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64-523: Thruster may refer to: A thruster is a propulsive device used by spacecraft and watercraft for station keeping , attitude control , in the reaction control system , or long-duration, low-thrust acceleration. Spacecraft propulsion Several methods of pragmatic spacecraft propulsion have been developed, each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (often monopropellant rockets ) or resistojet rockets for orbital station-keeping , while

128-738: A ( m M ) 2 / 5 {\displaystyle {\overline {r_{\text{SOI}}}}=0.9431a\left({\frac {m}{M}}\right)^{2/5}} Consider two point masses A {\displaystyle A} and B {\displaystyle B} at locations r A {\displaystyle r_{A}} and r B {\displaystyle r_{B}} , with mass m A {\displaystyle m_{A}} and m B {\displaystyle m_{B}} respectively. The distance R = | r B − r A | {\displaystyle R=|r_{B}-r_{A}|} separates

192-432: A SpaceX Falcon 9 rocket. Rather than relying on high temperature and fluid dynamics to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or electromagnetic forces to accelerate the reaction mass directly, where the reaction mass is usually a stream of ions . Ion propulsion rockets typically heat a plasma or charged gas inside a magnetic bottle and release it via

256-551: A magnetic nozzle so that no solid matter needs to come in contact with the plasma. Such an engine uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities. For these drives, at the highest exhaust speeds, energetic efficiency and thrust are all inversely proportional to exhaust velocity. Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel. Electric propulsion

320-532: A charged propellant. The benefit of this method is that it can achieve exhaust velocities, and therefore I sp {\displaystyle I_{\text{sp}}} , more than 10 times greater than those of a chemical engine, producing steady thrust with far less fuel. With a conventional chemical propulsion system, 2% of a rocket's total mass might make it to the destination, with the other 98% having been consumed as fuel. With an electric propulsion system, 70% of what's aboard in low Earth orbit can make it to

384-417: A deep-space destination. However, there is a trade-off. Chemical rockets transform propellants into most of the energy needed to propel them, but their electromagnetic equivalents must carry or produce the power required to create and accelerate propellants. Because there are currently practical limits on the amount of power available on a spacecraft, these engines are not suitable for launch vehicles or when

448-438: A diverse set of missions and destinations. Space exploration is about reaching the destination safely (mission enabling), quickly (reduced transit times), with a large quantity of payload mass, and relatively inexpensively (lower cost). The act of reaching the destination requires an in-space propulsion system, and the other metrics are modifiers to this fundamental action. Propulsion technologies can significantly improve

512-807: A few use momentum wheels for attitude control . Russian and antecedent Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used electric propulsion such as ion thrusters and Hall-effect thrusters . Various technologies need to support everything from small satellites and robotic deep space exploration to space stations and human missions to Mars . Hypothetical in-space propulsion technologies describe propulsion technologies that could meet future space science and exploration needs. These propulsion technologies are intended to provide effective exploration of

576-473: A fixed amount of reaction mass. The higher the specific impulse, the better the efficiency. Ion propulsion engines have high specific impulse (~3000 s) and low thrust whereas chemical rockets like monopropellant or bipropellant rocket engines have a low specific impulse (~300 s) but high thrust. The impulse per unit weight-on-Earth (typically designated by I sp {\displaystyle I_{\text{sp}}} ) has units of seconds. Because

640-560: A given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations for a long time can often produce the same impulse as another which produces large accelerations for a short time. However, when launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used. Some designs however, operate without internal reaction mass by taking advantage of magnetic fields or light pressure to change

704-438: A human spaceflight propulsion system to provide that acceleration continuously, (though human bodies can tolerate much larger accelerations over short periods). The occupants of a rocket or spaceship having such a propulsion system would be free from the ill effects of free fall , such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones. The Tsiolkovsky rocket equation shows, using

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768-457: A large collection surface to function effectively. E-sails propose to use very thin and lightweight wires holding an electric charge to deflect particles, which may have more controllable directionality. Magnetic sails deflect charged particles from the solar wind with a magnetic field, thereby imparting momentum to the spacecraft. For instance, the so-called Magsail is a large superconducting loop proposed for acceleration/deceleration in

832-430: A long period of time some form of propulsion is occasionally necessary to make small corrections ( orbital station-keeping ). Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion. A satellite's useful life is usually over once it has exhausted its ability to adjust its orbit. For interplanetary travel , a spacecraft can use its engines to leave Earth's orbit. It

896-566: A number of critical aspects of the mission. When launching a spacecraft from Earth, a propulsion method must overcome a higher gravitational pull to provide a positive net acceleration. When in space, the purpose of a propulsion system is to change the velocity, or v , of a spacecraft. In-space propulsion begins where the upper stage of the launch vehicle leaves off, performing the functions of primary propulsion , reaction control , station keeping , precision pointing , and orbital maneuvering . The main engines used in space provide

960-410: A particle of reaction mass with mass m at velocity v is mv . But this particle has kinetic energy mv ²/2, which must come from somewhere. In a conventional solid , liquid , or hybrid rocket , fuel is burned, providing the energy, and the reaction products are allowed to flow out of the engine nozzle , providing the reaction mass. In an ion thruster , electricity is used to accelerate ions behind

1024-509: A rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun, or constantly thrusting along its direction of motion to increase its distance from the Sun. The concept has been successfully tested by the Japanese IKAROS solar sail spacecraft. Because interstellar distances are great, a tremendous velocity is needed to get

1088-597: A spacecraft needs a quick, large impulse, such as when it brakes to enter a capture orbit. Even so, because electrodynamic rockets offer very high I sp {\displaystyle I_{\text{sp}}} , mission planners are increasingly willing to sacrifice power and thrust (and the extra time it will take to get a spacecraft where it needs to go) in order to save large amounts of propellant mass. Spacecraft operate in many areas of space. These include orbital maneuvering, interplanetary travel, and interstellar travel. Artificial satellites are first launched into

1152-484: A spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival remains a formidable challenge for spacecraft designers. No spacecraft capable of short duration (compared to human lifetime) interstellar travel has yet been built, but many hypothetical designs have been discussed. Spacecraft propulsion technology can be of several types, such as chemical, electric or nuclear. They are distinguished based on

1216-615: A two-body approximation, ellipses and hyperbolae, the SOI is taken as the boundary where the trajectory switches which mass field it is influenced by. It is not to be confused with the sphere of activity which extends well beyond the sphere of influence. The most common base models to calculate the sphere of influence is the Hill sphere and the Laplace sphere , but updated and particularly more dynamic ones have been described. The general equation describing

1280-768: Is also known as the tidal forces due to body B {\displaystyle B} . It is possible to construct the perturbation ratio χ B {\displaystyle \chi _{B}} for the frame centered on B {\displaystyle B} by interchanging A ↔ B {\displaystyle A\leftrightarrow B} . As C {\displaystyle C} gets close to A {\displaystyle A} , χ A → 0 {\displaystyle \chi _{A}\rightarrow 0} and χ B → ∞ {\displaystyle \chi _{B}\rightarrow \infty } , and vice versa. The frame to choose

1344-421: Is attracted to point B {\displaystyle B} with acceleration a A = G m B R 3 ( r B − r A ) {\displaystyle a_{A}={\frac {Gm_{B}}{R^{3}}}(r_{B}-r_{A})} , this frame is therefore non-inertial. To quantify the effects of the perturbations in this frame, one should consider

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1408-425: Is commonly used for station keeping on commercial communications satellites and for prime propulsion on some scientific space missions because of their high specific impulse. However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission. The idea of electric propulsion dates to 1906, when Robert Goddard considered

1472-635: Is complex, but research has developed methods for their use in propulsion systems, and some have been tested in a laboratory. Here, nuclear propulsion moreso refers to the source of propulsion being nuclear, instead of a nuclear electric rocket where a nuclear reactor would provide power (instead of solar panels) for other types of electrical propulsion. Nuclear propulsion methods include: There are several different space drives that need little or no reaction mass to function. Many spacecraft use reaction wheels or control moment gyroscopes to control orientation in space. A satellite or other space vehicle

1536-401: Is denoted as g B {\displaystyle g_{B}} and will be treated as a perturbation to the dynamics of C {\displaystyle C} due to the gravity g A {\displaystyle g_{A}} of body A {\displaystyle A} . Due to their gravitational interactions, point A {\displaystyle A}

1600-500: Is highly toxic and at risk of being banned across Europe. Non-toxic 'green' alternatives are now being developed to replace hydrazine. Nitrous oxide -based alternatives are garnering traction and government support, with development being led by commercial companies Dawn Aerospace, Impulse Space, and Launcher. The first nitrous oxide-based system flown in space was by D-Orbit onboard their ION Satellite Carrier ( space tug ) in 2021, using six Dawn Aerospace B20 thrusters, launched upon

1664-411: Is not explicitly necessary as the initial boost given by the rocket, gravity slingshot, monopropellant/bipropellent attitude control propulsion system are enough for the exploration of the solar system (see New Horizons ). Once it has done so, it must make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments. In between these adjustments,

1728-852: Is possible to approximate the separating surface. In such a case this surface must be close to the mass A {\displaystyle A} , denote r {\displaystyle r} as the distance from A {\displaystyle A} to the separating surface. The distance to the sphere of influence must thus satisfy m B m A r 3 R 3 = m A m B R 2 r 2 {\displaystyle {\frac {m_{B}}{m_{A}}}{\frac {r^{3}}{R^{3}}}={\frac {m_{A}}{m_{B}}}{\frac {R^{2}}{r^{2}}}} and so r = R ( m A m B ) 2 / 5 {\displaystyle r=R\left({\frac {m_{A}}{m_{B}}}\right)^{2/5}}

1792-424: Is situated fairly deep in a gravity well ; the escape velocity required to leave its orbit is 11.2 kilometers/second. Thus for destinations beyond, propulsion systems need enough propellant and to be of high enough efficiency. The same is true for other planets and moons, albeit some have lower gravity wells. As human beings evolved in a gravitational field of "one g " (9.81m/s²), it would be most comfortable for

1856-457: Is still active as of this date). As further proof of the solar sail concept, NanoSail-D became the first such powered satellite to orbit Earth . As of August 2017, NASA confirmed the Sunjammer solar sail project was concluded in 2014 with lessons learned for future space sail projects. The U.K. Cubesail programme will be the first mission to demonstrate solar sailing in low Earth orbit, and

1920-561: Is subject to the law of conservation of angular momentum , which constrains a body from a net change in angular velocity . Thus, for a vehicle to change its relative orientation without expending reaction mass, another part of the vehicle may rotate in the opposite direction. Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum, so such systems are designed to "bleed off" undesired rotational energies built up over time. The law of conservation of momentum

1984-419: Is the effective exhaust velocity : the equivalent speed which the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However, Gravity well A sphere of influence ( SOI ) in astrodynamics and astronomy is the oblate spheroid -shaped region where a particular celestial body exerts

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2048-407: Is the Sun (until the object enters another body's SOI). Because the definition of r SOI relies on the presence of the Sun and a planet, the term is only applicable in a three-body or greater system and requires the mass of the primary body to be much greater than the mass of the secondary body. This changes the three-body problem into a restricted two-body problem. The table shows the values of

2112-437: Is the one that has the smallest perturbation ratio. The surface for which χ A = χ B {\displaystyle \chi _{A}=\chi _{B}} separates the two regions of influence. In general this region is rather complicated but in the case that one mass dominates the other, say m A ≪ m B {\displaystyle m_{A}\ll m_{B}} , it

2176-463: Is then allowed to escape through a high-expansion ratio bell-shaped nozzle , a feature that gives a rocket engine its characteristic shape. The effect of the nozzle is to accelerate the mass, converting most of the thermal energy into kinetic energy, where exhaust speeds reaching as high as 10 times the speed of sound at sea level are common. The dominant form of chemical propulsion for satellites has historically been hydrazine , however, this fuel

2240-512: Is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible). But space is not empty, especially space inside the Solar System; there are gravitation fields, magnetic fields , electromagnetic waves , solar wind and solar radiation. Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically

2304-723: The Oberth effect . A tether propulsion system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object. Beam-powered propulsion is another method of propulsion without reaction mass, and includes sails pushed by laser , microwave, or particle beams. Advanced, and in some cases theoretical, propulsion technologies may use chemical or nonchemical physics to produce thrust but are generally considered to be of lower technical maturity with challenges that have not been overcome. For both human and robotic exploration, traversing

2368-539: The Solar System and may permit mission designers to plan missions to "fly anytime, anywhere, and complete a host of science objectives at the destinations" and with greater reliability and safety. With a wide range of possible missions and candidate propulsion technologies, the question of which technologies are "best" for future missions is a difficult one; expert opinion now holds that a portfolio of propulsion technologies should be developed to provide optimum solutions for

2432-431: The radius of the sphere r SOI {\displaystyle r_{\text{SOI}}} of a planet: r SOI ≈ a ( m M ) 2 / 5 {\displaystyle r_{\text{SOI}}\approx a\left({\frac {m}{M}}\right)^{2/5}} where In the patched conic approximation, once an object leaves the planet's SOI, the primary/only gravitational influence

2496-536: The solar wind and deceleration in the Interstellar medium . A variant is the mini-magnetospheric plasma propulsion system and its successor, the magnetoplasma sail , which inject plasma at a low rate to enhance the magnetic field to more effectively deflect charged particles in a plasma wind. Japan launched a solar sail-powered spacecraft, IKAROS in May 2010, which successfully demonstrated propulsion and guidance (and

2560-473: The vacuum state . Such methods are highly speculative and include: A NASA assessment of its Breakthrough Propulsion Physics Program divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are not impossible according to current theories. Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods. Four numbers are shown. The first

2624-728: The Sun. The Sphere of influence is, in fact, not quite a sphere. The distance to the SOI depends on the angular distance θ {\displaystyle \theta } from the massive body. A more accurate formula is given by r SOI ( θ ) ≈ a ( m M ) 2 / 5 1 1 + 3 cos 2 ⁡ ( θ ) 10 {\displaystyle r_{\text{SOI}}(\theta )\approx a\left({\frac {m}{M}}\right)^{2/5}{\frac {1}{\sqrt[{10}]{1+3\cos ^{2}(\theta )}}}} Averaging over all possible directions we get: r SOI ¯ = 0.9431

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2688-514: The amount of thrust that can be produced to a small value. Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle. Nuclear fuels typically have very high specific energy , much higher than chemical fuels, which means that they can generate large amounts of energy per unit mass. This makes them valuable in spaceflight, as it can enable high specific impulses , sometimes even at high thrusts. The machinery to do this

2752-528: The change in momentum per unit of propellant used by a spacecraft, or the velocity of the propellant exiting the spacecraft, can be used to measure its "specific impulse." The two values differ by a factor of the standard acceleration due to gravity, g n , 9.80665 m/s² ( I sp g n = v e {\displaystyle I_{\text{sp}}g_{\mathrm {n} }=v_{e}} ). In contrast to chemical rockets, electrodynamic rockets use electric or magnetic fields to accelerate

2816-542: The desired altitude by conventional liquid/solid propelled rockets, after which the satellite may use onboard propulsion systems for orbital stationkeeping. Once in the desired orbit, they often need some form of attitude control so that they are correctly pointed with respect to the Earth , the Sun , and possibly some astronomical object of interest. They are also subject to drag from the thin atmosphere , so that to stay in orbit for

2880-434: The energy needed to generate thrust by chemical reactions to create a hot gas that is expanded to produce thrust . Many different propellant combinations are used to obtain these chemical reactions, including, for example, hydrazine , liquid oxygen , liquid hydrogen , nitrous oxide , and hydrogen peroxide . They can be used as a monopropellant or in bi-propellant configurations. Rocket engines provide essentially

2944-439: The first mission to demonstrate full three-axis attitude control of a solar sail. The concept of a gravitational slingshot is a form of propulsion to carry a space probe onward to other destinations without the expense of reaction mass; harnessing the gravitational energy of other celestial objects allows the spacecraft to gain kinetic energy. However, more energy can be obtained from the gravity assist if rockets are used via

3008-415: The highest specific powers and high specific thrusts of any engine used for spacecraft propulsion. Most rocket engines are internal combustion heat engines (although non-combusting forms exist). Rocket engines generally produce a high-temperature reaction mass, as a hot gas, which is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber. The extremely hot gas

3072-455: The law of conservation of momentum , that for a rocket engine propulsion method to change the momentum of a spacecraft, it must change the momentum of something else in the opposite direction. In other words, the rocket must exhaust mass opposite the spacecraft's acceleration direction, with such exhausted mass called propellant or reaction mass . For this to happen, both reaction mass and energy are needed. The impulse provided by launching

3136-412: The main gravitational influence on an orbiting object. This is usually used to describe the areas in the Solar System where planets dominate the orbits of surrounding objects such as moons , despite the presence of the much more massive but distant Sun . In the patched conic approximation , used in estimating the trajectories of bodies moving between the neighbourhoods of different bodies using

3200-649: The momentum flux density P of an EM wave is quantitatively 1/c times the Poynting vector S , i.e. P = S /c , where c is the velocity of light. Field propulsion methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft; however, because many of these phenomena are diffuse in nature, corresponding propulsion structures must be proportionately large. The concept of solar sails rely on radiation pressure from electromagnetic energy, but they require

3264-503: The orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination. Special methods such as aerobraking or aerocapture are sometimes used for this final orbital adjustment. Some spacecraft propulsion methods such as solar sails provide very low but inexhaustible thrust; an interplanetary vehicle using one of these methods would follow

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3328-806: The physics of the propulsion system and how thrust is generated. Other experimental and more theoretical types are also included, depending on their technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications. Almost all types are reaction engines , which produce thrust by expelling reaction mass , in accordance with Newton's third law of motion . Examples include jet engines , rocket engines , pump-jet , and more uncommon variations such as Hall–effect thrusters , ion drives , mass drivers , and nuclear pulse propulsion . A large fraction of rocket engines in use today are chemical rockets ; that is, they obtain

3392-536: The possibility in his personal notebook. Konstantin Tsiolkovsky published the idea in 1911. Electric propulsion methods include: For some missions, particularly reasonably close to the Sun, solar energy may be sufficient, and has often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are called nuclear electric rockets . Current nuclear power generators are approximately half

3456-420: The primary propulsive force for orbit transfer , planetary trajectories , and extra planetary landing and ascent . The reaction control and orbital maneuvering systems provide the propulsive force for orbit maintenance, position control, station keeping, and spacecraft attitude control. In orbit, any additional impulse , even tiny, will result in a change in the orbit path, in two ways: Earth's surface

3520-402: The ratio of the perturbations to the main body gravity i.e. χ A = | g B − a A | | g A | {\displaystyle \chi _{A}={\frac {|g_{B}-a_{A}|}{|g_{A}|}}} . The perturbation g B − a A {\displaystyle g_{B}-a_{A}}

3584-431: The solar system is a struggle against time and distance. The most distant planets are 4.5–6 billion kilometers from the Sun and to reach them in any reasonable time requires much more capable propulsion systems than conventional chemical rockets. Rapid inner solar system missions with flexible launch dates are difficult, requiring propulsion systems that are beyond today's current state of the art. The logistics, and therefore

3648-438: The spacecraft typically moves along its trajectory without accelerating. The most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit : the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to

3712-445: The spacecraft's momentum. When discussing the efficiency of a propulsion system, designers often focus on the effective use of the reaction mass, which must be carried along with the rocket and is irretrievably consumed when used. Spacecraft performance can be quantified in amount of change in momentum per unit of propellant consumed, also called specific impulse . This is a measure of the amount of impulse that can be obtained from

3776-438: The spacecraft. Here other sources must provide the electrical energy (e.g. a solar panel or a nuclear reactor ), whereas the ions provide the reaction mass. The rate of change of velocity is called acceleration and the rate of change of momentum is called force . To reach a given velocity, one can apply a small acceleration over a long period of time, or a large acceleration over a short time; similarly, one can achieve

3840-452: The sphere of gravity of the bodies of the solar system in relation to the Sun (with the exception of the Moon which is reported relative to Earth): An important understanding to be drawn from this table is that "Sphere of Influence" here is "Primary". For example, though Jupiter is much larger in mass than say, Neptune, its Primary SOI is much smaller due to Jupiter's much closer proximity to

3904-415: The total system mass required to support sustained human exploration beyond Earth to destinations such as the Moon, Mars, or near-Earth objects , are daunting unless more efficient in-space propulsion technologies are developed and fielded. A variety of hypothetical propulsion techniques have been considered that require a deeper understanding of the properties of space, particularly inertial frames and

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3968-517: The two objects. Given a massless third point C {\displaystyle C} at location r C {\displaystyle r_{C}} , one can ask whether to use a frame centered on A {\displaystyle A} or on B {\displaystyle B} to analyse the dynamics of C {\displaystyle C} . Consider a frame centered on A {\displaystyle A} . The gravity of B {\displaystyle B}

4032-490: The weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun. Chemical power generators are not used due to the far lower total available energy. Beamed power to the spacecraft is considered to have potential, according to NASA and the University of Colorado Boulder . With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits

4096-408: The weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass, with the same units as velocity (e.g., meters per second). This measure is equivalent to the effective exhaust velocity of the engine, and is typically designated v e {\displaystyle v_{e}} . Either

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