Gravity transfer frustration.

[Hour Hero Yes]

Are gravity transfers a really useful skill? Should all good pilots be able to do it? I've spent most of my day trying to launch from Eve to Jool, but don't get enough DV so I loop around Kerbin, and then I either have the phase angles wayyy off or I can't get my actual SOI exit lined up straight, and I'm probably just wasting fuel I could've spent boosting straight to Jool.

I use the Mechjeb maneuver node editor to try to get everything perfect, but I have no idea how you could calculate phase angles multiple planet transfers in advance. Is there a good way to go about learning how to do this? I really want to be able to do it but am struggling.

[Zeroignite]

The Protractor mod calculates phase angles for you

And yeah, if you want to have a non-gigantic fuel budget, you'll want to be able to do Hohman interplanetary transfers. That said, I often botch it for worlds that have highly elliptical or eccentric orbits...

[Peewee]

That's not a simple task. You might want to check out this tool to help with the math, but if you insist on doing it all yourself you can change the conics patch mode to see your flyby relative to the particular body, and increase CONIC_PATCH_LIMIT (in settings.cfg, located in the install directory) to see more than 3 SoI's ahead.

[Kerbonautical]

Are gravity transfers a really useful skill? Should all good pilots be able to do it? I've spent most of my day trying to launch from Eve to Jool, but don't get enough DV so I loop around Kerbin, and then I either have the phase angles wayyy off or I can't get my actual SOI exit lined up straight, and I'm probably just wasting fuel I could've spent boosting straight to Jool.

I use the Mechjeb maneuver node editor to try to get everything perfect, but I have no idea how you could calculate phase angles multiple planet transfers in advance. Is there a good way to go about learning how to do this? I really want to be able to do it but am struggling.

Don't expect Mechjeb to be able to make efficient use of your fuel reserves, especially when leaving the earths SOI. More often than not it will calculate your maneuver in accordance the the rotational direction of planetary body you were last in such as Kerbin and just sends you whichever way around the planet/satellite that it chooses, normally requiring you make a manual burn at some point, wasting yet more fuel. I'm not against MJ, I just think it sucks for going interplanetary in it's current unfinished build.

[Kegereneku]

Here is 2 websites which might help you.

One give you the phase angle and Dv budget for Hohmann transfer. The other give you the Dv Budget for all other launch windows, you can in theory use it for slingshot maneuver.

http://ksp.olex.biz/

http://alexmoon.github.io/ksp/

[Hour Hero Yes]

I do use Protractor for phase angles and such, but I don't think having just the angles is that helpful when you're already on your slingshot trajectories across the solar system. unless it is and I just don't know how to access the math! That MATlab program looks good, albeit complicated. Time to learn some more stuff today

As far as MJ being inefficient--I usually use the "pinpoint closest approach" to get a general maneuver time and then I go around editing it myself. It usually doesn't have me departing the SOI prograde to the planet, so i make some maneuver node changes myself. I didn't know that about the conics mode, though. Figuring out HOW I was going to be exiting the SOI was the most frustrating thing I was trying to deal with yesterday.

http://alexmoon.github.io/ksp/

One question about this site, what exactly do "Ballistic" "Mid-course Plane Change" and "Optimal" mean for transfer types?

[Frederf]

See post below

Edited August 3, 2013 by Frederf

[metaphor]

"Ballistic" puts the inclination change as a component on the transfer burn, so you do a single burn to get to the other planet.

"Mid-course plane change" makes the first burn only a straight prograde burn (no normal component), then adds a second burn at the ascending/descending node for plane change while in solar orbit.

"Optimal" is the option with the least delta-v required. It might put a normal component on the first transfer burn, and also have a second inclination change burn in solar orbit, depending on what is most efficient.

If you want to perform a gravity assist/slingshot with no burns, you want the first option (ballistic).

[SkyRender]

I just discovered the KSP Launch Window Planner last night myself. Makes things much easier, I must say! Some planets have very... interesting... transfer windows. Often for about 10% extra dV you can arrive much, much sooner than the absolute optimal timing would allow for. For example, the classic first Duna transfer window at around day 58 is nice, but if you wait 'til day 61, you can get to Duna about 25% sooner for a very small extra amount (we're talking sub-100m/s).

[metaphor]

Yep most launch windows are pretty flexible. The phase angles currently used are only for exact Hohmann transfers. But for a little extra delta-v, depending on how far you are from the ideal phase angle, you can still encounter the other planet on a modified Hohmann transfer. At the beginning of a new game Eve is in front of Kerbin (rather than 50 degrees behind for an ideal transfer) and yet you can still get to Eve for 1300 m/s (as opposed to 1040 m/s for an ideal transfer).

That's why in real life a crewed Mars mission is designed to use a modified Hohmann transfer to get a significant reduction in transfer time at the cost of very little extra delta-v.