2003-11-19
Fireworks That Spin
Fireworks that rotate, either for stability or effect, are amongst my favourite devices to build and to shoot. Most are also very easy to build and great crowd pleasers. Here is a brief discussion of the construction of several different devices which you can build yourself.
The Driver
The basis of all moving fireworks is the rocket. The gases from the burning composition expand through a nozzle carrying mass away from the device in a specific direction rather than just isotropic expansion. This is the basis of the reaction engine, unlike what is often quoted in many works about unequal pressures on the inside of the case due to the nozzle opening.
While rockets are optimized for thrust, drivers must burn more slowly to provide a long duration effect. The typical driver is a end-burning rocket with a BP variant propellant, often just meal with added charcoal, or perhaps with metal powders to produce spark effects.
Many rotating devices have the nozzle in the side wall of the casing, rather than through a refractory nozzle, such devices suffer from significant nozzle widening as the case material ablates. However such devices are designed to deal with this problem, and are typically not high performance devices requiring continuous high thrust output from the driver.
Many drivers have provisions for passing fire to the next stage of the device near their burn-out. This is easily achieved by a small hole in the bulkhead plug of the driver or the side wall just below the bulkhead. Drivers range in size from tiny 5 mm ID devices smaller than cigarettes, up to huge 60 pound gerb turning cases.
The Saxon
Probably the most simplistic rotating firework that can be made. Saxons can be knocked up in minutes and put on a great show. A saxon consists of nothing more than a charged case, plugged at both ends, arranged to rotate about a pivot in the middle and drilled with one or more vent holes, who's exhaust gases produce rotation of the case about the pivot.
Saxons offer many variations, single action, double action, and reversing. They also scale up and down very well, you can build them from 5 mm ID (1/5"), to 25 mm ID (1") or more depending on the size of the effect you desire. You can make the cases as long as you wish, as long as the walls can withstand the punishment for the full length of the burn. Saxon performance can be improved by clay nozzles formed in the end plugs, but it is by no means required.
Like all rotating pieces you can add metal powders to improve the effect, or add colour lances or whistles matched off the main fuse, or passfires along the length of the saxon for transitions during the burn. Basic greenmix, perhaps with extra charcoal makes a good propellant for saxons. Adding about 5% Aluminium flitters makes a firefly effect, while adding 5-10% Titanium makes a spectacular crackling white spark effect. Ferro-Titanium is a popular metal alloy additive for saxon propellant, as is sparkler steel. Coloured driver compositions can be tried, but most will just destroy the case faster than BP based ones.
The Wheel
The wheel, and all its variations from a single driver on a hub, to dozens in a large exhibition wheel, are based on small end-burning drivers mounted on a hub and matched together to burn in sequence. It isn't all that different from a saxon, except the driver nozzle is in the end plug, not the side wall of the casing.
As wheels require a hub there is somewhat more work in attaching the charged driver cases and matching them together, but it is easy to build long-duration wheels and transitioning effects with this kind of device. Much more complicated devices can evolve when you start adding lances, whistles or reports. The pinnacle of wheel evolution is the Maltese Irdieden. Many books could be written on wheels alone, but I'll leave it up to your imagination to come up with pleasing effects.
Common driver compositions are Firefly, Ferro-Titanium, Titanium, and Aluminium silver. Wheels adorned by complicated lance work often have straight meal or greenmix drivers so little actual effect is seen from the drivers, but there hiss and soft orange sparks can make a good background to bright colour effects.
Drivers are often 12.5 mm ID (1/2"), but 9.5 mm ID (3/8") and 19 mm ID (3/4") are commonly used as well. The matching between stages must be well protected from the sparks the wheel will produce in operation, usually each driver is nosed and the match piping tied into the nosing with string so there is no exposed match.
The Catherine Wheel
Also known as a pin wheel, these devices used to be very popular and easily available. Now days they are virtually dead. They are significantly more difficult to build than driver based wheels, with a great deal of experimentation required to perfect the thickness of the tube and the fierceness of the powder.
The driver tube is typically dry rolled from kraft, several layers thick. It must be sufficiently thick to stop the tubes from cross-firing, but thin enough to largely burn away with the composition. Once you have your kraft pipe about half a metre long and 5-8 mm in diameter you have to charge it with pin wheel composition. The composition is typically granulated but still very fine to facilitate easy loading by the wire and funnel method.
Once the pipe is charged, it is softened by laying in wet rags for a few hours, then rolled to flatten it, and finally coiled onto a former, tacked in place with a glue and left to dry.
I must warn you, actually getting a pin wheel to work properly can be a frustrating process!
The Hummer
The hummer is a unique device. It spins around its long axis, so it can achieve quite high RPM, so high in fact that it produces and audible humming or chirping sound. The secret to hummer construction is that the vent hole is drilled tangentially to the ID of the thick-walled case. The escaping gases produce a moment about the long axis center of mass and the case spins. Thick tube walls help in making a robust vent and keeping that thrust vector through the burn as the hole expands. Hummer tubes often have their wall thickness approaching their ID.
Hummers are often used as shell, mine, or candle inserts. However, they can be lit on a hard surface, stood up on one end, where they provide an addictive 'ground bloom flower'-like performance. Typically they are 12.5 mm ID (1/2") and about 25 mm long (1"). BP meal with an added 10% metal for effect is the most common propellant. Iron and Titanium are common additives.
The Z-Bomb
The Z-Bomb is a variation on the hummer, where two vents are drilled not only at a tangent, but also pointing downwards so the thrust has a propelling as well as rotating effect. They require a tapered clay plug to prevent hole enlargement, complicating their construction.
Z-Bombs are actually fairly efficient beasts, and can lift payloads or just fly to good heights with an effects tail. While straight BP meal is the most common propellant, metals can be added for effect. A 12.5 mm ID (1/2") is quite common.
Launching Z-Bombs can be a bit of a problem, depending on their length/diameter ratio they can be very unstable before they've spun up to sufficient RPMs for good stability. Some shoot them from tubes, others from a launch pin. I've had some success flying short and squat versions (I call them flying hummers because of their near-unity aspect ratio, construction and performance), which have sufficient stability to be launched straight off flat and smooth ground, like a concrete slab.
The Helicopter
The helicopter, or buzz bomb, is a cross between a Z-bomb, saxon, and tourbillion. Some helicopter designs use aerodynamic wings like a propeller to provide lift, however this is not required and is in my experience unreliable. Instead of just having the thrust spin the case about a pivot, some of that thrust can be dedicated to lifting the device as well, simply by vectoring some of the gas flow downwards. A stick or bar is still required to prevent the device turning over during the first moments of its spin-up, but it need not be large or aerodynamic, in fact there is good evidence that wings just slow the device down by adding extra air resistance.
Smaller helicopter devices, made with good quality tubes can get away with just drilling the spin vent through the case wall, like a saxon. However, for best performance a tapered clay plug is recommended, and is essential for devices over about 9.5 mm ID (3/8") unless the tubes are of very good quality. Some people fireproof the vent with sodium silicate solution, which indeed helps, but by using the clay plug the silicate treatment is not required. The clay works better than the silicate fire-proofing anyway.
BP meal is the usual propellant. Some add metals for effect, but it usually decreases their performance, most use straight meal to get better lifting capacity and add a flash report or small payload of stars. Crackling dragon eggs are a popular terminating effect in larger 1.4G commercial helicopters.
The Tourbillion
The true tourbillion is a four nozzle device, built from a single charged case. Two nozzles spin the device for stability, two provide lift. As there is significant work is drilling and matching all the holes together most people just build helicopter devices, so they are a bit of a dieing art.
However, tourbillions provide a special effect which makes them unique. As all the four holes burn in the same propellant grain, eventually there comes a point where the flame fronts break through into each other and the propellant flakes off the walls and burns very rapidly. This produces the unique burst of sparks near the burn-out of a tourbillion.
Tourbillions need a stabilizer stick like a helicopter, traditionally half the length of paint stirring sticks (often found in hardware stores) are used. Smaller tourbillions can use tongue depressors or paddle-pop sticks.
The term tourbillion (French for whirl-wind) is often used to describe any device which spins in a flat spin, especially helicopter devices and shell, mine, cake and candle inserts which spin about in the air once released.
Greenmix or BP meal with extra coarse charcoal is the most common propellant, sometimes with additives for effect. Most commonly standard 1 pound rocket tubes (19 mm ID - 3/4") are used for their construction, with simpler devices (Helicopters and Z-Bombs) used for smaller IDs.
The Stinger Missile
The Stinger is a spin stabilized rocket. It is basically a hummer in which a central core has been bored from one end. Eventually the flame-front from the spin vent breaks into the core and the device takes off. A special launch pin is used to allow the device to spin-up before it takes off.
The propellant is typically straight meal, but adding Titanium is a very popular variation. They can carry a small payload, but a report is more common as they have stability issues with increasing length, rather like a Z-Bomb.
The most popular stinger size is the '1 pound' version, 19 mm ID (3/4") but only about 70-100 mm long. However I find micro-stingers made from 9.5 mm ID, 25 mm long tubes a lot of fun as well, as they produce not just a humming sound, but a loud chirp as they take off.
The Girandola
The girandola is a complicated but very spectacular flying wheel. It is essentially an exhibition wheel with two or more rotating drivers around the rim, in addition, several drivers are matched together to provide lift and carry the rotating platform gently into the sky.
There are many variations, including using only angled drivers to produce both rotation and lift, or combinations of spin drivers followed by angled lift/spin drivers. It can get quite complicated, especially when you add lances, whistles, and terminal shell effects.
Building a girandola is a pretty large project, but the result is spectacular. Make one, if for no other reason than the practice involved in getting such a complicated piece to work properly. 12.5 mm (1/2") or 19 mm (3/4") ID drivers are the most common, burning BP meal with added charcoal or Ferro-Titanium, and occasionally Aluminium firefly flitters.
Rings of piped match are usually used in girandola construction to ensure the timing and reliability of ignitions. The ignition sequences can get very complicated once lances and shells are added. Like all multi-driver wheels the connecting match must be well protected from premature ignition by tieing into the driver nosings. Care must also be taken to ensure bursting match pipe doesn't wrench out connections or otherwise cause ignition failures.
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