2003-11-08
Spin-Stabilised Rockets
These devices originally appeared on the Asian fireworks market, called warhead launchers. They were probably developed to reduce the labour and materials involved with conventional rocket designs. They fell off the market for a while because of their fairly large flash payload, but were around long enough to develop quite a following. Eventually Warren Klofkorn wrote up the design in an article for AFN sealing their fate as a favourite for all pyro amateurs.
I am a little shaky of the details of their history, but at some point they got called "Stinger Missiles" and the name stuck. They are no relation to the sophisticated FIM-92A weapons system, AFAIK that stinger is not spin stabilised at all, while it might roll at high rates, and have several very high-tech on-board devices (like laser ring gyros) to deal with this, it has servo controlled fins to steer it.
The fireworks stinger is a more humble device, a cross between an insert hummer (or bee) and a conventional core-burning rocket. The high-speed axial rotation of the motor gives it sufficient angular moment to fly in a (roughly!) straight line without conventional aerodynamic stabilisers. Unfortunately this design limits the physical aspect ratio of the motor casing (about 4 calibres long max) and requires a balanced mass placement throughout the device. Poorly built stingers can fly apart, take off in unpredictable directions, or just fail to leave the ground.
The ignition flame is applied to a tangential side vent, which spins up the motor on a launch pin inserted through the nozzle and into the core as a pivot. It takes a moment for the flame front in the propellant to break out into the core, this gives the motor a chance to come up to a reasonable rate of rotation before the conventional core-burning rocket motor comes up to pressure and lifts the device skywards.
The bigger a stinger is the more moment of inertia it will have, and consequently the slower its rotation will accelerate, also the less RPMs it will actually need for good stability. Small stingers can rotate so fast they produce sounds around 1 kHz in frequency, heard as a very addictive chirp on take-off. Examining the spectrogram of the sound produced by a stinger can be very informative, displaying any precession modulation and giving you a general feel for the performance achieved.
With this rotation comes tensile forces in the case, that add to the already high hoop stress of the internal pressures. Fortunately these devices are small enough that this doesn't cause much of a problem. Any balance problem or shifting mass in a payload will be immediately magnified and begin precession of the rotation axis, often quite violent and fast enough to modulate the sound produced. Taken to the extreme the motor can spin-flip or tumble uselessly off the launch pin and onto the ground. Any payloads must be small, well packed and balanced.
My first attempt at building a stinger was a very small device. A modified hummer, 8 mm ID with 1.8 mm nozzles and core. Unlike the videos I had seen of 19 mm ID stinger launches I was surprised that mine made a wonderful chirping sound, along with the beautiful broad tail of orange and silver sparks. I've been addicted to these micro scale stingers ever since. I've now settled on the 9.5 mm ID 25 mm long size as I have a large quantity of matching paper tubes and the slightly larger diameter helps carry more propellant giving better performance.
The tube wall need not be all that thick, the burn is fast and furious, as long as it doesn't suffer structural failure thermal failure will likely not be a problem. Too thin and forming the spin vent can be a problem. A little waterglass (sodium silicate solution) can be used to fireproof the spin vent which improves the performance a lot. If you are making your own tubes consider rolling them with waterglass as the paste or at the very least PVA rather than flour paste. Every milligram of mass you save will make the motor spin up faster and fly higher. Don't be overly concerned though, just get one flying then worry about tuning the performance.
These micro stingers are fun. While you can get better performance from a conventional rocket, the sound and tail the micro stinger makes is a great effect. You can make 20 or more in an hour once tooled up, and shoot them off spent sparkler wires stuck into the ground. You can even drill the spin vent higher, at the half-way point, and launch them straight off the ground without a launch pin. It isn't recommended, but these devices are small enough to make it fairly safe with enough fuse to get the hell away. The thin fuse required could be a problem to find commercially. I believe there is a Chinese visco-like product that would be suitable, but I just make my own special thin blackmatch and glue it in using a little meal made into a paste with nitrocellulose lacquer (ping-pong balls dissolved in acetone works fine).
Micro stinger tooling is trivial and cheap. You need a short length of 9 mm timber dowel. Cut the ends square and polish one up with fine sandpaper. The other end can be taped to resist splitting, or a larger piece of dowel can be jointed and glued on top. The bottom plug is rammed flat on the bench, it need only be 4 mm thick if you use wax treated clay, experiment to find how thin you can make them before the motor explodes. The wax can be added to pulverised clumping cat litter by dissolving a candle in shellite (petrol or Coleman fuel works too I believe) mixing through and letting the solvent evaporate. A case extender is very handy for ramming the last of the propellant and the top plug, make one simply by pasting a little kraft or chipboard over the end of a piece of tube off-cut, a second tube can be used to facilitate construction then removed before drying. A 1.8 mm diameter drill is used for both the nozzle and core as well as the spin vent. The core runs the full length of the propellant grain. No drilling jig is required, you can do it by hand with some care. You can make up a thin nipple, mandrel and hollow drift if you wish, it is potentionally safer with metals in your propellant, but the drill works fine if you go carefully. Drilling the spin-vent before ramming is messy and in my opinion not the best way to do it, if your propellant is especially sensitive then by all means drill it first and use a mandrel pin for the nozzle and core.
Like most pyro amateurs I've purchased a commercial 19.5 mm stinger tooling kit from Skylighter. This makes a much larger device that can carry worth-while payloads, a few stars or a sizable report. Dan Williams has an excellent step-by-step walk through on his site that will have you making your own in minutes.
The dead-weight of a 19 mm stinger is about 24-30 g depending on the length of the tube you elect to use. About 14 g of propellant will be needed for a rocket carrying a payload, this amount just covers the spindle and an extra 5-8 g of delay composition is placed above that before the garnish. If you are making a stinger purely for its tail effect you can cut the tube shorter and use about 18 g of propellant for better performance. These numbers are based on mallet rammed grains, pressed grains might take 20 g of propellant.
Suitable propellant for smaller devices (up to about 25 mm ID) is pure blackpowder meal, the faster the better. Larger stingers or ones with high-performance cores (long cores) will probably explode on pure meal, so you can cut it with extra charcoal to slow it down as you would for a conventional rocket. It is recommended to add 3-5% Titanium to the propellant for a wonderful tail effect, I've used up to 10% myself for a very beautiful tail effect. Titanium is very hard, and will score up your tooling in no time, particularly any spherical stuff that makes it under the Aluminium ramming base, but the effect is more than worth it.
I also use my Rocket Propellant (or RP), mainly because I always have a batch of it prepared for normal core-burners. It offers a strange variegated tail of charcoal and Aluminium effect that changes throughout the flight. It performs well too, but my personal favourite is still the Titanium/meal propellant.
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