Have I found a better way home from the Mun? (no)

[Person012345]

[quote name='cantab']My usual approach is one burn from low Mun orbit, in the right position, to put my Kerbin periapsis at my chosen value for re-entry. This should take 270-310 m/s or so, depending on how high your Mun orbit is. Of course this method is hard without trajectory predictions and manoeuvre nodes.[/QUOTE]

This. I simply burn from mun orbit and drop my kerbin periapsis down into the atmosphere then freefall the rest of the way home. I don't know if this is the most efficient but I've never had any problems with it, the delta-V requirements for it tend to be negligable and afterwards I don't have to worry.

[BenCushwa]

[quote name='Alshain']Direct burns are not always best, New Horizons used gravity assists to get where it was going at a fraction of the cost in a fraction of the time. However, that is interplanetary travel. In the case of returning from a moon to it's parent planet, I'm not sure gravity assists are really all the useful (maybe at Jool).[/QUOTE]

I think that as long as there's one body between where you are and where you want to go, you can do a useful gravity assist*. I've done gravity assists from Kerbin to Minmus (and vice versa) using the Mun, but it was mostly for fun since it didn't save me a ton of deltaV.


*This is, as you indicated, just within the bounds of one planetary/lunar system. You can use a gravity assist from Jool to help get you anywhere if you have the time. ;^)

[KerikBalm]

[quote name='Brainlord Mesomorph']What you describe is, I believe, a Holman Transfer (if I have the name right). And I am expecting someone to explain to me whythat’s better.
OTOH: the two burns I show in that screenshotare 126 m/s and 10 m/s. And I *think*that I’ve gotten closer to home with less fuel. Now that I look at that screen shot, while my Pe is halfway home, my Ap is still almost in lunar orbit. I wonder if I could plot another lunar encounter that would slow my ship even more.[/QUOTE]

I can't fully tell what is going on in that screenshot> have you already done the burn to escape Munar SOI?
If so, you need to add the dV needed to do that as well to compare it to the 270 m/s figure.
Then you also need to do another burn at your apoapsis to actually get your PE in kerbin's atmosphere

Its always worse to escape an SOI, and then do another burn, when you could have achieved the same from a burn deep in the gravity well, thanks to Oberth...
Like going into a heliocentric orbit first from LKO, and then burning for duna, instead of burning for duna from LKO.

But maybe you could do a bigger prograde burn from low munar orbit, and then the ~15 m/s , to get a better gravity assist.

Then do a little burn at munar Pe again to get your kerbin PE in the atmosphere

[Superfluous J]

[quote name='BenCushwa']I think that as long as there's one body between where you are and where you want to go, you can do a useful gravity assist*. I've done gravity assists from Kerbin to Minmus (and vice versa) using the Mun, but it was mostly for fun since it didn't save me a ton of deltaV.[/QUOTE]

Well said. It's actually a bit more than that. If you are at a planet, and it'll take a certain amount of dV to reach your destination planet (orbiting the same star), and a different but less amount of dV to reach any other planet (again orbiting the same star), you can use that other planet to gravity assist to your destination. It may take several passes, and even another pass by the original planet, to build up the speed required but it can be done.

Galileo, for example, went to Venus once and returned to Earth twice to get the velocity required to get to Jupiter. Venus isn't technically "between" Earth and Jupiter but it is less costly to get to, so it could be used.

This will also work in KSP, but here's the catch: It's really hard to do. You need to think and calculate and then nail very precise burns, and I for one prefer the "hit the gas and go" method :D

Note: As was stated earlier, this all also works at Jool between the moons, and even works with Mun and Minmus, though the savings is laughably small, so small that you could use up the savings doing course corrections. The nice part about Jool is, you don't have to wait as long and the opportunities are far more likely to just happen due to luck.

[GoSlash27]

[quote name='Brainlord Mesomorph']New Horizons : IRL! I know it works IRL. I’m frustrated by not being able to make it work in this game.

That, and I’m embarrassed that I keep thinking that I’ve made gravity assists work in this game.[/QUOTE]

Brainlord,
There is no reason to be embarrassed. None of us hatched out knowing how to do this stuff. We all made mistakes and we all learned from them. You're just going through the same process.

Best,
-Slashy

[Snark]

[quote name='Brainlord Mesomorph']Time and time again in this game I think I’ve figured out some little bit of interplanetary billiards that does something, and time and time again I’m shown that direct burns and doing everything at perigee is best. But if that is the case, how did NASA get Voyager where it was going? And why can’t I seem to make that work for me in this game?[/QUOTE]

NASA gets its craft where they're going because they have large numbers of extremely smart people with advanced degrees in astrophysics and very very expensive computers, who spend large amounts of time working out orbits based on extraordinarily precise measurements of planetary positions, and every bit of rocket control is handled by super-precise computers that have worked things out ahead of time.

Whereas you are a guy with a mouse and a pair of Mark One eyeballs. :)

Except in fairly gross cases with wide margins of error (like using a Tylo reverse assist to capture to Jool), gravity assists require extremely precise control to make them work, because even tiny errors in timing or approach vector will result in very large errors in the outcome. KSP simply doesn't have that kind of precision. And I would contend that it shouldn't, either, because adding it would make it less fun.

Oh, and incidentally, in the particular case of Voyager: they had an extraordinarily rare configuration of planets that had them all lined up just right. Happens only once every several centuries, we were just really lucky that it happened right around when we had the technology to take advantage of it. Still not something you could pull off easily in KSP, it still took a whole lotta expertise.

[PLAD]

Interplanetary billiards is a good expression for what we're doing here (ha, I just remembered the Red Dwarf episode where it's used literally..). I have to agree with a point several here have made though, it's often a lot of work for only a small gain. The standard for getting from LKO to LMO is about 857+273=1130m/s in 8 hours, using every trick I could think of only reduced that to 1066m/s, and the trip took 322 hours! The way back is a better gain percentage-wise, from the standard 273m/s in 8 hours to 229m/s, but took 276 hours!. The only practical reason I would try it is to beat a challenge or try to get an underpowered ship there and back. For pretty much anything that stays in Kerbin's range of influence, flyby tricks and bi-elliptic paths won't save you more than about 100m/s. Perfecting your ascent from the KSC to orbit can make a bigger difference than that. Properly using the Oberth effect can save you more than that.
That being said it can make a huge difference when going to other planets. Cutting a trip to Jool from 2000m/s to 1050m/s is a huge gain. It is still quite tricky and takes me a couple hours in real-world time to pull off, I suppose it depends on whether you enjoy the journey more than the destination, as it were. I personally love poking the laws of physics in the eye so I'll sit for hours planning a path and then nursing my ship through 0.2m/s course corrections to make an optimal flight. Have you seen the sneaky, sneaky trick the Juno mission is using to get to Jupiter? It can be simulated in KSP. You can cut nearly 500m/s off of a trip to Jool by using only a Kerbin flyby! A sharp KSP player posted a guide [URL="http://forum.kerbalspaceprogram.com/threads/122780-Juno-style-Kerbin-fly-by-to-Jool"]here[/URL].
In summary, if you want to learn flybys it will take some study and a lot of practice, but if you've got the time it can be a lot of fun. I've found that KSP is accurate enough to simulate the way it would work in real life quite nicely, indeed, most every path I find in stock KSP can be used similarly in the Real Solar System mod and the real world as well (allowing for the different planet arrangements). If you have some time you could check my Imgur album (linked to in post 20 of this thread), not only could you see how I started slow and gradually got better at it, I try to give enough details of what I'm doing to allow the reader to do the same thing. The guide for (blush) [URL="http://forum.kerbalspaceprogram.com/threads/80978-Flyby-Finder-for-KSP"]Flyby Finder[/URL] shows how to find, set up, and start a flight too.
Oh yes, and you will absolutely need to use a node editor like Mechjeb's or [URL="http://forum.kerbalspaceprogram.com/threads/47863-1-0-2-Precise-Node-1-1-3-Precisely-edit-your-maneuver-nodes"]Precise Node[/URL].

[GoSlash27]

I've been thinking about this today, and I think there might be a way to use a munar slingshot to reduce the DV requirement.
1) eject to an orbit with half the period of the Mun's orbit
2) wait an orbit and a half
3) do a small prograde burn at Pe to raise Ap a bit and get a new Munar encounter
4) sling around the Mun's leading edge to set up a free return trajectory.
It'd take over a week, be a major PITA to set up, and not really save much DV... but it should work. Theoretically...

Best,
-Slashy

[Superfluous J]

[quote name='GoSlash27']I've been thinking about this today, and I think there might be a way to use a munar slingshot to reduce the DV requirement.
1) eject to an orbit with half the period of the Mun's orbit
2) wait an orbit and a half
3) do a small prograde burn at Pe to raise Ap a bit and get a new Munar encounter
4) sling around the Mun's leading edge to set up a free return trajectory.
It'd take over a week, be a major PITA to set up, and not really save much DV... but it should work. Theoretically...

Best,
-Slashy[/QUOTE]

Yup. Technically you could do it by JUST escaping from Mun, then over multiple orbits do little slingshots over and over. I ain't gonna spend the time on it though :D