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Why ammonium perchlorate is used as a propellant for propelling rockets

Every chemical rocket propulsion is based on the implementation of mostly two different substances. Of the various reaction types that are possible, oxidation is mostly used in practice. One energy-rich substance is reacted with a second, whereby energy and reaction products with a lower energy content are released. In principle, however, any chemical reaction or principle that releases gas at high speed can be used to propel a rocket.

In rocket technology, the high-energy first substance is usually referred to as the actual fuel and the second as the oxidizer.

The chemical reaction that underlies all rocket fuels is an oxidation. An oxidizing agent tries to take over electrons from a reducing agent. The result is a combination of reducing agents and oxidizing agents that cannot be further oxidized and energy. Good oxidants are rare. Only 3 are recommended here: oxygen, fluorine and chlorine.

Oxygen itself is the main oxidant,compounds that contain a lot of oxygen are also good oxidizing agents. In the air, it causes oxidation in everyday life: the rusting of iron, the discoloration of apples, all of these are oxidations caused by atmospheric oxygen. Oxygen is only liquid at low temperatures of -183 ° C. However, this temperature is still well-controlled and liquid oxygen is therefore a popular oxidizer in rocket technology.

Fluorine is the best oxidizer imaginable. However, it is also more aggressive than oxygen and provides more energy than it. Similar to oxygen, it is liquid and very dense only at temperatures of -189 ° C (1.51 g / cm³). However, fluorine is so aggressive that it reacts spontaneously with many materials.

Chlorine releases significantly less energy than oxygen when burned, but is more than twice as heavy. Pure chlorine gas would be a liquid fuel, but since chlorine forms the aggressive hydrochloric acid and acts as an ozone depleting substance in the stratosphere, chlorine is not used as a liquid fuel. Perchlorate - salts of an acid from chlorine and oxygen are the oxidizer in many solid rockets.

Ammonium perchlorate (NH4ClO4) plays a special role. It is a solid oxidizer and fuel in one compound. When it burns, it breaks down into:
NH4ClO4 »2 H2O + N + O2 + Cl

The oxygen and chlorine released can then oxidize other fuels such as aluminum or polybutadiene. You usually combine both. Aluminum provides very high heat when burned and keeps the reaction going. However, the end product is aluminum chloride or aluminum oxide of high molecular weight and therefore has a poor specific impulse. A plastic like polybutadiene forms a rubber-like mass and, when burned, supplies carbon dioxide and water, comparable to the burning of kerosene.

The specific impulse of this combustion is the highest of solid fuels so far. Up to 2900 m / s was achieved in a vacuum, but this value is still far below that of liquid fuels. Speaks in favor of the ammonium perchlorate, which is very simple to manufacture based on solid-state engines, and which is why they are often used as boosters (auxiliary rockets to launch).

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