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24.2
CJ6Qh ion pl'b;pPelaion tho applied thrust
IWs'olO QPG Cl000 to the drag force
avoragexi durinq one orbit porlod, i.e.
k?lroo 3 eo t5 aM Pot B typical mall SAR
aoeellieo ineondodl to Ply at orbit
aAt6tudw km low eo I00 M. Thrusts of
thio ordou can $s modularly achieved by
puttinq low power (< 10 mN) ion thrusters
in parsllol end implementlnq throttling or
on-off modulatlon accordlnq to the
requ I red duty .
.-----.I --.--. --cons~dcring
typacnl values from Tnhle
2.-1. it can bo necn that , lor orbit-
Orbit Drag f.cs Delta-V Delta-V
keeping teaks. nrcjctn
height avgd. ov8r
Rrn S C I V ~ P C power
6 mo. 5 years
~ ~
limited nnrl are ttrrthor pennli7wl by the (Km) 1 orbit(nN) (m/s)
poor lop value. SfTe qc~d f.vntc?n*.lc.rs
a) b) a) b) a) b)
to ion thruntarr, in that thcry cin provide
hlqhor thrusts for the snmo I#: pc>wer at a
lmor moo, but the lover lop valuo can bo 275 30 15 1155 185 11550 852
critical for long-duration whnnlono. For 300 2l 10.5 815 272 8150 2721
orbit ~nnoeuvaring tho cpvn61rrbis OC powor 350 14.2 7.1 551 184 5508 1840
&a juot at the llait where ~rcjoto bacon0 aoo 11.7 5.9 457 152 4752 1522
intonooU~ng: but SPTn nlno orpear to bo 4 50 8.9 4.4 145 119 3452 1152
qodl cautaidlmtsrP, In vieu oe eho bettar Iop
500 7 J.5 270 90 2700 902
they con offer. Other coneidorntione. ouch
an tho tine to transfer./ muo?. then b@ Asounptions:
taken into account, in t.h@ nygtcn trade- case a) : s/C crosmction 4 m"2,
OfPS, e0 choose the riqht
drymirnn 400 K9:
PQChnr)ltwly.
cane b): S/C croeoectlon 2 rn-2,
J,TII!fC&L-E,P., AP_F_LI.ICATI O?tS*.TU I.fC.tfTSATS_
drymaso 600 Uq;
Q worst year nun activity:
>,a, ~rlpqla- co?Jng!>-on. - for- . Obn@rvEAk?!~
seeollitso-
Cone-olfoctivs observat ion mission:& can bo
concoivod with individuinl ncit.eII itcn,
leunchd on-dcmand, or bancd on emall
ontollitao conatellations c?rr)'inq f.AR [I]
or optical sensors. In both C.PBQPB the
oioaion roquiroa very low orbit mleleudos,
ooy in tho 200 to 600 Kn rmgo, to
onhowce tho optical ground rooolution or,
in the SCUR cmo: to reduce tho transnitted
per compatibly vith ehca limited
resources available on a small ~neolllte.
At thene altitudes tho residual
stheongheric drag beromen the Ilmitlnq
factor for thO mlssion duration. Even
aasualg o slender platform daslgn and an
orientation of tho appendaqen (I.e. solar
arrays, antenniao) such as to axhibit a
mininun cross omtion In the dlrecsion of
tho flight path, tho dolta-velocity
recpirQd to counteract the roaldual drag
becowo rapidly unmanageable with chatnlcal
propulsion for dosion dureeionn greatarr
than a Pew months: see Tablo 3.1-1. which
is rolwent to two typical liqhterat
configurations. The determining fmtor
bscowa the propellant mass, given the
launch maes limits.
Tho high specific impuloo of ion
propulnion allows reducing, by a factor of
about 10 w.r.t. chemical propuloion, tho
propallant nasn required to inpart a given
tots1 volocity incremont. AS a
consewonco, mission lifetinan of 'J year0
can ba achieved, which is inatrummtal to
iwplesnant small satellite conneallationa
for sprnanents observation ninaione. Itn
coaperiaon, the other E.P. tachnologion do
not porform woll in thia Qgglicetion
cheractorized by very high required
velocity increments.
Table 3.1-1 Drsq forces vs. altitude and
rcqu i red del tn-volocit ies for
their compensation
Table, 1.1-2 qives propellant mass and DC
power requircmants OP ion propulsion vs.
orbit altitudo lor a prospective small SAR
oaetsllite. For the consldered thr?rst
lOV015, tho CC powor requirements are
coapatiblo with spacocraft allocations. In
concluoion, ion propulsion has to be
conoidorod an enabling technology to
isplonont long duration observation
missions in vary low Earth orbits.
W0VorthQleeQ the operating time, Of the
order of 40000 hours, is OP concern and
ohould bo properly addressed by further
dovelopmonta. E?'orts to further Improve
tha efficiency OK low power thrusters are
also strongly raccornended.
Orbit Propell. Thrusters N'/ AVge DC
heiyht mass avqe orb.duty power
(h) (K9)(1) (W) , (2)
-
275 90 248m~. 1.94 450
300 62 248mN/0.66 320
390 41 8NW/0.88 240
400 34 8mM/O. 7 4 200
450 25 0mN/O. 55 150
500 20 0mN/O. 4 4 120
Nota 1): boo0d on 2800 e. Isp
Note 2): based on 30 W/mN
Table 3.1-2 Dtag compensation ion
propulsion requirements for
system b) of Table 3.1-1
.-