In preparation for a series of static tests this week, I’ve been working on some miscellaneous details. I updated the small test stand today to use a load cell for thrust measurement, instead of the state-of-the-art bathroom scale that was previously used, but I’ll talk about that later. I also realized that I’m running horribly short on igniters, and being the cheapskate that I am, I didn’t want to use electric matches for these tests. I remembered reading about using low-watt carbon resistors as igniters somewhere, and a quick Google search confirmed my speculation.

The main theory for resistor igniters is explained in an excellent whitepaper by Bob Dahlquist, archived at Aerocon’s site. I’ve also taken the liberty to put this article into a pretty, portable PDF. Paul Kelly has also done some research using resistors as igniters, archived at Backyard Ballistics. Contrail Rockets also uses a resistive ignition system for their hybrids, so the idea seems to be feasible. I mean, all igniters are resistors in one form or another, right? Granted, I have nichrome and pyrogen. But nichrome is difficult to solder, and bridgewire igniters are quite fiddly to assemble consistently. I wanted something quick, cheap, and brainless.

Realizing I had to come up with a bunch of igniters quickly, I headed down to the local Radio Shack to see what the resistor offerings were. My existing launch system is a 12-volt relay based affair, with lead acid batteries. According to Dahlquist’s papers, I needed a 1/4-watt resistor in the neighborhood of 1 ohm. Scanning the very limited selection (Radio Shack… what do you expect…) they didn’t have anything near these specifications. However, they did have some 1/8th watt 10 ohm resistors. A quick mental check said P=V²/R (I’m trying to install LaTeX on this server, give it time…) or 144/10 = 14.4 watts. Dahlquist’s paper recommends overpowering the resistor by at least a factor of 200, so a 1/8th watt resistor should need 25 watts… well, shucks. It’s about half of what the theory claims is required, but a pack of 5 is 99 cents, so I grabbed two and headed back to the shop.

The next step was to integrate them into something relatively igniter-like. Using some leftover shooter’s wire, I rigged up a “match head” by wire-wrapping the wire ends onto the resistor legs, and folding it into a narrow strip. Once assembled, the resistor barely protruded from the wire:

Dahlquist claims that resistors carry plenty of energy to light HTPB propellant by themselves, but remember that I’m not overpowering this one the same way that he suggests. To boost the igniter slightly, I drilled a hole in the bottom of a sliver of propellant, and slid the assembled igniter head inside. Pyrodex pellets would probably also work, as would a standard igniter dip. The size of the assembled igniter is pretty much controlled by the size of the propellant piece it sits inside.

Elated with my success thus far, I set up the launch system to try the device out:

Hey! It worked! My launch battery recorded 12.34 volts beforehand. I didn’t measure the internal resistance of the launch system and battery, but assuming maybe 1.5 ohms, the total power delivered through the resistor at button-press was 13.24 Watts — still short of the number required, but it still worked. You can clearly see that the resistor was underpowered — there’s a significant delay between when I say “one” and when big fire starts. More voltage or a smaller resistance value (aka jack up the power) should help here. I don’t have any decent measurement equipment with me right now, but when I return to the lab I’ll hook one up to an oscilloscope to see what sort of current we’re talking about. Since I had made fire, I declared success and called it a night. Possible improvements include:

• Increase the battery voltage — By doubling the batteries at the pad to 24 volts (two 12 Volt lead-acid batteries in series), the power delivered jumps by a factor of 4 to 57.6 watts. At that point the resistor should explode nearly instantaneously, rather than heating up slowly.
• Cast the resistor into the igniter propellant — This ensures intimate contact between the resistor and booster propellant. Since resistors get plenty hot to ignite APCP (as opposed to a simple nichrome bridge, which seems to require a more sensitive compound), this igniter would seem to be more durable than an NC-lacquer based system. (Another thought — I wonder if enough energy is delivered fast enough to light thermite?)
• Switch to SMD resistors — Though they aren’t available at the local Radio Shack, SMD resistors seem to hold more potential for igniters. They have a lower power rating, so they would be easier to fry with normal equipment; the downside is that they may not deliver enough energy to ignite HTPB-based propellants.

More investigation is needed. Stay tuned.

Posted January 8, 2008 in Ignition, Media. Add Comment / Trackback