Planning rendezvous from launch

[SabreRedleg]

I'm planning an Apollo style Mun mission in the BTSM mod, where life support and electric power will both be pretty limited on the lander. So I want to dock with the command module as quickly as possible on ascent. I've gotten pretty comfortable setting up a rendezvous between two ships in different orbits, but sometimes it can take several orbits for their relative positions to make a rendezvous feasible. Any tips on planning launch timing in order to make the rendezvous in the first orbit?

Thanks!

Edited April 30, 2014 by SabreRedleg

[Dkmdlb]

Wait until the target ship is about 1/8th or less* of its orbit away from passing over the launchpad and launch to the same altitude as the target.

* This will vary depending on factors such as target altitude, orbital eccentricity, launch vehicle TWR, launch profile, body characteristics, etc.

Edited April 30, 2014 by Dkmdlb

[FREEFALL1984]

I just tend to do it by eye, depending on the body your lifting from. If I have to overcome a thick atmosphere I always launch early, if not I like to launch late and keep travelling towards the target marker. it generally takes an orbit or two to get docked, but remember the fastest launchpad to ISS was 6 hours, which is 4 orbits of the earth (wherever that is) and docking in 4 orbits on KSP is fairly easy with some nice timing and a little luck.

[Claw]

Yeah, sometimes I even wait till the target ship is right overhead or just a bit past to be safe. In my opinion, it's better to undershoot and take a half orbit to catch up, than to overshoot and wait forever because you are slightly ahead. You'll end up with the worst case timing if you are slightly ahead and end up having to make crummy choices to fix it.

But like Dkmdlb said, it'll depend on your ships capabilities and the target's parameters.

[arise257]

It just takes practice. It really helps if you know exactly how long it takes for your craft to reach the same altitude as your orbital station (X), and what angle the pink target reticule on the navball indicates at t-x. For instance, my Mun lander takes about 120 seconds to reach an apoapsis of 50km, and the target reticule is at ~40 degrees, retrograde. I just wait for those conditions and launch. Usually, I'll come within 8km of the station - close enough to rendezvous in an orbit or two.

[Tortoise]

Im not sure, but make sure its a little bit infront of you or ahead, because you'll come in at a higher velocity than your target.

I never plan, I just launch and rendezvous

[Pecan]

*whisper* use MJ to 'practice'. F5, let it do the launch to rendezvous for you, not the position of the target ship, F9, do it yourself.

[EdFred]

Like Tortoise, I just eyeball it. If I'm a little ahead on my guess, I just push my apoapsis a ~5km higher (and round it out if I launched waaaay too soon) and let the other ship catch up. If I got a late launch, I bring down the periapsis 5km or so, and chase the other ship down.

[Sirrobert]

Yeah, sometimes I even wait till the target ship is right overhead or just a bit past to be safe. In my opinion, it's better to undershoot and take a half orbit to catch up, than to overshoot and wait forever because you are slightly ahead. You'll end up with the worst case timing if you are slightly ahead and end up having to make crummy choices to fix it.

But like Dkmdlb said, it'll depend on your ships capabilities and the target's parameters.

If you are ahead, get into a higher orbit, and the target will catchup to you instead

[LethalDose]

Yeah, sometimes I even wait till the target ship is right overhead or just a bit past to be safe. In my opinion, it's better to undershoot and take a half orbit to catch up, than to overshoot and wait forever because you are slightly ahead.

This is what I do and would be my advice: Go in to a lower (faster) orbit behind the target then Hohmann transfer up at the right time (some period before you pass under the target).

[Claw]

If you are ahead, get into a higher orbit, and the target will catchup to you instead

You are correct sir, but I didn't want to muddle my opinion with all the reasoning behind it. My preference is to risk launchig a bit late, because the only penalty is waiting. If I go early, the penalty is waiting, expending extra dV, having to flip my ship around two extra times, and other rendezvous complications. (Hence the "crummy choices.")

It's not always a big deal, but I find it's easier to not deal with the extra hassel if I don't have to. (Especially when docking huge parts.)

[Sirrobert]

You are correct sir, but I didn't want to muddle my opinion with all the reasoning behind it. My preference is to risk launchig a bit late, because the only penalty is waiting. If I go early, the penalty is waiting, expending extra dV, having to flip my ship around two extra times, and other rendezvous complications. (Hence the "crummy choices.")

It's not always a big deal, but I find it's easier to not deal with the extra hassel if I don't have to. (Especially when docking huge parts.)

Fair enough. For me it depends in what kind of orbit the target is, if I launch ahead or before.

In a low-ish orbit I prefere to be in a higher orbit than the target (I don't know why, but I just don't feel comfortable about 75km orbits. Mine are almost always above 100km)