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Some simple pyrotechnic projects

Rocket fuel - precipitation method

This simple rocket fuel is based on the classic mixture of KNO3, charcoal and sulphur. This method is designed to obviate the need for using a ball mill to produce the fuel since mills can be liable to explode.

A simple mixture of the three ingredients does not achieve an intimate enough mixture so a method is used whereby the KNO3 can be precipitated out of a concentrated solution in the form of micro-crystals.

The ingredients used are as follows:

Method

It is recommended to start with 100g of dry ingredients, i.e. 75g KNO3 (e.g. stump remover), 15g charcoal (e.g. hardwood maple barbeque - not briquettes) and 10g sulphur ( e.g. garden variety dusting powder).

First, grind the charcoal and sulphur together in a pestle and mortar to as fine a powder as possible in a reasonable amount of time. After the grinding, the powder will be a uniform very dark greenish colour.

Next, heat 35ml of low-sodium mineral water in a small stainless steel saucepan over a low heat on an electric ring. It's important to avoid using an open flame for heating the solution. Then add the KNO3 to the water - it's in the form of small beads. It will look as though there is far too much material to dissolve. Gently heat the pan to boiling point, stirring continuously. Use a wooden chopstick or something similar as a stirrer since it's best to avoid using metal.

When the water is at simmering point and after all the nitrate has dissolved, carefully and slowly added the charcoal/sulphur mixture. It is best to add this slowly to avoid creating a cloud of charcoal/sulphur dust in the air.

Keep stirring until all of the charcoal/sulphur has become wetted and is in suspension in the water. Boil very gently for around four minutes, stirring continuously.

Turn off the heat source. Remove all sources of ignition. Utilize a fume extractor if available. Allow the suspension to cool for a couple of minutes, but keep stirring. Now add the first part of the iso-propanol. For 100g of dry ingredients, you will require 40ml. The hot suspension will cause significant evaporation of iso-propanol which will present a fire hazard if there are any sources of ignition present. It's best to wear face and hand protection whilst performing this procedure.

Stir the mixture continuously for at least ten minutes. After this time, place the pan in an ice bath and add the second part of the iso-propanol. Keep stirring for several minutes more until the mixture is cold.

Transfer the mixture to some sort of paper filter. Coffee filters are about the right size for 100g of dry ingredients. It is best to use two filters inside each other for extra strength. When most of the liquid has drained away, squeeze as much remaining liquid as possible from the mixture.

When as much liquid as possible has been removed, the damp mixture can be pressed through a screen or sieve onto a drying tray.

Let the particles of powder dry completely. This can now be used for rocket fuel. Place into an airtight container for storage.

[ original procedure reference: unknown internet source ]

Simple core-burning rocket

This procedure is adapted from The incredible 5 cent sugar rocket by the Teleflite Corporation.

The first procedure is to make the propellant as described above. The original article used a sugar-based propellant. The design described here works well with the black powder based propellant discussed above.

To make it easier to understand the procedures, here is a picture of a completed engine after loading with propellant.

assembled rocket engine

Casings

The next stage in the procedure is to make the paper casing. Two inch wide gummed brown packaging tape is ideal for this application. It should be available in a hardware store or similar. However, if it is difficult to find (I had great trouble in finding it) a good place to look is in an artist supply shop (where it is used for mounting canvases). The tape should be the type coated on one side in a water-soluble glue. Duct-tape, masking tape and similar "sticky" tapes are not suitable. The following materials are required:

Cut a one foot length of 1/4" diameter hardwood dowel (available at from hardware store). Cover it with several strips of scotch tape. This is to make the inside diameter of the engine casing slightly larger than 1/4" to allow for shrinkage of the casing as it dries.

Next, cut a 10" length piece of the gummed tape and lay it, gum-side down, on a smooth flat surface such as a kitchen table. Dampen the paper side of the tape with a sponge but leave the last inch of the tape dry.

To help prevent the tape from sticking to the scotch-tape covered dowel, apply a little bit of cooking oil (such as some sort of frying spray). Now, turn the tape over and starting at the dampened end, begin rolling it up around the dowel. As soon as one turn of the dowel has been made, moisten the gummed side of the rest of the tape and roll it around the dowel as tightly and neatly as possible. This will take some practice to prevent it from telescoping. The reason the last inch was left dry was to make it easier to remove the casing from the dowel. The original article describes methods to overcome telescoping and other errors.

Before proceeding to the next stage, the casings should be allowed to dry thoroughly overnight. Alternatively, they can be baked in a 200F oven for around half and hour.

Exhaust Nozzles

The next stage is to make the exhaust nozzles. The following materials are required

The idea of the nozzle is that it constricts the exit of the casing allowing a faster flow of gas through the exhaust. Also, it allows a core to be formed in the propellant which leads to sufficient combustion speed to cause the rocket to fly, even with relatively slow-burning propellants.

It is important to create a nozzle of the correct thickness. Too thin and it will be blown out of the casing. Too thick and it will add too much weight to the engine which will hinder performance. For an engine of 1/4" in diameter, the nozzle should be 1/4" thick. To do this, insert a piece of the 1/4" dowel into the paper casing prepared earlier. Insert the dowel so that it is fully inside the case. Now mark on the dowel the position of the top of the casing. Remove the dowel, measure 1/4" inch from this mark to the end which was inserted into the case and make a new mark. Now wrap scotch tape around the dowel behind this position to a thickness such that when the tape covered dowel is inserted into the case, the end of the dowel will be 1/4" from the end of the case and it won't be possible to insert the dowel any further.

Next, mix up a small amount of the water putty to the consistency of a thick paste. With the marked dowel inserted into the case, fill the remaining 1/4" of the case with the water putty and press it in hard. Now remove the dowel. The fact that the engine casing is a little bit larger than 1/4" prevents the formation of a vacuum which would suck out the water putty when the dowel is removed. Allow the water putty to dry overnight. It can be dried faster in an oven, but may crack.

The next step is to drill a 1/8" hole in the water putty to form the exhaust. Ideally, this should be done in a drill press since the hole should be centered and aligned to the axis of the casing. An off-centre hole will give poor flight performance. The hole drilling should be done carefully to avoid cracking the putty. A sharp drill bit helps.

Core forming tools

The next stage is to assemble the core forming tool. This consists of a spindle which fits inside the casing, passing through the exhaust hole. A piece of brass or aluminium rod can be used. 3/32" sized pop-rivets make a good spindle because the rivet shaft is 1/8" in diameter and is typically smoother than a nail. Also, the rivet head can be shaped to support the nozzle. Such a rivet, carefully hammered into a piece of wood makes an effective tool - the length of the spindle should be about 1 1/2" so that a core of 1 1/4" will be formed above the nozzle. Again using a drill press if possible, drill a 1/8" hole in an 8" piece of dowel to a depth of 1 1/2". This should be as close to centered and parallel as possible. This rod will become the former for compressing fuel into the engine, the hole in the former will surround the rod, allowing the production of a core in the fuel. Make sure the former will fit over the spindle easily without binding. It helps if the spindle is polished somewhat.

Propellant loading

The next stage is to load the engine with fuel. The following diagram illustrates the procedure.

rocket engine loading

Next make a small paper funnel and place it over the open rocket casing. Fill it with about 1/8 teaspoon of the propellant. Using a small wire or stick, poke enough down into the casing to fill it about half full, remove the funnel, and insert the dowel tamp you just made (drilled end down) into the end of the casing. Using a small hammer, pack the propellant down around the spindle with four or five good, solid hits.

Repeat this filling and tamping procedure until the casing is full to within 1/4" of the top. Pack the rest of the space with kleenex, again tamping this firmly. When the casing is full, glue a piece of paper over the end. A small piece of gummed tape can be used and then trimmed when dry.

A small paper cone can then be attached to the top of the engine. This will give better aerodynamics if the engine is not used in a model rocket kit. If flying the engine on its own, a balance stick is required. The 12" long bamboo kebab skewers used for barbecue-ing are ideal. This can be attached with a piece of gummed tape.

Priming

Finally, the engine has to be primed and fused for firing. A standard Estes-style electric igniter can be used. Otherwise, crush a small amount of the propellant and pour this carefully through the exhaust hole. Insert a piece of visco type fuse into the exhaust hole and you're ready to go. If it won't fit, proceed as follows. Mix the crushed propellant with a drop of water until it forms a thick paste. After pouring the dry crushed propellant into the exhaust, seal the hole with the paste. Allow to dry. Attach a piece of visco to the end of the rocket and stick it in position with some tape so that the end of the visco touches the propellant sealed exhaust hole.

Firing

To launch in a model rocket kit, follow the procedure describe with the kit. To launch an engine with a stick, simply place the stick in a soda straw placed in the ground or place the stick in the neck of a bottle. Aim away from any people and property, light the fuse and retire to a safe distance. Always make sure there isn't the chance of starting a fire if the rocket will land somewhere unexpected.

As always, please follow all local, state and federal fire and safety regulations, and be sure to consider the health and safety of yourself, everyone around you, and their property. If you set a fire with one of these rockets or injure yourself or someone else in the process of making one, it will reflect badly on the hobbies of model and amateur rocketry as a whole and may result in even more repressive legislation than already exists.

Performance tips

The rocket's performance can be modified by lengthening or shortening the core. If it flies too fast or explodes, reduce the length of the core. If it flies too slowly or doesn't take off at all, lengthen the core. Also, check that the nozzle isn't too thick.

Disclaimer

Caution: The material in these web pages is not for everyone. Due to the nature of high energy materials, accidents can and will happen. The author accepts no responsibility for any accidents or injuries caused by following the procedures described here. A description here does not imply that the author has in any way attempted to follow that procedure. The author strongly urges you not to attempt any of the procedures described here. If in doubt, DON'T DO IT.You have to be able to think, have some common sense and some manual skills to do this stuff.

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