KOI Burn efficiency?

[GarrisonChisholm]

Just a quick question- I *think* I know the answer, but it seems so counter-intuitive that I feel I have to ask to be certain.

I just entered Minmus Orbital Influence on a Stop-And-Go sub-orbital jaunt, with my Ap @ 1800km, Peri (after recovering from Sub-O trajectory) at 12km. To head on a return to Kerbin, is it Really more fuel-efficient to let my altitude drop all the way to 12km before burning, rather than right here at the edge of KOI (~1800km above Min)?

Forgive me for the simplicity of the question, I am still "learning as I go".

(@OhioBob - fyi, I determined my own answer to my Stellar Evolution question, so decided not to make it a forum-post as I had indicated- I will PM a synopsis!)

[Agost]

If your PE is in the right position it's more efficient to burn there rather than at AP

However higher orbits have more energy, so you shouldn't circularize at 12km if it's what you mean, just do the escape burn at PE

p.s.: KOI? It's SOI.

[radonek]

As far as minmus go, orbital energy changes are so cheap it does not really matter. Mun or other big moons are different story, but you can wait it out. Since your orbit is fixed relative to planets own orbital motion, you can just wait until burn point is close to Pe (as close as inclination allow).

[Red Iron Crown]

It's the Oberth Effect that makes the lower altitude burn more efficient. I wrote a little synopsis about it a while back, you might find it worth a read.

[Champ]

Ap=1800 km, PE = 12km

If you burn at Ap, you will raise Pe. So, you will have to raise Pe from 12km to "high enough to exit minmus SOI"

If you burn at Pe, you will raise Ap. So, you will have to raise Ap from 1800 km to "high enough to exit minmus SOI"

Of course, burning at Pe is better. No need for advanced concepts like the Oberth effect or things like that to understand why.

This is if your goal is just to exit minmus SOI.

You said your goal is to get back to kerbin. To answer this question, you must look at the position of each node (where they are compared to kerbin and minmus) : you want to exit minmus SOI in minmus'retrograde direction.

Will try to make a little drawing to explain better

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Sorry, i made 3 wonderful drawings (i have amazing MS paint skills), but it seems that KSP forum wont let me post pictures from my computer : when i click on the "Insert image" button and select the "from my computer" tab, there is nothing to select the file (Firefox 40.0.3).

I will try to explain with words, then.

You said you want to get back to kerbin. The most fuel-economic way is to leave minmus' SOI by the "back" side (back if you consider the rotation of minmus around kerbin). This way, when you leave minmus' SOI, you will be on a lower orbit around kerbin. Then, burn retrograde at your new Ap around kerbin to set your Pe near 35 km. If you have problems surviving atmo reentry, say so, we can help.

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It's worse with IE : IE dont even show the "from my computer" tab...

Edited September 4, 2015 by Champ
Typo - Thanks Stilgar

[_stilgar_]

If you burn at Ap, you will raise Pe. So, you will have to raise PE from 12km to "high enough to exit minmus SOI"

If you burn at Pe, you will raise Ap. So, you will have to raise PE from 1800 km to "high enough to exit minmus SOI"

Sorry, i made 3 wonderful drawings (i have amazing MS paint skills), but it seems that KSP forum wont let me post pictures from my computer : when i click on the "Insert image" button and select the "from my computer" tab, there is nothing to select the file (Firefox 40.0.3).

Typo: PE ->AP in second line

You can upload your images here: http://www.imagebam.com/ - it will create a BB code, paste it into your post.

[Champ]

Typo: PE ->AP in second line

You can upload your images here: http://www.imagebam.com/ - it will create a BB code, paste it into your post.

Corrected. Thank you Stilgar.

Will try imagebam (i really wanna show my amasing MS paint skills !)

Beware ! The Harkonnens, the Space Guild and the Kraken allied against us !

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This is the escape trajectory you want :

The thick-black-dotted line is minmus' trajectory around kerbin (kerbin is not represented). The red line is your escape trajectory. The blue circle is minmus.

So, if the situation is like this :

then burn at Pe

If the situation is like this :

then burn at Ap.

If the situation is like this :

Then it is more complicated... play with manoeuvre nodes (if you have them) or accept to waste a little fuel in a not-optimal manoeuvre.

PS : Thank you Stilgar. Didnt know imagebam. Great tool.

Edited September 4, 2015 by Champ
Typo - again !

[GarrisonChisholm]

Ap=1800 km, PE = 12km

If you burn at Ap, you will raise Pe. So, you will have to raise Pe from 12km to "high enough to exit minmus SOI"

If you burn at Pe, you will raise Ap. So, you will have to raise Ap from 1800 km to "high enough to exit minmus SOI"

Of course, burning at Pe is better. No need for advanced concepts like the Oberth effect or things like that to understand why.

Wow. I can't believe I didn't think of it that way. All I could think of was that I was only 5-10km away (at AP) from leaving Minmus SOI; why can't I just go there?!? -I didn't visualize that what I should be thinking about was just whether to expend the energy to either raise my AP (at PE) or to move my PE until *IT* was the AP, which Obviously is a much greater move.

Thank you Champ for going to the effort to draw the diagrams. I have to say, this is the most helpful, coolest gaming community I've ever seen.

Reading your article now RedIronCrown; thank you too!

[Champ]

You are welcome.

As i said, burning at Ap or Pe is important if you just want to escape minmus' SOI. If you want to get back to Kerbin, the important thing is you want to exit minmus SOI in the good direction (the "back" direction by Minmus' point of view).

If you are in the third situation, i think you better burn at periapsis, then burn retrograde as soon as you leave minmus SOI (if you dont you may leave kerbin SOI)

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If you are in the third situation, i think you better burn at periapsis, then burn retrograde as soon as you leave minmus SOI (if you dont you may leave kerbin SOI)

Thinking of it, i'm not so sure... May be better to burn at Ap to circularise, then burn when you are at the good angle to leave minmus SOI in the good direction. May depend on your Ap/Pe altitude. F5/try/F9/retry

Edited September 4, 2015 by Champ

[tbarcello]

http://imgur.com/SHQpEyQ

When your escape is parrallel to body's orbital trajectory you are more likely to get apoapsis, but not for bodies with too big eccentricity. I guess...

[Hagen von Tronje]

Champ put it very nicely. Hohmann transfers work best when you burn at the opposite node of the one you wish to change, so if you want to raise your Ap, burn at Pe. Leaving orbit = raising Ap higher than the body's SOI.

I personally find that it does feel much more "commonsense" when you think of orbits not in terms of where you and your destination are, but where you're going to be. The plain ol' stock maneuver node actually does a surprisingly good job of making this transparent, especially with patched conics, one reason why I advocate for flying by hand rather than MJ, I have learned a lot of things, even things I already knew in my mind but had never truly visualized and absorbed until attempting them myself.

Another nice commonsense corollary is that if you want to avoid thinking too much about this, make your orbits reasonably circular whenever possible so you can plan based on desired transfer window and angle without the complicating factor. At least, I don't think there's any reason that's a bad idea - though I wouldn't waste dV correcting, when creating an orbit I aim to make it close to circular for this reason. Maybe one of the gurus knows better than I do and will tell me eccentric is better and I'll have something to chew on.

[match]

For efficiency, you can wait until your Pe escape burn puts you into the optimal return trajectory. (Retrograde in relation to Kerbin. ) but you may have to wait a while until it aligns.

Since you already have an eccentric orbit, you may as well put it to good use!

[Clipperride]

The effects of your burns are most obvious half an orbit later. Remember that golden rule and you shouldn't go far wrong.

I know what you mean by intuition leading you astray though. Returning from Minmus today and placed a bit of a random node to exit the SOI. Noticing the ∆v was higher than normal, I zoomed out a bit, thought about it and halfed the energy I needed to get where I wanted. Not a big deal with Minmus, but it is elsewhere.