How to change inclination for cheap ?

[Tatonf]

Hello there !

I'm having a bit of a problem here : I have a ship in a very eccentric orbit around Eve (Pe near the atmosphere, Ap near the bound of the SoI), but it's inclined by 13° relatively to Eve. I would like to make it equatorial, and I know that changing incilnation is cheaper if you're going slow rather than going fast.

I didn't know where to correct the inclination, so I tried to replicate this situation around Kerbin (so I can get my nodes while targeting Mun), and the nodes happened to be very close to the planet, meaning I'm going fast, meaning the inclination change is quite expensive...

So I don't understand why people say I should do it while I go slow if I don't have the choice of where I can do it... I tried to correct the inclination while being halfway between my Pe and my Ap, and I could barely bring it to 11° before it starts going up again...

What should I do ?

[wumpus]

9 minutes ago, Tatonf said:

Hello there !

I'm having a bit of a problem here : I have a ship in a very eccentric orbit around Eve (Pe near the atmosphere, Ap near the bound of the SoI), but it's inclined by 13° relatively to Eve. I would like to make it equatorial, and I know that changing incilnation is cheaper if you're going slow rather than going fast.

I didn't know where to correct the inclination, so I tried to replicate this situation around Kerbin (so I can get my nodes while targeting Mun), and the nodes happened to be very close to the planet, meaning I'm going fast, meaning the inclination change is quite expensive...

So I don't understand why people say I should do it while I go slow if I don't have the choice of where I can do it... I tried to correct the inclination while being halfway between my Pe and my Ap, and I could barely bring it to 11° before it starts going up again...

What should I do ?

Best guess is:

1. increase Ap

2. Change inclination (at Ap)

3. circularize if necessary.

Typically you get better results by combining burns, but since you need to be in different places for 1 and 2 this isn't going to happen.  My only suggestion is to set up two separate maneuver nodes (one at Pe and the second at Ap) and adjust them until they add to the smallest number.  Or you can work up a spreadsheet/small program and grind out the math.  But that is how real satellites move from KSC inclination to GSO inclination.

PS: Inclination changes just aren't done "on the cheap".  This should be the cheapest way, but there is always going to be a steep delta-v cost in a place with Eve's gravity.

Edited August 1, 2016 by wumpus

[Laie]

Basics:

The AN/DN are where you cross the equator. If you don't have any tools to help you (Precise Node or Mechjeb or...) you can only eyeball it and do your best (then again, I guess you use some tools because how else would you know that you have 13deg inclination?).

If you perform your maneuver at three degrees north (or south, doesn't matter), the lowest inclination you can hope to achieve is three degrees.

The task at hand:

Doing the plane change at high apoapsis is indeed cheap, but if the AP happens to be at 7° that's the lowest inclination you can achieve there.

Ideally, you should have paid attention during your approach to Eve and taken care that your PE is as close as possible to the equator (because then AP will be too) to ease later plane changes. But that ship has sailed.

The only "cheap" way I can think of is to combine burns. IF you want to lower your orbit to something circular, you can combine that with a plane change and get both in one go. The final maneuver should be about 10-20% more expensive than merely lowering AP, and much cheaper than doing first one, then the other.

 

[mk1980]

you can only change your inclination at the ascending or descending node. that's basically the 2 points where your current orbit intersects the plane of the desired (equatorial) target orbit.

technically, it's possible that the 2 nodes are close to AP and PE if the major axis of the ellipse is (almost) on the plane of the target orbit. they probably aren't in your case. so the next best thing is to do the plane change at the slower of the 2 nodes.

don't remember if the game displays AN and DN markers relative to the equator. probably not. around kerbin, you can set the mun as target to get the nodes (it's orbit is almost exactly equatorial). i think the orbit of gilly isn't equatorial, so that little trick won't help around eve. if you have some satellite already in an equatorial orbit, you can use that as your target so the game shows the AN/DN. if not, you'll probably have to eyeball it or find it by trial and error (set some arbitrary normal/antinormal burn and move the maneuver node along the orbit until you find the point where the change to the displayed inclination angle is largest)

[Tatonf]

33 minutes ago, Laie said:

Basics:

The AN/DN are where you cross the equator. If you don't have any tools to help you (Precise Node or Mechjeb or...) you can only eyeball it and do your best (then again, I guess you use some tools because how else would you know that you have 13deg inclination?).

 

Yes, I use KER, , but it doesn't show the nodes.

33 minutes ago, Laie said:

ing the plane change at high apoapsis is indeed cheap, but if the AP happens to be at 7° that's the lowest inclination you can achieve there.

Ideally, you should have paid attention during your approach to Eve and taken care that your PE is as close as possible to the equator (because then AP will be too) to ease later plane changes. But that ship has sailed.

It IS close to the equator :

33 minutes ago, Laie said:

The only "cheap" way I can think of is to combine burns. IF you want to lower your orbit to something circular, you can combine that with a plane change and get both in one go. The final maneuver should be about 10-20% more expensive than merely lowering AP, and much cheaper than doing first one, then the other.

Yes you're right, I forgot about that, I'm gonna do that.

EDIT : Well, the combination of retrograde plus normal or anti-normal doesn't work. My orbit stay inclined whatever I do. I don't know what to do.

Edited August 1, 2016 by Tatonf

[TopGull]

There's another way that no one has mentioned yet... When you're close to Ap, burn normal/antinormal so that your orbit becomes MORE inclined and crosses the equator closer to Ap. Then burn again when you cross the equator to get your desired inclination. I can't say for sure, but this might work out cheaper than the other options here.

[bewing]

With the orbit you have described, what you want to do first is move the Ap, so that it is aligned with the node.

You typically move the Ap by burning prograde before the Ap (pushes the Ap away from you -- if you are fairly close to the Ap, you won't raise it much), or by burning radial (in or out) halfway between the Ap and Pe.

Once the Ap is over the node, you do your +/- Normal burn there to fix your inclination.

[pincushionman]

I second @TopGull's suggestion, counter-intuitive as it is. You can also burn at Ap to raise your Pe, and with it your An/Dn, which may help. Remember, if your Ap is out of plane, one of thr nodes is already going to be better than the other for this task. Make sure you pick the appropriate one.

[Laie]

16 hours ago, Tatonf said:

It IS close to the equator :

It is indeed. And still way off -- yet another argument in favor of more data displays.
 

16 hours ago, Tatonf said:

EDIT : Well, the combination of retrograde plus normal or anti-normal doesn't work. My orbit stay inclined whatever I do. I don't know what to do.

You need to do your burn while you're pretty close to, preferably right on top of, the equator. KER will at least tell you your latitude if nothing else. That's no where your PE is, but if you want zero inclination that's where you have to do the maneuver.

Suggestion: use Hyperedit and place a probe (or any piece of debris, really) into the target inclination, just to have something you can use as indicator.

[LN400]

You say you use KER. You can edit the Orbit window to include both Time To Equatorial AN/DN and Angle To Equatorial AN/DN.

Edited August 2, 2016 by LN400

[SnakyLeVrai]

Have you tried a gravity assist with Gilly? When you get out of the SOI of an inclined moon, you can use it to throw yourself in polar orbit of the main planet for quite cheap. Takes a little planning. Never tried it with such a small rock though... Minmus is great for that.

Just remember real life comets can have their orbits turned from prograde to retrograde if they are eccentric enough and pass real close to Jupiter. The same applies here