Weaps that just disappear soon as you drop/fire rocket mean DP is not set up to use that weapon type. Sounds like you added a rocket to an aircraft that doesn't have a rocket gunstation. You need a gunstation.# that is set up to use the weapon type and matching [guns.#]
The below should fix it. Add them to DP of the aircraft. Replace the # with the next open number.
gunstation.#=2,-1,4,0.00,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
[guns.#]
gun.0=6,0,0,0,0,0,0,0