Monopropellant rocket
A monopropellant rocket (or "monoprop rocket") is a rocket that uses a single chemical as its propellant.
For monopropellant rockets that depend on a chemical reaction, the power for the propulsive reaction and resultant thrust is provided by the chemical itself. That is, the energy needed to propel the spacecraft is contained within the chemical bonds of the chemical molecules involved in the reaction.
The most commonly used monopropellant is hydrazine (N2H4), a chemical which is a strong reducing agent. The most common catalyst is granular alumina coated with iridium. These coated granules are usually under the commercial labels Aerojet S-405 (previously made by Shell) or W.C.Heraeus H-KC 12 GA (previously made by Kali Chemie). There is no igniter with hydrazine. Shell 405 is a spontaneous catalyst, that is, hydrazine decomposes on contact with the catalyst. The decomposition is highly exothermic and produces an 1000 °C (1800 °F) gas that is a mixture of nitrogen, hydrogen and ammonia. Another monopropellant is hydrogen peroxide, which, when purified to 90% or higher concentration, is self-decomposing at high temperatures or when a catalyst is present.
Most chemical-reaction monopropellant rocket systems consist of a fuel tank, usually a titanium or aluminium sphere, with an ethylene-propylene rubber container or a surface tension propellant management device filled with the fuel. The tank is then pressurized with helium or nitrogen, which pushes the fuel out to the motors. A pipe leads from the tank to a poppet valve, and then to the decomposition chamber of the rocket motor. Typically, a satellite will have not just one motor, but two to twelve, each with its own valve.
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