Duna transfer: from Kerbin, Mun or Minmus?

[DeepSpaceDutch]

New user here, hi everybody!

So I have been playing, getting frustrated and being totally euphoric with KSP for about 3 weeks now. I am HOOKED! Apparantely this has been the game I have been waiting for years for, without it even knowing myself.

In the last weeks I have succeeded in orbital flight, rendezvous and docking, setting up a station around Kerbin and the Mun and even landing a 30 ton roving-habitat on the Mun.

Now that I feel I can just handle the basics I have set my sights on a permanent Duna presence (station and base). So I'm slowly getting to grips with phase angles and launching practice missions to Minmus and the Mun. But I'm still left with a few questions, the biggest being;

Generally speaking what would be more efficient before the interplanetary injection burn?

• Orbit and refuel around Kerbin
  
• Orbit and refuel around the Mun
  
• Orbit and refuel around Minmus
  

Also, is interplanetary injection burn the correct term?

Thanks guys!

Edited May 14, 2013 by DeepSpaceDutch

[Richy teh space man]

Do it from around Kerbin orbit below the Mun, launch windows occur more regularly then from the moons themselves

[Tw1]

Interplanetary injection burn is probably fine, I use transfer burn myself though.

But I don't see why there would be any difference between taking of from the Mun, and from LKO.

You might not get as close to the ideal transfer though, as it could take up to 1.6 days for the Mun to be in the right position to perform the transfer burn.

In LKO (low kerbin orbit), the ejection angle is quicker to get to.

[DeepSpaceDutch]

Ok thanks guys, LKO it is!

I could imagine it would save on some delta-v transferring from the Mun or Minmus, but orbital mechanics is still a bit hazy to me.

Edited May 14, 2013 by DeepSpaceDutch

[georgTF]

Going to Minmus would require quite a bit of Delta-V (1000 approx.), and you would require a fuel station up there as well so that's 2 complications. Another complication is Minmus' position in relation to Kerbin which could be a minor annoyance or a big problem. Then there's the chance of the Mun also getting in the way of your vessel as it drops "down" to Kerbin and altering your trajectory. And the final problem is inclination. Mun is not a problem but Minmus is in an inclined orbit and boy can that mess up your slingshot.

The only positive is that you would get a nice boost as Kerbin accelerated you to about +3000 m/s before doing the ejection burn (and i guess the slingshot as well but that might not even work). That's not really enough to outweight the negatives. At least not for Duna which is fairly easy to reach (again around about 1000 m/s in my experience).

-fuel/getting to Minmus

-logistics (Minmus base setup & supply)

-Minmus position

-Mun position

-inclination

+Oberth effect

+Kerbin slingshot

I've done it, liked it, but i've also done it from a low orbit around Kerbin and that is preferable. A Duna bound ejection burn does not benefit much from the Oberth effect, although Jool or Eloooo might, idk.

[Oggula]

Assemble in low Kerbin orbit because you don't want to waste fuel braking into mun orbit then escaping again. However, you can use the mun to slingshot you out to Duna. If you do this right it will save you about 150 dv and result in a faster transfer, but its so difficult I wouldn't recommend it for going to Duna

[Tristavius]

Start with Kerbin orbit.

Refueling at Minmus and sling shotting round Kerbin can result in a craft leaving Kerbin SOI with the vast majority of it's tanks full, but it's a hard trick to learn, needs a much greater understanding of the mechanics and there's a lot more to go wrong. You of course also have to set up a supply chain to refuel at Minmus (unless your a modder using Kethane in which case it becomes a lot more viable).

Kerbin orbit will do you fine to get anywhere you want to go. Once you've done a ton of interplanetary transfers and are very familiar with the principals, when you're bored some night then go and learn the more advanced tricks

[Oggula]

How does refuelling at Minmus and then dropping back down to Kerbin help in any way? It just means you waste fuel on the deorbit and the Oberth effect you gain is almost exactly the same as a normal transfer? Ignore them DeepSpaceDutch darling, they probably learned the term 'slingshot' from a comic book.

[Flixxbeatz]

LKO. So resupply missions won't need extra dV to transfer to Mun/Minmus.

[DeepSpaceDutch]

-fuel/getting to Minmus

-logistics (Minmus base setup & supply)

-Minmus position

-Mun position

-inclination

+Oberth effect

+Kerbin slingshot

Ok cool, thanks georgTF, learning a lot!

Once you've done a ton of interplanetary transfers and are very familiar with the principals, when you're bored some night then go and learn the more advanced tricks

Gotcha! Still a lot to learn, do and explore...loving it

Ignore them DeepSpaceDutch darling, they probably learned the term 'slingshot' from a comic book.

Ok honey bunny

[Stochasty]

How does refuelling at Minmus and then dropping back down to Kerbin help in any way? It just means you waste fuel on the deorbit and the Oberth effect you gain is almost exactly the same as a normal transfer? Ignore them DeepSpaceDutch darling, they probably learned the term 'slingshot' from a comic book.

It's a lot easier to leave Kerbin's SOI from Minmus than it is from LKO. If you're refueling in Minmus orbit, the lost delta-v of going to Minmus first doesn't matter, so you'll have more fuel to work with once you get to your target. You can still utilize the Oberth effect by exiting Minmus' SOI on a Kerbin-grazing trajectory and executing your burn to transfer orbit at closest approach, but it requires good timing to pull off.

[Sof]

Burning from Kerbin is always the most efficient way.

[Eric S]

Oggula is either overlooking something or not understanding what you want to do. Done properly, refueling in minmus orbit, dropping your kerbin periapsis to 80-100km, then burning at periapsis gets you into a planetary transfer orbit with the least amount of delta-V expended after the point of refueling than most other methods. There are a few disadvantages to this method, and in general, I don't think it's worth it, but it can be done.

The first disadvantage is obvious enough, it takes a lot more setup to do this unless you've already got a fuel base at Minmus, and the only time that makes sense is if you're doing a lot of operations around Minmus or if you're using the Kethane mod to create fuel on Minmus.

The second isn't as obvious. It takes meticulous timing to get this right, and if you get it wrong, your correction burn could easily use up all the fuel that you saved, if not more. Basically, you need to time your ejection from Minmus so that it puts you into a Kerbin transfer orbit that will hit Kerbin periapsis right as you're hitting your ejection angle.

The third disadvantage is that this isn't actually efficient because of the delta-V to get everything into Minmus orbit (unless you're mining kethane on Minmus), it's just getting you out of the system with almost full fuel tanks. Drop tanks of some sort would probably be much easier to do and achieve the same effect.

That said, here's how it works, and how it gets you into a transfer orbit with so little delta-V expended from the point of refueling.

A burn to a Duna transfer from LKO takes 1000-1060 m/s delta-V depending on your parking orbit, so that's our benchmark.

So, starting at the point where you've refueled, You'll burn 100-250 delta-V to go from Minmus orbit to a Kerbin transfer orbit, again depending on parking orbits, target periapsis, etc. A higher Minmus orbit is more efficient here, but it also means your Minmus orbit has a longer period, making it harder to perfectly time your transfer burn.

Assuming you got the transfer right, you'll be hitting the periapsis of your Kerbin orbit with enough velocity to take you back out to Minmus, but at just the right time for your transfer to Duna. Since you've already got about 900 m/s velocity over what a normal LKO would have, you only need to provide the missing 150 m/s delta V, for a total expenditure of about 400 m/s since refueling.

Why is that not efficient overall? Because you spent over 900 m/s to reach the refueling point, and that isn't even counting the delta-V required to get the fuel there. However, if getting a transfer orbit with as close to a full fuel tank is the only measuring stick, it does quite well.

[Atanar]

I have gone from LKO to use the Mun gravity to get out of Kerbins SOI, is that efficient or am I wasting my time?

[automcdonough]

go straight from kerbin.

http://en.wikipedia.org/wiki/Oberth_effect

[Brofessional]

It depends on the size of the payload.

For small spacecraft I just go straight from LKO.

For larger craft I find it easier to refuel than to try building a massive vehicle, so I send them to Minmus first to refuel from a kethane lander that I keep there. The low gravity makes Minmus more efficient than the Mun. After refueling at Minmus I can usually get into solar orbit with only about 120 units of fuel/oxidizer.

Since time is not an issue for the Kerbals I usually just go into solar orbit, match inclinations and time-warp until I have a transfer window.

[DMagic]

You do have a very high orbital velocity at your periapsis when you return from a moon, but you could achieve the same effect by making a 2-step orbital transfer. Burn once from LKO to a highly elliptical orbit, then the second time around burn until you hit escape velocity and set up a Duna encounter. The difference is that this 2-step burn doesn't include the delta-v necessary to enter and leave orbit around the Mun/Minmus. That said, I do have an orbital fuel station around Minmus, and I think it's cool to do more complicated maneuvers like this, just not necessarily better.

And the 'gravity boost' from Kerbin doesn't have any effect here. Just think about it, if a highly elliptical orbit somehow gave you a boost in your orbital velocity then every time you went around Kerbin your orbit would get a little higher. There are other threads that go into this more, but it's not as simple as it is sometimes suggested to be.

Edited May 14, 2013 by DMagic

[automcdonough]

And the 'gravity boost' from Kerbin doesn't have any effect here. Just think about it, if a highly elliptical orbit somehow gave you a boost in your orbital velocity then every time you went around Kerbin your orbit would get a little higher. There are other threads that go into this more, but it's not as simple as it is sometimes suggested to be.

I think you misinterpreted it. The orbit doesn't magically change by itself.

The d/v you impart at lower altitude has a greater effect than the same d/v at higher altitude. You can see it too.

1- Go into an elliptical orbit.

2- set up an orbital maneuver, just straight prograde for 300 d/v or so.

3- drag the start point of this node around and take note of how the predicted apoapsis changes.

In addition to this effect, d/v spent to circularize a higher orbit is just wasted if your goal is just to hike off to a different planet. The actual amount of fuel difference I have no idea. I've also done the kethane refuel at minimus thing..

Like you said there's a million threads on the matter if you search.

[DMagic]

It's not that the delta-v has a greater effect, it's that it takes less time to achieve that delta-v. Your engines produce a constant force for as long as you burn them (assuming you are at full throttle the whole time), the faster you are traveling, the more energy this imparts on spacecraft in a given amount of time. The energy imparted by your engines is calculated by W=Force x Distance. Force is constant from your engines, and it takes less time to travel a certain distance if you are already going faster.

There is, of course, a Scott Manley video on breaking Kerbin orbit from Minmus directly, vs returning to a low Kerbin orbit first before burning. It definitely saves fuel to do it this way, but it's trickier to pull off as others have said.

But I was talking about the gravity slingshot, or 'assist'. That doesn't have any effect here. Although, you might be able to first return from Minmus, then pass close behind the Mun on the way out and save a small amount of delta-v that way, and use that to alter your direction for a more optimal transfer to Duna. That would be hard though, and really awesome.

[Crush]

You can always save a bit delta-v with a well-timed mun encounter when leaving Kerbin, but it's not much because the vector of Mun is not much different from the vector of Kerbin on an interplanetary scale and the gravity well of Mun is not particularly deep. It's usually not worth the hassle.

Much better examples where a gravity assist really helps is when using Duna to get to Jool or when using Eve to get to Moho (passing over/under Eve when near its descending/ascending node to Moho can save you all the fuel you need to get the orbital inclination like that of Moho).

[georgTF]

I'm a little confused now. Is it not possible to use Kerbin for a slingshot if you start from it's SOI? Obviously not if you're in LKO bu i thought that going up to Minmus would give you a chance. Granted that fuel saved is porbably going to be spent correcting (esp. if you use wimpy engines) but it would be a fun maneuver to try anyway. Is it not possible?

In any case the one time i did use Kerbin's gravity i already had a fuel depot at Minmus so the trip back down was free except Minmus ejection, which is very little because the low gravity hardly puts up a fight. This netted about 3000 m/s at Pe which i thought would be a nice speed to start the Kerbin ejection with. I understand that this isn't a slingshot because i used gravity to build speed after refueling in a very high orbit, and not to change trajectory or build/lose extra speed, except in the loose sense. I was trying to pick the cheapest ejection.

Coincidentaly, i was flying a space station/fuel depot so it made sense to get it to Duna with as much fuel left on board as possible. That would be the only usefull reason to attempt this solution.

[Cdr_Zeta]

I just read thru this...

got lost on the way ...

then my head spun around ...

and now I am back where I started from !

I plan on doing 'simulation' runs in the Kerbin Space Academy's SIMULATOR - working out the timings for grav assists to the various planets.

I am hoping that all beginning games start out with totally random planetary/moon placements ... thus the times wouldnt be the same but the ideas would be.

There seems to be variations on grav assist themes; sometimes like doubling back on Kerbin and swinging out a second time (from Kerbin) etc.

It still is taking me time to work the manuver nodes; the Mun always just gets in the way.

I was lookin at using Minmus to sling around; just have to find the right time to do that; this is what I call the grav assist timetables for interplanetary travel for my 'Kerbin Mission'; some of these 'launch windows' only come up once a year or more ! We dont plan on going too far too soon, but I havnt run this SIM yet.

Cdr Zeta

[DMagic]

I'm a little confused now. Is it not possible to use Kerbin for a slingshot if you start from it's SOI?

That depends on what you mean by slingshot. If you are talking about a passive gravity assist, where you simply fly close to the planet, then no; how far away your starting point is doesn't matter. If you are talking about a powered maneuver, where you wait until you are as close as possible to the planet (and going as fast as possible), then igniting your engines, then yes, you can; this is what people are talking about when they refer to the Oberth effect.

Flying out to Minmus first won't help by itself, but if you do what you are planning, refueling in Minmus orbit, then it can definitely be useful. You will lose about 150 m/s getting out of Minmus orbit, but after that it will only take another 100-200 m/s to get to Duna.

[PakledHostage]

I hosted a “Minimum Delta-V to Duna†challenge thread this past winter that, unfortunately, was lost along with so many good threads.

There was a diagram floating around that indicated that the minimum delta-V required for a direct transfer from a 100 km high circular orbit around Kerbin to Duna is about 1040 m/s. One entrant in the challenge managed about 1100 m/s using a direct transfer.

I managed to save about 60 m/s off the theoretical minimum for a direct transfer to reach Duna’s surface from a 100 km high circlular orbit about Kerbin using a gravitational assist off the Mun.

I managed to get from a 100 km orbit about Kerbin to Duna's surface using just 980 m/s. I could have maybe saved a dozen m/s or so more delta-V had I been a bit more precise, but not much more than that.

I still have the screen shots:

Starting Fuel

Starting Orbit

Post transfer burn fuel

Planned Munar slingshot trajectory

Actual Munar slingshot trajectory

Munar periapsis

Transfer trajectory – Post gravitational assist

Transfer trajectory – Duna arrival

Arrival fuel

Landed on Duna

Math

[Tristavius]

As ever, Scott Manley has a good video on the subject...

Edited May 15, 2013 by Tristavius
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