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Lagrange points not simulated


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Of all the Lagrange points, only L1 and L2 are actually impossible in KSP. Putting a satellite into an orbit that mimics the others is entirely feasible as you can match the orbital period of a planet or moon quite easily, though they won’t stay in place as they would if there was a real Lagrange point there.

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I wonder if Lagrange points could be faked in on top of the KSP Keplerian model as small SOIs with an assigned Mu bound to the relationship of the 2 bodies involved, so not n-body, but "sort of" 2.x-body?  The point itself would have an SOI like a body, but would not be a true 3rd body as it would have no gravity affecting the 2 main bodies; only a tiny amount of gravity affecting craft in its tiny SOI.    So the rails code would have to have a bit more to it to keep track of the points, not necessarily for all bodies (maybe just those above a certain mass, or only those the player enabled).  Limiting to only points that allow stable orbits in N-body (L4, and L5?) would narrow down the overhead also.  The KSP orbits in the lagrange point SOI would likely not need to be realistic compared to real lagrange orbits (I seem to remember that real lagrange orbits are a bit wonky and not Keplerian at all), but instead just normal ellipses around a pseudo body.  Just throwing it out there as a possible half-way meeting point.  KSP already compromises on n-body, understandably, so a further compromise that splits the difference between Keplerian an N-body doesn't seem crazy to me

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One of the early Developer Insights posts (link) explained how KSP2 intends (or at least intended at the time) to simulate gravity well enough to have the Lagrange points associated with the new planets Rask and Rusk.

That makes me guess that they will be simulating the "restricted n-body problem" where planets move "on rails" in Kepler orbits using the sphere-of-influence approximation, but our craft move under the gravity of several planets simultaneously.  The 'n' in "restricted n-body" would be 3 (one craft plus two planets) at least near Rask and Rusk.

Edited by OHara
I guess the n in "restricted n-body" counts the low-mass body among n, because we say the 2-body problem was solved by Kepler and Newton
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1 hour ago, OHara said:

One of the early Developer Insights posts (link) explained how KSP2 intends (or at least intended at the time) to simulate gravity well enough to have the Lagrange points associated with the new planets Rask and Rusk.

That makes me guess that they will be simulating the "restricted n-body problem" where planets move "on rails" in Kepler orbits using the sphere-of-influence approximation, but our craft move under the gravity of n planets simultaneously, where n is at least 2 3 (one craft plus two planets) at least near Rask and Rusk.

That sounds like it could leave a door open for a mod to do something like I wrote about that is lighter weight than a full RO install, but still have L4 and L5 on systems large enough, if the aspects for dealing with Rask and Rusk are leveragable by a mod for other body pairs.  Who knows?  Time will tell

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  • 4 weeks later...

Honestly, nah. I feel like trying to bridge the gap between n-body and (simplified)Keplerian is just wasteful. Either go the full Principia way, or stick to SOIs. Any systems that would ape realistic behavior in an unrealistic model is just... I don't know. Look at Rask&Rusk and how they said they would need to make a one-off solution space for them to 'work.' A game that will lampshade its own shortcomings...

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  • 4 weeks later...
On 12/9/2021 at 5:34 AM, Axelord FTW said:

Honestly, nah. I feel like trying to bridge the gap between n-body and (simplified)Keplerian is just wasteful. Either go the full Principia way, or stick to SOIs. Any systems that would ape realistic behavior in an unrealistic model is just... I don't know. Look at Rask&Rusk and how they said they would need to make a one-off solution space for them to 'work.' A game that will lampshade its own shortcomings...

It's a tough call that we don't have to make.  The issues of trying to code a game that will appeal to 7 year old up to 290 year old players (being optimistic on that last) and runs on both top of the line gaming computers and box store game consoles is not an enviable task.  People have a lot of fun and learn a lot playing KSP with its non-N-body mechanics using very entry level hardware.  My suggestion was along those lines.  Honestly, I think the product should be split into a simple version and a far more sophisticated version with a lot of compatibility (craft files, etc).  But that would be another can of worms, and I wonder i they'd ever make their money back on the more sophisticated version

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On 12/9/2021 at 9:34 PM, Axelord FTW said:

Honestly, nah. I feel like trying to bridge the gap between n-body and (simplified)Keplerian is just wasteful. Either go the full Principia way, or stick to SOIs. Any systems that would ape realistic behavior in an unrealistic model is just... I don't know. Look at Rask&Rusk and how they said they would need to make a one-off solution space for them to 'work.' A game that will lampshade its own shortcomings...

For comments from Nat the current system isn’t sound and flys apart if they try and run it in the games new nish-body spacial graph.  I assumed from that all the other new systems will be sound.

I get that they want it to be familiar but seems to me it would worth having an option for a gravitationally sound starting system let everyone play a fresh. 

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On 12/9/2021 at 10:34 AM, Axelord FTW said:

I feel like trying to bridge the gap between n-body and (simplified)Keplerian is just wasteful.

1 hour ago, mattinoz said:

the current system isn’t sound and flies apart if they try and run it in the games new n-ish-body spatial graph.

There is a natural place in that gap that KSP2 might have chosen:
let the planets and moons feel gravity of the single parent body only, in Kepler orbits 'on rails', but
let our craft feel gravity of all nearby(*) celestial bodies.

That would seem to let players do the interesting things, like park craft near Lagrange points and play with tidal forces on orbits, without any worries about the orbits of planets being unstable.

(*)I suspect the criteria for 'nearby' might be rather strict, maybe only Rask and Rusk, so that satellites in low-Kerbin orbit are not perturbed by the gravity of Mun.  Or maybe not.  It is not obvious to me whether very weak effects of far away celestial bodies would be interesting or just frustrating.

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3 hours ago, OHara said:

That would seem to let players do the interesting things, like park craft near Lagrange points and play with tidal forces on orbits, without any worries about the orbits of planets being unstable.

And all you have to do is worry about the orbit of the vessel itself. It could have a stationkeep toggle that locks the orbit (for a fuel fee as a difficulty option) but this would still stray from being simple and intuitive. Overall: just don't do N body. Just keep it to Rask and Rusk, and other extreme cases.

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