egg changed the topic of #principia to: READ THE FAQ: http://goo.gl/gMZF9H; The current version is Christoffel. We currently target 1.2.2, and 1.3.1. <scott_manley> anyone that doubts the wisdom of retrograde bop needs to get the hell out | https://xkcd.com/323/
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<GregroxMun>
hi
<egg>
hi
<GregroxMun>
so I'm trying to add axial tilt to Alternis Kerbol.
<egg>
I've been looking at TRAPPIST lately
<egg>
I might come up with a principia config at some point
<GregroxMun>
I've looked at gravity_model.cfg for the axial tilt, do you have to define GM, reference instant, radius, reference angle, and angular frequency? Or can I just do Pole RA and Dec?
<GregroxMun>
I do find using PoleRA and Dec kind of confusing.
<GregroxMun>
Is there a way of converting the plane of a planet's orbit to the perpindicular axis RA and Dec?
<GregroxMun>
so for instance, a planet with 0 axial tilt for any given inclination I can find the tilt to give it
<GregroxMun>
So far I've just tried to steal the tilt and the inclination+LAN from RSS
<egg>
I think I told you already that you do not need to define all of those, only RA and dec, if you want to keep the ones from the game
<GregroxMun>
alright thanks
<GregroxMun>
well it is not working
<egg>
as for RA/dec of the normal to a given orbit, ask UmbralRaptor
<egg>
how is it not working
<GregroxMun>
in-game, all planets have the default tilt.
<egg>
anyway, would you be interesting in making a TRAPPIST-1 mod (essentially a mod with your slippist models and a principia TRAPPIST-1 configuration) once I get around to computing that
<GregroxMun>
Possibly
<GregroxMun>
It could be done by just increasing the size and mass of the planets in Seven Worlds of Slippist One.
<GregroxMun>
actually I still need to update that mod for newer versions, I think it broke at some point.
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<egg>
GregroxMun: well you still need a proper principia config
<egg>
remember you tried your mod with principia and it had the wrong periods everywhere
<GregroxMun>
oh yeah
<egg>
you need a correct config if you want to have the correct transits at the correct periods
<GregroxMun>
Right Ascension = Longitude of Ascending Node - 90 degrees
<egg>
;rpn 281.0097 90 -
<kmath_>
egg: 191.0097
<egg>
um sorry
<egg>
;rpn 281.0097 90 +
<kmath_>
egg: 371.0097
<egg>
;rpn 281.0097 90 + 360 -
<kmath_>
egg: 11.00970000000001
<egg>
kinda
<egg>
the thing has some tilt so close enough
<egg>
okay so you have your formula for tiltless then :D
<GregroxMun>
yes
<GregroxMun>
and I'm good with that
<GregroxMun>
you know what I'm not really
<GregroxMun>
I think it's fair to assume that if you have a planet with x,y axial tilt in a 0 inclination orbit, its axial tilt will be x+dec, y+ra, where dec and ra is the normal of the nonzero inclination orbit.
<GregroxMun>
So say I just want to say "the planet has a tilt of 20 degrees from its zero-inclination orbital plane"
<GregroxMun>
and I want to make RA and Dec values for that axial tilt
<GregroxMun>
I would assume it should be RA^2 + Dec^2 = Tilt^2
<GregroxMun>
but I don't know if the nature of spheres would force trig functions into it.
<GregroxMun>
or would it just be the average of the angles (RA+Dec) / 2
<GregroxMun>
no it cant be that
<egg>
it definitely isn't that; put 20 deg into declination, or if you want to do fancier things, try it and have a look at it
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<GregroxMun>
the trajectory that Bop follows once the orbit fails looks nothing like how it should fall in the real world. Here it is being pulled and pushed around wildly. I'm not talking about the orbit line, but the actual path Bop takes through space.
<GregroxMun>
and also, this happens with a setup as simple as two binary planets orbiting the sun at a far distance (but only if they're close together)
<GregroxMun>
by two binary planets, I mean two worlds orbiting each other, not two pairs of planets.
<egg>
GregroxMun: send me the solar system and I'll take a look, but it looks mostly unstable tbh
<GregroxMun>
I know I've gotten this system to be stable in the past. Bop at least.
<GregroxMun>
But I need some clarification--should principia accurately portray the results of an unstable orbit?
<GregroxMun>
not in the orbit line, but in the paths that the bodies follow
<GregroxMun>
because if it's supposed to do that, then it looks like its failing.
<egg>
it's hard to tell whether it's accurate, multibody trajectories get *weird*
<egg>
and to answer your question, depends how unstable, if it goes *really* haywire the state of the multistep integrator can get corrupted
<GregroxMun>
it looks to me like Bop goes almost immediately from "stable circular orbit" to "haywire" within 5 hours.
<GregroxMun>
there's no in between where it follows a realistic looking trajectory
<egg>
it's a high loosely-bound orbit, doesn't take much boosting by the central planet to kick it?
<egg>
I mean if you want me too look at it send me the system and I'll try with different integrators to see if there's an issue there
<GregroxMun>
is it possible then that the reason Minmus and Bop failed isn't instability, but proximity to their parent?
<GregroxMun>
also, there was no error reporting like there was with the 1,200,000m sma test
<egg>
the linear multistep method has a 10 min timestep
<GregroxMun>
what does that mean exactly
<GregroxMun>
in terms of how it affects modders using principia
<GregroxMun>
does it mean the period can't be too short? How short can it get?
<egg>
I don't know?
<egg>
try not to be shorter than Io's period
<egg>
because then you're in uncharted territory, we test Io though
<egg>
actually even phobos should be fine
<egg>
kinda
<GregroxMun>
is it possible to change the timestep?
<egg>
yes, if you build your own principia
<egg>
(no)
<egg>
GregroxMun: uh I get an apocalypse on your system
<GregroxMun>
what happens exactly?
<egg>
yeah mun and laythe collide
<UmbralRaptor>
Unauthorized high precision Principia distribution with a 1 second timestep.
<egg>
didn't you see the warning message?
<GregroxMun>
that happens if the principia patch doesn't load for some reason
<GregroxMun>
what date is the collision at?
<GregroxMun>
but I dont know why the oh
<GregroxMun>
oh
<GregroxMun>
oh
<GregroxMun>
I left a @semiMajorAxis *= 2 in the config
<GregroxMun>
on Laythe in the principia patch
<GregroxMun>
go to AlternisKerbolRekerjiggered/ModCompatibility/PrincipiaCompatibility.cfg and comment out the line @semiMajorAxis *= 2 on line 15
<GregroxMun>
or delete the line
<GregroxMun>
egg ^
<egg>
yeah yeah got it, compiling principia with the Blanes and Moan 2002 SRKN11B right now
<GregroxMun>
after correcting that line the system, aside from Minmus and Bop, is stable for at least 160 days with no sign of change. This is unsurprising really because this should be identical to Laythe, Vall, and Tylo with the resonance breaker patch in Principia
<GregroxMun>
and I know in the past Bop at least has been shown to be stable
<GregroxMun>
and Minmus too IIRC.
<GregroxMun>
So I suppose that means the problem must be with the 10 second timestep
<egg>
well, it's with the switch from 45 min timestep on McLachlan & Atela 1995 order 5 optimal to Quinlan and Tremaine whichever with a 10 min timestep
<egg>
lower timestep, but a symmetric linear multistep integrator instead of a SPRK/SRKN
<egg>
UmbralRaptor: Quinlan! Tremaine!
<egg>
astronomers everywhere \o/
<GregroxMun>
I understand most of those words individually
<egg>
well google sequences of them while I wait for the thing to compile, you'll learn things
<GregroxMun>
ok so, and I suspect this is related to their inclination, this happens
<GregroxMun>
I'm gonna try it with the default (<1 deg) inclination
<GregroxMun>
happens to a lesser extent with Tylo's two moons
<egg>
GregroxMun: welp tried it with Blanes & Moan 2002's SRKN11B, and it looks stable (at least for 58 6-hour days)
<egg>
GregroxMun: you know what I'll just add an option to choose your integrator and timestep in the gravity models config if your system is too wonky for the default
<GregroxMun>
I like that idea
<GregroxMun>
thank you
<egg>
I'm surprised that Phobos behaves semi-sanely but not this bop
<egg>
too close to chaos, confusing the SLMS?
<egg>
I'm starting to think that SLMS methods are a bit dangerous :-p