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X-Ray Calorimeter Mission

X-Ray Calorimeter Mission. Attitude Control Philip Calhoun, Dave Olney, Joe Garrick Attitude Control Systems Engineering Branch Code 591 2 – 6 April 2012. ACS Overview. Sensors Coarse Sun Sensor (CSS) 8 units aligned to provide 4 π steradian coverage of sky

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X-Ray Calorimeter Mission

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  1. X-Ray Calorimeter Mission Attitude Control Philip Calhoun, Dave Olney, Joe Garrick Attitude Control Systems Engineering Branch Code 591 2 – 6 April 2012

  2. ACS Overview • Sensors • Coarse Sun Sensor (CSS) • 8 units aligned to provide 4πsteradian coverage of sky • Used for Safe mode sun acquisition • Gyro • 1 internally redundant unit • Sense the attitude rate of change • Used in Kalman Filter to propagate and smooth state • Star Trackers • 1 electronics and four heads • A star tracker head is used for attitude determination (inertial frame) • Second star tracker head is used for alignment between science instrument components • Two pairs of star trackers provide fully redundant capability • Actuators • Thrusters • 12 – 4.5 N (1 lbf) thrusters for attitude control and orbit maneuvers • Thrusters full on for orbit maneuvers, off-pulsed for attitude control • Thruster on-pulsed for attitude control during momentum unloading • Reaction Wheels • 4 – 75 Nms, 0.2 Nm reaction wheels • Used to attitude actuation

  3. ACS Functional Block Diagram Actuators Sensors Reaction Wheels 4 total (pyramid about Z) ACS FSW CSS Tot.8 Safe Mode Att. Determination Gyro Attitude Control Mission Attitude Determination Star Tracker Thrusters 12Total (See slide #18) Momentum Management Orbit Maintenance

  4. Observatory Coordinate System and Key Terms Observatory Coordinate System Origin is at the Mirror Node +YOBS points from the Mirror Node, forming a right handed orthogonal frame with X and Z Y is the axis for PITCH (side S/A’s are aligned w/ Y) Target +ZOBS points from the Mirror Node to the Target Z is the axis for ROLL (the Boresight is aligned w/ Z) “FORE” is the +Z (FMA) end of IXO “AFT” is the –Z (Instruments) end of IXO +XOBS points from the Mirror Node towards the Sun, X is the axis for YAW Sun

  5. Operations Requirements • Launch • Direct insertion into transfer orbit in full sun with continuous ground contact (have TDRSS capability) • Indefinite duration safe mode available immediately after LV separation • Deployments start right after LV separation • Cruise to L2 • Start with one month commissioning phase for checkouts, calibrations • Continuous DSN contacts during commissioning, then twice daily for 30 minutes for OD during cruise • Correction burns as required • Mirror cover deployed after observatory outgassing • No exposure of aperture to sun light allowed for remainder of mission • Science observations may start during cruise • L2 Insertion • Performed in Operational configuration (10-3 g level forces only) • Observations • Pointing at a target for < 106 seconds • 1 – 20 observations per week, re-pointing accomplished in less than an hour • Observing efficiency 85% • EOL disposal • Passivate observatory, impart 1 m/s towards deep space • Mission Ops • Highly autonomous observatory, 8 x 7 ground staffing • Data latency 2 weeks required, 72 hours goal from completion of observation to product delivery, excludes bright source observations

  6. Requirements and Considerations • Requirements • 3 year (5 year goal) mission lifetime at L2 • Attitude Requirements • Pointing: 10 arcsec, Pitch/Yaw (3-sigma); Roll number not provided by customer • Knowledge: 3 arcsec, Pitch/Yaw (3-sigma); Roll number not provided by customer • Jitter: 1 arcsec, all axes (3-sigma) over 1 seconds Slew Requirement • Complete a 60 degree (yaw) slew in 60 minutes (including settling) • Considerations • Impact of changing pitch field of regard to +/- 45 degrees • Impact of Solar force center of pressure (CP) to Center of Mass (CM) offsets • Assumptions • HGA not slewing during science observations • Adequate calibration slews and observation time for sensor alignments

  7. Requirements and Considerations (cont) . . . Target . . . . . . . . . . . . . Pitch: +/-25, -25  . . Sun Earth H . . . . . . . . . . . Moon . . Yaw: +/-180  . . . . . . . . . . . . . . . . . . . . Roll: +/-10  . . . . . . . . . . . . . . .

  8. ACS Modes, Sensors and Actuators • Orbit Adjust Maneuvers (Delta-V Mode) • Gyros and thrusters • Full-on thrusters for orbit maneuvers • Off-pulse thrusters for attitude control • Momentum Unloading (Delta-H Mode) • Gyros, thrusters and wheels (spin down) • Attitude control on thrusters as wheels spin down to commanded momentum level • Science/Mission (Science Mode) • Gyros, star tracker, wheels, Kalmanfilter • Inertial Pointing (point at sun during coast phase; target pointing during science) • Calibration slews (science and ACS sensor alignments) • Re-pointing slews (slewing to next science target) • Safe (Safehold Mode) • CSS, gyros and wheels • Point solar array at the sun

  9. ACS Sensors and Actuators

  10. Instrument Pointing Star Tracker Specifications Micro-Advanced Stellar Compass (μASC)

  11. SIRU Instrument Pointing Gyro Specifications

  12. Honeywell Reaction Wheel

  13. Coarse Sun Sensor: Adcole • Adcole analog CSS detectors provide close to 2πsteradiancoverage (85 deg half angle) • 8 detectors distributed across theobservatory provide near full4πsteradiancoverage • 4 CSS on SA panel facing sunward • 4 CSS on body facing anti-sunward

  14. Solar Radiation Torque and its Effect on Wheel Momentum Storage • Solar Array size and position have been selected to make the center of mass (Cm) and center of pressure (Cp) nearly coincident along longitudinal axis for a pitch angle = 0 degrees • Cp / Cm offset • along X axis (toward Sun) ~= 1m • along Z axis (11.5 cm) • For pitch ≠ 0the Cp and Cm will not coincide •  increased Solar Torque Baseline (Pitch = 25 deg) Ref Cm Cp 0.43 + 0.115 m Projected silhouette toward sun 25 o

  15. Four Reaction Wheels – Momentum Storage • RWA body alignment • RW1 RW2 RW3 RW4 • | 0.6124 0.6124 -0.6124 -0.6124 | • | 0.6124 -0.6124 0.6124 -0.6124 | • | 0.5000 0.5000 0.5000 0.5000 | • (bias to X and Y axis, momentum accumulates mainly on these axis) • Baseline (Pitch < 25 deg) • Momentum per wheel = 100 Nms, (HR-16) • Torque per wheel = 0.2 Nm • Plot shows momentum capacity (min = 130 N-m-sec) • > 165 Nms within adequate range of +/- Y axis • Trade (Pitch < 45 deg) • Momentum per wheel = 125 Nms (HR-16) • Torque per wheel = 0.2 Nm • Impact of 125 N-m-sec wheel • mass & imbalance increase Allowable Range for Momentum Buildup along Y axis Momentum Capacity ( 1 whl = 100 N-m-sec) Elevation (deg) Azimuth (deg)

  16. Slew Capability • Requirement: Slew 60 deg in 60 deg (allow time to settle) • Design: • Goal: Slew should not significantly impact momentum / control authority usage • Use 5% of minimum Momentum Capacity (~6.5 N-m-sec) • Use 10% of torque capability (~0.02 N-m) during ramp • 5 min ramp time to minimize slew transients • Slew completes in 58 min, 2 min for settle

  17. Pitch / Yaw Error Budget Uncorrelated items can be RSSed but a more conservative estimate is to Sum items SUM SUM SUM SUM Few things in our favor: Large inertias (torque produces small angles) Motion in star tracker filters out biases Large observation times Note: RWA jitter is allocation (based on experience of similar missions)

  18. Thruster Configuration 8 Thrusters canted 10⁰ in two planes Couple 4 Thrusters canted 45⁰ in one plane Solar Array Notional CG Lines of Action

  19. Momentum Unloading • Thruster Isp = 210 sec • Wheel torque = 0.2 Nm • Momentum accumulation = 125 N-m-s about Y axis (worse case) every 21days (Baseline: Pitch < 25 deg) = 180 N-m-s about Y axis (worse case) every 21 days (Pitch < 45 deg) • For 5 year mission, every 21 days is about 87 unloads • Max Thruster torque about each axis ~= +/- [1.4, 8, 8 ] N-m • Minimum on time thrusters = 5 m-sec • Thruster Pulsing Duration = 15.6 sec of thruster on-time to remove 125 Nms (Baseline: Pitch < 25 deg) • = 22.4 sec of thruster on-time to remove 180 Nms (Pitch < 45 deg) • Accuracy of unloading to < 0.03 N-m-s • Time to spin down wheels = 125 / 0.2 = 625 sec, about 10.5 minutes (Baseline) 180/ 0.2 = 900 sec, about 15 minutes (Pitch < 45 deg) • Fuel usage = Total momentum (5 yr) / ( Isp * r * 9.8) = (125*87) / (210 * 2 * 9.8) = 2.6 kg (baseline) = (180*87) / (210 * 2 * 9.8) = 3.8 kg (baseline) • Assume r = 1m

  20. Summary Potential future work: • Need Jitter Assessment to determine impact to Pointing Budget • Reaction Wheel Imbalance • Cyrocooler • Passive isolators could be used to reduce jitter if needed • Ex: Chandra Reaction wheels mounted on isolator • Evaluate launch vehicle tip-off rate damping, thrusters and/or wheels Issues/concerns: • Momentum Buildup due to Solar Pressure is driving to larger wheel size for off pointing about Pitch axis  Jitter increase

  21. Supplementary Slides

  22. Kalman Filter Performance

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