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.