Rescue a kerbal in a highly eliptical solar orbit (no, not a contract)

[juanml82]

So, at some point during the 1.0 release I decided to send a cheap ship in my career game to get the science from "low over the sun" while I was still getting used to the new atmo and (changing) heat systems.

So I've designed a 1.25 stack with a nerv engine, a bunch of science stuff in a service bay, an mk1 landing can and a shielded docking port. No need for heatshields, right? It would just take a bit of fuel to circularize at Kerbin during Ap, or I could just aerocapture, right??

Well, months have passed and, long after that ship was dispatched, I learnt the obvious: no, you can't forgo a heatshield (or several!) in such a mission profile and, no, it doesn't take "a bit" of fuel to circularize while zooming through Kerbin's SOI within a couple hours. It takes 5,000 dV, in a ship that has some 1,200 dV remaining.

And now that I'm trying to rescue my pilot (and the science!), I wonder exactly how.

I could try a rendezvous in solar orbit, which would cost an horrendous amount of dV, but is doable with ion engines (actually, I have a ship designed: the Rhino from the ascend stage makes it to orbit and kicks first, then an orange tank with MRS 2.5 nukes and finally the ions). Should I send that ship out of Kerbin right after the poor Sundiver ship zooms through?

The other option I'm thinking is to try to keep using gravity assists to raise the Ap to Jool, see if I have enough fuel left to get a slingshot out of Tylo and get rescued by whatever ship happens to be there within the 7-10 in-game years it would take for Sundiver to make it there.

Any other ideas? Oh, and obviously, I'm not using any life support, else my poor lost kerbal would be dead before I get to him. Here's a screenshot of Sundiver's orbit after a slingshot from Kerbin slightly raises the orbit and a projected burn for which I don't have enough fuel

[wibou7]

Wait, kerbin PE at 75 797m? That's awfully close...
You could send a very small ship with a tremendous TWR to encounter with your sundiver while it is close to Kerbin. Match orbit, then transfer the Kerbal, then burn so your rescue ship get a PE deep (20km?) inside Kerbin atmosphere. If you pack enough heatshield on your rescue ship, it should make it.

It shouldn't need that much DV to perform. Hell, if you use the Klaw, you should be able to manage it with the 1200 DV left on the sundiver.

 

[juanml82]

32 minutes ago, wibou7 said:

Wait, kerbin PE at 75 797m? That's awfully close...
You could send a very small ship with a tremendous TWR to encounter with your sundiver while it is close to Kerbin. Match orbit, then transfer the Kerbal, then burn so your rescue ship get a PE deep (20km?) inside Kerbin atmosphere. If you pack enough heatshield on your rescue ship, it should make it.

It shouldn't need that much DV to perform. Hell, if you use the Klaw, you should be able to manage it with the 1200 DV left on the sundiver.

 

I refined the Pe to get the most of the gravity slingshot.

I tried to rendezvous a ship, but matching velocities requires a 7,500 dV burn. I'm now thinking about putting Sundiver's Pe rather high above Kerbin, say between the Mun and Minmus. If I'm right, and I'm often not right, matching velocities would still require a huge burn, but the rescuer would already have burned a lot to make it to the RV point... but does that even make sense? Sundiver would still be travelling at fast solar orbit speeds, even if they are slower relative to Kerbin, and, supposedly, the Ap has the lower orbital speed so the rescuer should still need a lot of dV to match speeds

[Snark]

I've been in a very similar situation just recently, as it turns out, though for a very different reason.  I've been playing with the Outer Planets mod, and was sending a craft home from Sarnus, and didn't want to wait a decade to do it, and got ... impatient.  I'm too embarrassed to say how fast I was going when I hit Kerbin's SoI, but suffice to say it was faster than you're dealing with.  By, um, a lot.  And it turns out that at those speeds, a heat shield is pretty much useless.  I was able to shave off maybe 1 km/s of speed by aerobraking, but that was the best I could do-- any deeper, and the ship just exploded, heatshield or no heatshield.  So a rescue mission was in order.

What you need is not a high TWR, just a high dV.  Keep the rescue ship as light as possible, maybe even just give it a command chair in lieu of a pod.  If you make it ion-powered and use drop tanks (to cope with the absolutely abysmal mass ratio of xenon tanks), it's no trouble at all to build a craft with 15 km/s or more of dV.

Set it up to start accelerating away from Kerbin before the stranded ship has even arrived, so that you can get a matched-velocity intercept with it right away.  As soon as you rendezvous, you move your kerbal across, brake to a halt relative to Kerbin, then turn around and go home.

The amount of dV your rescue ship needs is basically 2x the stranded ship's Kerbin-relative velocity (once to match velocity with the target, then a second time  to brake to a Kerbin-relative halt), plus however much dV you want to use to come home (can be low if you want, it's basically a question of how long you're willing to wait to get home after the rendezvous).

[FancyMouse]

The reason you need far more deltaV than needed is because your trajectory is not tangent to Kerbin's orbit. This means your own velocity (relative to Sun) can't cancel out Kerbin's velocity effectively, and what's more, orthogonal component is introduced to increase your velocity relative to Kerbin.

When doing Solar orbit stuff, my habit is to always control in such a way that either Pe or Ap (depending on whether my destination is farther or closer to the Sun than Kerbin) is tangent to Kerbin's orbit. In this way, I can just control orbital period to be a multiple of a year and make sure I can reenter Kerbin at the right angle.

A typical sundiving for me would be, eject to Sun low orbit first, complete one orbit going back to Kerbin orbital height (but Kerbin is not there of course) - then I'll adjust my periapsis there so that I can encounter Kerbin a couple of periods later.

Another scenario is previous hilarious contract when tourists ask for sub-orbital Sun flight. I tackle this by eject to an orbit of 5-year period, complete the sub-orbital part at apoapsis, then restore back to the previous orbit, and finish the tour for 5 years in total.

Now given this, I don't know how much it can help your scenario. I don't know if it's possible for you to bring down Ap to Kerbin orbital height, then adjust Pe so that you can encounter Kerbin a couple of periods later. Not sure if 1.2km/s dV is enough for the whole series of maneuvers. My guess is yes, but I'm not sure. You need to plot the maneuver node and see.

Edited February 22, 2016 by FancyMouse

[StarManta]

14 hours ago, FancyMouse said:

Now given this, I don't know how much it can help your scenario. I don't know if it's possible for you to bring down Ap to Kerbin orbital height, then adjust Pe so that you can encounter Kerbin a couple of periods later. Not sure if 1.2km/s dV is enough for the whole series of maneuvers. My guess is yes, but I'm not sure. You need to plot the maneuver node and see.

Given where OP's periapsis is in the original image, I'm certain it will be easier for him to raise his apoapsis up to Jool than to lower it down to Kerbin. From there he could probably pretty easily attain an eccentric Jool orbit, to be rescued by a future Jool mission.

If he times the launch of the rescue craft right he might not even need Jool - he could send the rescue craft up so that its apoapsis is close to the Kerbal's. This would require some pretty lucky timing though - getting them to reach the apoapsis at the same time would be a trick and a half.

[juanml82]

I've made it!

I change the approach of Sundiver to have a Pe near the Mun orbit, as trying to match in LKO meant I'd dip into the atmosphere, but I think I went too far - the matching velocities burn required 8800 dV, so I should have tried to aim to a Pe around 500-1000 km and matching velocities with the rescuer there.

Anyway, after burning almost all those 8800 dV (basically, emptying an orange tank with MRS' quad nukes) I've managed an intercept some 10 minutes after exiting Kerbin's SOI.

 

 

And then I've manually plotted a 4,200 dV burn to return to Kerbin in a more circular orbit which requires some 2,000 dV to capture... but I later tried MJ and it beat me with a 4,400 burn which ends, once refined to lower the Kerbin Pe, into a less than 1,000 capture burn

[DancesWithSquirrels]

I've built some 16-17km dV ships with a lander can and a single nuke engine, using 8x asparagus staging on TT-70 mounts (all Mk1s) around a 4x Mk1 liquid fuel fuselage, with a pair of thermal control modules out on cubic octagonal strut outriggers. I had to rescue a poor little green man out between Jool and Eeloo. Don't forget some solar panels and a full heat shield for re-entry!!!

TWR is horrible for the first couple of stagings, but improves considerably...

Edited February 28, 2016 by DancesWithSquirrels