dave@stcns3.stc.oz (Dave Horsfall) (01/28/88)
An article in "Electronics Australia" Jan 88 talks about Aussat 3, and
starts off with saying that Aussat 3 was launched by Arianespace, as
distinct from the previous two which were shuttle-launched.
Then follows this statement: "The choice of Arianespace followed the
consideration of various economic factors, including the longer life
expectancy of a rocket-launched satellite compared with that of a
shuttle-deployed satellite."
Forgive me if this has been hashed over before, but that statement
does sound a little odd. Are shuttle-deployed satellites really less
reliable than rocket-launched ones? How come?
--
Dave Horsfall (VK2KFU) ACS: dave@stcns3.stc.OZ.AU
STC Pty Ltd ARPA: dave%stcns3.stc.OZ.AU@uunet.UU.NET
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North Sydney NSW 2060 AUSTRALIA munnari!stcns3.stc.OZ.AU!davehenry@utzoo.uucp (Henry Spencer) (02/03/88)
> ... Are shuttle-deployed satellites really less > reliable than rocket-launched ones? How come? Note that the original quote said "longer life expectancy", not "higher reliability". Barring equipment failures, the lifetime of a comsat is usually limited by its supply of maneuvering fuel. (No, it doesn't just stay where it's put, not in the presence of perturbing factors like the Moon's gravity and Earth's non-spherical shape.) Other things being equal -- they often aren't -- Ariane puts a satellite closer to its final orbit, meaning that less fuel is needed to get it up there and hence more is left for station-keeping. This is not because Ariane is somehow better, but because Kourou is much nearer the equator than Kennedy. A satellite launched from Kennedy starts out in an orbit inclined twenty-odd degrees to the equator, while launch from Kourou ends up in essentially the plane of the equator. Clarke orbit, the destination for most comsats, is in the plane of the equator. Plane changes take a lot of fuel. I don't know for *certain* that this is the underlying reasoning, but I'd be surprised if it was anything else. -- Those who do not understand Unix are | Henry Spencer @ U of Toronto Zoology condemned to reinvent it, poorly. | {allegra,ihnp4,decvax,utai}!utzoo!henry
karn@thumper.bellcore.com (Phil R. Karn) (02/04/88)
> Are shuttle-deployed satellites really less > reliable than rocket-launched ones? How come? Reliability is not the issue. It's station-keeping fuel, at least for geostationary satellites. Most satellites run out of fuel before their components fail and must be deliberately switched off lest they interfere with other satellites operating on the same frequencies from other orbital locations. Ariane is launched from Kourou, French Guiana, about 5.5 degrees north of the equator. Cape Canaveral is at about 28.5 degrees north latitude. Spacecraft launched on Ariane therefore require smaller kick motors to reach geostationary orbit from the launcher transfer orbit than do spacecraft launched from the Cape, and this translates directly into extra mass and volume for holding stationkeeping fuel. Another factor unique to Shuttle-launched satellites is the 45-minute (1/2 orbit) coast phase between shuttle deploy and PAM firing, intended to allow the shuttle to separate to a "safe" distance. During this period the spacecraft must continue spinning stably about its longitudinal axis. Physics says that bodies instead "prefer" to spin about the axis having the greatest moment of inertia (i.e., in a flat spin). When you combine this tendency with the gravity-gradient and drag perturbations due to the low altitude, the spacecraft must expend a nontrivial amount of hydrazine to hold attitude, fuel that will not be available later for stationkeeping. When you start looking at factors like these, you realize just how ill-suited the Shuttle is for launching satellites, and wonder just how anybody got the idea to make it our sole launcher. Some of us were even saying this BEFORE Challenger... Phil
allen@mmm.UUCP (Kurt Allen) (02/04/88)
In article <217@stcns3.stc.oz> dave@stcns3.stc.oz (Dave Horsfall) writes: > >Then follows this statement: "The choice of Arianespace followed the >consideration of various economic factors, including the longer life >expectancy of a rocket-launched satellite compared with that of a >shuttle-deployed satellite." > >Forgive me if this has been hashed over before, but that statement >does sound a little odd. Are shuttle-deployed satellites really less >reliable than rocket-launched ones? How come? While not knowing all the facts, I think that that remark is in reference to Shuttle launched rockets being launched in an orbit that typically requires more fuel to reach Clark orbit from. The more fuel used by the satellite means less fuel for station keeping when it is in it's final position. I believe that the majority of non functioning satellites in orbit became non functional because of lack of fuel to maintain their orbital positions and attitude. Just as an aside, the shuttle was to have tried in orbit refueling of satellites to revitalize older satellites. I don't recall whether it occured or not. -- Kurt W. Allen 3M Center ihnp4!mmm!allen