I find the term "Gravitational slingshots are fuel saving." to be inaccurate.

[LethalDose]

The brute force method works for that too. :v

Not if you burn all your resources using the brute force method. That's the point of being efficient, accomplishing the same task at the cost of fewer resources. Lower resource cost means greater resource surplus.

[deadshot462]

I've used EVE many times to either to an inclination change and/or lower my solar orbit to something closer to Moho. Quite a bit of fuel saved there.

[Vanamonde]

This discussion of how to play the game has been moved to the how-to discussion sub-forum.

[Dunbaratu]

I brute force it all the time and get better results than trying to be "efficient", and the only reason we don't do that irl is because for every pound you are lifting it costs an exorbitant amount of money to do so.

That entire sentence is self contradictory. You claim it gives better results, followed by claiming the reason it's not done is because it takes so much more fuel… what?

[Dunbaratu]

The solution to this is to increase your conic patch limit in the config file. If you want to do any sort of gravity assist travel that's essential.

Ah. I see there was an unstated assumption I forgot to put in my claim. That you're doing it without the assistance of a maneuver node. So you have to work out before you start your burn what will happen without the game engine showing it to you. The hard part there is that you're not just trying to make your periapsis hit the Mun, because that involves a lot of drifting up to higher orbit wasting the Oberth effect away, but instead trying to thrust enough to entirely leave Kerbin's SOI, but in such a way that as you do so on a fast pass overshooting the Mun, you get getting very near it as you do so. If you use a maneuver node to watch what you happen ahead of time then KSP is doing all that math for you.

Edited April 13, 2014 by Steven Mading

[Aedile]

They are useful, on the expense of time, and have a bit higher chance of messing up. IRL they are very unpractical for manned flights. Very useful for changing inclinations. Also depends on the weight of your ship. For example if you are sending a ship and a lander, few hundred dV, of the whole ship translate to thousands of DV of the lander. I usually use life support mod, so usually send a fuel ship(or few) using grav assists, and the manned flight directly. Since cost does not matter, it's still easier to directly send few fuel ships.

[MarvinKitFox]

You say "I find the term "Gravitational slingshots are fuel saving." to be inaccurate. "

That is like stating that "using the freeway is fuel saving" to be inaccurate, then using the example of taking a 30 miles detour on the wrong freeway to get to the convenience store one block over as your argument.

Let's rephrase that first statement. Gravitational slingshots can be fuel saving, if done right.

Using Mun to get to MinMus can save you 230m/s

Assuming you are willing to align the orbits down to the second, and launch your initial transfer accurate to << 1 m/s, and wait several hundred orbits for a fortuitous alignment of orbital planes and eccentricities.

As my "great octo migration" experiment proved, gravitational slingshots can take you from a barely Mun-crossing orbit to a Laythe surface landing(impact). all it needs is ******exact****** timing and orientation of the initial orbit.

[MarvinKitFox]

Just trying to plan a slingshot with a single body is tricky enough. I can't imagine the amount of time NASA must have spent planning the trajectories of probes designed to go on any kind of "tour" of a group of moons or the whole solar system.

Accurate solar system data, and a ton of computing power. Easy, really, to calculate. Its the gathering of the gravitational data that is hard, as you need to account for *all* significant objects, including the non-spherical nature of planets & the sun, relativistic effects, deep space "air" friction, solar wind speed and composition and density, light pressure, Yarkovsky–O'Keefe–Radzievskii–Paddack effect, Poynting-Robertson effect, etc, etc...

Once you have everything figured out though, and if you are patient, you can go from virtually *anywhere* to *anywhere else* in the solar system, at a final cost of virtually zero delta-v.

http://en.wikipedia.org/wiki/Interplanetary_Transport_Network

[cantab]

As my "great octo migration" experiment proved, gravitational slingshots can take you from a barely Mun-crossing orbit to a Laythe surface landing(impact). all it needs is ******exact****** timing and orientation of the initial orbit.

Linkay?

Once you have everything figured out though, and if you are patient, you can go from virtually *anywhere* to *anywhere else* in the solar system, at a final cost of virtually zero delta-v.

http://en.wikipedia.org/wiki/Interplanetary_Transport_Network

Note that this isn't exactly the same as a "regular" gravity assist, and that ITN-style transfers are not possible in Kerbal Space Program since it doesn't implement n-body gravity.

[TerLoki]

Here's a chance pic of a good fuel saving maneuver from one of my Jool probes:

That maneuver took about 230m/s, and I'd say it saved at least that much dropping my orbit down for an easier intercept with Tylo. Just think of it like this: If Vall wasn't there, how much additional dV would be required to reach an orbit in that area? I know it would take at least two burns as opposed to one, that's for sure.

[MarvinKitFox]

Linkay?

http://forum.kerbalspaceprogram.com/threads/69816-A-preliminary-study-into-the-Migratory-Habits-of-the-common-Octo-2

As for my other comment about the Interplanetary Transfer Network.. That was in response to vger's comment on how much time NASA has to spend on working such multi-encounter trajectories..

Sadly, yes, none of us can afford the multi-yottaflop supercomputers that would be needed to implement accurate astrodynamics in real time, much less at the sort of time acceleration KSP wants

[Kasuha]

Note that this isn't exactly the same as a "regular" gravity assist, and that ITN-style transfers are not possible in Kerbal Space Program since it doesn't implement n-body gravity.

I don't think there's anything in KSP implementation of space physics that would prevent building ITN. It's the same kind of problem as free return trajectory, except order of magnitude more complex to prepare.

[cantab]

Well Wikipedia mentions the ITN being based around the Lagrange points, and the orbits being described by the restricted three-body problem, neither of which we have in KSP which uses the simpler patched conic approximation.

[Kasuha]

Well Wikipedia mentions the ITN being based around the Lagrange points, and the orbits being described by the restricted three-body problem, neither of which we have in KSP which uses the simpler patched conic approximation.

Okay I admit I have not checked the link right away and have mistaken it for network of Cyclers.

But I still think gravity slingshots provide us a good equivalent in KSP - you need slightly more dv than the minimum to get to Eve and you're good to go anywhere in system using series of gravity slingshots, including favorable intercept paths/speeds for the target body of your choice.

[Starstrider42]

Okay I admit I have not checked the link right away and have mistaken it for network of Cyclers.

But I still think gravity slingshots provide us a good equivalent in KSP - you need slightly more dv than the minimum to get to Eve and you're good to go anywhere in system using series of gravity slingshots, including favorable intercept paths/speeds for the target body of your choice.

Oooh... now I want to work out a cycler network for the Kerbol system. How hard could it possibly be?

[Vector]

But I still think gravity slingshots provide us a good equivalent in KSP - you need slightly more dv than the minimum to get to Eve and you're good to go anywhere in system using series of gravity slingshots, including favorable intercept paths/speeds for the target body of your choice.

I agree that apart from the technical definition of an ITN, the gravity slingshots provide something effectively equivalent. The only thing that is missing (and doesn't come for free for real-life ITN either) is a program to calculate all the available paths. It is straightforward to plan gravity assists a few intercepts out, but mapping out potential options is tedious and not especially fun. For example, deciding between Kerbin-Eve-Kerbin-Duna-Jool, or Kerbin-Duna-Kerbin-Eve-Jool, or Kerbin-Eve-Kerbin-Eve-Kerbin-Jool? Among Jool's moons the possibilities explode and there is not a tool to easily map out the options.

[Kasuha]

It doesn't really feel that complicated to me. I'm still not sure if you can gain any "slingshot energy" by alternating slingshots off two or three bodies but what I already know is that on certain orbit and using slingshots off just one body you can alternate between orbit with apoapsis on that body and with periapsis on that body, with all orbits in between these two extremes crossing its orbit at varying angles. So it's usually a bit like climbing a ladder. Raise your apoapsis towards body on higher orbit, and "invert" your orbit around it using several slingshots to change that apoapsis to periapsis, which will move your apoapsis at or above trajectory of other body. So you can theoretically climb Kerbin -> Duna -> Dres -> Jool. Except Duna and Dres are not very good for that purpose and it's easier to get down to Eve, use powered slingshot around it to gain some energy, then slingshot around Kerbin right to Jool.

In Jool system there are two main objects of interest - Tylo and Laythe, with eventual small help of Vall. They are so close to each other that it's almost irrelevant which one you decide to use, just take what comes first.

[vger]

Once you have everything figured out though, and if you are patient, you can go from virtually *anywhere* to *anywhere else* in the solar system, at a final cost of virtually zero delta-v.

http://en.wikipedia.org/wiki/Interplanetary_Transport_Network

Wow... has NASA not ever presented a 3D animated model of this? Went looking for one and didn't find any.

[SpaghettiFace]

While on the topic of gravity slingshots, this mission report is an excellent, hilarious example. (Not mine, was linked on another forum)

That was literally the most hilarious thing I have ever seen.

[FREEFALL1984]

I frequently slingshot out of kerbin SOI, with some precise node execution and some good timing, (making sure the moon will be traveling prograde or retrograde to kerbins orbit more or less when you hit the interplanetary launch window) can allow for the bulk of interplanetary transfer Dv to be saved and just requires a much larger corrective burn when in interplanetary space which if you master it, it will still only be a fraction of the DV required to perform a standard transfer,

[wasmic]

I disagree with gravitational slingshots being useless. I slung off of Tylo, Vall, and then Tylo again for a near-circular orbit around Jool. It required only about 100 m/s of delta-V. So, they're definitely not useless.

[Rokker]

I'm pretty sure most of these arguments are just straight up ignoring the simple fact that if gravity assists didn't save fuel, then a good portion of the current non-Earth orbiting space missions would not exist or would be/have been much more limited. Missions such as Mariner 10, Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Messenger, Cassini, Rosetta-Philae, Dawn (debatable), New Horizons, Juno, Cassini–Huygens, Stardust, the upcoming ISEE-3 recapture attempt, Giotto, Vega 1, Vega 2, STEREO-A, STEREO-B (both of which used a lunar slingshot to enter solar orbit) BepiColombo. That list right there proves that gravity slingshots work and save fuel. The obvious issue is that the poster is not attmpting the slingshot at the proper time for a Minmus encounter. The math for it isnt necessarily easy, but that doesnt automatically make it incorrect.