Most Efficient Way Of Travelling From A Moon To Another Moon or Another Planet

[NeoMorph]

I have to admit that I am a bit at a loss regarding to returning to the planet when I have finished my visit to the Mun. I used to just aim for Kerbin and burn. Then I realised that if I burn into the retrograde of the Muns orbit I get a better fuel use a lot of the time. But it is still a rough guess as to when to start my burn.

So, rocket scientists. What is the best way to do transits from a moon back to its host planet?

On another matter, is it more effiecient to travel directly from moon to moon (ie Mun to Minmus) or is it better to go back from the Mun to Kerbin first and then go out to Minmus from there?

[Flaglore]

Hey Neomorph,

To your first question, the best way to return from the mun (assuming you are looking to land back on kerbin or go back to low orbit) is to wait until you are positioned in your mun orbit so that the mun is travelling directly toward you and you are there is a 90 degree angle between the kerbin-mun line and mun-spacecraft line. (That was probably confusing but hopefully you got it.) At that point burn propogade to escape, and you should move into an ellipticall orbit with apoapsis at mun level, and a fairly low periapsis that you can adjust to suit yourself.

As for your last question, it is more efficient to travel from the mun to minmus, but as max warp speeds can be quite low in mun orbit and the mun minmus window doesn't come round very often, it might be more convenient to just return to kerbin.

Hope that helps. feel free to ask any more questions!

[entaran]

I tend to burn prograde as kerbin is "setting" (as i zoom around the side of the moon), seems to use less dv to get back. You only need to break the orbit, you can fix your trajectory 6 hours later when you're at REALLY extreme kerbin orbit distance.

Also from the mun to minmus it's better to go direct, from minmus to the mun, you use kerbin for oberth effect. It's nearly impossible to miss the mun if you leave minmus (at pretty much any point).

Edited June 9, 2013 by entaran

[tavert]

Assuming you're returning from a fairly low circular prograde equatorial Mun orbit and want to get back to Kerbin with a periapsis low enough to aerobrake, you'll need about 270 m/s. Place the maneuver node wherever, add prograde delta-V until it's around the right number, then shift the node around your orbit until the Mun escape trajectory points retrograde relative to the Mun's orbit around Kerbin.

Going between Mun and Minmus it's best to do a direct transfer. If you drop your Kerbin orbit low for a gravity assist, then your orbital insertion when you get to your destination will be much more expensive.

[NeoMorph]

Assuming you're returning from a fairly low circular prograde equatorial Mun orbit and want to get back to Kerbin with a periapsis low enough to aerobrake, you'll need about 270 m/s. Place the maneuver node wherever, add prograde delta-V until it's around the right number, then shift the node around your orbit until the Mun escape trajectory points retrograde relative to the Mun's orbit around Kerbin.

Going between Mun and Minmus it's best to do a direct transfer. If you drop your Kerbin orbit low for a gravity assist, then your orbital insertion when you get to your destination will be much more expensive.

Interesting. Knowing the burn figure actually means you can fit the solution to the result rather than the other way around. I wonder if anyone has made a table of the correct figures for transferring from one body to the next. Would help a lot I would think.

[Frederf]

It's only useful in a small number of cases. Basically the Mun has an orbit velocity which is X m/s too much lateral speed if you were at the Ap of an orbit around Kerbin at the orbital radius of the Mun. Leaving the Mun's SOI with ~0 m/s relative (which takes some dV from a LMO) still produces an orbit around Kerbin that doesn't capture which requires additional dV depending on where you want to capture in Kerbin's atmosphere. The interesting data I think is where in position angle to burn for various dVs such that your escape vector from the Mun is exactly retrograde to its orbit. Low dVs will require you burn earlier in the escape than higher ones.

[Lexif]

I wonder if anyone has made a table of the correct figures for transferring from one body to the next. Would help a lot I would think.

This great web app will calculate everything you need for you -- if you are transfering between two bodies with the same parent. (Mun-Minmus works, Mun-Duna doesn't.)

http://alexmoon.github.io/ksp/

This page is much the same, but doesn't regard the orbital inclinations and excentric orbits of the planets. That's only needed for Minmus, Moho and Eeloo, though. But it explains all the parameters very well.

http://ksp.olex.biz/

If you just want to get a quick idea of the minimum delta-v requirements, try this "subway map". That's a type of chart were you add up the delta-v listed for each step to get from one place to another. Again, excentric and inclined orbits are not accounted for.

[deepfield]

Anybody care to continue on with how you would go about transferring from a moon to another planet? I would like to set up a refueling station at Minmus but am not sure how to go from there to say Duna.

[Empiro]

Essentially, when you exit the Mun's SOI, you want to be close to parallel with the Mun's orbit (in the retrograde direction). For the absolute maximum savings, follow this procedure:

1. Create a maneuver node for 270 m/s as suggested above. Zoom out a bit so that you can see your orbit around Kerbin after you exit the Mun's SOI.

2. Drag the node around so that the Apoasis around Kerbin is at a minimum.

3. Note your periapsis. If it's 20-30km around Kerbin, then you're done.

4. Otherwise, if the periapsis is too low (into the ground, for example), reduce the amount you burn. If it's too high, increase the amount you burn. Go to step 2.

What you're doing is that if you imagine you're in high circular orbit around Kerbin, you'd simply burn retrograde to lower your periapsis most efficiently. However, if you're don't burn exactly retrograde and add a radial component, you'd still lower your periapsis, but you'd also increase your apoasis at the same time. When you're trying to escape the Mun, the Mun will deflect your course a bit so it's hard to tell if you're exiting purely retrograde. By following the above procedure, you make sure that you exit the Mun with only a retrograde component.

[Kasuha]

For Minmus, it is ineffective to burn straight to another planet. It is too far and you lose on Oberth effect. Duna or Eve, maybe. But anywhere else it is better to lower your Kerbin periapsis first and make the transfer burn at that periapsis - the lower you set it, the more you get from Oberth effect. And it is not simple to set it up to eject in the right direction at the right speed. In certain cases, Minmus may be in an unsuitable place on its orbit and since it is really slow at that distance from Kerbin, it may stay in such place the whole transfer window. Then you'd want to transfer to lower orbit first, circularize, and eject from suitable spot on that orbit. But in that case it's better if the whole refueling station is in that place.

IMO the best place for refueling station is between 120 - 650 km.

[Red Iron Crown]

Anybody care to continue on with how you would go about transferring from a moon to another planet? I would like to set up a refueling station at Minmus but am not sure how to go from there to say Duna.

I'm somewhat surprised this hasn't been mentioned yet, but a Hohmann transfer is generally the most fuel efficient way to transfer between bodies. Basically, you burn to make your orbit just touch the orbit of the destination on the opposite side of the parent body (the "transfer burn" or "ejection burn"), arriving just as the destination body meets you, then doing another burn to match speeds (the "orbital insertion" or "capture burn"). Timing is important to ensure the destination body is in the right place when you arrive (correct "phase angle"), which is where those calculation tools linked above come in.

[Superfluous J]

When I set up a Kethane refueling operation at Minmus I got pretty good at ejecting at the right time. I don't know if I could explain it in any way that would be understandable though The trick as has been said is to leave Minmus at a time when your periapsis around Kerbin is pointing in the direction you'd want to burn to get an ejection to your target planet. Basically, you need to figure out when Kerbin and your target planet will be in the correct alignment, and more importantly where kerbin will be at that time, and then figure out the time before that when Minmus will be on the opposite side of Kerbin from the proper ejection burn angle.

...and if you've not gone cross-eyed by now I salute you. Just typing that has started to turn my brain into jelly.

[KerbMav]

.

Is there a movie to this book?

[Kasuha]

When I set up a Kethane refueling operation at Minmus I got pretty good at ejecting at the right time. I don't know if I could explain it in any way that would be understandable though The trick as has been said is to leave Minmus at a time when your periapsis around Kerbin is pointing in the direction you'd want to burn to get an ejection to your target planet. Basically, you need to figure out when Kerbin and your target planet will be in the correct alignment, and more importantly where kerbin will be at that time, and then figure out the time before that when Minmus will be on the opposite side of Kerbin from the proper ejection burn angle.

...and if you've not gone cross-eyed by now I salute you. Just typing that has started to turn my brain into jelly.

Your post got me thinking. Minmus has orbital period 50 (Kerbin) days, that's enough to miss a transfer window by wide margin. But if you drop your periapsis low above Kerbin and then don't burn exactly at the periapsis but slightly to one or the other side, you get a range of directions in which you can eject while still getting good Oberth boost. And you can drop your periapsis even more and make your orbit around Kerbin suborbital or even retrograde, extending the range even further (of course you need to plan your ejection burn correctly to not hit Kerbin). So clearly there's dependency between the angle at which you need to eject and effectivity you can reach, some angles being very effective and some angles being very ineffective. I'll need to experiment with it a little when I get home.

[cantab]

If you want to transfer from a moon to another planet, or vice-versa, the charts here are helpful.

http://www.reddit.com/r/KerbalAcademy/comments/1qu5jv/deltav_charts/

They're more complicated that the chart posted previously, but it's explained in the reddit thread. Basically you've got two routes: you can go directly to or from the moon, or you can go via a low periapsis over its parent planet. The former approach is only suitable with larger moons, and you'll see the charts don't give the values for Gilly or Minmus.

Just remember what's never a good idea is to via a low circular orbit over a planet unless you launched from it.

[Empiro]

I'd actually recommend setting a refueling station around Kerbin, and ship the fuel from Minmus. There's a bunch of reasons why this makes sense:

-Your refueling ships are simple (just big tanks), and it costs very little fuel to go from Minmus -> LKO due to aerobraking

-Your return trip is relatively cheap because you're just shipping empty tanks back

-As Kasuha points out, due to the Oberth effect it actually costs more to go from Minmus to any planet other than Duna/Eve

-You don't have to fiddle with trying to lower your periapsis at just the right time, or having to worry if Minmus is in the right position

-Most of all, your explorer ship is probably heavy (habitat modules, labs, etc.), and not needing that extra 900+ m/s DV to make Minmus orbit means that either you can have a much smaller launch vehicle, or you can bring along lots more empty tanks to refuel, and give your explorer ship far greater range

Edited August 22, 2014 by Empiro

[Kasuha]

Okay, I ran the "numbers" (actually just KSP "simulation") and here is my conclusion.

Direct transfer to Jool from Minmus costs about 2500 m/s dv under ideal conditions.

Transfer to Jool from 70 km LKO costs about 1900 m/s dv

To drop your periapsis from Minmus to 70 km above Kerbin you spend 200 m/s (prograde pass) or 300 m/s (retrograde pass). With that trajectory, you're about 900 m/s faster at periapsis than orbital speed, therefore you need to spend only about 1000 m/s to get to Jool - that means 1200-1300 m/s instead of 2500.

However.

To spend that 1000 m/s usefully, you need to burn near periapsis. If I am considering burning anywhere below 600 km still effective, you can wiggle the trajectory in about 45 degrees sector - 45 degrees in prograde pass, another 45 degrees in retrograde pass, i.e. about 90 degrees corridor where you're able to exit Kerbin effectively from Minmus, and that corridor is relative to Minmus position. You have about 75% chance that when your Jool transfer window comes, Minmus will not be in position where you can exit it effectively towards Jool.

It also takes 9 days to coast to that periapsis.

Jool transfer windows are wide - you can wait till Minmus moves enough to allow you efficient ejection. In case of exiting towards Moho, though, things may be more complicated.

Images (note I had a ship in 600 km orbit there as marker):

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[Superfluous J]

Your post got me thinking. Minmus has orbital period 50 (Kerbin) days, that's enough to miss a transfer window by wide margin. But if you drop your periapsis low above Kerbin and then don't burn exactly at the periapsis but slightly to one or the other side, you get a range of directions in which you can eject while still getting good Oberth boost. And you can drop your periapsis even more and make your orbit around Kerbin suborbital or even retrograde, extending the range even further (of course you need to plan your ejection burn correctly to not hit Kerbin). So clearly there's dependency between the angle at which you need to eject and effectivity you can reach, some angles being very effective and some angles being very ineffective. I'll need to experiment with it a little when I get home.

This (and your more detailed analysis in the next post) is essentially what I did. I literally just looked at the map and figured "Kerbin will be about here when the transfer needs to take place, and I'll want to burn this way, so I want my periapsis about here." I then left Minmus when it was 180 degrees away from that and (with the added complexity of a mun encounter. seriously. Every time I tried this I think Mun got in the way) then when the time came, I found my guesstimations were off by up to 20-30 degrees. I did the burn anyway and still got huge fuel savings.

With a Kethane (Or now Karbontie) operation at Minmus, you can essentially just build your ship so it can get to Minmus. Once it's there and you fill the tanks, it can literally get anywhere.

EDIT: Changed the phrase "literally eyeballed" to "literally looked at" because I realized that literally eyeballing something is just gross.

Edited August 22, 2014 by 5thHorseman