Mun slingshots.

[RedDwarfIV]

This technically also applies to Ike, since it has a significant amount of mass and is close to its parent planet. I'm not including the Joolian moons because, for some reason, those do often get used despite it being uncommon elsewhere.

When I watch YouTube videos of other peoples' interplanetary missions, I've noticed that they often burn straight from Kerbin periapsis. This is different from what I usually do, which is do a Munar gravity assist slingshot which greatly reduces the delta-V neccesary to reach the target planet. I could get two almost-full Jumbo-64s out to Duna, and there's no reason I couldn't use the same method to make reaching Moho, Eve, Dres, Jool and Eeloo easier.

So why don't most people do this? is it because they don't want to spend time getting the Mun into position? Is it because they want to get an intercept straight from Kerbin?

What do you do?

[Good_Apollo]

It's probably ignorance. I understand the concept but I do what I know how to do in KSP, which is a straight injection burn from Kerbin. If someone threw me a guide to using bodies like moons as slingshots in KSP I sure as hell would utilize them.

I was also under the impression that KSP didn't simulate gravitational forces well enough to perform slingshot maneuvers between multiple bodies. As in, you could use the Mun if you wanted to but you wouldn't gain additional DV. Inclination/eccentricity are what would throw me off the most, but then, I've still only gotten around to Eve, Jool, Duna, and only the Mun/Minmus as far as moons go. I tried to get to Moho once, but I never even got close and since then my efforts have been focused mainly around LKO and Duna operations.

Edited June 19, 2013 by Good_Apollo

[Sir Nahme]

the problem is calculating the phase angle for an ideal insertion using 2 bodies. Most people just don't want to do the math, some try and wing it and it doesn't work so they just do without, and some are simply too impatient to CARE about phase angles and want to do it the Jeb way with MOAR BOOSTERS!

[technicalfool]

What do you do?

I build 2300+ ton launchers, because there is no kill like overkill.

Getting a slingshot is nice when it happens. Mostly ends up as a way to get from one Joolian moon to another or a Mün to Minmus slingshot though, rather than an interplanetary transfer. Hey, free delta V is nice when you can get it.

[FlamedSteak]

I never thought to do an assisted Mun sling shot out of Kerbin's gravity, but I've just constructed a big ass rocket 100k above Kerbin and I know the delta v is going to be massive to get all that weight out.

A mun assisted sling shot might just do the fuel saving trick.

[Vanamonde]

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.

[Werwolf]

I slingshot off the Mun every time I leave Kerbin's SOI. Sometimes I can even set up a double shot, off the Mun and then off of Minmus. I also slingshot from moon to moon in the Joolian system... I knew the benefits of the maneuver when I got the game though, so I made sure to get comfortable with it.

[Echostatic]

I have no idea how to go about taking advantage of something like that... But it sounds like it would be useful.

[technicalfool]

I have no idea how to go about taking advantage of something like that... But it sounds like it would be useful.

The second image in this XKCD what-if is a pretty damned simple explanation of how a gravity slingshot works.

[TwoHedWlf]

I usually use a slingshot around the Mun if it ends up being somewhere close, but given the difficulty of planning the launch to the right time I don't always.

[Echostatic]

The second image in this XKCD what-if is a pretty damned simple explanation of how a gravity slingshot works.

I mean, I know how to do one, I just don't know how to do one that actually helps get me somewhere I want to go.

[Tw1]

There's gotta be a way to judge them using mods like protractor and KAC. My theory for traveling around the Jool system is get a rough launch window for a moon to use for the assist, and time things so you arrive at that moon roughly when there would be a window for transferring from that moon to the second moon. Hoping this will save a fair bit of delta V, especially with inclination changes. But if anyone has better advice, please share it with us.

[Hedd]

If I'm setting up for a planet and the Mun happens to be useful at that moment, I'll make an attempt. It usually very rare that is the case though and most of the time I'm either stress testing planetary vessels or heading off to explore so extended planning that could be foiled by a single malfunction is best left for tried and tested ships. Still hammering out the bugs.

[AustralianFries]

I just wait for the right launch window. Usually, I wait till the Mun is about to rise over the horizon, and launch. I transfer to the Mun almost immediately and slingshot to where ever I'm goin. Sometimes, it takes a bit more Delta-V to set up a slingshot though.

[CatastrophicFailure]

Just curious, does anyone have any empirical data as to how much extra delta v is spent or saved by doing a Mün (specifically) slingshot? I've read some opinions on here that treat it as common sense, and others that say any delta v gained is offset by the extra needed to make it in the first place.

Myself, figuring out the right launch window just isn't worth the math. I'll just strap an extra nuke on.

[wasmic]

I've only done a gravitational slingshot twice - that is 200 % of my interplanetary flights. That is because my game crashed after the first (successfull) münar slingshot. My second one worked out quite well, too. I just eyeballed it both times, and got out of Kerbin SOI for around 850 m/s of ÃŽâ€-V.

[tntristan12]

I used to do this for the longest time when going to Duna. The trick is to launch when the Mun is closest to the sun (or opposite the sun if you're going to the inner planets) so that by the time you get there, the slingshot will launch you prograde (or retrograde) relative to Kerbin.

My usual approach was to do a three point burn. The first point was an 800m/s delta-v burn to the Mun, which would set me up for a slingshot out of the kerbin system. My second burn would be at munar periaps, to raise my appehelion to Duna which usually cost me 300m/s. If necessary, I would then do a correction burn at Kerbin periaps.

All told that approach costs about 1100m/s of delta-v. More if I'm not being careful with my timing. Just out of curiosity I did a straight burn to Duna one time and discovered that it actually cost me a little bit less delta-v (about 1000m/s) I chalk it up to the fact that you just aren't moving as fast when you round the Mun, and so you don't get nearly as much help from Oberth.

That said, it might be worth the extra delta-v if you have it to spare, and you're flying a large craft with slow acceleration. You get to break apart your ejection into two or three smaller parts that allows you to make a more gradual exit.

It really boils down to personal preference.

[altazure]

I think Scott Manley said in one of his videos that at best, a munar assist saves you a couple dozen dV, and in an average case you end up actually spending more than using a straight injection due to the difficulty of setting the encounter up perfectly, so you're better off doing the whole burn at LKO.

[thrashing mad]

How about using gravity assist from Eve to get to Moho cheaper, like Mariner 10 did with Venus and Mercury? Would this work under game's mechanics?

[Uberick]

I was doing mun gravity assists, but didn't find them giving me much in the way of delta v. Most of the time, it actually cost me MORE delta v in correction burns, so I stopped doing them. Gravity assists around other planets can be helpful though, especially on long duration grand tour probe missions.

[Mr Shifty]

Some maths:

1) A Hohmann transit to, say, Duna from LKO (100km) requires an ejection burn of 1048 m/s.

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

So the Mun gravity assist saves you 1048-920 = 127 m/s

It's worth noting, however, that if you have a refueling station in orbit around the Mun at 10km, you're already going 557 m/s relative to the Mun, so you can get to Duna transfer velocity with a mere 200 m/s burn.

Edited June 19, 2013 by Mr Shifty

[metaphor]

It's always better in terms of delta-v to do a Mun slingshot, but not by much, at most around 200 m/s delta-v. But there are a bunch of reasons not to do it. One, it's a lot harder to calculate the burn time and angle needed for a successful transfer. If you don't do it exactly the right way, it will probably cost you more than the delta-v you could have saved. So it's a lot of extra effort for very little gain. Another reason is the Mun moves around Kerbin relatively slowly, and waiting until it's at the right time in its orbit may not be the ideal time for a transfer window to another planet. So overall, I don't think it's worth it. These are also some of the reasons why in real life, NASA doesn't generally use lunar gravity assists for interplanetary missions.

I've seen many people use a gravity assist from the Mun just to get into interplanetary space, and only burn towards the target planet from Sun orbit. That is a pretty big waste of fuel since you're not using the Oberth effect, and it would save a lot of delta-v to just do your transfer burn from low Kerbin orbit without worrying about the Mun.

How about using gravity assist from Eve to get to Moho cheaper, like Mariner 10 did with Venus and Mercury? Would this work under game's mechanics?

It's entirely possible. I use gravity assists all the time from other planets for long interplanetary missions. Here's a mission I did to Moho using gravity assists from Eve, and

.

I think more people should use interplanetary slingshots, because they are more effective than Mun slingshots, and also easier to set up. For example, in order to get to Jool you would need about 1900 m/s burn from LKO, but you could do a 1200 m/s burn to Eve or Duna and use those planets' gravity to slingshot you to Jool for free. You can just do an ordinary transfer from LKO to Eve or Duna, then when you are about 1 day from those other planets do a small correction burn (should be less than 10 m/s) to give you the right escape direction from the planet.

[Merinsan]

As I understand it, the amount of delta v you save is marginal, and it's hard to set up, and easy to get wrong meaning you often waste fuel.

Last night however when I went to Pol, I saved a ton of delta v from all the assists I got.

1. Laythe assist put me on a good solid Jool orbit, without even burning.

2. Small burn at periapsis got me a Tylo assist out to Pols orbit.

3. Small burn at periapsis again to get an encounter.

4. Bigger burn to get capture.

The most fuel I used once I got to the Jool SOI was in the actual Pol capture. The rest of the maneuvers combined used less fuel. Not planned though, just blind luck.

On every Mun encounter I have had, it's never given me a particularly good assist. Normally It's sending me the wrong way out of the Kerbin SOI, so I ignore it.

[CatastrophicFailure]

How about using a planetary gravity assist for a free return mission, ala the scrapped Apollo Venus flyby planned in the 70's? Would that be possible with KSP's setup?

[tntristan12]

Some maths:

1) A Hohmann transit to, say, Duna from LKO (100km) requires an ejection burn of 1048 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

...

So the Mun gravity assist saves you 1048-920 = 127 m/s

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)

Now, slingshotting around Ike when coming in is a very good delta-v investment. Assuming you don't want to aerocapture, which will become kind of difficult once proper re-entry heat is implemented, a 500-600m/s burn to slow down right as you enter Duna's SOI will give you an intercept with Ike that puts you on an elliptical orbit straight through Duna's atmosphere, allowing you to aerobrake at much more sane speeds.

I have yet to determine if slingshotting around the Mun upon return to the Kerbin system saves you any appreciable delta-v, but at least it makes aerobraking a more sensible in terms of heat stress on the spacecraft.