What does it mean for a transfer window to "close"?

[AlpacaMall]

I'm a few months into KSP and I think I've gotten the hang of the basics. That said, I've seen a lot of people talking about having a "wide" transfer window, or having a transfer window "close", and I'm not sure what that would mean.

Right now, when I want to go to another planet, I check the dV map, send a ship into orbit, check the transfer window planner, then - depending on my mood - leave Kerbin SOI either at the lowest dV, or as early as I can get with the dV on my ship. From what I've done, it seems like launching too early or too late just results in higher dV requirements, but I haven't noticed anything that would make me think "I can transfer within this window, but it's much harder outside if I wait past this point in time".  

[Superfluous J]

If you *just* leave Kerbin's SOI, then from Sun SOI burn to a planet, it takes more (and some times a lot more) m/s (i.e. fuel) to get to your destination. So it's better to leave when Kerbin and the target world are in specific places, such that leaving Kerbin in one single burn that reaches the target world's orbit will reach that orbit at the same time the target world is there.

That correct time is called the transfer window. It opens when it's possible to do it kinda cheaply. It then tends to get better and better (though not significantly) for a while (a few days generally) and then it gets less and less well, and then it gets SUPER EXPENSIVE at which point the window closes again.

For reference, check out the Alexmoon calculator and the Transfer Window Planner mod.

https://alexmoon.github.io/ksp/

 

[18Watt]

“Transfer Window” can mean different things to different players.  
One goal might be using the least propellant, another might be minimizing time spent en-route.  There might be other goals or criteria.  For most players, the search is for a way to reach their target using as little fuel as possible.  In that case, there are definitely times which are much better to depart than others.  That’s the “window”.

It’s absolutely possible to depart at other times, but the cost in fuel or DV will be higher.

[Jacke]

A given launch vehicle has a certain performance on interplanetary missions.  The max delta-V it can provide is with no payload.  As payload mass increases, the delta-V that can be delivered to that payload goes down.

To go from one planet to another takes a variable amount of delta-V as time passes, with minimums of delta-V being the centre of the launch window.  At that point, the payload mass that can be sent is maximum.

When designing the mission, a given payload is decided upon and the launch vehicle is intended to give it up to certain delta-V, greater than the minimum, with a small reserve.  That design delta-V being above the minimum means there's a stretch of time during which this particular design can be launched and successfully delivered the payload on its way to the target.

That stretch of time is the launch window.  At the start and end of the window, it starts cutting into the reserve delta-V, which isn't large and is intended to deal with non-destructive mishaps during launch.

Edited February 20, 2021 by Jacke

[Cavscout74]

The "transfer window" is basically the darker blue area of the plot:

 

 

This is a plot for Kerbin to Duna, the dV listed is for both the transfer & the capture at destination.  During the window the dV cost is as low as 1678 but increases quickly as you go further outside the window - as high as 24000 in the bounds of this plot

[AlpacaMall]

Thanks for the responses everyone!

On 2/19/2021 at 11:34 PM, Jacke said:

At the start and end of the window, it starts cutting into the reserve delta-V, which isn't large and is intended to deal with non-destructive mishaps during launch.

(this is what I was looking for, thanks!)