Okay, so I think I can script this up really fast for you but let me get everything straight.
You want it so when you press the jet key, the jetpack appears on your back. When you let go, the jetpack disappears. The jetpack is also on a nonregenerating energy bar separate from the player's normal jet fuel bar, which can be refueled through gas canisters?
And I assume you have all the emitters set-up so that isn't needed?