Anyway, often I'll throw some rockets in the car when I'm going to the slope, so that we all have toys to play with if we find ourselves hanging around with not enough wind to fly airplanes. Al saw one go off, and got bit by the rocket bug (tee hee).
He's built a couple of launchers for himself, one of which is a really easy to build and (I think) excellent design. Here's the version I built:
There's an o-ring on the launch tube for a seal, and the hold-down is 1/8" music wire (about a buck at the hardware store), which sits in a slotted piece of 1.5 inch PVC. Here are the parts separated out:
The launch tube has an o-ring on it, and has a stop for the bottle to rest against, made from a slice of 1/2" female slip coupling glued to the launch tube.
The music wire is very springy, and in its relaxed state is "opened" (as above). In the locked position (first picture), the free ends of the music wire are held together with a bar made from a scrap of metal or plastic or wood, and the launch string is attached to the protruding end of the bar. Pulling the string causes the bar to slip off one end of the wire, letting it spring open, releasing the rocket. I epoxied a small washer to one end of the music wire so the trigger bar can't slip off. Al, of course, machined a nice little brass stop on his.
The piece of 1.5" PVC is simply slotted on opposite sides to receive the music wire. (Note: I already had the plywood base with the large metal pipe flange on it, that's my "Littlefoot" launcher designed to hold the trigger mechanism from my Bigfoot launcher. So I used a threaded PVC fitting that fit in the flange for the slotted piece, rather than building a whole new launcher base. If you're building this from scratch, you'll probably just use a chunk of regular 1.5" PVC pipe. The only trick is fastening it securely to some sort of base, centered around the launch tube.)
That's basically it. Very simple, very cheap and easy to build, and a very positive locking mechanism. I added a safety catch bent from sheet brass, as you can see, so that when I accidentally trip over the launch string (which happens more than I'd care to admit) the rocket stays on the pad.
I've left out lots of details in the interest of brevity (like for instance how to get a tire valve hooked up to PVC pipe), so be sure to browse the other launchers that are out there, most of them have the details. The point here is just the easy-to-make trigger mechanism.
By the way, the music wire is a real bitch to bend: Al's version has a full loop bent in the center, like a proper spring closure, but I mangled a piece trying to do the same thing. Then I discovered I didn't need it. I suspect you could get away with thinner wire, too, if you like. Don't heat it to bend it: you'll destroy the temper and springiness.
Just for interest, Here's the first launcher Al built, a real beauty, but hard to make without a machine shop:
There's an o-ring on the launch tube for a seal, again, but the hold-down mechanism is really sweet. (I've seen this sort of mechanism before, but I don't know what it's called. It's very similar to the metal quick-connects used in pneumatic hoses, the ones that use ball bearings to lock on, but instead of the outer sleeve sliding back, this one rotates.) There are 4 ball bearings that protrude into the interior and lock over the ring on the bottle neck. The outer sleeve keeps them protruding into the center. When the outer sleeve is rotated, the cavities you can see in it line up behind the ball bearings, allowing them to slip outwards, releasing the bottle. A really slick, gorgeous, smooth-working release, but difficult for mere mortals to construct.
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