NASA

Technical

Memorandum

NASA TM- 100389

FEASIBILITY OF USING EXTREME ULTRAVIOLET

EXPLORER REACTION WHEELS TO SATISFY SPACE

INFRARED TELESCOPE FACILITY MANEUVERREQUIREMENTS

By W. D. Lightsey

Preliminary Design Office

Program Development Directorate

February 1990

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t ASANat_ona! Aeronautics andSpace Adm;nistrabon

George C. Marshall Space Flight Center

MSFC- Form 3190 (Rev. May 1983)

https://ntrs.nasa.gov/search.jsp?R=19900010240 2020-06-13T14:50:29+00:00Z

TABLE OF CONTENTS

Page

INTRODUCTION .............................................................................................. 1

PSC SIMULATION ............................................................................................ 1

HELIUM SLOSH ............................................................................................... 6

SIMULATION RESULTS .................................................................................... 7

CONCLUSION ................................................................................................. 17

PRECEDING PAGE BLANK NOT FILMED

iii

LIST OF ILLUSTRATIONS

Figure

I.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

Title Page

SIRTF maneuver requirements .................................................................... I

PCS block diagram (ideal system) ................................................................ 2

Gyro dynamic model ................................................................................ 2

Solar array dynamic model ......................................................................... 3

Solar pressure disturbance torque calculation parameters ...................................... 3

Wraparound spacecraft ............................................................................. 4

Reaction wheel configuration ...................................................................... 5

Maneuver acceleration command profiles ....................................................... 5

Slosh disturbance signal ............................................................................ 7

Fluid/tank interaction ............................................................................... 7

Right ascension change for 7-arc-min maneuver ................................................ 8

Right ascension attitude error ...................................................................... 9

Right ascension attitude error (magnified view) ................................................. 9

Declination error signal ............................................................................. 10

Roll error signal ...................................................................................... 10

Reaction wheel torque profiles .................................................................... 11

Change in roll attitude for 45-degree roll maneuver ............................................ 12

Roll attitude error .................................................................................... 13

Right ascension error (magnified view) .......................................................... 13

Declination error signal ............................................................................. 14

Roll error signal ...................................................................................... 14

Reaction wheel torque profiles .................................................................... 15

Wheel No. 3 momentum profile, 7-arc-min maneuver ......................................... 16

Wheel No. I momentum profile, 45-degree roll maneuver .................................... 16

iv

TECHNICAL MEMORANDUM

FEASIBILITY OF USING EXTREME ULTRAVIOLET EXPLORER (EUVE)REACTION WHEELS TO SATISFY SPACE INFRARED TELESCOPE

FACILITY (SIRTF) MANEUVER REQUIREMENTS

INTRODUCTION

This effort investigates the feasibility of using the extreme ultraviolet explorer (EUVE) reac-

tion wheels to provide the control torques for the space infrared telescope facility's (SIRTF) atti-

tude control system (ACS). Use of the EUVE reaction wheels will result in a lighter ACS than if

space telescope (ST) reaction wheels are used. This lighter ACS is desirable since the high altitude

(100,000 km) SIRTF vehicle is weight critical.

In September 1989 the maneuver requirements for SIRTF were revised. Figure I shows a

summary of the "new" maneuver requirements along with a summary of the previous requirements.

Under the "old" requirements the 120-degree slew in 480 s is the most demanding of the slew

maneuvers in terms of the actuator torque required to perform the maneuver. Under the "new"

requirements the 7-arc-min maneuver in 30 s becomes the reaction wheel torque "driver." This

7-arc-min maneuver is the same for both the "old" and "new" requirements. Since the "new" roll

requirement is less stringent than the "old," the "new" requirements as a whole are relaxed as

compared to the "old" requirements.

OLD

SLEW 120 ° IN 480 SEe

30.0 ARC-SEC ACCURACY

0.15 ARC-SEC STABILITY

NEW

SLEW 180 ° IN i000 SEC

0.25 ARC-SEC ACCURACY

0.15 ARC-SEC STABILITY

SLEW 7 ARC-MIN IN 30 SEC

0.25 ARC-SEe ACCURACY0.15 ARC-SEe STABILITY

ROLL 67.5 ° IN 270 SEC

30.0 ARC-SEC ACCURACY

0.15 ARC-SEC STABILITY

SLEW

ROLL

7 ARC-MIN IN 30 SEC

0.25 ARC-SEe ACCURACY

0.15 ARC-SEC STABILITY

45 ° IN 600 SEC

0.25 ARC-SEC ACCURACY

0.15 ARC-SEC STABILITY

Figure 1. SIRTF maneuver requirements.

PCS SIMULATION

A digital simulation was developed to analyze SIRTF's pointing control system (PCS).

Figure 2 shows a simpified block diagram of the PCS implemented in the simulation. Maneuvers

are controlled by the maneuver acceleration command _bc shown in the block diagram. The gains

Kt, Kp, and Kn are normalized with respect to the vehicle inertia so that inertia changes will not

require the gains to be changed.

UMITER

INTEGRALUMITER GAIN

PROPORTIONAL

RATE GAIN

F ....VB't_LE

TdlM

Figure 2. PCS block diagram (ideal system).

VeHCL__.O_CS-- --]

-I

Simple models were used for the actuators and sensors. Figure 3 shows the second-order

dynamic model used for the gyros. The simulation also includes a quantization level and a

sampling period for the gyros. A second-order model was also used to represent a single vehicle

bending mode about each axis. The vehicle rate resulting from the bending mode is added to the

rigid-body vehicle rate at the gyro node.

Figure 4 shows the model used to represent the solar array bending. As with the bending

mode models, the resultant vehicle rate caused by the solar array bending is added to the vehicle

rate at the gyro node. Since solar pressure will be the dominant environmental disturbance at the

high SIRTF orbit (100,000 km), a solar pressure disturbance is included in the simulation. Since

the viewing constraints of SIRTF will keep the solar array within 30 degrees of perpendicular to

the Sun line, a constant value was assumed for the solar pressure torque. The parameters used in

the solar pressure calculation are shown in Figure 5.

___! I c0.IS2+2 _('_nS+ ('0n_ I

(JL)s+ 2 _ COn fJ)s + f-On2 (JL)s= O)n 2 (L)v

(_:) 1-2_0)n -0_0n21 /('bs_+I(j0n21

Q)n = 27.96H z ; _ --- 1.0

Figure 3. Gyro dynamic model.

2

CGVEHICLE

T K

$2+ 2 _ (onS + O)n2m,,.--

(on = 0.5 Hz

= 0.005

K= 4.9 Xl0 "7 N/m

Figure 4. Solar array dynamic model.

2.94m /

-q--- 1.2m---_

i

WRAP AROUND,"---'-- EQI IIPMENT MODULES

(A2)

I

I

I

q

I

I

I

I

d

I

q

I

I

....... |

I

I

%%

I I

I I

I

I

I

I

I J

t II

I

i

pl

--. 3.98m

SOLARARRAY (As*`)

2.79m

As,, =11.14m 2A:_=3.53m2XSA=2mX2---O.Sm

TORQUE DUE TOSOLAR PRESSURETsp=9.27 X 10-sN.M.APPROXIMATELY CONSTANT(SA ALWAYS WITHIN 30° OF SUN)

Figure 5. Solar pressure disturbance torque calculation parameters.

3

The reactionwheelsare assumedto be ideal exceptfor a torquequantizationlevel and amaximum-outputtorquelimit. The simulationalsocontainsa distribution law which attemptstodistribute the residualwheelmomentumequally amongthe four reactionwheels.

Many of the parametervaluesusedin the modelspreviouslydiscussed(i.e., damping,natural frequencies,etc.) come from ST data.WhereST datadid not seemreasonableor wasnotavailable, valueswere assumedbasedon intuition. This wasnecessarybecause,at this early stage,actual valuesfor manyof the parameterssimply arenot available.

The vehicleconceptusedin the simulationis shownin Figure 6. This is the "wraparound"conceptwherethe spacecraftstructureis mountedaboutthe peripheryof the aft end of the tele-scope,and the solar array is side-mountedto the telescope.The vehicle inertiasaregiven in thefigure.

The reactionwheel configurationusedin the simulationis shownin Figure7. This con-figuration is adaptedfrom ST. Mathematically,the arrangementis equivalentto a pyramidalcon-figuration, but the physicalarrangementis due to packagingconsiderationson ST. Under the "old"maneuverrequirementsit wasnecessaryto increasethe 20-degreeinclination angle(Fig. 7),placing more torque along the roll axis, in order to meet the roll maneuverrequirement.Becauseof this, the torque availablefor the slew maneuverswas reduced.Under the "new" requirements,the roll maneuveris lessstringent, allowing the inclination angleto be returnedto its 20-degreevalue.

/-- 80LAR ARRAYAND

BUN SHADE

CENTAURIHTERFACE

L216.929

_. 711oSt._m7 X_2)

I¢, 7m _._2110,m xg-_)

Figure 6. Wraparound spacecraft.

4

Vt (LINE OF SIGHT)

V3

IFigure 7. Reaction wheel configuration.

The maneuver acceleration command profile _c (Fig. 2) is shown in Figure 8. This profile

determines the reaction wheel torque profile. Figure 8(a) shows a smooth profile with no dwell

time at peak acceleration. This is a jerk-minimizing profile that is desirable because it will

minimize the excitation of bending modes. The modified profile of Figure 8(b)-was necessary

under the "old" maneuver requirements. The reaction wheels were ramped up to their peak torque

value and held at that value for a period of time. This was necessary because of insufficient con-

trol torque. This profile results in increased jerk on the vehicle and may not even be feasible if the

ramp-up time becomes smaller than that which the actual reaction wheels can provide. This mod-

fled profile is not required to meet the "new" maneuver requirements. A similar profile could be

used, however, if an increased settling period at the end of a maneuver becomes necessary. This

would be a trade since the modified profile increases vehicle disturbance and may require a longer

settling time.

M z 10 .4

A O.8-N

E 0.6-UVE 0.4-R

0.2.AC •C 0.0-.

C -0.2OMM -0.4AND -0.S

(r, eV_-0.8

100

\

M x 10 .4

A 0.4.N

E 0.3

V

E 0.2R

A 0.1

C

C O.0

_ -0.!

MM -11.2!

AND -O.3

-0.4

02oo 3oo 4oo so0 6o0 7oo loo 2oo 3oo 400 5oo

Tree(SEC) _ME(SEC)

(a) ST reaction wheels. (b) EUVE reaction wheels.

Figure 8. Maneuver acceleration command profiles.

6OO 7OO

HELIUM SLOSH

A major concern regarding attitude control is the effect that the superfluid helium motion

will have on the vehicle attitude. Little data can be found concerning the behavior of superfluid

helium in a zero-g environment. The superfluid helium exhibits properties uncommon to those of a

"normal" fluid such as water. Because of this, propellant slosh models derived for "normal" fluids

may be useless for modeling superfluid helium "slosh."

In order to get some idea of the settling times that may be required at the end of maneuvers

due to the helium slosh, a highly simplified slosh disturbance model is assumed. This model strives

to represent a "worst case" disturbance that might occur due to sloshing, but in no way attempts to

represent the actual dynamics of the superfluid helium. If acceptable performance can be obtained

under the "worst case" model, then it is not likely that the disturbance due to the actual helium

slosh will adversely affect the vehicle's pointing ability.

The disturbance torque due to sloshing that occurs at the end of a maneuver is assumed to

be in the form of a sinusoid that decays over time, and is expressed as

Ta = T, lo e --{c°'r sin (teat) .

A typical disturbance signal Ta is shown in Figure 9. Since little is known about the

behavior of the helium, values must be assumed for co, and _. The values used in this effort are

co, = 0.25 Hz and _ = 0.1. To obtain a value for Tao the entire fluid mass of 580 kg is assumed

to be a point mass (Fig. 10). This mass is assumed to be moving relative to the tank wall at a

velocity equal to the maximum tangential velocity that it would obtain during the maneuver. Any

friction or interaction between the fluid and tank is neglected. At the instant the maneuver termi-

nates, the fluid collides with the tank wall, transferring its momentum to the vehicle. Using an

impact duration of 3 s along with the vehicle geometry, a value for Ta,, of approximately 0.05 N-mresults.

Although the above model is intended to represent a "worst case" slosh disturbance, several

assumptions were made and some guess work was required to derive the model. In light of this, it

is questionable whether or not the model is truly a "worst case" model. It is probably best to state

that, rather than neglecting slosh entirely, some disturbance model is included here, and the

performance of the system in the presence of this disturbance can be assessed. A parametric study

could result in some "maximum allowable disturbance" being determined.

O. 10

0.08

f 006

OS 004L0S 0.02H

000

N-0.02

M

-O04

-OO8

!

1" r - i

60 62 64 66 68 70 72 74 76 78 80

TIME (SEC)

Figure 9. Slosh disturbance signal.

Figure 10.

7"e4a//¢

Fluid/tank interaction.

SIMULATION RESULTS

A 7-arc-min right ascension slew is shown in Figure 11. To simulate a "limiting case" slew

maneuver, the roll attitude is fixed at 45-degrees and the declination change is zero. In this con-

figuration two of the reaction wheels are perpendicular to the maneuver axis and, therefore, cannot

contribute any torque along this axis. This leaves two reaction wheels to bear the load of

maneuvering the vehicle. There exists an infinite number of these "limiting" cases. The rightascension maneuver was used to make the results easier te visualize.

7

RI

GHT

A

SCEN

SI0N

DEO

0.12

0. t0

0.08

006

0.04

0.02

0.00

O_ POOR QUALITY

/_ i

/

JL

r [ .... r

3O 40 50

TIME (SEC)

60 70 80 90 t00

Figure 11. Right ascension change for 7-arc-min maneuver.

The maneuver begins at 40 s into the run and terminates at t = 60 s, allowing a 10-s

settling period. Figure 12 shows the right ascension attitude error being driven to zero at t = 60 s.

In Figure 13 this plot has been magnified at the maneuver ending period. It can be seen that at the

end of the settling period (t = 70 s) the error is well within the 0.15 arc-s stability and 0.25 arc-s

accuracy requirements.

Figure 14 shows a similar magnified view for the declination error. The effect of the slosh

disturbance is seen here. At t --- 70 s, the pointing requirements are being satisfied, but the

stability is only marginally within its bounds. The roll error is shown in Figure 15. Again the slosh

disturbance shows up in the roll response. The stability requirement is again marginally satisfied.

Notice that the disturbance torque effects do not show up in the right ascension error

response (Fig. 13). This is because the same disturbance signal is being applied about each of the

vehicle axes. Since the roll altitude is 45 degrees, the disturbance components along the right

ascension axis cancel while the components along the declination axis complement each other.

The reaction wheel torque signals are shown in Figure 16. Notice that wheels one and two

are not being used during the maneuver since they are perpendicular to the axis about which the

vehicle is being rotated. Wheels three and four reach peak torque values of about 0.23 N-re. The

maximum torque available from the EUVE reaction wheels is 0.296 N.m. Therefore, a small mar-

gin exists which could be used to provide a slightly longer settle-out period by ending the

maneuver sooner. Ideally, this is feasible, but in the real world some torque margin will be neces-

sary. (Available torque is a function of wheel speed.)

RI

OHT

ASCENSI

0N

ERR0R

ARCSEC

550

300

25O

200

150-

100-

50-

0-

0

/10 20 30 40 50 60 70 80

TIME(SEC)

Figure 12. Right ascension attitude error.

9O 100

0.01R

I

G 0.00HT

A -0.01SCE -0.02NS

I -0.030N

E -O.04RR0 -0.05

R

- -0.06AR

C -0.07SEC

60 65 70 75 80 85 90 95

TIME (SEC)

Figure 13. Right ascension attitude error (magnified view).

100

9

ORIGINAL F,_GE _S

OF POOR QUALITY

0.20 -

D

E 0.15C

L

I 010NA

I 0.050 F '

N j0.00 -- _ _ A ............

E l V"/'v

R-o.o5 J J0R

-010 .......................................

ARC -0.15 lS

cE -0.20

- 60 6.5 70 75 80 $5 90 95[

TIME (SEC)

100

Figure 14. Declination error signal.

0.15

0. t0

R0L 0.05L

E 0.00RR0R -0_05

AR -0.10CSEC -0.15

-0.20

J60 65 70 75

JI - • | "r

80 85

TIME (SEC)

Figure 15. Roll error signal.

90 95 100

!0

o

i

i

' o

l

o

o._

0

I

0 .o 0 (D 0 0

I I i

o

!o

i

o

0

t1 °

i

Fo

L

i°

t_

o o 0 0 d o o o o 0 oI I I i

_TuJun.J -_ _--O(ZC_Dua J Z _ I

<D

0

o]

,o on

7

li°

0 o 0 0 o o o 0 o o oI I I I I

E

.,I=

0

I!

Figures 17 through 22 show the resultsof the 45-degreeroll maneuver.In this casethemaneuverbeginsat t -- 50 s and terminates at t = 620 s, allowing a 30-s settling period. Since

the roll attitude is 45 degrees at the end of the maneuver, the slosh disturbance is once again

cancelled about the right ascension axis. By the end of the settling period (t = 650 s), all three

error signals have settled well below their required values.

A look at the wheel torques in Figure 22 shows that the peak torque of each wheel duringthe maneuver is about 0.13 N-m, well below the 0.296 N.m limit. Because of this, the inclination

angle in the reaction wheel configuration could be reduced from 20 degrees, thus providing more

torque for slew maneuvers. This would allow an increase of the settle-out period for the 7-arc-rain

slew maneuver, thus allowing the stability requirement to be better satisfied.

In addition to having sufficient control torque to perform maneuvers, the momentum

capability of the EUVE reaction wheels must also be sufficient. Figure 23 shows the momentum of

reaction wheel three for the 7-arc-min slew maneuver. The wheel four momentum profile is similar

(since the wheels were torqued the same) and the momentum of wheels one and two is essentially

zero, since these two wheels were only slightly torqued during the maneuver. The momentum stor-

age capability of the EUVE reaction wheels is 81.4 N-m-s which is well above the peak

momentum value of 1.15 N.m.s reached by reaction wheels three and four.

In Figure 24 the momentum of wheel one for the 45-degree roll maneuver is shown. Since

all the wheels were torqued equally (in magnitude) during the maneuver, the remaining three

wheels will have similar momentum profiles. Again, only 19 N.m.s is required for the manuevers,

which is about 23.5 percent of the EUVE wheel capability.

R0

LL

40

35

3O

2b

20

15.

I0-

//

.............. ..............................

//

0

0 t O0 200 300 400 500 600 700 800

TIME (SEC)

Figure 17. Change in roll attitude for 45-degree roll maneuver.

12

4X10

9

0-

0 I00 200 300 400 500

TrUE (SEC)

Figure 18. Roll attitude error.

600 700 800

0.000RI

gH -0.005T

AS -OO]O

CE

N -0 015S

O

N -0 02O

ERR -0.025OR

-0030

AR

C -0035SEC

620 6.30 640 650 660 670 680 690

TIME (SEC)

Figure 19. Right ascension error (magnified view).

70O

13

0.20

D[ 0.15CL

I 010 [ATI 0O5 1

0 oooLil/,R -0.05

oR - !

_ -0.10 iARC -0.15SEC -0.20- 620

L^nAAn^_^^-w ..... _ ......... 7-

630 640 650 660 670

TIME (SEC)

Figure 20. Declination error signal.

680 690 700

0.15

010 I I_

R0L 0 05L [

E 0 O0RR

0

R -0.05

AR -0.10CSEC -0.15

-0 20

620 65O 640 650 660 670

TIME (SEC)

Figure 21. Roll error signal.

680 690 700

14

orN

0

uR

0

C

Q

-----q i

J I

tf-j

_I_ _ _O_ODW IZ _I

0

Tg

io

nio-----g

g

I o

io

I-LI

!o

oI

0

I T

fi

C '\

ff

JJ

o

Tg

I o

I

F

o

L

i ,

0 o o d 0i I

_: Ic_J_Ju _ _--Or_C_l_l I Z _ I

b

i

0

i 0

0I

T

II

I

I

f_

jJ

f

C

0 0 o o 0I

0

JJ

g

Io

I

0

z

G

WF-

0

!

io

i

F

FF

0 u_

0 o

i

fJ

Ct

ff

JJ

o-0

00

o

0

_o

o

-o

0-0

6 o 0 o 0 0I I I

¢.;im

0L.

L0

¢,)

0

U¢,)

l"q

°_

15

W11EEL

M0MENTUM

N

M

02

0.0

-0,2

-04

-06

0.8

-1.0

_. .....

SE

C -12

-4

0 10 20 30 40 50 60 70 80 90 100

TIME (SEC)

Figure 23. Wheel No. 3 momentum profile, 7-arc-min maneuver.

WHE

EL

M

0ME

NTUM

(')_,

--6- ....

-14-

-16 .......

-20.

0 I O0 200 500 400 500 600 700

],ME (SEC)

Figure 24. Wheel No. 1 momentum profile, 45-degree roll maneuver.

800

16

A simulation of the third required maneuver, 180-degree slew in !,000 s, was made to see

if sufficient momentum storage is available for this maneuver also. This maneuver required

37 N.m.s of momentum storage which is 45.5 percent of the EUVE wheel capabilty.

CONCLUSION

The results generated with the present system model indicate that, with respect to wheel

torque and momentum storage capability, the EUVE reaction wheel is an attractive candidate for

S1RTF. A more detailed model of the reaction wheel will be necessary to assess the wheels

behavior in fine-pointing situations (torque ripple, etc.).

For each of the three required maneuvers ample torque and momentum is available to

complete the maneuver and allow a settle-out period for transients that may occur at the end of the

maneuver. In the present analysis, the settle-out periods were sufficient to allow the transients due

to the disturbance model to decay to acceptable levels.

These results were generated using a set of four reaction wheels as previously discussed.

Although no "wheel out" studies were performed under the "new" maneuver requirements, it

appears obvious, especially for the 7-arc-min slew, that the required maneuver could not be

performed in the event of a wheel failure. For this reason, a configuration consisting of more than

four wheels (probably six) will probably be required. This will increase the ACS weight but the

system will still be lighter than if four ST reaction wheels are used.

17

APPROVAL

FEASIBILITY OF USING EXTREME ULTRAVIOLET EXPLORER REACTION WHEELS TOSATISFY SPACE INFRARED TELESCOPE FACIUTY MANEUVER REQUIREMENTS

By W.D. Lightsey

The information in this report has been reviewed for technical content. Review of any

inlormation concerning Department of Defense or nuclear energy activities or programg has "been

made by the MSFC Security Classification Officer. This report, in its entirety, has been determined

to be unclassified.

,_ . _-,_'77

CHARLES R. DARWIN

Director, Program Development

18"_" U.S. GOVERNMENT PRINTING OFFICE 1990--731-061/20049

Report Documentation Pager_d:, ,i,_,,.. i. it_r

1. Report No ] 2. Government Accession No.

NASA TM- 100 3_8_9 ..........14. Title and Subtitle

Feasibility of Using Extreme Ultraviolet Explorer (EUVE)

Reaction Wheels to Satisfy Space Infrared Telescope

Facility (SIRTF) Maneuver Requirements

7 Ai,thor[si

W. D. Lightsey

3. Recipient's Catalog No.

5. Report Date

February 1990

6. Performing Organization Code

8. Performing Organization Report No.

9. Performing Organization Name and Address

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

12. sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546

151 -SuppiementarY Notes--

10. Work Unit No.

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

Prepared by Preliminary Design Office, Program Development Directorate.

16 Abstr,wl

A digital computer simulation is used to determine if the extreme ultraviolet explorer

(EUVE) reaction wheels can provide sufficient torque and momentum storage capability to

meet the space infrared telescope facility (SIRTF) maneuver requirements. A brief description

of the pointing control system (PCS) and the sensor and actuator dynamic models used in the

simulation is presented. A model to represent a disturbance such as fluid sloshing is

developed. Results developed with the simulation, and a discussion of these results are

presented.

17, K[;y Werds (Suggested by Author(s))

Pointing requirements, reaction wheels,

dynamics, momentum storage,vehicle rates

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