Re: STS-74, anticipated appearance

Philip Chien (kc4yer@amsat.org)
Mon, 13 Nov 1995 19:05:02 -0400 

Walter said:

>You will note that for the November 15 pass, the anticipated times for the
>shuttle and the space station differ by more than 2 minutes.  Experience
>from previous shuttle missions, and from the previous rendezvous missions
>in particular, suggests that the Mir elset has much higher reliability.
>It is true that Mir is reboosted from time to time, but perhaps we can
>expect that the RSA wouldn't want to complicate things for JSC during or
>just prior to a rendezvous mission?  I suppose CAMs (collision avoidance
>maneuvers) are still a remote possibility.

Quite true.  Typically the Progress spacecraft brings the propellant for
altitude maintenance, and it's performed whenever Mir's altitude gets too
low.  The Phase I agreements specify the maximum altitude for Mir to be at
specific dates when joint missions are planned to permit NASA to budget the
proper amount of propellant for the rendezvous portion.

>Also, previous experience and energy requirements suggest that the shuttle
>orbiter will chase Mir from a lower and faster orbit.  Thus, prior to the
>docking, Atlantis will appear behind Mir.  After undocking, it will again
>return to a lower and faster orbit, thus appearing ahead of Mir.

As a rule it's much easier to put the chaser vehicle in to a lower (faster)
orbit which phases to the proper point where it's raised in to the same
orbit as the target vehicle.

After undocking the primary reason to change the shuttle's orbit to a lower
one (as opposed to a higher orbit) is very obvious - eventually you have to
reduce your altitude to come home.  If you go up then you have to spend
more propellant without any obvious advantages, while lowering your orbit
helps you prepare for entry.  Typically the amount of altitude lowered, and
the exact time for the maneuver is designed to permit as many landing
opportunities as possible on the nominal and contingency landing days.


Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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