khodges
Posts: 4979
Joined: 7/3/2003
From: newton,
NC, USA
Status: offline
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Barry--- were you flying these things when you had to make your own ignitors? At the time I was flying about the only thing commercially available was the bimetallic strip that came with Aerotec disposables. They weren't very reliable, easy to short, and not long enough for the bigger engines. Until the "electric match" ignitors started coming out in the early '90s, I used flashbulb ignition.
For those rocketeers who don't know how this worked, it was cool. First you had to find a source of the little AG-1B flashbulbs like you used on Kodak Instamatic cameras from back in the late 60's and early '70s. Since the advent of self contained electronic flash in most small cameras, these things were getting hard to find.
Then you had to find a source of Thermalite. This stuff was like a fuse. There was fast and slow types; one burned at a rate of one inch per second and the other burned at two inches per second. It had a very small wire in its center so you could bend it and it would maintain the bend, and there was five strands of nickel-chromium wire, spiral wrapped around the outside. The whole thing was about 1/8" diameter. You could use it on black powder by unravveling the wire strands and connecting them to your electrical clips, stuff a short piece in the nozzle and you were good to go. Didn't work like that for composite engines (see below)
The last thing was sheathing, which was a very small teflon tubing that the Thermalite just fit into. I don't know why , but when Thermalite was sheathed, the burn rate (length per second) was almost instantaneous, at least, for pieces a foot long or so.
Composite propellant (Ammonium or Potassium Perchlorate and binder) engines don't ignite at the nozzle end, like black powder (Estes engines) do. The ignition has to occur at the top of the propellant grain, and the engine burns from top to bottom. There is a channel from the nozzle up through the propellant grain, and the ignitor is inserted all the way up to the top.
The homemade ignitors were made by cutting a piece of thermalite long enough to fold over one end about an inch to double the amount that could burn, and then long enough to stick out the nozzle about an inch or so. A piece of sheathing was slid over the length of Thermalite, leaving a "tail" of unsheathed at the nozzle end. A flashbulb was then taped to the unsheathed end and the whole thing was secured to the engine nozzle.
Now, your electrical leads from the launch controller were clipped to the small wire tabs on the flash bulb (after the motor was loaded into the rocket and it was on the launcher). Now, all you had to do was arm the switch and push the button. If everything worked like it should, you'd see a bright flash as the bulb fired, igniting the Thermalite. The sheathed portion would instantaneously transfer the flame front ot the folded over portion at the top of the propellant, and the propellant would ignite. The whole sequence took about three seconds. PUSH---FLASH----(pause)-----WHOOSH!!!!
< Message edited by khodges -- 1/28/2008 2:44:24 AM >
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Club Saito #2, WACO Brotherhood #20. What other trouble can I get into?
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All Forums >> Radios, Batteries, Clubhouse and more >> Model Rockets & Rocketry >> Lots of crossover with RC 
Lots of crossover with RC - 
with projects from 10" to 22'...here's some pics of my projects. 
ignition... in fact I had a scale Nike-Ajax that used a mercury switch and flashbulb ignition for the upper stage that got a lot of attention at NARAM-18 (the one in Orlando). 
