What Is a Rocket Engine?

Rocket engine A jet engine that comes with propellant (energy) for the aircraft and does not use outside air. It can work in the space outside the dense atmosphere, and the energy is converted into the kinetic energy of the working medium (working medium) in the rocket engine, forming high-speed jet discharge and generating thrust. ^[1]

Chinese name: Rocket engine
Foreign name: rocket engine
working principle: Impulse principle

Classification: Chemical rocket, nuclear rocket engine and electric rocket
Advantage: Comes with fuel and oxidant, no need to draw oxygen
Fuel classification: Solid fuel liquid fuel

Rocket Engine Introduction

The rocket engine is a jet engine that uses the principle of impulse, has its own propellant, and does not rely on outside air. A rocket engine is a type of jet engine that turns reactants (propellants) in a propellant tank or vehicle into a high-speed jet and generates thrust due to Newton's third law of motion. Rocket engines can be used for spacecraft propulsion, as well as missiles and other aircraft flying in the atmosphere. Most rocket engines are internal combustion engines and there are also non-combustion engines.

How Rocket Engines Work

Most engines rely on exhausting high-temperature and high-speed gas to obtain thrust. Solid or liquid propellants (composed of oxidant and fuel) are burned in the combustion chamber at high pressure (10-200 bar) to generate gas.

Rocket engine feeds propellant into combustion chamber

Liquid rockets use a pump or high-pressure gas to cause the oxidant and fuel to enter the combustion chamber. The two propellant components are mixed and combusted in the combustion chamber. The solid rocket propellant is mixed in advance and placed in the combustion chamber. Solid-liquid hybrid rockets use solid and liquid propellants or gas propellants. There are also high-energy power sources used to send inert reaction materials to heat exchangers for heating, which eliminates the need for a combustion chamber. Rocket propellants are usually stored in a propellant tank before they are burned and discharged to generate thrust. Propellants are generally chemical propellants. After exothermic chemical reactions, high-temperature gases are generated for rocket propulsion.

Rocket engine combustion chamber

The combustion chamber of a chemical rocket is generally cylindrical, and its size must be sufficient for the propellant to burn fully. The propellant used has different sizes. Describe combustion chamber size with L *

formula Here:

Vc is the combustion chamber capacity

At is the nozzle area

L * range is typically 25-60 feet (0.6-1.5 m)

The pressure and temperature of the combustion chamber usually reach extreme values. Unlike the suction jet engine, which has enough nitrogen to dilute and cool the combustion, the temperature of the rocket engine combustion chamber can reach the chemical standard. And the high pressure means that heat is conducted very quickly through the walls of the combustion chamber.

Combustion chamber shrinkage

The shrinkage ratio of the combustion chamber refers to the ratio of the cross-sectional area of the combustion chamber to the area of the throat of the nozzle. When the pressure of the propellant and the combustion chamber is constant, the shrinkage ratio is inversely proportional to the mass flow density. When the mass flow density is selected, the combustion chamber shrinkage ratio is also selected. However, it is more direct and convenient to use the shrinkage ratio to select the combustion chamber diameter. The selection of the shrinkage ratio is mainly based on experimental or statistical methods. The following data are recommended:

For the large thrust and high pressure combustion chambers of most pumped supply systems, the shrinkage ratio is usually 1.3 to 2.5.

For combustion chambers with centrifugal nozzles, the shrinkage ratio is usually 4 to 5.

Rocket engine nozzle

The shape of the engine depends mainly on the shape of the expansion nozzle: bell-shaped or tapered. In a tapered and widened nozzle with a high expansion ratio, the high-temperature gas produced by the combustion chamber is discharged through an opening (nozzle).

If the nozzle is provided with a sufficiently high pressure (2.5 to 3 times higher than the confining pressure), nozzle choke and supersonic jets will form, and most of the thermal energy is converted into kinetic energy, thereby increasing the speed of the exhaust. At sea level, it is not uncommon for engine exhaust to reach ten times the speed of sound.

Rocket engine Part of the rocket thrust comes from the pressure imbalance in the combustion chamber, but mainly comes from the pressure on the inner wall of the extrusion nozzle. When the exhaust gas expands (adiabatic), the pressure on the inner wall causes the rocket to move in one direction, while the exhaust gas moves in the opposite direction.

Rocket Engine Propellant Efficiency

For the engine to use the propellant effectively, it is necessary to use a certain mass of propellant to generate the maximum possible pressure on the combustion chamber and the nozzle. In addition, the following methods can also improve the propellant efficiency:

Heat the propellant to the highest possible temperature (use high-energy fuel, hydrogen, carbon or certain metals such as aluminum, or use nuclear energy)

Use low specific gravity gas (as much hydrogen as possible)

Use of small molecule propellants (or propellants that can be broken down into small molecules)

Because all measures are taken to reduce the quality of the propellant; pressure is proportional to the accelerated propellant dose; and because of Newton's third law, the pressure on the engine also acts on the propellant. The speed at which the exhaust gas exits the combustion chamber appears to be determined by the combustion chamber pressure. However, the speed is obviously affected by the above three factors. Taken together, the exhaust speed is the test engine

Rocket engine The best proof of efficiency.

Due to aerodynamic reasons, the exhaust gas has a blocking effect at the nozzle. The speed of sound increases with the square root of temperature, so using high-temperature exhaust can improve engine performance. At room temperature, the speed of sound in the air is 340 m / s, and in the rocket's high-temperature gas, it can reach more than 1700 m / s. Most of the rocket's performance is due to high temperature. In addition, rocket propellants usually use small molecules, which also makes the speed of sound in the exhaust gas higher than the speed of sound at the same temperature.

The expansive design of the nozzle doubles the speed of the exhaust, usually 1.5 to 2 times, resulting in a quasi-hypersonic exhaust jet. The speed increase is mainly determined by the area expansion ratio, that is, the ratio of the nozzle area to the nozzle outlet area. The nature of the gas is also important. The nozzle with a large expansion ratio is larger in size, but it allows the exhaust gas to release more heat, thereby increasing the exhaust speed.

Nozzle efficiency is affected by operating altitude because atmospheric pressure decreases with altitude. But because the exhaust 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 exhaust gas pressure is different from the confining pressure, the exhaust gas can become fully inflated or excessively inflated.

Rocket engine back pressure and optimal expansion

For best performance, the exhaust pressure at the nozzle tip needs to be equal to the confining pressure. If the exhaust pressure is less than the confining pressure, the vehicle will slow down due to the air pressure difference between the front end and the end of the engine. If the exhaust gas pressure is greater than the confining pressure, the exhaust gas pressure that should have been converted to thrust is not converted, and energy is wasted.

In order to maintain the balance between exhaust pressure and confining pressure, the nozzle diameter needs to increase with height, so that the exhaust gas has a sufficient distance to act on the nozzle to reduce pressure and temperature. This makes design more difficult. Eclectic approaches are often used in actual design, thus sacrificing efficiency. There are many special nozzles that can make up for this deficiency, such as plug nozzles, step nozzles, diffusion nozzles, and tile nozzles. Each

Russian rocket engine Special nozzles can adjust the confining pressure and allow the exhaust gas to spread more widely in the nozzle, generating additional thrust at high altitudes.

When the confining pressure is low enough, such as vacuum, there will be some problems: one problem is the shear of the nozzle. In some carriers, the weight of the nozzle also affects the efficiency of the engine. The second problem is that the exhaust gas expands and cools adiabatically in the nozzle. Certain chemicals in the jet will condense to produce "snow", which will cause the instability of the jet, which must be avoided.

Rocket Engine Power Cycle

Relative to the loss of thermal energy at the nozzle, the pumping loss is minimal. Engines used in the atmosphere use high-pressure power cycles to increase nozzle efficiency, while vacuum engines do not. For liquid engines, there are four basic forms of power cycle for injecting propellant into the combustion chamber:

Rocket engine Squeeze cycle-the propellant is squeezed out of the gas in the built-in high-pressure cylinder.

Expansion cycle-The propellant flows through the main combustion chamber to expand and drive a turbo pump.

Gas generator cycle-a small part of the propellant is burned in the pre-combustion chamber to drive the turbo pump, and the exhaust gas is eliminated through a separate pipe, resulting in loss of energy efficiency.

Staged combustion cycle-The high-pressure gas of the turbo pump is sent back to the driving self-starting cycle, and the high-pressure exhaust gas is directly sent to the main combustion chamber without energy loss.

Rocket engine overall performance

Rocket technology combines high thrust (million Newtons), high exhaust velocity (10 times the speed of sound at sea level), high thrust-to-weight ratio (> 100), and the ability to work outside the atmosphere. And often it is possible to make one performance higher by weakening another.

Rocket Engine Specific Impulse

An important indicator for measuring engine performance is the impulse produced by the propellant per mass, which is the specific impulse (usually written as Isp). Specific impulse can be measured in speed (Ve meters per second or feet per second) or time (seconds). Engines with larger specific shocks tend to perform better.

Rocket Engine Net Thrust

The following is the approximate formula for calculating the net thrust of the engine:

formula Because the rocket engine has no air inlet for the jet engine, there is no need to deduct punch resistance from the total thrust because the net thrust is equal to the total thrust (excluding static back pressure).

Rocket engine throttling

The engine can achieve throttling by controlling the flow of propellant (usually measured in kg / s or lb / s).

In principle, the engine can be throttled to reduce the outlet pressure to one-third of the confining pressure (nozzle flow separation), and the upper limit can be the maximum allowed by the engine's machinery.

In fact, the range in which the engine can be throttled 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 unstable combustion), but the nozzle can often be adjusted and tested within a larger range. And it is necessary to ensure that the nozzle outlet pressure is not too much lower than the confining pressure to avoid flow separation problems.

Rocket Engine Energy Efficiency

The rocket engine is a highly efficient thermal engine that generates high-speed jets, resulting in high combustion chamber temperatures and high compression ratios as in the Carnot cycle. If the speed of the vehicle reaches or slightly exceeds the exhaust speed (relative to the vehicle), the energy efficiency is very high. At zero speed, the energy efficiency is also zero. (This is true for all jet propulsion)

Rocket engine cooling system

Rocket Engine Material Technology

The reaction temperature of the reaction material in the combustion chamber can reach about 3500 K (~ 5800 ° F). This temperature is well above the melting point of the nozzle and combustion chamber materials (except graphite and tungsten). It is true that suitable propellants can be found within the tolerance range of certain materials, but it is also important to ensure that these materials do not burn, and that they melt or boil. Material technology determines the upper limit of chemical rocket exhaust temperature.

Another method is to use common materials such as aluminum, steel, nickel or copper alloys and use a cooling system to prevent the material from overheating. Such as regenerative cooling, the propellant passes through the combustion chamber or the pipe on the inner wall of the nozzle before combustion. Other cooling systems such as water curtain cooling, film cooling can

Rocket engine Extend the life of the combustion chamber and nozzle. These technologies can ensure that the temperature of the thermal boundary layer of the gas does not affect the safety of the material when it contacts the material.

Heat flux in rockets is often the highest in engineering, with a range of 1-200 MW / 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 velocity of the exhaust gas at the nozzle (causing a thin boundary layer) and high temperature.

Most other jet engine gas turbines operate at high temperatures, but because of their large surface area, it is difficult to cool, so they have to lower the temperature and lose efficiency.

Common cooling methods for rocket engines

No cooling: for short-term operation or testing

Ablation wall: Ablation material is on the wall of the chamber, which can continuously absorb heat and fall off

Radiation cooling: make the wall of the room reach white heat to radiate heat

Heat sink cooling: a propellant (usually liquid hydrogen) is poured down the walls of the chamber

Regenerative cooling: propellant flows through a cooling jacket inside the chamber wall before combustion

Water curtain cooling: The propellant injector is specially placed to reduce the temperature of the gas around the chamber wall

Film cooling: the wall of the chamber is wet with liquid propellant, and the liquid evaporates heat to cool it

All cooling measures are to form a layer of insulation (border

Rocket engine Layer), as long as this layer of insulation is not damaged, the wall will not be a problem. Unstable combustion or failure of the cooling system often results in the interruption of the protection of the boundary layer, which subsequently leads to the destruction of the chamber walls.

The regenerative cooling system also has a second boundary layer, which is the cooling pipe wall surrounding the chamber wall. Since this boundary layer serves as the isolation layer between the chamber wall and the coolant, its thickness must be as thin as possible, which can be achieved by accelerating the coolant flow rate.

Rocket engine mechanical problems

Rocket combustion chambers work under high pressure, usually 10-200 bar (1--20 MPa). The higher the pressure, the better the performance (because more efficient nozzles can be used). This places the outside of the combustion chamber under a large circumferential stress. Also due to the high temperature working environment, the tensile strength of structural materials is significantly reduced.

Rocket Engine Acoustics

The extreme vibration and acoustic environment inside the rocket engine causes its peak stress to be much higher than the average, especially the problems of pipe-like resonance and airflow disturbance.

Rocket engine combustion is unstable

There are several types of unstable combustion:

Rocket engine intermittent combustion

This is the low-frequency vibration of the pressure in the combustion chamber caused by the change in the pressure of the propellant tube caused by the change in the acceleration of the carrier. The carrier thrust can be changed periodically, resulting in damage to the load and the carrier. Intermittent combustion can be prevented by using a high-density propellant coupled with an inflatable damping turbopump.

Rocket engine buzz

This is due to insufficient pressure in the propellant injector. It is mainly unpleasant and does not cause any substantial harm. However, in some extreme cases, combustion may enter the injector, causing the unit's propellant to explode.

Rocket engine oscillating combustion

This condition often causes direct damage and is difficult to control. It is often an acoustic process that accompanies the chemical combustion process and is the main driving force for energy release. Can lead to unstable resonances, thinning the insulating boundary layer, with tragic consequences. This effect is difficult to analyze in advance during the design phase, and can only pass protracted testing and continuous correction. Correction usually involves fine-tuning the ejector to change the propellant

Rocket engine Chemical properties, or evaporate to a gaseous state before injecting the propellant into a Helmholtz damper (to change the resonance state of the combustion chamber).

Another common test method is to detonate a small amount of explosive in the combustion chamber to determine the impulse response of the engine and estimate the response time of the chamber pressure: the faster the recovery, the more stable the system.

Rocket engine exhaust noise

Rocket engines (except extra small) are more noisy than other engines. The supersonic exhaust gas is mixed with the surrounding air to form a shock wave. The sound intensity of the shock wave depends on the size of the rocket.

When Saturn V was launched, this noise was detected by a seismometer far from its launch point. The intensity of the sound produced depends on

Rocket engine Depends on rocket size and exhaust speed. The characteristic sound of shock waves heard at the scene was mainly a crackling sound. The peak value of this noise exceeds the permissible limit of microphones and audio electronics, so this noise is weakened or disappeared during recording or broadcast audio playback. The noise of a large rocket launch can directly kill people around. The noise around the base when the shuttle took off exceeded 200 dB (A).

Rockets usually have the loudest noise near the ground because the noise radiates from the plume and is reflected by the ground. Also, when the carrier slowly rises, only a small amount of propellant energy is converted into the kinetic energy of the carrier (the useful work P is transferred to the carrier P = F * V, where F is the thrust and V is the speed), so most of the energy is dispersed Into the exhaust gas, and then interact with the surrounding air to generate noise. This noise can be reduced by means of a covered flame isolation tank, spraying water on the plume, and deflecting the plume angle.

Rocket engine test run

The engine is usually statically tested on a rocket engine test bench before being put into production. For high-altitude engines, shorten the nozzle or test in a large vacuum chamber.

Rocket Engine Safety

Rockets give the impression that they are unreliable, dangerous and catastrophic. Rockets for military use are highly reliable. But one of the main non-military uses of rockets: orbital launch, in order to increase the payload weight, it is necessary to reduce its own weight, and reliability and lowering its weight cannot be met at the same time. And if the vehicle is flying a small number of times, the probability of accidents caused by design, operation or manufacture is high. In fact, all launchers are based on flight tests based on aerospace standards.

The error rate of the X-15 rocket aircraft was only 0.5%, and it only failed in one ground test. The shuttle's main engine has been free of accidents in more than 350 flights.

Rocket Engine Chemistry

Rocket propellants require the use of high specific energy (energy per unit mass) matter, because ideally all reactive materials are converted into exhaust kinetic energy. Except for unavoidable losses, engine design flaws, incomplete combustion and other factors, according to the laws of thermodynamics, a part of the energy is converted into the kinetic energy of the molecules, which cannot generate thrust. A monoatomic gas such as helium has only three degrees of freedom, which is equivalent to a three-dimensional space coordinate {x, y, z}. Only this spherically symmetric molecule does not have this loss. Diatomic molecules such as H2 can rotate around the axis of the connection direction and the axis in the vertical direction. According to the law of equalization of statistical mechanics, the effective energy will be divided equally to each degree of freedom. Therefore, this molecule has 3/5 energy in thermal equilibrium. Converted into one-way motion, 2/5 into rotary motion. Triatomic molecules such as water molecules have six degrees of freedom. Most chemical reactions are a third condition

Rocket engine condition. The function of the nozzle is to convert free thermal energy into unidirectional molecular motion to generate thrust. As long as the exhaust gas remains in equilibrium when it expands, the diffuser nozzle is large enough to allow the exhaust gas to expand and cool sufficiently. The lost rotation can be restored to the maximum kinetic energy.

Although the propellant specific energy plays a key role, the reaction products of low average molecular weight still play a significant role in determining the exhaust gas velocity. Because the engine operates at extremely high temperatures, and the temperature is directly proportional to the molecular energy, a certain amount of energy at a certain temperature is distributed to more low-quality molecules and eventually a higher exhaust gas velocity can be obtained. Therefore it is better to use low atomic mass elements. Liquid hydrogen (LH2) and liquid oxygen (LOX or LO2) are the most widely used propellants with respect to exhaust gas velocity. Other substances such as boron and liquid ozone are theoretically more efficient, but there are many problems with putting them into use.

Rocket engine ignition system

Ignition can take a variety of ways: pyrotechnic charge, plasma flame moment, electric plug. Some fuels and oxidants meet and burn, and for non-self-igniting fuels, fuel nozzles can be filled with spontaneous combustion materials (commonly used in Russian engines).

For liquid and solid-liquid hybrid rockets, the propellant must be ignited immediately after entering the combustion chamber. The ignition delay of milliseconds after the liquid propellant enters the combustion chamber will cause excessive liquid to enter, and the high-temperature gas generated after ignition will exceed the maximum design pressure of the combustion chamber, causing catastrophic consequences. This is called a "hard boot".

The gas propellant does not have a hard start because the total area of the nozzle is smaller than the area of the nozzle, and even if the combustion chamber is full of gas before ignition, high pressure will not be formed. Solid propellants are usually ignited using disposable pyrotechnic equipment.

After ignition, the combustion chamber can maintain combustion, and the igniter is no longer needed. After a few seconds of engine shutdown, the combustion chamber can automatically focus fire. However, once the combustion chamber has cooled, many engines can no longer be ignited.

Rocket Engine-Plume Physics

The exhaust gas of kerosene is rich in carbon, and its plume is orange according to its emission spectrum. The plume of rockets based on peroxide oxidants and hydrogen fuel is mostly water vapor, which is almost invisible to the naked eye, but appears bright in the ultraviolet and infrared fields of view. Solid rocket propellants contain metallic elements such as aluminum, which burn and emit white light, so their plumes are highly visible. Some exhaust gases, especially the plume of alcohol fuel, are diamond-type shock waves.

Rocket plume shape depends on design height, altitude thrust, and other factors. At high altitudes, all the tail flames of the rocket were inflated at an excessive degree, and they ended at the tail.

Rocket Engine Classification

The energy is converted into the kinetic energy of the working medium (working medium) in the rocket engine, which forms a high-speed jet discharge and generates power. Rocket engines are divided into chemical rocket engines, nuclear rocket engines, and electric rocket engines according to the type of energy that forms gas flow energy.

Chemical rocket engines are the most mature and widely used engines. The principle prototype of the nuclear rocket has been successfully developed. Electric rockets have been applied in the field of space propulsion. Specific impulse of the latter two types of engines is much higher than that of chemical rockets. The chemical rocket engine is mainly composed of a combustion chamber and a nozzle. The chemical propellant is both an energy source and a working medium. It converts chemical energy into thermal energy in the combustion chamber, generates high-temperature gas, expands and accelerates through the nozzle, and converts thermal energy into gas flow energy. (1500 to 5000 m / s) is discharged from the nozzle, and generates thrust. Chemical rocket engines are further divided into liquid rocket engines, solid rocket engines, and hybrid propellant rocket engines according to the state of the propellant. The liquid rocket engine uses liquid storable propellant at normal temperature and low temperature propellant which is liquid at low temperature. It has the characteristics of strong adaptability, can be started multiple times, and can meet the requirements of different launch vehicles and spacecraft. The solid rocket motor propellant uses an organic colloidal solid solution (double-based propellant) containing fuel and oxidant in the molecule or a mixture of several propellant components (composite propellant), which is directly installed in the combustion chamber. The structure is simple and easy to use. It can be stored for a long time and it is suitable for various strategic and tactical missiles. Hybrid propellant rocket engines are rarely used.

Rocket Engine Advantage

Compared with the air jet engine, the biggest feature of the rocket engine is that it has both fuel and oxidant. It depends on the 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 a cosmic vacuum outside the atmosphere. This is not possible with any air jet engine. The launching artificial satellites, lunar spacecraft, and propulsion devices used in various space vehicles are all rocket engines.

Rocket engine modern machine

Modern rocket engines are mainly divided into solid propellant and liquid propellant engines. The so-called "propellant" is the collective name of fuel (combustion agent) plus oxidant.

Rocket Engine Solid Rocket Engine

Solid rocket motors are chemical rocket engines that use solid propellants. Solid propellants include polyurethane, polybutadiene, hydroxyl-terminated polybutadiene, nitrate plasticized polyether, and the like.

A solid rocket motor consists of a pellet, a combustion chamber, a nozzle assembly, and an ignition device. A grain is a hollow cylinder made of a propellant and a small amount of additives (the hollow part is the combustion surface, and its cross-sectional shape is round, star, etc.). The charge is placed in a combustion chamber (generally the engine casing). When the propellant burns, the combustion chamber must withstand a high temperature of 2500 to 3500 degrees and a high pressure of 102 to 2 × 107 Pa. Therefore, it must be made of high-strength alloy steel, titanium alloy or composite material, and be placed between the grain and the inner wall of the combustion Equipped with thermal lining.

The ignition device is used to ignite the charge column, and usually consists of an electric ignition tube and a powder box (containing black powder or pyrotechnic powder). After electrification, the black powder is ignited by the electric heating wire, and then the black powder is ignited by the black powder.

In addition to accelerating the expansion of gas to generate thrust, in order to control the thrust direction, the nozzle is often combined with a thrust vector control system to form a nozzle assembly. This system can change the angle of gas injection, thus changing the thrust direction.

After the pellets burned, the engine stopped working.

Compared with the liquid rocket engine, the solid rocket engine has the advantages of simple structure, high propellant density, propellant can be stored in the middle of combustion, and it is convenient and reliable to operate. The disadvantage is that the "specific impulse" is smaller (also called specific thrust, which is the ratio of engine thrust to the weight of propellant consumed per second, in seconds). The solid rocket motor has a specific impulse of 250 to 300 seconds, a short working time, a large acceleration, which makes it difficult to control the thrust, and it is difficult to start repeatedly, which is not conducive to manned flight.

Solid rocket motors are mainly used as engines for rockets, missiles and sounding rockets, as well as boosters for spacecraft launches and aircraft takeoffs.

The solid rocket motor is mainly composed of four parts, including the shell, solid propellant, nozzle assembly, and ignition device. Among them, solid propellant formula and molding process, nozzle design and use of materials and manufacturing processes, shell materials and manufacturing processes are the most The key link directly affects the performance of solid-state engines. The performance of solid engines mainly depends on two aspects, thrust and specific impulse. For engines with special requirements such as ballistic missiles or anti-missile interceptor missiles, fast-burning performance will also be pursued.

The materials used in solid engine casings have evolved from high-strength metals (ultra-high-strength steels, titanium alloys, etc.) to advanced composite materials that are always high-performance carbon fibers. However, for space launches, solid rocket engines do not seek to reduce the weight of the shell, so many solid rockets are still using high-strength steel as the shell, such as the S-125 booster used by the Indian GSLV rocket, using the M250 type. High-strength steel. Lightweight high-strength carbon composites are mainly used in ballistic missiles, especially third-stage engines.

Propellants of solid engines can be divided into low energy, medium energy, and high energy propellants according to energy. Specific impulse greater than 2450 N / s / kg (250 seconds) is high energy, and 2255 N / s / kg (230 seconds) to 2450 N Medium energy per second / kilogram, low energy less than 2255 N / sec / kg; divided into smoke, micro smoke, and smokeless propellant according to characteristic signal There is no small loss on specific impulse; according to the material formula combination, it can be divided into single base, double base, composite propellant, single base propellant consists of a single compound, such as cotton, and the specific impulse is too low to be applicable. The double-base propellant is composed of cotton wool or nitroglycerin and some additives. The specific impulse is still insufficient, and there are not many applications. Composite propellant is a combination of separate combustion agent and oxidant material. It uses liquid polymer binder as fuel, adds crystalline oxidant solid filler and other additives, and fuses and solidifies into multiphase objects. To increase energy and density, some powdered light metal materials can be added as flammable agents, such as aluminum powder. Composite propellants are usually named after the chemical name of the binder fuel, such as HTPB (hydroxyl-terminated polybutadiene). The oxidant is mainly perchlorate such as amine perchlorate. Composite propellants are generally made by pouring and are the absolute mainstream of solid propellants. In addition, there are two types of modified double-based propellants, including composite modified double-based propellants (CMDB) and cross-linked modified double-based propellants (XLDB). On the basis of double-base propellant, the proportion of basic cotton and nitroglycerin is greatly reduced, and high-energy solid components such as oxidant perchlorate and fuel aluminum powder are added to form a composite modified double-base propellant. Adding a polymer compound as a cross-linking agent becomes a cross-linked modified double-base propellant. NEPE (nitrate-plasticized polyether) in cross-linked modified double-base propellant is the solid propellant with the highest practical specific impulse. The third-stage engine of China's DF-31A missile uses NEPE (China number N -15) Propellant.

The rocket engine nozzle belongs to the convergence-diffusion nozzle (ie Laval-DeLaval nozzle). It consists of the inlet section (convergent section), throat (throat lining), and exit cone (diffusion section or expansion section). It converts the thermal energy of the combustion products into the kinetic energy of the high-speed jet to generate thrust. The expansion ratio, that is, the area ratio of the throat and 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 using cooling nozzles, solid engines use ablative nozzles, and the inner wall of the nozzle is coated with ablative material, which absorbs heat through the material's ablation evaporation to prevent the nozzle from overheating and burning. Generally speaking, bell nozzles are used for engine nozzle expansion

Rocket Engine Liquid Rocket Engine

Liquid rocket engines refer to chemical rocket engines with liquid propellants. Commonly used liquid oxidants include liquid oxygen, dinitrogen tetroxide, and the like. Combustion agents include liquid hydrogen, dimethyl hydrazine, and kerosene. The oxidant and burner must be stored in separate storage tanks.

Liquid rocket engines generally consist of a thrust chamber, a propellant supply system, and an engine control system.

The thrust chamber is an important component that converts the chemical energy of a liquid propellant into a propulsive force. It consists of a propellant nozzle, a combustion chamber, and a nozzle assembly. The propellant is injected into the combustion chamber through the injector, and the combustion products are generated by atomization, evaporation, mixing and combustion, etc., and are ejected from the nozzle at a high speed (2500-5000 m / s) to generate thrust. The pressure in the combustion chamber can reach 200 atmospheres (about 20 MPa) and the temperature is 3000 to 4000 ° C, so it needs to be cooled.

The function of the propellant supply system is to deliver the propellant to the combustion chamber at the required flow rate and pressure. According to different conveying methods, there are two types of supply systems: squeeze type (pneumatic type) and pump type. Squeeze-type supply system uses high-pressure gas to be decompressed by a pressure reducer (the flow rate of oxidant and combustion agent is controlled by the pressure set by the pressure reducer) and enters the oxidant and combustion agent storage tanks and squeezes them into the combustion chamber respectively. in. The squeeze supply system is only used for small thrust engines. High-thrust engines use a pumped supply system, which uses a hydraulic pump to deliver 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 phases of engine starting, working, and shutting down. This process is performed automatically according to a predetermined procedure. The working parameters mainly refer to the thrust and the mixing ratio of the propellant.

The advantages of liquid rocket engines are higher specific impulse (250 to 500 seconds), large thrust range (single thrust of 1 gram to 700 tons of force), repeated starting, control of thrust, and longer working hours. Liquid rocket engines are mainly used for spacecraft launch, attitude correction and control, orbit transfer, and so on.

The liquid rocket engine is the mainstream of aerospace launching, and its structure is much more complicated than that of a solid engine. It is mainly composed of an ignition device, a combustion chamber, a nozzle, and a fuel delivery device. The ignition device is generally a gunpowder igniter. For upper-level engines that require multiple starts, multiple gunpowder igniters are required. For example, the J-2X engine of the Ares rocket of the United States has two gunpowder igniters to achieve two start functions. YF-73 and YF-75 are also equipped with two gunpowder igniters, which have the ability to start twice; the combustion chamber is where the liquid fuel and oxidant burn and expand. In order to obtain a higher specific impulse, generally it has a high Pressure, even ordinary engines, usually have high pressures of dozens of atmospheres. For the Soviet Union's RD-180 and other engines, the combustion chamber pressure is as high as more than 250 atmospheres. Combustion under high pressure is more complicated than under normal pressure. At the same time, as the volume of the combustion chamber increases, the combustion instability becomes more and more serious, and it is more troublesome to solve. There is no reliable mathematical model to analyze the combustion stability problem, which is mainly solved by a large number of engine combustion tests. The F-1 engine of the Saturn 5 rocket in the United States has undergone a ground test platform fire test of up to 200,000 seconds, and the RD-170 engine of the Soviet Energy rocket has also undergone a ground test platform fire test of more than 100,000 seconds. During repeated combustion tests, various engine parameters are continuously optimized to alleviate unstable combustion. However, the unstable combustion phenomenon is not obvious for engines with low room pressure and low thrust. Unstable combustion is one of the main problems restricting the increase of thrust of liquid engines. Liquid rocket engine combustion chambers are cooled with liquid fuel or oxidant. Before they enter the combustion chamber, they flow through the wall of the combustion chamber to cool down; the nozzles of liquid engines are also Laval nozzles, and the expansion section is generally bell-shaped, but The cooling nozzle is used to cool the liquid fuel or oxidant.

Liquid engine fuel delivery is divided into four modes: extrusion cycle, gas generator cycle, staged combustion cycle, and expansion cycle.

The extrusion cycle uses high-pressure gas to reduce the pressure of the pressure reducer into the oxidant and burner storage tanks, and squeezes them into the combustion chamber. Due to the material of the storage tank, it is impossible to achieve a high pressure, so it is only used in small On low-performance engines.

In the gas generator cycle , a part of the fuel and oxidant flow through a gas generator. After combustion, the fuel pump and the oxidant pump are driven to run. The fuel pump and the oxidant pump press the fuel into the combustion chamber, and the pre-combustion exhaust gas is directly discharged. Some of the initial fuel and oxidant flows are squeezed through the tank, and some are guided by natural gravity.

The staged combustion cycle is also called the supplementary combustion method. It is also the combustion of fuel and oxidant in the pre-burner, driving the fuel pump and the oxidant pump, but the difference is that the gas in the pre-burner is not directly discharged, but is forced into the combustion chamber. This avoids the waste of fuel and oxidant, and can achieve a larger specific impulse. The pursuit of high specific impulse engines generally adopts a staged combustion cycle method. In order to pursue a higher specific impulse during staged combustion, the pressure of the combustion chamber is generally higher than that of the gas generator cycle.

The expansion cycle is the flow of fuel or oxidant through the wall of the combustion chamber and the nozzle, where the combustion chamber and the nozzle are cooled, and the temperature increases by itself, pushing the fuel pump and the oxidant pump to operate. Obviously, gas generation The burner and staged combustion cycle will also flow through these high-temperature parts, but can be driven by the high-pressure gas of the pre-burner, which can achieve much greater thrust. Expansion combustion cycle engines generally have a high specific impulse, and theoretically the highest specific impulse when the other conditions are the same, but the thrust is difficult to increase, such as the RL10-B-2 in the United States, which has the highest among the used liquid engines. The specific impulse is 465.5 seconds, but the thrust is only 24750 pounds, which is about 11.2 tons.

Speaking of liquid engines, the circulation method and the pressure of the combustion chamber and the design of the nozzles 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 dinitrogen tetroxide, had a specific impulse of at most about 300 seconds in a vacuum, and hydrazines were highly toxic. Nitrogen tetroxide was also very corrosive and had been gradually eliminated. China's Long March New generation rockets such as No. 5 will also phase out the existing Long March rockets with hydrazine fuels in the next few years; kerosene fuel is higher than rush, and kerosene is not much higher than hydrazine, only about 20 seconds. The main feature is that it is cheap and non-toxic. It is very suitable for liquid engines. The current commercial rocket companies use liquid oxygen kerosene engines as a favorite. Methane engines are more specific than methane, and methane is a hydrocarbon fuel. The medium shock is the highest, but it is not much higher than kerosene, which is also about 20 seconds. At the same time, it needs low temperature storage. The volume is much larger than kerosene. The main cost is much higher, so it is rarely asked, but the cold war Later, various aerospace countries began the pre-research work on methane engines; the highest specific fuel combination was the liquid hydrogen and oxygen combination, not to mention liquid hydrogen fuel than kerosene, which is much more expensive than hydrazine, and storage Huge volume, but higher than the hydrogen oxygen impulse than liquid oxygen kerosene too, in a vacuum, generally can reach more than 420 seconds, more than a third higher than. In contrast to the Ziolkovsky formula, this means that the load can be driven into orbit with much less fuel. However, due to the expensiveness of liquid hydrogen, liquid hydrogen fuel was mainly used in the upper stage of the rocket (the first stage or higher is called the upper stage). With the advancement of technology, the price of liquid hydrogen has decreased. Hydrogen fuels, such as Japan's H-II, Europe's Ariane5, etc., the first stage of China's Long March 5 rocket will also use liquid hydrogen fuel. In the United States, the Delta4 rocket, a large-scale rocket that also uses liquid hydrogen fuel, has appeared, and its performance is very superior.

Rocket engine other energy

Rocket Engine 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 chemical rocket engines, the energy and working fluid of this engine are separated. Electricity is provided by the aircraft, which is generally obtained from solar, nuclear, and chemical energy through conversion devices. Working fluids include hydrogen, nitrogen, argon, mercury, ammonia and other gases.

The electric rocket engine consists of a power source, a power converter, a power regulator, a working fluid supply system and an electric thruster. The power supply and power converter supply electrical energy; the function of the power regulator is to start the engine according to a predetermined procedure, and continuously adjust various parameters of the electric thruster to keep the engine always in a specified working state; the working fluid supply system is to store working fluid and Conveying working medium; the function of electric thruster is to convert electric energy into kinetic energy of working medium, so that it generates high-speed jet flow and generates thrust.

There are three types of electric rocket engines: electric rocket engines, electrostatic rocket engines, and electromagnetic rocket engines. Electric heating rocket engines use electric energy to heat (resistance heating or arc heating) working fluids (hydrogen, amine, hydrazine, etc.) to vaporize them; after the nozzles expand and accelerate, they are discharged from the nozzles to generate thrust. The working fluid (mercury, cesium, hydrogen, etc.) of the electrostatic rocket engine is ionized into ions from the input chamber of the storage tank, and then accelerated into a high-speed ion current under the action of the electrostatic field of the electrode to generate thrust. The electromagnetic rocket engine uses the electromagnetic field to accelerate the ionized working fluid to generate a jet, which forms a thrust. The electric rocket engine has a very high specific impulse (700-2500 seconds) and extremely long life (it can be restarted tens of thousands of times, and the accumulated work can reach tens of thousands of hours). But the generated thrust is less than 100N. This engine is only suitable for attitude control and position maintenance of spacecraft.

Electric rocket engine

Rocket Engine Nuclear Rocket Engine

Fission: Fission rocket engines are essentially miniaturized nuclear reactors and placed on rockets. Nuclear rocket engines use nuclear fuel as the 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 fluid delivery system and a control system installed in a thrust chamber. In a nuclear reactor, nuclear energy is converted into thermal energy to heat a working fluid. After being heated and expanded by a nozzle, the heated working fluid is expelled from the nozzle at a speed of 6500 to 11,000 meters per second to generate thrust. Nuclear rocket engines have a high specific impulse (250-1000 seconds) and a long life, but the technology is complex and is only suitable for long-term spacecraft. Due to nuclear radiation protection, exhaust pollution, reactor control, and the design of high-efficiency heat exchangers, these engines have not been resolved and are still in trials. In addition, solar-heated and photon rocket engines are still in the theoretical exploration stage.

Nuclear rocket engine

Fusion type: Fusion nuclear rocket engine is considered to be the most promising rocket engine to achieve flight in the solar system. Its principle is similar to chemical rockets, except that the fuel is converted into hydrogen isotopes deuterium, tritium, and helium. The huge energy released to propel the rocket is orders of magnitude higher than the chemical rocket.

Because the materials produced by fusion nuclear reactions are neutrons, protons, and helium, they cannot be used in the earth's atmosphere, but the universe is full of various kinds of radiation, so there is nothing wrong with its use in space. The main problems that need to be solved for fusion rocket engines are the two issues of ignition and fuel chamber high temperature resistant materials (the temperature of the reaction chamber is as high as tens of millions to hundreds of millions of degrees Celsius), which are still in the theoretical exploration stage.

Rocket Engine Latest Achievements

Rocket Engine China

On July 4, 2006, the Aerospace Propulsion Technology Research Institute, which undertook the development of a new generation of large launch vehicle power systems, revealed that the "120-ton liquid oxygen kerosene engine" used to propel China's new generation of large launch vehicles was first developed at the academy The machine test run was successful.

On July 17, 2018, we learned from the Sixth Academy of China Aerospace Science and Technology Corporation that China's first high-thrust, high-performance liquid oxygen kerosene high-altitude engine has been successfully implemented for the first time. It is reported that this is China's first high-thrust, high-performance liquid oxygen kerosene high-altitude engine with a thrust of 120 tons, which is used to carry the rocket core II. ^[2]

Rocket engine usa

According to foreign media reports, the world's largest rocket built by NASA has now entered the critical review stage and is expected to be completed in 2018 with a mass of approximately 5.5 million pounds, a height of 98 meters, and a thrust of 8.4 million pounds. This is a historic moment, and in the past 40 years we have again obtained the super rocket because we are about to land on Mars. The critical design review of the SLS rocket has now completed all the steps. When the SLS rocket was launched, it began the era of exploring Mars. This is our most powerful carrier rocket, capable of sending nearly 100 tons of cargo into low-Earth orbit, and its carrying capacity is unprecedentedly strong.

This will be the most powerful rocket currently available, and can be paired with the Orion spacecraft to form a vehicle for exploring beyond Earth's 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 link. We have completed the first round of engine testing. The next step is to manufacture and test SLS rockets in 2017 and pass design certification. . Eventually, 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 low temperature liquid hydrogen liquid oxygen engine. Using the RS-25 engine , NASA is preparing to conduct a second round of evaluation of the SLS rocket propulsion and complete some structural tests. ^[3]

Rocket engines are world-renowned

F-1 rocket engine

The world's largest single-chamber liquid rocket engine developed by the United States is used for the Saturn 5 rocket with a single thrust of 700 tons. It uses kerosene as fuel and liquid oxygen as oxidant.

F-1 detailed data:

Combustion form: open cycle of gas generator, 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 of 70 engines)

Vacuum 304.1 seconds

Diameter: 3.645 meters

Length: 5.598 meters

Total weight: 8451.66 kg

Propellant flow during work: kerosene: 838.2 kg / s, liquid oxygen 1784.7 kg / s

Turbo pump power: 46225 kW

Design starts: 20

Design life: 2250 seconds

RD-170 rocket engine

The world's largest thrust liquid rocket engine developed by Russia, using kerosene + liquid oxygen, a single thrust of 800 tons (using a four combustion chamber, four nozzle design, it is also thought that it is four engines in parallel, but share a gas generator and turbo pump) For the first stage of the energy launch vehicle and Zenith launch vehicle (RD-171 rocket engine, an improved version of RD-170).

Its derivative models are the RD-180 rocket engine, with a thrust of 400 tons, which is equivalent to splitting the RD-170 into two, dual fuel chambers and dual nozzles. The first stage of the Optimus II and Optimus III launch vehicles.

The RD-191 rocket engine, with a single thrust of 200 tons and a single chamber and single nozzle, is equivalent to dividing the RD-170 into two, which is used for the Russian Angara launch vehicle. A variant of the RD-191, the RD-151, was sold to South Korea for the first stage of the Luo old carrier rocket.

RS-68 Rocket Engine

The world's largest thrust liquid hydrogen liquid oxygen engine developed by the United States, with a thrust of 300 tons, is used for the first stage of the Delta IV carrier rocket.

RD-0120 rocket engine

Russia's largest thrust liquid hydrogen liquid oxygen rocket engine, with a thrust of 200 tons, is used for the main engine of the energy carrier rocket.

Space Shuttle Main Engine (SSME)

The main engine of the American Space Shuttle uses liquid hydrogen and liquid oxygen, with a thrust of 200 tons. The biggest feature is reusability.

Space shuttle solid rocket motor

The world's largest thrust rocket engine, a single thrust of up to 1200 tons, can be reused 10 times, for the US space shuttle bundle booster, its improved version is used for the Ares 1 rocket active engine and Ares 5 rocket bundled boost Device.