Most launchers I'd seen up till now, are built mostly from PVC pipes of various diameter, and many use a circle of tie-wraps in a clamp to hold down the rocket while it's being pumped up. Some take a heft yank on the launch string, and then seem prone to tipping over, pointing the rocket straight at you

We tried to avoid most of these problems with this design. It's a heavier wood frame made from 1x6's that sits low and long. Uses a steel-rod "hairpin" to lock the rocket while pumping to avoid the alignment and fragility problems of the tie-wraps in the Clark-type launcher. A small aluminum piece clamps the ends of the hairpin lock together, very easy to pull off with a string, you don't need a huge heave. The air hose is 25 feet long (as is the trigger string), but only 1/4" outside diameter (0.017" inside), so has minimal volume that must be pumped up, only about a tenth of the volume of a 2-liter soda bottle. If the bottle goes boom while you're pumping, you're 25 feet away.
Drew up a parts list and instructions for building the thing, it's not too complex. I've attached the document here, hope it comes out OK.
The launcher is adjustable where it needs to be - you can raise the lock up or down by adding fender washers under it, the launch tube can be raised or lowered by putting a different-length pipe nipple in the flange base. Have a look at the instructions, you'll see how it goes.
Nearly all parts are from Home Depot, their SKU numbers are in the parts list, and current prices. A tire valve is from NAPA Auto Parts, and a few other pieces are from the local hobby shop, pretty common.
If somebody builds this, can you let me know how it goes? How understandable ar the instructions, how well does the launcher work, any suggestions etc.?
Hope I didn't mess it up too bad!
