Basic Question About Docking

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kiwigavin

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OK, I've got the hang of orbital rendezvous (in theory anyway). Buzz Aldrin talks at length in "Men From Earth" about concentric orbits and the fun early Gemini missions had trying to eyeball a rendezvous. Now excuse me for a dumb newbie question but I can't find anything via Google about this: I want to know how we dock. The physics not the mechanics. Not computers, not radar not automation just plane (sic) direction of travel and the direction of the forces we apply to get there.<br /><br />So we've rv'ed and are now station keeping. Here's my questions:<br /><br />1. What is our orientation wrt direction of travel? Are we lined up so the "axis of approach" is the same as direction of travel or are we 90 degrees out with one vehicle closer to earth than the other?<br /><br />The latter seems more feasible. That way to dock you just speed up and wait for you orbital radius to increase by the necessary hundreds of feet. However, all the photos of the shuttle docked seem to show it along the direction of travel.<br /><br />2. Is there a distance between objects below which you can just point and go? I assume when the CSM pulled the LM free it just moved forward a few dozen feet, did a 180, then moved forward back the way it came. <i> Even grammar can't handle orbital mechanics it seems </i>. Is it the same when you get within a few feet of the ISS?<br /><br />If anyone can point me at something WID PIKTURES that explains it I'd be jolly grateful.
 
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CalliArcale

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From following Shuttle-ISS missions, I've noticed that the Orbiter always approaches the station from in front of it. Approach is on the axis of travel. Of course, the Orbiter is lucky in that it docks along the station's long axis. Most of the Soyuz/Progress ports are not so conveniently positioned. (And, of course, in the very early days of ISS construction, it sometimes docked along other axis. It was docked on the nadir when it delivered Destiny.) I don't think the ISS gets reoriented for those dockings. (Axis of docking obviously has to match the axis of the docking port, so a Soyuz, for instance, may approach from below.) But I could be wrong about that. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"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."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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kiwigavin

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OK, That's interesting -I thought the various Progresses/Soyuz/Shuttles etc docked at one point and then were later re-positioned.
 
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kiwigavin

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Thanks SG. So, if they're approaching from the R bar do they thrust along it? Or do they still thrust along the direction of travel and allow the vehicle to move up the R bar?<br /><br />PS was at your "joint" this weekend and got a Commander's Pass for the year. So if you want to meet and explain using coffee and donuts give me the word...
 
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CalliArcale

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<blockquote><font class="small">In reply to:</font><hr /><p>OK, That's interesting -I thought the various Progresses/Soyuz/Shuttles etc docked at one point and then were later re-positioned.<p><hr /></p></p></blockquote><br /><br />Initially, Soyuz/Progress had the choice of Zvezda's aft and the nadirs of Zarya and Zvezda. (Zvezda's aft is often preferred for Progress, but this is because it's the only port that can accept propellant transfers into Zvezda's tanks). The Pirs module was installed on Zvezda's nadir port a few years ago. Pirs functions as both a docking module and an airlock for the Russian suits. So there are still three ports for Soyuz and Progress. The vehicles are occasionally shuffled around as needed to allow vehicles to dock to Zvezda's aft port.<br /><br />The Space Shuttle Orbiters must dock to one of the PMAs (Pressurized Mating Adapters). There are two at the station currently, but in practice only the forward one is used. I imagine this has mostly to do with clearance; the Orbiter is pretty big.<br /><br />The ATV (Automated Transfer Vehicle), built to deliver European cargo, will dock probably around March 30. It will dock to Zvezda's aft port. I'm not sure if it can dock to any of the others. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"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."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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