Whats the deal with Tranfer Windows?

[Workable Goblin]

[quote name='AbacusWizard']Transfer windows are one of the few things about KSP-level space travel that still mystifies me. I understand the basic idea conceptually, and I've heard of porkchop plots; I just have no idea how to run the numbers, or even where to start. Can anyone recommend a good resource for figuring it out? (I don't mean a mod that will do it for me or a calendar of useful launch dates; I want to know how to calculate them myself.)[/QUOTE]
You really, really don't. Finding launch windows is equivalent to solving [url=https://en.wikipedia.org/wiki/Lambert%27s_problem]Lambert's problem[/url] repeatedly (due to the patched conics approximation used by the game), which is generally difficult (in a mathematical sense). There's a reason that one of the first things that JPL did with electronic computers was program them to numerically solve this problem and calculate flight trajectories without needing people to do it by hand.

[GoSlash27]

Abacus Wizard,
I do know how to calculate them, but AFAIK there's no tutorial available for it. Explaining the math gets pretty unwieldy on this forum because it's not well- suited to mathematical equations.

Basically...
You start with the semimajor axis for both planets.
Then you figure out the semimajor axis for the transfer orbit.
With that, you have the transfer time from one planet to another.
Using that time, you figure out how much the destination planet will orbit in that time.
This gives you the alignment phase of the transfer.
Given this phase and the relative phase rate, you can work out how often transfer windows happen.

For figuring the transfer DV, you need to figure out how much velocity it takes to escape your home planet's SoI and how much DV to do the Hohmann transfer to the destination planet.
These are vector-added pythagorean style and then the initial orbital velocity is subtracted. The retroburn at the destination planet is computed the same way.

This method leaves a lot of variability due to orbital eccentricities and inclinations and doesn't give a specific date, but it's good enough for government work. A lot of the mods do it this way.

Best,
-Slashy

[AbacusWizard]

Thanks all; that should give me a good starting point for further investigation.

[Hannu]

[quote name='AbacusWizard']Can anyone recommend a good resource for figuring it out? (I don't mean a mod that will do it for me or a calendar of useful launch dates; I want to know how to calculate them myself.)[/QUOTE]

I used [URL="http://www.braeunig.us/space/interpl.htm"]Braeunig's[/URL] information to learn and program my own Porkchop calculator. Edited November 24, 2015 by Hannu
I corrected the link again

[Pds314]

Here's the thing. You don't strictly NEED to be efficient, but it really helps. A 10-ton payload with a mainsail needs a 2.5-m grey tank to get a bit under 2 km/s. It needs a PAIR of orange tanks to get a bit under 4, and a dozen orange tanks to go a bit under 6. It's a bit better if they're all drop tanks, but it's still highly nonlinear. Assuming massless decouplers and infinitely small tanks dropped as their fuel is exhausted, a bit under 2 km/s requires 15 tonnes of tankage. A bit under 4, requires 53 tonnes. A bit under 6, requires 128 tonnes.