First, the solid rocket engine.
Solid rocket engine is a chemical rocket engine using solid propellant. Solid propellants include polyurethane, polybutadiene, hydroxyl-terminated polybutadiene and nitrate plasticized polyether.
Solid rocket engine consists of grain, combustion chamber, nozzle assembly and ignition device. The grain is a hollow cylinder made of propellant and a small amount of additives (the hollow part is a combustion surface, and the cross-sectional shape is round and star-shaped. The grain is placed in the combustion chamber (usually the engine casing). When the propellant burns, the combustion chamber must bear the high temperature of 2,500 ~ 3,500 degrees and the high pressure of 102 ~ 2× 107 Pa, so it must be made of high-strength alloy steel, titanium alloy or composite materials, and an insulation lining must be installed between the grain and the combustion inner wall.
The ignition device is used to ignite the grain, and usually consists of an electric ignition tube and a powder box (filled with black powder or pyrotechnics). After electrifying, ignite the black powder with the heating wire, and then ignite the black powder to burn the grain.
In addition to accelerating gas expansion to generate thrust, the nozzle is usually combined with a thrust vector control system to control the thrust direction. The system can change the gas injection angle, thus realizing the change of thrust direction.
When the grain burns, the engine will stop working.
Compared with liquid rocket engine, solid rocket engine has the advantages of simple structure, high propellant density and convenient and reliable operation. The disadvantage is that "specific impulse" is small (also called specific thrust, which is the ratio of engine thrust to propellant weight consumed per second, in seconds). The specific impulse of solid rocket motor is 250 ~ 300 seconds, which has short working time and large acceleration, and it is difficult to control thrust and start repeatedly, which is not conducive to manned flight.
Solid rocket engines are mainly used as engines for rockets, missiles and sounding rockets, as well as booster engines for spacecraft launch and take-off.
Second, liquid rocket engine.
Liquid rocket engine refers to chemical rocket engine with liquid propellant. Commonly used liquid oxidants include liquid oxygen and nitrogen tetroxide. Combustion agents include liquid hydrogen, unsymmetrical dimethylhydrazine, kerosene, etc. Oxidants and combustibles must be stored in different tanks.
Liquid rocket engine generally consists of thrust chamber, propellant supply system and engine control system.
Thrust chamber is an important part that converts chemical energy of liquid propellant into propulsion. It consists of propellant nozzle, combustion chamber and nozzle assembly, as shown in the figure. Propellant is injected into the combustion chamber through an injector, and after atomization, evaporation, mixing and combustion, combustion products are produced, which are rushed out of the nozzle at a high speed (2,500-5,000m/s) to generate thrust. The pressure in the combustion chamber can reach 200 atmospheres (about 20OMPa) and the temperature is 300 ~ 4000℃, so it needs to be cooled.
The function of the propellant supply system is to deliver propellant to the combustion chamber according to the required flow and pressure. According to different modes of transportation, there are two types of supply systems: extrusion (pneumatic) and pumping. The extrusion supply system uses high-pressure gas to decompress through the pressure reducer (the flow rate of oxidant and combustion agent is controlled by the pressure set by the pressure reducer) to enter the oxidant and combustion agent storage tanks and then extrude them into the combustion chamber respectively. The extrusion supply system is only used for small thrust engines. High thrust engines use a pump pressure supply system, which uses a hydraulic pump to transport propellant.
The function of the engine control system is to adjust and control the working procedures and parameters of the engine. Working procedures include engine starting and working. Shutdown three stages, this process is carried out automatically according to the predetermined procedure. Working parameters mainly refer to thrust and propellant mixing ratio.
The advantages of liquid rocket engine are specific impulse (25o ~ 5o seconds), large thrust range (single thrust is 1g ~ 700t), repeated starting, controllable thrust and long working time. Liquid rocket engine is mainly used for spacecraft launch, attitude correction and control, orbit transfer and so on.
Third, rocket engines using other energy sources.
(1) electric rocket engine
Electric rocket engine is a rocket engine that uses electric energy to accelerate working medium and form high-speed jet to generate thrust. Unlike the chemical rocket engine, the energy and working fluid of this engine are separated. Electric energy is provided by aircraft, and is generally obtained by solar energy, nuclear energy and chemical energy through conversion devices. Working fluids include hydrogen, nitrogen, argon, mercury, ammonia and other gases.
The electric rocket engine consists of power supply, power exchanger, power regulator, working medium supply system and electric propeller. Power supply and power converter provide electric energy; The function of the power regulator is to start the engine according to the predetermined program, and constantly adjust various parameters of the electric propeller to keep the engine in the specified working state; The working medium supply system stores the working medium and transports the working medium; The function of electric thruster is to convert electric energy into kinetic energy of working medium, so that it can produce high-speed jet and thrust.
According to the different ways of accelerating working medium, there are three types of electric rocket engines: electric rocket engines, electrostatic rocket engines and electromagnetic rocket engines. Electric rocket engine uses electric energy to heat (resistance heating or arc heating) working medium (hydrogen, amine, hydrazine, etc.). ) make it vaporize; After the nozzle expands and accelerates, it is discharged from the nozzle to generate thrust. Working fluid (mercury, cesium, hydrogen, etc. Ionized from the tank into ions, and then accelerated into high-speed ion flow under the action of electrostatic field of the electrode, resulting in thrust. Electromagnetic rocket engine uses electromagnetic field to accelerate ionized working medium to produce jet and form thrust. The electric rocket engine has a very high specific impulse (70O~250O seconds) and a very long service life (it can be repeatedly started for tens of thousands of times and accumulated work can reach tens of thousands of hours). But the thrust generated is less than 10ON. This kind of engine is only suitable for attitude control and position maintenance of spacecraft.
(2) Nuclear rocket engine
Nuclear rocket engine uses nuclear fuel as energy source and liquid hydrogen, liquid helium and liquid ammonia as working fluids. A nuclear rocket engine consists of a nuclear reactor, a cooling nozzle, a working medium conveying system and a control system. Which is installed in the thrust chamber. In a nuclear reactor, nuclear energy is converted into heat energy to heat the working medium. After the heated working medium expands and accelerates through the nozzle, it is discharged from the nozzle at the speed of 6500 ~ 1 100 m/s to generate thrust. Nuclear rocket engine (250 ~1000s) has a long specific impulse, but its technology is complex, and it is only suitable for spacecraft working for a long time. This kind of engine is still being tested because of problems such as nuclear radiation protection, tail gas pollution, reactor control and high-efficiency heat exchanger design. In addition, solar heating and photon rocket engine are still in the stage of theoretical exploration.