Efficient orbit change

[The Aziz]

So I sent a probe to orbit Jool for the first time ever. After final burn I got something like this:

I'm with almost 3k of d-V left, as you can see, but it seems that orbit corrections in Jool SoI take a lot more. I tried some manevuers in a halfway, just before capture, etc, but nothing helped. Is there any really efficient way to change my.. inclination (is that it?) to desired?

[Dfthu]

Pretty sure you have to burn to change it. If your trying to get to moon you can maybe still get to it. I was in a odd orbit but I still made it

[Sanic]

The earlier you execute the inclination burn, the less deltaV it'll take. I prefer to execute the inclination burn just as I enter the SOI, so it's not hyper sensitive.

[paul23]

Inclination is one of the things you particularly wish to get "roughly correct" before actually entering the SOI (or in case of lift-off, during sub-orbit). A small change in speed when leaving kerbin means you are either before or after jool, changing inclination by 180 degrees. And anything between that is possible by changing the approaching plane. Using moons you can slow down keeping yourself in Jool's SOI for free, but this is really a "trial and error" approach: you might waste more because of inclination changes due to fly-bys. (so test that).

Other than that getting the perijovian (perijoolian?) to be as small as possible is of course most efficient.

[The Aziz]

..after some reading, that's not inclination, but I don't know what that is.

Well. I want to enter SOI along with that blue line:

Is it possible without a lot of effort?

[KerikBalm]

Its the argument of perapsis that is the problem here.

The most efficient way to change your inclination is to capture with your apoapsis way out, and do the plane change at apoapsis... that will get you part of the way there, but then the real problem is your argument of perapsis, which you need to change with a burn at the AN/DN

In some cases, it may be worth it to burn just before/after/at an An/DN (whichever is deeper in the gravity well), to push your orbit out so that the AN/DN is at apopasos (ie, you make a new apoapsis that is at the AN or DN).

Then you perform the plane change when moving slowly at your (new) apoapsis, and then circularize (if you could aerobrake, it would save dV, but for Jool, I've heard you need to be really careful and can't go more than half a km into the atmosphere before overheating destroys your craft)

[Snark]

If you're not in a hurry, do this:

1. While you're still far away from Jool, make a minor tweak to your orbit to get your Jool periapsis as low as you can without actually frying in atmosphere.

2. At Jool periapsis, do just enough retro-burn to capture to Jool orbit, but leave your apoapsis as high as you can get away with (i.e. just inside Jool's SoI).

3. Coast up to apoapsis (this will be slow).

4. At apoapsis, burn prograde to circularize (to a very very large, slow circular orbit around Jool).

5. Coast until you get to the AN/DN node of that orbit with your target one.

6. Do a burn that makes a plane change to get you coplanar with the target orbit, and also enough retrograde to get your periapsis back down to as-low-as-possible, just above the atmosphere.

7. Fall down to periapsis.

8. Do a retro burn to drop apoapsis down to your target orbit, and you can figure out the rest.

There may be some opportunity in steps 2 and 8 to save a little dV by aerobraking, but as KerikBalm points out, Jool's pretty toxic for aerobraking these days.

[KerikBalm]

Yea... I no longer aerobrake... do a gravity assist from Tylo or Laythe to capture... you could probable use one of them to change inclination and argument of perapsis too.

https://en.wikipedia.org/wiki/Argument_of_periapsis

If you get argument of perapsis down to 0 with respect to the target orbit(use the target orbit as a reference, not the planets equator), then its just a simple inclination change

[The Aziz]

I have similar situation with Eve approach today, it will be a perfect moment to try your advice before Jool one.

[cantab]

Reaching a specific inclined orbit - as opposed to merely a specific inclination - around a planet is indeed tricky.

I would start with a correction to make your Jool periapsis touch the target orbit, or as close to that as you can get. From there you could either

a) Combine your capture and plane change into a single burn at your AN/DN relative to the target orbit. You benefit from Pythagoras - the total burn is less than the sum of the prograde and normal components, and indeed probably won't be much more than the cost of a plain capture.

Make an initial capture into an elliptical orbit with an apoapsis high over Jool. Make your plane change when high over Jool. Then lower your orbit to match the target. You benefit because plane changes take less delta-V when you're going slower and you're going slower when you're higher up.

[paul23]

To append to the above: also remember that the "target orbit" around another body is fixed to the local reference plane. This reference plane however will rotate over the course compared to the "main body" (kerbol in case of jool). So launching at a different time might incur a better orbit, though at the price of not being able to do a perfect hohmann transfer. (OR of course the stars have to align perfectly, wait for million years...).

Would be fun if KSP started supporting J1 & J1.1 orbital pertubations: than we'd be able to actually have/plan free inclination changes & make sun-synchronous orbits. Given the huge rotational speed of Kerbin, the planet would be flattened a lot, so J1 is quite high. (Maybe not as much as Haumea but it should still be larger than earth).

Edited October 25, 2015 by paul23

[Red Iron Crown]

Adjusting the longitude of ascending node is tricky, I have not discovered any rules of thumb for doing so. I can only do so by trial and error, and have found the longer before the encounter the correction is done the cheaper it is to do.

My method (Using PreciseNode):

Wait until out of Kerbin's SoI.

Make a maneuver node a few hours or so in the future so there is no time pressure while fiddling.

Focus on the target (Jool in this case)

Adjust the node in the finest steps available, and watch how the resultant trajectory changes.

When the inclination and LaN are good, execute the burn (I often thrust limit engines for small burns like this).

Usually it will take a combination of node directions to get the desired encounter, but the cost for doing so is quite small (often 50m/s or less for a Jool correction).

[GoSlash27]

I have a radical idea:

Circularize around Laythe instead of Jool. It will carry you around to the target orbit's plane for free.

Then eject from Laythe to match your Pe to the target orbit. as soon as you escape Laythe's SoI, fix your inclination while the velocity is low.

This is the cheapest solution I can think of.

Best,

-Slashy

[paul23]

Adjusting the longitude of ascending node is tricky, I have not discovered any rules of thumb for doing so. I can only do so by trial and error, and have found the longer before the encounter the correction is done the cheaper it is to do.

My method (Using PreciseNode):

Wait until out of Kerbin's SoI.

Make a maneuver node a few hours or so in the future so there is no time pressure while fiddling.

Focus on the target (Jool in this case)

Adjust the node in the finest steps available, and watch how the resultant trajectory changes.

When the inclination and LaN are good, execute the burn (I often thrust limit engines for small burns like this).

Usually it will take a combination of node directions to get the desired encounter, but the cost for doing so is quite small (often 50m/s or less for a Jool correction).

Changing the longitude of ascending node without using J1 pertubation, and while already in an orbit is simply on of the most expensive manoevre. (In reality it's free, using J1).

The only way to go about it: First you make sure you have the same inclination as the wanted orbit. Then at the highest latitude point (so not periapsis, but the one nearest to the poles) you do an (anti)normal burn to change the orbital plane to the wanted orbit.

[Red Iron Crown]

Changing the longitude of ascending node without using J1 pertubation, and while already in an orbit is simply on of the most expensive manoevre. (In reality it's free, using J1).

That's why I am suggesting adjusting it before arriving and capturing in orbit.