Two 3' fire arrows

History

Instead of fins, fire arrows use a stick for stabilization, not unlike many fireworks rockets still do today. Until World War I ended, most rockets used sticks, rather than fins. This leads me to believe there must be a body of literature out there on the design of stick-stabilized rockets. I'd be extremely appreciative of any references, especially those that discuss stability. Sticks have less restoring force than fins do, therefore stick-stabilized rockets are inherently less stable than fin-stabilized rockets. They should only be flown in relatively calm wind conditions. However, every time I fly one, some people always seem to think that it won't fly. Remember, stick-stabilized rockets have a longer history than fin-stabilized rockets.

I've built a number of these rockets, most of them small ones as described in this article. I have built a full-scale model with a 6' x 1" bamboo pole, and a 4" diameter fiberglassed airframe. This one had 2x29mm motor mounts, and I flew it successfully several times. However, eventually it was destroyed in a crash when only one of the two G80 motors ignited on launch.

The fire arrow was a successful military weapon. It was the first use of black powder to propel a military projectile, and actually predates firearms. The Chinese used them to repel the Mongols. The Mongols adopted the technology and used them in an invasion of Japan. Later, the Mongols used fire arrows against the Arabs, who in turn, adopted the weapon themselves, and used them against the French in the seventh crusade. A variety of warheads were carried - incendiary, explosive, and shrapnel.

Wife repelling Mongol Hordes with fire arrows.

The particular fire arrow I've been modelling is from an illustration in the Chinese military classic Wu-ching Tsung-yao (The Complete Compendium of Military Classics), written in 1045 A.D.

The British Congreve rockets, from which we get the phrase "The rocket's red glare" in our national anthem were stick stabilized rockets directly descended from the fire arrow. By this time, the heads of the rockets were being constructed of iron, and contained bursting charges and carbine balls or incendiary materials.

While most Chinese fire arrows had spear points fitted on the end of the stabilizing stick, a few didn't. I've chosen to model ones without spear points for safety reasons. Another safety point I'd like to make is that several people have pointed out to me that these rockets resemble fireworks rockets. Despite this appearance, these fire arrow models are model rockets, not fireworks. They do not carry any pyrotechnic effects, and they use conventional parachute recovery. It is amazing that modern fireworks rockets still resemble the first Chinese rockets - they have not yet adopted "modern" features such as fins.

Anyone who's seen me fly rockets knows that I make heavy use of recycled materials for rocket construction. While I'm sure you could go out and spend bucks on glassine tubes and such, I've spec'd this one out using common household items. The materials cost on each rocket should be about a dollar. The parts list and tools required include the materials for building a launcher. Since these rockets don't use a launch rod, you'll probably have to build a launcher (if you're at a PHITS launch, you're welcome to use mine). 

Parts List:

• 3 - toilet paper tubes.
• 3 - 4" x 4" squares of thin (1/16" thick or less) cardboard.
• 2 - Spent 24mm (Estes D size) or 18mm (Estes A-C size) rocket motor casing.
• 1 - 3' x 3/16th" bamboo garden stake (select the straightest one you can find at a garden store).
• 1 - Nose weight (clay, or sand mixed with epoxy, etc.)
• 1 - Plastic grocery bag for making chute (I use Hobbytown USA bags).
• 1 - Length of string (for making shroud lines).
• 1 - Roll duct tape. Essential for rocketry.
• 1 - Aluminum soda pop can.
• 1 - 2x4, 18" long.
• 1 - 1/2" nominal copper plumbing pipe, 2 1/2' long.
• 1 - fishing line "snap swivel" for chute attachment.
• 1 - 24" of 1/8" sewing elastic (shock cord).

Tools and Supplies:

• Yellow (recommended) or white glue.
• "Super Glue" (Cyanoacrylate adhesive).
• Compass
• Ruler
• Pencil
• Clothespin
• Rubber bands
• Paint or construction paper
• Scissors
• Razor saw, Hacksaw, or other small saw for cutting expended engine casing
• Hammer
• Tubing cutter, for copper plumbing pipe
• 1/2" drill bit and electric or hand drill.

Body tube

The body tube is formed from one and a half TP tubes spliced together.

Cut one of the TP tubes in half (each piece will be 2 1/4" long). Mark one of these pieces, "A" and the other "B".

Slit the wall of tube "A". Then, cut it in half. This will result in two curved pieces of cardboard 1 1/8th" long. Call these parts "A1" and "A2".

Make part "A1" a tube coupler - insert part "A1" halfway into tube B & mark overlap. Remove part "A1", and smear glue on the overlap, and all over the outside. Insert part "A1" halfway into part "B", and then slide one of the other tubes on. The resulting tube should be 6 3/4" long.

Nose cone

The nose cone is formed from a cardboard cone and a tube.

Lay out the nose cone with a compass and ruler on one of the sheets of thin cardboard as shown in the figure. Cut the semicircle out of the cardboard, and form it into a cone with a slight overlap. Mark the overlap, smear it with glue, and then form the cone again. Allow the nose cone to dry with a clothespin holding it in place.

Slide part "A2" into the remaining TP tube & mark the overlap. Remove A2, and smear glue on the overlap. Part "A2" must be a slide fit inside a TP tube when dry. Allow part "A2" to dry with a clothespin holding it in place.

When both the cone and part "A2" are dry, they can be glued together to form the nose cone. Glue part "A2" into the bottom of the cone to form a "mushroom" like shape. Once the nose cone assembly has dried, fillet around the joint between "A2" and the cone until it has sealed. Failure to seal this joint will result in a parachute ejection failure.

Motor mount

Mark two 1 11/16" diameter circles on the remaining two sheets of thin cardboard. Cut them out.

Cut a length of the remaining TP tube to be the same length as the spent engine casing. Slit the wall of this tube and wrap it around the expended engine casing. Cut the tube so that there is only about 1/4" of overlap when wrapped around the engine casing. Glue this overlap sparingly, so as not to get glue inside the tube, and inadvertently gluing in the spent casing. Hold it tight while it dries by wrapping rubber bands around the tube.

When the tube has dried, remove the expended engine casing, and cut off 1/4" of it with a saw. This will form the motor mount block. Glue the 1/4" piece into the end of the tube formed above.

Measure the diameter of the motor tube, and cut circles with this diameter in the two 1 11/16" diameter circles you made above. Slide these onto the motor tube, and glue in place, 1/2" from either end.

Major assembly

Glue the completed motor mount tube into the body tube so that the end of the motor mount is flush with the end of the body tube. It works best to push the motor mount into the end of the body tube farthest from where it is spliced.

Take the bamboo garden stake, and sand a flat spot the length of the body tube (6 3/4") on the side of the largest (thickest) end. Glue this to the side of the body tube. Make sure it is glued on straight. Rubber bands are useful for holding this together as it dries. Once the glue dries, fillet the bamboo stake to the body tube. It is very important that this be secure, or it will tend to break off on landing.

Recovery System

Form the recovery system attachment point: Cut a 1/4" by 1/2" piece of aluminum from a soda pop can. Punch a small hole near one end that is large enough and close enough to the end for the clip end of a snap swivel to attach to it. Super glue the end of this tab without the hole to the base of the nose cone, on the inside.

Cut a hexagonal chute from the plastic grocery bag, or Hobbytown bag. Make 3 shroud lines, each one being four times the diameter of the chute. Attach the shroud lines to the chute with duct tape, and thread the lines through the loop end of the snap swivel.

Form two shock cord anchor points (the Estes style trapezoid with two folds) with some of the remaining scraps of TP tube. Glue the shock cord into the two shock cord anchors. Glue one anchor inside the body tube (make sure it's far enough down it won't interfere with the fit of the nose cone). Glue the other anchor inside the nose cone.

Clip the parachute snap swivel to the aluminum tab on the nose cone.

It's not worthwhile to attempt streamer recovery of this model. Use a parachute. I tried a 3"x30" streamer, and it fell too fast. I feel that the fact that there's a long bamboo stick falling fast is unsafe.

Weighting the nose

Determining if a fire arrow will fly stably is a challenge. Unlike "regular" rockets, there are no canned mathematical calculations or computer programs to determine stability. You can't even use the old cardboard cutout method, because it fails with the stick.

In addition to the oddity of shape, there is another important effect of the placement of the rocket motor in a fire arrow. In a rocket with the motor in the rear, the rocket is least stable at launch. This is because the weight of the engine pulls the center of gravity to the rear. The fire arrow, however, is most stable when the rocket is launched. As the fire arrow flies, propellant in the motor burns, and the center of gravity shifts aft. This is exactly the opposite of a conventional rocket.

This means that it is VERY important to measure the center of gravity of the fire arrow with an EXPENDED rocket motor casing in place, instead of a full one.

Take the second expended rocket motor, and put it in the fire arrow. Pack the chute into the nose, with wadding. Measure the center of gravity of the rocket. To be stable, a good location for the center of gravity is 1" forward of the aft end of the body tube. To move the center of gravity forward, add weight to the nose cone with your favorite method (sand & epoxy or clay, etc.)

When you fly your fire arrow, observe the flight path. If it initially flies straight, but tends to shoot off at odd angles at altitude, you can improve the stability by adding more nose weight.

If you decide to design your own fire arrows, please be careful. As it is difficult to determine if they will be stable before flight, conduct initial flights of new designs according to the safety code. In particular, do not do it around groups of people.

Finish

I like to finish the fire arrows by wrapping construction paper around the body tube. Paint is another option. Try to think of unconventional finishing techniques for this unconventional model rocket.

Launcher

I've found the easiest way to launch fire arrows is with a tube that the stick slides down into. I have one small launcher, which I will describe how to build here, that I use for fire arrows that use 3' bamboo garden stakes. I have another one made out of larger plumbing parts and 2x6's that I have used to launch up to a 6' fire arrow with a 4" body tube.

Drill a 1/2" diameter hole in the length of 2x4.

With a hammer, pound the 1/2" nominal copper tubing into the hole in the 2x4. Because the outside diameter is actually slightly larger than 1/2", this will take some pounding. The end you're pounding on will tend to get deformed - this is OK.

With the tubing cutter, cut off the deformed end of the copper pipe.

Flight

I've successfully flown a fire arrow of this design on a 13mm Estes A10-3T motor. While not very spectacular, the chute did (barely) have time to eject. In general, use short delays for for your fire arrow. 18mm motors I've used include Estes B4-4, A8-3, and even an Aerotech composite E. 24mm versions fly well on Estes D12-3 and D12-5.

Fire arrows don't fly well in the wind. They are more sensitive to windy conditions than finned rockets, because the stick has less restoring force than fins.

The body tube is quite short in fire arrows, so the ejection gases are still quite hot when it hits the chute. Use as much recovery wadding as you can. In my 6' model, I had problems keeping the chute from getting toasted due to the ejection charge of two Aerotech G80-4's in a 16" long body tube.

Don't try to catch fire arrows as they land - they're strong and I've never had a stick break, even when landing on concrete. The stick could be hazardous to your eyes, etc. Let the fire arrow come to rest on the ground before chasing after it.

References

History of Rocketry And Space Travel (Revised Edition) Wernher Von Braun & Fredrick I. Ordway III 1969, Thomas Y. Crowell Company, New York