WARNING! Anyone intending to make use of this technology should first be familiar with standard safety precautions for electricity, rocketry, and pyrotechnic materials. Neither the author, Aerocon Systems, Tripoli Central California, the Webmaster, nor the Reaction Research Society will be responsible for any accidents or damages, as the use of this technology by the reader is beyond their control. If you do not agree, you are advised to stop reading now.
Note: Bob Dahlquist died a couple of years ago of unexpected health complications. He leaves this white paper behind as part of his legacy to the rocketry community. David Reese has been kind enough to reformat these pages as a downloadable pdf - Aerocon Systems, Editor, 2008
THE CARBON RESISTOR IGNITER
Bob Dahlquist © 1998
Member: Reaction Research Society and Tripoli Central California
The use of carbon resistors as ignition initiators has been developed by the author over the past three years. Additional testing and experimentation on pyrotechnic coatings was done by Niels Anderson and Stan Currey. The resistor initiator has now been quantified and perfected to the point that it is ready for practical use.
The resistor initiator is useful for most pyrotechnic and rocketry purposes. Some examples are:
When various applications are contemplated, if the system is designed for the most difficult application, then it will usually work with the same reliability in the less difficult applications.
Note that the reliability under the above conditions was not 99% or 99.9%, but 100%. There were zero premature ignitions and zero ignition failures. Under other conditions, this level of reliability has not been achieved.
The main advantages of the resistor initiator, in addition to reliability, are:
The ability to easily and conveniently supply so much energy by electrical means alone allows a relatively insensitive first fire composition to be used. (The resistor igniter will produce fire even without any pyrotechnic coating, thus in many applications it can be used without any first fire composition; it can ignite the booster compound directly.)
The first fire composition (if used) can be chosen such that its no-fire energy, in Joules, is higher than the electrostatic spark energy available from a human body charged to 30,000 volts; while the same compound's all-fire energy is lower than the energy provided by the resistor initiator.
In this way, an igniter can be produced which can not be ignited accidentally by electrostatic discharge from a human body, yet will ignite without fail when intentionally fired.
Such an igniter will also be insensitive to other sources of extraneous energy, such as induced currents from nearby radar or radio transmitting antennas.
The high energy density provided by the resistor initiator eliminates the need to use any of the highly sensitive primer compositions (unless, of course, you are making detonators; and for that, a bridgewire is preferred anyway).
The resistor releases heat energy over a longer period of time than electric matches, which (consistent with their original application) only produce a very brief flash. This allows ample time for heat to transfer from the resistor to the booster compound, making ignition more reliable.
The availability of resistors manufactured to close tolerances adds to the reliability and repeatability of the resistor igniter for any given application.
However, there is no such thing as a standard resistance for all resistor initiators; the proper resistance depends upon the application and system in which it is to be used. The optimum value can vary from 1 to 1,000 ohms. Equations for predicting the optimum resistance for use in two different classes of firing systems (constant-voltage and capacitor discharge) were developed by the author.
The author has also developed several different electrical firing systems for use with resistor igniters (in addition to systems for use with other types of igniters).
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