I plan on building a circuit to air-start some Estes motors. Does anybody have experience with firing Estes igniters? I ran some tests with a 9-volt battery (actually an 8.4 NiMH battery) and it works fine. However, this requires a significant fraction of a second to heat up. I need the igniter to fire in less than a tenth of a second.

I am planning on using a voltage doubler or tripler circuit to charge a capacitor to 16 or 24 volts, which I hope will do the job. Has anybody else done this with an Estes igniter? Any idea on the amount of capacitance needed?

Dave Hein

I use Estes ignitors exclusively for my deployment charges, using a single small, low-resistance li-poly cell. It puts out about 3.5-4 Amps, and I consistently get ignition times about 0.2 seconds. Here is a typical plot from a recent flight:

The brown line is related to the voltage upstream of the switch I use for the ejection charge. You can see that the switch closed when the onboard vertical velocity estimate went below zero, and the charge fired right about 0.2 seconds later, based on the axial acceleration and the pressure spike. The second drop in the continuity voltage is when the baro-based apogee detection kicked in and turned on the transmitter.

The energy that went into the bridgewire is I^2*R*delta T, or 3.5^2*0.8*.2 = 1.96 Watt*seconds (Joules). If I were to use 2 of these low-resistance Li-ion cells in series, the voltage would double and the resistance would only go up by another 50 or so mOhms, so the power would go up by a factor of 2^2*(900/950)^2, or 3.8. The ignition time should go down by about that factor, or maybe more, since there's less time for the heat to escape as the burnwire heats up. These cells are 1.2 grams each, by the way, and I share the cell with my transmitter so I basically get this power source for free.

Now let's look at what sort of capacitor would be required to do the same job. The energy in a capacitor is 1/2CV^2, so to get 2 Joules with a cap charged up with a 9V battery, you would need 4/81 Farads, or 50 mF. That's a part that's 1" diameter and 2 inches long. That's on top of whatever battery you're using to keep it charged (typically a 45g 9V or an 8 gram, 12V A23 cell). A 20V solution has a little bit smaller cap, at 1" diameter and 1.5" long, but now you need 2 batteries to charge the cap, in addition to your cap, or a boost converter.

Supercaps store more energy for the same size and mass, but their ESR is so high in the small sizes as to make them useless for firing charges. So if size and weight are the primary factor, firing an ignitor directly from a li-poly cell will run circles around any capacitor solution.