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Glossary of aerospace engineering

## Summary

This glossary of aerospace engineering terms pertains specifically to aerospace engineering, its sub-disciplines, and related fields including aviation and aeronautics. For a broad overview of engineering, see glossary of engineering.

## B

• Balloon — In aeronautics, a balloon is an unpowered aerostat, which remains aloft or floats due to its buoyancy. A balloon may be free, moving with the wind, or tethered to a fixed point. It is distinct from an airship, which is a powered aerostat that can propel itself through the air in a controlled manner.
• Ballute — (a portmanteau of balloon and parachute) is a parachute-like braking device optimized for use at high altitudes and supersonic velocities. Invented by Goodyear in 1958, the original ballute was a cone-shaped balloon with a toroidal burble fence fitted around its widest point. A burble fence is an inflated structure intended to ensure flow separation.[25] This stabilizes the ballute as it decelerates through different flow regimes (from supersonic to subsonic).
• Beam-powered propulsion — also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a laser beam and it is either pulsed or continuous. A continuous beam lends itself to thermal rockets, photonic thrusters and light sails, whereas a pulsed beam lends itself to ablative thrusters and pulse detonation engines.[26]
• Bearing — In navigation, bearing is the horizontal angle between the direction of an object and another object, or between it and that of true north. Absolute bearing refers to the angle between the magnetic North (magnetic bearing) or true North (true bearing) and an object. For example, an object to the East would have an absolute bearing of 90 degrees. 'Relative bearing refers to the angle between the craft's forward direction, and the location of another object. For example, an object relative bearing of 0 degrees would be dead ahead; an object relative bearing 180 degrees would be behind.[27] Bearings can be measured in mils or degrees.
• Bernoulli's principle — In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.[28]: Ch.3 [29]: 156–164, § 3.5
• Bi-elliptic transfer — is an orbital maneuver that moves a spacecraft from one orbit to another and may, in certain situations, require less delta-v than a Hohmann transfer maneuver. The bi-elliptic transfer consists of two half-elliptic orbits. From the initial orbit, a first burn expends delta-v to boost the spacecraft into the first transfer orbit with an apoapsis at some point ${\displaystyle r_{b}}$ away from the central body. At this point a second burn sends the spacecraft into the second elliptical orbit with periapsis at the radius of the final desired orbit, where a third burn is performed, injecting the spacecraft into the desired orbit.[30]
• Big dumb booster — (BDB), is a general class of launch vehicle based on the premise that it is cheaper to operate large rockets of simple design than it is to operate smaller, more complex ones regardless of the lower payload efficiency.[31]
• Bleed air — produced by gas turbine engines is compressed air that is taken from the compressor stage of those engines, which is upstream of the fuel-burning sections.
• Booster — A booster rocket (or engine) is either the first stage of a multistage launch vehicle, or else a shorter-burning rocket used in parallel with longer-burning sustainer rockets to augment the space vehicle's takeoff thrust and payload capability.[32][33]
• Boundary layer — In physics and fluid mechanics, a boundary layer is an important concept and refers to the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. In the Earth's atmosphere, the atmospheric boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface. On an aircraft wing the boundary layer is the part of the flow close to the wing, where viscous forces distort the surrounding non-viscous flow.
• Buoyancy — In physics, buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. This pressure difference results in a net upwards force on the object. The magnitude of that force exerted is proportional to that pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.

## C

• Cabin pressurization — is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft, in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary, before it is distributed to the cabin by one or more environmental control systems.[34] The cabin pressure is regulated by the outflow valve.
• Cable lacing — is a method for tying wiring harnesses and cable looms, traditionally used in telecommunication, naval, and aerospace applications. This old cable management technique, taught to generations of linemen,[35] is still used in some modern applications since it does not create obstructions along the length of the cable, avoiding the handling problems of cables groomed by plastic or hook-and-loop cable ties.
• Camber — the asymmetric curves on the top and bottom, or front and back, of an aerofoil
• Canard — is an aeronautical arrangement wherein a small forewing or foreplane is placed forward of the main wing of a fixed-wing aircraft. The term "canard" may be used to describe the aircraft itself, the wing configuration or the foreplane.[36][37][38]
• Centennial challenges
• Center of gravity — A body's center of gravity is the point around which the resultant torque due to gravity forces vanishes. Where a gravity field can be considered to be uniform, the mass-center and the center-of-gravity will be the same. However, for satellites in orbit around a planet, in the absence of other torques being applied to a satellite, the slight variation (gradient) in gravitational field between closer-to (stronger) and further-from (weaker) the planet can lead to a torque that will tend to align the satellite such that its long axis is vertical. In such a case, it is important to make the distinction between the center-of-gravity and the mass-center. Any horizontal offset between the two will result in an applied torque.
• Center of mass — In physics, the center of mass of a distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero, or the point where if a force is applied it moves in the direction of the force without rotating. The distribution of mass is balanced around the center of mass and the average of the weighted position coordinates of the distributed mass defines its coordinates.
• Center of pressure — is the point where the total sum of a pressure field acts on a body, causing a force to act through that point.
• Centrifugal compressorCentrifugal compressors, sometimes called radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.[39] They achieve a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in potential energy/static pressure by slowing the flow through a diffuser. The pressure rise in the impeller is in most cases almost equal to the rise in the diffuser.
• Chord — is the imaginary straight line joining the leading and trailing edges of an aerofoil. The chord length is the distance between the trailing edge and the point on the leading edge where the chord intersects the leading edge.[40][41]
• Clean configuration — is the flight configuration of a fixed-wing aircraft when its external equipment is retracted to minimize drag and thus maximize airspeed for a given power setting.
• Cockpit — or flight deck, is the area, usually near the front of an aircraft or spacecraft, from which a pilot controls the aircraft.
• Collimated beam — A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. Such a beam cannot be created, due to diffraction.[42]
• Comet — is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail.
• Compressibility — In thermodynamics and fluid mechanics, compressibility (also known as the coefficient of compressibility[43] or isothermal compressibility[44]) is a measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress) change. In its simple form, the compressibility ${\displaystyle \beta }$ may be expressed as
${\displaystyle \beta =-{\frac {1}{V}}{\frac {\partial V}{\partial p}}}$, where V is volume and p is pressure. The choice to define compressibility as the opposite of the fraction makes compressibility positive in the (usual) case that an increase in pressure induces a reduction in volume. t is also known as reciprocal of bulk modulus(k) of elasticity of a fluid.
• Compression — In mechanics, compression is the application of balanced inward ("pushing") forces to different points on a material or structure, that is, forces with no net sum or torque directed so as to reduce its size in one or more directions.[45] It is contrasted with tension or traction, the application of balanced outward ("pulling") forces; and with shearing forces, directed so as to displace layers of the material parallel to each other. The compressive strength of materials and structures is an important engineering consideration.
• Compressor map – is a diagram showing significant performance parameters for a rotating compressor, and how they vary with changing ambient conditions of pressure and temperature.
• Computational fluid dynamics — (CFD), is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems.
• Conservation of momentum — The total momentum of objects involved in a collision remains constant regardless of friction and permanent deformation that may occur during the collision. The law of conservation of momentum can be used to analyse the interactions between objects, even in the presence of friction and other non-conservative forces. Conservation of momentum is a consequence of Newton's laws of motion.
• Constant speed drive — (CSD), is a type of transmission that takes an input shaft rotating at a wide range of speeds, delivering this power to an output shaft that rotates at a constant speed, despite the varying input. They are used to drive mechanisms, typically electrical generators, that require a constant input speed. The term is most commonly applied to hydraulic transmissions found on the accessory drives of gas turbine engines, such as aircraft jet engines. On modern aircraft, the CSD is often combined with a generator into a single unit known as an integrated drive generator (IDG).
• Control engineering — or control systems engineering, is an engineering discipline that applies automatic control theory to design systems with desired behaviors in control environments.[46] The discipline of controls overlaps and is usually taught along with electrical engineering at many institutions around the world.[46]
• Controllability
• Crew Exploration Vehicle
• Critical mach — In aerodynamics, the critical Mach number (Mcr or M* ) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it.[47] At the lower critical Mach number, airflow around the entire aircraft is subsonic. At the upper critical Mach number, airflow around the entire aircraft is supersonic.[48]
• Cylinder stress — In mechanics, a cylinder stress is a stress distribution with rotational symmetry; that is, which remains unchanged if the stressed object is rotated about some fixed axis.

## D

• Damage tolerance — is a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach to engineering design to account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage.
• Decalage — Decalage on a fixed-wing aircraft is the angle difference between the upper and lower wings of a biplane, i.e. the acute angle contained between the chords of the wings in question. Decalage is said to be positive when the upper wing has a higher angle of incidence than the lower wing, and negative when the lower wing's incidence is greater than that of the upper wing. Positive decalage results in greater lift from the upper wing than the lower wing, the difference increasing with the amount of decalage.[49]
• De Laval nozzle — (or convergent-divergent nozzle, CD nozzle or con-di nozzle), is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. It is used to accelerate a hot, pressurized gas passing through it to a higher supersonic speed in the axial (thrust) direction, by converting the heat energy of the flow into kinetic energy. Because of this, the nozzle is widely used in some types of steam turbines and rocket engine nozzles. It also sees use in supersonic jet engines.
• Dead reckoning — In navigation, dead reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time and course.
• Deflection — is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.
• Deformation (engineering) — In materials science, deformation refers to any changes in the shape or size of an object due to an applied force (the deformation energy, in this case, is transferred through work) or a change in temperature (the deformation energy, in this case, is transferred through heat).
• Deformation (mechanics) — in continuum mechanics is the transformation of a body from a reference configuration to a current configuration.[50] A configuration is a set containing the positions of all particles of the body. A deformation may be caused by external loads,[51] body forces (such as gravity or electromagnetic forces), or changes in temperature, moisture content, or chemical reactions, etc.
• Delta-v — (literally "change in velocity"), symbolised as v and pronounced delta-vee, as used in spacecraft flight dynamics, is a measure of the impulse that is needed to perform a maneuver such as launch from, or landing on a planet or moon, or in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of the vehicle.
• Delta-v budget — is an estimate of the total delta-v required for a space mission. It is calculated as the sum of the delta-v required for the propulsive maneuvers during the mission, and as input to the Tsiolkovsky rocket equation, determines how much propellant is required for a vehicle of given mass and propulsion system.
• Delta wing— is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ). Although long studied, it did not find significant applications until the jet age, when it proved suitable for high-speed subsonic and supersonic flight.
• Density
• Departure resistance – is a quality of an aircraft which enables it to remain in controlled flight and resist entering potentially dangerous less-controlled maneuvers such as spin.
• Derivative — The derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. For example, the derivative of the position of a moving object with respect to time is the object's velocity: this measures how quickly the position of the object changes when time advances.
• Digital Datcom — The United States Air Force Stability and Control Digital DATCOM is a computer program that implements the methods contained in the USAF Stability and Control DATCOM to calculate the static stability, control and dynamic derivative characteristics of fixed-wing aircraft. Digital DATCOM requires an input file containing a geometric description of an aircraft, and outputs its corresponding dimensionless stability derivatives according to the specified flight conditions. The values obtained can be used to calculate meaningful aspects of flight dynamics.
• Dihedral — Dihedral angle is the upward angle from horizontal of the wings or tailplane of a fixed-wing aircraft. "Anhedral angle" is the name given to negative dihedral angle, that is, when there is a downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft.
• Displacement (vector)
• Distance measuring equipment — (DME), is a radio navigation technology that measures the slant range (distance) between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 960 and 1215 megahertz (MHz). Line-of-visibility between the aircraft and ground station is required. An interrogator (airborne) initiates an exchange by transmitting a pulse pair, on an assigned ‘channel’, to the transponder ground station. The channel assignment specifies the carrier frequency and the spacing between the pulses. After a known delay, the transponder replies by transmitting a pulse pair on a frequency that is offset from the interrogation frequency by 63 MHz and having specified separation.[53]
• DME — distance measuring equipment.
• DO-178B
• DO-254
• Drag (physics) — In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid.[54] This can exist between two fluid layers (or surfaces) or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity.[55][56] Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.[57] Drag forces always decrease fluid velocity relative to the solid object in the fluid's path.
• Drag coefficient — In fluid dynamics, the drag coefficient (commonly denoted as: ${\displaystyle \scriptstyle C_{\mathrm {d} }\,}$, ${\displaystyle \scriptstyle C_{\mathrm {x} }\,}$ or ${\displaystyle \scriptstyle C_{\mathrm {w} }\,}$) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equation in which a lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. The drag coefficient is always associated with a particular surface area.[58]
• Drag equation — In fluid dynamics, the drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid. The equation is:
${\displaystyle F_{D}\,=\,{\tfrac {1}{2}}\,\rho \,u^{2}\,C_{D}\,A}$
${\displaystyle F_{D}}$ is the drag force, which is by definition the force component in the direction of the flow velocity,
${\displaystyle \rho }$ is the mass density of the fluid,[59]
${\displaystyle u}$ is the flow velocity relative to the object,
${\displaystyle A}$ is the reference area, and
${\displaystyle C_{D}}$ is the drag coefficient – a dimensionless coefficient related to the object's geometry and taking into account both skin friction and form drag. In general, ${\displaystyle C_{D}}$ depends on the Reynolds number.

## E

Given a domain ${\displaystyle \Omega \subseteq \mathbb {R} ^{n}}$ and a once-weakly differentiable vector field ${\displaystyle u\in H^{1}(\mathbb {R} ^{n})^{n}}$ which represents a fluid flow, such as a solution to the Navier-Stokes equations, its enstrophy is given by:[65]
${\displaystyle {\mathcal {E}}(u):=\int _{\Omega }|\nabla \mathbf {u} |^{2}\,dx}$
Where ${\displaystyle |\nabla \mathbf {u} |^{2}=\sum _{i,j=1}^{n}\left|\partial _{i}u^{j}\right|^{2}}$. This is quantity is the same as the squared seminorm ${\displaystyle |\mathbf {u} |_{H^{1}(\Omega )^{n}}^{2}}$of the solution in the Sobolev space ::::${\displaystyle H^{1}(\Omega )^{n}}$.
In the case that the flow is incompressible, or equivalently that ${\displaystyle \nabla \cdot \mathbf {u} =0}$, the enstrophy can be described as the integral of the square of the vorticity ${\displaystyle \mathbf {\omega } }$,[66]
${\displaystyle {\mathcal {E}}({\boldsymbol {\omega }})\equiv \int _{\Omega }|{\boldsymbol {\omega }}|^{2}\,dx}$
or, in terms of the flow velocity,
${\displaystyle {\mathcal {E}}(\mathbf {u} )\equiv \int _{S}|\nabla \times \mathbf {u} |^{2}\,dS\,.}$
In the context of the incompressible Navier-Stokes equations, enstrophy appears in the following useful result[18]
${\displaystyle {\frac {d}{dt}}\left({\frac {1}{2}}\int _{\Omega }|\mathbf {u} |^{2}\right)=-\nu {\mathcal {E}}(\mathbf {u} )}$
The quantity in parentheses on the left is the energy in the flow, so the result says that energy declines proportional to the kinematic viscosity ${\displaystyle \nu }$ times the enstrophy.
• European Space Agency
• Expander cycle (rocket) — is a power cycle of a bipropellant rocket engine. In this cycle, the fuel is used to cool the engine's combustion chamber, picking up heat and changing phase. The now heated and gaseous fuel then powers the turbine that drives the engine's fuel and oxidizer pumps before being injected into the combustion chamber and burned for thrust.

## H

${\displaystyle {\frac {\partial ^{2}u}{\partial t^{2}}}=c^{2}{\frac {\partial ^{2}u}{\partial x^{2}}}}$
The equation has the property that, if u and its first time derivative are arbitrarily specified initial data on the line t = 0 (with sufficient smoothness properties), then there exists a solution for all time t.
• Hypersonic speed — In aerodynamics, a hypersonic speed is one that greatly exceeds the speed of sound, often stated as starting at speeds of Mach 5 and above.[99] The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since individual physical changes in the airflow (like molecular dissociation and ionization) occur at different speeds; these effects collectively become important around Mach 5-10. The hypersonic regime can also be alternatively defined as speeds where specific heat capacity changes with the temperature of the flow as kinetic energy of the moving object is converted into heat.[100]
• Hypoxia — is a condition[101] in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body.[102] Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during hypoventilation training or strenuous physical exercise.

## I

• Impulse — Specific impulse (usually abbreviated Isp) is a measure of how efficiently a rocket uses propellant or a jet engine uses fuel. For engines whose reaction mass is only the fuel they carry, specific impulse is exactly proportional to exhaust gas velocity.
• Indicated airspeed — (IAS), is the airspeed read directly from the airspeed indicator (ASI) on an aircraft, driven by the pitot-static system.[103] It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.[103] An aircraft's indicated airspeed in knots is typically abbreviated KIAS for "Knots-Indicated Air Speed" (vs. KCAS for calibrated airspeed and KTAS for true airspeed).
• Instrument landing system — In aviation, the instrument landing system (ILS) is a radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a 12 mile (800 m) of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. Bringing the aircraft this close to the runway dramatically improves the weather conditions in which a safe landing can be made. Later versions of the system, or "categories", have further reduced the minimum altitudes.
• Interplanetary Transport Network — (ITN)[104] is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow. The ITN makes particular use of Lagrange points as locations where trajectories through space can be redirected using little or no energy. These points have the peculiar property of allowing objects to orbit around them, despite lacking an object to orbit. While it would use little energy, transport along the network would take a long time.[105]
• Interplanetary travelInterplanetary spaceflight or interplanetary travel is the crewed or uncrewed travel between stars and planets, usually within a single planetary system.[106]
• Interstellar travel — refers to the currently theoretical idea of interstellar probes or crewed spacecraft moving between stars or planetary systems in a galaxy. Interstellar travel would be much more difficult than interplanetary spaceflight. Whereas the distances between the planets in the Solar System are less than 30 astronomical units (AU), the distances between stars are typically hundreds of thousands of AU, and usually expressed in light-years. Because of the vastness of those distances, practical interstellar travel based on known physics would need to occur at a high percentage of the speed of light; even so, travel times would be long, at least decades and perhaps millennia or longer.[107]
• Ion thruster — An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
• ISRO — The Indian Space Research Organisation[d] (ISRO /ˈɪsr/) or (IAST : Bhāratīya Antrikṣ Anusandhān Saṅgaṭhan) is the national space agency of India, headquartered in Bengaluru. It operates under the Department of Space (DOS) which is directly overseen by the Prime Minister of India, while Chairman of ISRO acts as executive of DOS as well. ISRO is the primary agency in India to perform tasks related to space based applications, space exploration and development of related technologies.[108] It is one of six government space agencies in the world which possess full launch capabilities, deploy cryogenic engines, launch extra-terrestrial missions and operate large fleets of artificial satellites.[109][110][e]

## K

1. The orbit of a planet is an ellipse with the Sun at one of the two foci.
2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3. The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.
The elliptical orbits of planets were indicated by calculations of the orbit of Mars. From this, Kepler inferred that other bodies in the Solar System, including those farther away from the Sun, also have elliptical orbits. The second law helps to establish that when a planet is closer to the Sun, it travels faster. The third law expresses that the farther a planet is from the Sun, the slower its orbital speed, and vice versa.
Isaac Newton showed in 1687 that relationships like Kepler's would apply in the Solar System as a consequence of his own laws of motion and law of universal gravitation.
• Kessler syndrome — (also called the Kessler effect,[113][114] collisional cascading, or ablation cascade), proposed by NASA scientist Donald J. Kessler in 1978, is a theoretical scenario in which the density of objects in low Earth orbit (LEO) due to space pollution is high enough that collisions between objects could cause a cascade in which each collision generates space debris that increases the likelihood of further collisions.[115] One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges difficult for many generations.[115]
• Kinetic energy — In physics, the kinetic energy of an object is the energy that it possesses due to its motion.[116] It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body when decelerating from its current speed to a state of rest. In classical mechanics, the kinetic energy of a non-rotating object of mass m traveling at a speed v is ${\textstyle {\frac {1}{2}}mv^{2}}$. In relativistic mechanics, this is a good approximation only when v is much less than the speed of light.
• Kite — is a tethered heavier-than-air or lighter-than-air craft with wing surfaces that react against the air to create lift and drag forces.[117] A kite consists of wings, tethers and anchors. Kites often have a bridle and tail to guide the face of the kite so the wind can lift it.[118] Some kite designs don’t need a bridle; box kites can have a single attachment point. A kite may have fixed or moving anchors that can balance the kite. One technical definition is that a kite is “a collection of tether-coupled wing sets“.[119] The name derives from its resemblance to a hovering bird.[120]
• Kutta condition — is a principle in steady-flow fluid dynamics, especially aerodynamics, that is applicable to solid bodies with sharp corners, such as the trailing edges of airfoils. It is named for German mathematician and aerodynamicist Martin Kutta.
Kuethe and Schetzer state the Kutta condition as follows:[121]: § 4.11
A body with a sharp trailing edge which is moving through a fluid will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge.
In fluid flow around a body with a sharp corner, the Kutta condition refers to the flow pattern in which fluid approaches the corner from above and below, meets at the corner, and then flows away from the body. None of the fluid flows around the sharp corner.
The Kutta condition is significant when using the Kutta–Joukowski theorem to calculate the lift created by an airfoil with a sharp trailing edge. The value of circulation of the flow around the airfoil must be that value which would cause the Kutta condition to exist.
• Kutta–Joukowski theorem — is a fundamental theorem in aerodynamics used for the calculation of lift of an airfoil and any two-dimensional bodies including circular cylinders translating in a uniform fluid at a constant speed large enough so that the flow seen in the body-fixed frame is steady and unseparated. The theorem relates the lift generated by an airfoil to the speed of the airfoil through the fluid, the density of the fluid and the circulation around the airfoil. The circulation is defined as the line integral around a closed loop enclosing the airfoil of the component of the velocity of the fluid tangent to the loop.[122] It is named after Martin Kutta and Nikolai Zhukovsky (or Joukowski) who first developed its key ideas in the early 20th century. Kutta–Joukowski theorem is an inviscid theory, but it is a good approximation for real viscous flow in typical aerodynamic applications.[123]

## L

• Landerspacecraft designed to soft-land intact or almost undamaged on the surface of a celestial body and eventually take-off from it
• Landing — is the last part of a flight, where an aircraft, or spacecraft returns to the ground. When the flying object returns to water, the process is called alighting, although it is commonly called "landing", "touchdown"a or "splashdown" as well. A normal aircraft flight would include several parts of flight including taxi, takeoff, climb, cruise, descent and landing.
• Landing gear — is the undercarriage of an aircraft or spacecraft and may be used for either takeoff or landing. For aircraft it is generally needed for both. Also, for aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land, and taxi without damage. Wheeled landing gear is the most common, with skis or floats needed to operate from snow/ice/water and skids for vertical operation on land. Faster aircraft have retractable undercarriages, which fold away during flight to reduce drag.
• Lagrangian mechanics — Introduced by the Italian-French mathematician and astronomer Joseph-Louis Lagrange in 1788, Lagrangian mechanics is a formulation of classical mechanics and is founded on the stationary action principle.
Lagrangian mechanics defines a mechanical system to be a pair ${\displaystyle (M,L)}$ of a configuration space ${\displaystyle M}$ and a smooth function ${\displaystyle L=L(q,v,t)}$ called Lagrangian. By convention, ${\displaystyle L=T-V,}$ where ${\displaystyle T}$ and ${\displaystyle V}$ are the kinetic and potential energy of the system, respectively. Here ${\displaystyle q\in M,}$ and ${\displaystyle v}$ is the velocity vector at ${\displaystyle q}$ ${\displaystyle (v}$ is tangential to ${\displaystyle M).}$ (For those familiar with tangent bundles, ${\displaystyle L:TM\times \mathbb {R} _{t}\to \mathbb {R} ,}$ and ${\displaystyle v\in T_{q}M).}$
Given the time instants ${\displaystyle t_{1}}$ and ${\displaystyle t_{2},}$ Lagrangian mechanics postulates that a smooth path ${\displaystyle x_{0}:[t_{1},t_{2}]\to M}$ describes the time evolution of the given system if and only if ${\displaystyle x_{0}}$ is a stationary point of the action functional
${\displaystyle {\cal {S}}[x]\,{\stackrel {\text{def}}{=}}\,\int _{t_{1}}^{t_{2}}L(x(t),{\dot {x}}(t),t)\,dt.}$
If ${\displaystyle M}$ is an open subset of ${\displaystyle \mathbb {R} ^{n}}$ and ${\displaystyle t_{1},}$ ${\displaystyle t_{2}}$ are finite, then the smooth path ${\displaystyle x_{0}}$ is a stationary point of ${\displaystyle {\cal {S}}}$ if all its directional derivatives at ${\displaystyle x_{0}}$ vanish, i.e., for every smooth ${\displaystyle \delta :[t_{1},t_{2}]\to \mathbb {R} ^{n},}$
${\displaystyle \delta {\cal {S}}\ {\stackrel {\text{def}}{=}}\ {\frac {d}{d\varepsilon }}{\Biggl |}_{\varepsilon =0}{\cal {S}}\left[x_{0}+\varepsilon \delta \right]=0.}$
The function ${\displaystyle \delta (t)}$ on the right-hand side is called perturbation or virtual displacement. The directional derivative ${\displaystyle \delta {\cal {S}}}$ on the left is known as variation in physics and Gateaux derivative in mathematics.
Lagrangian mechanics has been extended to allow for non-conservative forces.

## M

${\displaystyle \mathbf {p} =m\mathbf {v} .}$
In the International System of Units (SI), the unit of measurement of momentum is the kilogram metre per second (kg⋅m/s), which is equivalent to the newton-second.
• Momentum wheel
• Monopropellant rocket — or monochemical rocket, is a rocket that uses a single chemical as its propellant.
• Motion — In physics, motion is the phenomenon in which an object changes its position. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and time. The motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame with change in time. The branch of physics describing the motion of objects without reference to its cause is kinematics; the branch studying forces and their effect on motion is dynamics.
• Multistage rocket — or step rocket[153] is a launch vehicle that uses two or more rocket stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Two-stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched.

## N

The Navier–Stokes equations mathematically express conservation of momentum and conservation of mass for Newtonian fluids. They are sometimes accompanied by an equation of state relating pressure, temperature and density.[154] They arise from applying Isaac Newton's second law to fluid motion, together with the assumption that the stress in the fluid is the sum of a diffusing viscous term (proportional to the gradient of velocity) and a pressure term—hence describing viscous flow. The difference between them and the closely related Euler equations is that Navier–Stokes equations take viscosity into account while the Euler equations model only inviscid flow. As a result, the Navier–Stokes are a parabolic equation and therefore have better analytic properties, at the expense of having less mathematical structure (e.g. they are never completely integrable).
A newton is defined as 1 kg⋅m/s2, which is the force which gives a mass of 1 kilogram an acceleration of 1 metre per second, per second.
This is a general physical law derived from empirical observations by what Isaac Newton called inductive reasoning.[158] It is a part of classical mechanics and was formulated in Newton's work Philosophiæ Naturalis Principia Mathematica ("the Principia"), first published on 5 July 1687. When Newton presented Book 1 of the unpublished text in April 1686 to the Royal Society, Robert Hooke made a claim that Newton had obtained the inverse square law from him.
In today's language, the law states that every point mass attracts every other point mass by a force acting along the line intersecting the two points. The force is proportional to the product of the two masses, and inversely proportional to the square of the distance between them.[159]
The equation for universal gravitation thus takes the form:
${\displaystyle F=G{\frac {m_{1}m_{2}}{r^{2}}},}$
where F is the gravitational force acting between two objects, m1 and m2 are the masses of the objects, r is the distance between the centers of their masses, and G is the gravitational constant.
Law 1. A body continues in its state of rest, or in uniform motion in a straight line, unless acted upon by a force.
Law 2. A body acted upon by a force moves in such a manner that the time rate of change of momentum equals the force.
Law 3. If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction.
The three laws of motion were first stated by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687.[161] Newton used them to explain and investigate the motion of many physical objects and systems, which laid the foundation for Newtonian mechanics.[162]
• Nose cone design — Given the problem of the aerodynamic design of the nose cone section of any vehicle or body meant to travel through a compressible fluid medium (such as a rocket or aircraft, missile or bullet), an important problem is the determination of the nose cone geometrical shape for optimum performance. For many applications, such a task requires the definition of a solid of revolution shape that experiences minimal resistance to rapid motion through such a fluid medium.
• Nozzle — is a device designed to control the direction or characteristics of a fluid flow (especially to increase velocity) as it exits (or enters) an enclosed chamber or pipe. A nozzle is often a pipe or tube of varying cross-sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them. In a nozzle, the velocity of fluid increases at the expense of its pressure energy.

## P

Define perpendicular axes ${\displaystyle x}$, ${\displaystyle y}$, and ${\displaystyle z}$ (which meet at origin ${\displaystyle O}$) so that the body lies in the ${\displaystyle xy}$ plane, and the ${\displaystyle z}$ axis is perpendicular to the plane of the body. Let Ix, Iy and Iz be moments of inertia about axis x, y, z respectively. Then the perpendicular axis theorem states that[174]
${\displaystyle I_{z}=I_{x}+I_{y}}$
This rule can be applied with the parallel axis theorem and the stretch rule to find polar moments of inertia for a variety of shapes.
If a planar object (or prism, by the stretch rule) has rotational symmetry such that ${\displaystyle I_{x}}$ and ${\displaystyle I_{y}}$ are equal,[175]
then the perpendicular axes theorem provides the useful relationship:
${\displaystyle I_{z}=2I_{x}=2I_{y}}$

## U

• UFO — An unidentified flying object is any perceived aerial phenomenon that cannot be immediately identified or explained. On investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.

## V

Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, being a coherent derived unit whose quantity is measured in the SI (metric system) as metres per second (m/s or m⋅s−1). For example, "5 metres per second" is a scalar, whereas "5 metres per second east" is a vector. If there is a change in speed, direction or both, then the object is said to be undergoing an acceleration.

## W

• Wave drag — In aeronautics, wave drag is a component of the aerodynamic drag on aircraft wings and fuselage, propeller blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves.[191] Wave drag is independent of viscous effects,[192] and tends to present itself as a sudden and dramatic increase in drag as the vehicle increases speed to the Critical Mach number. It is the sudden and dramatic rise of wave drag that leads to the concept of a sound barrier.
• Weight — In science and engineering, the weight of an object is the force acting on the object due to gravity.[193][194][195]
• Weight function — is a mathematical device used when performing a sum, integral, or average to give some elements more "weight" or influence on the result than other elements in the same set. The result of this application of a weight function is a weighted sum or weighted average. Weight functions occur frequently in statistics and analysis, and are closely related to the concept of a measure. Weight functions can be employed in both discrete and continuous settings. They can be used to construct systems of calculus called "weighted calculus"[196] and "meta-calculus".[197]
• Wind tunnels — are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft will fly. NASA uses wind tunnels to test scale models of aircraft and spacecraft. Some wind tunnels are large enough to contain full-size versions of vehicles. The wind tunnel moves air around an object, making it seem as if the object is flying.
• Wing — is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total drag on the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift.
• Wright Flyer — The Wright Flyer (the Kitty Hawk,[198][199]also known as Flyer I or 1903 Flyer) made the first sustained flight by a manned heavier-than-air powered and controlled aircraft—an airplane—on 17 December, 1903.[200] Invented and flown by Orville and Wilbur Wright, it marked the beginning of the "pioneer era" of aviation.
• Wright Glider — The Wright brothers designed, built and flew a series of three manned gliders in 1900–1902 as they worked towards achieving powered flight. They also made preliminary tests with a kite in 1899. In 1911 Orville conducted tests with a much more sophisticated glider. Neither the kite nor any of the gliders were preserved, but replicas of all have been built.

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1. ^ Geostationary orbit and Geosynchronous (equatorial) orbit are used somewhat interchangeably in sources.
2. ^ "Newtonian constant of gravitation" is the name introduced for G by Boys (1894). Use of the term by T.E. Stern (1928) was misquoted as "Newton's constant of gravitation" in Pure Science Reviewed for Profound and Unsophisticated Students (1930), in what is apparently the first use of that term. Use of "Newton's constant" (without specifying "gravitation" or "gravity") is more recent, as "Newton's constant" was also used for the heat transfer coefficient in Newton's law of cooling, but has by now become quite common, e.g. Calmet et al, Quantum Black Holes (2013), p. 93; P. de Aquino, Beyond Standard Model Phenomenology at the LHC (2013), p. 3. The name "Cavendish gravitational constant", sometimes "Newton–Cavendish gravitational constant", appears to have been common in the 1970s to 1980s, especially in (translations from) Soviet-era Russian literature, e.g. Sagitov (1970 [1969]), Soviet Physics: Uspekhi 30 (1987), Issues 1–6, p. 342 [etc.]. "Cavendish constant" and "Cavendish gravitational constant" is also used in Charles W. Misner, Kip S. Thorne, John Archibald Wheeler, "Gravitation", (1973), 1126f. Colloquial use of "Big G", as opposed to "little g" for gravitational acceleration dates to the 1960s (R.W. Fairbridge, The encyclopedia of atmospheric sciences and astrogeology, 1967, p. 436; note use of "Big G's" vs. "little g's" as early as the 1940s of the Einstein tensor Gμν vs. the metric tensor gμν, Scientific, medical, and technical books published in the United States of America: a selected list of titles in print with annotations: supplement of books published 1945–1948, Committee on American Scientific and Technical Bibliography National Research Council, 1950, p. 26).
3. ^ Cavendish determined the value of G indirectly, by reporting a value for the Earth's mass, or the average density of Earth, as 5.448 g⋅cm−3.
4. ^ ISO 15919: Bhāratīya Antarikṣ Anusandhān Saṅgaṭhan Bhāratīya Antrikṣ Anusandhān Saṅgaṭhan
5. ^ CNSA (China), ESA (most of Europe), ISRO, (India), JAXA (Japan), NASA (United States) and Roscosmos (Russia) are space agencies with full launch capabilities.
1. ^ It was shown separately that separated spherically symmetrical masses attract and are attracted as if all their mass were concentrated at their centers.