Instead of trying to fake the rotation, why not let the physics engine handle it for you? Simply make your projectile have some rotational inertia by making it have some length in the physic engine (make it a long box instead of a point or circle, or two joined points, or something like that). It may "just work" doing that. If not, you can probably get it to work by applying the launch forces to just one end of the projectile (the "front", I'd think). The other end should trail behind automatically. If that doesn't work, you could explicitly tell the physics engine that the projectile has some angular momentum, so it'll rotate during flight.