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rocket propulsion[′räk·ət pro‚pəl·shən]
The process of imparting a force to a flying vehicle, such as a missile or a spacecraft, by the momentum of ejected matter. This matter, called propellant, is stored in the vehicle and ejected at high velocity. In chemical rockets the propellents are chemical compounds that undergo a chemical combustion reaction, releasing the energy for thermodynamically accelerating and ejecting the gaseous reaction products at high velocities. Chemical rocket propulsion is thus differentiated from other types of rocket propulsion, which use nuclear, solar, or electrical energy as their power source and which may use mechanisms other than the adiabatic expansion of a gas for achieving a high ejection velocity. Propulsion systems using liquid propellants (such as kerosine and liquid oxygen) have traditionally been called rocket engines, and those that use propellants in solid form have been called rocket motors. See Electrothermal propulsion, Ion propulsion, Plasma propulsion, Propulsion
The performance of a missile or space vehicle propelled by a rocket propulsion system is usually expressed in terms of such parameters as range, maximum velocity increase of flight, payload, maximum altitude, or time to reach a given target. Propulsion performance parameters (such as rocket exhaust velocity, specific impulse, thrust, or propulsion system weight) are used in computing these vehicle performance criteria. The table gives typical performance values. See Specific impulse, Thrust
Rocket propulsion is used for different military missiles or space-flight missions. Each requires different thrust levels, operating durations, and other capabilities. In addition, rocket propulsion systems are used for rocket sleds, jet-assisted takeoff, principal power plants for experimental aircraft, or weather sounding rockets. For some space-flight applications, systems other than chemical rockets are used or are being investigated for possible future use.
|Propulsion system parameter||Typical range of values|
|Specific impulse at sea level||180–390 s|
|Specific impulse at altitude||215–470 s|
|Exhaust velocity at sea level||5800–15,000 ft/s (1800–4500 m/s)|
|Combustion temperature||4000–7200°F (2200–4000°C)|
|Chamber pressures||100–3000 lb/in.2 (0.7–20 MPa)|
|Ratio of thrust to propulsion||20–150|
|Thrust||0.01–6.6 × 106 lb (0.05–2.9 × 107 n)†|
|Flight speeds||0–50,000 ft/s (0–15,000 m/s)|
|*Exact values depend on application, propulsion system design, and propellant selection. †Maximum value applies to a cluster; for a single rocket motor it is 3.3 × 106 lb (14,700 kN).|
Liquid-propellant rocket engines
These use liquid propellants stored in the vehicle for their chemical combustion energy. The principal hardware subsystems are one or more thrust chambers, a propellant feed system, which includes the propellant tanks in the vehicle, and a control system.
Bipropellants have a separate oxidizer liquid (such as lique-field oxygen or nitrogen tetroxide) and a separate fuel liquid (such as liquefied hydrogen or hydrazine). Monopropellants consist of a single liquid that contains both oxidizer and fuel ingredients. A catalyst is required to decompose the monopropellant into gaseous combustion products. Bipropellant combinations allow higher performance (higher specific impulse) than monopropellants.
The three principal components of a thrust chamber are the combustion chamber, where rapid, high-temperature combustion takes place; the converging-diverging nozzle, where the hot reaction-product gases are accelerated to supersonic velocities; and an injector, which meters the flow of propellants in the desired mixture of fuel and oxidizer, introduces the propellants into the combustion chamber, and causes them to be atomized or broken up into small droplets. Some thrust chambers (such as the space shuttle's main engines and orbital maneuvering engines) are gimbaled or swiveled to allow a change in the direction of the thrust vector for vehicle flight motion control.
Solid-propellant rocket motors
In rocket motors the propellant is a solid material that feels like a soft plastic or soap. The solid propellant cake or body is known as the grain. It can have a complex internal geometry and is fully contained inside the solid motor case, to which a supersonic nozzle is attached.
The propellant contains all the chemicals necessary to maintain combustion. Once ignited, a grain will burn on all exposed surfaces until all the usable propellant is consumed; small unburned residual propellant slivers often remain in the chamber. As the grain surface recedes, a chemical reaction converts the solid propellant into hot gas. The hot gas then flows through internal passages within the grain to the nozzle, where it is accelerated to supersonic velocities. A pyrotechnic igniter provides the energy for starting the combustion.
The nozzle must be protected from excessive heat transfer, from high-velocity hot gases, from erosion by small solid or liquid particles in the gas (such as aluminum oxide), and from chemical reactions with aggressive rocket exhaust products. The highest heat transfer and the most severe erosion occur at the nozzle throat and immediately upstream from there. Special composite materials, called ablative materials, are used for heat protection, such as various types of graphite or reinforced plastics with fibers made of carbon or silica. The development of a new composite material, namely, woven carbon fibers in a carbon matrix, has allowed higher wall temperatures and higher strength at elevated temperatures; it is now used in nozzle throats, nozzle inlets, and exit cones. It is made by carbonizing (heating in a nonoxidizing atmosphere) organic materials, such as rayon or phenolics. Multiple layers of different heat-resistant and heat-insulating materials are often particularly effective. A three-dimensional pattern of fibers created by a process similar to weaving gives the nozzle extra strength. See Nozzle
Nozzles can have sophisticated thrust-vector control mechanisms. In one such system the nozzle forces are absorbed by a doughnut-shaped, confined, liquid-filled bag, in which the liquid moves as the nozzle is canted. The space shuttle solid rocket boosters have gimbaled nozzles for thrust-vector control, with actuators driven by auxiliary power units and hydraulic pumps.
Hybrid rocket propulsion
A hybrid uses a liquid propellant together with a solid propellant in the same rocket engine. The arrangement of the solid fuel is similar to that of the grain of a solid-propellant rocket; however, no burning takes place directly on the surface of the grain because it contains little or no oxidizer. Instead, the fuel on the grain surface is heated, decomposed, and vaporized, and the vapors burn with the oxidizer some distance away from the surface. The combustion is therefore inefficient.
Because flights of rocket-propelled vehicles are usually fairly expensive and because it is sometimes difficult to obtain sufficient and accurate data from fast-moving flight vehicles, it is accepted practice to test rocket propulsion systems and components extensively on the ground under simulated flight conditions. Components such as an igniter or a turbine are tested separately. Complete engines are tested in static engine test stands; the complete vehicle stage is also tested statically. In the latter two tests the engine and vehicle are adequately secured by suitable structures. Only in flight tests are they allowed to leave the ground.