Help with boosting a ridgid body

Ive created a script to control movement of a ridgid body and it is working well! Im trying to add features, one of which is when the player holds the space key its speed increases (boost).

Ive implimented some code but there doesnt seem to be any change when I hold space.

My Code:

 public float speed = 10.0f;
 public float maxVelocityChange = 10.0f; // Change this for boost
 public float rotationSpeed = 10.0f;
 public bool allowStrafe;
 public bool allowBoost;
 private bool grounded = false; // Grounding check, could be useful for later
 private float boostVelocityChange;
 private float boostSpeed;
 
 
 void Awake () {
     rigidbody.freezeRotation = true; // Stabalising object on awake
     boostVelocityChange = maxVelocityChange * 2;
     boostSpeed = speed * 2;
 }
 
 void FixedUpdate () {
     if (grounded) {
         // Calculate how fast we should be moving
         Vector3 targetVelocity = new Vector3(0, 0, Input.GetAxis("Vertical"));
         targetVelocity = transform.TransformDirection(targetVelocity); // What it would be if it moved
         if(allowBoost)
         {
             targetVelocity *= speed; // multiply speed to it to account for velocity
         }
         else
         {
             targetVelocity *= boostSpeed;
         }
          
         // rotate
         transform.Rotate(new Vector3(0, Input.GetAxis("Horizontal"), 0) * rotationSpeed);
         
         // Apply a force that attempts to reach target velocity
         
         //Boost code
         //
         //
         if(allowBoost)
         {
             if(Input.GetKey(KeyCode.Space))
             {
                 // Boost Active
                 Vector3 velocity = rigidbody.velocity; // hold velocity
                 Vector3 velocityChange = (targetVelocity - velocity); // hold target
                 velocityChange.x = Mathf.Clamp(velocityChange.x, -boostVelocityChange, boostVelocityChange); // round to fit the max velocity
                 velocityChange.z = Mathf.Clamp(velocityChange.z, -boostVelocityChange, boostVelocityChange);
                 velocityChange.y = 0; // no change in the Y axis
                 rigidbody.AddForce(velocityChange, ForceMode.VelocityChange); // Add the force to the object
             }
             else
             {
                 Vector3 velocity = rigidbody.velocity; // hold velocity
                 Vector3 velocityChange = (targetVelocity - velocity); // hold target
                 velocityChange.x = Mathf.Clamp(velocityChange.x, -maxVelocityChange, maxVelocityChange); // round to fit the max velocity
                 velocityChange.z = Mathf.Clamp(velocityChange.z, -maxVelocityChange, maxVelocityChange);
                 velocityChange.y = 0; // no change in the Y axis
                 rigidbody.AddForce(velocityChange, ForceMode.VelocityChange); // Add the force to the object
             }
         }
         else
         {
             Vector3 velocity = rigidbody.velocity; // hold velocity
             Vector3 velocityChange = (targetVelocity - velocity); // hold target
                velocityChange.x = Mathf.Clamp(velocityChange.x, -maxVelocityChange, maxVelocityChange); // round to fit the max velocity
                velocityChange.z = Mathf.Clamp(velocityChange.z, -maxVelocityChange, maxVelocityChange);
                velocityChange.y = 0; // no change in the Y axis
                rigidbody.AddForce(velocityChange, ForceMode.VelocityChange); // Add the force to the object
         }
         // Strafing code
         //
         //
         if(allowStrafe)
         {
             if(Input.GetKey(KeyCode.Q))
             {
                 Vector3 targetVelocityLeft = new Vector3(-1, 0, 0);
                 targetVelocityLeft = transform.TransformDirection(targetVelocityLeft); // What it would be if it moved
                 targetVelocityLeft *= speed; // multiply speed to it to account for velocity
                 
                 // Apply a force that attempts to reach target velocity
                 Vector3 velocityLeft = rigidbody.velocity; // hold velocity
                 Vector3 velocityChangeLeft = (targetVelocityLeft - velocityLeft); // hold target
                 velocityChangeLeft.x = Mathf.Clamp(velocityChangeLeft.x, -maxVelocityChange, maxVelocityChange); // round to fit the max velocity
                 velocityChangeLeft.z = Mathf.Clamp(velocityChangeLeft.z, -maxVelocityChange, maxVelocityChange);
                 velocityChangeLeft.y = 0; // no change in the Y axis
                 rigidbody.AddForce(velocityChangeLeft, ForceMode.VelocityChange); // Add the force to the object    
             }
             
             if(Input.GetKey(KeyCode.E))
             {
                 Vector3 targetVelocityRight = new Vector3(1, 0, 0);
                 targetVelocityRight = transform.TransformDirection(targetVelocityRight); // What it would be if it moved
                 targetVelocityRight *= speed; // multiply speed to it to account for velocity
                 
                 // Apply a force that attempts to reach target velocity
                 Vector3 velocityRight = rigidbody.velocity; // hold velocity
                 Vector3 velocityChangeRight = (targetVelocityRight - velocityRight); // hold target
                 velocityChangeRight.x = Mathf.Clamp(velocityChangeRight.x, -maxVelocityChange, maxVelocityChange); // round to fit the max velocity
                 velocityChangeRight.z = Mathf.Clamp(velocityChangeRight.z, -maxVelocityChange, maxVelocityChange);
                 velocityChangeRight.y = 0; // no change in the Y axis
                 rigidbody.AddForce(velocityChangeRight, ForceMode.VelocityChange); // Add the force to the object    
             }
         }
     }
 }
 
 void OnCollisionStay () {
     grounded = true;    
 }