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Date: 3 Feb 88 18:43:57 GMT
From: thumper!  (Phil R. Karn)
Subject: Re: Shuttle-launched satellites considered unreliable?

> 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...


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