Mun slingshots.

[Mr Shifty]

I'm not sure how you came up with those numbers. Especially the 78 m/s slow down in order to get to Duna. The best mun slingshot I've ever done has put my apehelion about halfway to Duna, and that was because I was very lucky and very careful. It took me another 300+ m/s burn at munar periaps to get it the rest of the way there, and another 100 m/s at my secondary kerbin periaps to get my periapsis at Duna to be where I wanted it (intersecting Ike's orbit)

I assumed maximal precision in timing; you have to get your transfer to within 10km of the Mun's surface and exit the Mun's SOI at the correct angle to leave Kerbin's SOI parallel to its prograde. My calculations came from a liberal use of the Vis-Viva equation and lining up velocity vectors at SOI edges.

[Nexustrimean]

I know they can save deltaV, but they are difficult to correctly set up. I tend to play KSP with less math, and more intuition, making the whole experience more enjoyable in my eyes.

[PakledHostage]

2) For Mun gravity assist, the ideal situation has you leave the Mun's SOI parallel to Kerbin's prograde travelling 619 m/s relative to Kerbin. (Which is the ejection burn velocity from Munar altitude: 11,400 km.)

* Since the Mun is already travelling at 542 m/s relative to Kerbin, you only need 77 m/s relative to the Mun at the Mun's SOI edge.

* To get 77 m/s at the Mun's SOI edge, and assuming you can time your burn to get your Munar periapsis down to 10km of altitude, you'll need to have 757 m/s at periapsis.

* If you start from 100km orbit around Kerbin, raising your apoapsis to the Mun's orbit requires an 842 m/s burn. Assuming the Mun's SOI catches up to your vessel near apoapsis (what you want), you're going 180m/s relative to Kerbin at that point, which is 362 m/s relative to the Mun.

* When you reach periapsis (10km) above the Mun, you'll be travelling 835 m/s, which is too fast, so you'll have to burn retrograde for 78 m/s to get your speed down to 757 m/s.

* So total delta-V for the Mun assist = 842 m/s to get up to the Mun + 78 m/s to adjust your ejection velocity = 920 m/s

Something about your analysis didn't quite sit right with me... Maybe the problem is with your description? If I understand you correctly, you're planning on using a Munar gravitational assist to enter an orbit with Kerbin Pe of 11400 km and speed at Pe of 619 m/s (relative to Kerbin). Trouble is that 619 m/s at Kerbin Pe of 11400 km is only enough to get you out to an Ap of ~22000 km. That's only 1/2 way to Minmus.

On the other hand, boosting by an ADDITIONAL 619 m/s from an initial 542 m/s circular starting orbit at 11400 km would get you into the right ballpark for a transfer to Duna. Did you make an oversight in your calculations? The rest of your numbers are consistent with such an oversight. Even though 757 m/s at a 10 km Munar periapsis is still an elliptical orbit, it is enough to escape the Mun's SOI. It isn't enough to also escape Kerbin's SOI, however.

[Mr Shifty]

On the other hand, boosting by an ADDITIONAL 619 m/s from an initial 542 m/s circular starting orbit at 11400 km would get you into the right ballpark for a transfer to Duna. Did you make an oversight in your calculations?

ARGH! You are so right! Thanks for checking my math. So that basically invalidates my whole post <sad face>

[Mr Shifty]

Corrections to my previous post about Munar slingshot. Thanks to PakledHostage for pointing the egregious error in my calculations.

You need 619 m/s relative to the Mun when you leave its SOI headed out-bound.

Thus at a Munar periapsis of 10km, you need to have a speed of 975 m/s, which means a prograde burn of 140 m/s at that point.

So, the (hopefully correct this time) actual total is 140 + 842 = 982 m/s, so the Munar assist can save you about 66 m/s maximum. Probably not worth it.

[Fuzzy Dunlop]

The mun is just too small to provide a significant delta-V boost. Kerbin, Eve, Jool and some of it's moons are the only objects big enough to make gravity assist worthwhile.

That said, slingshots from virtually any body are awesome for changing inclination. Like when you realise that your Kerbin return vessel is in retrograde Duna orbit, and you've launched into prograde.

[Ama Ram]

I often use the gravitational influence of the moons to brake my ship instead of aerobraking, for example, when going from Kerbin to Duna, to position a satellite I usually use Ike to brake instead of having to carry a heatshield to aerobrake.

[MisterKerman]

Officially my favorite topic right now. Scared of leaving Kerbin's SOI generally. Haven't the balls yet.

[NewtSoup]

On 6/19/2013 at 5:20 AM, Vanamonde said:

Quote

not including the Joolian moons because, for some reason, those do often get used

That's because there are 5 of the things, and it's hard NOT to bump into one. Ike is so large relative to its orbit that you frequently encounter its SOI as well. But the reason I don't use Mun slingshots is that you have to wait for 2 windows, Mun's and the target planet's, and then it's quite a pain to chart a Mun fly-by that ejects you at the proper angle.

Quite by accident I managed to use Ike to get into a lower eliptical Duna orbit with an ion powered craft.  Was really quite happy with the results.  I'd love to learn how to do it on purpose.

I've included the picture I took at the time - so a ~20m/s burn put me on an Ike flyby which catapulted me into the dotted line eliptical orbit.  That's such a change in AP and PE there must have been a fuel saving.  I captured into orbit really high up in Duna's SOI because of the TWR of just two ion engines powering the craft.

Edited June 17, 2018 by NewtSoup

[Tyko]

On 6/14/2018 at 5:35 AM, MisterKerman said:

Officially my favorite topic right now. Scared of leaving Kerbin's SOI generally. Haven't the balls yet.

Slingshots like this are an advanced maneuver with small benefits but more likely will cause you to spend a lot more fuel if you are off even by a few degrees. Lots of tricks that NASA / Roscosmos use work because they have their burns timed exactly using flight computers and have teams of orbital mechanics experts. Without the benefit of flight computers and those experts you're more likely to waste, rather than save, DV over a plain optimal direct transfer to Duna or Eve.

If you're just learning to do transfers I'd start with using Transfer Window Planner. This mod will let you plug in you're starting orbit and your target body/orbit. It will then tell you the best time to transfer and tell you how much DV it will cost to transfer out and then to capture. It's really accurate and saves you doing a lot of guessing or math. I usually give myself a 10% margin above what TWP recommends and I've been able to fly to most planets.

 

[Bill Phil]

I've reasoned to use Tylo gravity assists for Jool system missions. They're easy to set up and you can almost always use it to capture at Jool. Aerobraking is too risky thanks to heat.

As for the Mun? It's not all that worth it. Maybe if you're doing a KEKKJ run and you're trying to minimize Delta-V as much as possible it may be useful, but beyond that... not really worth it.

[Tyko]

Just wondering....If you're planning a slingshot anyway, wouldn't it be even better to start at your Mun station, plot a drop deep into Kerbin's gravity well just above 70km, and then burn your transfer at Kerbin PE?

Edited June 18, 2018 by Tyko

[Kesa]

3 hours ago, Tyko said:

Slingshots like this are an advanced maneuver with small benefits but more likely will cause you to spend a lot more fuel if you are off even by a few degrees. Lots of tricks that NASA / Roscosmos use work because they have their burns timed exactly using flight computers and have teams of orbital mechanics experts. Without the benefit of flight computers and those experts you're more likely to waste, rather than save, DV over a plain optimal direct transfer to Duna or Eve.

Direct transfer are simpler and probably the place to start for going interplanetary, but with simpler physics manoeuver nodes, you don't need NASA level tricks, experts and computers. In KSP, the same engine is used for the prediction of flight and the flight itself, that's a big difference with RL. With manoeuver nodes, you can even plot both the regular transfer and one with an assist and compare, only wasting time.

[FinalFan]

I was going to ask a question of a guy on Page 1 but then I noticed this was necroed from 2013.  

[MisterKerman]

3 hours ago, Tyko said:

Just wondering....If you're planning a slingshot anyway, wouldn't it be even better to start at your Mun station, plot a drop deep into Kerbin's gravity well just above 70km, and then burn your transfer at Kerbin PE?

Answer Tykooooo! I gotta know... :3

[Nich]

Oberth vs free energy

 

Mun slingshots really only make sense for Duna and Eve anything further out and the slingshot becomes very ineffecive and adds lots required accuracy

 

As for transfers from the mum if I remember correctly duna and eve are cheaper directly from the moon, Moho and Joul are cheaper to reduce orbit near kerbin.

 

From Minimus always reduce orbit to 70 km

[Eriksonn]

With n-body physics you can do mun to mun assists and save a bunch of deltav

This trip from lko to circular mun orbit took only 220 deltav(after the initial ~800 from lko), i know that i can optimize it to an elliptic mun orbit with 0 deltav but that is for another day...

N-body orbits are fun stuff 

[FinalFan]

4 hours ago, Eriksonn said:

With n-body physics you can do mun to mun assists and save a bunch of deltav

This trip from lko to circular mun orbit took only 220 deltav(after the initial ~800 from lko), i know that i can optimize it to an elliptic mun orbit with 0 deltav but that is for another day...

N-body orbits are fun stuff 

Based on my understanding of what you said, it would be fair to rephrase this to "This trip from mun encounter (from LKO) to circular mun orbit took only 220 delta-v", correct? 

[The Minmus Derp]

19 hours ago, Eriksonn said:

With n-body physics you can do mun to mun assists and save a bunch of deltav

This trip from lko to circular mun orbit took only 220 deltav(after the initial ~800 from lko), i know that i can optimize it to an elliptic mun orbit with 0 deltav but that is for another day...

N-body orbits are fun stuff 

Ermagerdthertlerksweird.

[Eriksonn]

On ‎6‎/‎22‎/‎2018 at 5:26 AM, FinalFan said:

Based on my understanding of what you said, it would be fair to rephrase this to "This trip from mun encounter (from LKO) to circular mun orbit took only 220 delta-v", correct? 

Yes that is correct. The entire trip took ~840dv for lko ejection and 220dv for mun injection, so ~1060dv total.

Today i managed to improve it somewhat to need ~840dv lko ejection and with only 10dv correction ended up on a circular psudo-orbit around mun.

The image only shows the final result, not the departure becauce that is similar to the previous image.

The yellow line is where i have been before, and as you can see it comes from outside the mun, that is becauce i flinged myself way out to the edge of kerbins soi and did a small correction there.

Edited June 23, 2018 by Eriksonn

[Superfluous J]

On 6/17/2018 at 12:03 PM, NewtSoup said:

I'd love to learn how to do it on purpose.

I learned this years ago when I got a fortunate Laythe encounter that put me in a fantastic Jool orbit. I posted about it and because I save everything, I have it right here. Substitute "Duna" for "Jool" and "Ike" for "Laythe" and this should work wonders.

On 11/23/2015 at 5:30 PM, 5thHorseman said:

Nail your Jool encounter way out. Preferably while still in Kerbin's SOI but at the midpoint or AN/DN should be fine. Burn to pass Jool on the right (so you're going the same direction as the moons) and get your trajectory passing right through Laythe or Tylo's SOI. Laythe's good because it's close and Jool's gravity gradient's more pronounced, and Tylo's good because it has more weight to throw (you) around.

Do that burn, and if you're lucky you'll actually get an encounter with that moon, and for a couple decimeters per second you can tweak the encounter (I suggest using a maneuver node to tweak, so you only have to burn once) to enter the moon's SOI from behind, and exit from the side putting you into a Jool orbit. If you're not lucky, no biggie. Just (again, use a maneuver node. Measure twice, burn once as they say) do a burn that is a combination of radial and -grade for - again - a few decimeters per second and you'll get an encounter in no time. Then tweak it as above.

If it's too squirrely (it's hard to do a perfect 2-decimeter burn with a massive hulk of a ship) just go forward about halfway to Jool, and the burn will take more dV and therefore be a bit easier to fudge.

One more thing: Give yourself at least 5 on the CONIC_PATCH_LIMIT. I actually keep it at 8. It's in the settings.cfg and you can also set it directly if you have PeciseNode installed.

[NewtSoup]

I shall remember this - I was just testing my MVL on Tylo to make sure it could land, refuel and take off again.  Takes about a year to refuel.

[Laie]

On 6/17/2018 at 6:03 PM, NewtSoup said:

Quite by accident I managed to use Ike to get into a lower eliptical Duna orbit with an ion powered craft.  Was really quite happy with the results.  I'd love to learn how to do it on purpose.

Half of the deal is knowing what a good assist looks like -- schematics showing a 180° U-turn are misleading, like @5thHorseman it didn't click for me until I got an accidental assist. Before, that, I've tried it from a wrong angle (heh), getting me nowhere. Example of a munar assist saving ~90m/s:

In order to set it up, I like to use a "marker" probe that has an ordinary direct burn for a maneuver.  My trajectory, post- assist, has to run very nearly parallel to that of the probe; this allows me to do most of the tweaking without constantly zooming and and out on the map.

On 6/18/2018 at 11:10 PM, Tyko said:

Just wondering....If you're planning a slingshot anyway, wouldn't it be even better to start at your Mun station, plot a drop deep into Kerbin's gravity well just above 70km, and then burn your transfer at Kerbin PE?

For the benefit of @MisterKerman: that's a totally different problem. If you start from a munar orbit, going through a low Kerbin PE is indeed the way to go, that's true. But it has nothing to do with gravity assists.

[MisterKerman]

31 minutes ago, Laie said:

For the benefit of @MisterKerman: that's a totally different problem. If you start from a munar orbit, going through a low Kerbin PE is indeed the way to go, that's true. But it has nothing to do with gravity assists.

Yeah I s'pose. More of an optimal method of escaping Kerbin's SOI I guess. Appreciate it! Stuff like that is helping me get the balls to one day go beyond Kerbin's moons.

Edited June 23, 2018 by MisterKerman

[NewtSoup]

52 minutes ago, MisterKerman said:

Yeah I s'pose. More of an optimal method of escaping Kerbin's SOI I guess. Appreciate it! Stuff like that is helping me get the balls to one day go beyond Kerbin's moons.

I was exactly the same, then I just went and did it.  Go to Duna, it's not much harder than going to Minmus really.  In the mission reports there's a record of my first trip to Duna that I did a couple of months ago.  I fairly simple.  It was far from perfect but Duna is quite forgiving..