Tell us about yourself!
I built one of these today to entertain my kids for a bit. About the only deviation is that i installed a regular 2 prong lamp plug and socket between the sprinkler solenoid and launch handle. I could then simply unplug the launch handle and hold onto it to keep anyone from unexpextedly launching. Just buy a cheap extension cord and cut it. I put the female plug on the solenoid end so no one would plug it into the wall and fry it. The launch handle, well... hopefully no one will do that and it is small enough to easily put out of reach/hide. Since radio shack no longer exists, i was unable to source 9v battery connectors locally. I just used electrical tape to tape wires to the batteries.
Oh, one more thing i thought of after builing: if the launch tube were made about 5 feet long, only adults would be able to get their eyes/head above it. Of course that would probably reduce range.