Transfer windows to the Outer Planets mod

[Red Dwarf]

I'm currently running a tour of the Outer Planets mod. My mothership doesn't have enough delta-v to make it out there all in one go, so I'm planet hopping, refueling the craft along the way. This has worked okay so far, and I'm currently en-route to Vall. The next step after my pit stop in the Jool system is transferring out to Sarnus, where I will top up the tanks again at Eeloo. Only thing is, I can't figure out the Jool --> Sarnus transfer window. Is there any way to calculate this?

[Rosco P. Coltrane]

Not at the computer at the moment, but AFIAK the Protractor mod gave you this out of the box. Or am I imagining things?

[Snark]

Some general-purpose math for working out transfer windows:

Let's say you're at planet 1 (Jool, in your case), transferring to planet 2 (Sarnus). Assuming that both planets are in circular orbits:

Let's define some terms:

t1 = the orbital period (length of year) for planet 1

t2 = same deal for planet 2

th = Hohmann transfer time (i.e. half of an ellipse)

Assuming that you know t1 and t2, you can calculate th as follows:

th = ((t1^(2/3) + t2^(2/3)) ^ 1.5) / sqrt(32)

"take t1 raised to the 2/3 power, add t2 raised to the 2/3 power, raise the whole shebang to the 1.5 power, and divide by square root of 32".

Now that you've calculated th, you can work out the phase angle for your launch window. I'm assuming here that planet 1 is the inner one and planet 2 is the outer:

Angle = 180 - (360 * th / t2)

This angle is the amount by which planet 1 should be trailing planet 2 at the time that you launch.

Just to show an example, imagine you're transferring from Kerbin to Duna:

t1 = 426 days (one Kerbin year)

t2 = 802 days (one Duna year)

Plugging in the equation above, this gives us:

th = 301 days

Angle = 180 - (360 * 301 / 802) = 44.5 degrees

(meaning "launch when Kerbin is trailing Duna by 44.5 degrees")

Just to check our math, http://ksp.olex.biz/ gives the same answer, so looks like it works out right.

So to solve your problem, just plug in Jool's year length for t1 and Sarnus' for t2, and there ya go.

If you want to know "how long until the launch window happens?":

The interval between launch windows, ti, is given by:

ti = 1 / (1/t1 - 1/t2)

(for example, in the Earth/Duna case, ti = 1 / (1/426 - 1/802) = 908 days.)

...so if you look at the map view and see "how many degrees off am I from the launch window", just take that, divide by 360, and multiply by ti to get the time until launch.

For example, if you're going from Kerbin to Duna, and you see that Kerbin trails Duna by 100 degrees right now: well, the target angle is 44.5 degrees (per above discussion), which means you're 53.5 degrees off, so the time until the window is 908 * (53.5/360) = 135 days.

Edited July 22, 2015 by Snark

[Red Iron Crown]

AFAIK the Transfer Window Planner mod will calculate windows for modified or added planets.

[Drew Kerman]

so will KSPTOT, you just have to ask it to generate a new bodies.ini file in the File menu while the game is loaded in the flight scene