Why can't we have Lagrange points again?

[Martian Emigrant]

I was reading another new post for Lagrange point simulation and.....It got me thinking some more about that.

I know can't be done....Realisticaly and you can't do funky orbits like in the "real" Universe with what we have....It is a game....

But...

But why can't we have new ephemeral moons with no surface and a small SOI.

If it's small enough you can't fly through it unintentionally. It as to be on purpose. And you could orbit in there.

At or close to the theoretical centre or on the edge of the SOI.

I am not asking for 4 points for every body of the system. How about say the leading and trailing point for every "Planet". Dres, Eeloo and Moho don't get any.

Comment?

 

ME

[cubinator]

Disclaimer: This is only my rather limited understanding, which may be wrong in some areas or not give the full picture.

Each celestial body in KSP is a surface "shell" with a point-mass in the center. The pull of gravity decreases with distance from that point mass. On Kerbin, it's 1 g at 600 km distance and 0 g at the edge of the SOI. The problem with doing away with the surface is that gravity is actually infinite at the center of the point mass, no matter how small you make it elsewhere, so by flying close to it you could accelerate to "sanic fast" very easily. So the point mass would have to be either so massive you get big delta-V savings from a gravity assist on nothing, or too small to be useful as a place to put spacecraft.

[Vanamonde]

This suggestion for the game's development has been moved to Suggestions & Development. 

[RatchetinSpace]

What cubinator said. The way gravity is simulated just prevents this from working.

How about, to simulate lagrange points, you fly a spacecraft out to pretty much that perfect orbit legitimately, and then use the cheat menu to tweak your orbit so you never fall out of sync with the body of choice. You can use the KSP wiki for orbital information, then you just have to adjust the position of the craft within the orbit. I know it's not ideal but it will get your crafts in the orbits you want.

[Martian Emigrant]

2 hours ago, cubinator said:

Disclaimer: This is only my rather limited understanding, which may be wrong in some areas or not give the full picture.

Each celestial body in KSP is a surface "shell" with a point-mass in the center. The pull of gravity decreases with distance from that point mass. On Kerbin, it's 1 g at 600 km distance and 0 g at the edge of the SOI. The problem with doing away with the surface is that gravity is actually infinite at the center of the point mass, no matter how small you make it elsewhere, so by flying close to it you could accelerate to "sanic fast" very easily. So the point mass would have to be either so massive you get big delta-V savings from a gravity assist on nothing, or too small to be useful as a place to put spacecraft.

 

6 minutes ago, RatchetinSpace said:

What cubinator said. The way gravity is simulated just prevents this from working.

How about, to simulate lagrange points, you fly a spacecraft out to pretty much that perfect orbit legitimately, and then use the cheat menu to tweak your orbit so you never fall out of sync with the body of choice. You can use the KSP wiki for orbital information, then you just have to adjust the position of the craft within the orbit. I know it's not ideal but it will get your crafts in the orbits you want.

Got'cha both.

Infinite gravity at centre. Can do gravity assist were none should exist. Check.

And Yes, I know I can edit files. I do, do it, on occasion. Sometime without help from the web. I just "Feel" my way through the code. But still....

 

I surrender. Thank you.

 

ME

[pincushionman]

I believe people have tried the invisible point mass before and the result was spectacular game breakage. Or was that when trying to make a double planet? No matter; the same problems arise.

You may want to take a look at the mod Principia. It's still WIP, but it adds N-body gravitation.

 

[eddiew]

Could you do it with an object a few km across (i.e. asteroids, which do tend to 'gravitate' to Lagrange points), with a pointlessly small 0.001g to avoid the temptation to slingshot around it?

Assuming you can set the SoI independently of the mass, that's the sort of thing you could handle with Kopernicus - although it would add a lot of bodies and you'd probably end up with a million contracts to visit your Lagrange rocks.

[stenole]

The Lagrange points aren't gravity wells. They are more like gravity saddles. The ships' movements nearby a fake Lagrange point wouldn't make sense. When approaching, the ship shouldn't accellerate towards it.

Edited October 24, 2016 by stenole
correction

[Superfluous J]

Really what people want is a spot where stuff tends to stay there, relative to the planet and moon. That's it.

One way to do this mathematically within the confines of the patched conic system is to have 2 concentric SOIs, the larger being about the size of about where stuff would tend to - if it wandered there on its own - stay in place. The smaller of the two SOIs would then have *0* gravity. These two SOIs would orbit the planet in step with the moon, 60 degrees ahead and behind it*.

Then anything within the center SOI would just slowly wander toward the outer SOI, and when it hit that SOI it would get gently nudged back. There'd be no crazy high pull at the center because of the concentric nature of the two SOIs and the fact that the inner one had no gravity.

I don't know if you can do this in Unity, or in KSP. I don't want it enough to go through all the work to try. But if it did work I'd probably install it for the novelty if for nothing else.'

One problem I see is high time warp. eventually you'll be warping to Eeloo and come back and find your Mun Lagrange station hit the SOI change too hard and is now ejecting from Sun.

*don't even ask about the other 3 points, they're not stable

Edited October 24, 2016 by 5thHorseman

[Choctofliatrio2.0]

On 10/22/2016 at 8:49 PM, cubinator said:

Disclaimer: This is only my rather limited understanding, which may be wrong in some areas or not give the full picture.

Each celestial body in KSP is a surface "shell" with a point-mass in the center. The pull of gravity decreases with distance from that point mass. On Kerbin, it's 1 g at 600 km distance and 0 g at the edge of the SOI. The problem with doing away with the surface is that gravity is actually infinite at the center of the point mass, no matter how small you make it elsewhere, so by flying close to it you could accelerate to "sanic fast" very easily. So the point mass would have to be either so massive you get big delta-V savings from a gravity assist on nothing, or too small to be useful as a place to put spacecraft.

So there's a black hole in the center of every planet?

 

...cool.

[cubinator]

11 minutes ago, Choctofliatrio2.0 said:

So there's a black hole in the center of every planet?

 

...cool.

Not a black hole, because the physics don't disallow a naked singularity. If you fly through it you would experience infinite acceleration without getting pulled in, basically an infinite gravity "assist". Which causes problems for spacecraft trying to get close to it.

[Gaarst]

3 minutes ago, cubinator said:

Not a black hole, because the physics don't disallow a naked singularity. If you fly through it you would experience infinite acceleration without getting pulled in, basically an infinite gravity "assist". Which causes problems for spacecraft trying to get close to it.

Your craft would probably be ripped apart by tidal forces before that point.

[Superfluous J]

2 hours ago, Gaarst said:

Your craft would probably be ripped apart by tidal forces before that point.

Actually would it? We're talking KSP here. If the ship was on rails, it surely wouldn't because the game treats it as a point. Off rails, I *think* that each part reacts to gravity based on the COM. Or, more accurately, the entire ship reacts to gravity as one unit. First, it would simplify calculations a LOT, and second, it would keep down on weird inter-part force problems.

Of course in reality the ship would be torn apart, but in reality there aren't mathematical not-black-holes in the centers of all planets

[cubinator]

3 hours ago, Gaarst said:

Your craft would probably be ripped apart by tidal forces before that point.

If not by tidal forces, then by some other problem involving infinite acceleration.

[Shpaget]

Well, the infinite gravity can be sidestepped.

if (SOI == L4 || SOI == L5)
 {
	if (forceOfGravity > bajillion) //perhaps consider overflow here
	 {
		forceOfGravity = somethingSane;
	 }
 }

 

Edited October 24, 2016 by Shpaget

[Darnok]

5 hours ago, stenole said:

The Lagrange points aren't gravity wells. They are more like gravity saddles. The ships' movements nearby a fake Lagrange point wouldn't make sense. When approaching, the ship shouldn't accellerate towards it.

Then Lagrange points should be small space spheres where you are not affected by any SOI?

Would that be game breaking feature?

[Martian Emigrant]

6 hours ago, eddiew said:

Could you do it with an object a few km across (i.e. asteroids, which do tend to 'gravitate' to Lagrange points), with a pointlessly small 0.001g to avoid the temptation to slingshot around it?

Assuming you can set the SoI independently of the mass, that's the sort of thing you could handle with Kopernicus - although it would add a lot of bodies and you'd probably end up with a million contracts to visit your Lagrange rocks.

Funny you should mention that.

I was going to let this tread die but after rereading the first few comments I was thinking like you.

I found this:

It allows the creation of planets.

So I was thinking to use it and create a "Trojan-Planetoid-Asteroid-Potato-whatever" or "co-orbital asteroids" called "Kerbin L4" and "Kerbin L5" with minute mass so you can't land on it (Less than 0.01G on the surface) and an SOI smaller than  Gilly. BTW being Planets means you can't "Grab" them either.

We can pretend that it's good enough. No need to do a Lissajous orbits.

 

Of course I have a bit much on my plate with the "Real life". My wanting to learn about KOS and what not.

 

ME

 

Edited October 24, 2016 by Martian Emigrant

[eddiew]

31 minutes ago, Martian Emigrant said:

So I was thinking to use it and create a "Trojan-Planetoid-Asteroid-Potato-whatever" or "co-orbital asteroids" called "Kerbin L4" and "Kerbin L5" with minute mass so you can't land on it (Less than 0.01G on the surface) and an SOI smaller than  Gilly. BTW being Planets means you can't "Grab" them either.

I believe Hale from OPM is about 0.002g, and it's still 'landable' if you can be bothered with the descent and use x4 timewarp for a few minutes until it settles   But you can turn down the science multiplier to zero and remove all resources so that it becomes, at best, a curiosity. Some people would put a base on it just because it's an object, others would use the 5m/s orbital velocity to park things comfortably. Although I secretly love the idea of a mine-able asteroid a few km across in one of the Lagranges. Make a nice change from Minmus mining! Although departing from it would lose you the Oberth effect of being near Kerbin, so it would be a minor trade off of efficiency. The main problem is how to NOT get contracts for it, to which I have no answer.

It's actually really weird trying to rendezvous around such tiny objects however...

Spoiler

 

[RoboRay]

5 hours ago, Choctofliatrio2.0 said:

So there's a black hole in the center of every planet?

 

...cool.

No.  In the real world if you go deep below the surface, you will be pulled upward by the gravitational attraction of the portion of the planet's mass that's above you while still being pulled downward by the portion below you.  If were able to make a hollow cave at the center of the planet, you'd be able to float in zero gee right in the middle, being pulled equally in all directions.  Once you move slightly out of the center, you'll be pulled gently toward that side, away from the center.  If you made the cave big enough, you could walk around on the walls of the cave as you would feel slightly more pull "upward" because you're closer to that mass than to the mass on the opposite side of the cave.

1 hour ago, 5thHorseman said:

Actually would it? We're talking KSP here. If the ship was on rails, it surely wouldn't because the game treats it as a point. Off rails, I *think* that each part reacts to gravity based on the COM. Or, more accurately, the entire ship reacts to gravity as one unit. First, it would simplify calculations a LOT, and second, it would keep down on weird inter-part force problems.

Of course in reality the ship would be torn apart, but in reality there aren't mathematical not-black-holes in the centers of all planets

Gravity is applied to a single point of the ship (the CoM).  Individual parts do not feel gravity variations, so there are no tidal forces within a single ship.  This is why you can't use gravity-gradient stabilization for space stations to keep them always facing the same side down at the planet (like the ISS does).

However, if you tether two craft together with a KAS winch, the two craft do experience the gravity gradient and you can tidally stabilize them.  I played around with this a while back.

Edited October 24, 2016 by RoboRay

[Shpaget]

10 minutes ago, RoboRay said:

No.  In the real world if you go deep below the surface, you will be pulled upward by the gravitational attraction of the portion of the planet's mass that's above you.  If were able to make a hollow cave at the center of the planet, you'd be able to float in zero gee right in the middle, being pulled equally in all directions.  Once you move slightly out of the center, you'll be pulled gently toward that side, away from the center.  If you made the cave big enough, you could walk around on the walls of the cave as you would feel slightly more pull "upward" because you're closer to that mass than to the mass on the opposite side of the cave.

Actually, anywhere in a uniformly massive shell (hollowed out planet) there is exactly zero g. While it is true that if you drift to one side you get closer to some part, there is more mass on the other side. The resulting force happens to be 0 for any point inside the shell.

Edited October 24, 2016 by Shpaget

[Kerbart]

5 hours ago, Choctofliatrio2.0 said:

So there's a black hole in the center of every planet?

Aside from semantics about singularities... modeled planets have all their mass concentrated in a single point in the center. Real planets do not. There would actually be no gravity in the center of a planet as the gravitational forces pulling on you from all directions would cancel each other out.

[RoboRay]

Just now, Shpaget said:

Actually, anywhere in a uniformly massive shell (hollowed out planet) there is exactly zero g. While it is true that if you drift to one side you get closer to some part, there is more mass on the other side. The resulting force happens to be 0 for any poinr inside the shell.

Oh, right...  I was only thinking linearly.  You're absolutely correct about the spherical effects.

[Martian Emigrant]

2 hours ago, Shpaget said:

Actually, anywhere in a uniformly massive shell (hollowed out planet) there is exactly zero g. While it is true that if you drift to one side you get closer to some part, there is more mass on the other side. The resulting force happens to be 0 for any point inside the shell.

You mean to say that Pellucidar doesn't exist and I can't go there anymore?

 

ME

Edited October 25, 2016 by Martian Emigrant

[Jonfliesgoats]

What about a massless fake planet at those Lagrange points?  0g would exist all radii from the SOI com.  Those fake Lagrange points could move on rails, like other planets.

The problem with this can be observed in the Jool system.  Imagine how you would have to manipulate Lagrange points around the many moons of Jool!  The idea of having Lagrange points in Kerbin but not in the Jool system isn't acceptable since the Lagrange points in Jool would be fantastically useful for developing Joolian infrastructure.  

Having exploitable lagrange points in one system and not the other wood feel strange.  Having fixed Lagrange points in Jool where nearby moons magically don't perturb your orbit would feel strange too.

If KSP 2.0 ever gave physics to celestials, then we'd have good Lagrange points.  Current computers would turn into puffs of purple smoke, though.

[stenole]

You can have parking orbits in KSP as it is now. And you can do it at any point of the orbit of the planet outside the planet's SOI. You just have to match the planets orbit perfectly. This synchronicity will eventually fail with timewarp. The same is also true for Lagrange points in the real world. A ship will get perturbed by other bodies in the solar system and the ship will start floating away. The Lagrange point is not really stable so the drift will eventually increase over time. In order to get a ship to be properly parked at a Lagrange point, it needs to adjust its position using thrusters regularly.

This active park mode could be implemented in KSP right now with more elegance than it would be to create fake N-body physics using special SOIs with different gravity rules.