The following is copied from an email to Frank Whitby, 10 Aug 06
The piston launcher project was fun. We both came up with some ideas, mostly to get around what seems to be the biggest hassle with the usual piston launcher design, that is the problem of hooking up a deeply embedded igniter. I was pleasantly surprised with the innovative thinking that Tobin (age 13) was capable of--it truly was a joint effort. Did you see the article recently in Sports Rocketry about the "PVC Spider" for reliable cluster ignition of black powder motors? You ignite a small amount of Pyrodex (*not* black powder) in a closed chamber with many small tubes directing the combustion into each nozzle. The flash point of Pyrodex is low enough for an Estes igniter to flash, but higher than black powder, and it burns much less explosively than BP when not constrained in a pressure vessel. We combined that idea with our way of making ejection mortars, which involves opening the tip of a mini xmas light bulb, attaching an extension (vinyl tubing works best) and surrounding the filament with powder. In this case, we cut off the tip of the bulb and didn't add any tubing, but inserted it into a spent 18mm BP motor case which had been cut in two and its nozzle bored for a snug fit around the bulb. The open half of the spent casing surrounds the light socket and is glued onto a dowel which carries the wires and supports the piston head. The BT-20 wall has an engine block glued to the bottom, and slides up over the piston head. You fit the wall over the protruding bottom of the motor inside your rocket with a precise friction fit, the slide the rocket/chamber assembly down to butt against the piston head, which lines up the Pyrodex-charged light bulb with the motor nozzle. We just loaded enough Pyrodex to touch the filament, but I can imagine that you could use more, and kick the rocket off the pad. NAR probably has rules about how much pyrogen an igniter can legally contain. Also, it doesn't take much of a charge to blow out the walls of 18mm Estes body tubes.