Why is launch window important?

[guitarxe]

There are quite a few calculation tools which tell you at what time (day, hour, minutes) to launch if you want to go to some planet/moon. I'm a newbie to this so I'm curious why is this important, and what exactly is it calculating?

Does it tell you at what time the planets are closest to each other in their orbits, thus saving you on fuel? In that case, it's not like I'm not going to make it to my target if I don't launch on these indicated dates, as long as I have enough fuel to get there, right?

Edited July 20, 2014 by guitarxe

[Boomerang]

Your last sentence is key. Given enough delta v, you can get just about anywhere by leaving at any time that you like. But the launch windows are calculating when the most efficient time to perform a transfer is. This doesn't matter much within the Kerbin system, you can usually eyeball going to the Mun and Minmus if you know what you're doing, but in order to launch yourself on a transfer to another planet, the location of Kerbin in relation to your target, as well as your position in orbit around Kerbin makes a huge difference in efficiency.

[Kryxal]

Those are the points in time when things are set up for the lowest delta-v requirements for direct transfers ... they may have a certain amount of give to them for minimal additional delta-v, but they're what to aim for for "cheap" transfers.

[technion]

Although we call them "launch windows", note they would more accurately be called "the manoeuvre node that you make to get from Kerbin to planet x". I always just launch "now" and float around in orbit until the window comes up.

[Geschosskopf]

I'm a newbie to this so I'm curious why is this important, and what exactly is it calculating?

First off, take note of the below:

Although we call them "launch windows", note they would more accurately be called "the manoeuvre node that you make to get from Kerbin to planet x". I always just launch "now" and float around in orbit until the window comes up.

I call them "transfer windows" because they're for transferring to other planets.

Anyway, the thing is, all the planets orbit at different distances from the sun, which means they're all going different speeds, with those closer to the sun going faster than those further out. This means that over the course of time, the inner planets lap the outer planets, so their positions relative to each other are constantly (if sometimes slowly) changing. The relative positions are defined by "phase angles", which is the angle between a line drawn 1st from the starting planet to the sun, then from the sun to the target planet. If you make the transfer burn (in the right direction, see below), when the phase angle is correct, then your ship will encounter the target planet and will spend the least delta-v doing so. Otherwise, it will miss and will require 1 or more extra burns to intercept the target planet, plus perhaps a long time (years maybe, depending on how bad you missed) waiting to do these correction burns. Thus, the "launch" or transfer window is the point in time when the correct phase angle exists between the starting and target planets.

So that's phase angle. The other angle is called "ejection angle", which is about burning in the correct direction. This is the point around your orbit, measured from the direction your starting planet is heading, where you place the maneuver node for the transfer burn. For going to Duna, Dres, Jool, and Eeloo, the ejection angle is positive, meaning it will be somewhere on the night side of Kerbin. When going to the inner planets, the ejection angle is negative so it will be somewhere on the day side of Kerbin. Placing the node at the correct ejection angle is essential to using the minimum amount of delta-v for the trip as shown by the calculator. If you're a little bit off, then your velocity vector after the burn won't quite be in the right direction, so you'll have to do another burn later to correct your path.

[cantab]

This is a Hohmann transfer, which is generally the most fuel-efficient transfer without using gravity assists. As you can see it leaves the starting orbit, goes round the primary to the other side (thus completing half of the transfer orbit), and enters the target orbit.

For interplanetary travel, you want the target planet to be there when you reach its orbit, and that means you need to leave at the right time. There's some leeway before you get a noticeable impact, so that's your transfer window. If you leave at another time you'll have to take a different transfer orbit that costs more fuel to reach. (Or spend several orbits round the Sun waiting for your encounter.)

In real life spacecraft can't always hang around waiting in LEO forever, so you can get a launch window based on the transfer window. In stock KSP that's not an issue.

[Halaberiel]

When you launch can effect your travel time & d/v requirements. Below is a good website to show these values:

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

As an example leaving everything on that website on default & pressing "Plot it!" you will see there are infinitely many launch windows for Duna but they all cost different d/v & they all have differing travel times. By default that website shows you the optimum wrt d/v, but you may sometimes want to sacrifice d/v to leave earlier (ie have your launch further to the left), or to have a shorter journey (further down). Zoom out & notice for Duna transfers the d/v requirements are fairly lax, ie they're usually not very high, and you have a lot of leeway when you launch. Other more difficult planets such as Moho are not so lax with prohibitive costs if you launch at the wrong time (5k total vs 25k for instance, blue vs green).

Edited July 20, 2014 by Halaberiel