A rocket engine, or simply "rocket," is a jet engine A jet engine is a reaction engine that discharges a fast moving jet of fluid to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets. In general, most jet engines are internal combustion engines but non-[1] that uses only propellant A propellant is a material that is used to move an object. The material is usually expelled by gas pressure through a convergent,divergent nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid or gelled mass for forming its high speed propulsive jet A jet is a coherent stream of fluid that is projected into a surrounding medium, usually from some kind of a nozzle or aperture. Jets can travel long distances without dissipating. In the Earth's atmosphere there exist jet streams that travel thousands of miles. Rocket engines are reaction engines A reaction engine is an engine which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force" and obtain thrust in accordance with Newton's third law Newton's laws of motion are three physical laws that form the basis for classical mechanics. They are:[note 1]. Since they need no external material to form their jet, rocket A rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction. Rocket engines push rockets forwards simply by throwing their exhaust backwards engines can be used for spacecraft propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through as well as terrestrial uses, such as missiles In common military parlance, the word missile describes a powered, guided munition that travels through the air or space, whilst the word rocket describes a powered, unguided munition. Unpowered, guided munitions are known as guided bombs. Powered munitions that travel through water are called torpedoes. A common further sub-division is to. Most rocket engines are internal combustion engines The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the, although non combusting forms also exist.
Rocket engines as a group, have the highest exhaust velocities, are by far the lightest, and are the most energy efficient (at least at very high speed) of all types of jet engines. However, for the thrust they give, due to the high exhaust velocity and relatively low specific energy Specific energy is defined as the energy per unit mass. Common metric units are J/kg. It is an intensive property. Contrast this with energy, which is an extensive property. There are two main types of specific energy: potential energy and specific kinetic energy. Others are the gray and sievert, measures for the absorption of radiation. The of rocket propellant, they consume propellant very rapidly.
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Terminology
Chemical rockets are rockets powered by exothermic In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in the form of light (e.g. a spark, flame, or explosion), electricity (e.g. a battery), or sound(e.g. burning hydrogen). Its etymology stems from the Greek prefix ex- (meaning "outside") and the Greek word chemical reactions of the propellant.
Rocket motor (or solid-propellant rocket motor) is a synonymous term with rocket engine that usually refers to solid rocket engines.
Liquid rockets (or liquid-propellant rocket engine) use one or more liquid propellants that are held in tanks prior to burning.
Hybrid rockets have a solid propellant in the combustion chamber and a second liquid or gas propellant is added to permit it to burn.
Thermal rockets are rockets where the propellant is inert, but is heated by a power source such as solar Solar thermal propulsion is a form of spacecraft propulsion that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar energy, such as concentrators and mirrors. The or nuclear power In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. The nuclear reactor's energy replaces the chemical energy of the reactive chemicals in a chemical rocket engine. Due to the higher energy density of the nuclear fuel or beamed energy Beam-powered propulsion is a class of spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy. Most designs are rocket engines where the energy is provided by the beam, and is used to superheat propellant that then provides propulsion, although some obtain propulsion directly from light.
Principle of operation
How rocket engines work Rocket engines give part of their thrust due to unopposed pressure on the combustion chamberRocket engines produce thrust by the expulsion of a high-speed fluid A fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids exhaust. This fluid is nearly A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel—the engine of the rocket—is usually a used plastic soft drink bottle. The water is forced out by a pressurized gas, typically compressed air always a gas which is created by high pressure (10-200 bar) combustion of solid or liquid propellants Rocket propellant is mass that is stored in some form of propellant tank, prior to being used as the propulsive mass that is ejected from a rocket engine in the form of a fluid jet to produce thrust. A fuel propellant is often burned with an oxidizer propellant to produce large volumes of very hot gas. These gases expand and push on a nozzle,, consisting of fuel Fuel is any material that can be used to generate energy to produce mechanical work in a controlled manner. The processes used to convert fuel into energy include chemical reactions, such as combustion, and nuclear reactions, such as nuclear fission or nuclear fusion. Fuels are also used in the cells of organisms in a process known as metabolism and oxidiser In the above equation, the iron has an oxidation number of 0 before and 3+ after the reaction. For oxygen (O) the oxidation number began as 0 and decreased to 2−. These changes can be viewed as two "half-reactions" that occur concurrently: components, within a combustion chamber The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber. This pressure can be used to do work, for example, to move a piston on a crankshaft or a turbine disc in a gas turbine. The energy can also be used to produce thrust when.
The fluid exhaust is then passed through a propelling nozzle A propelling nozzle is the component of a jet engine that operates to form an exhaust jet and maximise the velocity from the engine which typically uses the heat energy of the gas to accelerate the exhaust to very high speed, and the reaction to this pushes the engine in the opposite direction.
In rocket engines, high temperatures and pressures are highly desirable for good performance as this permits a longer nozzle to be fitted to the engine, which gives higher exhaust speeds, as well as giving better thermodynamic efficiency.
Introducing propellant into a combustion chamber
Rocket propellant is mass that is stored, usually in some form of propellant tank, prior to being ejected from a rocket engine in the form of a fluid jet to produce thrust.
Chemical rocket propellants are most commonly used, which undergo exothermic chemical reactions which produce hot gas which is used by a rocket for propulsive purposes. Alternatively, a chemically inert reaction mass Working mass is a mass against which a system operates in order to produce acceleration. All acceleration requires an exchange of momentum, which can be thought of as the "unit of movement". Momentum is related to mass and velocity, as given by the formula P = mv, where P is the momentum, m the mass, and v the velocity. The velocity of a can be heated using a high-energy power source via a heat exchanger, and then no combustion chamber is used.
A solid rocket motor.Solid rocket A solid rocket or a solid-fuel rocket is a rocket with a motor that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Indians, Chinese, Mongols and Arabs, in warfare as early as the 13th century. All rockets used some form of solid or powdered propellant up until the 20th century, propellants are prepared as a mixture of fuel and oxidizing components called 'grain' and the propellant storage casing effectively becomes the combustion chamber. Liquid-fueled rockets A liquid-propellant rocket or a liquid rocket is a rocket with an engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight pumps to pump the propellant from the tanks into the engines, which means typically pump separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Hybrid rocket A hybrid rocket is a rocket with a rocket engine which uses propellants in two different states of matter - one solid and the other either gas or liquid. The Hybrid rocket concept can be traced back at least 75 years engines use a combination of solid and liquid or gaseous propellants. Both liquid and hybrid rockets use injectors A liquid-fuel rocket or a liquid rocket is a rocket with an engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume and hence the mass of the tanks to be relatively low, resulting in a high mass ratio. Liquid rockets have been built as monopropellant rockets using a single type of to introduce the propellant into the chamber. These are often an array of simple jets- holes through which the propellant escapes under pressure; but sometimes may be more complex spray nozzles. When two or more propellants are injected the jets usually deliberately collide the propellants as this breaks up the flow into smaller droplets that burn more easily.
Combustion chamber
For chemical rockets the combustion chamber is typically just a cylinder, and flame holders All continuous-combustion jet engines require a flame holder. A flame holder creates a low-speed eddy in the engine to prevent the flame from being blown out. The design of the flame holder is an issue of balance between a stable eddy and drag are rarely used. The dimensions of the cylinder are such that the propellant is able to combust thoroughly; different propellants require different combustion chamber sizes for this to occur. This leads to a number called L * :
where:
- Vc is the volume of the chamber
- At is the area of the throat
L* is typically in the range of 25–60 inches (0.63–1.5 m).
The combination of temperatures and pressures typically reached in a combustion chamber is usually extreme by any standards. Unlike in air-breathing jet engines, no atmospheric nitrogen is present to dilute and cool the combustion, and the temperature can reach true stoichiometric Stoichiometry is a branch of chemistry that deals with the quantitative relationships that exist among the reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers. For example, in a reaction that forms ammonia (NH3), exactly one. This, in combination with the high pressures, means that the rate of heat conduction through the walls is very high.
Rocket nozzles
Main article: Rocket engine nozzle A rocket engine nozzle is a propelling nozzle usually of the de Laval type used in a rocket engine to expand and accelerate the combustion gases, from burning propellants, so that the exhaust gases exit the nozzle at hypersonic velocities Typical temperatures (T) and pressures (p) and speeds (v) in a De Laval NozzleThe large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape.
In rockets the hot gas produced in the combustion chamber is permitted to escape from the combustion chamber through an opening (the "throat"), within a high expansion-ratio 'de Laval nozzle' A rocket engine nozzle is a propelling nozzle usually of the de Laval type used in a rocket engine to expand and accelerate the combustion gases, from burning propellants, so that the exhaust gases exit the nozzle at hypersonic velocities.
Provided sufficient pressure is provided to the nozzle (about 2.5-3x above ambient pressure) the nozzle chokes and a supersonic jet is formed, dramatically accelerating the gas, converting most of the thermal energy into kinetic energy.
The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound of sea level air The speed of sound is the rate of travel of a sound wave through an elastic medium. In dry air at 20 °C , the speed of sound is 343 metres per second (1,125 ft/s). This equates to 1,236 kilometres per hour (768 mph), or about one kilometer in three seconds and about one mile in five seconds. This figure increases with temperature (equations are are not uncommon.
Rocket thrust is caused by pressures acting in the combustion chamber and nozzle. From Newtons third law, equal and opposite pressures act on the exhaust, and this accelerates it to high speeds.About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle (see diagram). As the gas expands (adiabatically In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid. The term "adiabatic" literally means impassable, coming from the Greek roots ἀ- , διὰ- ("through"), and βαῖνειν ("to pass"); this etymology corresponds) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other.
Propellant efficiency
For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. This can be achieved by all of:
- heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and sometimes metals such as aluminium, or even using nuclear energy)
- using a low specific density gas (as hydrogen rich as possible)
- using propellants which are, or decompose to, simple molecules with few degrees of freedom to maximise translational velocity
Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass of propellant present to be accelerated as it pushes on the engine, and since from Newton's third law the pressure that acts on the engine also reciprocally acts on the propellant, it turns out that for any given engine the speed that the propellant leaves the chamber is unaffected by the chamber pressure (although the thrust is proportional). However, speed is significantly affected by all three of the above factors and the exhaust speed is an excellent measure of the engine propellant efficiency. This is termed exhaust velocity, and after allowance is made for factors that can reduce it, the effective exhaust velocity Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the impulse—change in momentum—per unit of propellant. When referring to the specific impulse it just means to divide the impulse by the unit mass or unit weight. The higher the specific impulse, the less propellant is needed to gain a given amount of is one of the most important parameters of a rocket engine (although weight, cost, ease of manufacture etc. are usually also very important).
For aerodynamic reasons the flow goes sonic ("chokes") at the narrowest part of the nozzle, the 'throat'. Since the speed of sound The speed of sound is the rate of travel of a sound wave through an elastic medium. In dry air at 20 °C , the speed of sound is 343 metres per second (1,125 ft/s). This equates to 1,236 kilometres per hour (768 mph), or about one kilometer in three seconds and about one mile in five seconds. This figure increases with temperature (equations are in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at room temperature the speed of sound in air is about 340 m/s while the speed of sound in the hot gas of a rocket engine can be over 1700 m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air.
Expansion in the rocket nozzle then further multiplies the speed, typically between 1.5 and 2 times, giving a highly collimated Collimated light is light whose rays are nearly parallel, and therefore will spread slowly as it propagates. The word is related to "colinear" and implies light that does not disperse with distance , or that will disperse minimally (in reality). A perfectly collimated beam with no divergence cannot be created due to diffraction, but hypersonic exhaust jet. The speed increase of a rocket nozzle is mostly determined by its area expansion ratio—the ratio of the area of the throat to the area at the exit, but detailed properties of the gas are also important. Larger ratio nozzles are more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity.
Nozzle efficiency is affected by operation in the atmosphere because atmospheric pressure changes with altitude; but due to the supersonic speeds of the gas exiting from a rocket engine, the pressure of the jet may be either below or above ambient, and equilibrium between the two is not reached at all altitudes (See Diagram).
Back pressure and optimal expansion
For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure: if the exhaust's pressure is lower than the ambient pressure, then the vehicle will be slowed by the difference in pressure between the top of the engine and the exit; on the other hand, if the exhaust's pressure is higher, then exhaust pressure that could have been converted into thrust is not converted, and energy is wasted.
To maintain this ideal of equality between the exhaust's exit pressure and the ambient pressure, the diameter of the nozzle would need to increase with altitude, giving the pressure a longer nozzle to act on (and reducing the exit pressure and temperature). This increase is difficult to arrange in a lightweight fashion, although is routinely done with other forms of jet engines. In rocketry a lightweight compromise nozzle is generally used and some reduction in atmospheric performance occurs when used at other than the 'design altitude' or when throttled. To improve on this, various exotic nozzle designs such as the plug nozzle The ideal contour of a plug nozzle is a long tapering 'spike' with a doughnut-shaped combustion chamber situated at the base, hence sometimes this nozzle is also called a "spike nozzle". To save weight, this design is shortened without a large drop in efficiency, stepped nozzles A stepped nozzle is a de Laval rocket nozzle which has altitude compensating properties, the expanding nozzle The expanding nozzle is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of altitude compensating nozzles, a class that also includes the plug nozzle and aerospike. While the expanding nozzle is the least technically advanced and simplest to understand from and the aerospike The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. It is a member of the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low altitudes, where most missions have the greatest have been proposed, each providing some way to adapt to changing ambient air pressure and each allowing the gas to expand further against the nozzle, giving extra thrust at higher altitudes.
When exhausting into a sufficiently low ambient pressure (vacuum) several issues arise. One is the sheer weight of the nozzle- beyond a certain point, for a particular vehicle, the extra weight of the nozzle outweighs any performance gained. Secondly, as the exhaust gases adiabatically expand within the nozzle they cool, and eventually some of the chemicals can freeze, producing 'snow' within the jet. This causes instabilities in the jet and must be avoided.
On a De Laval nozzle, exhaust gas flow detachment will occur in a grossly over-expanded nozzle. As the detachment point will not be uniform around the axis of the engine, a side force may be imparted to the engine. This side force may change over time and result in control problems with the launch vehicle.
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