Getting to Duna

[Giacomo]

All the tutorials I have seen have Kerbin around 43 degrees away from Duna. Is there a way I can get to Duna without the planets being like this, or a way where I can easily get the planets into the needed position?

Edited January 18, 2016 by Giacomo

[RizzoTheRat]

Kerbal alarm Clock will let you set an alarm for the best launch window, so you can accelerate time and it'll drop you back to real time at the right point.

Alternatively you can use a calculator to work out when the next launch window is and forward to tat time manually http://alexmoon.github.io/ksp/

The tutorials show that point because it's the case in which you'll use the least fuel, you can go any time you like if you accept that it'll take longer and use more fuel.

Edited January 18, 2016 by RizzoTheRat

[Giacomo]

Okay, I will try it. Do you know a way of getting there without having to wait?

Thanks!  

[katateochi]

45 minutes ago, Giacomo said:

Okay, I will try it. Do you know a way of getting there without having to wait?

Thanks!  

Do you mean a way of jumping directly to a particular time, rather than having to timewarp?  At full timewarp (for which you either need to be landed, or in a very high orbit, or from within the tracking station) you will get there pretty quickly, infact without Kerbal Alarm Clock the risk is overshooting! 

There is a way, but it involves editing your save file (so make a backup first!).  IT's been a while since I did this, but IIRC; open your persistent.sfs file, search for FLIGHTSTATE and then under that look for UT = X where X will be a decimal value representing time in seconds since the start of the save.  I think that if you alter that value and then reload the save you should find yourself in the future, which may be as odd as that sounds, not sure how changing that will mess with the state of already launched craft.  TBH I think switching back to the tracking station and then timewarping at max speed with Kerbal Alarm clock there to automatically stop the warp, will actually take less time and won't result in anything odd happening to your save.

[Evanitis]

1 hour ago, Giacomo said:

Do you know a way of getting there without having to wait?

Sure - use the transfer window planner. Generally it's used to find the time where it takes the less dV to get where you want to, as waiting is easier than adding more fuel. But if that doesn't concern you, just enter your current orbit details, pick the target, and click the bluest part of the 'leftmost' side of the colorful graph. It will tell you what ejection angle you need, and how much dV to burn if you want to go -right now-.

[colonel0sanders]

21 minutes ago, Evanitis said:

Sure - use the transfer window planner. Generally it's used to find the time where it takes the less dV to get where you want to, as waiting is easier than adding more fuel. But if that doesn't concern you, just enter your current orbit details, pick the target, and click the bluest part of the 'leftmost' side of the colorful graph. It will tell you what ejection angle you need, and how much dV to burn if you want to go -right now-.

There's actually a possible transfer window to Duna, which doesn't take any more dv than the optimal one, which you can launch immediately (year 1, day 1) - but it takes about 2.5 years to get there.  It actually takes less time to wait until the regular window opens up around year 1 day 236.  To see the year 1 day 1 launch option, open the transfer window planner, hit the 'Advanced Settings' option, and enter 1500 days as the max 'time of flight'

Edited January 18, 2016 by colonel0sanders

[Evanitis]

As an eyeballing enthusiast, I never dug that deeply into the planner... Thanks for calling my attention on that - I'm much interested how that trajectory looks and where else I could use such long but efficient transfers.

[Snark]

Yah, your options basically boil down to:

1. Timewarp to the window, then launch.  This doesn't take very much of your time at all, it gets there pretty quick at max warp.  But it will take up half a year of Kerbin's time.
2. Launch without waiting for the window, and brute-force it.  You can go to Duna at any time, you don't have to wait for the window.  It's just that it'll take you a lot more dV to do it.

So really, all you need to decide is:  are you more stingy than impatient, or vice versa? 

[regex]

4 hours ago, Giacomo said:

Is there a way I can get to Duna without the planets being like this

Of course.  Just bring lots of fuel.

Edited January 18, 2016 by regex

[Giacomo]

Thanks for all your help!

[Hannu2]

17 hours ago, colonel0sanders said:

There's actually a possible transfer window to Duna, which doesn't take any more dv than the optimal one, which you can launch immediately (year 1, day 1) - but it takes about 2.5 years to get there.  It actually takes less time to wait until the regular window opens up around year 1 day 236.  To see the year 1 day 1 launch option, open the transfer window planner, hit the 'Advanced Settings' option, and enter 1500 days as the max 'time of flight'

There are infinite number of such trajectories if you plot porkchops up to longer travel times. They are near Hohmann orbits but instead of encountering the target after 1/2 orbits they encounter after integer + 1/2 orbits.

There are some practical uses for such orbits, both in real space exploration and in KSP. Sometimes they can save dv if target is on inclined orbit (Moho). They are also often necessary in slingshot maneuvers because dv change during an encounter is limited and it is not possible to get many encounters at optimal positions in first orbit (suitable positions of three planets can happen once in thousands of years).

[MrOsterman]

On 1/18/2016 at 11:15 AM, Snark said:

Yah, your options basically boil down to:

1. Timewarp to the window, then launch.  This doesn't take very much of your time at all, it gets there pretty quick at max warp.  But it will take up half a year of Kerbin's time.
2. Launch without waiting for the window, and brute-force it.  You can go to Duna at any time, you don't have to wait for the window.  It's just that it'll take you a lot more dV to do it.

So really, all you need to decide is:  are you more stingy than impatient, or vice versa? 

Hmm...

So here's my usual mission plan for Duna:

Get out of Kerbin's SOI, usually Behind It so that I'm nearly on the same orbit as Kerbin just floating along later in that orbit.  Then I create a note to make an intercept path/ hohfman transfer to Duna and I move that manuever node around until I get it done with the least dV I can pull off.  Lastly I time warp up to that manuever.  

I think I'm probably doing the same thing as waiting for a transfer window I'm just doing my waiting while I'm in a solar orbit.  Am i really wasting that much fuel this way?

[Snark]

14 minutes ago, MrOsterman said:

So here's my usual mission plan for Duna:

Get out of Kerbin's SOI, usually Behind It so that I'm nearly on the same orbit as Kerbin just floating along later in that orbit.  Then I create a note to make an intercept path/ hohfman transfer to Duna and I move that manuever node around until I get it done with the least dV I can pull off.  Lastly I time warp up to that manuever.  

I think I'm probably doing the same thing as waiting for a transfer window I'm just doing my waiting while I'm in a solar orbit.  Am i really wasting that much fuel this way?

No no no no, this is bad bad wrong.  Don't do this.  It's the worst possible thing you can do.  Hermann Oberth is spinning in his grave. 

Yes, you're wasting huge amounts of fuel this way.  Hundreds of m/s of dV.

You want to make your Duna burn from low Kerbin orbit.  The lower the better.  You'll get far more bang for your buck that way.

Get into LKO, something nice and low like 80 km.  Then set up a single maneuver node there that flings you free of Kerbin's SoI and all the way to Duna.  (If you need to do a little mid-course correction en route to Duna, for example to correct inclination, then that's okay.  But you want the lion's share of the total dV to be in that single big burn in low Kerbin orbit.)

Moral of the story:  Oberth effect is your friend.  Wherever possible, you always want to be doing all of your major burns when you are at low altitude over a heavy planet.

There are a lot of transfer planners out there, but my personal favorite is http://ksp.olex.biz/. Just put in the altitude of your parking orbit, and the origin and destination planets, and it tells you exactly when to burn, in what direction, and how much.

Note that if you're going to Duna from an 80 km Kerbin orbit, you only need 1055 m/s of dV.  That's total dV all the way to Duna.  Whereas, for comparison, consider the approach you describe:  First you have to burn a minimum of about 950 m/s just to escape Kerbin's SoI to get into a Kerbin-like orbit, and then you need to do a burn to get to Duna.  And I betcha that burn's a lot more than 100 m/s. 

In other words:  it only takes about 100 m/s more to get to Duna than it does just to escape Kerbin's SoI, if you do it right.  Assuming that you aerobrake when you get there, it actually takes slightly less dV to land on Duna than it does to land on Minmus.

Edited January 19, 2016 by Snark

[Plusck]

12 minutes ago, MrOsterman said:

Hmm...

So here's my usual mission plan for Duna:

Get out of Kerbin's SOI, usually Behind It so that I'm nearly on the same orbit as Kerbin just floating along later in that orbit.  Then I create a note to make an intercept path/ hohfman transfer to Duna and I move that manuever node around until I get it done with the least dV I can pull off.  Lastly I time warp up to that manuever.  

I think I'm probably doing the same thing as waiting for a transfer window I'm just doing my waiting while I'm in a solar orbit.  Am i really wasting that much fuel this way?

Yes, hugely.

The optimum single burn from LKO to get to Duna costs slightly more than 1000 m/s delta-v.

The burn to escape Kerbin's SOI is slightly more than 950 m/s delta-v (IIRC).

So however much more than 100 m/s your transfer burn is, from interplanetary space, is what you're losing.

And that is me being conservative with the numbers.

[MrOsterman]

18 hours ago, Snark said:

Moral of the story:  Oberth effect is your friend.  Wherever possible, you always want to be doing all of your major burns when you are at low altitude over a heavy planet.

 

Okay so along those lines, if I plan to refuel after getting into orbit, I should fly out to Minmus, refuel, set up an orbit around Kerbin, and, if I time it all right, burn hard close to Kerbin to fling around it and out into a Duna intercept course?

If I'm burning, hypothetically, fuel to finish my orbit circulization, then my best bet is to get back into that orbit of Kerbin to maximize the Oberth Effect and set up a solid trip to Duna.  Likewise, coming home from Duna, I should be pushing off of Ike with a full tanks, getting into a Duna orbit, and then "Pushing off" of Duna to make my trip home.

In my defense..... it's a lot easier to find an intercept course when you want it while in Solar orbit than it is to do so from Kerbin orbit if you're not warping into an ideal transfer window.....

But yes, absolutely it seems to be around 1000 m/s of dV to get out of Kerbin, and then another 1000 m/s give or take to get to Duna.  I've been working on the assumption that going directly from Kerbin to Duna would be in the ball park of 2000 m/s anyway so I haven't thought too much about it.

Time to rethink...

 

[Plusck]

29 minutes ago, MrOsterman said:

Okay so along those lines, if I plan to refuel after getting into orbit, I should fly out to Minmus, refuel, set up an orbit around Kerbin, and, if I time it all right, burn hard close to Kerbin to fling around it and out into a Duna intercept course?

If I'm burning, hypothetically, fuel to finish my orbit circulization, then my best bet is to get back into that orbit of Kerbin to maximize the Oberth Effect and set up a solid trip to Duna.  Likewise, coming home from Duna, I should be pushing off of Ike with a full tanks, getting into a Duna orbit, and then "Pushing off" of Duna to make my trip home.

In my defense..... it's a lot easier to find an intercept course when you want it while in Solar orbit than it is to do so from Kerbin orbit if you're not warping into an ideal transfer window.....

But yes, absolutely it seems to be around 1000 m/s of dV to get out of Kerbin, and then another 1000 m/s give or take to get to Duna.  I've been working on the assumption that going directly from Kerbin to Duna would be in the ball park of 2000 m/s anyway so I haven't thought too much about it.

Time to rethink...

On refuelling at Minmus then plotting a low-Kerbin highly eliptical orbit for the burn: absolutely yes, that would be the best way to do it.

The obvious two-fold problem is that you'll be virtually guaranteed to miss the optimal launch window due to Minmus's orbital period being so long, and most times your final orbital inclination will not be what you want it to be. So you get to keep 750-800 m/s more fuel onboard, but you might have a plane change and a less efficient transfer burn, so maybe only half that in the end. The question is whether all that planning and waiting is worth those savings.

Since Ike is so close to Duna, the waiting problem is far less acute. However, since Ike is bigger and closer than Minmus, you need a significant push to get back to an eliptical Duna orbit, and you'd want to use Ike as a gravity assist from Duna orbit anyway, so timing gets complicated. Finally (though I haven't tested and don't have the numbers) pushing off from Ike and doing one toe-scaping burn could well be more efficient than doing all those manouvres. The dv charts for the Kerbol system don't give the right information to tell, but they strongly suggest that the effort of returning to Duna would be wasted.

[WhiteKnuckle]

15 minutes ago, Plusck said:

On refuelling at Minmus then plotting a low-Kerbin highly eliptical orbit for the burn: absolutely yes, that would be the best way to do it.

The obvious two-fold problem is that you'll be virtually guaranteed to miss the optimal launch window due to Minmus's orbital period being so long, and most times your final orbital inclination will not be what you want it to be. So you get to keep 750-800 m/s more fuel onboard, but you might have a plane change and a less efficient transfer burn, so maybe only half that in the end. The question is whether all that planning and waiting is worth those savings.

Refuel at the Mun, then drop down to Kerbin. Solves the long orbit/high inclination issue and still gives you most of the benefits.

[Plusck]

10 minutes ago, WhiteKnuckle said:

Refuel at the Mun, then drop down to Kerbin. Solves the long orbit/high inclination issue and still gives you most of the benefits.

Do you know of any tutorial or have any hints for this? I have just been reading this thread and there seems to be some disagreement about how to go about it, or the benefits you gain.

As with Ike, it strikes me that you'd need to start with significantly "outward" escape from the Mun to get a decent eliptical orbit around Kerbin, otherwise you're just wasting fuel bringing your Pe down before raising Ap outside its SOI. So with the Mun I'm having difficulties picturing the best way to do this, and with Ike it's evenless clear.

[Plusck]

additional info:

From this thread: http://forum.kerbalspaceprogram.com/index.php?/topic/33699-efficient-hohmann-transfer-altitudes/#comment-456841

It seems the optimal altitude to burn from Kerbin to Eve or Duna is actually right around the Mun's orbit, so just burning straight from the Mun is better than trying to get back to an eliptical Kerbin orbit.

And the optimal altitude from Duna to kerbin is 580 km - suggesting that yes, it might be worthwhile making a first burn radially out from Ike's orbital path to get an eliptical Duna orbit, then completing the interplanetary burn from there.

Edited January 20, 2016 by Plusck

[Guest]

20 hours ago, Snark said:

No no no no, this is bad bad wrong.  Don't do this.  It's the worst possible thing you can do.  Hermann Oberth is spinning in his grave. 

 

I would suggest that it's not quite this bad. Are you trading some efficiency for convenience? yes. There is a way to get the best of both worlds however. Use the ship parked in solar orbit (maybe just a cheap little probe) to get an eyeball on the position of the transfer window. Then when Kerbin reaches that point, launch the main ship from LKO.

[WhiteKnuckle]

Just now, Plusck said:

additional info:

From this thread: http://forum.kerbalspaceprogram.com/index.php?/topic/33699-efficient-hohmann-transfer-altitudes/#comment-456841

It seems the optimal altitude to burn from Kerbin is actually right on the Mun's orbit, so just burning straight from the Mun is better than trying to get back to an eliptical Kerbin orbit.

And the optimal altitude from Duna is 580 km - suggesting that yes, it might be worthwhile making a first burn radially out from Ike's orbital path to get an eliptical Duna orbit, then completing the interplanetary burn from there.

I think there may be some confusion about the benefits of the Oberth effect vs. the benefits of a gravity assist.

Honestly I am the wrong person to answer this. But just based on playing around with maneuver nodes last night, burning retrograde from LMO would result in being flung out of the Kerbin system for less than 300m/s dv.

[Plusck]

10 minutes ago, WhiteKnuckle said:

I think there may be some confusion about the benefits of the Oberth effect vs. the benefits of a gravity assist.

Honestly I am the wrong person to answer this. But just based on playing around with maneuver nodes last night, burning retrograde from LMO would result in being flung out of the Kerbin system for less than 300m/s dv.

No bother - I was just hoping you knew more. I can't see an obvious best approach starting from Ike, and I'd rather avoid feeling the need to go back and test a number of strategies

[MrOsterman]

3 hours ago, Otis said:

I would suggest that it's not quite this bad. Are you trading some efficiency for convenience? yes. There is a way to get the best of both worlds however. Use the ship parked in solar orbit (maybe just a cheap little probe) to get an eyeball on the position of the transfer window. Then when Kerbin reaches that point, launch the main ship from LKO.

Oh I like!  It may give me a way to deal with any "from space" contracts and I can get a probe up for under 20k to park there and use to get an idea of good transfer windows to warp up to and launch from.  I am really really adverse to the idea of spending too much time out side of the game planning so having a probe I can use to create maneuver nodes to find the best courses from would really be a nice compromise and save me on some of the fuel I'm burning.

As to the using gravity and all, it's possible to line up a moon (any moon) and the planet you want to use so that you launch from the moon's orbit, leave the moon's SOI, fly around the main body, and at the PE, do a hard burn to push out and end up on an intercept plot to your target.  It requires that the moon and the target planet end up in the right relative positions to create that path, of course, but it is more than possible, and would minimize burns to pull in the AP of that "main body partial orbit".

The idea would be that you never burn retro because that's fuel used to slow what should be a constant acceleration plan.

I need to start figuring out how to get to Eve.... that feels like it is mostly retro grade burning.... but that's a task for a probe, and some time playing with maneuver nodes.

Plus the idea of being able to get around without having to empty my tanks every time would be nice.  Right now the Athena can get me to Duna and land safely, but it uses every drop of fuel with my current flight plan.  Maybe the fix isn't to build another space ship but to see if the Athena can do another trip out there safely and then start to work on it as my core design.

[Snark]

So, first things first, just one quick sanity check I want to make sure of, based on the OP's most recent comment:

5 hours ago, MrOsterman said:

Okay so along those lines, if I plan to refuel after getting into orbit, I should fly out to Minmus, refuel, set up an orbit around Kerbin, and, if I time it all right, burn hard close to Kerbin to fling around it and out into a Duna intercept course?

If I'm burning, hypothetically, fuel to finish my orbit circulization, then my best bet is to get back into that orbit of Kerbin to maximize the Oberth Effect and set up a solid trip to Duna.  Likewise, coming home from Duna, I should be pushing off of Ike with a full tanks, getting into a Duna orbit, and then "Pushing off" of Duna to make my trip home.

Just want to make sure that the OP has the right idea here.  When you say "get into an orbit" (of Kerbin, or Duna), then yes, that might be a good thing (or might not, see comments below), if what you mean by "an orbit" is a highly elliptical one, with the Ap out at the moon you just ejected from, and the Pe in low orbit just above the atmosphere.

It would not be the right thing if what you mean is "leave the moon, go down to low orbit around the planet just above the atmosphere, and circularize there before doing the interplanetary burn."  That would be extremely counterproductive.

I'm assuming you meant the former, and that's great.  Just a sanity check to make sure you didn't mean the latter. 

5 hours ago, MrOsterman said:

In my defense..... it's a lot easier to find an intercept course when you want it while in Solar orbit than it is to do so from Kerbin orbit if you're not warping into an ideal transfer window.....

Easier how?  It requires a bit more fiddling, but I don't think it's fundamentally harder.  Certainly it's more inconvenient, if you want to warp to a good window, since if you're in an orbit low enough to get Oberth benefit, you can't do high-speed warp, and have to switch to the tracking station or something to warp ahead to where you need.

There are a few little games you can play to make the problem easier.  If you're finding it difficult, it might be helpful if you describe exactly what your process is like for setting up the burn, and what about it you find difficult.  Then the helpful folks here might be persuaded to come up with a hint or two. 

 

...Anyway.  Here's my understanding of the current open question we're discussing:

"I'm in orbit around <moon>, having just refueled.  Should I burn to drop down to <planet> for an Oberth-effect benefit, and do my interplanetary burn there?  Or should I just eject directly from <moon> for the interplanetary burn?"

In other words:  Which is better, an Oberth maneuver or direct ejection?

The answer is:  It depends.  Specifically, it depends on several things:  the mass of the planet, the mass of the moon, and the orbital radius of the moon around the planet.  It may or may not be a net positive.

(I'm setting aside, for the moment, other practical concerns such as timing.  If you were considering dropping from Minmus to Kerbin, for example, @Plusck makes an excellent point that Minmus has a slow period and you'll likely miss the window, and also orbital-inclination issues would be problematic.  Those are real, genuine problems.  But for purposes of this discussion, just to keep it simple, I'm purely considering the dV required.  In other words, I'm simply defining "better" here as "requires less dV".)

So, let's tote up the math for a direct ejection:

• You get a boost due to the moon's orbital velocity around the planet.
• You get a boost due to the the ship's orbital velocity around the moon.
• You get a "force multiplier" effect due to Oberth effect from the moon.
• You need fairly minimal energy to escape the planet's gravity well (since the moon is orbiting fairly high up).  That is, there is an energy cost for climbing from the boundary of the moon's SoI to the planet's SoI, but it's going to be quite small compared with all the other numbers, so I'll just skip this.

On the other hand, if you go for an Oberth maneuver by swinging low past the planet, we have this:

• You have to eject from the moon to get into the low-grazing orbit around the planet.  This is direct overhead cost; it's sending you in the wrong direction, by lowering your orbital energy around the planet.  So why are we even considering it?  Because it's necessary in order to get a big benefit for the Oberth burn in low planet orbit.  Spend a little to gain a lot, is the idea.
• Your Oberth effect from the moon helps reduce the cost of the moon ejection burn described above, so that's a positive.
• You get a boost from your orbital speed around the planet.
• You get a big "force multiplier" boost from Oberth effect when you do your burn at planetary periapsis.

 

So:  There are some things that help you, and some things that hurt you.  Which approach is better (direct ejection or Oberth maneuver) will depend on "which one gives you better numbers when you add up all the components."  And the reason I said "it depends", before, is that factors such as moon orbital radius and planet mass will affect these various components, making some of them bigger and some of them smaller, which can tip the balance one way or the other.

In general, the Oberth maneuver if best if the following applies:

• The planet is very massive (because then the Oberth boost from the planet becomes much larger)
• The orbit of the moon is very high above the planet  (because then, much less of a burn is needed to drop planetary periapsis; also, you get more of a speed boost when you fall from the moon's orbit down to planetary periapsis.)
• The moon's mass is very low relative to the planet (because if it were higher, you'd get more Oberth benefit from the moon itself, and less reason to use the planet instead)

So from those criteria:

• Minmus would be perfect:  Kerbin's a massive planet, Minmus is in a very high orbit, and it's a tiny moon.  (Unfortunately it has orbit-inclination issues, and the long period makes for practical difficulties around launch windows.  But that's a navigation problem.  From the pure standpoint of dV, if all you care about is "eject from Kerbin with the most dV for the least fuel" and you're orbiting Minmus, you should clearly use an Oberth maneuver.)
• The Mun is in-between.  It's a lot more massive than Minmus, and in a much lower orbit.  This will reduce the benefit of the Oberth maneuver.  The question is, how much?  Without actually doing calculations (or an in-game experiment), I don't know off the top of my head which would be better.  My guess just off the top of my head is that it would still be worth it-- that is, you'd need less total dV for Oberth than for direct ejection-- but I don't know by how much, might not be a lot.
• Ike is the worst case here.  Duna's a lot less massive than Kerbin; Ike is smaller than the Mun, but it's a lot bigger relative to its planet than the Mun is.  Also, Ike is in a very low orbit around Duna, so the retrograde burn to lower Pe is going to be relatively expensive.  Again, I don't know for sure how the numbers would work out without doing the calculation or running an in-game experiment, but off-hand I'd guess that a direct ejection would probably make more sense than an Oberth maneuver here.  But that's just a guess, should run the numbers.

 

Edited January 20, 2016 by Snark

[MrOsterman]

42 minutes ago, Snark said:

Just want to make sure that the OP has the right idea here.  When you say "get into an orbit" (of Kerbin, or Duna), then yes, that might be a good thing (or might not, see comments below), if what you mean by "an orbit" is a highly elliptical one, with the Ap out at the moon you just ejected from, and the Pe in low orbit just above the atmosphere.

 

Not the OP, just someone who's generally clueless about orbital mechanics.

And yes that was what I meant. An orbit is an orbit so for using the gravity of the planet to Oberath my way out to Duna would require, if I'm guessing right, a push off of Minmus when it's roughly between Duna and Kerbin, then creating an really big elipse around Kerbin to set it up so that a Prograde burn at PE pushes the AP of the orbit out of the Kerbin SOI, and into an intercept with Duna.

As for easier, I find it much easier to take my current orbit of the Sun, drop a manuever node, set the prograde to get me on an Hoffman transfer to Duna's orbit, and then move it forward and backward until I get my actual intercept.  From there I can do a correction burn to get me into a nice tight PE to Duna, and then at that point do a retrograde burn to pull myself into an orbit.  If I'm going to try for a launch directly from Kerbin orbit I have to do more zooming in and out to get the node to line up with the flight path I want and plan the relative path which for me using the vanilla game ~feels~ considerably harder.  But as Ive had a few trips to Duna already, maybe it's time to up my game, take the Athena back out with a new crew and repeat my last trip but do it directly from the Mun (where my current group of tourists want to visit first.)