Timing Transfer Burns for Interplanetary Journeys

[Clipperride]

I think I understand the basics of using http://ksp.olex.biz to find the relative angles for a burn that leaves Kerbins SOI, but I'm still a little unsure about the ejection angle.

It says "The ejection angle is the angle, at which you want to start your transfer burn in your origin planet's or moon's orbit.". But doesn't it depend on your ships TWR? A burn could vary between 1 and several minutes depending on your design and goals. Should I always start my burn when I reach the correct angle or set a manoeuvre node for the desired ∆v at the correct angle and start the burn ½burntime before the node?

With other things going on in my game, I'm not wild about using to much time warp, so I'm hoping to get it right first time.

One more thing I wondered about - what about less than optimum flights? Is there a "Brute force" method for planets that are only generally in the right place relative to my starting point? I've seen ideal transfer windows given to an accuracy of seconds in both time and required angles. However precise do I have to be?

As always, advice is greatly appreciated.

Edited August 31, 2015 by Clipperride
Quickly and fully answered

[cantab]

It says "The ejection angle is the angle, at which you want to start your transfer burn in your origin planet's or moon's orbit.". But doesn't it depend on your ships TWR? A burn could vary between 1 and several minutes depending on your design and goals. Should I always start my burn when I reach the correct angle or set a manoeuvre node for the desired ∆v at the correct angle and start the burn ½burntime before the node?

I suspect that's a minor error on the website, and you should take the second option - set up a manoeuvre and burn it like you normally would.

One more thing I wondered about - what about less than optimum flights? Is there a "Brute force" method for planets that are only generally in the right place relative to my starting point? I've seen ideal transfer windows given to an accuracy of seconds in both time and required angles. However precise do I have to be?

"Transfer Windows" are necessarily fuzzy. You'll generally be fine within a few days, maybe even a few tens of days, for anywhere but Moho (which requires special treatment to get an efficient transfer anyway). Just bear in mind that a transfer that deviates from the fuel-ideal Hohmann Transfer may require not only a bigger departure burn but also a bigger capture burn or harsher aerobrake. The further your apoapsis (or for Eve periapsis) is beyond your destination planet's orbit the bigger the capture burn will be.

[Laie]

One more thing I wondered about - what about less than optimum flights? Is there a "Brute force" method for planets that are only generally in the right place relative to my starting point? I've seen ideal transfer windows given to an accuracy of seconds in both time and required angles. However precise do I have to be?

Let me answer your last question first: there is exactly one precise moment that's the best, but launching hours or days sooner or later usually only costs you very little. Simple rule of thumb: for everywhere except Moho, you have several Kerbin days on either side during which the dV requirement barely changes. If you want to know more, I suggest you try Alexmoon's Porkchop Plotter (also available as an in-game mod called Transfer Window Planner).

It says "The ejection angle is the angle, at which you want to start your transfer burn in your origin planet's or moon's orbit.". But doesn't it depend on your ships TWR? A burn could vary between 1 and several minutes depending on your design and goals. Should I always start my burn when I reach the correct angle or set a manoeuvre node for the desired ∆v at the correct angle and start the burn ½burntime before the node?

The latter. Not only for transfer burns, by the way: whenever a maneuver takes longer than a few seconds and/or precision is important, you should start your burn so that about similar work happens on either side of the node. If in doubt, it's better to start a little too early than too late.

If you happen to have a really low TWR, so the entire burn will take you more than 10-15 minutes, more elaborate measures are called for. Though from the sound of your post I don't expect you to be in that camp (yet), so I'll skip this bit.

[Clipperride]

Thanks for the answers folks! I've just noticed that the Interactive illustrated Interplanetary guide and calculator for KSP gives the burn information for transfers between the Mun and Minmus, which is ideal for practising the technique without having to use a load of time warp.

[Brainlord Mesomorph]

see the tutorial in my sig

- - - Updated - - -

If you happen to have a really low TWR, so the entire burn will take you more than 10-15 minutes, more elaborate measures are called for. Though from the sound of your post I don't expect you to be in that camp (yet), so I'll skip this bit.

see the tutorial in my sig

[Clipperride]

Cheers Brainlord - I must say I love the whiteboard graphics. Like being back at school, but a whole heap more fun :-)