ZANL188 From United States of America, joined Oct 2006, 3775 posts, RR: 0
Reply 1, posted (7 years 8 months 4 weeks 2 hours ago) and read 17367 times:
Quoting Flexo (Thread starter): I've been wondering why it takes roughly three days for the Space Shuttle to reach the international space station while there is only about 400 km distance to cover (if the timing is right).
What is it that delays the arrival for so long?
What's the hurry? I'm sure they could do first orbit rendezvous (demonstrated during Gemini and used routinely during Apollo) if there was a need.
The launch window is pretty small to do it on first orbit, a bigger launch window gives more operational flexibility.
Then there are tasks that need to be done before docking - TPS inspection comes to mind.
There's also the small matter of Space Adaptation Syndrome, not smart to attempt a docking (or other hazardous task) while the pilot is vomiting. Best to let them get done vomiting and then dock. This may also be the reason Hans' first spacewalk was delayed on the last mission. Vomiting in your helmet while on a spacewalk, 30 minutes from the nearest airlock, is a bad thing...
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Thorny From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 8 months 3 weeks 6 days 21 hours ago) and read 17270 times:
Quoting ZANL188 (Reply 1): What's the hurry? I'm sure they could do first orbit rendezvous (demonstrated during Gemini and used routinely during Apollo) if there was a need.
That only works when you launch one orbit after your target... as Gemini did with Agena. Apollo did it at the Moon (LM meeting CSM in orbit) but that was the moon, with 2-hour orbital periods, nearly perfectly equatorial orbits, generally only two or three days after landing, and with the CSM doing "phasing" burns to keep flying over the landing site. This was one problem that Apollo would have faced had far longer missions been planned: the CSM would have needed a lot more fuel to keep "phasing" its orbit to stay in reach of the LM.
After the first orbit, the target drifts farther and farther away from the launch site with each orbit due to precession. When the launch site rotates beneath the target's orbit, the target is now hundreds or thousands of miles away and rendezvous is no longer possible on the first orbit (at least, not with contemporary vehicles limited to standard 200-mile-high orbits like Soyuz and Shuttle.) This can be mitigated somewhat by changing the orbit of the target (speeding up or slowing down by raising or lowering the orbit a little), but there's a limit due to fuel and other factors how much this can be done. So in most cases, rendezvous is going to take a day or two. They often adjust the Station's orbit to make 2 day rendezvous possible (especially for Soyuz and Progress) instead of 3-day rendezvous.