X-Ray Gratings Mission

1. X-Ray Gratings Mission Attitude Control Dave Olney, Scott Starin, Joe Garrick Attitude Control Systems Engineering Branch Code 591 19 – 23 March 2012

2. 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 12 Total Momentum Management Orbit Maintenance

3. ACS Summary • 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 two heads • A star tracker head is used for alignment between science instrument • Second star tracker head is used for attitude determination in inertial frame • Actuators • Thrusters • 12 – 4 N (1 lbf) thrusters for attitude control and orbit maneuvers • Thrusters on full for orbit maneuvers and off pulsed for attitude control • Thruster on pulsed for attitude control during momentum unloading • Reaction Wheels • 4 – 50 Nms, 0.2 Nm reaction wheels • Used for attitude actuation

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 103 to 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 (not required by NASA) • 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 • 5 year mission lifetime at L2 • Attitude Requirements • Pointing: 6 arcsec, Pitch/Yaw (3-sigma); Roll number not provided by customer • Knowledge: 1.3 arcsec, Pitch/Yaw (3-sigma); Roll number not provided by customer • Jitter: 0.2 arcsec, all axis (3-sigma) over 2 seconds Slew Requirement • Complete a 60 degree (yaw) slew in 60 minutes (including settling) • Considerations • Impact of changing pitch to as much as +/- 45 degrees • CP-CG Offset- Impact on SC design • Use of solar panels to balance CP-CG offset • 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. 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

13. Spacecraft Mass PropertiesDeployed Configuration C.G. 163 cm origin

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) coincident for a pitch angle = 0 degrees. For non-zero pitch the Cp and Cm will not coincide. reference Cm Cp 0.143 m 25 o Projected silhouette toward sun

15. Momentum Unloading • Solar panel locations have been adjusted to place the center-of-pressure (as viewed from the sun) close to the center-of-mass making the solar pressure torque negligible at a pitch angle of zero. • For other pitch angles, however, the torque is not zero, consequently, angular momentum (= integral of torque over time) stored in the attitude control reaction wheels will grow over time. Over time wheel speeds can saturate and loss of attitude control will occur unless either a momentum unloading is performed or the torque is reversed (by a 180 degree yaw). The growth rate is maximum if the yaw angle is constant. Scheduling science observation targets which require large changes in yaw angle will mitigate this effect due to the torque reversing effect. • The worst case accumulation is at the maximum expected pitch angle of 25 degrees holding yaw fixed (inertially). At this attitude, in 30 days the total accumulation would be 81 Nms which exceeds the individual base-lined wheel capacity of 50 Nms. • Some of this capacity (~5 nms for a 60 deg/60 min slew rate) must be reserved for angular momentum storage during a slew. • Four operating wheels provide 65 Nms .

16. Four Reaction Wheels Arranged on the Faces of a Pyramid 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) Momentum per wheel = 50 Nms Torque per wheel = 0.2 Nm Torque along each body axis = [ 0.49 0.49 0.4] Nm Plot at right shows momentum capacity sphere in body = 65.5 Nms (use this for accumulation of momentum) Maximum momentum capacity on each body axis = [ 122.5 122.5 100.0 ] Nms (use this for slew time computations)

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 rigid body response; no flex body jitter included

18. Roll Error Budget SUM SUM SUM SUM Note: RWA jitter is rigid body response; no flex body jitter included

19. Kalman Filter Performance

20. Thrusters

21. Summary Potential future work: • Consider whether an aperture cover should be incorporated to be deployed whenever a safehold occurs. Issues/concerns: • None identified Considerations for CAT configuration: • Change in mass does not have significant impacts on ACS design or performance