SpaceSim | body simulation of the solar system

I have the following scene https://rawgit.com/bicarbon8/SpaceSim/MomentumProblem/babylon.html that I've setup with the ability to rotate using the following keys:

• i = pitch down
• k = pitch up
• j = roll left
• l = roll right
• u = yaw left
• o = yaw right
• / = cancel all rotation

and to move with the following keys:

• q = move forwards
• a = move backwards
• s = strafe left
• f = strafe right
• e = move up
• d = move down
• \ = cancel all movement

which is working fine to move the mesh in a direction that it is pointing, but I would like to initiate motion based on the rotation of the mesh, but then continue the movement in the original direction irrespective of the change in rotation to the mesh that happens after motion is initiated (maintain momentum).

I'm tracking the motion in each direction via a separate object with x, y, and z values so that each press of the keyboard for motion only modifies this object which is then applied to the mesh for each loop of the animation cycle, but because I'm using:

mesh.locallyTranslate(new BABYLON.Vector3(x, y, z))

when the mesh is rotated the motion rotates with the mesh. How can I convert the directionality of the mesh's rotation into a Vector3 that I can use with World-based translation such that the subsequent rotation of the mesh does not affect the motion it already has?

NOTE: this should simulate motion in a microgravity / no gravity environment

Thanks in advance for any help.