Although physics are executed in world space, you can get the forward vector of a spatial node like this:
var forward = -get_transform().basis.z
Or, if you are unsure:
var forward = -get_global_transform().basis.z
Then you can use it to apply a force.
Note: I wrote a
- in front of the Z axis, because the engine uses the OpenGL axis convention, so for example a camera looks at -Z instead of +Z, just FYI