A monopropellant rocket (or "monoprop rocket") is a rocket that uses a single chemical as its power source and propellant. Usually the propellant is admitted to a decomposition chamber that contains a silver or platinum sponge catalyst. The most commonly used monopropellant is hydrazine (N2H4), a chemical which is characterized as "strongly reducing" (sic). The most common catalyst is granular alumina coated with iridium (e.g. S-405 or KC 12 GA). 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 1800 °F (1000 °C) 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 monopropellant rocket systems consist of a fuel tank, usually a titanium or aluminium sphere, with an ethylene-propylene rubber bladder 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.
The attitude control rocket motors for satellites and space probes are often very small, an inch or so in diameter, and mounted in clusters that point in four directions (within a plane).
The rocket is fired when the computer sends direct current through a small electromagnet that opens the poppet valve. The firing is often very brief, a few thousandths of a second, and - if operated in air - would sound like a pebble thrown against a metal trash can; if on for long, it would make a piercing hiss.
Monopropellants are not as efficient as some other propulsion technologies. Engineers choose monopropellant systems when the need for simplicity and reliability outweigh the need for high delivered impulse. If the propulsion system must produce large amounts of thrust, or have a high specific impulse, as on the main motor of an interplanetary spacecraft, other technologies are used.
Full article ▸