Number of Kerbin-Duna Transfers

[palioxis1248]

Starting with Kerbin, Duna and Kerbol in a straight line, how many transfer windows are there from Kerbin to Duna until the system returns to the same state? How about from Duna to Kerbin? Also, how about Kerbin to any other planet?

Just something I would like to know. Can't say much more than that.

Edited February 8, 2014 by palioxis1248

[Taki117]

Each transfer window only ever occurs once, and the amount of time between transfers varies between planets. I'm not entirely sure what is being asked, so that's my best answer until amplifying information is given.

[PrivateFlip]

I think what's being asked if you start out with from the moment kerbin passes duna to the moment kerbin passed duna once more how many times you would have an efficient transfer window. The way I see it there would only be one relative configuration from which a hohmann transfer would intersect with the target planet so only one transfer until planets align again.

[EndOfTheEarth]

If you brought enough fuel with you, it's always a transfer window!

Honestly though, if we're going for a maximum-efficiency flight, then there is only one period of time where everything lines up in a convenient way, and you can determine that with Olex's web calculator.

[Captain Sierra]

How many times theres a transfer window depends on if you give a crap about your kerbals. I made a Jool transfer with a probe that took 500 days. I had to swing almost all the way out to Eeloo but I did get an intersect. Now, I could have waited for optimal transfer, which lets me make the same move in ~230 days.

[ComradeGoat]

Duna and Kerbin have quite similar orbital periods, so it takes a long time between transfer windows, 2-3 kerbin orbits.

Contrast with Jool, which has just under 1 transfer window per orbit of Kerbin, and Moho, which has one every 30 days or so.

[KalamazooViper]

Unfortunately Moho also requires insane delta v.

[Taki117]

Unfortunately Moho also requires insane delta v.

Not if you do it right, I did my Moho mission (Including landing and return) for about 7km/s dV

[ComradeGoat]

Just been experimenting by throwing stuff at Duna well outside the launch window (after it by making aphelion most of the way to Dres, letting Duna catch up, and intercepting on the downswing).

I expected it to take a lot more fuel. It really doesn't; couple of hundred delta V extra, if that. Think I might give up on waiting for Duna launch windows and just send stuff there when I feel like it.

The transfer time gets quite a bit longer though.

[EtherDragon]

Starting with Kerbin, Duna and Kerbol in a straight line, how many transfer windows are there from Kerbin to Duna until the system returns to the same state? How about from Duna to Kerbin? Also, how about Kerbin to any other planet?

Just something I would like to know. Can't say much more than that.

Precisely one. If you start with Kerbin, Duna, and the Sun all lined up - there will be one window when you will have the most efficient Hohmann Transfer opportunity before Kerbin lines up with Duna again.

[Champ]

Just been experimenting by throwing stuff at Duna well outside the launch window (after it by making aphelion most of the way to Dres, letting Duna catch up, and intercepting on the downswing).

I expected it to take a lot more fuel. It really doesn't; couple of hundred delta V extra, if that. Think I might give up on waiting for Duna launch windows and just send stuff there when I feel like it.

The transfer time gets quite a bit longer though.

Yes, i think transfer windows are mainly about travel time, not much about fuel efficiency.

[WafflesToo]

Yes, i think transfer windows are mainly about travel time, not much about fuel efficiency.

You have it backwards. There are faster routes than leaving at the transfer window, but they're more expensive in dV

[WafflesToo]

Unfortunately Moho also requires insane delta v.

About 8km/s if my calculations are correct. But as Taki117 points out, you don't necessarily need rocket fuel to get it all.

[capi3101]

In general there's one transfer window from Kerbin to Duna and one transfer back roughly each Earth calendar year (for reference, there's ~3.5 Kerbin years per Earth year). As has been pointed out, it's due to the similar orbital periods of the two bodies -from my observations and use of Protractor, I can say that there are fewer optimal transfer windows between these two bodies than between Kerbin and any other planet in the system.

As for Moho... http://forum.kerbalspaceprogram.com/threads/61478-Oh-bugger-Injection-burn-at-Moho?p=835667&viewfull=1#post835667

[Tank Buddy]

There is one launch window. If you miss it you have to wait for Duna to be 44 degrees ahead of Kerbin. You can still make it if you a few degrees off, but you miss the window, you have to wait for the planets to come around again.

[palioxis1248]

Um ok. I think I didn't phrase this right.

I understand that between the occurences of Kerbin and Duna lining up there can only be one transfer window. The thing is if you observe everything from an inertial reference frame above Kerbol, the next time Kerbin and Duna line up they will be at a different position.

So my question in reality is this:

Assuming unchanging camera angle from above Kerbol, and given Kerbin and Duna start out aligned, how many transfer windows are there before they return to that same configuration(i.e. without changing camera angle they are once again in the same position and alignment on the screen)?

[DoToH]

There is one launch window. If you miss it you have to wait for Duna to be 44 degrees ahead of Kerbin. You can still make it if you a few degrees off, but you miss the window, you have to wait for the planets to come around again.

That is not true. You can go to Duna or any other given body at any time. It is just more time or fuel consuming and maybe not a hoffman transfer, but is affordable in many cases.

Check the great launch planner (by alexmoon) for a graphical solution to non-hoffman transfers between bodies. Since I read his post here i never had to wait for so long to send my missions. It gives all the data required for the transfer burn an calculates time of travel and D/V. Link to forum thread HERE

I used this calculator to launch missions to Eve, Moho, Duna, Jool and never missed one single transfer ... and you can select the transfer that suits more your needs (in DV or time)

Assuming unchanging camera angle from above Kerbol, and given Kerbin and Duna start out aligned, how many transfer windows are there before they return to that same configuration(i.e. without changing camera angle they are once again in the same position and alignment on the screen)?

Answer to this question is: For optimal hoffman transfer, one launch window, but for non-optimal transfers you can burn at any time if you can afford the increase of DV needed.

Edited February 4, 2014 by DoToH
edited to correct spelling (my english is not very good)

[Taki117]

Um ok. I think I didn't phrase this right.

I understand that between the occurences of Kerbin and Duna lining up there can only be one transfer window. The thing is if you observe everything from an inertial reference frame above Kerbol, the next time Kerbin and Duna line up they will be at a different position.

So my question in reality is this:

Assuming unchanging camera angle from above Kerbol, and given Kerbin and Duna start out aligned, how many transfer windows are there before they return to that same configuration(i.e. without changing camera angle they are once again in the same position and alignment on the screen)?

It's a tad confusing as to why you would need to know this. If you're looking for how often Duna and Kerbin align for optimum transfer it's about every 225 days (As stated previously) however, you can transfer to Duna any time you wish (It will just take more dV) as stated previously. If you are looking for how many times, based on the Kerbal year that Duna and Kerbin line up a Kerbal day is about 6 RL hours, so in one RL day there are 4 kerbin days, now assuming that the year length is the same that means that every 91ish days is a new Kerbin Year so duna aligns with Kerbin for optimum transfer about every 2.5 years. (unless my math is horribly wrong

[alexmun]

So my question in reality is this:

Assuming unchanging camera angle from above Kerbol, and given Kerbin and Duna start out aligned, how many transfer windows are there before they return to that same configuration(i.e. without changing camera angle they are once again in the same position and alignment on the screen)?

In that case you're asking when the synodic period of Duna will line up with its sidereal period. The synodic period is how long it takes for Kerbin and Duna to reach the same angle again, and there is one transfer window per synodic period. The sidereal period is just how long it takes Duna to go around the sun.

The simple answer to your question is never. A slightly more accurate answer is about a million years (Earth-years, not Kerbin-years). That would work out to around 1.6 million transfer windows. You'd get a smaller number if you'd accept an approximate alignment (say +/- 1 degree from the original alignment), but that gets much harder to calculate.

[Rakaydos]

Out of curiosity, is the fuel costs for a non-hoffman transfer dependant entirely on how close you are to a proper hoffman transfer, or are there other points in the orbit where, say, a Bi-eliptic transfer (or other tranfer types) will cost less Dv than they would otherwise?

[palioxis1248]

It's a tad confusing as to why you would need to know this.

I'm trying to make a ring of fuel silos near the edge of the Kerbin System, so I need to know how many silos I'll need to account for all the transfer windows (because from an inertial reference frame the orbits aren't going to rotate once per Kerbin year).

In that case you're asking when the synodic period of Duna will line up with its sidereal period. The synodic period is how long it takes for Kerbin and Duna to reach the same angle again, and there is one transfer window per synodic period. The sidereal period is just how long it takes Duna to go around the sun.

The simple answer to your question is never. A slightly more accurate answer is about a million years (Earth-years, not Kerbin-years). That would work out to around 1.6 million transfer windows. You'd get a smaller number if you'd accept an approximate alignment (say +/- 1 degree from the original alignment), but that gets much harder to calculate.

That certainly puts a monkey wrench into things. Any chance that extending the transfer window period to +5 degrees from the perfect alignment makes it more or less cyclic?

Edited February 5, 2014 by palioxis1248

[palioxis1248]

Well, since alexmun more or less answered it, anyone know how to change this thread to answered?

I'm still open to suggestions on this though!

[palioxis1248]

Yes, i think transfer windows are mainly about travel time, not much about fuel efficiency.

That doesn't make much sense.

Assuming you don't overshoot the target orbit and swing back around, Hohmann transfers have the longest transfer windows of any other transfer orbit.

[Taki117]

I'm trying to make a ring of fuel silos near the edge of the Kerbin System, so I need to know how many silos I'll need to account for all the transfer windows (because from an inertial reference frame the orbits aren't going to rotate once per Kerbin year).

Honestly, you really only one fuel station because of how the transfer windows work. But any dV you save is almost entirely negated by the fact that you have to rendezvous with your target. (Discussed at length here)

[MockKnizzle]

Why make fuel silos out by Minmus when you can refuel in LKO? Burning up there, circularizing to rendezvous, and then leaving for Duna is gonna be less efficient than doing the same thing down in LKO due to the Oberth effect, plus it's a bigger pain to time everything.