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Multi-day rendezvous



 
 
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  #1  
Old February 9th 10, 09:06 PM posted to sci.space.station
Neil Fraser
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Posts: 38
Default Multi-day rendezvous

Why does it take the space shuttle so long to reach the space
station? Endeavour doesn't reach ISS until flight day three on this
mission. Some missions don't dock until flight day four.

By comparison, the world's first docking (Gemini 8) occured five and a
half hours after launch. And the first moon landing occured four days
after launch. Why has docking gotten so much harder?
  #2  
Old February 9th 10, 11:11 PM posted to sci.space.station
Eddie Lyons
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Posts: 29
Default Multi-day rendezvous

Docking hasn't gotten harder. The trajectory is optimised for fuel
efficiency. There's no particular rush to get there so why waste fuel just
to get there quicker? The time is also used to conduct the RMS/OBSS
inspection of the thermal protection system, to check out and prepare the
EMUs that will be used for EVA, and to prepare the docking system.

The Russian Soyuz and Progress also take two days to reach the station;
sometimes, again for greater fuel efficiency, Progress takes a three day
trajectory. When ATV and HTV are flying regularly, expect to see a similar
strategy.

It will be interesting, though, to see what trajectory Space X adopts for
the Dragon spacecraft. If they succeed in turning it into a manned ferry,
given the presumption it would carry seven crew (though it is doubtful this
would be needed, since Soyuz will still be flying) I can imagine it would be
preferable to have a shorter launch-to-docking period, given how cramped the
cabin interior would be.


Eddie Lyons
Portsmouth, UK


"Neil Fraser" wrote in message
...
Why does it take the space shuttle so long to reach the space
station? Endeavour doesn't reach ISS until flight day three on this
mission. Some missions don't dock until flight day four.

By comparison, the world's first docking (Gemini 8) occured five and a
half hours after launch. And the first moon landing occured four days
after launch. Why has docking gotten so much harder?



  #3  
Old February 9th 10, 11:28 PM posted to sci.space.station
Glen Overby[_1_]
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Posts: 152
Default Multi-day rendezvous

Neil Fraser wrote:
Why does it take the space shuttle so long to reach the space
station? Endeavour doesn't reach ISS until flight day three on this
mission. Some missions don't dock until flight day four.


They launch into the plane of ISS, but in a lower orbit. It takes them that
long to 'catch up'. If ISS was in a higher orbit, there could be a bigger
difference in orbital period so they could catch up faster. I think Jim Oberg
discusses that in the Orbitology chapter excerpt on his web site.

NASA avoids day 1 or day 2 rensdevous due to space sickness. They don't want
a sick commander trying to rensdevous.

Glen
  #4  
Old February 10th 10, 03:35 AM posted to sci.space.station
Neil Fraser
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Posts: 38
Default Multi-day rendezvous

On Feb 9, 2:11*pm, "Eddie Lyons" wrote:
Docking hasn't gotten harder. The trajectory is optimised for fuel
efficiency. There's no particular rush to get there so why waste fuel just
to get there quicker?


I don't see how a perfectly executed one hour ground to docking flight
plan would use any more or less fuel than a four day chase. Why would
a slow continual-thrust spiral be more efficient than raising the
apogee to station orbit in one burn, then raising the perigee to
station orbit on another burn 45 minutes later? It's not like a
planetary proble that is sent on a circuitous path around the solar
system to pick up multi-planet slingshots.

The down-side of a four day chase is that one is burning lots of
consumables. CO2 canisters and food on the crew, hydrogen and oxygen
for the fuel cells. That's a significant amount of mass on a vehicle
like the shuttle which does not have solar panels or recycling.

The upside of a four day chase is that one has tons of time to tweak
the orbit (but if Armstrong managed to get it correct on the very
first attempt in the 60s, it can't be that hard to do with modern
computers). Time to do OBSS inspection is certainly put to good use
(but FD-3/FD-4 rendezvous predates STS-107, so that can't be the
reason). Glen suggests that it allows for elimination of space
sickness during docking (but according to Wikipedia, "In most cases,
symptoms last from 2–4 days" which would mean a FD-1 rendezvous would
be far preferable).

  #5  
Old February 10th 10, 08:17 AM posted to sci.space.station
John Doe
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Posts: 1,134
Default Multi-day rendezvous


I don't see how a perfectly executed one hour ground to docking flight
plan would use any more or less fuel than a four day chase.



The earth rotates once every 24 hours. Once you launch, you stop
rotating with the earth and stay in that orbital plane.

So when the shuttle is on the ground, it needs to wait until the earth's
rotation has put KSC right under the orbital path of the ISS. At that
precise time, the ISS may be anywhere along that orbital path. It might
be right over KSC, it might be over australia etc.

Once you are in space, your speed is dictated by your altitude. You want
to go faster than the ISS ? You need to have a lower altitude. You want
to go slower ? You need to go higher.

You can only go so low before you hit atmospehere. And going higher
costs you a lot a fuel (and you'll have to spend fuel later to drop back
down).

Either way, there are realistic limits to the orbital speed differences
between a shuttle and station.

And once you are ready to dock, you need to ensure that the speed
difference with ISS is minimal, and more importantly, that you have fine
tunes your orbit to match precisely that of the ISS. It isn't enough to
be in the same place at same time, you need to be going in the exact
same direction in a 3D environment. (otherwise, when shuttle docks, its
momemtum will try to pullthe station in a different direction and put
lots of stress on the structure).


Could they shorten the time to docking ? It is possible that on some
flights, the relative postion of both vehicles at time of launch might
alloow quickler docking. But you need to also remember that after
launch, the crew complete the on-orbit transformation fo shuttle and
then gosub sleep for some 8 hours because they have already had a very
long day.
  #6  
Old February 10th 10, 08:19 AM posted to sci.space.station
Jorge R. Frank
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Posts: 2,089
Default Multi-day rendezvous

Neil Fraser wrote:
Why does it take the space shuttle so long to reach the space
station? Endeavour doesn't reach ISS until flight day three on this
mission. Some missions don't dock until flight day four.

By comparison, the world's first docking (Gemini 8) occured five and a
half hours after launch. And the first moon landing occured four days
after launch. Why has docking gotten so much harder?


It is due to launch window. Due to the high inclination of ISS, the
planar launch window winds up being the driving constraint, and you
pretty much have to accept whatever phase angle you get within that
window. At large phase angles, flight day 1 or 2 rendezvous is simply
not possible - phasing rate is proportional to delta-H and is therefore
limited by the height of ISS and the minimum safe orbit for the shuttle.
So given that you will need flight day 3 rendezvous on many days anyway,
and given the desire to standardize the timeline, flight day 3
rendezvous was standardized.

Soyuz/Progress also do flight day 3 rendezvous, for exactly the same reason.
  #8  
Old February 10th 10, 11:22 PM posted to sci.space.station
Eddie Lyons
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Posts: 29
Default Multi-day rendezvous

It's not a "slow continual-thrust spiral", but rather a number of shorter,
fuel efficient orbit raising and phasing burns conducted during the days
leading up to docking. This is also why Soyuz/Progress, ATV and HTV conduct
multi-day RV manoeuvres.

Your Gemini example is not a good one. The Agena docking targets were
launched just one or two orbits before the Gemini spacecraft, so the
missions were designed to allow an early docking. The need for orbit
raising, and more especially phasing burns were designed out of those
mission profiles. They took advntage of short-term orbital mechanics in
these mission designs. Such short-term advantages do not exist for long-term
programs like the ISS (or the earlier Salyut and Mir programs). The
realities of orbital mechanics in reaching a permanent habitat at an
altitude of 350 to 400 kilometres dictate the rendezvous profiles used.

Eddie Lyons
Portsmouth, UK


"Neil Fraser" wrote in message
...
On Feb 9, 2:11 pm, "Eddie Lyons" wrote:
Docking hasn't gotten harder. The trajectory is optimised for fuel
efficiency. There's no particular rush to get there so why waste fuel just
to get there quicker?


I don't see how a perfectly executed one hour ground to docking flight
plan would use any more or less fuel than a four day chase. Why would
a slow continual-thrust spiral be more efficient than raising the
apogee to station orbit in one burn, then raising the perigee to
station orbit on another burn 45 minutes later? It's not like a
planetary proble that is sent on a circuitous path around the solar
system to pick up multi-planet slingshots.

The down-side of a four day chase is that one is burning lots of
consumables. CO2 canisters and food on the crew, hydrogen and oxygen
for the fuel cells. That's a significant amount of mass on a vehicle
like the shuttle which does not have solar panels or recycling.

The upside of a four day chase is that one has tons of time to tweak
the orbit (but if Armstrong managed to get it correct on the very
first attempt in the 60s, it can't be that hard to do with modern
computers). Time to do OBSS inspection is certainly put to good use
(but FD-3/FD-4 rendezvous predates STS-107, so that can't be the
reason). Glen suggests that it allows for elimination of space
sickness during docking (but according to Wikipedia, "In most cases,
symptoms last from 2–4 days" which would mean a FD-1 rendezvous would
be far preferable).


  #9  
Old February 11th 10, 11:44 PM posted to sci.space.station
Dr J R Stockton[_59_]
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Posts: 2
Default Multi-day rendezvous

In sci.space.station message bKSdnRDlaoSPw-_WnZ2dnUVZ_h2dnZ2d@giganews.
com, Wed, 10 Feb 2010 01:19:45, Jorge R. Frank
posted:

At large phase angles, flight day 1 or 2 rendezvous is simply not
possible - phasing rate is proportional to delta-H and is therefore
limited by the height of ISS and the minimum safe orbit for the
shuttle.


"Phasing rate" is fairly close to 10 (actually 3 pi) km of distance
along the orbit, per orbit, for each km difference of orbit height. For
the metrically-challenged, that also works in miles.

URL:http://www.merlyn.demon.co.uk/gravity3.htm#Ov refers.

With a height difference of 100 km (which must be within half an order
of magnitude of the greatest possible), that gives 1000 km per orbit or
about 16,000 km per day. Earth's circumference is 40,000 km nominally;
and an ISS orbit is about 5% longer.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.
  #10  
Old February 17th 10, 04:31 PM posted to sci.space.station
Jorge R. Frank
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Posts: 2,089
Default Multi-day rendezvous

Cap'n Ahab The Non-Arab wrote:
On Tue, 9 Feb 2010 22:11:53 -0000, "Eddie Lyons"
wrote:

Docking hasn't gotten harder. The trajectory is optimised for fuel
efficiency. There's no particular rush to get there so why waste fuel just
to get there quicker? The time is also used to conduct the RMS/OBSS
inspection of the thermal protection system, to check out and prepare the
EMUs that will be used for EVA, and to prepare the docking system.


...One must still ask if an M=1 launch is capable, with the Shuttle
arriving within one orbit from launch.


No. Not enough time to do even the post-insertion configuration of the
orbiter. And the amount of braking required would produce enough plume
impingement to seriously damage just about any target. M=4 or M=5 might
be doable, on a very crowded timeline. Plume impingement would still be
a problem.
 




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