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Thoro are sovoral additlonal positive, and
neqativo factors, to bo V.T)ten ' into
acccrunt, vhlch may affect thn trip time:
but in conclusion the PMslbiltty of usinq
low power arcjets irl potmr limited
liqhtante Cor lov altitudo circular to
elliptical orbits coplanar trennfers ha0
to be considered still problemntic.
3.6 o~b_lc~-ci rcu la r i LA t Ion-
Thio propulsion task refers to sdtellituo,
injected in elliptical transfer orbit by a
I..v., whom final dentination otbit is
circular with a radius normally coincident
vith the T.O. aygee. Typica1 cases are
goostatlonary orbits anti medium altitude
circular orbits of about 10000 Km as
envlSm$@d, for example, by TRW's Odyssey
and ESh'o MGSS-14 constellntion [5).
Electric propulsion is ~onpnrcd to
chemical in Tnble 3.6-1. for thnno tua
typical mlsnions. Arcjetn nre found to
perform acceptably well in this case,
olnco trlp tlmos around 200 dayn cnn be
achieved lony with niqniflcant mass
oav,Inqn w.r.t. a chmlcal propulsion
alternative. The bQttQr mnsn envinq
achievable with SPTs Is, Instead, piid
wlth an excessive trip t inn dur;rt Lon.
___.I_____.. --_.--.-. ..."..-. . - -.- _.-.I
4. T.0. chnracterlstlcn:
- Periqee (Kn) C679 6778
- Apqao (Kn) 42164 I 7 2 M
** DostInation Orbit
characteristics:
- Radius (Km) 42164 16728
- Voloclty incromont,
idartl trnnnf.(a/s): 1476 1174
** T. 0. mass/propel 1 . miss
(W) (1)(2):
- chnnlcal R44/144 '1 G o/ 2 6 0
- arcjet 700/200 655/155
- SPT 555/55 544/ 14
*4 Trip tines (days):
' -
- chenlcal /1 1
arcjet 227 1R2
- SPT n.a. 453
Note I): Satellite drymase 500 Kq;
" 2): Including a 15\ increnno In delta
velocity due to non-Ideal tranfern
Table 3.6-1 E.P. alternativee for Orbit
C1 rcular I zat ion ,
4,RE;'XEW OF SYSTEM NEEDS
Prom thip ovorviev two E; P. technologies
appbar to have a future 'on power limited
lightsnts:
- small ion thrusters for drag
compensation, limltod orbit raining, orbit
circularization and fine trlmming of
orbital parameters of small satellites.
The required thrust lov0lo ara in tho 2 to
10 mN range. a modular approach anablinq
parallol operation oP thruetcra is also
required. Full thrust control, over a 30%
to 120 a range of the design thrust leveA,
is an important design requirement. An
Ovorall specific power of 40 w/mN maximum
(30 W/mN as a goal), and a spoclPir: system
mass of less than 1 Kq/mN maximum (0.7
Kg/mN as a goal), including power supply
24-5
end lgic should be sot as near tern
dovo~oprnont objectives. Isp valuos bettor
than 3609 sec., and very .on9 lifetimes,
aP the ordar of 40000 houra, are also
pr imary roqu ironenta:
- IOW QOWQr arCjOtS in the I to 1.2 KW
ranqe. for orbit mnnoeuverinq tnnks. For
such UOVICQS oflorts should tn! dcvotcd to
possibly brlnq the Isp closer to 600 sec.
for thrusts in the 150 to 2r~0 nl.J ranqe,
vith an overall speci:ic pover better than
8 W/mN includinq power supply.
On the other hand stationary plasma
thrusters do not Seem to have a clear role
for liqhtsats, at least in their present
power and thrust level ranqe. Scaling down
SPTS to lov thrust vnluos (say below io
nl'), while keeping unchanged thnir
ex:ellent parformance, in particular
efficiency and simplicity, is yet unproven
but miqht be worth beinq investlqatcd.
For the considered thru-t ranfin, new
devices bused on tho Electron Cyclotron
Resonanco (ECR) (2) are presently under
evnluation, nimfnq to further improve the
1 on thruster's per formancc .
The ECR tect iiquo nllowe oporatinq over a
vider thrust ranqe by chanqing the mass
flow rate in tho diocharqa ch;imber
enhancing the electrlcal efficiency and
qaa conoumption over a widor pressure
ranqo than tho conventlonnl RF nnd Knufman
techno 1 oq les . pn r t 1 cu 1 n r 1 y
ECR appears
indicated for sinal: sIz@ Ion thrusters,
for which it io aloo onaler to achieve the
optimum static maqnotlc flold necessary
for resonance.
The ECH technique consists in applying a
ntatlc magnetlc field I orthoqonal to the
direction of an oscillating RF field, so
that electrons are forcod to rotate within
the thruster's discharqe chamber, around
the magnetic field lines at a cyclotron
frequency givon by: Fc- eB/2nm. The mean
energy per collision, transferred to an
electron, is:
Wc- (e*E1/4m)(1/(4nI (F-Fc)+(l/il)), with:
0- magnetic Pield; e- olectron charqo:
m- electron mass; E- applied electric
field peak amplitude: 7- mean time between
two consecutive collisions, inversely
proportional to gas pressuro; F- frequerlcy
of the appliod RF field.
At re60nance F=Fc, and the quantity WC
assumes its maximuin value: Wc - (cEi)'/4m,
which depends only on collision interval
and electric field peak value. Clearly the
maximum bonoeitn of tho ECR effoct is
achieved in the low presouro ranqo, being
the col~ision fi-squency proportional to
the operating qas pessure.
For a ion propulsion system in the
millinowton range a RP excitation in the
V"p range proves advantageous for the
rollovinq considerations: