Lagrange Points in KSP

[Space4Rockets]

Hello Kerbonauts,

I recently saw this video on youtube http://youtu.be/mxpVbU5FH0s which got me thinking that I might like to put a space station at one of the Lagrange Points for kerbin. I know the scale of the kerbin solar system is smaller than the real one. Is anyone out there who might be able to help me calculate the location of these points for permanent base around kerbin?

I hope you enjoy the video and please post if you can help with any of the maths involved in finding a Lagrange Point.

[ElJugador]

There aren't any, as KSP doesn't use multiple-body physics.

[Space4Rockets]

are you positive?

[Pezzhippo]

Unfortunately so, I too would have liked to put something in Kerbin's L4 or L5 points. As it stands, celestial bodies only have Spheres of Influence (SOI) with craft alternating between them when traveling.

[IsaacHeron]

I wonder if the Squad could put an invisible planet/moon where the Lagrange are? No collision mesh, etc.

Would this even behave like a Lagrange point?

[Pezzhippo]

I wonder if the Squad could put an invisible planet/moon where the Lagrange are? No collision mesh, etc.

Would this even behave like a Lagrange point?

Yes and no, I would think. Indeed, if you were to orbit this point, it would be fairly similar to doing the same at an actual Lagrange Point, where the object is not truly stationary, but rather, oscillates around the point. The downside to such a method, though, is that you would have an artificial SOI as opposed to the conflicting gravitational forces between two bodies that forms real world points.

From a gaming/technical perspective also, I would foresee there being issues if a craft were to pass too close to the center of this new SOI. The other planets and moons have surfaces that prevent you from reaching the center of their SOIs, but if we look at Jool, we can already see that the laws of physics tend to break down rapidly the further you dive towards its core.

And lastly, one has to remember how many bodies there are in the system. Using the proposed method, you would need to add 5 Lagrange points per pair of interacting bodies. To use Jool as an example again, that would mean setting up 5 points for each of its moons, totaling 25 points for those, and another 5 for the Jool-Kerbol relationship. You can see this would mean a lot more work for SQUAD ;3 Sad as it is, unless KSP adopts multi-body physics, Lagrange Points are likely to only be a novel dream.

[inigma]

Can KSP adopt multibody physics? Else I'll have to scrap the L4 Space Telescope idea under the Space Telescopes campaign in the wiki.

[SuperFastJellyfish]

Can KSP adopt multibody physics? Else I'll have to scrap the L4 Space Telescope idea under the Space Telescopes campaign in the wiki.

AFAIK because of the physics calculations on every single part of your ship plus the nature of n-body physics you would need a supercomputer to make it playable as a game.

[Pyre]

AFAIK because of the physics calculations on every single part of your ship plus the nature of n-body physics you would need a supercomputer to make it playable as a game.

Actually, the physics calculations would be perfectly doable. The problem is that you can't do them in advance without them being ridiculously inaccurate.

[A Fat Pokemon]

Yes, the calculations for n-body physics are actually pretty simple, its just that you cannot predict future orbits at all really, as the equations for position-time only work when there is only one body being calculated for. You still can of course predict the future orbit as it would play out, but it would require you to calculate how ever far into the orbit you want to see in advance, which is likely to make your game run quite slowly.

[Motokid600]

Yes and no, I would think. Indeed, if you were to orbit this point, it would be fairly similar to doing the same at an actual Lagrange Point, where the object is not truly stationary, but rather, oscillates around the point. The downside to such a method, though, is that you would have an artificial SOI as opposed to the conflicting gravitational forces between two bodies that forms real world points.

From a gaming/technical perspective also, I would foresee there being issues if a craft were to pass too close to the center of this new SOI. The other planets and moons have surfaces that prevent you from reaching the center of their SOIs, but if we look at Jool, we can already see that the laws of physics tend to break down rapidly the further you dive towards its core.

And lastly, one has to remember how many bodies there are in the system. Using the proposed method, you would need to add 5 Lagrange points per pair of interacting bodies. To use Jool as an example again, that would mean setting up 5 points for each of its moons, totaling 25 points for those, and another 5 for the Jool-Kerbol relationship. You can see this would mean a lot more work for SQUAD ;3 Sad as it is, unless KSP adopts multi-body physics, Lagrange Points are likely to only be a novel dream.

Ive never been to Jool..yet. But can you actually enter its "infinite" atmosphere? Like a proper gas giant? It should thicken to the point were you actually float... saying you don't get crushed.

[NeoMorph]

Squad has said repeatedly that they won't be including n-body physics so that means no Lagrange points.

Edited June 8, 2013 by NeoMorph
Must learn to splel

[NeoMorph]

Ive never been to Jool..yet. But can you actually enter its "infinite" atmosphere? Like a proper gas giant? It should thicken to the point were you actually float... saying you don't get crushed.

Actually yes you can... you get crushed but your wreckage bobs about in the clouds... and yes, I only just found this out heh.

[Motokid600]

Actually yes you can... you get crushed but your wreckage bobs about in the clouds... and yes, I only just found this out heh.

That's really awesome. How far down did you get? Sorry to go a little off topic, but I must know lol. Is it possible to create craft that can go farther down then others or is there a defined crush point?

[NeoMorph]

Defined surface by the looks of things. But man, does it take ages to get down. I was just dropping a probe and forgot the parachute too but it STILL took ages. Temps and pressures were mind boggling.

Oh yeah, and aerobraking is FUN using Jool. Your heat shield burns for AGES!

[sporkafife]

An orbit at the L4 and L5 points can sort of be fudged on KSP by just putting your satellite in a circular orbit identical to Kerbins, just 60° in front or behind it.

Ironically enough, this is possible because the patched conics system doesn't model the gravitational perturbation of Kerbin when you are outside it's SOI, and the planet orbits perfectly around the sun instead of them both orbiting the barycentre as if Kerbin were massless. So L4 and L5 orbits are possible aesthetically, only because of KSP's innacurate physics

However it is purely aesthetic, and you are not actually orbiting around the Lagrange point, it just looks as if you are from a long distance off.

Edit: And I see you are a Sixty Symbols viewer, good on you! I had the chance to go to Nottingham Uni, but unfortunately let it pass as I didn't think I would get the required grades to be let in... annoyingly enough, I got the grades needed after turning it down

Edited June 8, 2013 by sporkafife

[CoolMatthew]

@sporkafife: That is a good point that people seem to miss a lot in these discussions.

Re: No N-Body=No Lagrange Points: I still hold out hope that at some point Squad will a implement a special case of the sphere of influence concept with special effects that model Lagrange Points.

[Unicorntr00per]

@sporkafife: but if L4 and L5 "work" (and by your reasoning they would) then L3 would "work" as well. That means like 60% of Lagrange points are feasible in KSP and I can still set up my sinister space station in the shadow of the sun

[jheriko]

AFAIK because of the physics calculations on every single part of your ship plus the nature of n-body physics you would need a supercomputer to make it playable as a game.

Yes, the calculations for n-body physics are actually pretty simple, its just that you cannot predict future orbits at all really, as the equations for position-time only work when there is only one body being calculated for. You still can of course predict the future orbit as it would play out, but it would require you to calculate how ever far into the orbit you want to see in advance, which is likely to make your game run quite slowly.

These kinds of answers are a little frustrating. I very strongly doubt that it is impractical to solve this problem in a way that would be good enough for KSP. I base this on having 1000s of points gravitating with each other @ 30fps 10 years ago with mediocre programming skill at best. With the clever optimisation of having a fixed path when not accelerating and having the planets be on rails I'm confident that it could be implemented whilst giving a similar quality of experience to what we have with the conics atm - manoeuvre nodes and all - and without a severe impact on framerate.

Personally I would not want this though - it adds little value besides being able to do weird and wonderful things at Lagrange points and actually breaks lots of niceness like being able to have simple, stable orbits around all bodies, having orbits be periodic, only having to worry about gravity inside of the relatively small spheres of influence etc.

Squad has said repeatedly that they won't be including n-body physics so that means no Lagrange points.

This is a good answer.

[Mephane]

Is it not possible to add Langrange points without true multi-body physics? Like a mere dummy object, invisible with no hitbox, non-targettable, but with its own gravity, in the specific position, and then handle this object otherwise just like another moon or planet? You'd have stuff like a "L5 SOI" and you could orbit this dummy object, thus place probes or stations in this area, without invoking the actual physics.

Edited June 8, 2013 by Mephane

[Paul Kingtiger]

Is it not possible to add Langrange points without true multi-body physics?

Yes it is. With true n-body physics you are calculating the gravity of n number of objects, which means their mass, distance and relative position to each other, that's all a bit of a headache. But you don't need to do any of that.

For a start to get working Langrange points that are accurate enough for us you only need 3 bodies, your ship and the 2 bodies with a largest effect on your ship. That could be Kerbin and the Mun, or Kerbin and the Sun, depending on your orbit.

We can make it even easier by ignoring the mass of your ship (tiny compared to the mass of everything else) and the effect of other objects on each other (because planets and moons are on rails and don't change their vector)

So now we only have to calculate the net effect of two other objects on your ship, a much easier problem to solve but still good enough to give you Langrange points. How easy is this to implement in game? A whole lot easier than n-body physics! Easy enough to make it worth while? I can't answer that.

[Pbhead]

I tried to simulate a L1 and L2 point by trying to stick something in a high enough orbit so that it would have an orbital period of one year, but kerbin (and no body, really) seems to have a SOI large enough to allow for it. which... makes sense, really, if you think about it. but still. its like. hurrrg, so close, yet so far.

[tntristan12]

No... Tiny invisible planets are not the same thing as Lagrange points... You don't orbit Lagrange points either. You need to continuously correct your orbit to stay there, which is something you can't really do unless you want to spend all your time in that mission and accomplish nothing else.

The real problem is time warp. There is nothing saying you can't have n-body physics in KSP, but you wouldnt be able to warp time since there is no analytical way to predict where you will be. It simply breaks the rails system.

[Pbhead]

ok. first. you often do orbit Lagrange points. 2. there is no station keeping anywhere in this game, so this would be no different, we just assume stable and that as close enough. 3. dont need n-body physics, just 3 body restricted... and often not even that.

[Endeavour]

Because of the lack of n-body physics, L3, L4, and L5 all work. Just put something into orbit where it should be, and it'll stay there. Unfortunately, L1 and L2, the useful ones, don't work.