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# Finding an efficient plane-change altitude

## Question

Hallo - odd question here. I've been toying with blasting around the local system using minimum rockets and minimum fuel usage - odd for me since my motto has always been "Don't just build...overbuild!"

I'm currently heading to Minmus in an attempt to grab all possible science (3X over with 2 storage containers) from the moon on a single trip; I think my record is 2000 but I can't quite recall.

Anyhoo; my usual launch is to boost into parking orbit - 100kms - set the Mint Frosted Moon as target, burn for alignment then boost via a normal transfer. I'm pretty sure however that if I boosted into a higher orbit - say 1000kms - it would take less fuel to burn into alignment since my velocity is lower.

A)  is that correct? And if so

B) Is there - mathematically speaking - an optimum altitude to change my plane with minimum fuel usage?

Thanks!

Edited by NorthernDevo
Speling mistayks

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the optimal altitude to align with Minmus is zero.  Ie launch into the correct plane as KSC passes under Minmus ascending/descending node.

Failing that Minmus is fairly low inclination, boosting yourself up to any other parking orbit would cost more than you would save on the plane change. Best to plane change at minmus altitude.

If you need to save DeltaV and you must use an equatorial parking orbit, and don't want to wait for a window to encounter Minmus at an ascending/descending node.  then your best bet would be to trade off for time, boost youself up to Munmus alt at one node then at the other, raise your peiapsis until you get an encounter on your next orbit

Edited by Rhomphaia

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Thanks for the info - yes; I've always done it this way because I've never been good at launching into the correct plane from the ground; heading East is easy! Now that you say it, I think I ought to spend a few flights working that out.

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I usually end up putting a cheap satellite into low Kerbin orbit that's aligned on Minmus' orbital plane. I find it a lot easier to eyeball the satellite's much closer orbit and then launch directly into its orbital plane.

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1 hour ago, NorthernDevo said:

I'm pretty sure however that if I boosted into a higher orbit - say 1000kms - it would take less fuel to burn into alignment since my velocity is lower.

Sort of. You don't need to be in an orbit. It partially depends on how good you are at creating a circular 71km orbit with your craft. Let's say that you're like me and use plenty of SRBs, so your Ap tends to be pretty high. When you are raising your Pe to just barely above (or below!) the top of Kerbin's atmosphere, you have some opportunity to adjust the direction that your Ap-Pe line points. So you point it 45 to 60 degrees ahead of the Minty Moon (you also do this by timing your launch, of course). You burn at the Pe to raise your Ap to 46Mm without doing any plane changing first. You have to trust yourself to know that Minmus will be in the right place when you get there. Then, when you are somewhere between 10Mm and 20Mm altitude, you burn north or south to fix your trajectory for a nice close intercept (and do any little east-west fiddling at the same time). But as you say, you are going nice and slow at that point, so this mid-course adjustment costs you very little.

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Gotcha - I've been aware I haven't been doing it as efficiently as possible; you've given me some tips to follow; thanks

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Hi all; update:

I built a simple Mk1 firecracker and sent Beaslie into orbit, following the path of Megan's derelict - she's been stuck up there for about a week now in a highly inclined orbit at around 5000kms. I discovered quickly that by rotating the ship in the VAB prior to rollout lining up with the erstwhile kerbonaut was surprisingly easy. Thanks for the help fellows; this is so much easier than I thought!

Now I'll build a recovery rocket and get the poor rocketeer down.

Cheers!

Edited by NorthernDevo

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