Rigidbody movement issues

Hi,

Firstly, thanks for looking, and secondly, I hate having to ask for help, but I have now spent a couple of days trawling the anwsers site, as well as searching the internet for the clues.

I am attempting to make a topdown space shooter, with frictionless movement. I started out by using transform.translate, and then moved to rigidbodies. (first time writing a game, and firstime writing JavaScript, so this is all a learning experiance.) I have the Rigidbodies moving as expected, the problem comes, when I want to set a max speed, so things cannot accelerate indefinetly, this works o.k, until i want to change direction, whilst at top speed, due to the way i have my code currently, if my velocity is equal to my ships top speed, no input is accepted. Below is the code;

 var ShipTopSpeed:float = 50;

 var RevShipTopSpeed:float = -10;
 var ShipAcceleration:float = 100;
 var ShipDecceleration:float = 1.5;
 var ShipRotationPosSpeed:float = 50;
 var ShipRotationNegSpeed:float = -50;
 var ShipRotationAcceleration:float = 5;
 var ShipRotationDecceleration:float = 1;
 var ShipRotationCurrentSpeed:float; 
 var velocity:Vector3;
 function Update(){
 }
 function FixedUpdate () {
 cross = rigidbody.angularVelocity;
 
 if (Input.GetButton('a')){
 rigidbody.AddRelativeTorque(Vector3.down* ShipRotationPosSpeed);
       }
 if (Input.GetButton('d')){
 rigidbody.AddRelativeTorque(Vector3.up * ShipRotationPosSpeed);
 }    
 if(Input.GetButton('w')) {
 if (rigidbody.velocity.magnitude < ShipTopSpeed){
 rigidbody.AddRelativeForce(Vector3.forward * ShipAcceleration);
 }
     }    
 if (Input.GetButton('s')){
 rigidbody.AddRelativeForce(Vector3.back * ShipDecceleration);
 }
 print (cross.y);
 }

I need a way of determining if the angle of rotation has changed, that then allows me to apply force until the max speed in that direction has been achived.

Im guessing i need to do something with Vector3.dot?

Any suggestions are welcome. and thanks in advance.