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[komatii]

I've got what is essentially a binary planetary system, although done as a very large moon orbiting the primary planet rather than using the binary orbit tool.
However I can't manage add any other moons that will maintain an orbit.

I go to the add tool with my main planet selected, select moons, and select any of the default moons [generally ones around 300-3000km in diameter].
When I place the mouse around the planet it displays the moon with the short trail indicating where it will travel, the arc looks correct for a circular orbit around my planet at the given distance. But after placing the moon, when I hit play they aren't even close to orbiting. They either quickly degrade and get suck into a collision or near collision with the planet, or take a weird path of a very light arc with repeated humps of approx 45-80 degrees.
  If I select them and click auto-orbit, it adjusts their parameters but they still do pretty much the same thing and don't orbit the parent planet.
  Time step is the default 2 minutes. My main planet is about 1.4 Earths mass, and the large moon is about 1.02 Earths in a close (53,000km) 21.6 hour orbit. It's orbit is completely stable.

komatii

[komatii]

Some other info...

On my large moon, if I grab the "mean" slider and move it back and forth it changes position along it's orbital path.
But on any of the other moons, when I move the "mean" slider they generally move along an expected orbital path until the slider gets close to 0 degrees, and which point the semi-major axis value jumps to NaN and the other sliders go grey with no slider control drawn.

[mudkipz]

generally, close orbits are very unstable because the planets interact with each other. Look at our solar system. the earth is almost a hundred million miles away from the sun? do you have that? if you have something larger than the earth, an even larger space maybe required for stable orbits

"O" is the auto orbit key btw, and it won't work if you have all the planets too close, its just not possible

[komatii]

It happens if I put a tiny moon at the maximum distance it can be and still maintain an orbit.
If I delete my extremely large moon [planet is 1.4x earth mass, large moon is 1.02 in a fairly close orbit as I'm trying to simulate a system where the larger planet is very large when viewed in the sky from the smaller one] then new moons all behave exactly as expected.

komatii

[mudkipz]

yes
close orbits are inherently less stable
the moons affect each other

pressing O
will help

[FiahOwl]

generally, close orbits are very unstable because the planets interact with each other. Look at our solar system. the earth is almost a hundred million miles away from the sun? do you have that? if you have something larger than the earth, an even larger space maybe required for stable orbits

"O" is the auto orbit key btw, and it won't work if you have all the planets too close, its just not possible

Error, about 90 million miles away.

[mudkipz]

approximation...
to the nearest 100m

no actually the it's 92,955,807 miles

[Dan Dixon]

Auto-orbit doesn't work very well with binary orbits as it only considers the influence of a single large body.

Having that other large moon makes any other nearby orbits inherently unstable.

Can you save an upload the simulation so we can see where you're wanting to place the smaller bodies?