So, clearly I need help with retrograde orbits

[James_Eh]

To try to put it as succinctly as possible:

Approaching Jool with a massive craft intending to carry out the Jool 5 challenge. I fooled about with approaches for the longest time, and finally found what I was looking for: a direct encounter with Laythe. It is critical to the mission that the first orbit I establish is around Laythe so that I can shed the weight of that lander, and possibly use the returning part of it to help with the Tylo mission. I don't have the dV to lug all 3 landers (Laythe, Tylo, other 3) from orbit to orbit. So I was happy to be able to skip the Jool aerobrake and head right to Laythe.

All went well except just as I entered the Laythe SOI, I realized that I was heading for a retrograde aerobrake. I wanted to change that prior to the event but it seemed it was going to cost me well over 1000 m/s. I assume that was because the window of encounter at Laythe is small when you are screaming in at interplanetary speeds. So I went ahead with the aerobrake thinking that as long as I created a highly elliptical orbit, the inclination change would be relatively cheap.

Boy was that wrong. I am in a 2000x30 km orbit and it seems that it will cost me upwards of 3000 m/s to correct. Does this not seem excessive?

So now I am wrestling with choices:

1) do everything at Laythe retrograde. This would be initially cheapest, but the next burn to Tylo would cost more.

2) drop the massive Tylo lander and smaller lander, do the inclination change with just the drive and the Laythe lander, land and return, then find the cheapest way to re-dock everything. Same dV but far less fuel consumption. It seems to me that this would only be the best option if it all wound up prograde at the end.

3) abandon orbit! Accelerate at Laythe peri to leave Laythe SOI, then set up another encounter. I tend to favor this one...

4) is there some kind of cheap orbital change I can make from my current highly elliptical orbit to wind up with a lower net dV requirement to switch to prograde??

Any other options? Or advice?

Edited April 16, 2014 by James_Eh
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[Claw]

You should be able to reverse your orbit at AP for double whatever your orbital velocity is at that point. So if you keep your AP really high, your orbital velocity will be low. The further your AP, the lower your velocity, the less dV you need. You double it because you basically kill your orbital velocity to zero, then set that same orbital velocity going the other direction.

[cantab]

I really don't think it matters that much. Laythe's rotational velocity at the equator is 60 m/s, so the ascent to a retrograde orbit is going to cost you 120 m/s extra. The descent, of course, costs you nothing more since you're aerobraking. Hopefully you aren't that marginal on delta-V that 120 m/s means the difference between success and failure.

And as for burn to leave Laythe and go on to Tylo, I'm pretty sure it makes virtually no difference at all. Just you'll have to burn on the other side of Laythe to if you were in a prograde orbit.

[Jimbimbibble]

You can also aerobrake into a prograde Laythe orbit if you come in on the correct side of the planet. It might not take very much to do this.

[Kasuha]

I assume you were trying to reverse your orbit from periapsis as doing that from apoapsis should not take more than several tens of m/s. But you're already travelling together with Laythe so you have its orbital speed around Jool - for further transfers to other moons it's almost irrelevant which direction is your orbit around it. So the cheapest solution is to do what you need on Laythe from your current orbit and then continue to other moons normally. You can either use your current elliptical orbit (then you might want to wait for suitable moment in Laythe orbit to eject) or circularize and in that case you just need to eject in the right direction which would be opposite side of Laythe compared to if you were in prograde orbit.

[James_Eh]

* facepalm * I was so locked into the usual way that I make small changes to inclinations (at the equatorial nodes) that I gave zero thought to the fact that I could wait until apoapsis and just stop dead and restart. Funny thing is, I knew that I wanted a very eccentric orbit to make the manouver cheap - then I proceeded to try to set it up in the most expensive way possible. Thanks to all for pointing out what should have been blazingly obvious.

[mellojoe]

Conic_Patch_Mode = 0

I find this helps tremendously with approaches. You can then use TAB to focus in on the particular body (ie: Laythe) and see exactly how your path coorelates with that body. Tiny tweaks from far away can translate into large changes. I find turning on RCS and making small tweaks can ensure I enter the sphere of influence in the correct direction (prograde or retrograde to the orbit, whichever you prefer for that mission) and adjust the PE to a high degree of accuracy with just tiny bursts from the RCS and/or thrusters.

[Claw]

Thanks to all for pointing out what should have been blazingly obvious.

It's okay, that's why we are all here, to help eachother out. Like when someone is looking for their glasses while they are wearing them.

[cantab]

Conic_Patch_Mode = 0

I find this helps tremendously with approaches. You can then use TAB to focus in on the particular body (ie: Laythe) and see exactly how your path coorelates with that body.

In .23.5 you no longer need to make the setting change. With the default configs, if you're focussed on the body you're encountering it automatically switches to mode 0.

[Kasuha]

In .23.5 you no longer need to make the setting change. With the default configs, if you're focussed on the body you're encountering it automatically switches to mode 0.

The need to do so is definitely lower but sometimes I'm still finding the new system less comfortable than simple mode 0.

And conic patch limit is definitely set too low. Planning Jool intercept from Kerbin LKO with Mun slingshot is completely out of question with it, for instance.

[mellojoe]

In .23.5 you no longer need to make the setting change. With the default configs, if you're focussed on the body you're encountering it automatically switches to mode 0.

I did not know this. Thats great to hear. I'll play with that some tonight.