SNAP ACS

Slide1: 

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

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. ACS Overview

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 ACS Driving Requirements

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 ACS Driving Assumptions

ACS Selected Configuration & Rationale: 

ACS Selected Configuration & Rationale Control mode recommendation Design Approach for science mode Updated component recommendation (*) Solar torque assessment (*) Wheel sizing (*) Isolation package (*) Jitter analysis

ACS Control Mode Recommendation: 

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

ACS Control Mode Recommendation continued: 

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

ACS Design Approach for Science Mode: 

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

ACS Component Recommendation: 

ACS Component Recommendation

ACS Solar Torque Assessment Assumptions: 

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

ACS Solar Torque Assessment: 

ACS Solar Torque Assessment

ACS Wheel Sizing Criteria: 

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

ACS Vibration Isolation Package Consideration: 

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

ACS Component Placement: 

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

ACS Requirements Imposed On Other Sub-Systems: 

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)

ACS Technologies Required: 

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

ACS Risk Assessment: 

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

ACS Issues and Concerns: 

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

ACS Labor Cost: 

ACS Labor Cost Note: Estimated cost derived from existing programs, such as MAP.

Slide20: 

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): 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