Mid-transfer rendezvous?

[NFunky]

Hey all,

So I got KSP a couple months ago and have been enjoying the hell out of it! In fact, I haven't been this into a game in years and I'm still discovering new things each time I start it up.

Anyway, I've got a career going (my second career, but first with contracts) and I've been doing okay, despite the fact that it's basically been a comedy of errors throughout. Most of my mistakes tend to stem from lack of planning and attention to detail, which brings me to my current dilemma. Here's the situation...

I sent a science lander to the Mun, everything went fine and then I tried to head home. Lo-and-behold, I guess I used a bit more fuel than I should have during decent and landing, cause it turned out I only had enough to get into low munar orbit, but not escape velocity!

So, I've got plenty of cash and science, so I unlock the AGU and some other stuff to allow me to launch a rescue/refueling mission. All goes well, and I make a very nice rendezvous in munar orbit and split the remaining fuel between the two craft, which is more than enough to bring both back (and decelerate them, since I'm using FAR and deadly reentry). So I disengage the AGU and set up return burns for each ship individually. This means that, while they'll be on relatively similar transfer orbits, they'll be seperated by quite a distance for much of the trip.

This all looks fine, except for the fact that I forgot I was using TAC life support and never transfered life support resources when I did the fuel, and one of my guys only has about 3/4 enough to get back home! And my last save was after leaving the mun's SOI, and the ships were already quite far from each other.

Long story (really long, sorry about that) short, I need to re-rendezvous my ships, but they're now on long, Kerbicentric transfer orbits. I have absolutely no idea how to do a rendezvous when essentially not in orbit. I guess what I need to know is how to rendezvous within less than half an orbit, where both orbits are extremely eccentric. Is this even possible? Am I better off launching a new resupply ship and attempting a rendezvous like that? Is my Kerbal just doomed to die within shouting distance of home and safety? HELP!

[Technical Ben]

Depends on how far out you are. You could manually tweak nodes until it shows you reaching them. I'm not sure if KSP shows that info when not in orbit...

Is it possible? I've done an aero capture on Laythe when I launched my rover and forgot to attach a landing rocket (so sent another probe on an intercept course about 2 mins after the first). Catching that was FUN [hard]! So anything is possible.

[Starman4308]

As I see it, your best bet is to burn one vehicle directly towards the other, with only small consideration for orbital mechanics. Orbital mechanics starts to screw "burn straight for it" maneuvers proportional to the fraction of an orbit completed, and you probably have at best 1/4 of an orbit to catch up. The more delta-V you have left, the better: you can make more aggressive, more direct maneuvers, and you don't have the time for efficiency.

Assuming you use the trailing vehicle, I imagine your maneuver would be mostly prograde with a bit of radial-in; tweak it to get a good intercept, and don't be afraid of correction burns. Do, of course, ensure you have enough delta-V to cancel relative velocities at the rendezvous: this will have the advantage of putting the rendezvousing craft back on a Kerbin reentry orbit.

If you just do not have the delta-V on your two vehicles to manage that, you're going to need to build a delta-V monster capable of meeting the troubled vehicle halfway, and then completely reversing trajectory. What I would do is mark the target's periapsis, and set a maneuver node to match the target orbit, just going the opposite way*.

*Make sure you launch in the correct direction for this: you might have to launch west from KSC instead of east.

Edited November 2, 2014 by Starman4308

[Plusk]

Depending on how far apart they are, and how different ejection orbits were, either set manouver nodes correctly, or simply target the thing you need to chase down and BUUUUUURN As long as there enough dv and youre far away from Kerbin, there shouldnt be a problem. But to be honest i never done that, as im always planning my missions ahead in detail. One cannot underestimate the power of pen and paper (and a calculator) in KSP

Edit: Its 110% doable on interplanetary orbits, however on the timeframe you have during inter-Kerbin orbits, it all depends on how much dv you are willing/able to spend.

Edited November 2, 2014 by Plusk

[Kryxal]

I think you only have a few hours to make the maneuver in ... target the one that needs saving, what's the range and relative motion? If you can't get close with 40% of your fuel in time, I don't think it's worth trying with that ship, go for the delta-v monster.

[NFunky]

Hey guys, those were some quick answers! Thanks!

I have an fl400 split between the two small vessels, so I think I'll have enough delta-v. I'm going to try it when I get home tonight. I've got hope again!