O-10 MonoPropellant Engine/RCS blocks - PhysicsSignificance = 1? What's going on here

[SciMan]

After installing 0.24, one of the first things I did was look at the new parts in a new sandbox save.

As soon as I saw the new O-10 MonoPropellant Engine I was very confused because it's so much bigger than it was in Scott Manley's 0.24 preview video that showcased the new parts. So I took a look at the config, and as I suspected, it's been rescaled.

However, I found a single line in the config file that confused me even more than the part's re-scale.

"PhysicsSignificance = 1" Putting this line into a part's config causes the game to treat it as having zero mass, and disables physics calculations on it.

Calculating physics on a very small structural part can be buggy, so putting that line on the cubic octagonal strut for example, makes perfect sense.

However, those parts don't change mass or apply forces to the craft they're attached to, so disabling physics only changes the craft's effective mass. Not a big deal with very low mass parts.

Fuel tanks change mass as their contents are drained/refilled. No fuel tanks in the game that I know of have disabled physics.

Engines and RCS thrusters apply force to the craft they're attached to. There are 3 RCS thruster parts and 1 engine part that have disabled physics.

The parts are:

O-10 MonoPropellant Engine (New part)

RV-105 RCS Thruster Quad

PA-7 Linear RCS Port

VR-N1ER "Vernor" RCS thruster. (New part)

The O-10 engine's mass is stated as 0.03 in it's config. The LV-1/-1R has the same mass, but doesn't have disabled physics.

The PA-7 Linear RCS and RV-105 RCS quad both have masses of 0.05 according to their configs. That's more than the LV-1/-1R.

The Vernor thruster has a mass of 0.08 in its config. That's only 0.01 away from the mass of the Rockomax 24-77.

The decision to disable physics calculation on these parts is problematic for at least two reasons.

Firstly, simple math tells us that parts that have Zero mass and can produce thrust, will have a functionally infinite thrust-to-mass ratio.

For: Thrust = X, Mass = 0, TWR = Thrust-to-Mass ratio

Thrust / Mass = TWR

X/0 = "Undefined", "NaN", "+or-Infinity depending on sign of X"

Infinite thrust for zero mass penalty? That's impossible to balance without changing the "for zero mass" part.

Secondly, I wouldn't be surprised if the game engine started acting strangely in some way if it has to deal with a part that doesn't have any mass. AFAIK the symptoms could be anything from crashes to Kracken attacks to phantom forces, and anything in between. Nothing in the VAB or SPH, however. Only when game physics is running.

(the smallest Solar panels and the small radial battery shouldn't have physics disabled either, but that's a less severe issue)

I'm not sure if this is a bug or not, so I put it here where it might get some attention from those who know more than me about why it's considered to be OK to disable physics on these parts. If I put it in the wrong place, feel free to move it.