[FFT] Fire-in-the-Hole igniter construction

Back in April, there was a thread in which a couple of people wanted to know how the Fire-in-the-Hole igniters were constructed.

Recently, I used one for a J110 and it failed to light the motor. I think this was due to the age and condition of the igniter - it was from 2002, I believe, and the pyrogen had started to crumble.

In any case, I had a second FITH which I decided to disassemble. And since it appears that no one is actively producing FITH igniters, I thought I would share the results.

The dipped portion of the igniter was ~ 1.25 to 1.5 inches in length and ~

1/8 to 3/16 inches in diameter. The lead wire was standard solid zip-type.

Now the interesting part (be patient, as this may be hard to describe):

In this little diagram, the "---" lines represent the lead wires, with the bare ends at the right and the pyrogen at the left.

======== a------------- A b-------------------------- B

One lead (labeled "A") is about an 1" shorter than the other (labeled "B"). The double line, made of "=", represents a separate bare wire, not connected to anything but coated with a layer of pyrogen. This structure I'll call the "core" and seems to be the main physical differentiation from those seen at

The bridge wire, which I believe to be nichrome based on its resistance, is attached at points "a" and "b" by first wire-wrapping then just a touch of solder. But it does not go directly from "a" to "b", but rather is wrapped about 6 times around the "core" and the B lead. Thus the bridge wire is actually about 2.5 inches in length (resistance of this length is around 2-3 ohms).

Finally, the region from "a" to "b" has a coating of pyrogen.

Now the speculation about how this was assembled:

I think the core was fabricated (and dried) prior to igniter assembly. This is based on 1) observation that the pyrogen flaked off the offside in a clean layer, leaving the core intact and 2) that bridge wire separated cleaning from the core (was not embedded).

Next I think a 2.5" length of nichrome was wirewrapped to "a" and "b" and soldered in place. Then the core was placed next to "B" and the lead wires twisted so as to force the bridge wire to tightly wrap around the core.

Finally, the assembly dipped in pyrogen.