Igniters – A Layman’s Guide
Any old igniter will not do! What do you need to know about igniters?
- An igniter must be properly sized for the rocket motor. Just because it will fit doesn’t mean it is okay to use. Rocket motors operate under pressure. Motor casings can only withstand certain pressures. The smaller the nozzle opening, the greater the pressure (all else being equal). The larger the igniter in a nozzle, the smaller the nozzle opening becomes. Do you see where this is going? Yes, putting too large an igniter in a motor can block the nozzle, increasing the pressure, maybe to failure. So the next time you see a motor fail, before you blame the #$!* motor manufacturer, find out if the correct igniter size was used. And remember that just because an igniter of the wrong size worked once, doesn’t mean it will work reliably day in and day out.
- Igniters have to burn long enough to ignite the propellant. Who cares how hot an igniter is if it doesn’t burn long enough to get your motor going. Worry less about hot, more about whether it burns long enough.
Roadrunner recommends igniters no larger than 26 gauge be used in its motors. All testing was conducted with 26 gauge igniters. Not only was there no overpressurization, no motor ever failed to ignite the first time! Of course, this means if your igniter burns, but fails to ignite the motor, Roadrunner has no idea how easily it will ignite the second time around!
Roadrunner did test other manufacturer’s igniters. Each Roadrunner motor was ignited using 1) a Copperhead, 2) a First Fire Jr, 3) a Quickburst Twiggy (at 22 gauge, larger than Roadrunner’s recommended igniter size), 4) a Quickburst Popper, and 5) an Ellis Mountain igniter. Blastoff without a hitch. The Roadrunner propellant motors ignited slightly (but measurably) slower on the Quickburst Popper. It was not a statistically valid sample size, but better than no testing.