Question on Transfer Windows

[Tazin]

I haven't yet done any trips to other planets, just the moon and minmus. I have installed the Kerbal Alarm Clock mod to notify me of when transfer windows become available. My question is, how important are transfer windows? is it impossible to capture yourself to another planet outside the window? does it take tons more of delta v? also, how big are these windows, time wise? do i have to be in orbit already when the window comes, or is it open for several days so that i can send more than one ship?

Edited July 2, 2015 by Tazin

[Falkenherz]

Look here, for dV requirements outside of those transfer windows.

[Streetwind]

A "transfer window" is a point in time when the dV cost for a given transfer is minimized. You don't have to take the window; you can fly at any time you want. But going outside the window gradually increases dV costs as you need to fly a less optimal trajectory.

Usually, a transfer window is given as a 'porkchop plot', which is a graph that relates time in transit with dV spent for a specific departure date.

A calculator that shows you a porkchop plot usually defaults to a specific trajectory, namely the ideal Hohmann trajectory, because it is the cheapest dV-wise. It is however not the fastest; in fact, it's the slowest you can go without deliberately overshooting and flopping around the target. You can also spend extra dV to fly a route that gets you there significantly faster, for example (a so-called high energy transfer). But, the ideal time for that accelerated trajectory does not have to coincide with the ideal time for a Hohmann transfer.

As such, a transfer window is always trajectory-specific. You decide on how long exactly you want to be in transit, and the "window" is the specific trajectory solution on the specific date of departure that offers the minimum dV spent overall. If you don't care how long you are in transit, the window always describes the ideal Hohmann transfer and pinpoints the absolute minimum dV cost among all possible solutions.

How big the window is and how fast the dV cost ramps up as you deviate from it varies strongly depending on where you are, where you want to go, and what trajectory you want to fly. Moho and Eeloo are typical candidates for generating highly variable windows depending on when you want to leave, because of their eccentric and inclined orbits that present you a different picture everytime they are in alignment with Kerbin.

Go play with the linked calculator, give it some initial conditions, let it draw a graph, then mouse over it and let it recalculate a high energy transfer by clicking into the graph at a lower in-transit time, for example. It'll help you understand how to read the graph. Then let it re-draw the graph with different initial conditions and see how that makes the minimum appear in a different spot.

Edited July 1, 2015 by Streetwind

[Laie]

is it impossible to capture yourself to another planet outside the window? does it take tons more of delta v? also, how big are these windows, time wise?

Both getting there and achieving capture become more and more expensive if you don't launch at the right time. How much more expensive? That depends.

Falkenherz has already linked to Alex Moon's launch window planner. I suggest you give it a try. It will provide definite answers to all your questions.

[mrclucks]

Transfer windows are just recommendations on when to go that would take the least amount of delta v. You can almost always go to your destination whenever you want however it will take somewhere between a little more delta v to many times the amount(example: you can go in basically a straight line to another planet but it would take a lot of delta v to accelerate up to speed and a lot to slow down, which crafts generally don't going to have)

Only times when you cannot go to your planet that instant is when you would collide with another planetary body, which is rare)

The "pork chops" in the plots will generally span at least a day or so with variations of about 10 m/s delta v, so if you're right in one, you should be able to launch a craft, circulize and plot the course without losing out on any saved delta v. It's similar to the effects of the oberth when burning for 5seconds at a minute to periapsis and burning for 5second a few seconds from periapsis. It's not the most efficient but you shouldn't lose out on much delta v

[Red Iron Crown]

In addition to the good advice given above, you should note that transfer windows get shorter the closer your target is to the Sun. You can miss an Eeloo transfer window by several days and not have much of an effect on the cost of the transfer; in contrast, a Moho window can't be missed by more than an hour or two without significant penalties (this is one of several reasons why Moho transfers are hard to do efficiently).

[Snark]

Another really handy transfer calculator, with an intuitive interface:

http://ksp.olex.biz

...you tell it:

- your origin body

- your target body

- the height of your parking orbit

...it tells you:

- what relative positions the origin and target should have for the ideal launch window (with a nice graphical display)

- what position in your parking orbit you should initiate the burn (also nicely graphical)

- how many m/s of dV you will need

- a little warning if the target body has significant orbital inclination to your origin body (i.e. if you'll need to do an orbital-plane adjustment partway through)

As previous posters have said, missing the window just means that the dV requirement goes up. This can mean a more expensive burn to depart, and/or a more expensive burn to match velocities at the target.

Sometimes this matters more than others. For example, aerobraking can take care of a lot of the dV expense on arrival. Let's say you're going to Duna and you're significantly off the optimal launch window. You may be able to find a good trajectory that doesn't take a hugely bigger expensive burn to depart Kerbin, but which results in a very high-velocity intercept with Duna. Well, if you can aerobrake on Duna, that takes care of a lot of that dV. It means a bigger challenge designing your ship to handle very high-velocity aerobraking, but the dV requirements are mitigated.

Also note that these launch windows all assume simple Hohmann transfers. Two-burn transfers can also be an option. For example, if you're going from Kerbin to Duna or Jool, and you're really really off the window, and you don't feel like waiting, one approach may be to eject from Kerbin retrograde (so that your periapsis is lowered down towards the sun), then do a prograde burn at solar periapsis to adjust your orbit. Yes, this process will be more expensive than an ideal Hohmann transfer... but it will be a lot cheaper than trying to do a direct intercept if the alignment is really wrong.

[Tazin]

WOOOW that launch window planner really explains it all. Thanks guys! One question, what do the three transfer types mean? Ballistic, mid course, and optimal? for my transfer to Duna they showed no changes in the plot.

[DeMatt]

WOOOW that launch window planner really explains it all. Thanks guys! One question, what do the three transfer types mean? Ballistic, mid course, and optimal? for my transfer to Duna they showed no changes in the plot.

The first two options determine when you make an inclination change to match your target orbit - either immediately in Ballistic, or partway there in Mid-Course. "Optimal" merely chooses the less dV-intensive of the two.

Since Duna is in the same plane as Kerbin, you do not need to make any inclination change, and thus the two options are equal.

[Evanitis]

While not completely related to the original question, I'd like to call your attention on gravity assists too. I was aware of the existence of the technique for long, but I always undervalued it's efficiency in conserving dV while thinking that it's a complicated, 'really pro-thing' to do.

Once I read into it, I found that it's easy to preform, and saves much fuel - especially these days since 1.0.4 made the atmospheres hit like a hammer, thus making aerobraking- and capturing harder.

So that wiki page is worth a read - with a bit of practice this trick can get your carft to Duna for the dV cost of a Mun flyby. And it can achieve much more later.

Edited July 2, 2015 by Evanitis