Dr. Aprille Ericsson Eric Stoneking June 28, 2001

1. SuperNova/ Acceleration Probe (SNAP) Attitude Control Systems Dr. Aprille Ericsson Eric Stoneking June 28, 2001

2. ACS Overview • Will meet the requirements with some modifications: • ACS can acquire the target within the instrument FOV. • The instrument will be used as the fine pointing sensor. • Tip Off and Solar Pressure Momentum • Wheel sizing and Wheel location • Isolation Package • Reviewed full labor cost • Future studies/trades recommendation • Detailed jitter analysis and fuel analysis needs to be performed. SNAP, June 25-28, 2001Goddard Space Flight Center

3. ACS Driving Requirements • Pointing Accuracy • Yaw & Pitch : 1 arc-sec (1) • Boresight Roll: 100 arc-sec (1) • Attitude Knowledge • Yaw & Pitch : 0.02 arc-sec (1) • Boresight Roll: 2 arc-sec (1) • Jitter/Stability -Stellar (over 200 sec) • Yaw & Pitch : 0.02 arc-sec (1) • Boresight Roll: 2 arc-sec (1) • Sun Avoidance • Earth Avoidance • Moon Avoidance SNAP, June 25-28, 2001Goddard Space Flight Center

4. ACS Driving Assumptions • Orbit: 19x57 Re-baseline • Inclination: 65º • Coordinates: Roll (Z) axis, instrument boresight axis Pitch (Y) axis, is sun pointing Yaw (X) axis, YxZ=X velocity vector is moving • Inertia (kg-m2) [3600, 3300, 2100] • Effective Area: 20.6 m2 • Tip off rate: Sea Launch & Delta III - 0.6º/sec • Slew 180 degrees in one hour including settling • 6 degree/minute slew rate • 30 minutes for settling with a 0.5 Hz bandwidth controller SNAP, June 25-28, 2001Goddard Space Flight Center

5. ACS Selected Configuration & Rationale • Control mode recommendation • Design Approach for science mode • Updated component recommendation (*) • Solar torque assessment (*) • Wheel sizing (*) • Isolation package (*) • Jitter analysis SNAP, June 25-28, 2001Goddard Space Flight Center

6. ACS Control Mode Recommendation • Science mode - • Three axis stabilized • Stellar pointed • Instrument shielded from sun • Use wheels to slew into position • Rate null/Sun acquisition - • Null the rate and point solar array normal to the sun • Use propulsion to damp the tip off rate and slew with wheel • Acquisition time is less than one hour, assuming 0.6 deg/sec tip off rate and 180 degree away from the sun • Safehold mode - • Use CSS and wheel to point solar array normal to the sun, similar to sun acquisition SNAP, June 25-28, 2001Goddard Space Flight Center

7. ACS Control Mode Recommendation continued • Eclipse mode - • Perform Delta H mode prior to eclipse period • Use Star Tracker, IRU and wheels to maintain position • Delta H mode - • Momentum unloading once or twice a day • Use thrusters to dump momentum and use wheels to slew into position • Delta V mode - • Use wheels to slew to burn position, perform delta V, then perform Delta H SNAP, June 25-28, 2001Goddard Space Flight Center

8. ACS Design Approach for Science Mode • Reaction wheels are used as control actuators, and for 180 degree slew (four wheels with the apex of the pyramid along roll axis) • Star Tracker and gyro are used as attitude sensors • Use Stellar Instrument guide signal as feed forward information to correct the steady state position error • Thrusters are used for wheel momentum unloading SNAP, June 25-28, 2001Goddard Space Flight Center

9. ACS Component Recommendation SNAP, June 25-28, 2001Goddard Space Flight Center

10. ACS Solar Torque Assessment Assumptions • Solar force equations from Wertz • Sun angle varies only with s/c pitch axis but assumed worse case of 90° • The radiant energy is either reflected or absorbed • Sunshield is a flat, specular surface • Net Solar Torque is along roll axis (Note: only considered a normal force contribution) • CG offset: 1.5 m • Sun exposed Area: 20.4 m2 • Total momentum accumulated every day (worse case): 19.1 Nms • Total propellant mass required for momentum unloading per year: 3.5 kg SNAP, June 25-28, 2001Goddard Space Flight Center

11. ACS Solar Torque Assessment SNAP, June 25-28, 2001Goddard Space Flight Center

12. ACS Wheel Sizing Criteria • Wheel torque capability is not an issue • Small solar torque, worse case is 2.22e-4 Nm • Slew 6°/minute requires torque of 0.024 Nm • Wheel momentum capability is an issue • Total momentum accumulated with 1 slew per day is 25.4 Nms • Need to bias speed at least a decade above the lowest structure mode (1 Hz) to avoid structural mode excitation • Need to have enough margins to avoid wheel saturation and zero crossing • Wheel power usage and wheel jitter are also an issue SNAP, June 25-28, 2001Goddard Space Flight Center

13. ACS Vibration Isolation Package Consideration • Active just too expensive and involved • Passive, no power required • Lockheed Martin Eureka Isolation System • Weight: 10 Kg • Heritage: STRV-2 spacecraft in the fall of 1997 • TRW Chandra Isolation Package • Weight: 5 Kg • No Heritage; Specific design for NGST/NEXUS • Lord Isolators (4) • Weight: 0.45 kg • Heritage associated with launch effects: OV-3, VCL, QuickTOMS • Should be placed under wheel assembly SNAP, June 25-28, 2001Goddard Space Flight Center

14. ACS Component Placement • Wheels shall be located as close to the center of mass as possible to reduce wheel induced jitter • Four wheel option shall be in pyramid configuration with the apex of pyramid along the roll axis • Star tracker’s boresight shall be perpendicular to the instrument boresight • Gyro shall be mounted on the tracker optical bench • Vibration isolation package should be placed under wheel assembly SNAP, June 25-28, 2001Goddard Space Flight Center

15. ACS Requirements Imposed On Other Sub-Systems • Lowest structural mode shall be 5 Hz, one decade higher than the controller bandwidth • Wheels and Propellant tank shall be as close to center-of-mass as possible • The product of area and cpcg offset shall not exceed 40 m3 (based on 20.4 m2 area and 1.5 m cpcg offset) SNAP, June 25-28, 2001Goddard Space Flight Center

16. ACS Technologies Required • New Generation Integrated Wheel • Impact on design • Assumed Dynamic & Static Imbalance disturbance torques and forces are based on the Triana wheel • Larger wheel may have somewhat higher disturbances • Alternative / Ithaco B-wheel • Higher Power Consumption • Higher disturbances • Feedback to technology developer • Jitter Requirements • Mass Target • Power Target • Momentum & Torque Requirements SNAP, June 25-28, 2001Goddard Space Flight Center

17. ACS Risk Assessment • Most of the hardware will be flight qualified, the risk of hardware failure is low • Wheels will be modified technology • Isolators do not have heritage for this application • Three axis stabilized spacecraft have been done so often that the risk of control failure is very low • Reliance on instrument star guide data adds complexity to mission but can be done SNAP, June 25-28, 2001Goddard Space Flight Center

18. ACS Issues and Concerns • Jitter • Isolate fundamental wheel frequency through detailed analysis from manufacturer • Must tune isolator - type, size and interface • Flexible mode Analysis • Require extensive analysis to avoid control/structure resonance • cpcg-cg offset • Smaller offset will minimize thruster firing frequency and propellant required for momentum unloading • Offset will migrate with mission life, will get better with fuel depletion • Fuel slosh Disturbance Analysis • Minimize fuel tank Cg offset • 3 jitter values • Use current Star tracker with a very accurate Kalman Filter • Augment Star Tracker data with instrument data for fine pointing • May need replace gyro with SKIRU-DII • Use of Instrument guide data • Possible mitigation by use of more sophisticated focal plane-sensors • Non-white and non-bias errors must be carefully accounted SNAP, June 25-28, 2001Goddard Space Flight Center

19. ACS Labor Cost Note: Estimated cost derived from existing programs, such as MAP. SNAP, June 25-28, 2001Goddard Space Flight Center

20. Attitude Determination & ControlSubsystem Summary • Technology Readiness Level: Bus=TRL9 except EVD & wheel=TRL7 • Type of Materials Used: Wheel - stainless steel • Mass (kg.): 73 kg • Orbit Average Power consumption (W): 118.1 W for average • Primary Sensors: Star Tracker, IRU, DSS, CSS • Stabilization Type: 3-axis stabilized • Flight Heritage: wheels-Triana, guide telescope-Trace & Nexus • Complexity: Middle • Risk: (Ease of fallback; Can we use another technology/process and not sacrifice performance?) Yes, modified explorer wheels SNAP, June 25-28, 2001Goddard Space Flight Center