What is the most efficient way to briefly escape Kerbin's SOI for science?

[Rabada]

I've been playing KSP for a very long time, and there is one orbital maneuver I cannot seem to figure out. In most of my career mode games I usually do a mission where I will have Jeb escape Kerbin's SOI to gather some good science. Usually it seems to take about 950 ish dV to get on a Kerbin escape trajectory, and another 950 dV for me to plot a kerbin return. I can't figure out a way to plot a return trajectory after escaping Kerbin's SOI that doesn't use a lot of dV, and I feel like it should be possible. I usually play with lifesupport mods, so I don't want Jeb to be on too long of a mission.

What is the most efficient way to briefly escape Kerbin's SOI and safely return?

Edited October 14, 2015 by Rabada

[Snark]

The most efficient way to escape is also the slowest. Launch as if you're going into the lowest possible LKO you can manage (just above the atmosphere), but keep accelerating to raise your apoapsis higher and higher until it just barely escapes Kerbin. This will be slow (will take a couple of weeks), but makes the return trip the cheapest.

(You can use a Mun slingshot maneuver if you like, but it's not really going to save much dV, and unless you tweak it just right, you're likely to end up ejecting Kerbin's SoI fast enough that you need more dV to come home.)

This means when you exit Kerbin's SoI, you're moving really really slowly and with only a small radial-outward component to your motion.

So as soon as you're outside, grab your science and point the nose of your ship straight at Kerbin, then do a modest burn (say, 100 m/s). Then wait. You'll re-enter the SoI pretty soon, and then can fine-tune your periapsis. (A more scientific way, once you're outside, is to go to map view and add a maneuver node close in front of your ship, then adjust it to send you to Kerbin.)

By the way-- while you're doing it, make sure to make it a crewed vessel. Stepping outside Kerbin's SoI is the only way to get to level 3-- planting flags on Mun and Minmus will only take you to 2.

[Rabada]

The most efficient way to escape is also the slowest. Launch as if you're going into the lowest possible LKO you can manage (just above the atmosphere), but keep accelerating to raise your apoapsis higher and higher until it just barely escapes Kerbin. This will be slow (will take a couple of weeks), but makes the return trip the cheapest.

(You can use a Mun slingshot maneuver if you like, but it's not really going to save much dV, and unless you tweak it just right, you're likely to end up ejecting Kerbin's SoI fast enough that you need more dV to come home.)

This means when you exit Kerbin's SoI, you're moving really really slowly and with only a small radial-outward component to your motion.

So as soon as you're outside, grab your science and point the nose of your ship straight at Kerbin, then do a modest burn (say, 100 m/s). Then wait. You'll re-enter the SoI pretty soon, and then can fine-tune your periapsis. (A more scientific way, once you're outside, is to go to map view and add a maneuver node close in front of your ship, then adjust it to send you to Kerbin.)

By the way-- while you're doing it, make sure to make it a crewed vessel. Stepping outside Kerbin's SoI is the only way to get to level 3-- planting flags on Mun and Minmus will only take you to 2.

Ah! That makes sense! I was mostly messing around with ejecting prograde and retrograde, I never thought to try ejecting anti-radially. I had an ejection angle mostly anti-radial and a bit prograde relative to Kerbins orbit. This put Jeb on an orbit slightly larger than Kerbin's and a bit offset once he escaped Kerbin's SOI. To return, I only had to burn 80 m/s retrograde with a little bit radial burn and I got a Kerbin intercept with a 25km periapsis 9 hours after making the escape. This makes perfect sense to me now. By ejecting radially with a bit of prograde, I eject Jeb ahead of Kerbin's orbit, but I sent him into an orbit that is higher and thus slower than Kerbin's, so that allows Kerbin to catch up. And the little burn seemed to speed up this process.

Thanks!

Edit: Also, the reason I am doing this in my game is mostly to get a bunch of Kerbals some experience to level them up, thanks again!

Edited October 14, 2015 by Rabada

[Van Disaster]

The absolute most efficient way is probably a slingshot from Mun *and* Minmus - I've used Munar slingshots to Minmus a few times for economy challenges. A bit awkward if Minmus is a long way off plane, mind. If you got it right you could probably borrow Minmus to reorient on the way back in too, given it's pretty near the edge of SoI as it is.

[Pecan]

You know how orbital mechanicss is utterly insane?

Like, if you want to catch up with something ahead of you in orbit you should slow down? Or, if you want to meet something behind you in orbit you should speed up?

Guess what - just leaving Kerbin (any body/planet) SOI by going prograde = going faster = once you leave the SOI, with a slightly higher solar Ap, you're also going to have a slightly longer/slower orbital period so will automatically fall back into the SOI as the planet catches up with you. Going to Duna, for instance, you have to make sure your Ap is so high you 'dodge' the whole SOI, otherwise it's all back to Kerbin.

Which also means ... just leaving a body's SOI by going retrograde = going slower = catching up = you'll also re-enter the SOI; unless your Pe has become low enough to transit to, for instance, Eve.

TL;DR - "just" leaving a SOI prograde/retrograde, there's no need to burn back into it at all, you'll fall into it automagically.

[Rabada]

The absolute most efficient way is probably a slingshot from Mun *and* Minmus - I've used Munar slingshots to Minmus a few times for economy challenges. A bit awkward if Minmus is a long way off plane, mind. If you got it right you could probably borrow Minmus to reorient on the way back in too, given it's pretty near the edge of SoI as it is.

I personally never found Munar gravity assists that useful. The amount of dV you save never really seemed worth it to me. However I do find reverse gravity assists to be incredibly useful. I sent out 4 probes to Jool earlier today for contracts and science around the moons. I was able to set up a Laythe Reverse gravity assist to capture each probe by only spending about 10 M/s of dV. I should write up a guide on how to do that.

[Van Disaster]

I personally never found Munar gravity assists that useful. The amount of dV you save never really seemed worth it to me. However I do find reverse gravity assists to be incredibly useful. I sent out 4 probes to Jool earlier today for contracts and science around the moons. I was able to set up a Laythe Reverse gravity assist to capture each probe by only spending about 10 M/s of dV. I should write up a guide on how to do that.

Apparently saved me ~100dV ( and a fair bit of travel time ) going to Minmus in the last one I made notes of. I doubt that was anywhere near perfect because I was more interested in getting there quicker than cheaper ( which was daft, because the trip was about getting there cheaply, but I didn't want to spend all year setting it up ).

And yes, Jool system pinball is fun too

[CaptainArchmage]

The best way is to be going as close to zero m/s as you leave Kerbin's SOI. If you have the upgraded tracking centre just get the parabola out of the SOI barely, else accelerate until you see "escaping" on the map view status instead of "orbit" or "sub orbit". Leave, do science, and thrust back towards Kerbin.

Probably been ninja'd a few times now.

[Empiro]

I personally never found Munar gravity assists that useful. The amount of dV you save never really seemed worth it to me. However I do find reverse gravity assists to be incredibly useful. I sent out 4 probes to Jool earlier today for contracts and science around the moons. I was able to set up a Laythe Reverse gravity assist to capture each probe by only spending about 10 M/s of dV. I should write up a guide on how to do that.

Gravity assists in Jool are much more effective because the inner moons are much more massive and have a much faster orbital velocity.

[Snark]

Gravity assists in Jool are much more effective because the inner moons are much more massive and have a much faster orbital velocity.

Amen to that!

If you ever play the New Horizons mod (lots of fun, it completely rearranges the solar system in addition to adding a bunch of planets and moons), it has a lot of really nice slingshot opportunities. For example, it has one hell-planet orbiting toe-scorchingly close to the sun, which takes a metric crapload of dV to get to (think Moho, squared). But there's also a hot gas giant orbiting super close too, which you can use for a reverse slingshot to save a bundle.

[maltesh]

You know how orbital mechanicss is utterly insane?

Like, if you want to catch up with something ahead of you in orbit you should slow down? Or, if you want to meet something behind you in orbit you should speed up?

Guess what - just leaving Kerbin (any body/planet) SOI by going prograde = going faster = once you leave the SOI, with a slightly higher solar Ap, you're also going to have a slightly longer/slower orbital period so will automatically fall back into the SOI as the planet catches up with you. Going to Duna, for instance, you have to make sure your Ap is so high you 'dodge' the whole SOI, otherwise it's all back to Kerbin.

Which also means ... just leaving a body's SOI by going retrograde = going slower = catching up = you'll also re-enter the SOI; unless your Pe has become low enough to transit to, for instance, Eve.

TL;DR - "just" leaving a SOI prograde/retrograde, there's no need to burn back into it at all, you'll fall into it automagically.

If you leave the SOI with a low relative velocity, moving in the world's prograde direction, not only are you at your closest point to the world in question, but you're moving faster than the world around the sun, which means your apoapsis is going to be further from the world's orbit than the radius of the SOI.

Similarly, if you leave the SOI with a low relative velocity, moving in the world's retrograde direction, you're again at your closest point to the world in question at the SOI boundary, and moving slower than the world in question. Your periapse, by necessity, is going to be further from the world's orbit than the radius of the SOI.

Can I ask if this is something you've successfully done in KSP, and which worlds you've tried it out with?

[Rabada]

If you leave the SOI with a low relative velocity, moving in the world's prograde direction, not only are you at your closest point to the world in question, but you're moving faster than the world around the sun, which means your apoapsis is going to be further from the world's orbit than the radius of the SOI.

Similarly, if you leave the SOI with a low relative velocity, moving in the world's retrograde direction, you're again at your closest point to the world in question at the SOI boundary, and moving slower than the world in question. Your periapse, by necessity, is going to be further from the world's orbit than the radius of the SOI.

Can I ask if this is something you've successfully done in KSP, and which worlds you've tried it out with?

Thanks to advice in this thread, I successfully sent Jeb from an 80 km Low Kerbin Orbit to briefly exiting Kerbin's SOI and entering the Sun's SOI for a couple hours, and then I was able to return Jeb to Kerbin. All together it took a little over a thousand m/s of dV. The ejection burn from Low Kerbin Orbit to a Kerbin escape trajectory cost about 950 m/s of d/v, then once Jeb escaped Kerbin's SOI, I needed a 90 m/s burn to plot a return trajectory back to Kerbin with a 25 km periapsis for an aerocapture.

When I first tried to accomplish this, I originally attempted to plot escape trajectories similiar to the ones you describnes, the escape trajectories at first had Jeb exit Kerbin's SOI moving in Kerbin's prograde direction, and then I tried again having Jeb exit in Kerbin's retrograde direction. However to return to Kerbin in a short amount of time from either direction cost a lot of dV.

After I posted this thread, I read Snark's reply where he suggested that I exit Kerbin's SOI moving radially out from Kerbin. By exiting radially out, Jeb ended up in an orbit with a slightly higher apoapsis and a lower periapsis than Kerbin's orbit, but a very similiar orbital period. After Jeb exited Kerbin's SOI, it only took a small retrograde burn to return Jeb to Kerbin from the Sun's SOI.