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, nor Tripoli Central California, nor 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. - Aerocon Systems, Editor, 2004
CAPACITOR DISCHARGE FIRING SYSTEMS
Bob Dahlquist © 1998
For launching big, powerful rockets safely, one needs a safe and reliable igniter. When you push the button, you don't want the rocket to just sit there while you wonder whether or not you have a hang fire. Nor do you want it to ignite accidentally due to a discharge of static electricity. An accidental ignition can instantly incinerate anyone standing near a really big rocket.
Igniters that require a lot of electrical energy to fire are safer. Nichrome wire, Thermalite wire-wrap, Firestarô and resistor igniters are four examples. When such igniters are used in low-voltage systems, heavy batteries and heavy wires are normally required, to deliver enough energy to the igniter to assure rapid and reliable ignition.
High energy can be provided with less weight, lighter wire and greater portability by using a high voltage capacitor-discharge system. A large-value capacitor stores energy during the arming sequence, releasing it in a fraction of a second when the launch button is pushed.
The energy stored in a capacitor is proportional to the square of the voltage. Thus, at 350 volts, a 600 microfarad capacitor smaller than a salt shaker can store more energy than a bulky 300,000 microfarad capacitor bank at 12.6 volts.
But, there is a catch. At the high voltages that make capacitors efficient for storing energy, most igniters aren't efficient in utilizing the energy. To use the energy efficiently, the igniter must be designed for high voltage; it must have high resistance. And to produce reliable ignition, the resistance must be concentrated in a small space, not spread out.
The resistor igniter is very easy to make with high resistance. In addition, it may be unique in its ability to concentrate a high resistance within a very small volume; ensuring that the conversion of electrical energy to heat occurs within a very small volume as well. This produces a very high heat flux per unit area.
The high resistance of the resistor igniter causes the stored energy in the capacitor to be released in a smooth and controlled manner rather than creating a shock wave or micro-explosion.
In addition, the high resistance of the igniter ensures that most of the stored energy will be transferred to the resistor rather than being dissipated in the resistance of the wires. This allows the igniter to be built with a high no-fire energy, greatly reducing the chances of accidental ignition.
Igniters made from 1/4 watt carbon film resistors require more than 10 joules to fire. A human body carrying a static charge of 30,000 volts has only about 0.045 joule of stored electrical energy. Thus an accidental electrostatic discharge from a human body has less than 1/200 of the energy needed to fire an uncoated resistor igniter.
Because the resistor igniter uses so much electrical energy, it produces a good flame without needing any sensitive pyrotechnic compound. The energy in the flame comes from the firing current alone.
Yet, the resistor igniter will work at a distance of 2 miles, and a pair of resistor igniters will work at a distance of 2,000 feet, when used with an appropriate capacitor discharge firing system.
A relatively insensitive pyrotechnic compound can be used as a booster. For greatest safety, the booster compound should be electrically non-conductive, and have low sensitivity to electrostatic discharge.
First, the voltage must not exceed the dielectric strength (peak voltage rating) of the wire you are going to use. If you use a ballast resistor in the firing box, the capacitor voltage can be allowed to exceed the voltage rating of the wire by 5% to 15%, depending on how much the resistor drops the voltage.
Second, the voltage must not exceed the voltage rating of available capacitors.
Having chosen your voltage, calculate the required energy for the number of igniters you intend to fire on one circuit. You need at least 12 and no more than about 20 joules per igniter, delivered to the igniters (when using 1/4-watt resistors). Very simply,
Now you must multiply by a reasonable factor to allow for energy lost in the wires and in the ballast resistor. So let's make thatU = 14 Nwhere U = Energy in joules (delivered to each igniter)N = Number of igniters to be fired
Now calculate the required capacitance. The energy stored in a capacitor is:U = 1.2 (14 N) or
Solving for C givesU = 1/2 V^2 Cwhere U = Stored energy in joulesV = Capacitor voltage
With a capacitor-discharge system, the negative temperature coefficient of the carbon resistor causes the igniter resistance to decrease as the capacitor voltage is decreasing, which maintains the current flow more or less constant while the first 3/4 of the energy is discharged. Therefore we need only use the initial value of resistance (Ri) in the calculations (unlike the resistor igniter for use at constant voltage).C = 2U / V^2
Each resistor should be overloaded by a factor of 400; thus
The voltage applied to the resistor igniters will be somewhat less than the capacitor voltage due to the ballast resistor and the resistance of the wires. Divide the capacitor voltage by 1.2 for a first approximation, assuming you will be connecting all the igniters in parallel.Ri = E^2 / 400 Pr orwhere Ri = Initial igniter resistance, ohms
The load resistance, RL will be:
The total resistance should not be more than about 1.2 times the load resistance. Therefore the sum of the ballast resistance and wire resistance should equal 10% to 20% of the load resistance.RL = Ri / Nwhere Ri = Resistance of a single igniter, ohmsN = Number of igniters connected in parallel
The minimum ballast resistance is determined by the current rating of your firing contacts. This resistance serves to limit the line current if an arc develops. The type of arc most likely to occur results from the resistor opening while there is still considerable energy remaining in the capacitor. The minimum ballast resistance for protecting the contacts against this kind of arc is based on the assumption that half of the energy has already been discharged, and that the EMF across the arc is 40 volts. Therefore the minimum sum of ballast and wire resistance is:
The resistance of 1,000 ft. of #18 zip cord is 13 ohms; the resistance of 1/2 mile is 34.4 ohms. The resistance of other gauges of wire can be found in standard electrician's tables; the little blue book, Pocket Ref, by Thomas J. Glover, contains such a table.Rb + Rw = E/Iwhere E = (V/1.414) -40 andV = Capacitor voltage at full charge
When calculating the ballast resistance you need, base your calculation on the shortest firing leads you will ever use. Then check to make sure that the sum of ballast and wire resistance does not exceed about 20% of the load resistance with the longest firing leads you intend to use. If it does, you have two choices. Use heavier contacts, or always use a certain minimum length of firing leads, with less ballast.
For the ballast, you can use several 10-ohm 10-watt wire-wound resistors in series. For balanced circuits (neither lead grounded), half of the resistance should be connected in series with each lead.
Most switch and relay contacts are not designed to interrupt more than 30 volts when used with DC; higher voltages can strike an arc. Therefore you must take care not to open the contacts while the capacitor is still discharging. Keep the contacts closed until the capacitor has discharged to 30 volts or less.
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