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Quick Question on Munar Transfer


wired2thenet

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Recently, I\'ve been encountering situations in which during my initial launch, I\'m at or very near approximately 78km, but haven\'t yet circularized the orbit, when the Mun rises. First time it happened, I immediately continued my burn directly to Munar Transfer, and saw my Periapsis grow to around 78km doing so.

My question is, when this happens, do you do the same? Is there any drawback to going for it immediately (moment of opportunity), or would it be better to do an orbit before that burn (ie, fuel efficiency).

In one instance, I took stock of my fuel levels during direct transfer, and then repeated it after a single orbit. In the direct transfer, I noticed just slightly better fuel performance; the problem is that direct transfer was done without the mishap on staging when I did the circularize orbit (stupid thumb staged when I didn\'t want it to).

*edited due to stupidity :D

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Delta-v for a Perfect TMI after one orbit (with perfect circularization burn) would be the same as delta-v for a perfect TMI right when you should have begun the circularization burn. Which makes sense, if you burn the rocket the same amount at the same place in orbit it doesn\'t matter if it\'s two separate burns 30 minutes apart, or one big one.

Note, however, the 'perfects' in that summary. If you assume you\'ll make slight errors with every burn, doing it as one burn might be more efficient. This is definitely what I do when the chance comes up. In fact, I take it a step further; if the Mun\'s in the right general neighborhood, I\'ll eyeball my Ap point on the suborbital to be lined up correctly for TMI.

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Delta-v for a Perfect TMI after one orbit (with perfect circularization burn) would be the same as delta-v for a perfect TMI right when you should have begun the circularization burn. Which makes sense, if you burn the rocket the same amount at the same place in orbit it doesn\'t matter if it\'s two separate burns 30 minutes apart, or one big one.

Note, however, the 'perfects' in that summary. If you assume you\'ll make slight errors with every burn, doing it as one burn might be more efficient. This is definitely what I do when the chance comes up. In fact, I take it a step further; if the Mun\'s in the right general neighborhood, I\'ll eyeball my Ap point on the suborbital to be lined up correctly for TMI.

That\'s what I\'ve been thinking, seeing as how many times this has happened to me recently. Fuel consumption between the two should be nearly zero, regardless. But after nearly 10 successful transfers (and 9 successful returns), I\'m finding I\'m dropping stages a lot quicker than I usually do, and sometimes with MUCH more remaining fuel (could be a bug, though I doubt it ... may just be more confident on the direct approach than waiting one orbit ... timing is critical on the latter, and I do tend to 'Kerbal' it up more often than now). I\'m currently playing around with a design from the Exchange (can\'t remember the rocket name off-hand, but it\'s a very elegant design, and VERY easy to fly ... I wanted to thank the author of it, but I\'d be necro\'ing it :D). I\'m starting to wonder if I should time my launches now, depending on the craft, until the Mun will end up rising about the time a 'lonely little island' (my terminology) pops up in the great ocean. Every time I see that island, I know I have about 10 seconds before the Mun rises (depending on the launch schedule), and can time the burn almost down to the second.

*Edit* Ahh, it\'s this one: http://kerbalspaceprogram.com/forum/index.php?topic=10638.msg161771#msg161771 To the author, if you read this, this is a very cool rocket!

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In the 0.13.3 demo version, every launch starts at the same time of the same day. So we could launch directly to TMI and skip the Kerbin orbit. The Mun is a little bit high in the sky at that moment, so my personal method was to interrupt the gravity turn at 45° and hold it there until AP is at the Mun\'s orbit. It seems like this should be inefficient, but the fuel penalty was only slight.

There was one video tutorial where I watched the guy waste fuel fiddling to make his orbit perfectly circular while the TMI window opened and closed. Arg! ;P

I think it helps to do a burn closer to Kerbin, but I forget the name of that effect. Anyone? Anyone?

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Found it:

Oberth Effect

Basically, the same chemical energy does more effective work because the craft starts with more velocity. If it seems counter intuitive to you too, read the proof in the Wikipedia article.

So, if I understand correctly, my inefficient 45° burn that got the right trajectory was compensated for by low altitude and high escape velocity. While we were using the 0.13.3 demo, this was a nice way to do a quick Mun shot.

Nowadays, it\'s better and easier to time warp on the ground until Mun is in the right spot, then do a proper launch to TMI, burning prograde the whole way.

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