Rocket engine is provided by aircraftpropellant(energy), a jet engine that does not use outside air.It can work in the space beyond the dense atmosphere. The energy is converted into the kinetic energy of the working medium (working medium) in the rocket engine, forming a high-speed jet discharge and generating thrust.[1]
Rocket engines useimpulsePrinciple, with propellant, independent of external airJet engine。Rocket engine is a kind of jet engine, which transfers the reactants in propellant tank or vehicle（propellant）BecomeHigh-speed jet, due toNewton's third law of motionAnd generate thrust.Rocket engines can be used for spacecraft propulsion, and can also be used for missiles to fly in the atmosphere.Most rocket engines are internal combustion engines, and there are also non combustion engines.
working principle
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Most engines obtain thrust by discharging high-temperature and high-speed gas. Solid or liquid propellant (composed of oxidant and fuel) burns in the combustion chamber at high pressure (10-200bar) to produce gas.
Feed propellant into combustion chamber
Liquid rocket makes oxidant and fuel enter combustion chamber respectively by pump or high-pressure gas, and two propellant components are mixed and burned in combustion chamber.The propellant of the solid rocket is mixed in advance and put into the combustion chamber.Solid liquid hybrid rockets use solid and liquid mixed propellants or gas propellants, and some use high-energy power supplies to send inert reactive materials to heat exchangers, which does not require combustion chambers.Rocket propellant is usually stored in propellant tank before burning and discharging to generate thrust.Chemical propellants are generally used as propellants, which generate high-temperature gas for rocket propulsion after undergoing exothermic chemical reaction.
combustion chamber
The combustion chamber of chemical rocket is usually cylindrical, and its size should meet the requirements of full combustion of propellant. Different propellants are used, and their sizes are different.Describe combustion chamber dimensions with L *
here:
Vc is the combustion chamber capacity
At is the nozzle area
The range of L * is generally 25-60 feet (0.6-1.5m)
The pressure and temperature of the combustion chamber usually reach the extreme value. Unlike aspirated jet engines, which have enough nitrogen to dilute and cool combustion, the temperature of the rocket engine combustion chamber can reach the chemical standard value.The high pressure means that the heat transfer speed in the combustion chamber wall is very fast.
Combustion chamber shrinkage ratio
The contraction ratio of the combustion chamber refers to the ratio of the cross-sectional area of the combustion chamber to the throat area of the nozzle.When the pressure of propellant and combustion chamber is constant, the contraction ratio is inversely proportional to the mass flow density, and the combustion chamber contraction ratio is determined when the mass flow density is selected.However, it is more direct and convenient to select the combustion chamber diameter by using the contraction ratio.The selection of shrinkage ratio is mainly based on experimental or statistical methods, and the following data are recommended:
For the large thrust and high-pressure combustors of most pump pressure supply systems, the contraction ratio is usually 1.3~2.5.
For the combustion chamber with centrifugal nozzle, the contraction ratio is usually 4~5.
injector
The shape of the engine mainly depends on the shape of the expansion nozzle: bell shaped or conical.In a narrowing and widening nozzle with high expansion ratio, the high-temperature gas generated by the combustion chamber is discharged through an opening (nozzle).
If enough high pressure is provided to the nozzle (2.5 to 3 times higher than the confining pressure), nozzle choke and supersonic jet will be formed, and most of the heat energy will be converted into kinetic energy, thus increasing the exhaust speed.At sea level, it is not uncommon for engines to exhaust at ten times the speed of sound.Part of the rocket thrust comes from the pressure imbalance in the combustion chamber, but it mainly comes from the pressure squeezing the inner wall of the nozzle.When the exhaust gas expands (adiabatic), the pressure on the inner wall makes the rocket move in one direction while the exhaust gas moves in the opposite direction.
Propellant efficiency
In order to make the engine use propellant effectively, it is necessary to use a certain mass of propellant to generate the maximum possible pressure on the combustion chamber and nozzle. In addition, the following methods can also improve the propellant efficiency:
Heat the propellant to the highest possible temperature (using high-energy fuel, hydrogen, carbon or some metals such as aluminum, or nuclear energy).
Use low specific gravity gas (containing hydrogen as much as possible).
Use small molecule propellant (or propellant that can be decomposed into small molecules).
rocket engine
Because all measures are taken to reduce the mass of propellant;The pressure is proportional to the accelerated propulsion dose;Also because of Newton's third law, the pressure acting on the engine also acts on the propellant.The speed of exhaust gas leaving the combustion chamber seems to be determined by the combustion chamber pressure.However, the speed is obviously affected by the above three factors.Taken together, exhaust speed is the best proof of engine efficiency.
Due to aerodynamic reasons, the exhaust gas has a flow blocking effect at the nozzle.The sound speed increases with the square root of temperature, so the use of high-temperature exhaust can improve engine performance.At room temperature, the sound speed in the air is 340m/s, while in the high-temperature gas of the rocket, it can reach more than 1700m/s. Most of the performance of the rocket is due to the high temperature.In addition, the rocket propellant usually uses small molecules, which also makes the sound speed in exhaust gas higher than that in air at the same temperature.
The expansion design of the nozzle doubles the exhaust velocity, usually 1.5 to 2 times, resulting in a quasi hypersonic exhaust jet.The increment of velocity is mainly determined by the area expansion ratio, that is, the ratio of nozzle area to nozzle outlet area.The nature of the gas is also important.The nozzle with large expansion ratio is larger in size, but it can make the exhaust gas release more heat, thus improving the exhaust speed.
The nozzle efficiency is affected by the working height, because the atmospheric pressure decreases with the height.However, as the tail gas is supersonic, the pressure of the jet will only be lower or higher than the confining pressure, and cannot be balanced with it.
If the tail gas pressure is different from the confining pressure, the tail gas can become fully expanded or excessively expanded.
Back pressure and optimum expansion
To obtain the best performance, the pressure of exhaust gas at the nozzle end needs to be equal to the confining pressure.If the exhaust pressure is less than the confining pressure, the carrier will decelerate due to the air pressure difference between the front end and the end of the engine.If the tail gas pressure is greater than the confining pressure, the tail gas pressure that should be converted into thrust is not converted, and energy is wasted.
Russian rocket engine
In order to maintain the balance between tail gas pressure and confining pressure, the nozzle diameter needs to increase with the height, so that the tail gas has a long enough distance to act on the nozzle to reduce the pressure and temperature.This increases the design difficulty.In actual design, a compromise approach is usually adopted, thus sacrificing efficiency.There are many special nozzles that can make up for this defect, such as plug nozzle, step nozzle, diffusion nozzle and tile nozzle.Each special nozzle can adjust the confining pressure and allow the exhaust gas to spread more widely in the nozzle, generating additional thrust at high altitude.
When the confining pressure is low enough, such as vacuum, some problems will occur: one problem is the shear weight of the nozzle. In some carriers, the weight of the nozzle also affects the engine efficiency.The second problem is that the exhaust gas expands adiabatic and cools in the nozzle. Some chemicals in the jet will condense and produce "snow", which will lead to instability of the jet. This must be avoided.
Power cycle
Compared with the heat loss at the nozzle, the pumping loss is very small.Engines used in the atmosphere use high-pressure power cycles to improve nozzle efficiency, while vacuum engines do not have this requirement.For liquid engines, there are four basic forms of power cycle to inject propellant into the combustion chamber:
Squeeze cycle - the propellant is forced out by the gas in the built-in high-pressure gas cylinder.
Expansion cycle - the propellant flows through the main combustion chamber to expand and drive the turbine pump.
Gas generator circulation - a small part of propellant is burned in the precombustion chamber to drive the turbine pump, and the exhaust gas is discharged through independent pipes, so the energy efficiency is lost.
Staged combustion cycle - the high-pressure gas from the turbine pump is sent back to the drive self starting cycle, and the high-pressure exhaust gas is directly sent to the main combustion chamber without energy loss.
Overall performance
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Rocket technology combines high thrust (millionNewton）, high exhaust velocity (10 times the sound speed at sea level), high thrust weight ratio (>100) and the ability to work outside the atmosphere.In addition, it is often possible to make one performance higher by weakening the other.
Specific impulse
An important index to measure the performance of an engine is the impulse generated by a unit mass of propellant, that is, the specific impulse (usually written as Isp).Specific impulse can be measured by velocity (Ve meters per second or feet per second) or time (seconds).Engines with large specific impulse often have excellent performance.
Net thrust
The following is the approximate calculation formula of engine net thrust:
Because the rocket engine doesn'tJet engineTherefore, it is not necessary to deduct the punching resistance from the total thrust, because the net thrust is equal to the total thrust (excluding the static back pressure).
throttle
The engine can achieve the purpose of throttling by controlling the propellant flow (usually in kg/s or lb/s).
In principle, the engine can reduce the outlet pressure to one-third of the confining pressure (nozzle flow separation) by throttling, and the upper limit can reach the maximum value that the engine mechanical force allows.
In fact, the throttling range of the engine varies greatly, but most rockets can easily reach their mechanical upper limit. The main limiting factor is combustion stability.For example, the propellant nozzle needs a minimum pressure to avoid causing destructive vibration (intermittent combustion and combustion instability), but the nozzle can often be adjusted and tested in a larger range.It is also necessary to ensure that the nozzle outlet pressure will not be too much lower than the confining pressure to avoid flow separation problems.
energy efficiency
Rocket engine is a kind of thermal engine with high efficiency, which produces high-speed jet, resulting in high combustion chamber temperature and high compression ratio just like Carnot cycle.If the velocity of the vehicle reaches or slightly exceeds the exhaust velocity (relative to the vehicle), then the energy efficiency is high.At zero speed, the energy efficiency is zero.(This is true of all jet propulsion)
Cooling system
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Material process
The reaction temperature of reaction materials in the combustion chamber can reach about 3500 K (~5800 ℉).This temperature is far beyond the melting point of nozzle and combustion chamber materials (except graphite and tungsten).It is true that suitable propellants can be found within the bearing range of some materials themselves, but it is also important to ensure that these materials will not burn, melt or boil.The material process determines the upper limit of chemical rocket exhaust temperature.
rocket engine
Another method is to use ordinary materials such as aluminum, steel, nickel or copper alloy and use a cooling system to prevent the material from overheating.For example, regenerative cooling is used to make the propellant pass through the inner wall of the combustion chamber or nozzle before combustion.Other cooling systems such as water curtain cooling and film cooling can extend the life of combustion chamber and nozzle.These technologies can guarantee the heat of gasboundary layerThe temperature will not affect the safety of materials when contacting them.
The heat flux in rockets is often the highest in engineering, and its variation range is 1-200MW/m2.The heat flux at the nozzle is the highest, usually twice that at the combustion chamber and nozzle.This is due to the high speed (resulting in a thin boundary layer) and high temperature of the exhaust gas at the nozzle.
Most other jet enginesgas turbineIt operates at high temperature, but its surface area is too large to cool, so it has to reduce the temperature and lose efficiency.
Common cooling methods
No cooling: for short-term operation or test
Ablative wall: there is ablative material on the chamber wall, which can absorb heat and fall off continuously
Radiation cooling: make the room wall reach white hot state to radiate heat
Heat sink cooling: pour a propellant (usually liquid hydrogen) down along the chamber wall
Regenerative cooling: propellant flows through the cooling sleeve in the chamber wall before combustion
Water curtain cooling: the propellant ejector is specially arranged to reduce the gas temperature around the chamber wall
Film cooling: the chamber wall is wetted by liquid propellant, and the liquid evaporates and absorbs heat to cool it
rocket engine
All cooling measures are to form a layer of isolation layer (boundary layer) with lower temperature than the room wall. As long as this layer of isolation layer is not damaged, the room wall will not have problems.However, combustion instability or cooling system failure will often lead to the interruption of boundary layer protection, and then lead to the destruction of the chamber wall.
The regenerative cooling system also has a second boundary layer, which is the cooling pipe wall surrounding the chamber wall.Since this boundary layer acts as the isolation layer between the chamber wall and the coolant, its thickness should be as thin as possible, which can be achieved by accelerating the coolant flow rate.
Mechanical problems
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The rocket combustion chamber works under high pressure, usually 10-200bar (1-20MPa). The higher the pressure, the better the performance (because more efficient nozzles can be used).This puts the outside of the combustion chamber under great circumferential stress.Due to the high-temperature working environment, the tensile strength of structural materials is significantly reduced.
Acoustic problems
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The extreme vibration and acoustic environment in the rocket engine cause the peak stress to be much higher than the average value, especially the problems of organ like pipe resonance and airflow disturbance.
Combustion instability
Combustion instability includes the following:
Intermittent combustion
This is the low-frequency vibration of the combustion chamber pressure caused by the propellant delivery pipe pressure change caused by the acceleration change of the launch vehicle.It can make the thrust of the launch vehicle change periodically, resulting in damage to the load and the launch vehicle.Intermittent combustion can be prevented by using high-density propellant coupled with gas filled damping turbine pump.
Buzzing phenomenon
This is due to insufficient pressure in the propellant ejector.It is mainly unpleasant and has no substantial harm.However, in some extreme cases, the combustion may enter the injector and cause the explosion of monopropellant.
Oscillating combustion
rocket engine
This situation often causes direct damage and is difficult to control.It is often an acoustic process accompanying the chemical combustion process, and is the main driving force for energy release.It may lead to unstable resonance and thin insulation boundary layer, resulting in tragic consequences.This kind of impact is difficult to analyze in advance in the design phase, and can only be achieved through long-term testing and constant correction.The correction methods usually include adjusting the injector carefully, changing the chemical properties of the propellant, or evaporating the propellant into a gas state before injecting it into the Helmholtz damper (to change the resonance state of the combustion chamber).
Another common test method is to detonate a small amount of explosives in the combustion chamber to determine the pulse response of the engine and estimate the response time of the chamber pressure: the faster the recovery is, the more stable the system is.
Exhaust noise
Rocket engines (except for ultra small ones) are very noisy compared with other engines.The hypersonic exhaust gas mixes with the surrounding air to form a shock wave.The sound intensity of the shock wave depends on the size of the rocket.
rocket engine
When Saturn V launched, a seismograph far away from its launch point detected the noise.The sound intensity generated depends on the size of the rocket and the exhaust speed.The shock wave signature sound heard at the scene is mainly a pop sound.The peak value of this kind of noise exceeds the maximum allowable limit of microphone and audio electronic equipment, so this kind of noise is weakened or disappeared in recording or broadcast audio playback.The noise of large rocket launch can directly kill people around.When the space shuttle takes off, the noise around the base exceeds 200dB (A).
Usually, the noise of the rocket near the ground is the biggest, because the noise is radiated from the plume and reflected by the ground.In addition, when the launch vehicle rises slowly, only a small amount of propellant energy is converted into the kinetic energy of the launch vehicle (useful power P is transferred to the launch vehicle P=F * V, F is the thrust, V is the speed), so most of the energy is dispersed into the exhaust gas, and then interacts with the surrounding air to generate noise.This noise can be reduced by means of a flame isolation groove with a roof, spraying water on the plume, deflecting the angle of the plume, etc.
Commissioning
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Before the engine is put into production, the static test is usually carried out on the rocket engine test bench.For high altitude engines, the nozzle needs to be shortened or tested in a large vacuum chamber.
Security
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Rockets give the impression that they are unreliable, dangerous and catastrophic.Military rockets are highly reliable.However, one of the main non military uses of rockets: orbital launch. In order to increase the payload weight, it is necessary to reduce the deadweight, and reliability and deadweight reduction cannot be met at the same time.Moreover, if the number of flight times of the carrier is very small, the probability of accidents caused by design, operation or manufacturing is very high.In fact, all launch vehicles are flight tests based on aerospace standard data.
The error rate of the X-15 rocket aircraft is only 0.5%, and only one failure occurred in the ground test.The main engine of the space shuttle has no accident in more than 350 flights.
Chemical problems
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rocket engine
Rocket propellant requires the use of materials with high specific energy (energy per unit mass), because under ideal conditions all reactive materials are converted into exhaust kinetic energy.In addition to inevitable losses, engine design defects, incomplete combustion and other factors, according to the law of thermodynamics, part of the energy is converted into kinetic energy of molecules, which cannot generate thrust.A monatomic gas such as helium has only three degrees of freedom, equivalent to onethree-dimensional spaceIn the coordinates {x, y, z}, only this spherical symmetric molecule has no such loss.Diatomic molecules such as H2 can rotate around the axis in the connection direction and the axis perpendicular to it. According to the equipartition law of statistical mechanics, the effective energy will be equally distributed to all degrees of freedom. Therefore, 3/5 of the energy of this molecule in the thermal balance will be converted into one-way motion, and 2/5 into rotational motion.Triatomic molecules such as water molecules have six degrees of freedom.Most chemical reactions are the third case.The function of the nozzle is to convert free thermal energy into unidirectional molecular movement to generate thrust. As long as the exhaust gas maintains a balanced state during expansion, the diffusion nozzle is large enough to allow the exhaust gas to fully expand and cool, and the lost rotational energy can be restored to kinetic energy to the maximum extent.
Although the propellant ratio plays a key role, the reaction products with low average molecular weight still play a significant role in determining the exhaust gas speed.Because the engine works at extremely high temperature, and the temperature is proportional to the molecular energy, a certain amount of energy at a certain temperature can be distributed to more low-mass molecules to finally obtain a higher exhaust speed.Therefore, it is better to use low atomic mass elements.Liquid hydrogen (LH2) and liquid oxygen (LOX or LO2) are widely used propellants with the highest efficiency relative to exhaust gas velocity.Other substances such as boron and liquid ozone are more efficient in theory, but there are still many problems when they are put into use.
ignition system
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There are many ways to ignite: initiating explosive charge, plasma flame moment, electric spark plug.Some fuels and oxidizers burn together. For non self igniting fuels, self igniting substances can be filled in the fuel nozzle (commonly used by Russian engines).
For liquid and solid-liquid hybrid rockets, the propellant entering the combustion chamber must be ignited immediately.The ignition delay of millisecond after liquid propellant enters the combustion chamber will lead to excessive liquid entering, and the high-temperature gas generated after ignition will exceed the designed maximum pressure of the combustion chamber, thus causing catastrophic consequences.This is called "hard start".
The gas propellant will not have a hard start, because the total area of the injection port is less than the area of the nozzle port, and even if the combustion chamber is full of gas before ignition, high pressure will not be formed.Solid propellant is usually ignited by disposable pyrotechnics.
After ignition, the combustion chamber can maintain combustion, and the igniter is no longer needed.After the engine stops for a few seconds, the combustion chamber can automatically focus on fire.However, once the combustion chamber is cooled, many engines cannot be re ignited.
Rocket Engine - Plume Physics
Kerosene exhaust gas is rich in carbon, according to itsEmission lineThe plume is orange.The plume of the rocket based on peroxide oxidizer and hydrogen fuel is mostly water vapor, which is almost invisible to the naked eye, but it is bright in the ultraviolet and infrared fields of vision.The solid rocket propellant contains metal elements such as aluminum, which burn and emit white light, so its plume height is visible.Some exhaust gases, especially the plume of alcohol fuel, present diamond shock waves.
The shape of the rocket plume depends on the design altitude, altitude thrust and other factors.At high altitude, all rocket exhaust flames are in a state of excessive expansion and end at the tail.
classification
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The energy is converted into the kinetic energy of the working medium (working medium) in the rocket engine, forming a high-speed jet to discharge and generate power.Rocket engines are divided into chemical rocket enginesNuclear rocket engineAnd electric rocket engines.
Chemical rocket engine is the most mature and widely used engine.The principle prototype of the nuclear rocket has been successfully developed.Electric rockets have been used in space propulsion.Last two enginesSpecific impulseIt is much higher than chemical rocket.Chemical rocket engine is mainly composed of combustion chamber and nozzle. Chemical propellant is both energy and working medium. It converts chemical energy into heat energy in combustion chamber, generates high-temperature gas, accelerates by nozzle expansion, converts heat energy into kinetic energy of air flow, and discharges from nozzle at high speed (1500-5000m/s) to generate thrust.Chemical rocket engine can be divided intoLiquid rocket engine, solid rocket motor andHybrid propellant rocket engine。Liquid rocket engines use liquid storable propellants at normal temperature and liquid cryogenic propellants at low temperature, which are characterized by strong adaptability and multiple starting, and can meet the requirements of different launch vehicles and spacecraft.Solid rocket motor propellant adopts organic colloidal solid solution containing fuel and oxidant in molecule（Double base propellant）Or a mixture of several propellant components (composite propellant), which is directly installed in the combustion chamber, has simple structure, convenient use, and can be stored in the ready to launch state for a long time, and is suitable for various strategic and tactical missiles.Hybrid propellant rocket engines are rarely used.
advantage
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withAir jet engineIn comparison, the biggest feature of the rocket engine is that it carries both fuel and oxidant, and it relies on oxidant to support combustion, and does not need to draw oxygen from the surrounding atmosphere.So it can work not only in the atmosphere, but also in the cosmic vacuum outside the atmosphere.This is impossible for any air jet engine.The launched artificial satellites, moon spacecraft and the propulsion devices used by various spacecraft are all rocket engines.
Modern machine
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Modern rocket motors are mainly divided into solid propellant and liquid propellant motors.The so-called "propellant" is the combination of fuel (combustion agent) and oxidant.
Solid rocket motor
The solid rocket motor is a chemical rocket motor using solid propellant.Solid propellants include polyurethanepolybutadiene、Hydroxyl terminated polybutadieneNitrate plasticized polyether, etc.
Solid rocket motor is composed 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 the burning surface, and its cross section is round, star shaped, etc.).The grain is placed in the combustion chamber (usually the engine housing).During propellant combustion, the combustion chamber must withstand a high temperature of 2500~3500 ℃ and a high pressure of 102~2 × 107 Pa. Therefore, it must be made of high-strength alloy steel, titanium alloy or composite materials, and heat insulation lining shall be equipped between the grain and the combustion inner wall.
The ignition device is used to ignite the grain, which is usually composed of an electric ignition tube and a powder box (containing black powder or pyrotechnic agent).After electrification, the black powder is ignited by the electric heating wire, and then ignited by the black powder.
In addition to accelerating the gas expansion to generate thrust, the nozzle assembly is often formed with the thrust vector control system in order to control the thrust direction.The system can change the injection angle of fuel gas to realize the change of thrust direction.
When the grain is burned, the engine stops working.
Compared with liquid rocket motor, solid rocket motor has the advantages of simple structure, high propellant density, propellant that can be stored in the middle of combustion and ready for use, and easy and reliable operation.The disadvantage is that the "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, the working time is short, and the large acceleration makes the thrust difficult to control, and it is difficult to start repeatedly, which is unfavorable to manned flight.
Solid rocket motor is mainly used forrocket, missile and sounding rocket engines, andSpacecraft launchAnd the booster engine for aircraft takeoff.
Solid rocket motor is mainly composed of four parts: shell, solid propellant, nozzle assembly and ignition device. Among them, solid propellant formula and forming process, nozzle design and materials used and manufacturing process, shell material and manufacturing process are the most critical links, which directly affect the performance of solid rocket motor.The performance of solid motors mainly depends on thrust and specific impulse. For motors with special requirements such as ballistic missiles or antimissile interceptors, they also pursue fast burning performance.
The materials used for solid rocket motor housing have passed from high-strength metal (ultra-high strength steel, titanium alloy, etc.) toAdvanced composite materialsIt is always the evolution of high-performance carbon fiber.However, for space launch, solid rocket motors do not seek to reduce the weight of the shell too much, so many solid rockets are still using high-strength steel as the shell, such as the S-125 booster used by India's GSLV rocket, which uses M250 high-strength steel.Lightweight and high-strength carbon composites are mainly used in ballistic missiles, especially the third stage engines.
The propellants of solid motors can be divided into low-energy, medium energy and high-energy propellants according to energy. The propellants with specific impulse greater than 2450 N/s/kg (i.e. 250 seconds) are high-energy, 2255 N/s/kg (i.e. 230 seconds) to 2450 N/s/kg are medium energy, and those with specific impulse less than 2255 N/s/kg are low-energy;According to the characteristic signal, it can be divided into smoke propellant, light smoke propellant and smokeless propellant. Generally speaking, smokeless propellant will have a considerable loss compared with smokey propellant;According to the material formula combination, it can be divided into single base, double base and composite propellants. The single base propellants are composed of single compounds, such as fire wool, and the specific impulse is too low to be applicable.The double base propellant is composed of collodion or nitroglycerin and some additives, but the specific impulse is still insufficient and is not widely used.Composite propellant is a combination of separate combustion agent and oxidizer materialsPolymerThe binder is used as fuel, and crystalline oxidant solid filler and other additives are added to fuse and solidify into a multiphase object.In order to increase energy and density, some powdery light metal materials can also be added as combustible agents, such as aluminum powder.Composite propellant is usually named after the chemical name of binder fuel, such as HTPB (hydroxyl terminated polybutadiene), and the oxidant is mainly perchlorate, such as amine perchlorate.Composite propellant is generally cast and is the absolute mainstream of solid propellant.In addition, modified double base propellants include composite modified double base propellants (CMDB) and cross-linked modified double base propellants (XLDB for short).On the basis of double base propellants, the proportion of basic components such as collodion and nitroglycerin is greatly reduced. Adding high-energy solid components such as oxidant perchlorate and fuel aluminum powder, it is a composite modified double base propellant, and then addingPolymer compoundAs a cross-linking agent, it becomes a cross-linked modified double base propellant.NEPE (Nitrate Plasticized Polyether), one of the crosslinked modified double base propellants, is a practical solid propellant with the highest specific impulse.
Rocket engine nozzle belongs to convergence diffusion nozzle (Laval DeLaval nozzle), which is composed of inlet section (convergence section), throat (throat lining) and outlet cone (diffusion section or expansion section). Its role is to convert the heat energy of combustion products into the kinetic energy of high-speed jet to generate thrust.The expansion ratio, that is, the area ratio of the throat to the nozzle, directly affects the performance of the engine. A well-designed nozzle has a great impact on the performance of the engine.In addition, unlike liquid engines, which use cooling nozzles, solid engines use ablative nozzles. The inner wall of the nozzle is coated with ablative materials, which absorb heat through ablative evaporation of materials to prevent the nozzle from overheating and burning.Generally speaking, the expansion section of engine nozzle adopts bell shaped nozzle.
Liquid rocket engine
Liquid rocket engine refers to chemical rocket engine with liquid propellant.Common liquid oxidants include liquid oxygen, nitrogen tetroxide, etc. Combustion agents include liquid hydrogenUnsymmetrical dimethylhydrazine, kerosene, etc.Oxidant and combustion agent must be stored in different tanks.
Liquid rocket engines generally consist of thrust chambersPropellant supply systemComposition of engine control system.
Thrust chamber is an important component that converts chemical energy of liquid propellant into propulsion force.It consists of propellant nozzle, combustion chamber, nozzle assembly, etc.Propellant is injected into the combustion chamber through the injector, and through atomization, evaporation, mixing and combustion, it generates combustion products, and rushes out of the nozzle at high speed (2500-5000m/s) to generate thrust.The pressure in the combustion chamber can reach 200 atmospheric pressure (about 20MPa) and the temperature is 3000~4000 ℃, so cooling is required.
The function of the propellant supply system is to deliver propellant to the combustion chamber according to the required flow rate and pressure.There are two types of supply systems: extrusion type (pneumatic type) and pump pressure type according to different delivery modes.The extrusion supply system uses the high-pressure gas to enter the oxidant and combustion agent storage tank after depressurization through the pressure reducer (the flow of oxidant and combustion agent is controlled by the pressure set by the pressure reducer), and then extrude them into the combustion chamber respectively.The squeeze supply system is only used for small thrust engines.The high thrust engine uses 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.The working procedure includes three stages of engine starting, working and shutdown, which is automatically carried out according to the predetermined procedure.The working parameters mainly refer to the thrust and the mixing ratio of propellant.
The liquid rocket engine has the advantages of high specific impulse (250~500 seconds), large thrust range (single thrust is 1 gram force~700 ton force), repeated starting, thrust control, long working time, etc.Liquid rocket engine is mainly used for spacecraft launch, attitude correction and control, orbit transfer, etc.
Liquid rocket engine is the mainstream of space launch. Its structure is much more complex than that of solid rocket engine. It is mainly composed of ignition device, combustion chamber, nozzle and fuel delivery device.The ignition device is generally a powder igniter. For the upper stage engine that needs to be started for many times, multiple powder igniters are required. For example, the J-2X engine of Ares Rocket of the United States has two powder igniters to achieve the function of twice starting. China's YF-73 and YF-75 also have two powder igniters to achieve the function of twice starting;The combustion chamber is the place where liquid fuel and oxidant are burned and expanded. In order to obtain higher specific impulse, it usually has a high pressure. Even for ordinary engines, it usually has a pressure of tens of atmospheres. For engines such as RD-180 in the Soviet Union, the combustion chamber pressure is more than 250 atmospheres.The combustion under high pressure is more complicated than that under normal pressure. At the same time, with the increase of the combustion chamber volume, the combustion instability becomes more and more serious, and it is more troublesome to solve.There is no reliable mathematical model to analyze combustion stability, which is mainly solved by a large number of engine combustion tests.AmericanSaturn 5 rocketThe F-1 engine of the Soviet energy rocket was tested on the ground test bed for more than 200000 seconds, and the RD-170 engine of the Soviet energy rocket was also tested on the ground test bed for more than 100000 seconds. During repeated combustion tests, the engine parameters were continuously optimized to alleviate unstable combustion.However, for engines with low chamber pressure and small thrust, the phenomenon of unstable combustion is not obvious, and unstable combustion is one of the main problems restricting the thrust increase of liquid engines.The combustion chamber of liquid rocket engine uses liquid fuel or oxidant for cooling. Before they enter the combustion chamber, they first flow through the combustion chamber wall to cool down;The nozzle of liquid engine is alsoLaval nozzleThe expansion section is usually bell shaped, but cooling nozzles are used, which are cooled by liquid fuel or oxidant.
The fuel delivery of liquid engine is divided into four modes: extrusion cycle, gas generator cycle, staged combustion cycle and expansion cycle.
Extrusion cycleHigh pressure gas is used to enter the oxidizer and combustion agent storage tanks after decompression through the pressure reducer, and then it is squeezed into the combustion chamber respectively. Due to the materials in the storage tank, it is impossible to achieve much pressure, so it is only used on small low performance engines.
Gas generator cyclePart of the fuel and oxidant flow through a gas generator, and after combustion, the fuel pump and oxidant pump are driven to run. The fuel pump and oxidant pump press the fuel into the combustion chamber, and the exhaust gas of pre combustion is directly discharged.Some of the initial fuel and oxidant flows are squeezed by the storage tank, and some are guided by natural gravity.
Staged combustion cycleAlso known as supplementary combustion, it is the same way that fuel and oxidant are burned in the pre burner to drive the fuel pump and oxidant pump. However, the difference is that the gas in the pre burner is not directly discharged, but is pressed into the combustion chamber, which avoids the waste of fuel and oxidant, and can achieve greater specific impulse.In pursuit of high specific impulse engines, staged combustion cycle is generally adopted. In order to pursue higher specific impulse during staged combustion, the general combustion chamber pressure is much higher than the gas generator cycle, also known as high-pressure afterburning mode.
Expansion cycleThe fuel or oxidant flows through the combustion chamber wall and nozzle wall, where it cools the combustion chamber and nozzle, and at the same time, the self heating has greater pressure to drive the fuel pump and oxidant pump to run. Obviously, the cycle of gas generator and staged combustion will also flow through these high temperature parts, but it can achieve much greater thrust by driving the high-pressure gas of the precombustion.Generally speaking, the engine of expansion combustion cycle has a very high specific impulse. Theoretically, other conditions are the same, which is the highest specific impulse. However, it is difficult to increase the thrust. For example, the RL10-B-2 in the United States has the highest specific impulse of 465.5 seconds among used liquid engines, but the thrust is only 24750 pounds, about 11.2 tons.
When it comes to liquid engines, the cycle mode, combustion chamber pressure and nozzle design certainly affect the specific impulse, but it is the liquid fuel that most affects the specific impulse of the engine.Early hydrazine fuels, combined with nitrogen tetroxide, had a specific impulse of about 300 seconds at most in vacuum, and hydrazine fuels were highly toxic, and nitrogen tetroxide was also highly corrosive, which has gradually been eliminatedLong March 5The new generation of rockets will also phase out the existing hydrazine fuelLong March Rocket；The higher specific impulse is kerosene fuel. Kerosene is not much higher than hydrazine. The specific impulse is only about 20 seconds. The main feature is that it is cheap, non-toxic, and suitable for liquid engines. At present, the engines of commercial rocket companies all choose liquid oxygen kerosene engines;The higher specific impulse is methane engine. Methane is the highest specific impulse among hydrocarbon fuels, but it is not much higher than kerosene, which is also about 20 seconds. At the same time, it needs low-temperature storage, which is much larger than kerosene, and the main cost is much higher, so there is little interest. However, after the Cold War, space faring countries began to pre research methane engines;The fuel combination with the highest specific impulse is liquid hydrogen and liquid oxygen. Liquid hydrogen fuel is not only more expensive than kerosene, but also more expensive than hydrazine, and its storage volume is huge. However, the specific impulse of liquid hydrogen and liquid oxygen is too much higher than that of liquid oxygen and kerosene. In vacuum, it can generally reach more than 420 seconds, more than 1/3 higher.contrastZiolkovsky formula, which means that the load can be put into orbit with much less fuel.However, due to the high cost of liquid hydrogen, liquid hydrogen fuel was mainly used in the upper stage of the rocket (above the first stage is called the upper stage) in the early stage. With the progress of technology, the price of liquid hydrogen has decreased. The first stage of new generation rockets generally uses liquid hydrogen fuel, such as H-II in Japan, Ariane5 in Europe, etc., and the first stage of China's Long March 5 rocket will also use liquid hydrogen fuel.The Delta 4 rocket, a large rocket whose booster also uses liquid hydrogen fuel, appeared in the United States, and its performance is very superior.
Other energy sources
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Electric rocket engine
The electric rocket engine is a rocket engine that uses electric energy to accelerate the working medium and form a high-speed jet to generate thrust.Unlike the chemical rocket engine, the energy and working medium of this engine are separated.Electric energy is provided by aircraft, generally by solar energy, nuclear energy and chemical energy through conversion devices.Working medium includes hydrogen, nitrogen, argon, mercury, ammonia and other gases.
Electric rocket engine is composed of power supply, power exchanger, power regulator, working fluid supply system and electric thruster.Power supply and power exchanger supply electric energy;The function of the power regulator is to start the engine according to the predetermined procedure, and constantly adjust various parameters of the electric thruster, so that the engine is always in the specified working state;The working medium supply system is to store working medium and transport working medium;The role of the electric thruster is to convert the electric energy into the kinetic energy of the working medium, so that it can generate high-speed jet flow and generate thrust.
Electric rocket engine
According to the way of accelerating working medium, there are three types of electric rocket engines: electric rocket engines, electrostatic rocket engines and electromagnetic rocket engines.Electrothermal rocket engines use electric energy to heat (resistance heating or arc heating) working fluids (hydrogen, amine, hydrazine, etc.) to gasify them;After being expanded and accelerated by the nozzle, it is discharged from the nozzle to generate thrust.The working medium (mercury, cesium, hydrogen, etc.) of the electrostatic rocket engine is ionized into ions from the tank input ionization chamber, and then accelerated into high-speed ion flow under the action of the electrostatic field of the electrode to generate thrust.Electromagnetic rocket engine uses electromagnetic field to accelerate ionized working medium to generate jet and form thrust.The electric rocket engine has extremely high specific impulse (700-2500 seconds) and extremely long service life (can be started repeatedly for tens of thousands of times and can work accumulatively for tens of thousands of hours).But the generated thrust is less than 100N.This kind of engine is only suitable for attitude control and position maintenance of spacecraft.
Nuclear rocket engine
Nuclear rocket engine
Fission type: Fission type rocket engines essentially miniaturize nuclear reactors and place them on rockets.Nuclear rocket engines use nuclear fuel as energy and liquid hydrogen, liquid helium, liquid ammonia and other working fluids.The nuclear rocket engine consists of the nuclear reactor installed in the thrust chamber, cooling nozzle, working medium delivery system and control system.In a nuclear reactor, nuclear energy is converted into heat energy to heat the working medium. After the heated working medium is expanded and accelerated by the nozzle, it is discharged from the nozzle at a speed of 6500-11000 m/s to generate thrust.The specific impulse of the nuclear rocket engine (250-1000 seconds) has a long life, but the technology is complex, and it is only suitable for long-term working spacecraft.This kind of engine is still under test due to the unsolved problems of nuclear radiation protection, exhaust pollution, reactor control, and the design of high-efficiency heat exchanger.In addition, solar heating andPhotonic rocket engineIt is still in the stage of theoretical exploration.
Fusion: fusion nuclear rocket engine is considered to be the most potential rocket engine to achieve flight in the solar system. Its principle is similar to that of chemical rocket, except that the fuel is transformed into three isotopes of hydrogen, namely deuterium, tritium and helium. The huge energy released by nuclear fusion reaction is used to push the rocket, which is several orders of magnitude higher than that of chemical rocket.
Since the materials produced by fusion nuclear reaction are neutrons, protons and helium, they cannot be used in the earth's atmosphere, but the space itself is full of various radiation, so it is not inappropriate to use them in space.The main problems that need to be solved for the nuclear fusion rocket engine are the ignition and the high temperature resistant materials of the fuel chamber (the temperature of the reaction chamber is as high as tens of millions to billions of degrees Celsius), which are still in the stage of theoretical exploration.
Latest achievements
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China
On July 4, 2006Aerospace Propulsion Technology Research InstituteIt was revealed that the "120 ton liquid oxygen kerosene engine" used to promote China's new generation of large launch vehicles was successfully tested for the first time in the Institute.
On July 17, 2018, it was learned from the Sixth Academy of China Aerospace Science and Technology Group that China's first high thrust, high-performance liquid oxygen kerosene aeroengine developed by the Academy has successfully carried out the first overall hot test run a few days ago.It is reported that this is China's first high thrust, high-performance liquid oxygen kerosene aeroengine, with a thrust of up to 120 tons, which is used for the second stage of the carrier rocket core.[2]
In November 2022, Tianbing Technology's 110 ton thrust liquid oxygen kerosene rocket engine "Tianhuo 12" successfully completed the whole system hot commissioning.At present, it is the largest liquid rocket engine with the largest thrust for commercial aerospace in China, and also the largest 3D printing engine in China.[5]
On November 26, 2022, the first two start-up tests of the 130 ton level reusable liquid oxygen kerosene staged combustion cycle engine independently developed by China were successful。[6]
On April 28, 2024, China's independently developed 130t pump rear swing liquid oxygen kerosene rocket engine four parallel ignition test was successful.[7]
U.S.A
According to foreign media reports, the world's largest rocket built by NASA has entered a critical review stage. It is expected to be completed in 2018, with a mass of about 5.5 million pounds, a height of 98 meters, and a thrust of 8.4 million pounds.This is a historic moment. In the past 40 years, we have won the super rocket again because we are going to land on Mars.At present, the critical design review of SLS rocket has completed all the steps.When the SLS rocket was launched, the era of Mars exploration began. This is our most powerful carrier rocket. It can send nearly 100 tons of cargo into low Earth orbit, and its carrying capacity is unprecedented.
This will be the most powerful rocket at present, which can be matched with Orion spacecraft to form a vehicle to explore beyond Earth orbit.The assistant deputy director of NASA's exploration system development department believes that all major components of the first flight are entering the production phase. We have completed the first round of engine testing. The next step is to manufacture and test SLS rockets in 2017 and pass the design certification.In the end, SLS will become a very powerful rocket. The SLS project manager believes that the rocket design team works very hard to accelerate the development of the rocket.
The core power of the rocket is a cryogenic liquid hydrogen and liquid oxygen engine, which usesRS-25 engineNASA is preparing for the second round of assessment of SLS rocket propulsion and completing some structural tests.[3]
the republic of korea
On March 30, 2022 local time, the Research Institute of the Ministry of National Defense of the Republic of Korea successfully test launched a self-developed solid fuel rocket for the first time in Tai'an County, Chungnam do, which will be used to carry small reconnaissance satellites.[4]
The world's largest thrust single chamber liquid rocket engine developed by the United States is used forSaturn 5The rocket, with a single thrust of 700 tons, uses kerosene as fuel and liquid oxygen as oxidant.
Detailed data of F-1:
Combustion type: gas generator open cycle, liquid-liquid combustion
Propellant: kerosene liquid oxygen
Thrust: 690.988 tons at sea level
Vacuum 793.683 tons
Specific impulse: 255.4 seconds at sea level (average value of 70 engines)
The world's largest thrust liquid rocket engine developed by Russia uses kerosene+liquid oxygen, with a single thrust of 800 tons (using four combustion chambers and four nozzles design, some people also think it is four engines in parallel, but sharingGas generatorandTurbopump）, forEnergy launch vehicleandZenit rocket （RD-171 Rocket EngineFor RD-170).
Its derivative models areRD-180 Rocket EngineThe thrust is 400 tons, which is equivalent to dividing the RD-170 into two parts, two fuel chambers and two nozzles.For the United StatesOptimus IIandOptimus III carrier rocketThe first level of.
RD-191 Rocket Engine, a single thrust of 200 tons, a single chamber and a single nozzle, which is equivalent to dividing the RD-170 into two parts for RussiaAngara carrier rocket。RD-191 derivativesRD-151Sold to Korea forKorea Space Launch Vehicle The first level of.
The world's largest thrust liquid hydrogen liquid oxygen engine developed by the United States, with a thrust of 300 tons, is used forDelta IV carrier rocketThe first level of.
RD-0120 rocket engine
The largest liquid hydrogen and liquid oxygen rocket engine in Russia, with a thrust of 200 tons, is used forEnergy launch vehicleThe main engine of.
American Space ShuttleIts main engine uses liquid hydrogen and liquid oxygen, with a thrust of 200 tons. Its biggest feature is that it can be reused.
Space Shuttle Solid Rocket Motor
The rocket engine with the largest thrust in the world, with a single thrust up to 1200 tons, can be reused for 10 times, and is used in the United Statesspace shuttleBundle booster, modified forAres 1 rocketMain engine andAres 5 rocketBundle the booster.
