Early Pe Kicks... when to do them?

[Scottiths]

I want to set up a mission to Moho. I have protractor and MechJeb 2. I want to set it up so that my final exit burn is very short. IE: I want to make my orbit very elliptical so that when it comes time to exit Kerbin SOI I just need a short burn. How do I judge where the ejection angle will be when the planet is in phase using the tools I have listed? Do I just burn when Theta is near zero? Or is it recording the wrong ejection angle if the planet is not in the correct phase angle yet?

[CalculusWarrior]

How many engines do you have, and what type? If you have sufficient acceleration, I would suggest not kicking your periapsis at all.

By making a very elliptical orbit, you increase that orbit's period to several days, thus throwing off your phase angles if you happen to be in the wrong place of the orbit at the right time.

My suggestion? Just wing it. Start periapsis kicking a few days before your phase angles line up, and if you don't get a close encounter right away, put a maneuver node immediately after you exit Kerbin's SoI and play with that until you have an encounter at your desired altitude.

[RoboRay]

The window for a near-optimal Jool transfer is about a week wide, so you don't need to worry overly much about hiring a precise target on departure time going there. It's tighter with the inner planets, though. Moho requires the greatest phase angle accuracy, as it changes so rapidly.

If you only push your Ap out to near the Mun's orbit, your period is still measured in hours, not days. If you want to push Ap out beyond Minmus, your period may reach a week.

You also only shorten your final injection burn by 150m/sec or so by pushing Ap beyond the Mun with kicks, so it's not a big deal if you only take it that far.

Edited May 12, 2013 by RoboRay

[Zephram Kerman]

Here's my suggestion: starting doing periapsis kicks when the phase angle is a little more than a day ahead of time. During the final kicker burn, watch the orbit period to make it match up (about one day) so that you return to periapsis on time for your transfer burn.

[Scottiths]

Well my problem is that I have multiple ships all waiting to go. I need to have them able to leave fairly rapidly in order to get them all in the window (I tried building a mother ship to carry it all at once but my computer had a heart attack).

An extreme example of the kind of information I am looking for is this: If I were 1 year away from a phase angle window, would it be possible to set up the elliptical orbit such that its Pe was at the appropriate ejection angle for the planetary phase angle in a years time?

Some of my probe craft are Ion powered and so have TWR of as low as .09, so I really need as much kick as I can get (I let Mech Jeb handle really long burns while I lurk the forums )

[Scottiths]

Sorry to double post but I just thought of a way to simplify the question.

Is the angle in protractor mode for Æ always correct or does it change over time with relative positions?

If it is always correct then I can just set up my kicks around that.

[Specialist290]

Well my problem is that I have multiple ships all waiting to go. I need to have them able to leave fairly rapidly in order to get them all in the window (I tried building a mother ship to carry it all at once but my computer had a heart attack).

An extreme example of the kind of information I am looking for is this: If I were 1 year away from a phase angle window, would it be possible to set up the elliptical orbit such that its Pe was at the appropriate ejection angle for the planetary phase angle in a years time?

Some of my probe craft are Ion powered and so have TWR of as low as .09, so I really need as much kick as I can get (I let Mech Jeb handle really long burns while I lurk the forums )

Unless you've built your probes such that you have a margin of error thinner than a knife's edge, you should be able to get away with setting up your orbits so that they bracket the ideal launch time by a few days at most on either side, giving yourself a few hours between each craft's burns. (In other words, if you have for instance five probes, set them up so that one puts you at periapsis on the ideal orbit right on the tick, then one each something like three and six hours before and after that point.) It's likely that you'd have to perform some midcourse corrections anyway, so it shouldn't be too much trouble to correct for if you get your final escape burn in the neighborhood of your destination. Unless you're a math wizard, though, I'd advise trying to set up the first "kicks" as close to the launch window as you can. If you don't have it already, Kerbal Alarm Clock has a number of nifty features that can help with this.

I'm not experienced enough with Protractor myself to really answer that last question, unfortunately.

Edited May 12, 2013 by Specialist290