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SNAP Attitude Control System Overview

This detailed overview encompasses the ACS driving requirements, assumptions, selected configuration, and control mode recommendations for SNAP conducted at Goddard Space Flight Center in June 2001. It includes analyses, recommendations, and configurations, aiming to ensure optimal spacecraft performance and stability in various operational modes. With a focus on pointing accuracy, yaw, pitch, and roll, this overview provides essential insights into the attitude control system's design, functionality, and driving factors.

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SNAP Attitude Control System Overview

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  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

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