294 lines
18 KiB
Plaintext
294 lines
18 KiB
Plaintext
PrimoPyro1990
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.... Composite Rocket Fuels ....
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DISCLAIMER: The following file contains information of harmful or illegal
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nature. Neither the BBS or author providing this information
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can be considered responsible for the use of this file.
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The person using this knowledge is solely responsible for
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it's use or misuse. This file is intended to educate only.
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1.) Introduction
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Composite propellants are solid rocket fuels that are composed of separate
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fuels and oxidizers mixed together in one homogenous mass. This propellant
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is then either molded into a grain to be inserted in an engine or cast in an
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engine casing and left to harden. The fuels and oxidizers taken separately
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are generally unreactive. Composite propellants are used in a number of
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engines. There are engines that use water for fuel and an oxidizer, air for
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an oxidizer like a ramjet, and a liquid/solid engine that can be throttled.
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The rocket motors discussed here a best built by the amateur with propellant
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weights below 2 lbs. and preferably under 1 lb. This is still powerful enough
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to shoot a sizable rocket to well over 4 miles altitude.
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Before I get into propellant mixtures a few terms to learn are:
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Specific Impulse - Defined as the impulse (force * time) delivered by
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burning a unit weight of propellant in a rocket engine.
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Volume Specific Impulse - The product of specific impulse and density. This
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is expressed in pound-seconds per cubic inch.
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If the propellant's weight is kept constant, a
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propellant with a lower Isp but a higher density
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may outperform one with a greater Isp but a
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lower density.
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Specific Force - This is a measurement of the ability for a gas to perform
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work. Specific force (F) is expressed in foot-pound per
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pound.
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3.) OXIDIZERS
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Composite propellants contain both an oxidizer and a fuel. The oxidizer may
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be a monopropellant and as such contributes power to the propellant mix.
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The ideal oxidizer should decompose into totally gaseous exhaust.
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Oxidizers used in composite propellants : Potassium perchlorate (KClO4).
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Potassium pechlorate was one of the first used oxidizers. One of it's draw-
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backs is the product of decomposition ( potasium chloride ) is not a gas at
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regular temperatures and does not contribute as a working gas. The KCl
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appears as a dense smoke in the rockets exhaust. Burning rates of propellants
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made with KClO4 are usually high at 0.8 - 0.9 in/sec at 1000 PSI. Densities
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of fuels made with KClO4 also tend to high at 1.8 - 2.0 gm/cc. Specific
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impulses are usually below 200 lb-sec/lb. Potassium perchlorate is hardly
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ever used im modern propellants.
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Ammonium Perchlorate NH4ClO4. This is the oxidizer of choice when possible.
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The products of diassociation of NH4ClO4 are 100% gas. The specific impulse
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of propellants using this oxidizer reaches 250 lb-sec/lb. Depending on the
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percentage of NH4ClO4 the burning rate may reach or exceed 0.5 in/sec. The
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products of exhaust are N2, CO, CO2, H2, H2O, and HCl. The HCl may pose some
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problem if the engine is used in high humidity as the HCl vapor may form
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visible hydrochloric acid fumes.
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Ammonium Nitrate NH4NO3. This oxidizer is useful as it is usually available
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in bulk weight. The products of disassociation of NH4NO3 are 100% gas. How-
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ever the temperatures produced by the propellant are low. For this reason,the
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specific impulse of NH4NO3 propellants are usually no greater than 180 lb-sec
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/lb and low percentage propellants have an Isp of 75 lb-sec/lb. The products
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of exhaust of NH4NO3 propellants are N2, CO, CO2, H2, H2O. These gases cause
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no special problems. The burning rate of NH4NO3 Propellants are low, ranging
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from 0.05 in/sec to 0.27 in/sec. The higher burning rates are possible if
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catalysts are used in the propellant. Prussian blue, chromium compounds
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(ammonium dichromate), or cobalt compounds are catalysts that are used.
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Ammonium nitrate is hygroscopic and undergoes a phase change if the temper-
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ature goes above 90 deg./F. Because of this phase change, some grains may
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crack if the temperature cycles about this temperature. The burning temper-
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ature of NH4NO3 propellants are lower than any other propellants especially
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at low percentages of oxidizer.
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Lithium Perchorate LiClO4. Some work has been done using lithium perchlorate
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as an oxidizer. The lithium chloride formed in the exhaust is a gas at high
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temperatures. Lithium salts are hygroscopic and must be protected from high
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humidity. Burning rates of LiClO4 propellants are similar to KClO4 mixtures.
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4.) FUELS
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Fuels Used in Composite Propellants : Since most rubbers and polymers are not
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available to the general public, some adjustments have to be made. A good
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source of plastics is an auto supply store. There you can find epoxy resin
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which can be used as a fuel. You will also find fiberglass resin. This is a
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liquid made from polystyrene and polyester resin. It is catalyzed with a few
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drops of hardener. PVC plastic can be dissolved in tetrahydofuran to make
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a thick paste. This can be mixed with an oxidizer and allowed to dry for an
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extended time to form a propellant grain. Asphalt was used in some JATO units
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about 30 years ago but it was found lacking when used at high temperatures.
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Some fuels used in commercial engines are polyurethane rubber, polysulfide
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rubber, and butadiene-acrylic acid. Non ferrous metals are added to propell-
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ants to increase the temperature of combustion and consequently the Isp. The
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metals most used are aluminum, magnesium, and copper. The metals are usually
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added in amounts of 5% - 25%. In engines designed to breath water as an oxid-
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izer, metal amounts to about 50% to 80% of the weight of the propellant. The
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other components are usually ammonium perchlorate and a polymer.
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Propellant Grain Geometry : If the grain is ignited from end on, like a
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candle burns, the thrust will be steady or neutral. If the grain has a hole
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in it extending end to end and the combustion takes place from the inside out
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then the thrust will rise to a peak or be progressive. This is because the
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surface area of the grain becomes greater as it burns whereas in a neutral
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grain the surface area remains the same. A cruciform shaped grain produces a
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large amount of thrust first then tapers off because the surface area becomes
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smaller. If the grain is tubular and the combustion takes place from both the
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inside out and the outside in, then the thrust will be neutral but fast
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burning.
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Wherever you wish the grain not to burn, it must be coated with a retardent.
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Epoxy works well as a retardent as does Elmers white glue. At least two coats
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of retardent should be used. An epoxy retardent can be used to retain a grain
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in a rocket engine. When tubular grains are used, the igniter is usually put
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towards the nose of the rocket and fires backwards towards the nozzle. This
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insures the grain is ignited completely.
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Inspect the propellant grain for any cracks or imperfections. A crack can
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cause the surface area of the propellant to increase astronomically. This can
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cause an explosion because of the increased pressure.
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5.) PROPELLANT MIXTURES
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The ratios of oxidizers and fuels depends on the type of engine desired. The
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amount of oxidizer can be as high as 90% as in some ammonium nitrate mixes to
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as little as 20% ammonium perchlorate as in some water breathing engines.
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A fast burning mixture: Potassium Perchlorate 20%
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Isp=200 Ammonium Perchlorate 55%
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Epoxy Resin/Hardener 17%
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Powdered Aluminum 8%
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This is very fast burning but the exhaust makes a fairly heavy smoke.
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A slow burning propellant. Great for sustainer engines.
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Isp=165 Ammonium Nitrate 70%
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Ammonium Perchlorate 10%
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Polyester Resin 18%
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Powdered Charcoal 2%
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Not very powerful but useful. The charcoal helps keep the combustion steady.
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A very powerful mixture: Ammonium Perchlorate 75%
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Isp=250 Powdered Aluminum 10%
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PVC in THF 15%
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All the ingredients should be dampened with THF (tetrahydrofuran) before
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mixing. Do this in an area with very good ventilation and wear rubber gloves
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to keep from contacting the THF with bare skin. This mixture is best used in
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a perforated grain to help the solvent evaporate.
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An ammonium nitrate based propellant: Ammonium Nitrate 70%
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Isp=160 Powdered Aluminum 5%
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Polyester Resin 18%
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Ammonium Bichromate 5%
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Powdered Charcoal 2%
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A good mix when perchlorates are not available.
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Do not under any circumstances use chlorates for rocket propellants. You will
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not make a rocket, just a pipe bomb with fins.
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6.) COMPOUNDING PROPELLANTS
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One thing to keep in mind when making a propellant, the volume of fuel/binder
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to volume of oxidizer and additives must not be too low. If it is then the
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mixture will be too dry to mix well. It will also hurt the strength of the
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grain. You may have to cut down on the amount of oxidizer depending on the
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fuel you are using.
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For rockets weighing 1 pound and less the easiest way to make the propellant
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is to obtain a suitable container for mixing and put in the bottom of it the
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correct amount of fuel/binder. The other ingredients are added one at a time
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to the fuel and mixed in. One thing that really determines the success of a
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propellant is the particle size of the oxidizer. It should be as finely
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powdered as possible. Continue mixing the propellant until it is a homo-
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geneous mixture. Now pour it or stuff it into the engine casing taking care
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to eliminate all air bubbles. Any mandrels needed to form the grain to shape
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shpould already be lubricated for release and in place. After waiting a suit-
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able time for the binder to harden, remove the mandrels and place the engine
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in a warm place to finish curing. Inspect the grain for any cracks or imper-
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fections.
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Some large propellant grains are constructed by cementing smaller grains to-
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gether. Disks of propellant can be glued and stacked to form a long grain.
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The disks can be drilled with a number of holes to make a progressive or
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regressive burning grain. The holes are lined up when the disks are stacked.
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If you construct a press with a number of guide rods to match the drilled
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holes, so much the better. The cement can be a very thin layer of the polymer
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used to make the grain. If you are using a PVC based grain, then dampen both
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mating surfaces with THF and press them together for a minute before adding
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the next disk.
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You can also load a cardboard casing with the propellant. After the prope-
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llant is cured, this cartridge is loaded into the engine.
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When drilling these propellants or using any power tool for shaping them, use
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the lowest speed while checking to make sure no heat is building up on the
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cutting surface. If care is used, machining propellants is safe.
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7.) ENGINE CONSTRUCTION
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The typical engine is designed to operate at 1000 psi. The casing of the
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engine should be able to withstand at least 3000 psi as a safety factor. A
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low carbon seamless steel tube with 1/16" walls can withstand that sort of
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pressure. If the tubing has a welded seam, test fire a few engines to see if
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the tubes can take the pressure. One drawback to using steel as an engine
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casing is if the engine explodes you have some very lethal shrapnel flying
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around. If you use a high strength/high heat plastic you can eliminate some
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of this danger. Epoxy can be used to wet down a mat of fiberglass then the
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fiberglass is rolled around a large dowel to form a casing. The dowel has to
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be coated with a lubricant to keep the epoxy from gluing the casing and
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dowel together. Or you can obtain a heavy cardboard tube with the correct ID
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and coat it with epoxy then wrap epoxy/fiberglass around it. If the tubes are
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constructed properly they can take quite a bit of pressure before splitting
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apart.
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An rocket engine is equipped with a nozzle to accelerate the exhaust out of
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the rocket at a high velocity. A nozzle has a convergent section that does
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this. A divergent section of nozzle is used to lower the exhaust pressure
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so the exhaust gases accelerate out of the engine at high speeds.
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The nozzle of the engine can be machined out of metal or made of a fireproof
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ceramic. If the nozzle and the casing are metal, they can be brazed together
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before the engine is loaded. The nozzle can also be screwed into place by
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using 4 - 6 screws going through the side of the casing into the nozzle. Care
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must be used to see that the screws don't break through the inside of the
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nozzle. On smaller rockets, you may be able to get away with plaster of paris
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nozzles or for more powerful motors try pressing a mixture of 90% kaolin and
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10% aluminum oxide into a nozzle shape in the casing. Dampen the mix with a
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little water before pressing. You can make a nozzle die by turning 2 pieces
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of hardwood into divergent/convergent sections. This die should be fitted
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with a dowel guide pin at the mating points to help keep the die straight.
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A nozzle can be made from just a divervent section. This can be easily made
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by drilling the required hole in a section of nozzle. Then by drilling out
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the first hole with larger drills without completely breaking through, a
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diverging nozzle is formed. Smooth out the ID of the nozzle after drilling
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the holes. This type of nozzle is pretty good on smaller engines with a 1" ID
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or less. By using some ingenuity, you should have no problem in making a
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servicable nozzle. A rule of thumb to use for the ID of the nozzle is to use
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a hole that has an area (repeat-area,not diameter) 1/3 the area of the ID of
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the rocket engine casing.
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Most propellants burn unsteadily at low pressures. Solid rocket engines are
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equipped with a blast plug that allows the pressure to build up in the engine
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before being blown out like a cork in a bottle. Sometimes the ignitor is
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combined with the blast plug in a single unit. A stiff plastic disk makes for
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a good plug. It should have a thickness of about 1/16".
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The engine is sealed with a plug in the fore section. Depending on the con-
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struction of the engine this plug may be made of wood, plastic, or metal. It
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is held in place with either screws or epoxy. This plug must make the casing
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gas tight. Remember most rockets develop 1000 PSI.
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The ignitor is simply an electric match. It can be made with nichrome wire
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or a small light bulb. The match is used to ignite a small charge of black
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powder that in turn ignites the propellant. The ignitors leads should be
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shunted together to eliminate premature ignition. A fuse can be used instead
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of electric ignition. If you go this route, be sure of the burning time of
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the fuse and allow yourself enough time to retreat to safety after igniting
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the fuse. I cannot recommend using a fuse because you cannot stop a fuse
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from burning if someone walks into your launch area. With electric ignition,
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everything is under your control until the time of launching.
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8.) Engine Design
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It would be nice to be able to give you the complete info on designing
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rocket engines. However, the required math would be a file about 300K in
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length. Also this file is mainly about propellants. The other info is gravy.
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The best I can offer is to check out your local library for design
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and engineering books. If you want to build a rocket to simply shoot off to
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stroke your pyro perversions, build a small engine containing no more than
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4 oz. of fuel. Use a paper casing to keep the danger down and chances are
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very good that if your construction is sound you'll get the thrill of seeing
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your rocket go out of sight. If you plan to hoist a payload into suborbital
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projectory however, learn about thermodynamics, interior ballistics, and
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propellant chemistry.
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I recommend trying to get the book Amateur Rocketry Handbook. This
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book is out of print but it was put together by the Fort Sill Artillery
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School and contains a lot of valuable info.
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9.) Testing and Firing
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You should construct a few engines exactly the same and test fire a
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number of them to find out what to expect when you finally do launch a rocket
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. The engines can be buried nozzle end up in the ground and fired. Time the
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burning of the engine to figure out the rate of combustion of the propellant.
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Inspect the casing to see how it stood up. If everything seems okay you can
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construct a static testing fixture to measure the thrust. Keep in mind that
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even a small engine can put out a few hundred pounds thrust for a split
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second. When you do launch a rocket, keep people away from the launch site
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and under cover. Check out the skies for airplanes or other traffic. Don't
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launch rockets under conditions of low visibility or heavy winds.
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*** Kilroy was here ***
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