• occulting the star.umbras.org/publications/poster/aiaa_poster_35x60_6i.pdfthis away from the...

Major F

unctional Com

ponents:

•Sun Shade

•O

cculting Screen•

Pedestal/Cassette &

Boom

•B

us•

Solar Arrays

•Propulsion M

odules & B

oom UM

BR

AS: D

esign of a Free-F

lying Occulter for Space Telescopes

Ian J. E. Jordan, H

elen Hart, A

lfred B. Schultz; C

omputer Sciences C

orporationFred B

ruhweiler; C

atholic University of A

merica

Dorothy Fraquelli, Forrest H

amilton, M

ark Kochte; C

omputer Sciences C

orporation

Abstract:

A free-flying occulter used w

ith a space-based telescope can enhance the contrast in the region close to a star,

allowing extrasolar planet searches. A

n occulting spacecraft design, emphasizing configuration, control, propul-

sion,mass

estimates,and

power

requirements,is

presentedrequiring

noextension

ofexistingtechnology

orexotic

engineering solutions. The design is scalable for use w

ith telescopes having 1 to 10 metre apertures. Several inno-

vationsare

employed

toblock

starlightandsuppress

scatteredsunlight.A

stationkeeping

controlmethod

ispossi-

bleusing

variousthruster

types.Solarelectric

propulsionenables

scientificallyusefulobservation

ratesand

allows

the occulting craft to be packaged on existing launchers with no on-orbit assem

bly.

Design

Launch Packaging

Control

Operations

Sun ShadeScalability

UM

BR

AS (U

mbral M

issions Blocking R

adiating Astronom

ical Sources) is a class of missions as

well as a technique. T

he general design is scalable for small as w

ell as large occulters. Below

is arepresentative set of m

issions classes and their general mission design characteristics.

Mission

TelescopeScreen

Mission

Class

Aperture

SizeD

uration

E-C

lass (Explorer)

~ 1 m~5-8 m

~1+ years

D-C

lass (Discovery)

1-4 m10-30 m

2+ years

N-C

lass (NG

ST)

4-10+ m

~45 mup to 6 years

Functionally, the occulting screen is used for tw

o purposes:

•O

cculting the Star.•

Hiding other spacecraft structures (e.g., bus, solar array) from

the telescope.

ML

I screen construction emphasizes:

•optim

izing science thermal characteristics for N

-class infrared missions.

•preserving science utility by m

itigating meteoroid im

pact damage

The Sun Shade allow

s minim

al scattering of sunlight toward the telescope since

it shades the entire side of the screen facing the telescope.

At left w

e see a cross-section forpackaging a large N

-class occulterinto a T

itan-IV fairing or Shuttle

payload bay. The solar arrays fold

flat against the bus. The propulsion

booms fold longitudinally, and the

cassette housing the occultingscreen is pulled in next to the bus.T

he Sun Shade is also folded to fitw

ithin the fairing.

At left is a diagram

of an N-class occulter

bus with the propulsion boom

s folded toplace

them

odulesalongside

thespacecraft

bus. Folding rigid booms allow

the num-

ber of flexible propellant line connectionsto

bem

inimized.

Placementof

Attitude

&T

ranslation Control (A

TC

S) thrusters onthe

busis

shown,w

ithnozzle

sizeexagger-

ated for clarity.U

MB

RA

S/SPID

ER

Features:

•O

cculting Screen unfurled from ‘w

indow shade’ cassette on Screen Pedestal.

•C

ontrolled 1-time unfurling of Screen &

Propulsion Boom

Deploym

ent•

‘Curtain track’ to create larger screen from

overlapping layers.•

Bistem

s or coiled Astrom

asts for shade/screen support.•

Sun Shade shields screen from direct solar exposure during observations.

•Screen/Pedestal articulates for:

* transit to provide symm

etry for thrust control.* observations to hide bus/array/propulsion from

telescope.•

Propulsion Module on boom

(s) minim

izes contamination &

interference.•

Xenon propellant tanks stored in bus.

•Solar E

lectric Propulsion offers significant observation rates.•

Attitude/T

ranslation Control thrusters m

ounted on Bus &

Propulsion Module.

•3-axis stabilization for stationkeeping, m

aneuvering, & transits.

Above

isa

controlblockdiagram

showing

aschem

efor

controllingthe

rel-ative position of the occulter w

ith respect to the telescope. Imaging of the

occulter against the background stars by cameras onboard the telescope

allows the occulter to be appropriately positioned.

Shown above is the general schem

e of coplanarforce-couples, w

ith the center of mass in the

plane of the couple, enabling attitude and trans-lational control am

idst science observations.T

hruster plume im

pingement on the screen is

avoided. Attitude control thrusters are located

onthe

propulsionm

odule(B

)and

thespacecraft

bus (A) and allow

some T-axis translational

motion w

ith little impact on the observation.

The occulter w

ould appearbrightto

atelescope

ifitwere

not shaded from the sun (a

problem for m

ost previousfree-flying occulter ideas).T

he plot at left shows the

equivalent brightness (inastronom

ical magnitudes) of

a sphere with the sam

e pro-jected area as a 45-m

etre N-class occulter at various distances

(log of separation in metres). A

n unshaded structure, is quitebright.

At left is a close-up of

the spacecraft alongw

ith its reflection off ofthe occulting screen.B

istem or astrom

astsupports for the screen-and-shade (shade notw

ithinfield

ofview)are

visibleboth

directlyand

inreflection

risingfrom

the cassette-pedestalstructure.

Occulterand

telescopem

ustoperatefarfrom

Earth to m

inimize the effects of differential

gravitation on stationkeeping. At right (not

toscale),the

sun,telescope,andocculter

liein a plane. T

he occulter is positioned suchthat the telescope is in the star-shadow

castby the occulter. T

he occulter spacecraft busis hidden from

the telescope by the screen.

The

transitconfigurationallow

ssim

plercontrolofthespacecraft.

The

diagrams

aboveand below

show the observing and transit configurations of U

MB

RA

S’ occulter fortw

o different mission classes.

AIA

A 2000-5230

Planview

sabove

areof

theobserving

configura-tion show

n from three different perspectives:

•“A

bove” occulter, looking from the sun.

•From

the telescope.•

From the “side”

The artist’s rendition above show

s the occulter viewed

from the target-star side of the screen in its observing

configuration. The underside of the bus and solar arrays

are visible in the foreground in front of the screen.A

stromasts or bistem

s are shown connecting and sup-

porting the top and bottom of the screen.

Characteristic Solar A

rray and Bus Sizes for SP

IDE

R in various m

ission classes.

Mission

Solar Array

Solar Array

Solar Array

Bus

Class

Y-w

idth+X

-wing

-X-w

ing

E-class

4 m4 x 0.8 m

4 x 0.4 m1.2 x 1.2 x 1.2 m

D-class

4 m6 x 1.0 m

4 x 0.5 m1.5 x 1.5 x 4 m

N-class

12 m5 x 1.0 m

4 x 0.4 m1.5 x 1.5 x 6-8 m

Atleft,the

N-class

occultertransits

with

thesun

inthe

planeof

theocculting

screento

minim

izesolar

heatingof the screen. T

his helps the screen reach a lower tem

-perature, enabling utility at longer w

avelengths than ifthe screen had to dissipate heat absorbed by m

oredirect solar exposure during transits.

Below

, the E- and D

-class occulters do not needthe extra tw

ist placing the sun in the plane of thescreen

thattheN

-classdo

sincesm

allertelescopesare not likely to be efficient planet hunters atw

avelengths where therm

al emission by the

screen would com

promise science objectives.

3-D renderings of an N

-class UM

BR

AS

Occulter courtesy of E

dward R

owles, M

ulti-Im

age Productions,http://starships.com

/Multi-Im

age/MIP_H

ome.H

TM

L

At right, is a view

from the sunw

ard side of thebus/array. SPID

ER

is configured for observing.

SPIDE

R operation

requiresarticulatingthe

occultingscreen

into a symm

etricconfiguration form

ovement betw

eentargets. A

t left is aconceptual dia-gram

showing

snapshots of an N-

class occulter’sconfiguration at dif-ferent tim

es asview

ed from above

the telescope-occulter-target plane. Phases (A

) and (I) represent successive stations on differentTarget-Telescope

Lines-of-Sight

(differentsciencetargets).

The

phasesin

between

show snapshots of the occulter at representative m

oments during the journey

between targets.

•A

rticulate the screen into transit position.•

Orient occulter tow

ard next Target-Telescope Line-of-Sight.

•A

cceleration toward next Target-Telescope L

ine-of-Sight.•

Mid-course turnover

•D

eccelerate to a halt.•

Alignm

ent of Occulter along Target-Telescope L

ine-of-Sight.•

Articulate screen into observing configuration.

Distinctphasesof

target-to-targettransitoperations:

CO

RV

ET

is a proposed demonstration m

ission that couldvalidate the external free-flying occulter technique.

http

://ww

w.stsci.ed

u/~

jord

an

/um

bra

s/

At left is the sam

e diagram as described

above, however the propulsion boom

shave

beendeployed.

Note

thatthepropul-

sionm

oduleshave

swung

intotheir

opera-tional place. T

he number of required

booms depends upon the num

ber ofengines and the packing factor w

ithin thelaunch fairing. H

ere, we assum

e 24required

engines.H

owever,few

ermay

beneeded for sm

aller missions, or for

NSTA

R engines w

ith longer life carbon-carbon cathodes replacing current m

olyb-denum

ones. Placement of A

TC

S thrust-ers on the propulsion m

odule is alsoshow

n.

UM

BR

AS em

ploys a sun shade to shield the screen fromsunlight. T

he Shade’s razor-like edge is designed to mini-

mize

sunlightscatteringfrom

theedge

intothe

telescope.

At right is show

n the bright-ness of diffuse and specularedges illum

inated by the sun,for tw

o different edge sharp-nesses. N

ote that the edge ism

uch less bright than thespheres show

n above. The

specular curves are for an ori-entation w

ith the brightness ata m

aximum

(viewer perpen-

diculartothe

edge).Properly

oriented,theshade

reflectsm

ostofthis

away

fromthe

telescope.V

apordeposition

ofa

micro-bead-

ing material on the edge could strike a balance m

inimizing over-

all light scattered into the telescope.

Best guess upper lim

its on the mass and pow

er budgets for 3 different sized UM

BR

AS O

cculter mission classes.

Coronagraph &

Occulting

Rover,

Visible

Exoplanet

Telescope

•Single launch (both craft) onboard A

tlas II AS

•N

ear-Earth Solar-orbit or L

2

•~ 1-m

etre space telescope (~1000 kg)o U

noccluded, off-axis primary m

irroro Focal plane coronagraphic im

ager

•O

cculter (<1300 kg) w

/o 5- to 8-m

etre occulting screeno 1 N

STAR

XIPS engine

o Fuel for 1-2 year mission

o 50 bright targets, 2 visits eacho Jovian search 0.25” - 4” atλ =

0.5µ

SPIDE

R =

Solar-

Pow

ered

Ion-D

riven

Eclipse

Rover

SolarElectric

Propulsionforprim

arypropulsion

issufficientto

achieve science goals. NA

SA/H

ughes NSTA

R engines are

baselined for this mission. X

IPS or cold-gas thrusters may be

used for the AT

CS (A

ttitude & T

ranslation Control Sub-

Mass &

Pow

er Estim

ates for UM

BR

AS M

issions

E-class

D-class

N-class

5-metre Screen

10-metre Screen

45-metre Screen

Mass/Pow

er/Dim

ensions:M

ass/Power/D

imensions:

Mass/Pow

er/Dim

ensions:P

ayload:extensible screen

13kg

52 kg360

kgshade

10 kg20 kg

60 kgscreen cassette

20 kg40 kg

90 kgbus-screen boom

30 kg30 kg

70 kgm

etrology (beacons)10 kg

[50 W]

10 kg[50 W

]30

kg[50 W

] (inc booms)

pedestal & m

asts50 kg

100 kg130 kg

Payload subtotal:123

kg252 kg

710kg

Bus +

Array:

Structure:array support

12 kg20

kg50 kg

bus + propulsion boom

75 kg100

kg200 kg

Power P

roduction & Storage:

Arrays (G

aAs @

18% B

OL

)168

kg{4.2 kW

}280

kg{7kW

}600

kg{15 kW

}Pow

er Control &

Conversion

175 kg300 kg

680 kgB

attery : (450 W-hrs N

iH2 )

10kg

10kg

10 kg

Propulsion:

NSTA

R @

92mN

(ηm

=.85)

17kg

(1)102 kg

(2 op. of 6)408 kg

(6 op. of 24)

Power C

onditioning/Control

30kg

[~0.3 kW] (1 units)

120kg

[~0.6 kW] (4 units)

300 kg[~1.5 kW

] (10 units)X

e propellant storage tanks~

30kg

~130

kg~1200 kg

Xe pressure &

feed system50

kg[100 W

]50

kg[100 W

]50 kg

[100 W]

Attitude &

position determination &

control:A

CS (16 U

K-10 @

25mN

)-

<120

kg(4 x 0.7 kW

)<

120 kg(4 x 0.7 kW

)A

CS aux (16 N

2 @5N

+ pinholes)

40kg

[100 W]

40kg

[100 W]

40 kg[100 W

]N

2 tanks + feed system

+ 25 kg N

2100

kg100 kg

100 kgsun sensors (4 units)

4kg

[12W]

4kg

[12W]

4 kg[12 W

]star trackers/optical navigation cam

eras3

kg[40 W

] (2 unit)6

kg[80 W

] (4 units)6 kg

[80 W] (4 units)

gyros & reaction w

heels26

kg[150 W

] (4 each)42

kg[150 W

] (6 each)142 kg

[500 W] (6 each)

Com

munications:

Com

munications (low

gain), 2 units50

kg[70 W

]50

kg[70 W

]150 kg

[170 W]

Com

mand, C

ontrol & D

ata I/O:

50kg

[50 W]

50kg

[50 W]

50 kg[50 W

]

Therm

al Control:---

10 kg20 kg

30 kg

Bus +

Array subtotal

853 kg1527 kg

4223 kg

Dry m

ass:986 kg

1779kg

4933 kg

Margin ~ 20%

(total dry mass):

197 kg356 kg

987 kg

Xenon Propellant

100kg

440 kg4000 kg

Total mass

(inc. propellant):< 1278 kg

< 2700 kg< 9860 kg

Differentsized

missions

usedifferentsized

launchers.T

heE

- or D-class occulters m

ay be launched from A

tlas II AS

sized boosters or smaller. T

he largest conceived N-class

occulter might need a shuttle, T

itan IV-class vehicle.