I'm about to start work on a 2.25" 29mm two stager and have been wondering how I'm going to handle the ignition of the sustainer. I could use a commercial rocket timer/stager but the $$$$ is a factor.
I started thinking to myself, exactly how hard would it be to build a simple CDI stager? IIRC in a old(early 80's) Model Rocketeer there was a article and plan for a CDI stager. I did a little googling thinking I'd pull up plenty of hits for such a device but so far, nada.
Anyone have experience in such a system? Anyone know of a online resource for such a system?
For ignition I only use my homemade cat5, nichrome wrap, Igniterman pyrogen igniters. I average a 1.2 ohm reading and never had any troubles with firing them off our clubs 12v launch system. I save my ematches for ejection charges.
I was chatting with some pyro guys and they mentioned the use of a CDI system for their displays. However, they were using ematches.
I really wish I still had that old Model Rocketeer that had the original CDI article. I *think* it was in MR.I've searched Nilfinger for that issue to no avail.
In those days it was most likely a flashbulb igniting thermalite fuse with (UGH!) a mercury switch as the detector for end of first stage thrust.
Anyway, the size of the capacitor it will take to reliably ignite your homemade ignitors will be substantial. You'd have to do some calculations and tests to determine that, but it ain't gonna be small for a 'regular' 12 volt igniter. Enough zap to, well, make your heart skip a lot more than a few beats.
Do yourself a favor and use modern electronics with an ematch. Safer, more reliable. Capacitive discharge can be kind of tricky; you can use both too little and too much zap. I've blown Estes igniters to bits without even igniting most of the pyrogen with capacitive discharges.
Most of the igniters and e-matches around are designed to heat a pyrogen to ignition. That's in contrast to the very rapid energy pulse delivered by a typical CDI system. You can get a very good "jolt" from a CDI system, but the feeling you get does not necessarily translate into an ability to heat something up. Nor will a device designed to heat at lower voltage/higher current respond the same to a high voltage/low current pulse. Even the transmission system (wires and leads) will respond differently.
There are designs for high voltage/low current pulse applications. For instance, Google for "exploding bridgewire".
Note that an igniter is a *system*. Not only is the heated element a part, the pyrogen itself is designed to work with the characteristics of the heated element.
I would do LOTS of ground tests before attempting CDI ignition in the sustainer of a staged rocket. There's enough to go wrong already. ;)
Have you thought about a PerfectFlite miniTimer3 w/ G-Switch? It's
1.5" x 1" and about $43. Add a 9V Duracell for power. The manual says firing current is 33.9 amps for one second. That should light most anything. I use an e-match dipped in Magnalite. That works for all
38mm and 54 mm motors I've ever tried. You can still experiment with a CDI unit, but at least you'll have a small, light; reliable, and fairly cheap stager to use. Larry Lobdell Jr.
Tried that with an Estes ignitor a couple years ago after salvaging a flash from a disposable camera. Zip, zilch, nada. If I recall correctly, I saw >10A current spike, but it was much too short to set off the ignitor.
Davel wrote in news: email@example.com:
I wonder how it would work with an Igniterman-style dipped igniter that has a conductive pyrogen bridge instead of a nichrome resistive element?
For the Estes igniter,the nichrome wire has to have enough power duration to heat up the pyrogen coating,but in the other igniter,the current has to go through the actual pyrogen.The pyrogen would directly dissipate the power. IMO,the ~300VDC flash discharge would be more effective on the Igniterman- style.
It can certainly be made to work. Back about 1969 or 1970, I built and flew a very simple unit. With parts salvaged from a bulb-type flash unit, it had a socket for an AG-1 flashbulb, fired by a 15V capacitor. Note that the capacitor was very small and light, and the battery was left on the ground - the capacitor was charged through a mini phone jack right before flight.
At the time, Centuri offered their Sure-Shot ignitor, which was a small rod coated with pyrogen. In normal use, you'd wrap nichrome wire around the base, and insert the rod into the engine. What I did was glue the rod to the top of the flashbulb. The whole assembly bit into a BT-50 tube, with a reducer to BT-20 where the bulb socket was. A mercury switch handled firing.
So - Standard (w/ejection charge) 18mm B or C engine in the booster; A through C in the sustainer. Bulb with Sure-Shot in socket; sustainer friction fit on top. Normal ignitor in booster engine. Put it all on the pad; connect the battery for several seconds to charge the capacitor. Launch.
At burn-out, the deceleration (should) trigger the mercury switch and launch the second stage. At appropriate times, both stages would fire their ejection charges and parachute down.
I never had a problem with ignition; the problem was with somewhat casual operation of the mercury switch. The second stage didn't always launch straight up. After it fired on a slightly downward angle I abandoned the project.
At 15 years old, the prospects of trying different mercury switches didn't appeal to me, and I rejected the idea of adding drag to the booster. After all, I wanted it to perform!
Ah yes, those low voltage "ultracaps." I forgot about those...they make it much more workable than with older technology electrolytic capacitors.
But it's still a balancing act. You have to dump enough energy into the bridge wire fast enough to get it hot enough long enough to ignite the pyrogen without either blowing the pyrogen off unlit or melting the bridgewire and opening the circuit before the pyrogen lights.
Test, test, test on the bench and keep records so you can plot the optimum response range. For a given type of igniter you want a plot of capacitance on one axis and charging voltage on the other. Two colors of dots represent successful ignition and failed ignition. (If you get really into it you can plot time to ignition on the third axis.) It'll be fairly obvious where you can get reliable ignition with a minimum capacitor mass. Oh, and igniters that narrowly fail to light can have their ignition characteristics altered by the heat so be careful about re-using these.
And Ted, please don't splatter mercury on the launch field! There are better ways to sense either burnout deceleration or staging separation than mercury switches. Not to mention safer...build safety into any staging electronics through smart design: "What happens if..."