How do you do this?

[JackBush]

I just read elsewhere: "I launch from Kerbin, slingshot myself around the Mun to increase my orbit of Kerbin without using a ton of fuel, then carefully set myself on a course for Minmus." How do you do this?

Edited November 13, 2015 by JackBush

[xiej]

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Edited November 15, 2024 by xiej

[eddiew]

It's not really worth it. A Mun encounter is about 850m/s, Minmus is around 920. Assuming you go from equatorial LKO and arrive at Minmus' AN/DN so's to avoid the plane change.

Unless you're really into ultra fine optimisations, I wouldn't bother (KSP is not well suited to ultra fine optimisations)

[Snark]

TL;DR: Don't bother, it's not worth it, unless you deliberately want to just for the sake of the challenge.

It takes around 850 m/s to get from Kerbin LKO to the Mun.

It takes less than 100 m/s more dV to go from Kerbin LKO to Minmus.

That means that even in the most optimum case, you can't possibly save more than around 100 m/s of dV.

In exchange for which, you'd be attempting a maneuver that's extremely finicky to get right, and if you're even slightly wrong, you end up costing yourself far more than you saved.

As for the actual how-to: you launch from Kerbin for a munar flyby, and arrange it so that your path past the Mun takes you behind the Mun in its orbit, rather than in front. This will give you a slingshot boost that will make your orbit bigger.

If the Mun and Minmus are lined up exactly right, and you get your trajectory exactly right, then it will go on to intersect Minmus. But "exactly" here is very finicky indeed, and the penalty for failure is large.

So if you're looking for a savings: really, it's not worth it.

[Reactordrone]

TL;DR: Don't bother, it's not worth it, unless you deliberately want to just for the sake of the challenge.

It takes around 850 m/s to get from Kerbin LKO to the Mun.

It takes less than 100 m/s more dV to go from Kerbin LKO to Minmus.

That means that even in the most optimum case, you can't possibly save more than around 100 m/s of dV.

In exchange for which, you'd be attempting a maneuver that's extremely finicky to get right, and if you're even slightly wrong, you end up costing yourself far more than you saved.

As for the actual how-to: you launch from Kerbin for a munar flyby, and arrange it so that your path past the Mun takes you behind the Mun in its orbit, rather than in front. This will give you a slingshot boost that will make your orbit bigger.

If the Mun and Minmus are lined up exactly right, and you get your trajectory exactly right, then it will go on to intersect Minmus. But "exactly" here is very finicky indeed, and the penalty for failure is large.

So if you're looking for a savings: really, it's not worth it.

You will also save a little on your Minmus capture since you'll have an orbit closer to the target orbit but it's still not much of a saving.

[Stoney3K]

Side note perhaps, but I have yet to see a good tutorial on how to do interplanetary gravity assist maneuvers. AFAIK, there are as of yet no mods that allow you to plan a proper gravity assited transfer.

Slingshotting from Mun to Minmus orbit is pretty straightforward but not worth the effort because the amount of kick you get from the Mun is marginal.

[Snark]

You will also save a little on your Minmus capture since you'll have an orbit closer to the target orbit but it's still not much of a saving.

In principle, yes. In practice, you're just as likely to lose dV because while you've raised your Pe up closer to that of Minmus, you're likely to have your Ap flung way out to heck-knows-where (or a complete escape trajectory out of Kerbin).

In practice, this maneuver is extremely difficult to get to work at all, and virtually impossible to carry out in a way that actually saves dV.

[JackBush]

Many thanks. It sounded very .... but I'm too much of a noob to want to go near it now that I've heard your sage advice.

[Snark]

Many thanks. It sounded very .... but I'm too much of a noob to want to go near it now that I've heard your sage advice.

Gravity slingshots can make sense, but they really, really don't for the specific case of Mun/Minmus. Not because the boost from the Mun is too small (it can actually give a pretty reasonable kick), but because Minmus is such a small target.

Here are the cases that you may run into, later in your career, where gravity slingshots become more relevant:

• Capturing to the Jool system. Doing that requires a lot of dV, and aerocapture is difficult without burning up. But Jool has a couple of moons (Laythe and Tylo) that are very massive and well-suited to gravity maneuvers. You can use one of them for a reverse gravity assist to slow down and capture to Jool. Big savings, and not very difficult to set up.
  
• If you have a ship orbiting Mun or Minmus and want to go interplanetary (e.g. go from Mun orbit to Duna or somewhere). In this case, you get more bang for your buck if you use a Kerbin slingshot: that is, instead of launching directly from Mun (or Minmus) to your destination, you instead drop down to low Pe over Kerbin and do your thrusting there.
  

Most of the Kerbin system is really not well set up to get gravity-assist benefits; it's just too finicky and difficult. Possible, but usually more trouble than it's worth, except for the above cases (might have missed a case somewhere).

If you run a mod that updates the solar system, there may be more opportunities. For example, New Horizons adds several gas giants to the system, and there are some good chances for gravity assists there. But if you're new to the game, I'd suggest giving the unmodded solar system a good thorough play-through before you start tinkering with it.

[Van Disaster]

It's really not *that* hard to do and it'll also get you there a bit faster - very worth doing if you're on an economy challenge ( saved me ~100dV last time, plane change was very cheap ) or really tight on fuel, otherwise just try it if there's a convenient alignment. Howto is fairly basic, establish a low Kerbin orbit, pop a node down & drag the AP out somewhere past Mun, move it around until you get a Mun encounter & adjust to pass close behind Mun. Fiddle with the AP until it's out at Minmus' orbit. Now you just have to move the node forwards ( use the next-orbit option and then tweak it's position and probably your AP ) until your exit from Mun gives you a Minmus encounter.

You'll probably find when you've actually done your Mun encounter you'll need a course correction, do it as early as possible & you can probably do it on RCS - it's a good time to fine tune your Minmus intercept anyway so you don't have to waste fuel realigning when you enter Minmus SOI. Don't forget to keep control of your AP...

[r_rolo1]

Side note perhaps, but I have yet to see a good tutorial on how to do interplanetary gravity assist maneuvers. AFAIK, there are as of yet no mods that allow you to plan a proper gravity assited transfer.

There is Arrowstar KSP Trajectory Optimization Tool, that unfortunately is not a in game too :/

[Van Disaster]

PreciseNode will let you adjust the number of visible conics samples.