Have I found a better way home from the Mun? (no)

[Brainlord Mesomorph]

My usual way home from the Mun is to leave lunar orbit retrograde (to the Mun) which leaves me in a roughly circular orbit of Kerbin just inside lunar orbit. From there, I’d do long retrograde burn at apogee to reduce perigee until it touches the atmosphere for aerobreaking.

But I had a ship that was low on fuel the other day, so I started playing around, and this is what I came up with:

We leave lunar orbit prograde (that seems to take less fuel) just barely enough delta V to get out of the SOI. That puts us in an elliptical Kerbin orbit just *outside* lunar orbit. Apogee around 26,000 km. Then at the perigee of that orbit, we plot a retrograde burn about 15 m/s. Play around in there, you’ll find there is a maneuver that puts you back into the Munar SOI. You can plot a ridiculously tight horseshoe shaped spoon-out around the Mun that cancels a huge amount of your forward velocity.

I turn that 26,000 km apogee into a 2000 km perigee with a 15 m/s burn! That's like two thirds of the way home! Granted, it takes a few days (OK, weeks), but the Kerbals don’t mind and I seem to be saving hundreds and hundreds of m/s of delta V.

In the past, I have stumbled spoon-out maneuvers around the Mun usually while coming home from Minmus. But those were just happy accidents. This is something I can do reliably every time, as if I know what I’m doing. :D

So am I right? Is this something other people should do?

Or (more likely) are one of you about to tell me that I’m fooling myself? “Blah blah blah Holman, blah blah blah Oberth, blah blah blah is really cheaper“

EDIT: NO, a direct burn is the same. Edited November 19, 2015 by Brainlord Mesomorph

[ineon]

Cool stuff. You've given me another method in my arsenal for trading time for dV too :). If you do manage to do it reliably can you post a screenshot of the maneuver (with the mechjeb planner numbers if you use it)?

[panzer1b]

I just approach these problems with giving my ships such absurd dV that i could go to laythe and come back after landing on the mun, but neat info nevertheless. Ill have to take a look at this sometime when im not too busy with capital ships, combat, and overall destruction that i like doing normally. Afterall, its not kerbal space program, its kerbal shooting program, a sci-fi spacecraft combat simulator :D

[cantab]

My usual approach is one burn from low Mun orbit, in the right position, to put my Kerbin periapsis at my chosen value for re-entry. This should take 270-310 m/s or so, depending on how high your Mun orbit is. Of course this method is hard without trajectory predictions and manoeuvre nodes.

[KSK]

I don't see myself using this personally - with the size of Mun ships that I normally use, I really don't need much fuel to get them home using Cantab's approach.

With that said, it does sound like a very cool bit of navigation!

[Brainlord Mesomorph]

[quote name='ineon']Cool stuff. You've given me another method in my arsenal for trading time for dV too :). If you do manage to do it reliably can you post a screenshot of the maneuver (with the mechjeb planner numbers if you use it)?[/QUOTE]

[IMG]http://benmargolis.com/ksp/screenshot329.jpg[/IMG]

Here.
The brown burn is first, leaving lunar orbit.
That puts you in the purple orbit.
Then the green burn puts you in the green orbit.
Which then becomes the red the spoon-out maneuver, and finally leaves you in the blue orbit more than halfway home. Edited November 19, 2015 by Brainlord Mesomorph

[ineon]

Nice. What is your patch conics limit (I think that was what it was called) set to? I'm guessing 5 or 6?

[Brainlord Mesomorph]

[quote name='KSK']I don't see myself using this personally - with the size of Mun ships that I normally use, I really don't need much fuel to get them home using Cantab's approach.

With that said, it does sound like a very cool bit of navigation![/QUOTE]

Thanks.

I'm in early career game. So delta V is my major issue right now. (usually my dual engine nuclear lander can go from LKO to Munar surface and back one tank of gas, the high delta V way)

EDIT: Cantab's approach?? Link pls. Edited November 19, 2015 by Brainlord Mesomorph

[Friend Bear]

[quote name='Brainlord Mesomorph']My usual way home from the Mun is to leave lunar orbit retrograde (to the Mun) which leaves me in a roughly circular orbit of Kerbin just inside lunar orbit. From there, I’d do long retrograde burn at apogee to reduce perigee until it touches the atmosphere for aerobreaking.

But I had a ship that was low on fuel the other day, so I started playing around, and this is what I came up with:

We leave lunar orbit prograde (that seems to take less fuel) just barely enough delta V to get out of the SOI. That puts us in an elliptical Kerbin orbit just *outside* lunar orbit. Apogee around 26,000 km. Then at the perigee of that orbit, we plot a retrograde burn about 15 m/s. Play around in there, you’ll find there is a maneuver that puts you back into the Munar SOI. You can plot a ridiculously tight horseshoe shaped spoon-out around the Mun that cancels a huge amount of your forward velocity.

I turn that 26,000 km apogee into a 2000 km perigee with a 15 m/s burn! That's like two thirds of the way home! Granted, it takes a few days (OK, weeks), but the Kerbals don’t mind and I seem to be saving hundreds and hundreds of m/s of delta V.

In the past, I have stumbled spoon-out maneuvers around the Mun usually while coming home from Minmus. But those were just happy accidents. This is something I can do reliably every time, as if I know what I’m doing. :D

So am I right? Is this something other people should do?

Or (more likely) are one of you about to tell me that I’m fooling myself? “Blah blah blah Holman, blah blah blah Oberth, blah blah blah is really cheaper“[/QUOTE]

I feel the way I do it is much more fuel efficient. Compare to this (at work so I can't do it, myself):

In a circular orbit of mun.
Use a maneuver node to make a burn at prograde at a point near between mun and kerb. Note, near, not directly in between. Just make a nav point with a nice prograde burn applied that puts you just outside mun SOI and drag it around until the hugely eliptical orbit has where you are about, now, at the furthest point out on the eliptical orbit of kerb. The closest point will be incredibly close to kerb. Once out of mun SOI, you can burn just a tad more to fine tune your eliptical orbit into the kerb atmosphere. If you do it delicately enough, you can enter kerb without burning retro to slow down since orbit of mun is not too distant as to cause high speed re-entry.

This was a bad explanation, so in essence, you burn prograde when around the mun until your new, kerb SOI orbit is eliptical with one end at mun distance and the other nearly in the kerb atmosphere.

Sample (kinda): [URL]https://www.youtube.com/watch?v=XXIV7Nfj3_Q[/URL] at 18:00 min, you see him setting up a node. While he reaches kerb atmosphere, this is a little less efficient than I do. BUT, at 18:00 exactly, look where his ship is. (looking downward), at a counter-clockwise orbit, where his ship is is about where the maneuver node should be that i explain above. you burn there, you will get his results, but with less delta-v used.

[I]Ultimately[/I], it is good practice to avoid large burn maneuvers with multiple burn stages. This applies everywhere. Say you want to go to Eve from Kerb. It is hugely inefficient to first leave Kerb and have a matched kerb orbit of kerbol then burn to hit Eve. Instead, you want to burn from inside kerb until your outside Kerb orbit is where you want it to be...meeting Eve. Similar idea here. You don't want to leave Mun then adjust. Burn inside Mun so that your outside Mun orbit is where you want it. Edited November 19, 2015 by Friend Bear

[GoSlash27]

Brainlord Mesomorph,
Here's the approach Cantab's talking about:
[img]http://i52.photobucket.com/albums/g13/GoSlash27/MunRetro_zpsoiss32ck.jpg[/img]
About 270 m/sec straight to reentry. How much is the DV for your 2 burn technique?

Best,
-Slashy

[Brainlord Mesomorph]

What you describe is, I believe, a Holman Transfer (if I have the name right). And I am expecting someone to explain to me whythat’s better.
OTOH: the two burns I show in that screenshotare 126 m/s and 10 m/s. And I *think*that I’ve gotten closer to home with less fuel. Now that I look at that screen shot, while my Pe is halfway home, my Ap is still almost in lunar orbit. I wonder if I could plot another lunar encounter that would slow my ship even more.

[Seajay]

[quote name='Brainlord Mesomorph']And I am expecting someone to explain to me whythat’s better. [/QUOTE]
Surely there is no need for convincing arguments. Trust in empirical data. The standard return costs 270, you've spent 136. If you can get your pe in to atmosphere for less than 134, your way is cheaper (assuming you both started from a similar orbit) . Edited November 19, 2015 by Seajay

[EditorRUS]

Isn't that just using reverse gravity assist of Mun?

[Friend Bear]

[quote name='GoSlash27']Brainlord Mesomorph,
Here's the approach Cantab's talking about:
[URL]http://i52.photobucket.com/albums/g13/GoSlash27/MunRetro_zpsoiss32ck.jpg[/URL]
About 270 m/sec straight to reentry. How much is the DV for your 2 burn technique?

Best,
-Slashy[/QUOTE]

YES!

This is what I meant.

[Alshain]

[quote name='Brainlord Mesomorph']What you describe is, I believe, a Holman Transfer (if I have the name right). And I am expecting someone to explain to me whythat’s better.
OTOH: the two burns I show in that screenshotare 126 m/s and 10 m/s. And I *think*that I’ve gotten closer to home with less fuel. Now that I look at that screen shot, while my Pe is halfway home, my Ap is still almost in lunar orbit. I wonder if I could plot another lunar encounter that would slow my ship even more.[/QUOTE]

It's better because it takes less time, it's simpler to execute, and the cost savings aren't that much (if any at all). Your screenshot doesn't show the completed trip home, so your two maneuvers weren't enough. You still have a 3rd burn ahead of you which would likely bring the cost to about the same. What cantab is describing takes one cheap burn to get all the way to the atmosphere.

It's spelled "Hohmann" by the way and I'm not entirely sure an escape trajectory counts as a Hohmann transfer. I'm actually not sure. I believe a Hohmann transfer requires a transition between two circular orbits. While the Mun is certainly in a circular orbit, re-entering the atmosphere is not. Edited November 19, 2015 by Alshain

[GoSlash27]

[quote name='Brainlord Mesomorph']What you describe is, I believe, a Holman Transfer (if I have the name right). And I am expecting someone to explain to me whythat’s better.
OTOH: the two burns I show in that screenshotare 126 m/s and 10 m/s. And I *think*that I’ve gotten closer to home with less fuel. Now that I look at that screen shot, while my Pe is halfway home, my Ap is still almost in lunar orbit. I wonder if I could plot another lunar encounter that would slow my ship even more.[/QUOTE]

Brainlord,
It would be a "Hohmann transfer" If I were to follow it up with a burn at periapsis to circularize in a new orbit. Really it's just a direct ejection to Kerbin reentry.

As for "why it's better", it's not necessarily. It certainly saves DV compared to the way you've been doing it previously, but using a gravity assist should save you some DV if done right. It's up to you to decide if the DV savings is worth the additional weeks of mission time.

Best,
-Slashy

[vixr]

[quote name='GoSlash27']Brainlord,
It would be a "Hohmann transfer" If I were to follow it up with a burn at periapsis to circularize in a new orbit. Really it's just a direct ejection to Kerbin reentry.

As for "why it's better", it's not necessarily. It certainly saves DV compared to the way you've been doing it previously, but using a gravity assist should save you some DV if done right. It's up to you to decide if the DV savings is worth the additional weeks of mission time.

Best,
-Slashy[/QUOTE]

I did this by accident once...except that instead of dropping straight into a descent I skipped off the atmo and slid into an orbit around Kerbin and the next time around I was able to splash down without any further Delta v. Coolest thing ever. Edited November 19, 2015 by vixr

[Brainlord Mesomorph]

I knew "Holman" wasn't right. And neither am I about this whole idea.


While the spoon-out maneuver is very dramatic and is canceling out a huge amount of forward velocity, it’s a huge amount of forward velocity I didn’t need to give myself in the first place. So the answer is no, I did not figure out a better way home from the Mun


Time and time again in this game I think I’ve figured out some little bit of interplanetary billiards that does something, and time and time again I’m shown that direct burns and doing everything at perigee is best. But if that is the case, how did NASA get Voyager where it was going? And why can’t I seem to make that work for me in this game?

[Alshain]

[quote name='vixr']I did this by accident once...except that instead of dropping straight into a descent I skipped off the atmo and slid into an orbit around Kerbin and the next time around I was able to splash down without any further Delta v[/QUOTE]

That's called a an [URL="https://en.wikipedia.org/wiki/Aerocapture"]aerocapture[/URL]. A short burn at your Ap afterward would have put you in a nice orbit.

[PLAD]

When I really want to cut the dV required for a Mun mission I use something similar to that, it's basically a bi-elliptic transfer with a flyby thrown in. In the attached album I show my lowest-dv path ever from Kerbin to Mun and back. I did it in back in KSP version 0.23.5, when the Kerbal-X simply could not do a Mun-and-return mission, I was seeing how close I could come without refueling. (Then I refueled to get it home). You can see that starting from an 9x9km orbit around Mun a 207m/s burn takes you out to a 58,000km Kerbin apoapsis where a 20m/s retrograde burn give you a low flyby of Mun that flings you all the way to Kerbin's atmosphere with no further burns necessary. I did need a little correction burn so my total from LKO to Kerbin was 229m/s total. For comparison I had the rescue craft run a standard Mun-and-return so you can see a direct path cost 273m/s, as Slashy says above.
The path you showed doesn't go out far enough, so the flyby of Mun doesn't lower your periapsis to Kerbin's atmosphere, I think you'll find adding just a couple more m/s will get you there, since just an extra 1m/s burn in LMO will add thousands of Km to your Kerbin apoapsis. There are very delicate adjustments of the burn sizes though, for those trying it a node editor will help a lot. And take care to aim for encountering Mun after Kerbin periapsis, not before, as I detail in the album, the path predictor gives a false solution otherwise!
The album is rather large, sorry about that, I also detail a very low cost bielliptic way of getting from LKO to LMO using numerous Mun flybys to reduce both the LKO departure dV (from the standard 857 down to 845m/s) and the Mun orbit insertion burn (from the standard 273m/s to 208+13=221m/s). Plus the rescue mission thrown in for comparison.

[IMGUR]wxGvv[/IMGUR]

You will note that I could have reduced the return dV a bit more by using the same multi-Mun flyby path I took to get there, but I didn't have the patience to try that.

[Alshain]

[quote name='Brainlord Mesomorph']Time and time again in this game I think I’ve figured out some little bit of interplanetary billiards that does something, and time and time again I’m shown that direct burns and doing everything at perigee is best. But if that is the case, how did NASA get Voyager where it was going? And why can’t I seem to make that work for me in this game?[/QUOTE]

Direct burns are not always best, New Horizons used gravity assists to get where it was going at a fraction of the cost in a fraction of the time. However, that is interplanetary travel. In the case of returning from a moon to it's parent planet, I'm not sure gravity assists are really all the useful (maybe at Jool).

[Brainlord Mesomorph]

[quote name='Alshain']Direct burns are not always best, New Horizons used gravity assists to get where it was going at a fraction of the cost in a fraction of the time. However, that is interplanetary travel. In the case of returning from a moon to it's parent planet, I'm not sure gravity assists are really all the useful (maybe at Jool).[/QUOTE]


New Horizons : IRL! I know it works IRL. I’m frustrated by not being able to make it work in this game.

That, and I’m embarrassed that I keep thinking that I’ve made gravity assists work in this game.

[Seajay]

[quote name='Brainlord Mesomorph']I wonder if I could plot another lunar encounter that would slow my ship even more.[/QUOTE]
Probably not, I have played with this a lot when I had a ship stuck halfway home and very low on fuel. On your first encounter your ap is higher than the mun so you're travelling faster than the mun at encounter. That means you enter the SOI prograde and leave retrograde which slows you down. Now that your orbit is inside the mun you're travelling slower at encounter i.e. It is catching up with you which means that you enter its SOI travelling kerbin retrograde relative to the mun. Using it to change your direction in that situation is only ever going to increase your (kerbin-relative) orbital velocity.

[Alshain]

[quote name='Brainlord Mesomorph']New Horizons : IRL! I know it works IRL. I’m frustrated by not being able to make it work in this game.[/QUOTE]

Well, you were the one that mentioned Voyager, but never mind, I'm done with this thread then.

[BenCushwa]

[quote name='Friend Bear'][I]Ultimately[/I], it is good practice to avoid large burn maneuvers with multiple burn stages. This applies everywhere. Say you want to go to Eve from Kerb. It is hugely inefficient to first leave Kerb and have a matched kerb orbit of kerbol then burn to hit Eve. Instead, you want to burn from inside kerb until your outside Kerb orbit is where you want it to be...meeting Eve. Similar idea here. You don't want to leave Mun then adjust. [B]Burn inside Mun so that your outside Mun orbit is where you want it[/B].[/QUOTE] (emphasis added)

When I'm leaving the Mun or Minmus to return to Kerbin, I wait to depart until my ship is as close to the moon's orbital retrograde as it can be and then burn straight up. (If I'm landed at a latitude of more than +/- 10 degrees or so, I'll shift my burn towards the orbital plane.) By the time I enter Kerbin's SOI I've effectively done a retrograde burn dropping my periapsis while at the same time escaping whichever moon I was on so I'm not wasting any deltaV. If I time/position my burn correctly, I can return to Kerbin's SOI, re-enter, and land with a single burn.