Brief HistoryThe first igniters that I used for my rocket motors were quite simple, consisting of a pair of electrical wires, with a short length of nichrome wire soldered the two leads. Since this concept does not guarantee ignition unless the nichrome wire is actually contacting the propellant grain, later igniters had a small "ball" of propellant cast onto the nichrome wire. This method of ignition was more reliable, however, there were still other drawbacks affecting the reliability. For example, if the power supply was, for some reason low (or if it was cold, such as during a winter launch), the propellant ball on the igniter would char, but not ignite. As well, since the nichrome wire is brittle, breakage would sometimes occur. There was another more significant drawback. I found that, after ignition, the motor would burn for several seconds to "build up pressure" before suddenly thrusting with full force. There was clearly propellant being wasted during this phase of the burn. This occurred because of the burnrate characteristic of this propellant (and most other propellants). The rate of burning is strongly dependant upon pressure. At atmospheric pressure, the propellant burns slowly, and only starts to burn rapidly once sufficient pressure has built up in the rocket motor. Only then does the motor produce appreciable thrust. To solve this problem of wasted grain burning, and therefore reduced specific impulse, I first tried installing a "burst diaphragm" in the motor. This is a thin sheet of brass that blocks the nozzle throat, and allows pressure to build up in the motor to achieve efficient burning. At that point, the diaphragm should burst and be ejected out ot the nozzle. I experimented with such a concept, but this did not work satisfactorily. In fact, such an arrangement caused a motor failure on one of my flights (Flight C-18, July 1980), The problem was in sizing the diaphragm to burst (reliably) at exactly the right pressure, which was not simple to achieve, at least not so with my small throated motors. I abandoned this concept, and tried an alternative approach--to devise an igniter that would more reliably and effectively ignite the propellant, and would serve an additional function--to simultaneously pressurize the motor. A simple Black Powder charge would achieve this. Black powder is easy to ignite, and by placing the charge in a sealed cartridge, it would burn nearly instantly, releasing a sizeable volume of hot gases. This served to pressurize the motor while very effectively lighting the grain. The motor then achieves full thrust almost immediately. Over several flights and static tests, this pyrotechnic igniter proved to be highly reliable and significantly boosted the performance of the rocket motors. Pyrotechnic Igniter 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: 
For added safety during storage, the bared ends of the wire leads are shunted, (twisted together) to eliminate the possibility of inadvertent current flow through the igniter The electrical power source for my firing system had consisted of four 1.25 V nicad batteries (see Launch Support System), but I then switched to the DURACELL Ultra 223 photo battery due to its excellent current delivery capacity and cold weather performance. More recently, I've used rechargeable nickel metal hydride batteries, two 3.6V cells (300 mAh each) in series which deliver a generous current at 7.2V. These "cell phone" batteries also have excellent cold-weather performance. 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). 
a) 3 gram igniter b) 7 gram igniter c) 15 gram igniter Ignition PowderThe Ignition Powder that I currently use is a modified Black Powder mixture, and consists of 80% Potassium Nitrate and 20% Charcoal, by weight. Sulphur is not added to this mixture. The function of sulphur in conventional Black Powder is to allow the powder to ignite more readily by a spark. This is great for flintlock guns, but for safety reasons, it is not such a good feature for rocket motor igniters, nor is it necessary.
Chamber Pressure due to Igniter CombustionThe combustion of the igniter charge serves two important purposes:
The pressure that is generated by combustion of the igniter charge may be determined by the following equation:
P = pressure in combustion chamber at time, t lbf/in2 r= density of charge material lbm/in3 D = loading density = C / V lbm/in3 C = original mass of charge lbm V = free volume of combustion chamber at time, t in3 R = universal gas constant in-lbf /° R-lbm M = effective molecular weight of combustion products (system mass divided by number of moles of gas) lbm / mole T = adiabatic flame temperature ° R G = fraction of original charge mass consumed by time, t dimensionless Pa = atmospheric pressure lbf/in2 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-lbf/lbm. Thus, the effective force for Black Powder is 100 000 * 12 = 1 200 000 in-lbf/lbm. Derivation of this equation Example calculation For convenience, an Excel spreadsheet is available which determines combustion pressure using this method:
Pyrogen IgnitionA 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.
  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. Mini-bulb Igniter  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 advantages of minimal cost and 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 design is based on a concept pioneered by rocketry experimenter Rob Furtak, who has used the Xmas-tree bulb with great success in his rocketry work. The igniter described here 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. Most Xmas bulbs have a very fine wire or piece of foil that is wrapped around the base of the two leads that the filament is attached to. The purpose of this "shunt" is to maintain continuity in case one bulb in a string of bulbs burns out. This shunt should be removed with a fine pick or pair of tweezers. Otherwise, the ohmmeter test may falsely indicate that the bulb's filament is intact. 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. Despite the fragile appearance of the mini-bulb filament, testing has demonstrated that it is very robust. In one experiment, 20 Xmas-tree bulbs were mounted (radially outward) on the circumference of a 8 inch (20 cm) disc, which was subsequently spun at high rotational speed on a lathe, subjecting the bulbs to over 120 g's. Afterward, the bulbs were tested and all were found to function normally. Ultra-low Current IgniterThe 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:
 "Ferocious" IgniterDuring 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: 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 65oC. (150oF.). 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.
  Bottom left: Pressing between poly sheets Right: Ferocious igniter in action "Spitfire" IgniterSurely the best igniter I've ever used for igniting composite propellant is the Spitfire. This igniter is the recent outcome of a prolonged research project into developing a hot burning, reliable igniter conducted by Rob Furtak. This igniter is based on the Minibulb igniter, but instead of using a Black Powder composition as the combustion agent, a pyromix is used to coat the bridgewire (filament). The pyromix consists of a neoprene binder combined with the following composition referred to as Thermex Powder:The Thermex Powder must first be well blended. I place the constituents into a tupperware container which is mounted to a rotating "tumbler", and allow the mixing to occur for one or two hours, depending on the batch size. The minibulb (Xmas bulb) must be broken open to expose the filament. I have found that the most reliable method of doing this is to place the glass ampule of the bulb into the jaws of a workshop vise, then carefully and slowly close the vise. The bulb glass ampule will suddenly fracture and leave the filament totally exposed. The glass tends to shatter so the bulb is covered completely with a suitable cloth to catch the glass fragments. Wear safety glasses when performing this step. I've broken open about a hundred bulbs using this method, without damaging a single filament. Note that most Xmas bulbs have a very fine wire or piece of foil that is wrapped around the base of the two leads that the filament is attached to. The purpose of this "shunt" is to maintain continuity in case one bulb in a string of bulbs burns out. This shunt should be removed with a fine pick or pair of tweezers. Otherwise, the ohmmeter test may falsely indicate that the bulb's filament is intact. The copper-plated electrical leads, which typically are quite oxidized, should then be lightly scraped with a hobby knife or sandpaper to ensure good electrical conduction.
 The Thermex Powder is then added to a small amount of neoprene contact cement at a ratio of approximately 2:1 by mass, and using a craft stick, is well blended. The resulting pyromix blend should be as stiff as toothpaste. Using a toothpick, the pyromix is then carefully placed onto the filament of the minibulb (a twirling motion around the filament seems to work well). The filament should be completely covered, and to help make the resulting igniter more resilient, the pyromix should also extend down to the base of the minibulb. Working time is quite short, only a few minutes due to the high evaporation rate of the solvent, so only a few igniters should be made at one time. The igniter is then set aside to cure. Curing takes place in a few hours or even less in an environment with a slightly elevated temperature. After the cure is complete, the igniter is tested for continuity using an ohmmeter. The resistance should be approximately 3 to 5 ohms. Prior to storage of the finished igniter, the leads should be shunted (twist the leads together) for safety.
  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. |
