Is there any technical reason that same job can't be done with 12V and a different resistor?

I emailed Contrail about that once but never did get an answer. I'd try one of their motors but I've got enough gse to drag out to the pad without adding another battery.

I don't know what type of resistor is being used: wire wound, metal film, metal oxide, carbon or carbon film, but there's no good technical reason why you couldn't use a 1-2 ohm 1/8-1/2 watt resistor on 12 volts to ignite the Pyrodex(R).

In my hands on, practical experience, a 1/4 watt 10 ohm resistor shorted across a 7Ah 12V gel-cell will burst into flame pretty convincingly after about 2 seconds. You have to get the right sort of resistor- the "flame proof" are right out.

A 1/8 watt resistor of the same value will go a lot faster, but doesn't produce as much flame as it's about 1/2 the size.

The 1/4 watt size will ignite pyrodex pellets if it's taped carefully to the side but there is a slight delay.

I'd use two regular igniters-dipped wirewrap sort or an igniter and ematch. Why waste nitrous with a failed ignition?

N

Contrails has chosen the resistor/Pyrodex(R) ignition route to avoid BATFE issues, however I don't consider a 10 ohm resistor that takes ~2 seconds to burn a suitable e-match replacement. E-matches actuate within a few milliseconds of current application, and an uninhibited Pyrodex(R) pellet will burn for 30-35 milliseconds, and a bore inhibited Pyrodex pellet will burn for 60-70 milliseconds, and is the preferred embodiment IMO.

Pyrodex ignites at ~600F. IMO you want your Pyrodex(R) pellet heater to reach >> than 600F in < 0.1 seconds. For this you need power!

A 1/4 watt resistor might have 0.04 grams of mass. If we estimate the heat capacity is ~1 J/gram, the heat capacity of the resistor is ~25 C/J assuming no losses and a close estimate of the real heat capacity. A heating rate of 14 watts from a 12 volt battery would raise the temperature ~350 C/s (~630F/s) (the minimum ignition temperature of Pyrodex(R)), or to ~700C (~1300 F) in two seconds which is where I would expect a resistor to flame. With the recommended 24 volt battery, these times are reduced by at least a factor of 4.

If the more standard 1.6 ohm resistance valve was chosen at 12 volts, the heating rate is 90 watts, and the times to temperature would be reduced by more than a factor of 6. The time to obtain pyrodex ignition would be ~0.15 seconds and the resistor flame time would be ~0.3 seconds. This latter resistor value is preferred.

Not all resistor types are readily available or cheap (both are necessary) in 1-2 ohm values, but 100 resistors will cost less than $10. Chose wisely and you have a nice cheap ignition system.

Bob