Question Locking rigid body rotation doesn't seem to work.

Hello,

I've got a head scratching problem that i can't find a solution for.

I have a top-down spacehip shooter (like retro asteroids type thing) with a little spaceship that rotates clockwise/anticlockwise on the Z axis. I have the X and Y axis locked (little tick box on rigid body) but often, after colliding with an asteroid the ship is pushed off axis and indeed rotates on either X and Y , or bit of both.
Is there a way to truly lock these axis?

Many thanks.

J.