control moment gyros(CMGs) provide control torques by changing the direction of the wheel momentumvector (gyroscopic torque). Both reaction wheels and CMGs can have large storedmomentum but the maximum reaction wheel torque available is limited by powerconsiderations, as shown in Figure 3-1.An example: RW Power (@ 6000 rpm) = 1 ft -lb @ 1300 watts or 3 ft-lbs @3900 wattsReaction wheels require large power to produce torques due to changingmomentum by accelerating/decelerating a spinning disc about a fixed axis. However,CMGs (see Figure 3-2) can produce very large torques by changing momentumvector direction (using fairly small input torques) of a large spinning disc. CMG discsusually take long periods for spin-up since the spin motors are not sized nearly aslarge as a reaction wheel motor.L-55