How to RV with high orbit capsule?

[Thucydides]

Hello, 
I have been following the Scott Manley tutorials on YouTube to get better at KSP. Unfortunately, after putting Bob onto Minimus I ran out of fuel on the way back and he is stuck in very wide orbit around Kerbal. I have been trying in vain to get a probe up to rescue him but I can't seem to figure out how to get this RV right. I have a screen cap of his orbit but I'm not sure how upload it here. Mun often obstructs my craft's orbit, or they run out of fuel before I can get them into a similar orbit with Bob's lander. Contact chevron's appear very far apart and I don't know how to bring them closer together. It's a lot hardert han rescuing a Kerbal from low orbit. 

Edited March 19, 2016 by Thucydides

[Vim Razz]

Put your rescue vehicle an orbit that touches your stranded vehicle's periapse.  Then set up a maneuver node where your orbits meet and see if you can get a reasonable encounter.  If you can't, wait for an orbit and see if you can set up a maneuver then, or if you need to wait another orbit and try again.

If the encounter indicators don't seem to move as you play with the maneuver node, you might need to pass them first before experimenting with the node for the next possible encounter.

[Thucydides]

Thank you Vim Razz, 
I have been trying something along these lines but still not having much luck. With limited power and fuel my rescue vehicle's orbit is much smaller than the target's, so whilst it can orbit many times during a single orbit of the target vehicle, it never seems to give me a good rv. Perhaps it's just trial and error?  I shall have another go with your suggested method tomorrow. Are there any 'tricks' like when going to Mun I was told it was best to wait till it was at about 90 from the intended intersect point? Does that work with this kind of thing? 

If I miss the time when the target is at its periapse, I have to wait a whole other year or at least, many months.

If I try and make my orbit match their's (someone recommended ont he KSP facebook page), it of course takes about 2 minutes burn and by the end, I don't have much at all. 

Thank you for your help and patience. 

[Vim Razz]

42 minutes ago, Thucydides said:

If I miss the time when the target is at its periapse, I have to wait a whole other year or at least, many months.

If I try and make my orbit match their's (someone recommended ont he KSP facebook page), it of course takes about 2 minutes burn and by the end, I don't have much at all.

Wow, that must be a rather large orbit around Kerbin if it's period is months long.

You will need enough fuel in the rescue vehicle to "match orbits" in the end, or at least come close enough for your stranded kerbal to EVA over from one ship to the other, and enough to get back to Kerbin again afterwards.  So if you're not certain that you have enough dV in your rescue craft, then that might be something to verify before getting another vehicle stranded in the rescue attempt.

How circular is the target vehicles orbit?.  I've assumed based on the "Minmus return" comment that it was pretty oblong with one end much closer to Kerbin than the other, but if that's not correct (that is, if it's more circular), then the periapse might not be the easiest place to catch it, timing wise.

[Snark]

To @Thucydides:  it would be easier to offer specific advice if you could post a screenshot of your orbit.  Here's how to do that:

 

[Plusck]

6 hours ago, Thucydides said:

Contact chevron's appear very far apart and I don't know how to bring them closer together. It's a lot hardert han rescuing a Kerbal from low orbit. 

The "easy" way to get the contact chevrons to close, or at least the easy way that I have found to do it, is as follows:

1) Assuming you have lower orbit for your initial craft, and a slightly higher orbit for the target craft, plot a node to touch the target orbit (as close as reasonable, best not to try too hard if it is far off). If you already have chevrons appearing, skip this step.

2) Place a new node between the point that your target's chevron appears and your own craft's chevron (i.e. the one that gives the distance to target). Leave it at 0 m/s for the moment.

3) Place another node a bit further along in the orbit. About a quarter should be fine. Leave it at 0 m/s. Now, the chevrons will indicate the target's position and distance at the end of that orbit, not at the beginning.

4) Go back to the node you placed in 2) above. If you add prograde/retrograde to it, the chevrons for the next orbit around will move. You should be able to get a decent appproach without using too much fuel.

4b) if this takes a lot of fuel, then one single orbit is not enough. Go to the node you placed in 3), right click it, and then click on the "next orbit" button on the bottom right. The chevrons now show the positions of your two craft two complete orbits later. If you go back to node 2), and add prograde/retrograde, you should get a match with a lot less fuel.

5) Burn! Once you've deleted the node at 2), you'll be sure to find a slight difference. If the target is still behind, burn a smidgeon prograde then click and hold node 3) to see what changes. If it is ahead you'll have to turn around (or use RCS) to burn back a bit. You can also click and hold the node 3) while burning to see what changes in real time.

If this makes sense (sort of) but you're not quite sure about any of these steps, I could post screenies. Don't hesitate to ask.

Edited March 20, 2016 by Plusck

[Plusck]

 

Oh and... forgot to add the final bit to the method.

Once you're closing in at the end, about an eighth or less of an orbit around (and no more), use the navball while watching the map, and turn so that you are facing the "anti-target" icon on the navball. When in "target" mode, the retrograde marker should be close to the anti-target marker. What you want to so is burn while aiming on the other side of the retrograde marker compared to anti-target. This has the effect of pushing retrograde over antitarget. Keep watching the map and mouse over the chevrons to make sure they are showing an ever closer encounter. Stop doing this when the result is close.

Once you have a really close encounter, drop a final maneuvre node on the two chevrons. Add anything to give a number of m/s equal to your speed relative to target. It doesn't really matter which direction, because what you really need to know is (a) exact time to closest approach, and (b) burn time to neutralise that amount.

You can then switch to external view, target mode, and turn to face retrograde. Looking back behind your craft, the target should be there. When it is close, do a full-thrust blast (again aiming to push retrograde over anti-target, if it has started to move off) to force a recalculation of burn time. If you burn retrograde about 50-60% of that time away from the target, you should finish your burn with the target extremely close, and with relative velocities cancelled.

[Kryxal]

I'd probably take the easy way out ... do a plane change before setting up the intercept (which means it's probably going to cost a couple hundred m/s assuming a similar inclination to Minmus) then just play with your transfer.  Since you're in the same plane, you can just make a node to get you out there and slide it around the orbit to improve closest approach.  A quickie delta-v budget from orbit is in the 2k m/s range (300 for plane change, 950 for Minmus-like apoapsis, 200 for rough velocity match, 200 for deorbit, and an extra 350).

A capsule on top of a terrier with one FL-T400 fuel tank puts you in the 2600 m/s range (2400 with heat shield with ablator at 40%, probably a good idea), even more to spare.

[Thucydides]

Thanks to Snark for instruction on how to upload the image: Hope this works. This is slightly altered now due to some 'getting out and push' in EVA, the orbit is more eliptical, with a lower periapse, but the problems remain largely the same from what I can tell. I'll load it up this evening and get another screenshot and try the above suggestions. Thank you for all your help thusfar.
 

[OhioBob]

13 hours ago, Vim Razz said:

Put your rescue vehicle an orbit that touches your stranded vehicle's periapse.  Then set up a maneuver node where your orbits meet and see if you can get a reasonable encounter.  If you can't, wait for an orbit and see if you can set up a maneuver then, or if you need to wait another orbit and try again.

When the rescue vehicle reaches the stranded vehicle's periapsis, what you can do is perform a burn to adjust the rescue vehicle's orbital period so that the two vehicles will meet at a controlled time.  For instance, let's say that when the rescue vehicle arrives at the stranded vehicle's periapsis, the stranded vehicle is exactly 120 hours from periapsis.  Change the rescue vehicle's orbital period so that it will complete some whole number of orbits in that 120 hour period.  For example, if the rescue vehicle's has an orbital period of 28 hours, then perform a burn to raise its periapsis until it has an orbital period of exactly 30 hours.  Now the two vehicles will meet at the same location and time after 4 complete orbits of the rescue vehicle.

[Thucydides]

Hello, 

So this is the new orbit of the craft; I'll now proceed to try to use your advice above to get a rescue sorted.  

[Thucydides]

19 hours ago, Plusck said:

The "easy" way to get the contact chevrons to close, or at least the easy way that I have found to do it, is as follows:

1) Assuming you have lower orbit for your initial craft, and a slightly higher orbit for the target craft, plot a node to touch the target orbit (as close as reasonable, best not to try too hard if it is far off). If you already have chevrons appearing, skip this step.

2) Place a new node between the point that your target's chevron appears and your own craft's chevron (i.e. the one that gives the distance to target). Leave it at 0 m/s for the moment.

3) Place another node a bit further along in the orbit. About a quarter should be fine. Leave it at 0 m/s. Now, the chevrons will indicate the target's position and distance at the end of that orbit, not at the beginning.

 

Hi Plusck, could you explain a bit more about where to place the manouvre node? Presumably that is on the rescue crafts orbit (sorry for the dumb questions) 

[Plusck]

34 minutes ago, Thucydides said:

Hi Plusck, could you explain a bit more about where to place the manouvre node? Presumably that is on the rescue crafts orbit (sorry for the dumb questions) 

Here are some screenshots of a highly, highly elliptical intercept. I tried to show each step. I hope it's clear... :erm:

 

The purpose of the third, 0 m/s node is essentially to make sure that you can see the intercept, because the intercept should logically happen more or less exactly where you placed the second node (the one that ends up being 265 m/s). Since it's at the same place, the game will otherwise only show the immediate crossing of paths, not the one you want the next orbit around.

Edited March 20, 2016 by Plusck