Thread: The last bit of an orbital rendezvous

1. The last bit of an orbital rendezvous

Just a quick question. Does anyone here know the dynamics of the last stage of a docking rendezvous? From 'in convoy' ----------> 'physically attached'.

I'm curious because if you're initially behind the target and speed up, you increase altitude and pass on the outside, and if you're behind and slow down, you pass the target on the inside.

So I'm guessing relative approaches that are tangential or radial to your orbits are out? Relative approach happens at some angle? And this has to be calculated with extreme accuracy? I mean, you can't just get within 50m, say, and then wing it by sight?

Unless of course the freaky orbital dynamics I mentioned above aren't that important once you get as close as 50m? In other words you can close 50m quickly enough, and that the change in relative altitude is small enough to ignore/easily adjust for?

P.S. Any technical links would be highly appreciated. Especially if they have pictures.

2. I found this....

http://www.bogan.ca/orbits/sstravel/rendezvous.htm

it seems to answer your question, but I just skimmed through it.

3. Originally Posted by NickW
I found this....

http://www.bogan.ca/orbits/sstravel/rendezvous.htm

it seems to answer your question, but I just skimmed through it.
That certainly helps, thanks.

4. The nasty rendezvous calculations are used to get within about 100-200 meters of the target with most of the relative velocity nulled (the 'in convoy' position). Once you get that close, it's possible to eyeball the docking if you have a visual docking target and some way of measuring the range and rate of closure.* You are correct that if the orbits are similar the counterintuitive effects of orbital dynamics are small enough to easily compensate for.

The shuttle usually uses an "R-Bar" approach, meaning the rendezvous point is about 180m directly below the target (along the radial vector). For STS-125, Atlantis will then slowly close with Hubble from below. I don't remember why, but when the LDEF was retrieved during STS-32, Columbia had to approach it from above.

Now that the shuttle docking port on the ISS faces forward (along the velocity vector, or V-Bar) docking shuttles execute a maneuver called TORVA (Twice Orbital Rate V-Bar Approach) to slowly rotate from the terminal point 180m below to a position 120m in front of the station. This maneuver is also performed visually.

The ATV uses a slightly different method, the V-Bar approach. The terminal rendezvous point is 250m behind the station, in line with Zvezda's rear docking port. IIRC, the Gemini program also used a V-Bar approach.

For diagrams of shuttle rendezvous, download any of the mission-specific rendezvous checklists from these pages:
http://www.nasa.gov/centers/johnson/...les/index.html
http://www.nasa.gov/centers/johnson/...a_archive.html

A diagram of the ATV approach and docking is available here:
http://esamultimedia.esa.int/docs/AT...ule_3April.pdf

*In a worst-case scenario, this could consist of calibrated markings on the window and a stopwatch.

5. Originally Posted by GeorgeLeRoyTirebiter
...
That was exactly what I was looking for. Thanks a lot GeorgeLRT.

Orbital dynamics don't seem to be involved at all during the final stage of the rendezvous. They track and target, using active sensors and thrusters.

In the Atlantis pack they also included the Rescue rendezvous. Interestingly, this is a Vbar approach, just like Orbiter-ISS, but unlike Atlantis-Hubble.

I've thumbnailed the four different types below, in case anyone's interested.

No1: Atlantis-Hubble. Rbar; Atlantis approaches from below.
No2: Rescue-Atlantis. Vbar; Rescue orbiter approaches Atlantis from the front (backs up if you will)
No3: Orbiter-ISS. Vbar; Orbiter approaches ISS from the front
No4: ESA ATV-ISS. Vbar; ATV approaches ISS from behind
Last edited by PraedSt; 2009-May-12 at 06:09 PM. Reason: Grammar

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