[net.columbia] SBS orbital mechanics

karn (11/12/82)

>From what I could discern by watching the SBS ejection, the orbital
mechanics and sun angles appear very similar to standard procedure
with expendable launch vehicles.

When the SBS payload was ejected, the shuttle was making a descending (north
to south) equator crossing over the Pacific Ocean.  The shuttle was oriented
with the velocity vector towards the bottom, the earth off the starboard
wing, and the sun off the port wing.  (It was local noon at the "launch
site.")

Thus, when SBS-3 was ejected, its engine's thrust vector was pointing
against its velocity vector. (If it had fired immediately, the payload
would have reentered). The 45 minute delay placed the actual firing 1/2
orbit later, near an ascending (south to north) equator crossing at
"local midnight" over the Indian Ocean. Since the spacecraft was spinning
with its axis fixed in inertial space, the engine's thrust vector was now
properly pointed in line with its velocity vector.

After the burn, SBS-3 went into an elliptical orbit with the burn point
becoming the perigee.  The semimajor axis of the ellipse is in the sun-
earth line, which is common for geostationary transfer orbits because of
thermal constraints.  It also minimizes orbital perturbations by the sun.
Apogee occurs at "local noon", so that the satellite does not spend much
(if any) time in eclipse.  Normally, this kind of orbit is achieved with
night launches on conventional launchers.

Due to the earth's oblateness, the apogee precesses several degrees
after several orbits to coincide with an equator crossing near the final
geostationary latitude.  At this point, the apogee kick motor on the
satellite is fired and it then stops moving relative to the earth.

Phil Karn