A possible Lagrange point workaround.

[tntristan12]

If you put them on rails in the correct orbit they will not be always in the right place. Both L4 and L5 are in a slightly higher orbit and therefor have a slower orbit.

http://upload.wikimedia.org/wikipedia/commons/thumb/7/78/L4_diagram.svg/403px-L4_diagram.svg.png

But since we're already 'faking' it you could of course put them in the same orbit as the planet and keeping them in the 'correct' position.

Edit: F! Got ninja'd by less than a minute.

False! Or rather, False in KSP. In real life L4 and L5 must *by definition* form an equilateral triangle with both bodies, but since KSP doesn't simulate n-body physics, the barycenter for, say, the Kerbin-Mun system would be exactly at the center of Kerbin, and since the Mun orbits in a perfect circle (which is one of the key assumptions of the restricted 3-body problem that gives rise to Lagrangian points in the first place) you *can* just put your satellites at 60 degrees ahead of or behind the Mun (or any other angle for that matter) and leave them on rails, to simulate those points. As for L1-3, those points are highly unstable anyway in real life, so the way KSP wipes them out of the picture using the Patched Conics Approximation isn't causing you to miss much.

As for the phantom body "model" (because it isn't actually a model at all), this would create a highly unphysical system that makes no sense within the KSP universe or in the context of orbital mechanics. It's quite possibly the worst way to "simulate" L points.

[tntristan12]

There might be a hack that does not involve a complete re-write of the basic game engine.

Don't put the L-points on their own rails but instead put them on their parents rails with a distance and angle off-set.

...I actually kind of like this idea.

[NeilC]

Don't put the L-points on their own rails but instead put them on their parents rails with a distance and angle off-set.

How is this not a major change to the game engine?

Right now there are exactly two types of bodies in KSP:

- The active craft and anything within load distance of it, which get force-based physics treatment

- "On-rails" orbital objects, which all obey Kepler's 2-body patched conic approximation

Neither of those objects could be used; you are not proposing a quick hack, but a significant development effort. Just because you can partially describe the desired behaviour of a new type of object in one sentence doesn't make it easy to implement.

For instance, your "distance and angle offset" - I know what you mean of course, in visual terms looking down at the Kerbol system. But the solver doesn't work that way. The rule "keep this object at the same angle around the sun as Kerbin, but at this distance" doesn't translate well into parameters like semi-major axis, eccentricity, and time since epoch.

Whenever we users come up with a super-simple idea like "Can't we just <x>?", you can be pretty sure that we didn't just solve some big challenge in a new, easy way the devs never thought of. You can be pretty sure the idea only seems easy because we have no idea what we're talking about and we aren't seeing all the problems with "just" doing that.

[Dunbaratu]

Nobody seems to be addressing the problem that KSP makes station-keeping impossible and thus this doesn't work.

Let's say you have a vessel where its velocity down to as close to zero relative to the lagrange point center as you can "see" on the instruments. The instruments round to the nearest tenth of a meter per second, so anything smaller than 0.05 m/s will show as zero as far as you can tell when piloting it.

But even at 0.05 m/s that's still 1 meter every 20 seconds. Meaning that to travel 100,000 meters, as one suggested SOI for the L points was, takes 2,000,000 seconds.

2,000,000 seconds is about 23.14 days.

Even if you are such an awesome pilot that you can get your speed down to the point where it *looks* like zero, you could still drift out of the L point in as little as 23.14 days.

So how do you correct for that in KSP where ships that are not the current focus of attention cannot DO anything? Their on-rails movement relative to the L point will drift them out of the L point if you ever time warp more than 23.14 days ahead without switching focus back to your L point station again to do a bit of station-keeping thrusting with it. Even an automated mod like kOS in which you could write a station-keeping autopilot is useless because ships can't thrust or maneuver when they aren't the center of attention.

[Wahgineer]

Phantom bodies wouldn't work as at the centre there would be a point of gravity in an infinitely small space (aka a black hole) This works fine for other planets as they have a surface to stop you getting their but this wouldn't be there for this.

I fell through the ground on minmus with a 300 part ship and started accelerating towards the center. As I got closer the acceleration approached infinity and the ship simply folded up once it got to close. This is what would happen if this was used as a Lagrange point.

(however if a black hole was ever added this would work pretty nicely. )

1: the gravity would be much lower

2: there is no surface, so you wont accelerate to light speed falling through the surface, you'll just orbit it normally.

3: if you ever got too close to the center, you'd either sling shot back out, or your orbit would be a straight line. No black holes.

[Kasuha]

in KSP. In real life L4 and L5 must *by definition* form an equilateral triangle with both bodies

L4 and L5 don't exist "by definition". They're result of natural forces.

This is how L4 and L5 look like for Jupiter. That equilateral triangle looks quite hairy to me.

Edit: even nicer picture. Trojans in green, Hildas in red.

I am pretty convinced there's no point to try to simulate that in KSP.

Edited January 13, 2014 by Kasuha

[Specialist290]

One thing that's sort of always at the back of my mind when I read these discussions but that never seems to actually get brought up: What about Minmus? Discounting the fact that KSP itself doesn't run on n-body physics at the moment, would the presence of a second moon of Minmus's size and mass be gravitationally significant enough to complicate the picture in a realistic model as compared to the (relatively) simple one we have in the real world for just the Earth-Moon-Sun arrangement?