Jool's Delta V

[LunaTrick]

Why is it when I go to Jool, most of the time the delta v needed is ~1900. But every now and then it is something like 4000. What is going on?

I should mention that I am using MechJeb v2.0.13252.1108.

[RoboRay]

Planets move at different speeds depending on how far away they are from the sun. So, the distance between your starting planet and destination planet is constantly changing, as are their relative positions around the sun.

If you time things right, waiting for an ideal alignment of the planets, you can travel there with very little fuel expended on a minimum-energy Hohmann transfer. If you don't wait for the right alignment, you'll need a lot more fuel. Or maybe even more than you could possibly carry.

This is why you may notice Mars missions coming up in the news every 18 months or so. That's the length of time between Earth/Mars transfer windows, so that's why the missions are spread out in regular, consistent intervals.

You can find your ideal transfer windows for KSP here: http://ksp.olex.biz/

Edited October 12, 2013 by RoboRay

[LunaTrick]

This is all understood, but MechJeb is said to be a miser on fuel. So one assumes that it would choose the best alignment, not the worst. This is more of a MechJeb question I guess.

[RoboRay]

No idea, then. I've never let MechJeb plan transfers for me.

You're going to get some variation based on eccentricity and inclination depending on where the planets are along their orbits, but it shouldn't be a factor of x2 or greater. Look at your phase angle... is MJ waiting for the Hohmann window?

On a side note, never assume MJ's choices are the best. They're workable, but you can generally do better.

[LunaTrick]

I use MechJeb because of problems I have with the user interface. If they fix my maneuver node issues in 0.22 I will return to using it. But as of now - it's MechJeb.

[Captain Sierra]

MechJeb likes to go early, by at least a week. i reccomend trying to do the ejection maneuver node yourself as mechJeb is NOT a miser with fuel, quite the opposite. it is precise, which inadvertently saves wasted dV but it is NOT fuel conservative.

[LunaTrick]

OK I want to monitor the phase angle to see just what MechJeb is up to.

Do I have this right - that I display the phase angle variable in a custom mechjeb window, watch the angle countdown to zero, then I launch. Is that how you use the phase angle?