Free return trajectories...

[MaverickSawyer]

Greetings, all. I have been looking high and low for a tutorial or guide on how to make free return trajectories... No luck, either with the search function or with manual browsing.

So, I pose this question: How would one go about making a free return trajectory without any advanced autopilots (Lazor, Mechjeb, kOS, etc.)? So far every time I go to the Mun or Minmus SoIs, I have to enter orbit in order to plot a path home. Otherwise, I'd be slung out into interplanetary space.

Edited January 26, 2014 by MaverickSawyer

[zaky133]

During your transfer burn or even before it, you should add a manoevre and mess around with the blue, pink and yellow markers until you see that the trajectory will enter the SOI of the moon, pass through it and then orbit around kerbin. If you are getting shot off into a path around the sun, it is most likely you are going too fast.

[Greenspan]

I was just about to ask the same thing! Does the Mun even have enough gravity to perform a free return?

[MaverickSawyer]

Hrm... I'm also skimming in real close, so the gravity slings me around more sharply...

However, I'm ultimately aiming for an Apollo-style "figure-8" run that will dump me back to Kerbin below 1,000 km. Haven't been able to make it work yet...

[Taki117]

It is possible to perform a free return trajectory from the Man, however the "Apollo figure 8" is not possible due to how KSP simulates gravity wells. What's going to end up happening is your Munar periapsis is going to be rediculously high.

Edited January 26, 2014 by Taki117

[regex]

You can do a free return fairly easily. Put a maneuver node up and aim for a retrograde Munar orbit ("ahead" of the Mun), then adjust it until your return puts you in Kerbin's atmosphere.

[maccollo]

It is possible to perform a free return trajectory from the Man, however the "Apollo figure 8" is not possible due to how KSP simulates gravity wells. What's going to end up happening is your Munar periapsis is going to be rediculously high.

That's mainly because of the scale of things in the KSP universe.

In the RSS you can do the figure 8 pretty easily, altho it can get a bit tricky if your inclination doesn't match the moon.

[Mako]

Mulbin achieved it with his Munbug with a burn of 878 m/s

MAJOR BREAKTHROUGH!

OK I've just hit a major goal today with the Munbug development. Munbug VIII just did this... purely with the third stage!

http://i1024.photobucket.com/albums/y302/bloobot/screenshot8_zps2f95c0c2.jpg

For those not familiar with the Apollo flight plans, what you are looking at is a Free Return Trajectory... just like the real thing

Munbug VI / VII didn't quite have the DV to achieve this (only standard Munar intercept). This now means I'm getting pretty close to the authentic Apollo flight plan... the third stage even has a tiny drop of fuel left to crash it into the moon!

[PDCWolf]

Starting from a 75km circular orbit, the smallest possible amount of deltaV for a free return is 868m/s. To get a free return with an aero-capture back at Kerbin, you have to tweak both the position of your ejection burn and it's duration. Also remember most of the time mid-course maneuvers are required to achieve perfection, those maneuvers are done by adding a few m/s on the radial/antiradial vector with RCS, or you can do them at your mun periapsis via prograde/retrograde burns (or RCS, depending on how much deltaV do you need).

Can't really show you anything right now because I just don't have media of that kind available.

[MaverickSawyer]

Wound up tinkering with the flight path... three MCCs scattered over nearly 2 days, and a pair of Munar flybys, left me with a PeA of 30km. Not a true free return, but far less dV spent than going through the orbital insertion, then leaving again. And, since the mission could be completed by a flyby, instead of landing, I could get away with this.

And thank you, Mako, for finding that. I'm going to go over that in detail and try to replicate it.

[maccollo]

There's the more complex free return however, which is a free return from a flyby to another planet. Basically like the 6 month trajectory in the mars direct plan where, you miss the Mars insertion you fly past it and come back to Earth after 1.5 years.

I've been trying to get this one working since I've been playing with RSS recently and getting anything more than a flyby is... Really hard.

But the maneuver is incredibly difficult to execute. Just flying out on a 2 year orbit is easy. The problem comes from the flyby encounter where the orbit is modified significantly from the gravity assist, and I've been having a very hard time to get this offset to match the 2 year interval.

[Mako]

Not a problem. It was the first thing that came to mind when I saw the question. I've flown a version, Munbug X I believe, and managed to get the free return trajectory exactly as Mulbin describes it. It's an amazing craft and I highly recommend it. It was a very fun, very difficult challenge to attempt and it took me quite a few tries to carry it out without any auto-pilot. I'm currently working on an free return trajectory Mun flyby on my first 0.23 Career save. I think it really illustrates the impressive skill and ability of the Apollo scientists and astronauts. Good luck!

[Roastduck]

All my Mun missions start out with a free return trajectory. It takes some correction burns on the way, but they only take maybe 20-30 m/s dV each.

[palioxis1248]

You can do a free return pretty easily by following these steps.

1. Place a maneuver node along your orbit somewhere where you're relatively certain it won't encounter the Mun.

2. Drag on the prograde arrow until the projected apoapsis is slightly above the Mun's orbit. (about 1000km does the trick, I think.)

3. Take the maneuver node and drag it to the area where the transfer window is. A free return trajectory is usually located somewhere near the end of the window.

4. Fiddle around with the node as necessary to get the periapse down to a dozen kilometers, to maximise Oberth effect at Munar periapse.

[Flixxbeatz]

Well you can tinker around with nodes. A bit of timing/luck is needed though, if you want your Periapsis to the destination very low yet you can still go back in without burning.

Edited January 26, 2014 by Flixxbeatz

[Superfluous J]

I do a free return trajectory - almost - in this video here. I linked it to 13 minutes in were I start to set up the node. The whole process takes about 7 minutes mostly because I was very finicky and didn't have RCS to fine tune my orbit.

I had a Mun periapsis of 1685(!) on the maneuver node. I don't know what the Kerbin periapsis was there but it was obviously low, because when I raised my Mun periapsis to something actually practical (10k) my Kerbin periapsis disappeared (again, !). I for some reason futzed with it more and got a Mun periapsis of 32k and a Kerbin return periapsis of 45k. Again, that does not seem "way too high" for me but it could have been lower. Obviously, because it WAS lower earlier.

After the burn, my actual mun periapsis ended up being 22k with no Kerbin periapsis. I must, at the time, have thought that was bad so I mucked with it and actually settled on 14k Mun periapsis and a 49k Kerbin periapsis, both of which are pretty awesome, IMO.

Of course, SOI periapsis change messed it up and I ended up having to burn at Mun to get my Kerbin periapsis back in the atmosphere. If I'd had Kerbal Alarm Clock installed to slow me down on SOI changes it probably would have been a true Free Return. Slow down at SOI changes, kids. It's not just a good idea. It's the law.

[CarlosMaximus]

.... Slow down at SOI changes, kids. It's not just a good idea. It's the law.

LOL!! and +1.

[SRV Ron]

Two types of free return to aerobraking landing are possible. The Apollo Figure Eight is the most difficult to set up using the Add maneuver mode.

Much more common.

[PDCWolf]

Since it's never enough people explaining the same thing:

Starting from a 75x75km, 868m/s are enough to take us there, but not to get us back to a reasonable aerocapture/aerobraking return. We have to tweak both the burn position and it's duration (or rather, the DeltaV used). Here's the planned maneuver with 873m/s. As you can see we get some really nice numbers.

Obviously we are definitely not mechjeb, so executing the maneuver perfectly to the same numbers is impossible, but I got rather close this time, I got some really nice numbers on both sides, and we can fix the Kerbin periapsis later!

Time for probe separation. Notice how I used a docking port as to diminish any perturbation made by ejection forces. Also, the probe has no engine, it's all RCS.

Mun periapsis getting closer, here's where we burn prograde to make our kerbin periapsis lower. Burning prograde on a mun-retrograde orbit means we are burning retrograde on our general Kerbin orbit, that's why Kerbin periapsis decreases. I got it down to 35km, great for a 2-pass aerobrake (I like it this way because I get to regain kerbin orbit and from there I can decide where to land). Notice how much monopropellant I used on the resources tab, it was at 100 units

[Smidge204]

It is possible to perform a free return trajectory from the Man, however the "Apollo figure 8" is not possible due to how KSP simulates gravity wells.

Spoiler: Might have to change the conic section draw mode so see it, but it's possible without too much effort.

Edit: After the burn:

=Smidge=

Edited January 26, 2014 by Smidge204

[MaverickSawyer]

Thank you much, everyone! I'm going to try out some of these suggestions.