"Physicsless" parts and CoM display

[pincushionman]

It is my understanding that several parts, including landing gear and some science instruments, are "physicsless" and do not contribute to the mass or drag of the spacecraft.

However, this does not seem to be reflected in the build scenes (VAB and SPH). When I drag landing gear across the surface of my vessel, the CoM indicator visibly moves along with it. This is troublesome, as if my understanding is correct, it no longer reflects the actual in-game center of mass. This could cause problems in a number of ways; if you're trying to balance some phisiced part with one that is actually phisicsless on a small vessel, your thrust vector could be all off. Or, as I seem to be having trouble with, you put the main gear of a plane where the wheels are just behind the CoM, but because the "apparent" mass point of the gear is ahead of the wheels, the displayed CoM moves forward and you end up with the wheels too far forward and the plane drops and flips back onto its tail when simulation starts (I know this also means my CoM and gear aren't actually in the ideal placement, I'm just trying to get something to work as an experiment).

Am I seeing this correctly and the displayed/actual CoM's don't match, or do they actually contribute to the CoM calculation and I'm making some other mistake which isn't as obvious to me? The displayed center of mass DOES move in the build scenes in either case.

[The_Rocketeer]

Solution 1: always add the gears last.

I believe gears are not massless when lowered, but they absolutely are when raised. This shouldn't cause you too many problems - honestly, how often do u need to balance something THAT precisely?

[kerbiloid]

"Massless parts"

http://wiki.kerbalspaceprogram.com/wiki/Massless_part

Parameter "@PhysicsSignificance" in cfg files.

Mod AddPhysics

http://forum.kerbalspaceprogram.com/threads/88645-Add-Physics

[Claw]

Am I seeing this correctly and the displayed/actual CoM's don't match, or do they actually contribute to the CoM calculation and I'm making some other mistake which isn't as obvious to me? The displayed center of mass DOES move in the build scenes in either case.

Depends on what version you are using. Massless parts are now correctly accounted for the the SPH/VAB. Landing gear shouldn't move the CoM around in the editor anymore. They used to be handled differently, but they've been set up the same as the other physicsless parts. (or unless you're using mods?)

I believe gears are not massless when lowered, but they absolutely are when raised.

Gear were never different extended or retracted. Although your solution holds true for earlier versions of KSP.

Cheers,

~Claw

[Red Iron Crown]

If you use FAR then the physicsless parts still affect aerodynamic drag, perhaps that was what The_Rocketeer was referring to as they have more drag when extended.