Monopropellant rocket

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^{[citation needed]} used monopropellant is hydrazine (N₂H₄), a chemical which is a strong reducing agent. 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.^{[citation needed]} 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.^{[citation needed]}

Most^{[citation needed]} chemical-reaction 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.^{[citation needed]}

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).^{[citation needed]}

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.^{[citation needed]}

Chemical-reaction 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.^{[citation needed]}

A concept to provide low Earth orbit (LEO) propellant depots that could be used as way-stations for other spacecraft to stop and refuel on the way to beyond-LEO missions has proposed that waste gaseous hydrogen—an inevitable byproduct of long-term liquid hydrogen storage in the radiative heat environment of space—would be usable as a monopropellant in a solar-thermal propulsion system. The waste hydrogen would be productively utilized for both orbital stationkeeping and attitude control, as well as providing limited propellant and thrust to use for orbital maneuvers to better rendezvous with other spacecraft that would be inbound to receive fuel from the depot.^[1]

Solar-thermal monoprop thrusters are also integral to the design of a next-generation cryogenic upper stage rocket proposed by U.S. company United Launch Alliance (ULA). The Advanced Common Evolved Stage (ACES) is intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA Centaur and ULA Delta Cryogenic Second Stage (DCSS) upper stage vehicles. The ACES Integrated Vehicle Fluids option eliminates all hydrazine and helium from the space vehicle—normally used for attitude control and station keeping—and depends instead on solar-thermal monoprop thrusters using waste hydrogen.^[2]

NASA is developing a new monopropellant propulsion system for small, cost-driven spacecraft with delta-v requirements in the range of 10-150 m/sec. This system is based on a hydroxylammonium nitrate (HAN)/water/fuel monopropellant blend which is extremely dense, environmentally benign, and promises good performance and simplicity.^{[citation needed]}

• Liquid-propellant rocket
• Mars Reconnaissance Orbiter
• attitude control
• momentum wheel