Shuttle time to reach ISS

zanderose · Nov 8, 2006

The shuttle after reaching orbit will take almost 2 days chasing the ISS. I know the ISS is also orbing the earth at very high speeds, However Nasa knows exactly where the ISS is at a particular moment. Why cant the launch the space craft either the Shuttle from Nasa or the Soyuz from Russia at least to reach orbit within a shorter distance from the ISS and dock within a day or just hours

rfoshaug · Nov 8, 2006

<blockquote>In reply to:<hr />The shuttle after reaching orbit will take almost 2 days chasing the ISS. I know the ISS is also orbing the earth at very high speeds, However Nasa knows exactly where the ISS is at a particular moment. Why cant the launch the space craft either the Shuttle from Nasa or the Soyuz from Russia at least to reach orbit within a shorter distance from the ISS and dock within a day or just hours<hr /></blockquote> That's a good question and it deserves a detailed answer. In this post I did a small "orbital mechanics 101", so I guess this is "orbital mechanics 102"... <img src="/images/icons/wink.gif" /> Imagine looking at a globe. Around the globe you hold a ring. This ring is large enough that it can go all the way around the globe without touching it. Imagine holding this ring along the equator of the globe. This ring would represent the orbit of a spacecraft. The spacecraft would move around the globe along the ring, heading east (almost all spacecraft are launched more or less eastward to take advantage of the earth's rotation, which of course is in that direction - that's why stars and the moon and sun rise in East and sets in West). Imagine launching a spacecraft from Kennedy Space Center (which is located at about 28° north of the equator) or Baikonur Cosmodrome (which is 46° north of the equator). If you had to go all the way down to the equator and then turn east, this would cost a lot of fuel. This is why they decided to place ISS in an orbit with a 51.6° inclination so that it can easily be reached from these launch sites. This inclination means that you must hold that ring not around equator, but tilt it 51.6 d <div class="Discussion_UserSignature"> ----------------------------------My minds have many opinions </div>

willpittenger · Nov 8, 2006

So what was the difference between an ISS flight and the Gemini missions where they had to rendezvous in the first orbit? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

MeteorWayne · Nov 8, 2006

Two things I can think of. First, we were pushing the envelope (and risking lives more in the process) for a defined goal. Beat the Russians to the moon. I wonder (but do not know) what the inclinations of the Mercury and Geminii missions were. {Note to self...find out!}, but I imagine they were in orbits that were the most efficient possible. I would also suspect that for a rendevous mission they kept the orbits as low as possible. Again, I don't know, but when you're pushing the envelope, push as little as you can. Other differences are that the 59 degree ISS inclination is not optimum for a launch from KSC, and the shuttle is MUCH more massive than a Gemini capsule. Mercury and Gemini were "shoot the rock at the target" type missions. They were small rocks with brave humans inside. We didn't really know how long everything would work right, so the time was worth more than the value and expense. Despite all it's shortcomings, once you get the shuttle in orbit, time is not as critical an issue. You can take some of that time, give the folks some rest, open the cooling sustem, check that everything's OK before you head toward a target. Relax a little <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

mrmorris · Nov 8, 2006

I have never been able to get an explanation that I'm truly satisfied with on why the shuttle and the Soyuz take so long to get to the ISS. I did a good bit of Googling on the subject when I was researching for my G-X3 thread, as I really wanted to be able to reasonably/realistically plan for G-X3 to make it from launch to docking within 10 hours, and preferebly closer to half that. SG stated the same thing then as now -- that the slower approach takes less dv. This statement doesn't really make any sense without being supported by something else. To reach the ISS, a spacecraft gets inserted into orbit A, generally about 185 km. It then has to issue a burn for a Hohman transfer orbit to 350-400km (said orbit will take about 45 minutes to complete). It then has to issue a set of smaller burns to rendezvous and dock. The positions of ISS and the spacecraft need to be at a given angle when the burn for the Hohman transfer is initiated, but it doesn't really matter whether that angle is met on the first orbit, or the hundred-and-first orbit, the actual energy required for the burn is the same. If the two are in the same orbital inclination, then the only issue is a single angle, and simply waiting for the spacecraft to 'catch-up' to put the ISS into the right positon for the HT. Presumably this could be targeted at launch time such that only an orbit or two would be required before the HT burn. It would require a more precise insertion... but precise insertions aren't really a problems... I don't think... but maybe. If the spacecraft is being inserted at an inclination *other* than 59 degrees, then you have a whole 'nother set of angles involved -- probably somebody has named them theta or sigmi, or maybe etta beta pi. At that point it would be a matter of the spacecraft catching up to the ISS *and* the two inclinations crossing. Even so -- it still seems to be a matter of insertion accuracy. As to the shuttle -- they do in

nacnud · Nov 8, 2006

I did read a very detailed answer to this question somewhere on nasaspaceflight.com but I can't seem to find it right now <img src="/images/icons/frown.gif" /> But there was a lot more reasons than just the propellant usage problem. There were operational constraints due to converting the shuttle to space based flight after launch. Crew considerations, for instance the sleep patterns of the crew, they normally sleep soon after entering orbit as they have been up for a long time preparing to launch also space sickness is worst in the first few days in zero gee. Phasing issues between the ISS and the shuttle, etc. Interestingly the soyuz and progress also take a similar time to dock with the iss.

CalliArcale · Nov 9, 2006

Operational constraints is a good point. I think the first flight day is pretty much spent just getting the thing into orbital configuration. (Well, what's left of flight day one after the launch is done, that is.) And now they do that RMS/boom inspection too. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

willpittenger · Nov 9, 2006

I should have mentioned the Genesis mission was meant to mimic the conditions encountered by an emergency launch from the Moon by a lander. Also, how did they get the rendezvous in before the first orbit was complete? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

qso1 · Nov 9, 2006

willpittenger: So what was the difference between an ISS flight and the Gemini missions where they had to rendezvous in the first orbit? Me: To add to what MeteorWayne said, orbital inclination for Gemini missions varied with mission but were within a range of 28 to 33 degrees, Mercury went into similarly inclined orbits. Mercury never maneuvered or docked with any spacecraft. Actual docking with a target did not occur until Gemini 8 which docked with an Agena vehicle some 6 hours after launch. I'd say the main reason for why Gemini met and docked with its targets faster is due to that being the primary objective of Gemini. Get the docking mission underway as rapidly as possible. Today the shuttle takes longer because safety overrides speed. The crew may have a lot of pre docking tasks on their plate and at least one pre dock sleep period may be necessary to be in prime shape for the docking with ISS and post docking busy period that usually follows. There is simply no need to get to ISS in a hurry. <div class="Discussion_UserSignature"> My borrowed quote for the time being:There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened. </div>

qso1 · Nov 9, 2006

willpittenger: Also, how did they get the rendezvous in before the first orbit was complete? Me: Gemini 8 took at least 3 orbits before docking with its GATV. Gemini 9 could not dock with its substitute docking vehicle because the shroud failed to separate but they did rendezveous with it well over 4 hours or more after launch, at least 2 to 3 orbits. Gemini 10 also got within maneuvering range of its Agena target after almost 5 hours. Similar timelines appear to have been the case for the last 2 Gemini flights. That is, typically 4 to 5 hours after launch, docking with the targets had been accomplished. <div class="Discussion_UserSignature"> My borrowed quote for the time being:There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened. </div>

MeteorWayne · Nov 9, 2006

Thanks for the info on the Mercury/Gemini inclinations, qso. I'd never even thought about that question before yesterday. Where did you dig that up? Basically, I think it comes down to the fact that in the Gemini missions, the docking was the primary mission goal, so get it out of the way so you can accomplish that even if the mission is cut short. With the Shuttle/ISS missions, docking is only the beginning, not the mission objective, which is to build the station. It takes time to ready the shuttle for space after orbit, and a rested crew is essential to achieve the mission objectives. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

mrmorris · Nov 9, 2006

"No, I said it takes more propellant." OK -- my confusion lies in the slippery realm of semantics. dv is being used with two different definitions (essentially 'potential dv' and 'actual dv'). 'Propellant' and 'dv' are normally used almost interchangeably when talking about a spacecraft altering its orbit (even though the two are obviously not synonyms). When asked how much maneuvering capacity the shuttle has, it's valid to aswer the question in terms of either dv or in propellant mass. Generally, the question would be answered in terms of dv, as that's one step closer to the intended use of such a question. However, dv when spoken of in this fashion would better be termed 'potential dv', or perhaps 'optimum dv'. If the propellant is burned in less than optimum fashion, then the actual change in velocity generated may be less than this figure. Anyway -- I see what you're saying. Are you aware of any online mission logs for the shuttle for ISS missions? I found some shuttle mission logs online, but pre-ISS. There *are* a couple of Mir missions in the logs, with detailed data on burn times and dv. At that time at least, the orbiters used a single long burn to match orbits. I'd like to see how the burns are done now for comparison.

qso1 · Nov 11, 2006

MeteorWayne: Where did you dig that up? Me: I knew it from way way back before the internet but I then looked for a web source which wasn't easy but I found one at: www-pao.ksc.nasa.gov/history/gemini/gemini.htm Unfortunately I couldn't pull it up on this computer but did so on my home computer. You might get it one yours, if not, let me know and I'll send you a link from home. <div class="Discussion_UserSignature"> My borrowed quote for the time being:There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened. </div>

qso1 · Nov 11, 2006

Can't seem to link up to the ISS stuff at the NASA website but there are some detailed ISS links there. Heres one link I managed to get to. http://www.hq.nasa.gov/office/pao/History/humansp.html. <div class="Discussion_UserSignature"> My borrowed quote for the time being:There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened. </div>

MeteorWayne · Nov 11, 2006

Thanks, qso1. I was able to dock with the Gemini site <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

qso1 · Nov 11, 2006

Good deal, glad I could help. <div class="Discussion_UserSignature"> My borrowed quote for the time being:There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened. </div>

gunsandrockets · Nov 13, 2006

"What was the difference between an ISS flight and the Gemini missions where they had to rendezvous in the first orbit?" Gemini XI launched 97 minutes after the launch of the Agena target vehicle, placing both vehicles in a path very close to each other. That is what allowed the Gemini XI to make a first orbit rendezvous. http://www.astronautix.com/flights/gemini11.htm When a launch window opens to the ISS, the Shuttle ends up in the same orbital plane as the ISS but the ISS could be orbiting as far away as the opposite side of the planet. So the Shuttle needs to catch up to the ISS before making the Shuttle's final orbital adjustment.

gunsandrockets · Nov 13, 2006

Well said! Thanx for making so clear a subject which seemed so arcane to me.

willpittenger · Nov 14, 2006

That still does not explain how they are different. How did they time Gemini XI's launch so that the target was close? Couldn't the shuttle do the same -- at least in an emergency? <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>

mrmorris · Nov 14, 2006

"How did they time Gemini XI's launch so that the target was close?" For one thing, while I couldn't verify it via Googling, I assume that Gemini XI was launched into a 28.5 degree inclination. At least I don't know why they wouldn't have. Assuming this, that would mean that instead of having two launch windows per day from KSC into the proper plane -- it would be essentially continuous. Even then, the rendezvous launch window for Gemini XI was tiny. from here, it's indicated that it was two seconds. "Couldn't the shuttle do the same -- at least in an emergency? " I just did *almost* these calcs in my Dragon thread in B&T, so I'll give the rough numbers here. 50% of the answer is a matter of orbits and launch windows. There are two opportunities each day to launch into the plane of the ISS, or 730 annually. The shuttle can only use one of those due to the ET-disposal and abort considerations... down to 365. The shuttle has to dock during daylight hours... down to ~182 annually. Of course the shuttle currently has to launch during daylight hours which would take us town to 90, but I'll assume this has been lifted for the 'emergency'. With 182 launches, this means that in any given month, there are about 15 launch windows (using only the above constraints). In my Dragon thread I worked out the time (hopefully correctly) required to 'catch-up' to the ISS if a spacecraft is launched into a 185km orbit. It came out to a worst-case scenario of ~36 hours. The time required *should* be spread very evenly over the timeframe of 0-36 hours. We can assume then that of our 15 launch windows for the month, ~7 will take 18 hours or less, ~4 will take 9 hours or less, and ~2 will take four hours or less. O

MeteorWayne · Nov 14, 2006

g&r hit it here, let's see if I can add a bit. Gemini XI launched exactly 1 orbit later, so the planes aligned at the time the target was nearly overhead. So it was kind of a "throw a rock at the target" launch. When launching to the high inclination ISS orbit, the most important thing is that the planes are aligned. That's the 5 minute launch window. Where it is in it's orbit, is nowhere near as important (energy wise), since catching up is easy. Just launch into a lower orbit and wait for a while to catch up. Hope this clarified things a bit. MW <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

MeteorWayne · Nov 14, 2006

I agree but with the shuttle to ISS launch, the constraining factor is the plane, rather than the position of the target in it's orbit. That's the point I was trying to make. The right velocity is not as important as long as you make orbit and have sufficient delta v available to boost the apogee and perigee to the target's orbit. All that is required is time. If you launch in the wrong plane, the propellant will be used to change the plane (complicated, and costly, fuel wise) rather than adjusting position and height. Position takes care of itself, height is what you save the propellant for. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

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