Rendezvous timing

[MrAnonymous]

Best way to estimate the timing for launching so you can rendezvous in orbit at just the right time

[lajoswinkler]

Yes. It exists.

[Vanamonde]

MrAnonymous, here's what I do: Save the game, launch and fly to the orbital height of the target, note how far the target has moved in that time, load the save, then launch again when the target is that margin away from KSC.

lajoswinkler, please keep in mind that not everyone on the forum has the same ability to articulate questions regarding the mechanics of spaceflight, especially since English is not everyone's native language. If you have something helpful to offer, please do so. Otherwise, simply skip to the next thread.

[Vlk]

I used to launch randomly, without timing, then make a higher/lower orbit, and wait. But lately i started to try to time launches, and compared to my previous rendezvouses, even a guess of correct time makes it much much easier and quicker (and effective). I think you don't have to be too precise, you can adjust the orbit even with very small deltavs. And you can decide how much deltav you can allow. (The lower deltav, the more time you need to wait)

[lajoswinkler]

Oh come on, I was just joking.

English is not my native language, either.

MrAnonymous, exact time can't be calculated because you're flying the vessel, introducing abberations in the flight plan which result in orbital deviations. Go Kerbal - trial and error.

[capi3101]

Best guess for a rocket - wait until the target craft is about 450,000 to 500,000 meters uprange of KSC before launching. That'll get you within a hundred klicks in my experience, close enough to adjust and make rendezvous within an orbit.

Spaceplane - haven't really tried to estimate a "best" range yet.

It is a matter of trial and error, and not a little bit affected by how exactly you fly.

[Kasuha]

If you want to make direct rendezvous just for the heck of it, then do what Vanamonde wrote. It means you will spend twice the time on it as you need to launch twice. You also need to perform the two launches the same way because if you time the gravity turn differently, you'll end up at different place.

If you just want to rendezvous in shortest real time, just take enough fuel for corrections and use time warp wisely. For rendezvous at LKO, raise your orbit high. For rendezvous at high orbit, start at LKO. Then it's matter of one orbit and a well placed maneuver.

[Claw]

If you are just looking for a rule of thumb to get you started, the lower your target is in orbit, the sooner you should launch when it passes over KSC. If it is higher in orbit, then you should wait for it to be further past KSC.

Imagine a target in a 100km orbit. If it is straight overhead when you launch, you will be only slightly behind it when you reach an 80km orbit because of the curved trajectory of the launch. Since you are lower, your relative speed will be slightly faster and you will slowly catch up until it is time to burn and match APs.

Now imagine something in geostationary orbit on the other side of Kerbin. That target stays 180 degrees on the other side of Kerbin from KSC. When you launch from KSC, you will end up slightly past your ideal burn time to match your apoapsis (because you are slighly downrange from KSC).

It's a rough start, but from there you can get some experience and do some trial and error like others have suggested. I'm sure someone created a mod or online app for that, but I don't know of any myself.

Edit: incline orbits are another challenge in themselves. I'm sure there is a host of techniques for that, but your biggest concern is launching when KSC is nearing the inclined orbit. The position of the target will be a second consideration if you can also match up the rule of thumb above.

Edited February 6, 2014 by Claw

[Taki117]

On average 20 to 25 degrees from directly above the KSC is about the best you're going to do.

[KerbMav]

Copy/Paste from a google hit that wont open on my office computer ...

"A general rule of thumb is to launch when your target is just rising in the west (unless your target is in a retrograde orbit)."

[SpeedDaemon]

I usually don't bother with trying to rendezvous directly, but on the first orbit, since it's much easier. Time the launch so you end up a little behind your target. Instead of circularizing, just burn enough to get you PE out of atmosphere and rendezvous when you hit AP the second time. If you end up too far behind, burn to get PE out of atmosphere, then burn again next orbit to rendezvous.

[Jimmy Jazz]

I found that the easiest way for me is to try for a direct rendezvous, but I'll usualy let the target ship stay a bit behind me when I reach it's hight. Then it's just a task of geting my apogee sligtly above the targeted orbit and burning at it until I get a close encounter, so the ship later catches up with me. When I get to the encounter I just kill my relative velocity, and voila.

[vetrox]

Doesnt mechjeb have the option for this?

[Vanamonde]

do what Vanamonde wrote. It means you will spend twice the time on it as you need to launch twice.

This is true, though after a few run you get a feel for it, and can get close enough without the rehearsal flight.

[Greenspan]

If your target is very low orbit ~70-90km, it's much better to launch too early and end up ahead of the target vs too late and end up behind the target.

If your ahead of it, it is easy to raise your AP and rendezvous in one orbit. If your behind it, it's a pain because you have to lower your PE into the atmosphere to catch up with it, which is doable but a PITA.

Don't think too much about doing a direct rendezvous, it's near impossible w/o mechjeb our some autopilot, your better off just eyeballing it and launching a little early, then use the maneuver nodes to account for the error and rendezvous in 1 orbit.

Edited February 6, 2014 by Greenspan