Rocket explosion goes off 'randomly' and not when i want to.

Firstly it's a really bad idea to be using gameObject.Find within OnCollisionEnter. The one you are using on Start is far from ideal but the time it takes to find objects would be enough to delay the particle effects visibly on collision when you have multiple rockets on screen. No guarantee with this that they will actually find the explosion object on themselves and not on another rocket instead.

Also not clear why you have Destroy(gameObject,time); in there twice. Seems like that is either going to be redundant or potentially call a null reference when the second destroy cannot find the item since it has already been destroyed (if it wasn't destroying itself it would anyway). If you want the rocket to destroy whatever it hit use Destroy(theCollision.gameObject). I also cannot see where you have actually set the value of time. If unset it will be 0 by default and therefore be destroying things instantly.

There are a few other ways to do this. First is for each rocket item to have SmokeTrail and Explosion on them as a particle system and assigned to variables in the inspector so you don't need to mess around with Find or GetComponent and can just enable/disable them directly. Or they could be child objects within the rocket prefab which would work much the same. That would work fine for the SmokeTrail.

For the explosion however I would probably look at instantiating a prefab with the explosion particle system on it at the point the rocket collided. Use Destroy(explosionInstance, 1); (or however long the particle animation is) to destroy the instantiated explosion on a timer or a script on the explosion prefab to do it. After instantiating the explosion destroy the rocket itself instantly rather than turning off the renderer and smoke trail.

EDIT:

 //Assign particles in the inspector instead of with Find
 //In the particle editor set them to Play on Awake
 
 //Child object of rocket with the trail particles on
 var rocketTrail : GameObject;
 
 //Not a child object but a prefab from your scene
 var explosion : GameObject;
 
 function OnCollisionEnter(theCollision : Collision){
        
     //Spawn explosion
     var explosionInstance : GameObject = Instantiate(explosion, transform.position, transform.rotation);    
     Destroy(explosionInstance, 2f);
     
     //Destroy rocket
     Destroy(gameObject);         
 }

My JS is a bit rusty so you may need to check that but this should remove the need to use Find which is slowing things down for you. Remove the explosion from the rocket itself and just have it assigned as a prefab from your project. The reference to rocketTrail above isn't doing anything currently but you may want it to stop emitting particles if the rocket has a limited range and is going to run out of fuel or something. Either way avoid using Find at all costs and assign things in the inspector where you can.