| Propellant Mixtures |
| ABOUT FUELS |
| Fuels Used in Composite Propellants : Since most rubbers and polymers are not available to the general public, some adjustments have to be made. A good source of plastics is an auto supply store. There you can find epoxy resin which can be used as a fuel. You will also find fibreglass resin. This is a liquid made from polystyrene and polyester resin. It is catalysed with a few drops of hardener. PVC plastic can be dissolved in tetrahydofuran to make a thick paste. This can be mixed with an oxidizer and allowed to dry for an extended time to form a propellant grain. Asphalt was used in some JATO units about 30 years ago but it was found lacking when used at high temperatures. Some fuels used in commercial engines are polyurethane rubber, polysulfide rubber, and butadiene-acrylic acid. Non ferrous metals are added to propellants to increase the temperature of combustion and consequently the Isp. The metals most used are aluminium, magnesium, and copper. The metals are usually added in amounts of 5% - 25%. In engines designed to breath water as an oxidiser, metal amounts to about 50% to 80% of the weight of the propellant. The other components are usually ammonium perchlorate and a polymer. |
| Propellant Grain Geometry : If the grain is ignited from end on, like a candle burns, the thrust will be steady or neutral. If the grain has a hole in it extending end to end and the combustion takes place from the inside out then the thrust will rise to a peak or be progressive. This is because the surface area of the grain becomes greater as it burns whereas in a neutral grain the surface area remains the same. A cruciform shaped grain produces a large amount of thrust first then tapers off because the surface area becomes smaller. If the grain is tubular and the combustion takes place from both the inside out and the outside in, then the thrust will be neutral but fast burning. Wherever you wish the grain not to burn, it must be coated with a retardant. Epoxy works well as a retardant as does Elmers white glue. At least two coats of retardant should be used. An epoxy retardant can be used to retain a grain in a rocket engine. When tubular grains are used, the igniter is usually put towards the nose of the rocket and fires backwards towards the nozzle. This insures the grain is ignited completely. Inspect the propellant grain for any cracks or imperfections. A crack can cause the surface area of the propellant to increase astronomically. This can cause an explosion because of the increased pressure. |
| The ratios of oxidizers and fuels depends on the type of engine desired. The amount of oxidizer can be as high as 90% as in some ammonium nitrate mixes to as little as 20% ammonium perchlorate as in some water breathing engines. |
| A slow burning propellant. Great for sustainer engines. Isp=165 Ammonium Nitrate 70% Ammonium Perchlorate 10% Polyester Resin 18% Powdered Charcoal 2% Not very powerful but useful. The charcoal helps keep the combustion steady. |
| A very powerful mixture: Ammonium Perchlorate 75% Isp=250 Powdered Aluminium 10% PVC in THF 15% All the ingredients should be dampened with THF (tetrahydrofuran) before mixing. Do this in an area with very good ventilation and wear rubber gloves to keep from contacting the THF with bare skin. This mixture is best used in a perforated grain to help the solvent evaporate. |
| A fast burning mixture: Potassium Perchlorate 20% Isp=200 Ammonium Perchlorate 55% Epoxy Resin/Hardener 17% Powdered Aluminium 8% This is very fast burning but the exhaust makes a fairly heavy smoke. |
| An ammonium nitrate based propellant: Ammonium Nitrate 70% Isp=160 Powdered Aluminium 5% Polyester Resin 18% Ammonium Bichromate 5% Powdered Charcoal 2% A good mix when perchlorates are not available. |
| Do not under any circumstances use chlorates for rocket propellants. You will not make a rocket, just a pipe bomb with fins. |
| PROPELLANT MIXTURES |