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

[WindShieIds]

Performing a gravitational slingshot from the Mun to Minmus would require you to first hohmann intercept to the Mun, that takes up some fuel, after that you have to pro grade or retrograde based on if you have an escape or not, after that you have to pro grade until your escape is lined up with Minmus.

Why do that when you can jut do a hohmann transfer to Minmus? It takes up no more fuel because both can be intercepted by going into an escape trajectory, even if it did take up more fuel it'd be very little.

In my opinion gravitational slingshots are not only inaccurate, but a waste of fuel.

What do you think about gravitational slingshots?

[TerLoki]

In that situation yeah, a slingshot really isn't worth it. But then again that's going between two moons around Kerbin, there's not much dV to be used anyway. On the other hand if you can get a slingshot off, say, Eve to get to Jool or Eeloo then you could save a pretty good margin of fuel. It really all depends on the mass of the body you're using for the assist and where you're going to begin with.

[WindShieIds]

In that situation yeah, a slingshot really isn't worth it. But then again that's going between two moons around Kerbin, there's not much dV to be used anyway. On the other hand if you can get a slingshot off, say, Eve to get to Jool or Eeloo then you could save a pretty good margin of fuel. It really all depends on the mass of the body you're using for the assist and where you're going to begin with.

But we have to account the fuel in order to get to the slingshot area, let's say we're going to use Kerbol as a slingshot, then we'll have to retrograde off quiet a bit of speed off in order to do that, and that'd take up fuel.

However if you're already at the slingshot zone and on an ultimate supership exploration mission that involves going to all the planets in one mission, well yea it's useful then.

[Wooks]

The slingshot is a tool in your toolkit, a mean to reach an end, not the end itself. Planning a mission to DO a slingshot sounds worthless, now performing a slingshot DURING a mission feels like a great achievement, you were smart enough to tweak orbital mechanics in your favor.

[Red Iron Crown]

There are definitely situations in which a gravity slingshot uses more fuel than it saves. Kerbin to its moons and back is one of those situations.

I find the most useful slingshots are those that use a planet in an orbit between origin and destination. Kerbin -> Eve slingshot -> Moho and Kerbin -> Jool slingshot -> Eeloo are good examples.

WindShieIds, I don't think you can use Kerbol for gravity slingshots.

[SkyRender]

There are definitely situations in which a gravity slingshot uses more fuel than it saves. Kerbin to its moons and back is one of those situations.

I find the most useful slingshots are those that use a planet in an orbit between origin and destination. Kerbin -> Eve slingshot -> Moho and Kerbin -> Jool slingshot -> Eeloo are good examples.

WindShieIds, I don't think you can use Kerbol for gravity slingshots.

Using Tylo as your exit vector from the Jool system also tends to save a bit of delta-V. Also, technically most transfers out of Kerbin orbit are using Kerbin as a gravitational slingshot. Though if you're trying to go from Duna or Dres to Eve or Moho, it can be a useful tool to that end since its gravity is so high.

[Moar Boosters]

It's easiest to see their benefits when returning from the Jool system. Moons like Tylo can give you thousands of m/s extra speed and it's not that difficult to set up slingshots there because Vall and Laythe are also viable targets. If your orbit around Jool is a sensible one it only takes a few course corrections to setup the slingshot which will do most of the work getting you back towards Kerbin.

The other easy one to setup is using Jool to get home from Eeloo. Instead of flying back to Kerbin straight from Eeloo, it's much cheaper to throw yourself into the Jool system and let gravity do the rest of the work.

[Red Iron Crown]

Using Tylo as your exit vector from the Jool system also tends to save a bit of delta-V. Also, technically most transfers out of Kerbin orbit are using Kerbin as a gravitational slingshot. Though if you're trying to go from Duna or Dres to Eve or Moho, it can be a useful tool to that end since its gravity is so high.

Kerbol, not Kerbin. I don't think it's possible to use the Sun for a slingshot.

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

Edited April 12, 2014 by Red Iron Crown

[Tw1]

Not really a term, but a statement. [/nitpicking]

[cantab]

Just look at a delta-V map. From LKO there's very little difference in what you need for the Mun and for Minmus, so there's very little delta-V that can be saved. In the wider solar system the differences are bigger, likewise I believe in the Jool system. And if you want to make a big inclination change a gravity assist gives big savings.

Why not try what I did. Throw together a little probe with a few hundred m/s of delta-V, put it on an Eve or Duna intercept with the previous stage, and see where you can get from there using just little course corrections and the energy of the planets around you.

[SirusKing]

I am sending a probe into deep space, outside Kerbols "SOI", and to do it I am using a gravity slingshot from jool. It only takes me about 3km/s of Dv to get an encounter and it flings me right out of the kerbol system, which would normally take MUCH more.

[Robotengineer]

Gravity slingshots can be annoying, especially unplanned ones. I recently sent a mission to Jool to visit some of its moons and I got repeated encounters with Laythe that flung me right into Jool.

[problemecium]

Some delta-V can be saved by performing a carefully timed Hohmann transfer that transfers you through the Mun's SOI and into Minmus's, but as the above respondents have said this isn't a good situation for it.

[bitbucket]

The people who think gravitational slingshots are a good idea are usually the ones who've been unintentionally thrown into interplanetary space by Mun encounters.

[maccollo]

The people who think gravitational slingshots are a good idea are usually the ones who've been unintentionally thrown into interplanetary space by Mun encounters.

They are great if you know how to use them properly, and also when to NOT use them.

Using Tylo to do an unpowered gravity capture around Jool is great and can save you a lot of fuel.

Using Mun as a slingshot to go to anywhere outside Kerbin is terrible.

Edited April 13, 2014 by maccollo

[Brapness]

I once performed a massive inclination correction around Jool by flying by Laythe. What should have cost me thousands of m/s of delta-v instead cost me just 200 to get the intercept.

I also once did a sun-dive (I don't think you can get a true 'slingshot' from Kerbol, but you can burn at perihelion and do a kind of bi-elliptic transfer) from Moho to try and intercept Duna, that was daft as Moho's orbital speed is high, so I needed to waste plenty of fuel to lower my perihelion. It was however really cool so no complaints from me, and had I done it from and to elsewhere I think it might have worked.

[Dunbaratu]

The difficulty is that trying to use the Mun as a slingshotter to leave Kerbin and go to another planet requires precise math that you cannot perform, even if you know how, without access to seeing the numbers "under the hood" in KSP, which you cannot do without a Mod. You need to know precise orbital parameters of Kerbin, the Mun, the target planet, and where they are in their current orbit at the moment of you're taking off, where Kerbin is within its daily rotation, and so on.

Because to do it right and get a savings, you have to be able to do better than you'd do from the Oberth effect of just doing your interplanetary injection burn directly from low Kerbin orbit, and that means you have to be able to predict ahead of time how the Mun will bend your path later on after you stretched your ellipse out from low Kerbin orbit quite a bit. Otherwise what happens is that you watch the map view, and burn until you like the path you see the Mun encounter causes, then stop the engines and coast until you go through the Mun's SOI, and then later after that's done you look at how much more burning you have to do from free space and complete your injection from there. It's that "stop the engines and coast" part that makes it less efficient than an injection directly from a low kerbin orbit burn.

To get the proper effect that real space agencies get, you need to know all the data ahead of time to make the mathematical prediction of when and were you start your burn before you begin. The usual KSP way of "watch the screen and stop when you like what you see" doesn't cut it for this.

[Kasuha]

Performing a gravitational slingshot from the Mun to Minmus would require you to first hohmann intercept to the Mun, that takes up some fuel, after that you have to pro grade or retrograde based on if you have an escape or not, after that you have to pro grade until your escape is lined up with Minmus.

Why do that when you can jut do a hohmann transfer to Minmus? It takes up no more fuel because both can be intercepted by going into an escape trajectory, even if it did take up more fuel it'd be very little.

In my opinion gravitational slingshots are not only inaccurate, but a waste of fuel.

What do you think about gravitational slingshots?

Correct gravity slingshot around Mun to Minmus means you do burn in LKO to intercepts Mun on a trajectory which angles around Mun (with no burn applied) and leaves you on trajectory which has apoapsis on Minmus trajectory and periapsis way higher than was the initial height of the orbit where you started the burn. So it really does save fuel as you burn less to get to Minmus, and burn less to brake.

The difference is not very big in this case but it counts.

Standard transfer to Moho takes about 4000 m/s dv. Using gravity slingshots around Eve can push it down to 3000 m/s. That is significant difference.

Edit: Here is one transfer to Minmus orbit (not with intercept but that's not important at this point) for 928 m/s and one carefully prepared Mun gravity slingshot towards Minmus for 860 m/s. Notice that in first case, the trajectory has periapsis at 70 km, while in the other case the trajectory has periapsis much higher, at 8700 km. So braking at Minmus will require less dv in second case, too.

Direct transfer

Mun slingshot

Edited April 12, 2014 by Kasuha

[LethalDose]

They are great if you know how to use them properly, and also when to NOT use them.

Using Tylo do an unpowered gravity capture around Jool is great and can save you a lot of fuel.

Using Mun as a slingshot to go to anywhere outside Kerbin is terrible.

This sums it up pretty well. They're helpful if you use them right, and just because the slingshot off the Mun from Kerbin isn't that efficient (it takes a lot of work to set up and Kashua shows it saves ~ 70 m/s, which is pretty paltry) doesn't mean that all gravity slingshots are inefficient.

Slingshots I've used haven't been off of satellites, but off of primaries. I usually have a fuel depot around Minmus for interplanetary trips. I break orbit around minmus, Drop my periapsis REALLY low around Kerbin (~ 70-80 km), then slingshot off of Kerbin into interplanetary space. Now, it takes more fuel overall, but I still leave the Kerbin system with an extra 500 m/s in the vessel, which is what matters in my book. I've also slingshot off of Duna on my return trip because Ike was my last stop on that trip. Not sure how much dV it saved, but it was a nice short burn.

Other slingshots I want to try are off of Eve on a approach to Moho and the sligshot off of Tylo that Maccollo mentioned sounds cool, too. In the Eve slingshot, my plan is to approach Eve going prograde, and get shot out behind it retrograde to further drop the perihelion. I'm not sure if this approach is going to save much dV compared to making the plane change at Kerbin, though... Anyway, it's on my todo list for my next Moho probe.

But note that most, if not all, the places where we're talking about using for the slingshot efficiently (Tylo, Eve, Kerbin) are all fairly high gravity bodies. I think the Mun simply doesn't have enough gravity to effectively slingshot vessels.

Finally "Gravitational slingshots are fuel saving" is a phrase or statement, not a "term".

[_Aramchek_]

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.

If we could just brute force irl without money or r&d constraints we'd have some nightmarish god of thunder type crafts out in the real world.

[vger]

The difficulty is that trying to use the Mun as a slingshotter to leave Kerbin and go to another planet requires precise math that you cannot perform, even if you know how, without access to seeing the numbers "under the hood" in KSP, which you cannot do without a Mod. You need to know precise orbital parameters of Kerbin, the Mun, the target planet, and where they are in their current orbit at the moment of you're taking off, where Kerbin is within its daily rotation, and so on.

Because to do it right and get a savings, you have to be able to do better than you'd do from the Oberth effect of just doing your interplanetary injection burn directly from low Kerbin orbit, and that means you have to be able to predict ahead of time how the Mun will bend your path later on after you stretched your ellipse out from low Kerbin orbit quite a bit. Otherwise what happens is that you watch the map view, and burn until you like the path you see the Mun encounter causes, then stop the engines and coast until you go through the Mun's SOI, and then later after that's done you look at how much more burning you have to do from free space and complete your injection from there. It's that "stop the engines and coast" part that makes it less efficient than an injection directly from a low kerbin orbit burn.

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.

[cantab]

Because to do it right and get a savings, you have to be able to do better than you'd do from the Oberth effect of just doing your interplanetary injection burn directly from low Kerbin orbit, and that means you have to be able to predict ahead of time how the Mun will bend your path later on after you stretched your ellipse out from low Kerbin orbit quite a bit.

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.

I somehow forgot before, but as it happens I did recently set up a Munar gravity assist onto a Eve assist to ultimately reach Moho. I only went by the Mun for the science, otherwise it really wouldn't have been worth it: it saved about 80 m/s from a 1020 m/s ejection burn, at the cost of spending ages setting things up right. And by ages I mean several hours of working with Precise Node and alexmoon's launch window planner, and this after I'd already worked out the desired transfer orbit and only needed to add in the Mun flyby.

[LethalDose]

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.

If we could just brute force irl without money or r&d constraints we'd have some nightmarish god of thunder type crafts out in the real world.

I try to be efficient so I have resources (mainly, fuel and oxidizer), in case things go wrong, so I can stash fuel at a station, or if other opportunities arise during a mission.

[_Aramchek_]

I try to be efficient so I have resources (mainly, fuel and oxidizer), in case things go wrong, so I can stash fuel at a station, or if other opportunities arise during a mission.

The brute force method works for that too. :v

[Kasuha]

... at the cost of spending ages setting things up right. And by ages I mean several hours of working with Precise Node and alexmoon's launch window planner, and this after I'd already worked out the desired transfer orbit and only needed to add in the Mun flyby.

I rarely need to set up the maneuver perfectly for the slingshot. It helps with only one thing - to verify that you can get the slingshot you need with periapsis above crash zone. But you can never perform your precisely prepared maneuver exactly (unless you use tools) and slingshot maneuvers are extremely sensitive. So if you already know that the slingshot is possible with safe periapsis, there's no need to set up perfect maneuver. Just set up maneuver to approximately the right place, perform the burn, delete the maneuver and fine tune your real trajectory using RCS.

Slingshots in KSP are easiest with orbits in between. Meaning, you perform slingshot not to hit some target, but just to intersect its trajectory at suitable spot. Then you put a correction to that intersection and tune the orbit by a few m/s for intercept in several orbits. After the intercept is about 3/4 orbit ahead, again fine-tune for another slingshot to the next target. Yes it takes a lot of Kerbal time. But it does not require any heavy calculations, just basic knowledge how gravity slingshots work.