This is an illustration of what is referred to as a pyrotechnic igniter which was used for igniting the B-200, C-400 and similar sugar-propellant rocket motors. It consists of a length of polyethylene plastic drinking straw filled as shown with a charge of Ignition Powder, and as such, is often referred to as a "straw igniter".

The igniter is sealed at both ends with polyethylene "hot-melt glue". The nichrome (nickel-chromium, high resistance) wire serves as the heating filament (bridgewire), and is soldered to the ends of the copper wire leads using "stainless steel" solder. Nichrome wire is quite inexpensive (about $0.20 /ft.) . Alternatively, a strand (or two) of coarse "steel wool" may be used in place of nichrome wire, or even a strand of fine copper "speaker" wire.

Nichrome wire can be tricky to solder. An easy and secure way to attach the nichrome wire to the electrical lead wires is as illustrated below:

1. After stripping insulation from the electrical leads, form closed loops using needlenose pliers. Thread the nichrome wire through the loops.

2. Adjust the spacing such that the span of the nichrome wire between loops is about 5 mm.

3. Brush the loops lightly with acid soldering paste. Using a soldering gun, heat the loops and fill with solder. Capillary action will make this easy to do. The nichrome wire will now be embedded in solder. Use a magnifying glass to inspect to make certain that the loops are fully filled with solder. Excess solder paste is then removed by swirling the igniter in alcohol.

Important note:   Use of an igniter containing a much greater charge of BP than shown in Figure 1 may result in an initial pressure "spike". This overpressurization of the rocket motor during startup may result in motor failure characterized by shearing of the safety bolts retaining the rocket motor head. The effect of igniter size on the motor chamber pressure curve can be significant for "small" rocket motors (such as the A-100, B-200 & C-400). This is illustrated in Figure 2 for a particular BEM (Ballistic Evaluation Motor).

Preparation of Ignition Powder

The charcoal must be "natural wood" variety (not briquettes). The charcoal may be broken into smaller pieces by wrapping within a sheet of cotton, then striking with a hammer. These pieces may then be pulverized by use of an electric coffee grinder. The potassium nitrate is also ground to a fine powder using a coffee grinder. The two constituents are then weighed out to 80/20 (KN/Charcoal) proportions, then blended thoroughly together. I utilize the same rotating mixer as I use for blending propellant mixtures. Recommended blending time is 3 hours per 100 grams.
After blending, the powder is placed in a plastic bowl, and enough water is added to form a fluid paste. The paste is well mixed by hand, then spread out onto a sheet of parchment paper, and allowed to dry completely. A mortar and pestle is then used to break the resulting clumps into coarse granular form. A flour sifter is then used to sift the granules to the required particle size.

The combustion of the igniter charge serves two important purposes:

1. To generate a heat flux in the form of hot, dense gases which rapidly ignite the propellant grain on all exposed (non-inhibited or bonded) surfaces. Convective heat transfer resulting from the gases flowing at high velocity over the grain surface toward the nozzle exit aids the ignition process.

2. To pressurize the chamber to a level such that the burn rate of the propellant is sufficient to maintain this pressure. Logically, this pressure level should be the design pressure of the motor.

The pressure that is generated by combustion of the igniter charge may be determined by the following equation:
(Ref. NASA SP-8051 Solid Rocket Motor Igniters)

P = pressure in combustion chamber at time, t    lb_f/in²

r= density of charge material    lb_m/in³

D = loading density = C / V lb_m/in³

C = original mass of charge    lb_m

V = free volume of combustion chamber at time, t    in³

l = R T/ M "effective force" (energy),     in-lb_f /lb_m

R = universal gas constant    in-lb_f /Ь R-lb_m

M = effective molecular weight of combustion products (system mass divided by number of moles of gas)    lb_m / mole

T = adiabatic flame temperature    Ь R

G = fraction of original charge mass consumed by time, t    dimensionless

P_a = atmospheric pressure    lb_f/in²

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The effective force, l ("lambda"), may be calculated from the values of M and T determined from a combustion analysis (e.g. using PEP program) for any charge material. Alternatively, it can be readily determined if the impetus for a particular charge material is known. The impetus for Black Powder (commercial grade) is approximately 100 000 ft-lb_f/lb_m. Thus, the effective force for Black Powder is 100 000 * 12 = 1 200 000 in-lb_f/lb_m.
If the Black Powder is not commercially made, rather prepared as described earlier in this Web Page, the impetus will probably be far less, say, between 50-75% of this value.

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Derivation of this equation
Example calculation   

For convenience, an Excel spreadsheet is available which determines combustion pressure using this method:
        Rocketry software IGNITER.XLS          68kbytes        MS Excel 5.0 file   

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Pyrogen Ignition

A pyrotechnic igniter, as described above, works very well for starting smaller sized rocket motors. However, for larger motors (i.e. K-class & larger), a pyrogen ignition system provide superior motor starts. A pyrogen is essentially a small rocket motor mounted at the bulkhead. Nearly instantaneous ignition of the motor grain is assured by the high velocity, particle-laden flame that emanates from the pyrogen. The pyrogen used for the Kappa rocket motor is shown in Figure 3.

Figure 3 -- Pyrogen/bulkhead assembly for Kappa motor

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The grain used for this particular pyrogen is cast KNSU propellant, chosen for its ease of ignition and rapid burn rate. The pyrogen grain is ignited by a black powder charge, initially contained within the pyrogen canister by a burst diaphragm. This charge additionally aids pressurization of the motor.

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Mini-bulb Igniter

      
Figure 4 -- Mini-bulb igniter

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An easy to make pyrotechnic igniter can be made in a manner similar to the igniter shown in Figure 1, except that instead of utilizing a nichrome bridgewire, a "Xmas-tree" mini-bulb can be used in its place. This particular light bulb has the advantage of very small size, which conveniently fits within a soda straw, and requires only a small electrical current to fire the charge. A 9V battery, for example, works well.

This igniter may be used for either motor ignition or for firing a parachute ejection charge.

To make this igniter (shown in Figure 4), the plastic base of the mini-bulb is first removed and discarded. This exposes the two copper wire leads, which are then scraped clean of oxide, and soldered to the electrical lead wires. The other ends of the lead wires should then be stripped, and shunted (twisted together) for safety.

The glass bulb is then carefully broken open. The tip is first snapped off, which relieves the vacuum within. A pair of wire strippers (or other appropriate tool) can then be used to break off the remaining tapered portion of the glass bulb, leaving the straight portion largely intact. Care must be taken to prevent damage to the filament bridgewire or to break the lower portion of the bulb. Initially wrapping the bulb with tape helps to avoid this sort of damage. An ohmmeter should subsequently be used to ensure the filament has remained intact (the measured resistance should only be a few ohms). Don't connect a battery across the leads -- the filament may burn up as it is now exposed to atmospheric oxygen.

The mini-bulb is then placed within a 2 inch (5 cm) length of polyethylene soda straw, and the end nearest the leads sealed with hot-melt polyethylene glue.

Ignition powder (approximately 1 gram) is then carefully loaded, and tamped every so often to eliminate voids. The final step in preparation of the igniter is to tamp in a small ball of glass wool (fibre glass), and seal the end with hot-melt glue.


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Ultra-low Current Igniter

The following is a description of an igniter that requires very low electrical power, requiring only 20 mA at 1.2V (= 25mW) to fire. As such, this design is exceptionally reliable and especially useful in cold weather operation, which greatly reduces a typical battery's available power.

This igniter may be used for either motor ignition or for firing a parachute ejection charge.

The "Ultra-low Current Igniter" was developed for EARco (Experimental Aerospace Research) by Ken Tucker to increase the safety of Rocketry.

Description

Safety is paramount in rocket experimentation, and so for reliable operation of parachute ejection systems, 2nd stage ignition, and for any system that may create a hazardous condition in the event of an igniter malfunction, a very reliable, redundant igniter may be desirable. Based on Radio Shacks Part # 272-1139 Mini Lamp (or equivalent), a 1.5 Volt, 25 Milliamp electrical igniter is described. These lamps come equipped with pre-attached wire leads to facilitate hook-up (see Figure 5). In practice, the lamp will only require 1.2 Volts and 20 Milliamps to function.

Construction:

Record the resistance of the bulb using an ohm meter, a reading of, 20 -30 ohms is usual. A hole is ground through the lamp bulb by carefully grinding the bulb on a fine file or polishing stone. A magnifying glass is a handy tool to check the progress of the grind. This requires some patience but after a little practice requires a total of 5 minutes. As the glass is ground, a hole will occur, at that moment the vacuum within the bulb draws in the air and some of the glass shards. To prevent the glass shards from impacting the bulb filament, the glass should be ground with the filament perpendicular to the hole.

Once a hole is observed, recheck the ohms reading of the bulb. It should be the same as previous.

Carefully place some black powder into the bulb through the hole. Just enough to be loose is fine, packing in too tightly could damage the bridgewire. The bridgewire will explode when heated and will land on the powder, so the powder does not need to be in direct contact with the bridgewire. Take an ohms reading again to make sure no damage was done to the filament. A little Scotch tape over the hole keeps the powder in.

When fired, the igniter will produce the equivalent of a match igniting, exhausting through the hole. This may be insufficient, so a secondary burn may be desirable. Place the igniter in a tube an inch or so long, about the same diameter as the igniter. Fill this with black powder, and seal the ends. This amplifies the potency of the igniter (see mini-bulb igniter, described above).

This igniter is quite shock-proof. Hard data is lacking, but this igniter survives being thrown against a concrete surface, which causes a few 100g's of deceleration. Its very low current and voltage requirements allow for numerous redundant igniters in parallel with the ignition power supply. For example 3 igniters will only require 1.2 volts and 60 ma (3 x 20 ma) to function.

Cold Soak Test

A semi dead "9V" battery (5.1 Volt with mini x-mas bulb load prior to freezing) was placed in the freezer at -4F for 6 hours. Using the mini x-mas bulb, with a resistance load of the igniter found the voltage to be 2.5 volts. It was very dim using the mini x-mas bulb.

Connecting an "Ultra-low Current Igniter" to this dead frozen battery and it exploded instantly.

Technical Details:
Using a mini x-mas bulb as electrical load:
New 9V battery....no load 9.8V......loaded 8.2V
Used 9V battery.....no load 8.8V......loaded 5.1V (nearly dead, not good for smoke detectors)
Cold Used battery..no load 5V.........loaded 3V


Figure 6-- Mini Lamp for Ultra-low Current Igniter

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"Ferocious" Igniter

During the development phase of the RNX epoxy composite propellant, it was learned early on that the "straw" igniter (described above) was not suitable for igniting RNX propellant grains. As this epoxy-based propellant has a higher decomposition temperature, a hotter and more sustained burn is necessary. The solution that arose was an igniter that used a more energetic oxidizer together with epoxy as a binder. The resulting product, aptly referred to as a "Ferocious" igniter, burns with a hot flame for a duration of about two seconds, reliably lighting an RNX composite propellant grain.

The developmental formulation utilized potassium chlorate (KC) as the oxidizer, but this was later replaced with potassium perchlorate (KP) for safety reasons. The Ferocious igniter is made as follows:

A nichrome bridgewire is soldered to a pair of electrical leads, similar to that used for the "straw" igniter. For motors that have a small throat diameter, or if the igniter is to be installed at the grain outside surface (rather than in the grain core), fine gauge wire such as 30 AWG "wrapping wire" (RS p/n 278-501) should be used. A pyromix coating is then prepared, using the following materials:


• Potassium perchlorate, very finely ground using mortar & pestle
• Sulphur, also finely ground
• High grade epoxy, such as West System (adhesive grade is not satisfactory)

A small quantity of epoxy mixture is prepared, using the manufacturer recommended resin/hardener ratio. One half gram of epoxy is sufficient for 4 or 5 igniters. A sprinkling of sulphur is then added to the epoxy, just enough to colour the mixture a distinct yellow or light amber. The mixture is then blended well using a wooden craft stick, for a minimum of two minutes. A plastic "pan" cut from a 2 litre soda bottle works well as a disposable mixing pan. The next step is to add the KP, a little at a time, until the resulting mixture resembles a stiff paste.

Squares of polyethylene sheet , 1" x 1" (25x25 mm) are cut out from a plastic sandwich bag (or similar), two per igniter. The igniter electrical leads are placed such that the bridgewire is centred onto one of these poly squares. A dab of pyromix (approximately the amount equivalent to a pencil eraser) is placed onto the bridgewire, as shown in Figure 7. The second poly square is then placed over the pyromix and carefully pressed down such that the bridgewire is fully embedded. The resulting thickness should be no greater than 3 mm to prevent fracture of the pyromix coating upon firing. If the igniter is to be installed at the grain outside surface, the pyromix should be pressed such that a thin disc is formed of approximately 2 mm thickness maximum. The disc form is recommended for both the Epoch motor and the Paradigm motor.

The igniter is then set aside to cure overnight at room temperature. The poly squares are simply peeled off after curing. The cured igniters can be readily trimmed as necessary. For a more rapid cure, the igniters may be placed inside a dedicated shop oven set at 65^oC. (150^oF.). Curing at this elevated temperature is complete after approximately 1/2 hour. .

The Ferocious igniter has proven to be highly reliable. In over 30 motor firings, not a single misfire has occurred.

  
Figure 7 -- Top left:   Applying pyromix
Bottom left:   Pressing between poly sheets
Right:   Ferocious igniter in action

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For an alternative "Electric Match" igniter, check out Dave's High Power Rocketry -- Making Your Own Igniters . These igniters utilize nitrocellulose lacquer for a hot and fast burn.