Rendevous Method

[Waxing_Kibbous]

Just looking for some feedback on this method I stumbled on by accident. I found that if I am a far off from my target that I can burn a bit to make an elliptical orbit to get a close approach at Pe as opposed to simply time accelerating and play catch up. When I near the target I kill my speed relative to it, getting me back into a similar orbit . The downside is that obviously more fuel needed, but it's not that much really. Anybody else have any tips for efficient orbital interception?

[Retread]

The most fool proof method I've found is this:

1. Match your target's orbit as closely as possible. At this stage, it doesn't matter how far out of phase the two vessels are.

2. At periapsis, burn prograde until you get a close intercept (which should also take place close to periapsis)

3. At intercept, burn retrograde to kill relative velocity, fine tuning as necessary to acchienve rendezvous.

Think of it in terms of orbital periods. For example, both you and the target are in the same orbit, taking 1 hour for a complete circuit of Kerbin. If the target is 15 minutes behind you, you need to lengthen your orbit by 15 minutes in order to arrive at the same place at the same time next time around.

If your target is ahead of you, you could also decrease your orbital period by making the initial burn retrograde. However, this is not possible in most cases because you will hit the atmosphere or the ground unless the target is very close.

[Alshain]

The most efficient orbital rendezvous is to do it when you launch. You don't establish orbit till you are slowing down your relative velocity. This takes some practice, on KSC wait till your target is just leaving the desert shores area as it circles around. Make sure you set it as the target right away, then launch your craft and do a normal ascent until your Ap crosses paths. You might have to burn just a little further but the closest approach should split into two before you find it. The procedure itself isn't all that tricky, it's just guessing when to actually launch. Even if you guess wrong, you can just extend your Ap a little and find an intersect and if that doesn't work you should be pretty close and you can just use the other method. Best to guess launching early than late. If you launch late, it gets a bit more difficult to find.

As you approach your target, slow your relative velocity like you always do but be warned, the speed difference will be much larger than using other methods so you need to slow down a lot. This will put you into orbit.

You can do this with your target in any orbit but the dynamic changes if it is not equatorial because you have to wait for the planet to spin under the target's orbit and it becomes a lot harder.

Edited October 26, 2015 by Alshain

[Waxing_Kibbous]

Alshane: Yeah, I've got pretty good at meeting up with a vessel at launch, but there are times, like doing a multiple rescue, or meeting up from one orbit to another where things are harder to sync.

That technique is nicely displayed here if anyone wants a video version:

Retread: I think you said what I did, only much more coherently But yeah, that's what I've been doing and it seems to work out well.

[Alshain]

Alshane: Yeah, I've got pretty good at meeting up with a vessel at launch, but there are times, like doing a multiple rescue, or meeting up from one orbit to another where things are harder to sync.

That technique is nicely displayed here if anyone wants a video version:

Retread: I think you said what I did, only much more coherently But yeah, that's what I've been doing and it seems to work out well.

Yep, that is it. Definitely the best way to go, if it is an equatorial. I have yet to meet up with a polar orbit like this, I've seen it done, but ultimately the craft is never in the right position while I'm under the orbit, so I just launch polar and do it the old way.

[Snark]

Alshane: Yeah, I've got pretty good at meeting up with a vessel at launch, but there are times, like doing a multiple rescue, or meeting up from one orbit to another where things are harder to sync.

So, in the specific case of doing multiple rescues in LKO (i.e. you have one ship and want to rescue 4 kerbals who are all in LKO orbits), here's a useful technique that lets you sweep them all up with very little dV expenditure:

1. Rescue kerbal #1 by doing a direct-from-launch rendezvous. You're now in a circular orbit in LKO, pretty close to the same orbit as the other kerbals, but offset from them by some amount of longitude.

2. Do a small burn to raise your Ap up, say by 50-100 m/s. Coast up to Ap, then when you're up there, lower your Pe so that it's slightly lower than the lowest remaining kerbal's orbit (if it isn't already).

3. You're now on an orbit that intersects all of their orbits, and with a slightly longer period than them.

4. Do as many orbits as you need for the next kerbal to have a close approach. You can do a very small Pe burn (prograde or retrograde) to get a close approach within 2.3 km.

5. When you get that close approach, don't try to match velocity. Just switch to the kerbal you're rescuing, and fly them to your ship using their EVA pack. Kerbals have around 300 m/s of dV in their EVA packs, and it's free fuel. Far more efficient to match the Kerbal's velocity to your ship than vice versa.

6. Repeat steps 4-5 until all the kerbals are swept up.

7. Profit!

Also: per Retread's advice, that's a good technique. If you're patient and are willing to exchange "need more orbits to get intercept" for "save fuel", you can do a smaller burn that raises your Ap only slightly, so that you get an intercept that's a few orbits in the future rather than next orbit.