Whats the deal with Tranfer Windows?

[Dfthu]

I don't get it, what's so special about transfer windows? I just send out ships whenever,not caring about transfer windows. Does it cut down fuel or time. If so, what's difference? I don't care time, we have timewarp for that and I always over engineer my rockets. Edited November 21, 2015 by Dfthu

[EuSouONumero345]

It's economical?

[klgraham1013]

It can affect the dV required and the length of the trip.

[SpaceplaneAddict]

They're practical, and they're easy?

[BenCushwa]

Transfer windows are nothing more than the most economical times to send a ship from the standpoint of minimizing deltaV. You certainly don't have to launch a mission within a transfer window, especially if you're doing a gravity assist to help you along, but you may end up burning ore fuel to get there.

[Choctofliatrio]

It makes getting to other planets faster. By this, I mean it makes PLANNING getting there faster. Takes a lot longer to work the maneuver nodes if you're disregarding transfer windows.

[LordFerret]

I'll do 'both', it depends on the mission and target and how long I'm willing to wait for the next window (I run multiple missions).

[MaxL_1023]

Transfer windows are periods where the alignment of your launch site and your destination allow low-energy, short period transfer trajectories.

For example, Kerbin to Duna:

[B]Kerbin and Duna both at 12 o'clock[/B]

1. You need to burn the delta-V to raise your AP to Duna's orbit
2. When you get to your AP, Duna isn't there. Since it was on a higher orbit, you passed it a month ago.
3. Revert Flight
4. Burn the delta V to raise your AP past Duna's orbit. Now, when you reach Duna's orbit, you are ahead of Duna. Duna catches up to you while you are outside. If you time it right, you fall back through Duna's orbit while passing close to Duna.
5. You enter Duna's SOI with a velocity of 5 or 6 km/s and fly off into space.
6. Revert Flight
7. You make 4 or 5 burns to gradually raise your orbit, until you eventually reach Duna with reasonable velocity.
8. Your Kerbal's Grandchildren land on Duna

[B]Kerbin at ~10:30, Duna at 9 o'clock (Rough Transfer Window assuming CCW orbits)

[/B]1. You need to burn the delta-V to raise your AP to Duna's orbit
2. When you hit Duna's orbit, you enter Duna's SOI. The head start Duna had at launch compensated for your lower orbit.
3. Since you are only moving slightly slower than Duna, you don't need to burn so much fuel to enter orbit

Basically, transfer windows let you get to another planet in half an orbit (for Hoffman) or one orbit (Bi-Elliptical). Otherwise, you need to do a resonance drift (set it up so that after 4 or 5 orbits you meet your target) or expend a lot of radial delta-v to shift your orbit in flight. If you have over-engineered rockets, you might just be doing hyperbolic transfers, where your orbit goes out way past your target but you meet in the middle. This is what probes like New Horizons did (with a gravity assist) to reach the outer planets in a reasonable time. However, if you want to enter orbit or land without wasting fuel you need to use a transfer.

You can get away with flying all willy-nilly "Jebsplosion style" but it makes Bob and Bill nervous.

[FancyMouse]

Fuel is the main concern. Giving a departure time and the journey time, there's a unique solution (ejection angle, required delta V, etc.) from one planet to another, and that's the theoretical foundation of those transfer window planning tools.
You can try any of those tools, look at the graph, and get an idea of how it really works. There is a significant portion of time where you'll need a lot more than the optimal fuel regardless of how long you're willing to travel - that's why such window exists

[EditorRUS]

It makes drastic difference when you perform your burn. Porkchop plots are perfect example of why launching time matters.
[IMG]http://i.imgur.com/OViKSgC.png[/IMG]
If you were to launch it right now and use the least amount of fuel, you'd still need ~3736 dV to get there and you'll spend 904 (!) days. It will take the least amount of fuel and time if you launch it at (in this case) Year 21, Day 322, 3:18:20.

[luizopiloto]

Try land a craft at KSC, coming from a high inclined orbit w/o deorbting at the right window... Will end up in the middle of nowhere... And in career mode, the distance you recover from KSC matters :P

[Foxster]

One of the big reasons for me is the speed you arrive in the SOI of the target planet.

If you sneak up on it from behind after a low energy transfer then you need a lot less dv to make orbit than if you come crashing into the system at a wild angle. You likely won't be able to slow for a capture at all.

[More Boosters]

[quote name='Foxster']One of the big reasons for me is the speed you arrive in the SOI of the target planet.

If you sneak up on it from behind after a low energy transfer then you need a lot less dv to make orbit than if you come crashing into the system at a wild angle. You likely won't be able to slow for a capture at all.[/QUOTE]

Aerocapture tho

Can't wait until they fix Jool, I have to fire my engines like a barbarian.

[GoSlash27]

Dfthu,
The reason I use transfer windows is the exact opposite of why you don't:

I don't overengineer my ships. I design them to meet the mission requirements. Without a transfer window, I have no idea how much DV I need. Without knowing how much DV I need, I can't design my ship.

Best,
-Slashy

[LostOblivion]

They're not only economical, but allows you to send kerbals to Duna in a matter of days.

[More Boosters]

[quote name='LostOblivion']They're not only economical, but allows you to send kerbals to Duna in a matter of days.[/QUOTE]

Citation needed. How many days and how much delta V? Or do you just mean less than a year?

[egoego]

I hope a follow up question is allowed. How do you get to your transfer window? Just time accelerate until you reach year 21 (if we go with EditoRUS' example)?

[Gaarst]

[quote name='egoego']I hope a follow up question is allowed. How do you get to your transfer window? Just time accelerate until you reach year 21 (if we go with EditoRUS' example)?[/QUOTE]

Either time warp or do another mission in the meantime.

Also, the greatest synodic period from Kerbin is to Duna and it is a little over 2 years; in other words, there is a transfer window to Duna every 2 years, and to other planets even more often, so you won't have to wait for 21 years to go interplanetary.
Even if you account for orbital inclination (for Moho and Eeloo, for example) and really want the lowest dV possible you might have to wait a little longer, but never for 20 years.

[Alshain]

So 9 people who have voted in this poll play Sandbox rofl.

If you just launch any time you want, either you don't play career or you spend a lot of time doing contracts.

[COLOR=silver][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

[quote name='egoego']I hope a follow up question is allowed. How do you get to your transfer window? Just time accelerate until you reach year 21 (if we go with EditoRUS' example)?[/QUOTE]

Little of both. First of all, Kerbal Alarm Clock + Transfer Window Planner. But after that I do missions to the Mun and Minmus to build up the KSC and Tech Tree, by the time the first Moho Window comes along, I'm done with most of that. I play with RemoteTech and ScanSat which adds more missions to do. There are other mods to add missions as well and before you know it you are going interplanetary. In my current game I've just launched all my communication and scanner sattelites to Moho as well as an unmanned orbital Science vessel and my tech tree isn't quite done yet (but close). I haven't time warped more than a week or so at a time, just to get to Minmus and back. 34 active flights so far.

By the way the transfer windows are quite large, you don't have to launch on the day, in the screenshot below, the 4 vessels on the left were sent 3 days ahead of the actual KAC alarm, and the one just approaching the Mun's orbit was sent 9 days after (was waiting on science for the narrow band scanner to come back from Minmus). If you are using Transfer Window Planner, it points out the most ideal window, but you don't have to take it. Moho at the start of the game for example has a fairly cheap window in about 30 days and 60 days, it's not the ideal, but it's close enough. If I only worked with the absolute ideal windows, I would have to timewarp a lot.

[IMG]http://images.akamai.steamusercontent.com/ugc/403432173259271168/D0F7F7497E35731D3AB220821CB121E9BFB7A9E6/[/IMG] Edited November 21, 2015 by Alshain

[Pthigrivi]

Yeah I play stock and just eyeball it. It's usually close enough, but things get a little weird with inclined planets. Man... Sock alarm clock. A man can dream.

[MaxL_1023]

For most reasonable transfers, the difference between "exactly on" and "about a week off" isn't huge - on the order of a couple hundred m/s of delta-v on both ends. The delta-v requirements are non-linear functions like parabolas - you get a broad trough with increasing slope when you get farther off the optimal point.

My solution in stock was to use the ion engine with a couple gigantor solar arrays - you get enough delta-V to correct almost anything. If you are operating near Jool/Eeloo you will have trouble powering the ions, but for Duna/Eve/Moho you can usually transfer and enter orbit easily. Landing needs chemical rockets (except for Gilly or maybe Minmus if your probe is really light) but orbiting with ions is fairly easy.

[AbacusWizard]

Transfer windows are one of the few things about KSP-level space travel that still mystifies me. I understand the basic idea conceptually, and I've heard of porkchop plots; I just have no idea how to run the numbers, or even where to start. Can anyone recommend a good resource for figuring it out? (I don't mean a mod that will do it for me or a calendar of useful launch dates; I want to know how to calculate them myself.)

[Kowgan]

@AbacusWizard: PurpleTarget has a great series of videos explaining plenty of stuff regarding Math in KSP. This might be very useful for what you're looking for:

[video=youtube;ZswG3PUGAOU]https://www.youtube.com/watch?v=ZswG3PUGAOU[/video]

1-7, as well as all the other videos on the series are worth watching and rewatching. Edited November 22, 2015 by Kowgan

[LostOblivion]

[quote name='LostOblivion']They're not only economical, but allows you to send kerbals to Duna in a matter of days.[/QUOTE]

For example, if ejecting from Kerbin around y1 d250 burning for 6000 delta-v, you'll be at Duna in about 35 days, whereas if you burned for the same out of window, you'd potentially need worst case 300,000 delta-v.

[cantab]

[quote name='AbacusWizard']Transfer windows are one of the few things about KSP-level space travel that still mystifies me. I understand the basic idea conceptually, and I've heard of porkchop plots; I just have no idea how to run the numbers, or even where to start. Can anyone recommend a good resource for figuring it out? (I don't mean a mod that will do it for me or a calendar of useful launch dates; I want to know how to calculate them myself.)[/QUOTE]I dunno about resources, but it's all in the timing.

Find out the orbital radii of your start and destination planets. From there find out the semi-major axis of your transfer orbit. From there find out half its orbital period, that's how long your transfer takes. From there work out how far your destination will move in its orbit over that transfer time. From there work out the angles.