mishkin%UUCP@YALE.ARPA (10/28/84)
From: Nathaniel Mishkin <mfci!mishkin%UUCP@YALE.ARPA>
While we're all wondering about some details of the satellite recovery
schedule for the next shuttle mission, I'd like to add my own wonders:
What are the constraints on the rendezvous? According to the PBS show,
the satellites are in quite an elliptical orbit. I got the impression
that the apogee of the orbit is well higher than the maximum possible
shuttle apogee. So presumably they have to time the rendezvous so
that the shuttle meets the satellite when the satellite is at a low
point in its orbit. How long do the astronauts have before the
satellite drifts too far away? How elliptical can the shuttle's
orbit be made? How much force needs to be applied to the satellite
to get it into the shuttle bay? (After all, it DID fire an engine so
it does have some momentum that has to be overcome, right? My physics
is, er, a bit rusty.)
How dangerous is all this? I mean, in the most recent shuttle mission
during the refueling experiment people commented on the "danger" of
dealing with the fuel in that environment. Mightn't there be some
unexpended fuel sitting in the satellite's booster? Would you want
to be staring down the gullet of an engine that didn't behave as
expected in the first place?
-- Nat
-------karn@mouton.UUCP (10/29/84)
My understanding is that the hydrazine thrusters originally intended for station keeping were used to drop the two satellites into lower orbits for retrieval. While I don't have specific information on Palapa or Westar, I do have a "NASA Facts" sheet on Galaxy I, which is also a Hughes HS-376 design. The spacecraft plus unfired apogee kick motor weighs 1,218 Kg, of which 135 Kg is hydrazine. Assuming a specific impulse of 150 sec (I don't have the exact number but monopropellant hydrazine thrusters aren't very efficient), that gives a delta V capability of 172 m/sec. However, if the AKM has been fired this would get rid of about 585 kg of solid propellant, increasing the delta V capability to 353 m/sec. Depending on the apogee of the bad orbit, dropping the satellite back into a 300 km circular orbit is well within the capability of these thrusters. For example, 83.5 m/sec is required for a 600 km apogee, and 189 m/sec for a 1000 km apogee. I don't know many of the actual figures involved; if anyone could send them to me I'd appreciate it as I'd like to do these calculations again with greater precision. The thing that interests me is how they will be able to match the planes of the Palapa-B and Westar-6 while still meeting the launch window constraints of the two other satellites the shuttle is taking up. Anybody have current orbital elements? Phil
eder@ssc-vax.UUCP (Dani Eder) (10/29/84)
> While we're all wondering about some details of the satellite recovery > schedule for the next shuttle mission, I'd like to add my own wonders: > > What are the constraints on the rendezvous? According to the PBS show, > the satellites are in quite an elliptical orbit. I got the impression > The satellites were placed in elliptical orbits by the first of two solid motors that were to be used in their original delivery. The second, smaller, motors were fired months ago, and placed the satellites about 600 km high (if memory serves me). The Hughes model 376 satellites carry fuel used to keep them in position or move about in GEO. This fuel was recently used to move the satellites down to an altitude the Shuttle Orbiter can reach. > How dangerous is all this? Probably less dangerous than the delivery of the new satellites which will also occur on this mission. The new ones carry 8000 lbs of very energetic propellants each. The old satellites have burnt out their motors, which means no fuel is left. Someone in another article asked why the RMS is being used to recover the satellites rather than the astronauts directly, and whether the satllites are spinning fast. The satellites mass 1000 lbm each. Even though they 'weigh' nothing, that mass still has to be pushed around. The nearest equivalent I can think of is pushing a boat around a dock by hand. At low speeds this is nearly frictionless in two dimensions. The astronauts are in these bulky suits, too boot. While it could be done, it is much easier to use the electrically powered RMS. The satellites will have been spun down to less than one RPM before the astronauts get to them. Once the stinger is firmly attached to the satellite, the astronaut merely pushes the 'stop' button on his Manned Maneuvering Unit. The MMU calculates which thrusters to fire to stop the residual spin. Dani Eder / Boeing Aerospace Company / ssc-vax!eder /(206)773-4545 *** REPLACE THIS LINE WITH YOUR MESSAGE ***