Equatorial Orbits Of Minmus

[The Flying Kerbal]

Simple question guys, how do you transfer out to Minmus so you arrive in an equatorial trajectory?

I can launch into an LKO which matches the orbit of Minmus perfectly, yet when I arrive I still am not doing so along the equatorial.  I've tried every combination of launch/inclination adjustment I can think of, but all to no avail.  And I'm not talking about a degree or two out here, this is an appreciable difference of maybe ten or more degrees of error.

So what am I doing wrong, or is it just a fact of life that because Minmus is on an inclined orbit, entering its SOI on an inclined plane is just a fact of life?

Thanks all.

[Capt. Hunt]

4 minutes ago, The Flying Kerbal said:

Simple question guys, how do you transfer out to Minmus so you arrive in an equatorial trajectory?

I can launch into an LKO which matches the orbit of Minmus perfectly, yet when I arrive I still am not doing so along the equatorial.  I've tried every combination of launch/inclination adjustment I can think of, but all to no avail.  And I'm not talking about a degree or two out here, this is an appreciable difference of maybe ten or more degrees of error.

So what am I doing wrong, or is it just a fact of life that because Minmus is on an inclined orbit, entering its SOI on an inclined plane is just a fact of life?

Thanks all.

yeah, but inclination changes at the edge of the SOI are very easy, usually less then 10 m/s.

[bewing]

I've been told that Minmus' equator is coplanar with Kerbin's. I've never checked because I don't care. But if it's true, then a perfectly equatorial launch from Kerbin to arrive at the Minmus node should get you into an equatorial orbit.

However, even a very small angular deviation from a pure equatorial orbit at Kerbin will always easily result in arriving in a polar or inclined Minmus orbit.

[Superfluous J]

You basically have to arrive at Minmus going UP when it's going DOWN, or DOWN when it's going UP. I don't know what your inclination should be, though. I suspect you'd have to be quite polar, though, as when you arrive at Minmus you're traveling quite slowly so won't have much north/south speed.

I'm sure there's math that'd tell you. Essentially you want your north/south velocity when you're at Minmus' orbit to be equal to and opposite to Minmus' north/south velocity.

But really it's far easier to just align your orbit when you get there, or learn to not care if you're equatorial or not.

EDIT: Also, you'd have to make sure you enter Minmus' SOI at the equator. Good luck with that I have no idea how to ensure it.

Edited April 13, 2018 by 5thHorseman

[Kryxal]

Even transferring such that you land on the equator from a zero-inclination orbit wouldn't help much ... the orbital plane may be the same, but it's going to be moving in the z axis relative to Kerbin.

[The Flying Kerbal]

As usual, great answers from everyone.  And it's good to know this is the norm, I was beginning to go slightly mad trying to figure out what I was doing wrong. 

[Gargamel]

1 hour ago, The Flying Kerbal said:

As usual, great answers from everyone.  And it's good to know this is the norm, I was beginning to go slightly mad trying to figure out what I was doing wrong. 

Nope, not doing anything wrong, just another step on the learning curve. 

[Zeiss Ikon]

For bodies with shallow gravity wells like Minmus, Gilly, and a couple of the Joolian moons, it's not really worth worrying about orbital plane unless you're on way too tight a dV budget.  You can reverse your orbit around Minmus for less than a small plane change in LKO.  The problem I see is actually detecting Minmus's equatorial plane.  It's not as if the mint moon was marked with latitude and longitude stripes.  And with a six day rotation period, it won't do you a lot of good to try to sit and watch it turn...

[Capt. Hunt]

17 hours ago, bewing said:

I've been told that Minmus' equator is coplanar with Kerbin's. I've never checked because I don't care. But if it's true, then a perfectly equatorial launch from Kerbin to arrive at the Minmus node should get you into an equatorial orbit.

However, even a very small angular deviation from a pure equatorial orbit at Kerbin will always easily result in arriving in a polar or inclined Minmus orbit.

all of the planets and moons have zero axial tilt, because of a limitation in the game engine, IIRC.  Unfortunately, since minmus isn't orbiting on the same plane as Kerbin-Mun you won't be able to catch it on the equator unless you manage to do so as Minmus is passing one of it's orbital nodes with Kerbin's equator.

Edited April 13, 2018 by Capt. Hunt

[Tyko]

You can set Minmus as your target then time your transfer to Minmus when it's at either the Ascending or Descending Node. This is when Minmus is exactly co-planar with Kerbin

NOTE: this is assuming your craft is at a zero inclination orbit around Kerbin. If you're orbit is tilted then you'll be seeing the AN and DN for your craft and will arrive off-angle.

[Superfluous J]

I did a thing.

 

[Snark]

@The Flying Kerbal,

One thing to bear in mind is that in terms of dV, it doesn't actually matter much whether you arrive perfectly equatorial.  Minmus is so small, with such light gravity, that even in low orbit your speed is only a couple hundred m/s.  Plane changes are really easy and cheap.  As long as you're not arriving in something close to a polar orbit, it'll probably only take you a couple dozen m/s to correct your inclination.

So, the simplest solution to "how do I get into equatorial Minmus orbit" is basically just "don't even bother worrying about it, and fix it after you arrive."

Unless, of course, your reason for wanting this is just for the challenge of it rather than practical reasons, in which case, carry on. 

[Zeiss Ikon]

3 hours ago, Tyko said:

You can set Minmus as your target then time your transfer to Minmus when it's at either the Ascending or Descending Node. This is when Minmus is exactly co-planar with Kerbin

NOTE: this is assuming your craft is at a zero inclination orbit around Kerbin. If you're orbit is tilted then you'll be seeing the AN and DN for your craft and will arrive off-angle.

Fortunately, you can set the Mun as your target to check your own craft's orbital inclination; since the Mun is in a perfectly equatorial orbit, your nodes relative to the Mun are the same as your nodes relative to Kerbin's equator.  Bad news is, the nodes only read in neaest tenth of a degree, and .05 degrees error still adds up to a good bit of inclination when you're going to Minmus -- even if you arrive perfectly at Minmus's node.

[Tyko]

3 hours ago, Zeiss Ikon said:

Fortunately, you can set the Mun as your target to check your own craft's orbital inclination; since the Mun is in a perfectly equatorial orbit, your nodes relative to the Mun are the same as your nodes relative to Kerbin's equator.  Bad news is, the nodes only read in neaest tenth of a degree, and .05 degrees error still adds up to a good bit of inclination when you're going to Minmus -- even if you arrive perfectly at Minmus's node.

Fair point...I use KER so can see my inclination to the a very small fraction of a degree - 1000th I think?

[Zeiss Ikon]

10 hours ago, Tyko said:

I use KER so can see my inclination to the a very small fraction of a degree - 1000th I think?

And I play stock except for Better Burn Time.

[Superfluous J]

I was able to get two maneuvers set up with a 3.6 degree final Minmus inclination, starting at a 60 degree Kerbin orbit. That took quite a while and a lot of fiddling.

I was never able to execute that maneuver, though, with anything resembling enough accuracy. I don't think I got anything close to even 15 degrees.

So my conclusion is: Don't. It's stupid-hard and there is zero benefit to it over just fixing your inclination once there (or, as I suggested earlier, going Zen and not caring anymore).

One thing I did determine, though, was that your lowest inclination was when you Pe was matched with the target equatoral orbit. At least, with my "approach from near polar" strategy.

Edited April 16, 2018 by 5thHorseman

[Tyko]

18 hours ago, 5thHorseman said:

I was able to get two maneuvers set up with a 3.6 degree final Minmus inclination, starting at a 60 degree Kerbin orbit. That took quite a while and a lot of fiddling.

I was never able to execute that maneuver, though, with anything resembling enough accuracy. I don't think I got anything close to even 15 degrees.

So my conclusion is: Don't. It's stupid-hard and there is zero benefit to it over just fixing your inclination once there (or, as I suggested earlier, going Zen and not caring anymore).

One thing I did determine, though, was that your lowest inclination was when you Pe was matched with the target equatoral orbit. At least, with my "approach from near polar" strategy.

I'll give it a try tonight