Help get Jeb and Bill home

[tomf]

Jeb and Bill are going to get home in exactly a year.

They are in a spaceplane with about 1000m/s of delta-v

After a series of gravitaional assists that seemed like a good idea at the time, they are have just left Kerbin's SOI in a resonant orbit with PE just outside Moho and AP somewhere around Duna. They will re-encounter Kerbin in a years time. If it is relevant they are currently on the outbound leg of their orbit.

The problem is that on the current course they will meet kerbin at ~6000m/s, far too fast to capture without exploding due to heat.

Does anyone have any ideas of a plan that might save them? Ideally one that doesn't involve chasing after them with a massive fuel truck, because I did that already to give them enough fuel to escape from Laythe and get back to Kerbin.

[Evanitis]

As you stated in another thread, gravity assisting will be the key here. Though I'm not really a math guy to tell if Mun can slow you down in itself to remain in Kerbin's SoI. Probably not. Though doing a slingshot also make your burns more effective. If you burned that 1000 m/s while slinging by the Mun, it would probably be enough. I guess.

Though you could get a better assist from a planet. It's really hard to tell without a picture of your Kerbol if Duna, Eve maybe Jool can be reached by a little burn (or by waiting a few orbits ). But planetary mass is a lot higher than munar one, so if I had to pick a place to burn most of my fuel, I chose one of them.

Edited February 19, 2016 by Evanitis

[Snark]

So.  The fact that they're going to be at such high relative speed is due to their orbit crossing Kerbin's at an angle (like an "X"), rather than being parallel to it.  So the only way you're going to reduce that velocity significantly (without spending many thousands of dV) is to adjust either the periapsis or their apoapsis to be at the level of Kerbin's orbit.

So, you need to either raise their Pe up to Kerbin, or lower their Ap down to Kerbin.  I don't have the numbers in front of me to figure out what those values would be.  How much dV does each one of those numbers need?  I suspect that 1000 m/s of dV is not gonna cut it, though.

Basically, you need to find a way to get into a less-eccentric, more-circular orbit for minimal fuel expenditure.

Don't worry about actually  encountering Kerbin, just get your orbit to where you need it to be.  Once you've got a good orbit, you can use multiple-orbit-ahead planning to get a Kerbin encounter for a relatively small amount of dV, as long as you're willing to wait a few orbits to do so.

The other option would be a rescue mission.  Don't try to tinker with your orbit, just go ahead and go for the 6 km/s Kerbin encounter, which your built-in dV can reduce to 5 km/s.  Build a rescue craft on Kerbin that has ~20 km/s of dV.  (Yes, that's a lot, but if you make it small and light, ion-powered, and use drop-tanks to deal with xenon tanks' abysmal mass ratio, it's not too hard).  Edit:  Actually, you don't really need 20 km/s:  the bare minimum is twice the spaceplane's Kerbin-relative speed, plus however much dV to come home after that; you can get by with less dV if you're willing to come home slowly.  For example, if the spaceplane is going past Kerbin at 5 km/s:  you'll need 5 km/s to match speed with it, then after the kerbals have transferred, you'll need another 5 km/s to stop fleeing home and kill your Kerbin-relative velocity; then you'll need however-much dV to head home (more if you want to come home faster).

All it needs is a couple of Mk1 lander cans to hold the kerbals (or if you really want to go bare-bones style, a couple of command chairs).  When the spaceplane goes shrieking past, it accelerates to match velocities, Bill and Jeb move over to the rescue craft, and then it turns around and goes home while the now-uncrewed spaceplane heads out beyond Eeloo.  If you use command chairs to keep it super-light, then you'd need to give 'em another vehicle to do reentry with once they get to LKO.

Edited February 19, 2016 by Snark

[tbob]

I like that so much I'm gonna have to get Jeb in some trouble just so I have an excuse to build that hot-rod convertible rescue sled.

[Snark]

38 minutes ago, tbob said:

I like that so much I'm gonna have to get Jeb in some trouble just so I have an excuse to build that hot-rod convertible rescue sled.

Well, calling it a "hot-rod" may be a stretch. 

Yes, it's got a lot of dV, but you're looking at a  lot of time doing ion burns.  Even a very lightweight ion craft takes a very long time to do a 5 km/s burn, and you have to do two of them, minimum (one to accelerate to match velocities with the target, then another to kill that velocity so you can come home), followed by whatever burns you do to come home again.  It won't actually require 20 km/s of dV; there's 10-from-LKO, minimum, just to catch up to the target and then kill the outward velocity, but at that point you could come home more slowly if you like.

The main thing to bear in mind if you want to build super-high-dV ion craft is to use drop tanks.  A lot.  Xenon tanks have really horrible mass ratios; the empty tanks are heavy.  If all you do is just load up your craft with lots of xenon tanks that stay with you, you're killing your dV because you're lugging around a lot of mass tied up in empty tanks.  You want to ditch those tanks as soon as they're empty.

Edited February 19, 2016 by Snark