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m, = f*U*d
Whcn liquid bipropellant cngines providing hi&r
spccific impulse are used, thc propellant mass is
:rbut 23 5 lower. In addition to altitude corrcc-
tions inclination corrections may ham to be pcrfor-
mcd. The inclination drift at allitudcs bctwccn BO/
km and 3'90 km amounts to about 0.2 dcdycar U?$,.
requires approxhntcly 27 dscc velocity incrcmcnbi,; a
for corrcction.
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22 Sprtcecnft Physicul Chnmcteristics ,
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It is, of coursc not the objcctivc s)f this papct to
claboratc on TACSAT rncchanical, struct~ral.
powcr and payload dcsign Ccaturcs. In vicw of.thc
fact, however, that thc AOCS is thc S/C subsyskrn
with thc largcst numbcr of funciion:rl ;itid hardwa-
rc intcrfaccs with othcr subsvstcms ;is wcll ;IS siitcl-
Iitc opcraiional prcxcdurcs. it is rcprtlcd ncccssa-
ry to cstnhlish at least a rouyh framc of thc cxbcc-
tcd SIC physical charactcristics:
- Thc S/C mass targctcd Tor ccmsidcratiod in
TACSAT missions is bctwccn 200 and 70(1 kg
i.c. within thc payload ranpc of dcdicatcd Iiiun-
-
-
chcrs like YEGrISUS. TAURUS etc.
Onboard clcctrical powcr gcncration capabilitv
of about 1 k W is rcgardcd necessary to ;daw
for half orbit cclipsc pcriods and
opcration of microwave pavloads (SAH typi-
cal avcragc powcr 500 W. peak powcr about
10 kW/pulsc)
Conscqucntly a solx array surfacc ;ma of
about X to 10 rn' will bc rcquircd corrcsponding
to a mass contribution of about 16 to 20 kg if ;I
tlTicai weight factor of 2 kgtn' and usc of convcntional
silicon solar cclls arc assumed.
solar cells providing about 30 '.T incrcasc in
power output per unit surfacc a m will Iiavc to
he cxcludcd for cost rcasons.
S/C gcornctricd configurations arc gcncrallv
detcrmincd by the constiaints irnposcd by
thc launchcr cargo bay hncnsions. shape
and acceleration load. .
thc payload rcquircmcnts (including poucr).
If thc fundamental configuration shall bc riqained
for thc diffcrcnt mission applications as
outlined under section 2, thc gcometrical requircments
associated with
optical observation payloads i.e. telcscopcs
of sufficicnt f d length, operating in the
visible and infrarcd region, and requiring
cooling of the detector mays (e.g. Liquid
helium dcwers) andlor
microwave payloads (SAR) with large area
(p,-wavefecd or phased array) antennas suffi-
ciently cnended in long track direction
arc cxpcctcd to be the dcsign drivers. Some
squarc-shaped. modular ccntral body truss struc-
ture with triangular, rectangular or hexagonal
cross-scctional area seems to be principly ade-
quate and will provide the necessary free surfa-
ces for the housing the folded solar arrnys and
cnsurc unobstructcd field of vicw for flttitudc
measurcmcnt scnsors and frce spacc for attitude
and orbit control rcaction jets.
Ouitc 3 numbcr of clcisting dcsings (e.g. LAND-
SAT 6 - ref.3, MOS-1, sec sccrion 3.2) are
cquippcd with a single wing solar array giving
risc to Iargc (solar) disturbancc torques and
clisturbancc torque changcs during cclipsc tran-
sitions. due to thc unsymmetrical configurat ion.
From the AOCS point of view a symmetrical
configuration as generally adopted for gcosyn-
chronous communication satellites is of course
highly dcsirablc. In vicw of thc rcquircd flcxibi-
litv for frcc sclcction of thc orbit inclination
according to thc tactical rcquircmcnts thc solar
array is assumed to be oricntahic not only as
usunl around its axis of spmciry, but aLco about
an axis pcrpcndicular to the first iixis of rotation.
hlotor drivcn dcployrncnt mcchmisrns iis for
instancc implcmcntcd in EUTELSAT I1 can bc
casily modificd to allow scrvo-controllcd uticu-
lath of the solar array at the yokc junction to
cnabk oricntation of thc solar array surfacc to
thc sun for a largc rangc of orbit inclinations.
Morcovcr by controlling thc solar array rotation
about two axis, solar pressure torque compcnsa-
tion tcchniqucs can bc applied to irnprovc attitu-
dc control pcrfarmancc and onboard angular
morncntum rnanagcmcnt. -
In lis. 2.2-la (Annex 1) such a variable solar array
configuration cap;ibility is schcmatical!y indicated.
Unsymmetrical dcflcction of oppositc solar arrays
w.r.1. the sun dircction will gcncrrrtc solar prcssurc
torqucs about an Luris pcrpcndicular to thc sun
incidcncc plmc, counter-rotation as shown in Fig.
2.2-lb will give risc to "wind mill torques" acting
around the sun line.
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