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. A (usually liquid) rocket propellant that can be used by itself, without the need for a second component. ... A Redstone rocket, part of the Mercury program A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. ... General Name, Symbol, Number silver, Ag, 47 Chemical series transition metals Group, Period, Block 11, 5, d Appearance lustrous white metal Atomic mass 107. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Atomic mass 195. ... A catalyst (Greek: καταλύτης, catalytÄ“s) is a substance that accelerates the rate (speed) of a chemical reaction without itself being transformed or consumed by the reaction (see also catalysis). ... Hydrazine is a chemical compound with formula N2H4 used as a rocket fuel. ... General Name, Symbol, Number iridium, Ir, 77 Chemical series transition metals Group, Period, Block 9, 6, d Appearance silvery white Atomic mass 192. ...

There are some unique chemical compounds that burn by themselves—no oxygen required! This is because the chemical when energized, spontaneously decomposes and then the decomposition products are exhausted to produce thrust. Another monopropellant is hydrogen peroxide, which when purified to 90% or higher is self-combustible at high temperatures, or with a catalyst. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... Hydrogen peroxide (H2O2) is a clear liquid, slightly more viscous than water, that has strong oxidizing properties and is therefore a powerful bleaching agent that has found use as a disinfectant, as an oxidizer, and (particularly in high concentrations as high test peroxide (HTP)) as a monopropellant in rockets. ... A catalyst (Greek: καταλύτης, catalytÄ“s) is a substance that accelerates the rate (speed) of a chemical reaction without itself being transformed or consumed by the reaction (see also catalysis). ...

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. A satellite is any object that orbits another object (which is known as its primary). ... In the context of spacecraft, attitude control is control of the angular position and rotation of the spacecraft, either relative to the object that it is orbiting, or relative to the celestial sphere. ...

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. General Name, Symbol, Number titanium, Ti, 22 Chemical series transition metals Group, Period, Block 4, 4, d Appearance silvery metallic Atomic mass 47. ... Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... poppet valve A poppet valve is the type of valve system used in most piston engines, used to seal the intake and exhaust ports. ...

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 look almost like 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. A computer is a device or machine for processing information from data according to a program — a compiled list of instruction. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. ... poppet valve A poppet valve is the type of valve system used in most piston engines, used to seal the intake and exhaust ports. ...

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. The specific impulse (commonly abbreviated Isp) of a propulsion system is the impulse (change in momentum) per unit of propellant. ...

See also

• thruster (competing technologies)
• spacecraft propulsion
• attitude control
• momentum wheel (a complementary technology)