Am I reading this dV chart correctly for getting to Duna?

[Shiv]

I'm looking at this chart for my transfer to Duna. It's my first interplanetary journey so I could be just doing this wrong period, but I think I'm doing this correctly.

So the chart says I'll need about 1060 m/s dV to get to Duna once in orbit around Kerbin, and that it should take me 75 days to get there. Kerbal Alarm Clock tells me that there is a transfer window in 3 days time. I launched my craft, got it into LKO, and then set up a transfer node to plot out my course to Duna. Using PreciseNode I went 3 days into the future and the closest intercept I can get is 160 days at 1100 m/s dV. If I go up to 3000 m/s dV I can get a 100 day intercept, but that's not really within the bounds of my craft.

I basically set up the maneuver node, set the prograde until I'm at 1100 m/s then play with the rotation until I get an intercept, same as I'd do to go to the Mun or Minmus, but I'm getting entirely different results than the established ones. Is there something I'm doing wrong? My craft only has life support for 270 days so I can't do a 160 day journey or my Kerbal will die on the way back.

Edited August 7, 2014 by Shiv

[cantab]

You have the game in Earth time, I hope.

When you get that 160 day trip, is your Duna encounter the first or second time you cross its orbit? If the decond, try and tweak it a little so you instead encounter on the first crossing.

If you're still stuck, try using Alexmoon's calculator.

[mhoram]

The definition of KSP days has changed in 0.23.5 and the chart you use is from a time before 0.23.5.

As for ÃŽâ€V: You will need according to the chart 1060 m/s to get an intercept with Duna. And additionally 370 m/s are needed to get into a low Duna-Orbit.

A quick test on http://alexmoon.github.io/ksp/ also shows a way longer transfer-time to Duna of around 270 days.

Another thing you should consider is that you need life support for a longer time than "flighttime to Duna + flighttime back", because usually there is no transfer-window for the way back at the time you arrive on Duna.

[Akalaël]

As mhoram says, you'll probably have to wait at Duna for the transfer window to get back to Kerbin, and it can sometimes take months.

Or you can go back with a non-homman transfer, but it cost you a LOT more delta-V.

[Sirrobert]

Going to other planets is a litle different than going to moons. Instead, you can compare it to going to Minmus from Mun, with Kerbin staying in the middle

[Shiv]

You have the game in Earth time, I hope.

Don't know how to tell that. I don't think I do, but when I get the intercept it says "T = 170 d, 15 h" or something like that.

When you get that 160 day trip, is your Duna encounter the first or second time you cross its orbit? If the decond, try and tweak it a little so you instead encounter on the first crossing.

The first time it crosses the orbit.

What's kind of odd is that I have a circular orbit but at certain points the projected second orbit (the one of the sun) completely shrinks down to the initial orbit of Kerbin. I guess that's where I'm shooting out of Kerbin retrograde and just get recaptured or some such since the dV isn't that different.

The definition of KSP days has changed in 0.23.5 and the chart you use is from a time before 0.23.5.

Do you know where a more recent chart is?

[mhoram]

Do you know where a more recent chart is?

In order to get the new travel duration, just multiply the number by 4 (Earth Day = 24 hours, Kerbal Day = 6 hours).

The ÃŽâ€V-Values did not change, so they are still in a reasonable range.

Here are some other ÃŽâ€V-Maps.

[LethalDose]

After some maths in a different thread, I feel comfortable stating there are some substantial inaccuracies in most of the dV maps published, mostly in terms of the required dV required to get back. These inaccuracies are somewhat moot, though, since you can aerobrake around Kerbin on the way back and recover most of that dV. The most accurate one I've found is Metaphor's map here.

Shiv, you also commented you're "Shooting out of Kerbin retrograde"? If you're going to Duna, you should be leaving prograde. I was actually surprised by how important your ejection angle is to getting a short and efficient transfer. That's why I like this transfer guide.

And you can check if you're using "earth days" or "Kerbin days" in the options from the main menu, though you're pretty obviously using earth days since your values have "15 hrs" listed.

[cantab]

The only other factor I can think of is that Duna's orbit is fairly elliptical. If you're meeting it when it's far from Kerbin you might take longer to get there. But I don't know if it's enough to explain the discrepancy.

[Shiv]

The only other factor I can think of is that Duna's orbit is fairly elliptical. If you're meeting it when it's far from Kerbin you might take longer to get there. But I don't know if it's enough to explain the discrepancy.

Isn't that the point of waiting for a transfer window though? So that the planet is the closest to Kerbin to reduce the amount of transfer time?

And you can check if you're using "earth days" or "Kerbin days" in the options from the main menu, though you're pretty obviously using earth days since your values have "15 hrs" listed.

Actually I was using Kerbal days, unfortunately. The 15 h was just a guess going off of memory and I guess I saw 5 h instead and just misremembered it.

Still can't get this transfer to be any shorter than 150 days. Completely boggles my mind. I'm using protractor, PreciseNode, AlarmClock, that link above, and everything short of just having MechJeb do it for me and still getting the wrong result.

Edited August 5, 2014 by Shiv

[Yasmy]

Picking a window depends on your mission criteria. Delta-v, time until next window, transfer time and relative orbital inclination are some possible criteria.

For many people, the next available reasonable delta-v maneuver is what people care about, much more than transfer time. This is typically a Hohmann transfer for which you will go half way around the sun to get to your target. So the transfer window occurs when the target planet will be half way around the sun from where you are now when you get there. This does not typically occur when the starting and ending planets are near each other.

If what you care about is transfer time, you could create a rocket with many times the minimum delta-v needed, and get there much faster. But it is very inefficient.

It sounds like what you really should look at (after figuring out how to get the transfer time down to nominal) is the soonest return launch date from Duna. Then budget life support for the number of days between launch from Kerbin and return to Kerbin from the next Duna to Kerbin launch window after Duna arrival. Extra fuel spent reducing transfer time probably won't pay off on the voyage out since you have to wait for a return window.

Edited August 5, 2014 by Yasmy

[mhoram]

The only other factor I can think of is that Duna's orbit is fairly elliptical. If you're meeting it when it's far from Kerbin you might take longer to get there. But I don't know if it's enough to explain the discrepancy.

Dunas orbit has an apoapsis of 21783Mm and Periapsis of 19669 Mm.

Kerbin has a circular orbit with a radius of 13599 Mm.

To calculate the duration for a Hohmann-Transfer for the shortest and longest possible paths, the following formula is used:

ShortTransferTime = SiderealPeriod(Apo=19669Mm, Peri=13599Mm)/2

LongTransferTime = SiderealPeriod(Apo=21783Mm, Peri=13599Mm)/2

And the sidereal period is calculated by 2À * Sqrt(a³/μ)

where a is the semi-major axis of the transferorbit and μ the Gravitational Parameter of the Body that is orbited (in this case Kerbol).

This result in transfer times of 288 and 316 Kerbin Days for Hohmann-Transfers from Kerbin to Duna.

Still can't get this transfer to be any shorter than 150 days. Completely boggles my mind. I'm using protractor, PreciseNode, AlarmClock, that link above, and everything short of just having MechJeb do it for me and still getting the wrong result.

If you want transfer times of 150 days or less, then you definitively do not use Hohmann-Transfers.

A good indicator for the question if a transfer is a Hohmann-Transfer is that Kerbol lies on the line between departure and arrival.

Isn't that the point of waiting for a transfer window though? So that the planet is the closest to Kerbin to reduce the amount of transfer time?

No because transfer windows in the usual meaning have nothing to do with closenes.

For a Hohmann-Transfer there exist ideal points in time where the ÃŽâ€V needed is smaller than in the range around it.

The farther away you are from this ideal time, the more ÃŽâ€V you need for the transfer (local Minimum).

The Transfer window is the timeinterval in which you need for the Hohmann-transfer less than "ideal-ÃŽâ€V + X", where you have to specify X.

Edited August 5, 2014 by mhoram

[cantab]

If you want transfer times of 150 days of less, then you definitively do not use Hohmann-Transfers.

I'm pretty sure the OP's working in Earth time though. The Hohmann transfer should take 75 Edays or thereabouts.

One thing left is that maybe the OP's not actually in a transfer window, and Alarm Clock is being misleading. Shiv, can you post the current date of your save? (You should be able to get it from the tracking station).

[Shiv]

I'm not working in Earth time, already checked on that. The current date of my save is something like Year 1, day 214 according to KAC.

[KingPhantom]

just a side question to this topic.

when i check the deltaV for eve orbit.... i do the following math:

4550 + 950 + 80 + 1310 = 6890 (and for the return i think i need 2000 to be safe. => 8890m/s)

start from kerbin + (leaving kerbin?) + (transfer to eve with intercept?) + get a low eve orbit.

are the 950 + 80 the transfer?

is that correct? i need near 10000m/s for that. and even more for gilly...

dres is much cheaper!

https://i.imgur.com/ENCurMH.png

[metaphor]

just a side question to this topic.

when i check the deltaV for eve orbit.... i do the following math:

4550 + 950 + 80 + 1310 = 6890 (and for the return i think i need 2000 to be safe. => 8890m/s)

start from kerbin + (leaving kerbin?) + (transfer to eve with intercept?) + get a low eve orbit.

are the 950 + 80 the transfer?

is that correct? i need near 10000m/s for that. and even more for gilly...

dres is much cheaper!

https://i.imgur.com/ENCurMH.png

It depends on where you want to go and how you do your transfer. If you just want to go to low Eve orbit, you can aerobrake when you get to Eve, so you don't need the extra ~1400 m/s of delta-v. But you might need an inclination correction on the way to Eve (and back), which would be about 300-400 m/s. If you want to get back from low Eve orbit to Kerbin, you'll need about 1400 m/s to escape Eve, and then possibly an inclination change of 300-400 m/s, for 1700-1800 m/s total. To get to Gilly, try this.

Dres is more expensive in terms of delta-v than that chart shows, more like 1200-1700 m/s to get captured from the transfer into a low orbit.

Edited August 6, 2014 by metaphor

[cantab]

I'm not working in Earth time, already checked on that. The current date of my save is something like Year 1, day 214 according to KAC.

OK, if you're in Kerbin time then yes, there's a transfer window on that date, and the other stuff you said in your first post matches what alexmoon's planner is telling me.

Playing around with the advanced settings (set the longest value under time of flight to the time you want to take), you can get a 100 kday transfer for 1600 m/s if you leave a bit later. Of course you'll need to check the return window too, and that's where I think you might have a problem.

[Mr Shifty]

Isn't that the point of waiting for a transfer window though? So that the planet is the closest to Kerbin to reduce the amount of transfer time?

No, transfer windows are the points in the relative orbits of the two planets when the Hohmann transfer orbit between them takes the right amount of time so that you and the destination planet arrive at the intersection together. For an illustration, look at Figure 5.1 on this page: http://www.braeunig.us/space/interpl.htm The transfer time is fixed and the speed of the destination planet is fixed, so you have to launch when the angle (phase angle) between the two planets is a particular number. Olex's calculator shows this: http://ksp.olex.biz/

I'm not working in Earth time, already checked on that. The current date of my save is something like Year 1, day 214 according to KAC.

The problem is that the delta-V map you're using is in Earth days, but you're using Kerbin days. 75 Earth days = 300 Kerbin days, which is the correct amount for a Hohmann transfer between the two.

[Shiv]

The problem is that the delta-V map you're using is in Earth days, but you're using Kerbin days. 75 Earth days = 300 Kerbin days, which is the correct amount for a Hohmann transfer between the two.

Ah, that's why the large discrepancy. Mystery solved, thanks!

[KingPhantom]

my gilly rocket:

RCS powered lander: 1000m/s

Transfer and Orbit Stage: 9900m/s

Lifterstage: 5000m/s

the first 400m/s of the transfer from kerbin to eve were from the lifter stage.

the return was a waste of fuel because i only want to go back fast. used 2000m/s and was back in LKO with 3000m/s left. three of the biggest of the smallest tanks (radial mounted) where still full