Dr. Aprille Ericsson Eric Stoneking June 28, 2001

Dr. Aprille Ericsson

Eric StonekingJune 28, 2001

SuperNova/ Acceleration Probe (SNAP)

Attitude Control Systems

SNAP, June 25-28, 2001Goddard Space Flight Center

ACSPage 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 MomentumWheel sizing and Wheel location Isolation PackageReviewed full labor costFuture studies/trades recommendation

Detailed jitter analysis and fuel analysis needs to be performed.

ACS Overview


ACSPage 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


ACSPage 4

Orbit: 19x57 Re-baseline Inclination: 65º Coordinates: Roll (Z) axis, instrument boresight

axisPitch (Y) axis, is sun pointingYaw (X) axis, YxZ=Xvelocity 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


ACSPage 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


ACSPage 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


ACSPage 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


ACSPage 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


ACSPage 9

ACS Component Recommendation

Component Model Qty

Mass

(kg)

Orbit

Avg

Power

(W)

Peak

Power

(W)

Night

Power

(W)

Coarse Sun Sensors Adcole 11866 8 0.037 0 0 0

Digital Sun Sensors Adcole 17061 2 0.644 0.13 1.4 0

Star Tracker Ball CT-602 2 10.8 16 18 16

Attitude Control Electronics (MAP ACE) 1 8 11 13 11

I nertial Reference Unit Litton SI RU (4 axis) 1 5.44 22 40 22

Reaction Wheels (30 Nms, 0.05 Nm) explorer (in house) 4 44 40 104 28

EVD for 8 thrusters MAP 1 3 13* 17 13*

I solation package Lord 4 0.45 0 0 0

Totals = 72.371 89.13 193.4 77*EVD power is insignificant during orbit


ACSPage 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 m2

Total momentum accumulated every day (worse case): 19.1 Nms

Total propellant mass required for momentum unloading per year: 3.5 kg


ACSPage 11

ACS Solar Torque Assessment

Momentum accumulated over 1 day vs cpcg

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00

cpcg-offset (m)

Tota

l Mom

entu

m (N

ms)

Htotal (Nms)


ACSPage 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 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 issueModel Cababilities Qty

Mass

(kg)

Power

Avg

(W)

Power

Peak

(W)

J itter

Amplitude

(arc- sec)

I thaco E-wheel 26 Nms, 0.3 Nm 4 42 120 600 1.65E-02

I thaco B-wheel 14 Nms, 0.05 Nm 4 23.6 60 240 1.30E-02

explorer (in house)30 Nms, 0.05 Nm 4 44 40 104 7.50E-03


ACSPage 13

ACS Vibration Isolation Package Consideration

Active just too expensive and involvedPassive, 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


ACSPage 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


ACSPage 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 m3 (based on 20.4 m2 area and 1.5 m cpcg offset)


ACSPage 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


ACSPage 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


ACSPage 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


ACSPage 19

ACS Labor Cost

Description Cost ($K)

Hardware analysis and Design

Sof tware analysis and Design

Systems analysis and Design

I ntegration & Testing

Lead Engineer

Total =

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


ACSPage 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): 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

Dr. Aprille Ericsson Eric Stoneking June 28, 2001

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