Liquid-Propellant Rocket Propulsion SystemsThese systems are commonly called rocket engines. They are usually preferred for large space launch vehicles, because they allow a higher specific impulse and thus a larger payload (or a more ambitious spaceflight mission). In some applications they also allow restarting in flight, random throttling (reduced thrust operation and thus better trajectory control), and reuse for another flight. Most use liquid bi-propellantsÑthat is, an oxidizing liquid (such as liquid oxygen or nitrogen tetroxide) burning with a liquid fuel (such as liquid hydrogen or kerosene). Some attitude-control rockets have used monopropellants (such as hydrazine). Monopropellants are stable liquids at ambient temperature; but when stimulated by a catalyst or heat, they decompose into hot gases.A liquid-propellant rocket engine system consists of one or more thrust chambers, one or more vehicle tanks that contain the propellants, a feed mechanism to force the liquids into the thrust chamber, a power source to furnish the energy required by the feed mechanism, suitable valves and piping to transfer the liquids, a structure to transmit the thrust forces, and control devices to start and regulate propellant flow rates.The thrust chamber consists of an injector, a combustion chamber, and a supersonic nozzle. It is where the liquid propellants are metered, injected, atomized, mixed, and burned to form hot gaseous reaction products. These products are then accelerated and ejected at a high velocity to impart thrust. The injector is usually an intricate assembly of internal passages and accurately oriented injection holes that introduce the propellants into the combustion chamber, atomizing and mixing them in such a way as to create a relatively uniform mixture of fuel and oxidizer in droplets that will readily evaporate and burn in the combustion chamber. The chamber may be cooled by circulating one of the propellants (usually the fuel) through cooling jackets or passages. Heat may also be absorbed by ablative materials. Alternatively, certain special high-temperature materials, such as molybdenum metal, can be used as another means of radiating away excess heat.Two principal types of feed systems are used for liquid-propellant rocket engines. One, which uses pumps for moving the propellants from their tanks to the thrust chamber, is usually found in high-thrust booster applications. The other, using high-pressure gas for expelling or displacing the propellants from their tanks, commonly has spacecraft attitude-control and maneuvering applications.
Because liquid propellants float in the zero-gravity environment of space, special devices are necessary to ensure that the outlet pipe will always be filled with liquid.The function of low-thrust attitude-control systems (ACS) is to achieve an accurate predetermined flight path and to precisely control the vehicle's rotational position (for example, to properly align a fixed telescope or antenna). This is done with low-thrust liquid-propellant rocket propulsion systems that can be randomly pulsed (multiple start-and-stop operations). The attitude-control engines are used in addition to the main rocket engines on most space vehicles, upper stages of multiwarhead ballistic missiles, or some guided tactical rocket missiles (surface-to-air missiles for ballistic missile defense), allowing them to be steered toward the intended target.The thrust chamber nozzles are usually mounted in pairs at the perimeter of a spacecraft. Two thrust chambers pointing in opposite directions are fired simultaneously to give a true turning movement to the vehicle. A typical€S may have between 4 and 16 thrust chambers, depending on the redundancy and on the number of rotational maneuvers that need to be controlled. It requires a minimum of 12 thrust chambers to allow control in two rotational directions about three perpendicular axes. Thrust levels can range from a fraction of a newton up to pehaps 1,000 newtons each. The mode of operation is to use a series of short-time pulses of thrust applications, often only for 20 to 100 milliseconds. In some long-duration satellite spacecraft, the ACS may have to be pulsed or restarted as often as 50,000 times to assure an accurate trajectory or orbit and to counteract flight-path perturbations or disturbances.All liquid propellants present some hazards. High-performance liquid propellants such as liquid oxygen or hydrogen are very cold (cryogenic), relatively nontoxic, and difficult to keep stored. Storable liquid propellants, such as nitric acid, nitrogen tetroxide, or hydrazine are usually toxic, corrosive, or flammable.