Monopropellant rocket

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 reaction 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". The most common catalyst is granular alumina coated with iridium (aka. Shell-405). There is no igniter with hydrazine. Shell 405 is a spontaneous catalyst, that is, hydrazine decomposes (combusts) on contact with the catalyst. The reaction is highly exothermic and produces an 1800 °F (1000 °C) gas that is a mixture of nitrogen, hydrogen and ammonia.

There are some unique chemical compounds that burn by themselves—no oxygen required. This is because the chemical, when energized or when in contact with a catalyst, spontaneously decomposes. The decomposition products are then exhausted to produce thrust. 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.

Engineers long ago realized the usefulness of monopropellant chemicals for satellite propulsion and attitude controls. Because only one chemical is used, the system is very simple, and thus very reliable.

Most monopropellant rocket systems consist of a fuel tank, usually a titanium or aluminum sphere, with a ethylene-propylene rubber bladder filled with the fuel. The sphere is then pressurized with helium, which pushes the fuel out to the motors. A pipe leads from the bladder to a poppet valve, and then to the reaction chamber of the rocket motor. Usually, there's not just one motor, but two to twelve, each with its own little 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. They bear a resemblance to small toys.

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 usually sounds like a pebble thrown against a metal trash can. If the motor stays on for long, it makes 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.

See also