How big are Transfer windows

[DrewBox]

This is my first Real attempt to acually go and maybe even land on another planet. I have Kerbal Alarm Clock to tell me when to make the transfer burn, but how long is a particulare transfer window open for?

[kahlzun]

Frankly, it depends a lot on the planets you're travelling between.

Moho has a pretty short one because it moves so fast, Eeloo has a long one because it moves slow (You have to look out for getting cockblocked by Jool though).

I think the Moho one is about 30 minutes, and the Eeloo one is about 4 days.

[FenrirWolf]

They're like maneuver nodes. There is an infinitely small instant in time that represents the most optimal transfer possible, and every moment before or after the window becomes less and less efficient the farther you get from that instant.

But in practice the window is "open" anywhere from a few hours to a few days before or after the the timer hits zero. A window to a big, slower moving planet like Jool will be more forgiving than a faster one like Moho.

[DrewBox]

Well, since Moho is the first planet Im going to, sounds like im about to have some "fun" with quick saves.

[FenrirWolf]

At least if you miss a Moho window, another one will come up pretty quickly afterwards.

[cantab]

Well, since Moho is the first planet Im going to, sounds like im about to have some "fun" with quick saves.

Indeed. Good luck, Moho's probably the hardest body in the game to enter orbit around.

[kahlzun]

Because Moho is so close to the sun, you enter it's SOI -screamingly fast-. It actually needs the most dV to get to, out of all the bodies, the usual figure quoted is about 7000 to ensure ejection, plane change, and capture, let alone landing.

Duna or Eve are probably the simplest: all an Eve lander needs is a parachute, and it's landed.

If you plan to come back, then Duna is your friend.

This site can show you the 'width' of the transfer windows; It's horizontal axis.

Edited June 29, 2014 by kahlzun

[LethalDose]

Well, since Moho is the first planet Im going to, sounds like im about to have some "fun" with quick saves.

Oh... oh gods of Kerbal.

2 bits of advice: First, don't make Moho your first interplanetary target, it's probably the most difficult planet in the game to reach, and it definitely has the highest dV cost to reach. Second, because of it's fast speed and high inclination, trying to do a Hohmann transfer to Moho is a complete nightmare and other approaches (e.g. a bi-elliptic transfer) are typically easier.

If this is your "first real attempt to travel to another planet", I'd strongly recommend targeting Eve or Duna instead of Moho. It's much easier to learn the basics of interplanetary transfers with these targets.

EDIT: aaaaand ninja'd twice.

It's probably worth mentioning Both Eve & Duna have moons that are very easy to land on and return from, as well.

Edited June 29, 2014 by LethalDose

[Tortoise]

Technically transfer windows are infinite, but you're just looking for the most efficient time. Those typically last..Kerbal hours?

[Eric S]

Technically transfer windows are infinite, but you're just looking for the most efficient time. Those typically last..Kerbal hours?

[MarvinKitFox]

Transfer windows are instantaneous.

Even a millisecond early or later, and your trip will start costing you more than the true minimum of delta-v.

How much difference does the time make?

What matters is the angle of both bodies relative to the sun, i.e. their phase angle in their orbit. The two orbits need to be at the correct angle to each other.

1 degree error is less than my own aiming error.

5 degrees is about 10% more delta-v needed.

40 degrees error and you pay penalty of about equal to your current planet's orbital speed. (9200m/s for kerbin)

If going from Kerbin to Moho, 5% error is 8 hours.

If going from Kerbin to Jool, 5% error is 6 days.

If going from Jool to Eeloo, 5% error is 180 days.

[Epthelyn]

40 degrees error and you pay penalty of about equal to your current planet's orbital speed. (9200m/s for kerbin)

If you're 40 degrees off on a transfer window then it's not so much a question of accuracy as "Why did you wait n whole days when the transfer window was at time x?"

[LethalDose]

Those typically last..Kerbal hours?

Kerbal hours are the same as Earth hours (1 hr = 60 mins = 3600 s), it's not until you get to days and years that things change.

[Kasuha]

If going from Kerbin to Moho, 5% error is 8 hours.

If going from Kerbin to Jool, 5% error is 6 days.

If going from Jool to Eeloo, 5% error is 180 days.

That's not very accurate. Especially the difference between Eeloo and Jool suggests there is some error involved because they should be about the same.

If we declare that the width of the window is 5 degrees shift from the optimal position on either side, we must take into account that all planets go around the Sun in the same direction, so the difference does not grow as fast as it would grow relatively to some fixed point.

My results are:

Moho: 22 hours

Eve: 113 hours

Duna: 151 hours

Dres: 88 hours

Jool: 77 hours

Eeloo: 75 hours

These numbers are total width, so if you go to Moho you should depart between 11 hours early and 11 hours late to have this accuracy. And since the shift can be fixed by some radial component of the ejection, you won't lose more than about 1% of dv if you do it right.

Calculation, e.g. for Dres:

Kerbin sidereal period (seconds) = Sk = 9203545

Dres sidereal period (seconds) = Sd = 47893063

Kerbin angular velocity (degrees per hour) = 360 * 3600 / Sk = 0.141

Dres angular velocity (degrees per hour) = 360 * 3600 / Sd = 0.027

Difference (degrees per hour) = abs(1296000 / Sk - 1296000 / Sd) = 0.113

Time for that difference to reach ten degrees = 10 / abs(1296000 / Sk - 1296000 / Sd) = 87.9

Edited June 29, 2014 by Kasuha

[BrianMcNett]

As others have already noted, transfer windows are a bit like maneuver nodes. There's an instant where, if you hit it exactly, you will get the minimum fuel transfer. The further off you are from that time, the more fuel it will take. Too far outside that window, and it takes impossible quantities of fuel.

Most folks have mentioned so far that the transfer windows for inner planets are shorter than for the outer ones. True! There are other considerations, however.

Moho is a very good example. It's not only inclined, but also eccentric, such that the most efficient transfer windows only take place every five years. So even though you have a transfer window every few months, these are often hugely sub-optimal You'll find this an issue mostly when it comes time to circularize your orbit around Moho. Unless you got the "lucky" transfer, you'll likely use almost three times the ideal delta-v to circularize your orbit around Moho. Yikes! So although it should take about 2200 m/s delta-v to circularize, unless you're watching really closely, and waiting to plan your Moho encounter at Moho's periapsis, come prepared to blow off 5000 delta-v or more.

Your easiest transfer is Duna, although the window isn't nearly as frequent. The launch window is several hours. Jool is also a good target, although you'll be a couple years in transit. There's almost a full day lee-way. The windows get bigger the further out the planets get, and a large target like Jool, with it's big SOI will be easier to hit throughout its transfer window.

I've left Jeb, Bob, and Bill stranded on the surface of Moho, and am planning a rescue mission. They're going to need a ton of delta-v, because I'm not waiting five years for the most efficient window.

[Kryxal]

For that, all you have to do is hit Moho's orbit at the point where the most efficient window would be, then reduce your apoapsis so as to return to that point next orbit when Moho is there ... it also makes your capture burn easier!

[kahlzun]

For that, all you have to do is hit Moho's orbit at the point where the most efficient window would be, then reduce your apoapsis so as to return to that point next orbit when Moho is there ... it also makes your capture burn easier!

That can be efficient, yes, but can take a long time to set up.