THE RESISTOR INITIATOR

The heart of the resistor igniter is a quarter-watt, carbon film 5% tolerance resistor. Its value in ohms is chosen such that it will draw enough current to produce a 200x to 400x overload in the resistor. For a quarter-watt resistor, this means that when the firing button is pushed, 50 to 100 watts will be dissipated in the resistor. This overload pyrolyzes the resistor's coating and converts it to flame in a fraction of a second.

The rate of heat release at 50 Watts dissipation is 12 (gram)calories per second. The surface area of the carbon film in a ¼-Watt resistor is roughly 5 mm2 or 0.05 cm2. The heat flux per unit area, or thermal energy density, is then

q"= (12 cal/sec) / (0.05 cm^2)

= 240 cal/cm^2-sec.

This is roughly 8 times the heat flux required to ignite HTPB propellant directly, if the resistor is cast into the propellant. But to have a short ignition delay overall, a faster burning pyrotechnic compound is needed as a booster when igniting rocket motors.

Experiments by the author showed that the energy required to pyrolyze the resistor and produce flames amounted to about 14 Joules. The time delay between application of the firing current and the appearance of flame varied inversely with the average power dissipation, approximately as follows:

t (sec.) = 14 Watt-sec./Pavg (Watts) (Eq.1)

When the resistors were immersed in black powder or coated with a pyrogen composition, the time delay for ignition was substantially less. In Stan Currey's experiments using a sensitive pyrogen compound, some brands of resistors ignited the coating with as little as 3 or 4 Joules. However, ignition was not reliable at such a low energy. Reliable ignition of the sensitive coating was achieved at about 7 Joules per resistor.

Carbon composition resistors were also tested by the author, and had much longer ignition delays. This is because the carbon, where the heat is released, is covered by a thick layer of insulating material, which has appreciable thermal mass and thermal resistance (see figure 1). Considerable energy is consumed in pyrolyzing this thick coating; so that Equation 1 is not valid for carbon composition resistors unless a much larger value for the energy is used.

Carbon composition resistors often broke in half without producing any flame. Carbon film resistors, on the other hand, have a ceramic substrate which normally holds the resistor together during and after firing. Often the carbon film resistor can be fired twice.

Flameproof and metal film resistors were tested also, but held true to their name; producing no flame at all, even at extreme overloads.

The resistance of the carbon film resistor was found to decrease during firing, to about half of its initial value. With the application of sufficient firing energy, the resistance then increased to several thousand ohms or more. For this reason, average power is used in equation 1.

When computing power dissipation, all the other resistances in the circuit must be taken into account. This includes the internal resistance of the battery or other power source, the resistance of the wires, and the contact resistances of all the switch and relay contacts, wire connections, and igniter clips.

As long as a DC power source is used (such as a battery or a capacitor), inductance and capacitance can be assumed to be insignificant for the purpose of this calculation.

At any given instant, the current flowing in the firing circuit will be equal to the open-circuit voltage of the power source, divided by the total of the resistances mentioned above.

I= E/Rt (Eq.2)

The instantaneous power dissipation will then be equal to the square of the current, multiplied by the actual resistance of the igniter resistor at that moment.

P= I2Ri (Eq.3)

Integrating the instantaneous power over the elapsed time gives the energy (in Watt-seconds or Joules) dissipated in the resistor. Dividing the energy by the elapsed time gives the average power in Watts.

The open-circuit voltage may be essentially constant (as when a battery system is used) or may vary from a high value to near zero during firing (as when a capacitor discharge system is used).

AC from an inverter or generator may also be used, with appropriate precautions. The author has not tested the use of AC with resistor igniters, but if 50 or 60 Hz is used, the equations for constant voltage DC given below should produce a resistance value that will be close enough for practical purposes. RMS values of voltage and current must be used.

When using AC, it is best not to connect the firing circuit to the same power source used by other important items such as computers and solenoid valves.

(Next) (Previous) (Back to Dahlquist Papers Main Page) (Back to Home Page)