Am I to replace one of my above scripts with your example? Sorry for sounding stupid but I'm unsure as to how to use your script

I've made a few changes according to your answer. I have a script called rotateCatapultArm which allows the player to choose an angle for the arm before firing (which should in turn affect the force on the projectile, raised arm = less swing and less force). I have added your curPos variable to this script as this position is what should be referenced as the 'initial position' before firing. The script is as follows:

var speed : float = 5.0f; // meters per second
var curPos : Vector3 = transform.position;

gameObject.tag == "catapult arm";

function Update()
{
	if(Input.GetKey ("up") && transform.rotation.eulerAngles.x < 85)
		{
		transform.rotation.eulerAngles.x += speed;
		}
	if(Input.GetKey ("down") && transform.rotation.eulerAngles.x > 10)
		{
		transform.rotation.eulerAngles.x -= speed;
		}
		
}

I have then used the remainder of your velocity calculation code within the projectileMovement script:

var projectile : Rigidbody;
var curPos:rotateCatapultArm = GetComponent(rotateCatapultArm);


function Update()
{

if(rigidbody.isKinematic ==false)
{
curPos.Translate(0,0,0);
var deltaPos: Vector3 = transform.position - curPos;
var calculatedVelocity = deltaPos / Time.fixedDeltaTime;

rigidbody.velocity = calculatedVelocity;

}
}

but this doesnt seem to do anything. For starters, I get an error in the console saying BCE0051: Operator '-' cannot be used with a left hand side of type 'UnityEngine.Vector3' and a right hand side of type 'rotateCatapultArm'.

Could you please assist me with this?

Well, do you have a 'Translate' method for your rotateCatapultArm script? You can't subtract a 'rotateCatapultArm' from a vector, now, can you? I think you should be using 'curPos.transform.position' for that...

In any case, my 'Release' method should be called from inside Update (or whatever) at the moment that you want to release the ball from the catapult. It manages all of the velocity calculation assuming that the arm continues to move. I'm not really sure why people keep changing my scripts and then wondering why they don't work any more.

Sorry, I did not mean to make things more problematic. My catapult arm does not carry on moving at the moment the projectile is "unchilded". Do you think it would be best if it was?

I've amended my LaunchSequence.js to

var scriptObject : GameObject;
function Update () {
	if(Input.GetKey (KeyCode.Space) 
		&& transform.rotation.eulerAngles.x =70)
		{
		scriptObject.GetComponent(changeProjectileState).Release();
		}
}

and then put the Release method in to changeProjectileState.js

gameObject.tag == "projectile";
function Release () 
{
	if(rigidbody.isKinematic == true)
//    yield WaitForFixedUpdate();
    var curPos : Vector3 = transform.position;
//    yield WaitForFixedUpdate();
    var deltaPos : Vector3 = transform.position - curPos;
    var calculatedVelocity = deltaPos / Time.fixedDeltaTime;
    transform.parent = null;
    rigidbody.useGravity = true;
    rigidbody.isKinematic = false;
    // this is the key bit-
    rigidbody.velocity = calculatedVelocity;
}

and then tried to point the scriptObject component to my wooden projectile in the inspector so that it knows what to call the Release method for, also removing my awkward if statement of isKinematic == false

Problem is, it now seems that the block doesn't move at all. I'm not sure if the reference to Release is working or not but the block doesn't even seem to fall due to the gravity that should turn on Maybe I'm making a few mistakes here but I feel like it is getting close