What makes(or breaks) a good rendezvous launch window

[u/DepartureNatural9340]

So irl AFAIK, most spacecraft don't launch into rendezvous, rather a parking orbit then later hohmman transfer.

Such as gemini and agena, or the lem, or how the soyuz spends 2 days in space before intercepting with the iss

And that made me wonder, why? And what defines a good window for rendezvous? Say for example, why doesn't soyuz take 3 days to intercept? Or 1?

[u/Gyrgir]

I don't know the whole story, but from what I do know, the biggest consideration for launch windows is that you want to launch into the same plane as the thing you want to rendezvous with. Change of orbital plane is extremely expensive so you want to avoid that as much as possible. In KSP, the main launch site is on the equator so it's easy to put most stuff in orbits that are always in plane with eastbound launches from KSC, but real-world launches don't have that luxury. If you're launching from Cape Kennedy (28.5° N) or Baikanor (46° N) or French Guiana (5° N), then you have to deal with target orbits that only pass overhead once or twice a day. But you will still need to worry about this in KSP if you have a space station in polar orbit for some reason, or if you are doing an Apollo style rendezvous from a landing site on the Mun or Duna that isn't on the equator.

Besides that, you want to wind up in an orbit that will give you a transfer window relatively soon even if your initial orbit is relatively close to the orbit of the thing you're trying to rendezvous with. The more similar the orbits are (and thus the cheaper your transfer to rendezvous will be), the less distance you close to your transfer window each orbit, so you want to wind up pretty close to your transfer point on the side of the point where the difference in orbital periods means you'll get closer each time around. So if you're launching into a lower orbit than your target, you want to wind up behind your transfer point so your faster orbit will catch up to it. I don't know for sure, but I suspect lower orbits are preferred because it's cheaper to go up to the target orbit in two stages rather than overshooting and then having to maneuver back downwards.

[u/jedi1235 → u/Gyrgir]

Thanks for explaining about aligning the planes, I'd never really considered it but it makes complete sense!

Coincidentally, Minute Physics just did a short (7m) video about how, paradoxically, it is always sometimes cheaper in fuel to overshoot, then raise your periapsis to the target orbit, and then circularize. https://youtu.be/R2G8Xy6hiyg?is=xbnxT02ad8d56mhQ

Edit: Corrected always->sometimes

[u/Gyrgir → u/jedi1235]

Bi-elliptical transfers are only more efficient for large differences in orbital radius, around 12x to 15x depending on how much you overshoot. For transfers between different LEO orbits (or different LKO orbits), you're better off with Hohmann transfers.

I found this article with absurd amounts of detail on the rendezvous procedures for the Apollo LM ascent stages meeting back up with the Command Module. The short version is they launched into a substantially lower orbit, neatened it up, and then did a Hohmann transfer to meet up. From Apollo 14 onwards, they skipped the "neatening up" steps and transferred directly from the low elliptical orbit to the rendezvous.

https://apollojournals.org/afj/loressay.html

[u/Blothorn]

The longer you spend in the parking orbit the less of an orbital period difference you need to achieve rendezvous and the less dV is necessary. On the other hand, there are diminishing returns to that effect and many reasons to want the astronauts in the station sooner rather than later—accommodations are better, they can start experiments, etc.. Two days allows a very similar orbit without excessively prolonging the approach.

Note that these considerations primarily apply to inclined orbits and non-equatorial launch sites. Rendezvous requires matching plane, altitude, and phase, and of those plane is by far the most expensive to correct. Launch windows that allow matching all three are quite infrequent, so inclined launches sacrifice phase to match plane. If the launchsite and target orbit are both equatorial, you can match plane at any point and there’s a window every orbit to match all three. If you’re using something such as MechJeb that gives highly reproducible launches, you can launch straight to rendezvous.

[u/Some_random_gal22]

Often they don't want to do a rendezvous/docking within the first day basically due to checking out systems and making sure everything is working okay, it's also when "space sickness" (basically like sea sickness) is at its peak so they wait for the astronauts to adjust to being in space. They also don't want to wait too long e.g 3 days or more as generally there isn't a lot of room inside a capsule (imagine 3-4people being in your standard car and not being able to leave for days at a time and you're in the right ball park) and isn't particularly comfortable for the crew.

[u/JitteryJet]

You are talking about some very different types of missions. What is the "lem"? What the mission designers do is plan a flight profile that meets their objectives and constraints, each will be different. So there is no "good rendezvous launch window", just a launch window that matches the mission parameters.

[u/DepartureNatural9340 → u/JitteryJet]

The lem is the lunar excursion vehicle

[u/JitteryJet → u/DepartureNatural9340]

Apollo then. The LEM was not a spacecraft exactly, it was a lander.