Developer Insights #4

[The Aziz]

29 minutes ago, KerikBalm said:

La grange points, really?

This interests me

One. one point. Somewhere in the gravitational middle between these two planets

[KerikBalm]

Quote

We’re enhancing our physics simulation above and beyond the original Kerbal Space Program to account for some more complicated orbital dynamics. One example that has already been shown in the trailer are the binary planets Rask and Rusk, which orbit each other. One approach to simulating this would be to have an invisible gravitational center point between the two worlds, but this would make orbiting Rask and Rusk just like orbiting any other body, with slightly different parameters for collision, and the side effect that ships would be drawn to the barycenter between the two bodies. We’re aiming for a higher degree of realism. Instead, in the case of Rask and Rusk, we’ll be calculating the gravitational pull of multiple bodies on our Kerbal vessels, so that developing a stable orbit in complex conditions like a binary planet system becomes a new and exciting challenge! In addition, attempting a landing on Rask or Rusk will be a different experience depending on the location of the sister planet in relation to your target for touchdown, and yes, there will be an astable Lagrage point between the two planets (if we pull this off correctly). Full system n-body gravity is, of course, not planned for KSP2, as it would be overly compute intensive and also require complex station keeping on all vessels in orbit that, we feel, distracts from the fun of the game

10 hours ago, The Aziz said:

One. one point. Somewhere in the gravitational middle between these two planets

Well, they only mention one point which seems like it would be L1, but the way they describe it, there should be all 5.

 

[Incarnation of Chaos]

1 hour ago, KerikBalm said:

Well, they only mention one point which seems like it would be L1, but the way they describe it, there should be all 5.

 

How do you get that out of "Full-System n-body gravity is not planned for KSP2"?

To me it sounds like they're using it as a workaround to prevent the barycenter from becoming a singularity and not much else, multiple points would imply that a larger number of elements in the system would be part of the simulation since they'd have to be calculated to get the forces to cancel at those other points.

Overall; i think we just need to sit tight and wait for more information. Because this is pretty interesting as far as i'm concerned, and breaks with many of our previously thought solutions.

[KerikBalm]

Quote

we’ll be calculating the gravitational pull of multiple bodies on our Kerbal vessels, so that developing a stable orbit in complex conditions like a binary planet system becomes a new and exciting challenge! In addition, attempting a landing on Rask or Rusk will be a different experience depending on the location of the sister planet in relation to your target for touchdown, 

7 minutes ago, Incarnation of Chaos said:

How do you get that out of "Full-System n-body gravity is not planned for KSP2"?

To me it sounds like they're using it as a workaround to prevent the barycenter from becoming a singularity and not much else, multiple points would imply that a larger number of elements in the system would be part of the simulation since they'd have to be calculated to get the forces to cancel at those other points.

L1-5 come from only 2 bodies. You can't get one without the others, unless they are doing something really weird, and they don't truly even get one.

They said they would have pull from multiple bodies, and their statement about landings being different depending on where the other planet is, implies that the effects are not limited to some pseudo SOI between the planets.

If they had ships simply orbiting a barycenter with a covered/non-naked singularity, it would be stable, not unstable like they say.

My guess is that they will mostly use patched conics, but within certain SOIs the ships trajectories will be subject to 2 to n body gravitation, while the planets themselves stay on "conic section rails"

 

[The Aziz]

I say the bodies will still be on rails, but in that rare occasion, you know...

[Incarnation of Chaos]

3 minutes ago, KerikBalm said:

L1-5 come from only 2 bodies. You can't get one without the others, unless they are doing something really weird, and they don't truly even get one.

They said they would have pull from multiple bodies, and their statement about landings being different depending on where the other planet is, implies that the effects are not limited to some pseudo SOI between the planets.

If they had ships simply orbiting a barycenter with a covered/non-naked singularity, it would be stable, not unstable like they say.

My guess is that they will mostly use patched conics, but within certain SOIs the ships trajectories will be subject to 2 to n body gravitation, while the planets themselves stay on "conic section rails"

 

Alright; i need to take some physics apparently. 

Though they could just have a seperate layer on each planet to simulate "Bumpy" gravity; there's a mod for KSP that does that with principia installed. So they wouldn't need to have more than 2 planets affecting the ship in theory. This would also make more sense for rask and rusk; since with them being partially molten you could have massive local variations in the density from place to place causing local gravitational anomalies.

Also when i said "Singularity" i meant it more in a mathematical sense than something like a black hole; since i was thinking of the massive unintended accelerations you could get from a patched conics workaround for a barycenter. But this does actually seem like it would allow them to keep them on rails.

[KerikBalm]

3 hours ago, Incarnation of Chaos said:

Alright; i need to take some physics apparently. 

FYI, these are the 5 points, in this case from just 2 massas, the sun and Earth

 

[Tonas1997]

This whole discussion reminded me of this section from the Principa FAQ:

Quote

I'd like the planets to follow their stock orbits

This would break physics. As an example, if planets were to do that, you would not get Lagrange points. It is an interesting exercise to compute the sum of the centrifugal and gravitational potentials for a body orbiting the centre of another (rather than their barycentre) in the reference frame that fixes both bodies and the orbital plane, and computing its gradient. It is easily seen that this gradient does not vanish in 5 points, but in only 3 instead.

So no, apparently if you want the full set of Lagrange points between two bodies you can't have them on rails.

Edited June 15, 2020 by Tonas1997

[KerikBalm]

Yes you can, but they have to orbit the barycenter, which isn't the stock way.

You could put them on rails orbiting a barycenter

[K^2]

4 hours ago, Tonas1997 said:

So no, apparently if you want the full set of Lagrange points between two bodies you can't have them on rails.

This is because of how KSP handles on-rail moons. KSP requires all planets to orbit the sun and all moons to orbit centers of planets. That's easy to fix in KSP2, with both planets in a double-planet system orbiting barycenter instead. Barycenter can orbit the star just like a planet would in KSP, but without having a heavenly body to correspond to it.

So yeah, we can easily have all possible Lagrange points in the Rask/Rusk system with both planets still being entirely on-rail. Whether or not SOI will be big enough to contain all of them is a separate question, but if they make SOI at least as big as system's Hill Sphere, and the system is tightly bound, I'm pretty sure we'll at least get L2 and L3 in addition to L1.

[Bill Phil]

Lagrange Points like in the Earth-Moon and the Sun-Jupiter systems are stable because of the enormous difference in mass between the primary and its satellite (specifically L4 and L5). If Rask and Rusk are within an order of magnitude in mass, there shouldn't be any stable Lagrange points.

Edited June 15, 2020 by Bill Phil

[mcwaffles2003]

53 minutes ago, Bill Phil said:

Lagrange Points like in the Earth-Moon and the Sun-Jupiter systems are stable because of the enormous difference in mass between the primary and its satellite (specifically L4 and L5). If Rask and Rusk are within an order of magnitude in mass, there shouldn't be any stable Lagrange points.

The L4 and L5 Lagrange points wouldn't just be orthogonal to the line between rask and rusk at the baycenter along the plane of rotation? 

[Superfluous J]

1 hour ago, mcwaffles2003 said:

The L4 and L5 Lagrange points wouldn't just be orthogonal to the line between rask and rusk at the baycenter along the plane of rotation?

Assuming they are exactly the same mass, then yes.

I'd need to actually do the math but I believe it's because (or at least related to) that (perfectly) twin worlds orbit each other such that "60 degrees east/west" on one world's orbit is actually also "60 degrees west/east" on the other world's orbit.

(60 degrees East and West on the orbit is where those Lagrange points are, BTW. Important point about them that I skipped over above)

[Bill Phil]

1 hour ago, mcwaffles2003 said:

The L4 and L5 Lagrange points wouldn't just be orthogonal to the line between rask and rusk at the baycenter along the plane of rotation? 

They might exist, but they would be unstable.

[K^2]

20 hours ago, Bill Phil said:

Lagrange Points like in the Earth-Moon and the Sun-Jupiter systems are stable because of the enormous difference in mass between the primary and its satellite (specifically L4 and L5). If Rask and Rusk are within an order of magnitude in mass, there shouldn't be any stable Lagrange points.

The L4/L5 will be unstable, yeah, but L1-L3 should still have dynamically stable-ish Lissajous orbits.

Edit: Revised. L4/L5 likely to simply not even be there.

Edited June 16, 2020 by K^2

[KerikBalm]

8 hours ago, K^2 said:

The L4/L5 will be unstable, yeah, but L1-L3 should still have dynamically stable-ish Lissajous orbits.

L1-3 are also unstable

11 hours ago, Bill Phil said:

Lagrange Points like in the Earth-Moon and the Sun-Jupiter systems are stable because of the enormous difference in mass between the primary and its satellite (specifically L4 and L5). If Rask and Rusk are within an order of magnitude in mass, there shouldn't be any stable Lagrange points.

Indeed, but since L1,2, and 3 aren't stable, but are still usefull, then L4 and L5 should still be usefull as well

According to wikipedia, the ratio between the primary mass (such as the Sun) and the secondary mass (such as the Earth) needs to be greater than 25:1. If we have yet another massive body (call it the tertiary mass) at the L4/5 point, then the secondary to tertirary body needs a mass ratio of >10:1, from what I read on the Theia impact hypothesis.

17 hours ago, Tonas1997 said:

This whole discussion reminded me of this section from the Principa FAQ:

So no, apparently if you want the full set of Lagrange points between two bodies you can't have them on rails.

Here is a diagram tht helps to explain where the L4 and L5 points are:

That its pulled to the barycenter (Earth+Moon mass tugging on it), and not the Earth, is what allows it to be farther from the Earth than the Moon is, and still have the same orbital period as the moon around the Earth. With the bodies on rails, you just need to have them on rails aroun the barycenter, and it will work out again.

Stock bodies don't have this behavior, but they could. 

 

 

 

[Xurkitree]

N BODY HYPE TRAIN RISE UP

ik its just for the system, but im so glad that they're attempting to make something really good.

Edited June 16, 2020 by Xurkitree

[K^2]

7 hours ago, KerikBalm said:

L1-3 are also unstable

[...]

Indeed, but since L1,2, and 3 aren't stable, but are still usefull, then L4 and L5 should still be usefull as well

I really think you are missing the point about quasi-stability, where it comes from, or how it's useful. L1-L3 are NEVER stable, but they ALWAYS exist and ALWAYS have quasi-stable orbits around them. L4 and L5 exist SOMETIMES and can be entirely stable for large enough mass difference. For two bodies of equal masses that are orbiting each other, there is no such thing as L4 and L5 at all, stable or otherwise.

So long as N-body simulation is accurate, we should have a quasi-stable point at L1 between Rask and Rusk that we can use for satellites that require minimal station-keeping.

So long as N-body simulation is accurate and SoI extends far enough, we will also have L2 and L3 to which all of the above applies.

Given what we know about Rask and Rusk, there will not be an L4 and L5 at all, because masses of two planets will probably be too similar.

[Guest]

I like what they're saying about Rask and Rusk. Sounds like a good compromise between realism and playability.

[Lewie]

Personally, I don’t care about all that fancy mumbo jumbo y’all are talking about....it always amazes me how smart you guys are!

I just can’t wait to go to Rask and Rusk....they look really cool...

[Bej Kerman]

47 minutes ago, Lewie said:

Personally, I don’t care about all that fancy mumbo jumbo y’all are talking about....it always amazes me how smart you guys are!

I just can’t wait to go to Rask and Rusk....they look really cool...

I wonder what the cracks and lava looks like up close...

[KerikBalm]

1 hour ago, K^2 said:

I really think you are missing the point about quasi-stability, where it comes from, or how it's useful. L1-L3 are NEVER stable, but they ALWAYS exist and ALWAYS have quasi-stable orbits around them. L4 and L5 exist SOMETIMES and can be entirely stable for large enough mass difference. For two bodies of equal masses that are orbiting each other, there is no such thing as L4 and L5 at all, stable or otherwise.

Umm... they still exist, in the case of them being equally massive, you have a point orbiting the barycenter, equidistant from both.

[Lewie]

40 minutes ago, Bej Kerman said:

I wonder what the cracks and lava looks like up close...

I think a couple kerbals might find them selves racing over those....