UmbralRaptop changed the topic of #principia to: READ THE FAQ: http://goo.gl/gMZF9H; The current version is Fano. We currently target 1.3.1, 1.4.x, 1.5.1, and 1.6.1. <scott_manley> anyone that doubts the wisdom of retrograde bop needs to get the hell out | https://xkcd.com/323/ | <egg> calculating the influence of lamont on Pluto is a bit silly…
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[Principia] josehaley forked the repository - https://git.io/fjZZn
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RiemannianRaptor is now known as SingularRaptor
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[Principia] pleroy opened pull request #2149: Checkpointing done right - https://git.io/fjZu4
<Gregrox>
you and your silly mod have forced me to make drastic changes to my planet mod >:v
<Gregrox>
(I only feign anger)
<Gregrox>
(I do not feign annoyance, but it's annoyance at myself for not noticing the problems earlier)
<Gregrox>
So here's a problem. I have a planet with moons that have exactly the same semimajoraxis ratios and mass ratios as the moons of Pluto (with the phase of the three resonant moons also set up correctly), and yet it seems to be unstable. Is there anything very obviously wrong with that setup?
<Gregrox>
that I missed
<Gregrox>
It's the double-gas giant Mandrake/Rutherford
<Gregrox>
Mandrake and Pluto's masses are both taken to be "1" and Rutherford, Lozon, Beagrid, Jancy, and Tatian have the same masses relative to Mandrake as Charon, Styx, Nix, Kerberos, and Hydra have relative to Pluto. In addition, the ratio between all five of Mandrake's moon's orbital periods are the same as the ratio between the periods of Pluto's five
<Gregrox>
moons.
<Gregrox>
Three of the four little moons are in resonance with each other. I have also duplicated the initial phase angles.
<Gregrox>
egg
<Gregrox>
egg|laptop|egg_
<egg|laptop|egg_>
setting up a resonance is hard, because the osculating elements won't really look like they are resonant if they are
<egg|laptop|egg_>
(again, osculating elements behave weird)
<Gregrox>
osculating elements?
<egg|laptop|egg_>
the orbital elements you give
<Gregrox>
mhm
<egg|laptop|egg_>
they're not the *actual* period or semimajor axis or anything
<Gregrox>
right
<Gregrox>
But is there a way to fix that, and is it a straightforward way
<egg|laptop|egg_>
lolnope
<Gregrox>
to the first or the second?
<egg|laptop|egg_>
well it's not straightforward in any case
<Gregrox>
To be clear I mean to fix my system, not fix the general problem of the elements not being quite right.
<egg|laptop|egg_>
yeah but to properly set up a resonance, I don't see anything easy
<Gregrox>
dang.
<Gregrox>
Well doesn't Pluto and its moons work in RSS+Principia?
<Gregrox>
Can those properties be stolen and rejiggered to work?
<egg|laptop|egg_>
it's not defined that way in RSS+Principia, it's defined from cartesian elements. Also we only have Pluton + Charon
<Gregrox>
dang
<Gregrox>
So there's another problem
<Gregrox>
I have a trojan moon that isn't working right.
<Gregrox>
I figured maybe it has something to do with the way I did the orbits, let me explain
<Gregrox>
The planet is Mesbin, the moon is Kerbmun, and the trojan moon is Troymin.
<Gregrox>
I gave Kerbmun and Troymin almost the same semimajoraxis, and they are in fact 60 degrees apart. But Kerbmun's orbit is a little higher because its mass is greater, and I use Kopernicus' relatively new orbital period feature to account for this and keep it orbiting with the exact same period as Troymin.
<Gregrox>
I wonder if this is problematic when putting that into Principia.
<egg|laptop|egg_>
maybe?
<egg|laptop|egg_>
how would I know :-p
<Gregrox>
you're supposed to be the expert on these things :v
<egg|laptop|egg_>
what are the mass ratios too
<egg|laptop|egg_>
yeah but if I don't see it I'm not going to figure out what's going on from tea leaves
<Gregrox>
Mesbin is 71 Mkerbin, Kerbmun is 1 Mkerbin, and Troymin is uh
<Gregrox>
negligible
<Gregrox>
it's a few km across
<egg|laptop|egg_>
okay so the masses are reasonable at least
<lamont>
that’s the branch of mathematics which deals with the question that you’re asking. unfortunately all i really know about it is that’s the thing you need to learn.
<Gregrox>
After running simulations with a series of varied orbits I'm becoming convinced that for some reason, Troymin is genuinely unstable as a Kerbmun trojan.
<Gregrox>
I haven't the faintest clue why
<Gregrox>
OK so I think it is actually a Principia problem, because in Universe Sandbox 2, even after 8 years, the system is fine.
<Gregrox>
Troymin oscillates a bit
<Gregrox>
the thing is, why would how I define orbits be different between US2 and Principia and why does that make a stability difference.
<Gregrox>
the difference is night and day--troymin is usually thrown out after a month or so in Principia whereas it shows no sign of falling apart after years in US2.
<egg|laptop|egg_>
which one has the smaller semimajor axis between the trojan and the big body
<egg|laptop|egg_>
Gregrox: ^
<Gregrox>
the trojan
<egg|laptop|egg_>
hm
<Gregrox>
I double checked--the periods are the same accounting for the extra mass of kerbmun
<Gregrox>
is it possible that troymin thinks kerbmun's mass should be added to Mesbin's?
<egg|laptop|egg_>
yeah but you don't want to account for that mass
<Gregrox>
why not
<Gregrox>
(keeping in mind that if the sma are the same, it falls apart somewhat quicker!)
<egg|laptop|egg_>
because it's clearly orbiting the primary in this situation as a first approximation, not far away from both
<Gregrox>
oh yeah I'm not accounting for mesbin+kerbmun for TROYMIN's orbit
<Gregrox>
I'm accounting for Mesbin+Kerbmun for KERBMUN's orbit.
<egg|laptop|egg_>
yeah because of the way principia interprets osculating elements the behaviour will change between slightly smaller and slightly larger sma
<Gregrox>
so what should I do then?
<egg|laptop|egg_>
just to check, these orbits are all reasonably slow, right?
<Gregrox>
period of something like 30 hours iirc
<egg|laptop|egg_>
for the inner two?
<Gregrox>
30 hours for Kerbmun and Troymin, but 28 minutes, 1.4 hours, and about 2.5 hours for Statmun, Thresomin, and Graymun.
<Gregrox>
and at the current integrator settings I have no trouble with those orbits
<Gregrox>
statmun and thresomin have negligible masses, graymun's mass is about 2 * Luna.
<egg|laptop|egg_>
and derbin
<Gregrox>
to be clear, I really meant 2 * 0.01 * Luna.
<Gregrox>
Derbin is about 8 * 0.01 * Earth
<egg|laptop|egg_>
... yeah that would screw with things
<egg|laptop|egg_>
what happens if you drop derbin
<Gregrox>
as I said before, the instability still happens, and at about the same time
<egg|laptop|egg_>
interesting
<egg|laptop|egg_>
and if you drop graymun?
<Gregrox>
I haven't tried that, but I guess it's worth a shot.
<Gregrox>
well
<Gregrox>
keeping in mind that both those moons are present in the US2 sim that did work.
<Gregrox>
I'll remove all the moons but those two and see what happens
<egg|laptop|egg_>
yes but the US2 sim doesn't have the same initial state
<egg|laptop|egg_>
in particular, assuming they interpret the initial elements as primary-centred osculating, since we interpret them as Jacobi, an inner moon will significantly change things
<kmath>
YouTube - Kerbmun and Troymin, no other moons.
<Gregrox>
egg|laptop|egg_
<Gregrox>
no change
<Gregrox>
still destabilizes around fortnight 270 (a fortnight is 14 * 1701 seconds or 6.6 hours, roughly equivalent to a kerbin day)
<egg|laptop|egg_>
okay so, keeping this state with just three bodies and no perturbators, try decreasing troymin's semimajor axis a bit
<Gregrox>
how much is "a bit"
<egg|laptop|egg_>
hey, I'm not the one who's trying to optimize an initial state by hand :-p
<Gregrox>
to be clear: troymin's orbit is already lower than kerbmun's--you want me to make it go lower still, right?
<egg|laptop|egg_>
yes
<Gregrox>
I guess I'
<Gregrox>
I'll double that difference
<Gregrox>
principia will just ignore the kopernicus period, right?
<egg|laptop|egg_>
Principia doesn't know about Kopernicus at all
<egg|laptop|egg_>
what happens is that Kopernicus sets the orbital elements of the planet
<egg|laptop|egg_>
if you tell Kopernicus to set a period, it will tweak the semimajor axis accordingly, and I'll see that
<egg|laptop|egg_>
if you tell Kopernicus both a period and a semimajor axis and they're not consistent, I don't know what happens so don't specify both, it's silly
<Gregrox>
kopernicus period is irrespective of sma
<Gregrox>
you specify both so you can do things like have a heavy body have a faster orbit at the same sma
<Gregrox>
i believe finalizeOrbit also does this
<Gregrox>
but
<Gregrox>
I did try it without defining the period, and it didn't seem to make a difference.
<Gregrox>
also doubling the difference in altitudes made the troypocalypse happen earlier.
<Gregrox>
about 190 fortnights in
<Gregrox>
At a certain point I feel like just giving up and making Troymin a Kerbmun moon.
<Gregrox>
>:v
<Gregrox>
As far as I'm concerned, I have now made sure that WW is realistic.
<Gregrox>
the one thing that was definitely a realism problem and not principia-specific has been fixed
<Gregrox>
the other two problems are proven plausible.
<Gregrox>
so I'm just gonna leave them the way they are in the stock physics and cheat to make them work in principia