Gravity assists: do you use them?

[Cirocco]

With the recent attention to the rosetta mission, I got to wondering: how often do KSP players use gravity assists?

I personally don't really use gravity assists because I rarely do multiple-body missions (except between planets and their moons, but I've never been to Jool either...). I did once use the Mun to gravity brake an asteroid in a stable orbit around kerbin, but that was completely unplanned and very, very lucky. I plan to use a gravity assist from jool when I decide to finally run a mission to send a prode out to interstellar space, but other then that, I don't really use any planned assists.

I imagine that most gravity assists would be used in the Jool system, using things like Tylo for a braking or slingshot assist, but I don't really know for sure.

So, do share ! Do you use gravity assists in KSP? If so, how often and in which missions? Do you manage multiple assists? If you use them, how do you calculate them, etc.

[Richy teh space man]

It's not that hard to get a craft into orbit with enough fuel to go to most places. If KSP was on the same scale as our solar system then it would probably be more useful.

That said I sometimes use it in the Joolian system to save a little fuel, and mostly just use the manoeuvre nodes.

[Mystique]

I usually use them to navigate between moons and their parent planet, though only if bodies involved are already in right position, because planning that stuff ahead and waiting for planets to get into exact position takes much more real life time than making usual transfers

P.S. Don't send anything to interstellar space, huge Krakens live there.

[KevinW42]

Do unplanned assists due to time warp bugs count? I had a maneuver node set up to give my probe a Laythe encounter, but Mechjeb warped me too fast and flung me out of the Joolian system onto a trajectory that only required a minor correction to take me to Eeloo.

[Lukaszenko]

I do in the Jool system, to return back to Kerbin. Can make a huge difference.

I also used it when sending scanning satellites to Jool. I only sent a couple of them and used gravity assists to send them to and scan all the moons.

Also I've used EVE to get to/ from Moho.

[ftunk70]

Yes I use them. It is really nice going shopping around the Joolian moons basically not spending delta-v to get new encounters. I visited all the moons and landed on three of them using the same probe using only small adjustments in the 40-200 delta-v range for each leg.

Also, it is very useful for getting to and from Moho. You can get half the slowdown and all of the inclination change for free via Eve. I think it can save a couple thousand delta-v. I like small rockets and this really keeps the part count down so that my aged laptop can keep up

[Superfluous J]

I only really use them in one situation, because that's the only situation that makes it easier: Getting into orbit around Jool. No aerobraking, no huge burns, just a simple 3-step process:

1. Coming in, get your ship so it's coming around the back of Jool at Laythe's orbit. Ideally, your hyperbolic orbit should be touching Laythe's somewhere but really just getting close is fine.
   
2. Unless you got lucky and got an encounter with Laythe already, as soon as you get into Jools SOI burn retrograde until you get a Laythe encounter.
   
3. Fiddle a little bit until your Laythe encounter gets you to where you want to go.

All other gravity assists take tons of planning and impeccable timing. No thanks!

[Jovus]

In real life, time is cheaper than dV. In KSP, dV is cheaper than time. I expect this explains the discontinuity.

[cantab]

Loads of times. It takes a bit of work, but it's very satisfying to pull off a good assist. I've made good use of them to send asteroids around, putting one orbiting Ike and another landed on Eve both thanks to Kerbin assists. And then there's this little guy:

Flew by every planet, plus the Mun and Ike. Its twin followed a similar route, missing Eeloo but instead impacting the Sun.

[Superfluous J]

I've made good use of them to send asteroids around

Oh! I forgot the other use of them I've done in the past and will likely do in the future. When you catch an asteroid and it's in a wonky orbit, it's a lot cheaper to get its orbit to intersect Mun's orbit in some way, than it is to change the asteroid's tilt and all that stuff. So I do that (intercept Mun) and use it to straighten out my orbit around Kerbin before burning to whatever orbit I want it to eventually actually be in.

[Cirocco]

Thanks guys, there's some good info in here! I never thought about using gravity assists to straighten out orbits or getting them inclined to a certain degree. Makes sense though, this could certainly help in things like a Moho return mission.

keep it up, I love reading about people's (possibly unplanned) gravity assists

[federicoaa]

I used once a mun gravity assist to capture an asteroid, it was beautiful.

[FunnyBunny14]

I use gravity assists to easily get out of Kerbin's SOI to do what I like to call the slingshot maneuver. I basically go out, do some science, and then go back. Really useful in early career and it saves you some fuel so you can return to Kerbin.

[kinnison]

in reality there are many days logged on supercomputers to plan some of these missions and multiple gravity assists with minimal fuel. the main reason is for every ounce on craft weight you save, means it is far cheaper to launch into space. less fuel = less money. For me it is mostly trial by error. I could spend hours fine tuning a maneuver node to save only a few tens of delta-V

[Red Dwarf]

I use gravity assists as much as possible. I usually use the Mun to get a little extra kick heading out of Kerbin's SOI, and from there, who knows? Bounce off Eve, swing round Duna, switch back to Kerbin, then off to Dres. I have zero ability to plan those maneuvers though, so the fuel I waste getting those encounters could probably be better used just getting to my target the traditional way

[cantab]

Of course we do have it easier in that the patched conic approximation is the reality in KSP, which means we get real-time course predictions ingame, and a tool like KSPTOT takes minutes on a home PC to find an optimal trajectory.

Indeed sometimes it's not worth it. Mun assists to the planets give tiny delta-V savings, though you may want to do science or contracts during the flyby. Planet-planet assists offer much more benefit, for example you can get to Jool via Eve and Kerbin assists and save 40% of the delta-V a direct Hohmann transfer would take.

Incidentally, Precise Node is nigh-essential for this kind of work. When you care about 0.1 m/s either way the stock widget doesn't really cut it.

[bsalis]

I used once a mun gravity assist to capture an asteroid, it was beautiful.

That is what NASA is planning!

As for me, I have used Tylo and Vall once each. Those are the only planned assists I have done.

[Bigbootie]

Not a lot but after reading this thread I'm going to start using them more frequently.

[electronicfox]

I once got a gravity assist from the Mun. It was accidental. My ship ran outta fuel, and the assist sent me lithobraking hard into Minmus.

[InterCity]

Since I mostly focus on close (duna, mun and minimus) colomies, it has no use for me to do it regularly. I once did it in the Jool system, but that was purely for the sake of doing it.

[Red Iron Crown]

I rarely use them. I appreciate the efficiency gains to be had from using them, but I find them tedious to set up properly.

[magnemoe]

Has used them sometimes, in Jool system or using Mun to change the orbit of astroids.

Far more common is dropping from the Minmus shipyard down to Kerbin then do the interplanetary burn. You need to drop from Minmus so your Pe matches the escape burn point pretty well, then adjust the orbit so you get at the burn point in correct time.

This is far easier now, use an dummy satelite and set up the burn, set it as target and you can see the burn point and direction, you might need to enter into an retrograde orbit but this is cheap from Minmus, This can also be done from other moons, the retrograde setup might be impractical.

Aerobraking is far more common, Laythe is often used to aerobrake right into Jool orbit, do an aerobrake who is just too weak to capture and you end up with an orbit pretty much like Laythe.

[LethalDose]

In real life, time is cheaper than dV. In KSP, dV is cheaper than time. I expect this explains the discontinuity.

Well said, bud.

[Broax]

I actually never used gravity assists. I wasn't so sure how helpful they'd be in KSP but judging from some of these replies maybe I should start thinking about it...

[Norpo]

Generally I find that whenever I would use them (duna, ike, jool, etc.) I can simply aerobrake instead. As I don't have Deadly Re-Entry, I have no reason to not aerobrake whenever I get the chance. I mean, free Delta-V, not like I can refuse, right?

However, I plan to add some new mods and start a new save when the next update comes out, and i'll play 6.4x Kerbol with deadly reentry, FAR, etc. Hopefully this will increase the difficulty enough that I'll have a good reason to use gravity assists, if only because I don't want my ship burning to a crisp in the atmosphere of Duna going at 2km/s.