0 Inclination Orbit around Inclined Planet - Efficiency

[ScottyDoesKnow]

So I've been working on an Eve ship lately and I don't know if I'm transferring in the most efficient way. I want to end up in a 0 inclination orbit around Eve, but since Eve's orbit around the sun is inclined I always come in at an angle (unless you get the perfect transfer window when they're in line, but you can't rely on that). Currently what I'm doing is aerobraking just enough to be captured and then doing my inclination change (around 300dV the last time).

It seems to me that this is the only way to do it, but just wanted to make sure I wasn't missing some trick.

[Mitchz95]

I usually do it the same way. Not sure if there's a better way, but I'd love to find out.

[FanaticalFighter]

Aerobrake so that your apoapsis is REALLY far away from eve. Do the plane change there. Shouldn't be a large burn at all. Then aerobrake into a lower orbit. Done.

[ScottyDoesKnow]

Aerobrake so that your apoapsis is REALLY far away from eve. Do the plane change there. Shouldn't be a large burn at all. Then aerobrake into a lower orbit. Done.

Unfortunately it doesn't work like that in this situation. If you're doing a standard transfer, you're going to be in a similar orbit to eve and then catch up to it or let it catch up to you. This means your periapsis and apoapsis are in front of and behind Eve and your ascending and descending nodes are on the sides (the points closest to and farthest from the sun, in line with Eve). So basically your aN and dN are never near your apoapsis.

[Kasuha]

To enter in Eve's equatorial plane, you need to match orbit inclination when transferring and then fine-tune the intercept to come at the right height. That's usually not very fuel efficient maneuver.

The other option is to fine-tune your intercept so your (atmosphere-grazing) periapsis is at an/dn and aerobrake to highly elliptic orbit where the other an/dn is on the far side of the ellipse. Then fix the inclination on the far side.

Unfortunately it doesn't work like that in this situation. If you're doing a standard transfer, you're going to be in a similar orbit to eve and then catch up to it or let it catch up to you. This means your periapsis and apoapsis are in front of and behind Eve and your ascending and descending nodes are on the sides (the points closest to and farthest from the sun, in line with Eve). So basically your aN and dN are never near your apoapsis.

That's not true. Your transfer defines the direction from which you approach, but you can fine-tune where exactly you will pass the planet and can set it up so you have the an/dn near apoapsis after the capture. Conic mode 0 is very useful for that, and starting with 0.24 we should be able to see it with default conic mode as well.

Edited March 2, 2014 by Kasuha

[Kryxal]

I'd think you could probably catch a near-equatorial aerocapture, then with a high apoapsis you just do a normal burn when you hit the equator again. The wonders of conic patch mode...

[m4rt14n]

Can u use mechjeb match inclination or change inclination while far away from the SOI? As in when you are transfering but not yet in the SOI (Eve) yet?

I always get this problem where all my transfers are all over the place in terms of orbital inclinations.

[Geschosskopf]

Can u use mechjeb match inclination or change inclination while far away from the SOI? As in when you are transfering but not yet in the SOI (Eve) yet?

Usually not, at least not efficiently. Whether you do Hohmann or ballistic transfers, you're always meeting the target planet at an angle to its orbital plane with neither the AN or DN between you and Pe.

Another thing is, the polar axes of all planets point straight N-S so even the planets themselves are thus inclined to their own orbital paths. Thus, even if you match planes with the target planet on the way there, you're still going to be inclined to the planet's equator by the same amount as the planet itself is inclined.

[Pecan]

Isn't the point of 'Spheres Of Influence' that only the current one is tracked and calculated, so all MJ orbit details - and maneuvers - will always affect your current orbit, not that projected for any future SOI?

[Kasuha]

Whether you do Hohmann or ballistic transfers, you're always meeting the target planet at an angle to its orbital plane with neither the AN or DN between you and Pe.

1/ when you match your target's orbital inclination, you approach it very close to its equatorial plane and you can fine-tune the orbit to intercept exactly at equatorial plane.

2/ whatever angle you are approaching from, you can always adjust your trajectory to have the an/dn exactly at periapsis.

Edit: I did an experiment and prepared Moho intercept which comes (almost exactly) at equatorial plane. It does not exactly match Moho's inclination but it is much closer to it than to inclination of other planets.

Second image in the album compares the plane of the intercept trajectory with inclination of Kerbin. There is some difference but it's just because I did not spend enough time fine-tuning it. I believe it is enough for most practical purposes.

Note also that Alexmoons transfer calculator optimizes for injection to equatorial orbit, so final inclination change is part of the braking burn.

This approach is best prepared from the opposite side of the orbit than where the intercept point is.

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Edited March 2, 2014 by Kasuha

[Xiphos]

When you first enter the SOI of Eve, do all of your inclination/aerobrake there. With just your eyeball you can get pretty close to horizontal. Also, at this point it should cost very little Dv to do so....

[ScottyDoesKnow]

That's not true. Your transfer defines the direction from which you approach, but you can fine-tune where exactly you will pass the planet and can set it up so you have the an/dn near apoapsis after the capture. Conic mode 0 is very useful for that, and starting with 0.24 we should be able to see it with default conic mode as well.

But any time that you're coming in from a different direction (more towards or away from the sun) you're not using the most efficient transfer. It means your apoapsis is higher than Eve and your periapsis lower. Off the top of my head, I can't see that being more efficient. But I guess that's what I'm asking, if doing something like that could be more efficient (in this case at least cost less than 300 dV).

I'd think you could probably catch a near-equatorial aerocapture, then with a high apoapsis you just do a normal burn when you hit the equator again. The wonders of conic patch mode...

The inclination of your capture is defined by the inclination of your approach, which is pretty set in stone for you to actually hit Eve (since Eve is inclined in regards to the sun). It's possible for your transfer window to get you there when Eve is in line with Kerbin, but you can't rely on that unless you want to wait through multiple windows (100s of days).

Isn't the point of 'Spheres Of Influence' that only the current one is tracked and calculated, so all MJ orbit details - and maneuvers - will always affect your current orbit, not that projected for any future SOI?

Only the current one affects you (gravity), but your future orbit is calculated from where and at what velocity you enter that SoI. So your future orbit is very much affected.

1/ when you match your target's orbital inclination, you approach it very close to its equatorial plane and you can fine-tune the orbit to intercept exactly at equatorial plane.

2/ whatever angle you are approaching from, you can always adjust your trajectory to have the an/dn exactly at periapsis.

1. When you match inclination, you approach somewhere between 0 and 2.1 degrees defined by your transfer window. You can't change this without waiting for a different window.

2. This just seems incorrect. The location of your an/dn in relation to your ap/pe is completely defined by your approach. If you approach from behind or in front of Eve it will be in the worst location (but is the most efficient transfer). For your an/dn to be exactly at ap/pe, you would have to be travelling directly towards or away from the sun, which is a horribly inefficient transfer.

[ScottyDoesKnow]

When you first enter the SOI of Eve, do all of your inclination/aerobrake there. With just your eyeball you can get pretty close to horizontal. Also, at this point it should cost very little Dv to do so....

You can only change inclination at your ascending and descending nodes.

[Kasuha]

2. This just seems incorrect. The location of your an/dn in relation to your ap/pe is completely defined by your approach. If you approach from behind or in front of Eve it will be in the worst location (but is the most efficient transfer). For your an/dn to be exactly at ap/pe, you would have to be travelling directly towards or away from the sun, which is a horribly inefficient transfer.

Don't forget you're not inside the SOI. You're coming from certain direction and all your possible periapses are on a paraboloid (I guess) with rotational axis on the direction from which you are coming. Equatorial plane is a plane and all intersects of this paraboloid with the equatorial plane are periapses which lie in equatorial plane. Not only you can put periapsis above equator, you can even choose at which altitude you want to have it.

These screenshots are ecactly that case. I am intercepting Moho at a rather inclined trajectory, but where exactly will I put that trajectory is up to me. Third and seventh screenshot have periapsis above equator, one on each side.

It was all achieved by RCS fine-tuning from 3/4 an orbit before the intercept.

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Edited March 2, 2014 by Kasuha

[ScottyDoesKnow]

I see what you mean, the adjustments in the picture will allow you to reduce the inclination afterwards. But what I'm talking about as being the problem is the inclination caused by the slope of the orange line in all the pictures, which will be something between 0 and 2.1 degrees that can't be removed in that way.

[Kryxal]

Note that THAT kind of inclined trajectory will make your life more difficult at Eve ... consider this:

You're coming in on a hyperbolic trajectory, but at a much earlier point you can relatively easily adjust your intercept without really changing the direction, so all such intercepts may be considered effectively parallel.

Plot the course such that you're plowing right into the planet.

Now alter the course so that you're skimming by at your required aerocapture altitude (direction not specified, we're about to use that point). Given the speed you're coming in (assuming 4908 m/s at 70 km above the surface pre-aerocapture, and the numbers should be close anyway), your course will be altered 62 degrees from SoI entry (actually a bit less, but let's be safe). If you want the math ... well, have fun playing with the equations. Suffice to say, my hyperbola had a=1 and b=.61 or so.

This means that altering from direct impact to an aerocapture trajectory, we move our peripasis to a circle at an altitude of 70 km, 152 degrees off the approach direction (28 degrees off the extension through the planet).

In this specific case, so long as you're within 28 degrees inclination, you CAN drop your periapsis for aerobraking right on the equator...