X ray calorimeter mission
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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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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

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


Acs functional block diagram

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


Observatory coordinate system and key terms

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


Operations requirements

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


Requirements and considerations

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


  • Requirements and considerations cont

    Requirements and Considerations (cont)

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    Target

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    Pitch: +/-25, -25 

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    Sun

    Earth

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    Moon

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    Yaw:

    +/-180 

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    Roll:

    +/-10 

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    Acs modes sensors and actuators

    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


    Acs sensors and actuators

    ACS Sensors and Actuators


    Instrument pointing star tracker specifications

    Instrument Pointing Star Tracker Specifications

    Micro-Advanced Stellar Compass (μASC)


    Siru instrument pointing gyro specifications

    SIRU Instrument Pointing Gyro Specifications


    Honeywell reaction wheel

    Honeywell Reaction Wheel


    Coarse sun sensor adcole

    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


    Solar radiation torque and its effect on wheel momentum storage

    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


    Four reaction wheels momentum storage

    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)


    Slew capability

    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


    Pitch yaw error budget

    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)


    X ray calorimeter mission

    Thruster Configuration

    8 Thrusters canted 10⁰ in two planes

    Couple

    4 Thrusters canted 45⁰ in one plane

    Solar Array

    Notional CG

    Lines of Action


    Momentum unloading

    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


    Summary

    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


    X ray calorimeter mission

    Supplementary Slides


    Kalman filter performance

    Kalman Filter Performance


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