| Oxidisers |
| Composite propellants contain both an oxidizer and a fuel. The oxidizer may be a monopropellant and as such contributes power to the propellant mix. The ideal oxidizer should decompose into totally gaseous exhaust. |
| Oxidizers used in composite propellants : Potassium perchlorate (KClO4). Potassium perchlorate was one of the first used oxidizers. One of it's draw- backs is the product of decomposition ( potassium chloride ) is not a gas at regular temperatures and does not contribute as a working gas. The KCl appears as a dense smoke in the rockets exhaust. Burning rates of propellants made with KClO4 are usually high at 0.8 - 0.9 in/sec at 1000 PSI. Densities of fuels made with KClO4 also tend to high at 1.8 - 2.0 gm/cc. Specific impulses are usually below 200 lb-sec/lb. Potassium perchlorate is hardly ever used in modern propellants. Ammonium Perchlorate NH4ClO4. This is the oxidizer of choice when possible. The products of disassociation of NH4ClO4 are 100% gas. The specific impulse of propellants using this oxidizer reaches 250 lb-sec/lb. Depending on the percentage of NH4ClO4 the burning rate may reach or exceed 0.5 in/sec. The products of exhaust are N2, CO, CO2, H2, H2O, and HCl. The HCl may pose some problem if the engine is used in high humidity as the HCl vapour may form visible hydrochloric acid fumes. Ammonium Nitrate NH4NO3. This oxidizer is useful as it is usually available in bulk weight. The products of disassociation of NH4NO3 are 100% gas. How- ever the temperatures produced by the propellant are low. For this reason, the specific impulse of NH4NO3 propellants are usually no greater than 180 lb-sec /lb and low percentage propellants have an Isp of 75 lb-sec/lb. The products of exhaust of NH4NO3 propellants are N2, CO, CO2, H2, H2O. These gases cause no special problems. The burning rate of NH4NO3 Propellants are low, ranging from 0.05 in/sec to 0.27 in/sec. The higher burning rates are possible if catalysts are used in the propellant. Prussian blue, chromium compounds (ammonium dichromate), or cobalt compounds are catalysts that are used. Ammonium nitrate is hygroscopic and undergoes a phase change if the temperature goes above 90 deg./F. Because of this phase change, some grains may crack if the temperature cycles about this temperature. The burning temperature of NH4NO3 propellants are lower than any other propellants especially at low percentages of oxidiser. Lithium Perchlorate LiClO4. Some work has been done using lithium perchlorate as an oxidizer. The lithium chloride formed in the exhaust is a gas at high temperatures. Lithium salts are hygroscopic and must be protected from high humidity. Burning rates of LiClO4 propellants are similar to KClO4 mixtures. |