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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
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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
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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
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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
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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
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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. |
#7
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Multi-day rendezvous
I don't think its harder, its all down to economy of fuel and whatever else
has to be done in between. Brian -- Brian Gaff....Note, this account does not accept Bcc: email. graphics are great, but the blind can't hear them Email: __________________________________________________ __________________________________________________ __________ "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? |
#8
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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
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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
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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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