Change Inclination before or after I make my orbit smaller?

[davidpsummers]

I need to lower and orbit. I also need to change the inclination. Do I change inclination on the larger orbit or wait until I make is small (to safe fuel). Part of me thinks, "changes in higher orbits are easier" while another part thinks the loss of dV going to the smaller obit means changes will be cheaper.

Also, just to check, if the orbit is elliptical, it is cheaper to change the inclincation at the apoapsis?

Edited June 28, 2014 by davidpsummers

[Claw]

Inclination changes are always cheaper when you are going slower. So "cheaper" when you are at the higher orbit, or at the AP of an elliptical orbit.

Depending on how much of an inclination change is required, it may also be cheaper to establish a highly elliptical orbit, change inclination, then circularize. That takes a bit of time though.

[mhoram]

The slower you are, the cheaper inclination changes are.

So I would suggest:

1. Drop your Periapsis to the target-orbit (so that you have an elliptical orbit).

2. Change the inclination at Apoapsis of the now elliptical orbit.

3. Drop your Apoapsis to the target-orbit.

As a side note: If you want to change the inclination by a lot (>60°) it might be more efficient to raise the Apoapsis up to SOI.

[Kasuha]

Assuming you're in high circular orbit and need to go to low circular orbit with different inclination, then

1/ coast to one of inclination points

2/ perform burn which will lower your periapsis to target and change the inclination at the same time

3/ coast to periapsis

4/ circularize

This was you'll get both in just two burns and it will be cheaper than if you do them separately

[Kryxal]

Combined burn sounds right to me, especially since you benefit from the sum-of-squares.

[Jean Deaux]

I too finding a combined burn more efficient when able. This can take a while to setup just the way you want, but I find uses less fuel vs two separate maneuvers.

[davidpsummers]

Yeah, I sometimes combine inclination changes with changes in orbit shape, but usually I just try and do one or the other to keep it simple. It was a surprise to me when I found that keeping your obit the same size was tricky with large inclination changes.

[davidpsummers]

"As a side note: If you want to change the inclination by a lot (>60°) it might be more efficient to raise the Apoapsis up to SOI."

I've not used this because I don't know of a rule of when it applies and I figure I'm more likely to waste fuel doing it wrong.

[davidpsummers]

The specific case here is I've gotten my first probe to Jool. I'm in a high orbit going to drop the probe into the planet. But I'm dropping my periapsis in stages to get in "free" flyby's of the moons. Once I get the periapsis down, I may use aerobraking to get better encounters with the moons. (I would be better if I hadn't gotten myself in a retrograde orbit :-).

[LethalDose]

If you want to get into a prograde orbit, you're in a good position to do it. Reversing orbital direction is basically a 180 degree inclination change, so you can do it all in one burn up high, or you can slingshot off of the moons into better orbits.