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th: burn-our (or dry) mm and subsystcm complc-
xity, the most appropriate solution prim;:rily
dcpcnds on thc total velocity incrrment to be
gcncratcd for the &ion in question.
In tablcs 3.2-2 and 3.2-3 characteristic parameters
of catalytic, monopropellant hydrakc- and liquid
bipropcllant thrusters respcctivcly arc summarizcd.
For casc of rcfcrcnce. again equipment available
from DASA in-house manufacturiug has bccn dscd
as a basis of comparison. The parameters of the
liquid bipropcUant rcaction jcts rcfcr IO "sccbnd
ycncration" thrusters. Thcy diffcr from prcviious
snd convcntional design in that thc combustion
chambcr is made of Platinum-Rhodium dloy,
which allows highcr opcrational tempcraturcs and
bettcr cfficicncy than thc classical rcgcncrarivc
chamber cooling principlc. High pulsc rcproducibi-
litv is cnsurcd by the swirl iitomizcr injection in-
stcad of cnnc injcction principlc xed.
The diagrams of fiy;lrcs 3.2-1 and 3.2-2 show thc
spccific impulse and impulse hit sizc rcspccitvclv
of thc DASA ION and 4N 2"' gcncration thrustcrs
in pulscd modc of opcration as function of thrustcr
ON-time in cornpanson to convcntional rcaction
jcts. Smilll minimum impulsc bit six (and ;iccurim
rcproducibilitv) are particularlv important fcaturcs
from thc AOCS point of vic~.
Figure 3.2-3 shows the graphicai rcprcscntation of
tradc-off rcsults, performed to idcntify thc bcncfit
of liquid bipropellant propulsion subsystcm total
mass including tanks. propellant. piping, and thru-
stcr systcm for a satcllitc with 8 small AOCS-thru-
sters as compared to mono-propellant tcchnoloq.
It turns out that thc liquid biprooctlant tccli~iolog
is alway supcrior and offcrs incrcasing m m bcnctit
ovcr monopropellant systcms for incrcasing total
impulse (Ss) to be gcncratcd.
While during orbit corrcction mancuvcrs thc di-
sturbancc torqucs cncountcrcd during AV-genera-
tion duc to SIC ccntrc of mass offscts arc usually
rclativcly high and ncccssitatc countcr;iction by
(thc samc) IhrJstcr systcm in ON- or OFF-modu-
latcd operation. cacrnal torqucs for attitudc con-
trol or momentum manayemcnt in normal opcra-
tion may also be gcncratcd by magcto-torquers.
Thc rclativcly high magnctic ficld strcngth of thc
ewth at low orbit altitudes is particularlv suitcd for
magnetic torquing.
Table 3.2-4 summarizes the most important para-
metcis of torquc rods as manufacturcd by ITHA-
CO. I-iigh pcrformancc attitude control anti stabili-
zlltion of all SIC axes in the neccssary dynamic
range is bat accomplishcd by angular momentum
storsgc deviccs i.c. rcaction- and momentum
wheels. Fig. 3.2-4 givcs a small selcction only of
possible wheel arrangements and combinations for
gcnerathg (idternid) control torques about one or
several S/C 3x6 andlor bias momentum perpendi-
cular to the orbit plane. As already mentioned with
the typical AOCS concepts of section 3.1 the bias
momentum principlc offcrs particular advantagcs if
attitudc reference about all thce S/C ruciS is not
continuously available. Therefore momcntum-/rex-
tion whccl combinations arc expectcd to cover most
applications. A summary of characteristic paramc-
tcrs of off-the-shclf cquipmcnt (manufacturer:
TELDIX) is given in tablc 3.2-4 of Annex 2.
23-1
323 Fcusibilify assessment of control co..tcepts for ,
TACSA 7 missions
From thc prcdous discussions of mission objcctivcs.
control concepts, altcrnativcs and cquiprncnt trade-
offs and thc AOCS rcquircmcnts as summarized in
tablc 1.3-1 it becomes obvious that
- thc classical bias momentum systcm with two-
rvd.; attitudc rcfcrcnce and WHECON control
can only satisfy thc nccds for communication
missions. whcrc no particularly stringcnt rcquirc-
mcnts arc imposcd on thc Linc-of-Sight (LOS)
stability.
- In thc case of wcathcr satcllitcs, for thc limitcd
periods of ..:arming or picture-taking adcquntc
LOS-stability must bc eacurcd. absolute attitudc
pointing accuracy is not so critical
- For carth obscntation missions. depcnding on
thc obscrvation payload concept (push-broom.
dctcctor array, scan mirror, mul!i-spcctral dc-
compnsiiion) and ground rcsolution both accura-
te pointing and LOS-stability bccomc incrca-
singly important. Payload operation'will have to
he intcrruptcd during orbit coh?ction ;i'nd trnn-
sicnt pcriods. In normal modc whccl control
about 311 three axcs accompanied with high
prccision altitude rcferencc has to bc forcsccn.
Suntcillnncc and vcrification bciny also carth
obscrvntion tasks ncccssarilv imposc at Icast the
siunc rcquircmcnts on the AOCS pnrtlv cvcc in
prcscncc of payload-induccd perturbation envi-
rocmcnt. p-wavc payloads additionally rcquirc
highly accuratc orbit data (GPS), LOS stability
and accuratc attitudc rcconstitution ovcr lor:
pcriods (minutcs). Payload data proccssing and
payloadhttitudc data fusion is gcncrally rcquircd
cotraining high dcmanh on onboard computa-