Hi Daz,
Welcome to our community! 
I don’t know in which lecture you are but do you have this piece of code in your Rocket class?
void ApplyRotation(float rotationThisFrame)
{
rb.freezeRotation = true; // freezing rotation so we can manually rotate
transform.Rotate(Vector3.forward * rotationThisFrame * Time.deltaTime);
rb.freezeRotation = false; // unfreezing rotation so the physics system can take over
}
If so, test this:
void ApplyRotation(float rotationThisFrame)
{
rb.angularVelocity = Vector3.zero; // set the angular velocity to (0f, 0f, 0f)
transform.Rotate(Vector3.forward * rotationThisFrame * Time.deltaTime);
}
Did this fix it?
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