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

SNAP, June 25-28, 2001 Goddard Space Flight Center ACS Page 2  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. ACS Overview

SNAP, June 25-28, 2001 Goddard Space Flight Center ACS Page 3  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 ACS Driving Requirements

SNAP, June 25-28, 2001 Goddard Space Flight Center ACS Page 4  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-m 2 )[3600, 3300, 2100]  Effective Area:20.6 m 2  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 ACS Driving Assumptions

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

SNAP, June 25-28, 2001 Goddard Space Flight Center ACS Page 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 m 2  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, 2001 Goddard Space Flight Center ACS Page 11 ACS Solar Torque Assessment

SNAP, June 25-28, 2001 Goddard Space Flight Center ACS Page 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 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, 2001 Goddard Space Flight Center ACS Page 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, 2001 Goddard Space Flight Center ACS Page 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, 2001 Goddard Space Flight Center ACS Page 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 m 3 (based on 20.4 m 2 area and 1.5 m cpcg offset)

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

SNAP, June 25-28, 2001 Goddard Space Flight Center ACS Page 20 Attitude Determination & Control Subsystem 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): 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