Re: Supersynchronous

Chris Peat (Chris.Peat@dlr.de)
Tue, 29 Sep 1998 09:28:45 +0200

>>>Anybody know what "supersynchronous" means?
>>
>>Exactly the above - ie place it in a transfer orbit far higher than
>>one would think needed - 140 x 54755km. Plane change maneuvers have a
>>lower penalty fuel-wise the further you are from the primary body - thus
>>a supersynchronous delivery to geostationary orbit can save on satellite
>>stationkeeping fuel and prolong the vehicles life.
>>
>I've noticed that earlier, but IF PAS 7 is going to geosynch, and was
>launched by an Ariane, and all Ariane's now launch from Kourou at 1 deg.
>latitude - why would they do a plane change ??

Actually, Kourou is nearer to latitude +5 deg. and for reasons connected
with the finite duration of the Ariane ascent trajectory the GTO reached
from Kourou usually has a 7 deg. inclination, so a small plane change
maneuvre is always required, and this is best done as far from the Earth as
possible where the velocity is lowest.
There is also another small benefit from a supersynchronous transfer, even
if one ignores plane changes. To get from a "standard" (140 x 35786km) GTO
into GEO requires a single maneuvre of 1478m/s at apogee. Using the numbers
given for the PAS 7 supersynchronous transfer orbit, it would require two
manuevres to reach GEO, the first one of 1186m/s at apogee to raise the
perigee to GEO height, followed by a second of 270m/s at the new perigee to
lower the apogee down to GEO. The total would thus be 1456 m/s which is
slightly less than the single maneuvre for a standard GTO.

One could argue, of course, that the launcher must provide a larger delta-v
to reach the supersynchronous orbit in the first place, but since PAS-7 was
the only payload, maybe there was some spare delta-V capacity which would
otherwise have been wasted. They could have also used the extra launcher
capability to put more fuel on the satellite, but perhaps the tanks were
full already, and could not be easily resized.


Chris

Chris Peat, Anite Systems
Currently working at the German Space Operations Centre near Munich