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.
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Above ground level – In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL) is a height measured with respect to the underlying ground surface. This is as opposed to altitude/elevation above mean sea level (AMSL), or (in broadcast engineering) height above average terrain (HAAT). In other words, these expressions (AGL, AMSL, HAAT) indicate where the "zero level" or "reference altitude" is located. Absolute humidity – describes the water content of air and is expressed in either grams per cubic meterWyer, S.S., "A treatise on producer-gas and gas-producers", (1906) The Engineering and Mining Journal, London, p.23 or grams per kilogram.Perry, R.H. and Green, D.W, (2007) Perry's Chemical Engineers' Handbook (8th Edition), Section 12, Psychrometry, Evaporative Cooling and Solids Drying McGraw-Hill, Absolute value – In mathematics, the absolute value or modulus of a real number is the non-negative value of without regard to its sign. Namely, for a positive , for a negative (in which case is positive), and . For example, the absolute value of 3 is 3, and the absolute value of −3 is also 3. The absolute value of a number may be thought of as its distance from zero. Acceleration – In physics, acceleration is the rate of change of velocity of an object with respect to time. An object's acceleration is the net result of any and all forces acting on the object, as described by Newton's second law. The SI unit for acceleration is metre per second squared Accelerations are vector quantities (they have magnitude and direction) and add according to the parallelogram law. As a vector, the calculated net force is equal to the product of the object's mass (a scalar quantity) and its acceleration. Acquisition of signal – A pass, in spaceflight and satellite communications, is the period in which a satellite or other spacecraft is above the local horizon and available for radio communication with a particular ground station, satellite receiver, or relay satellite (or, in some cases, for visual sighting). The beginning of a pass is termed acquisition of signal; the end of a pass is termed loss of signal. The point at which a spacecraft comes closest to a ground observer is the time of closest approach. Action – In physics, action is an attribute of the dynamics of a physical system from which the equations of motion of the system can be derived. It is a mathematical functional which takes the trajectory, also called path or history, of the system as its argument and has a real number as its result. Generally, the action takes different values for different paths.McGraw Hill Encyclopaedia of Physics (2nd Edition), C.B. Parker, 1994, Action has the dimensions of [energy]⋅[time] or [momentum]⋅[length], and its SI unit is joule-second. ADF – Automatic direction finder Advanced Space Vision System – The Advanced Space Vision System (also known as the Space Vision System or by its acronym SVS) is a computer vision system designed primarily for International Space Station (ISS) assembly. The system uses regular 2D cameras in the Space Shuttle bay, on the Canadarm, or on the ISS along with cooperative targets to calculate the 3D position of an object. Aeroacoustics – is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces. Noise generation can also be associated with periodically varying flows. A notable example of this phenomenon is the Aeolian tones produced by wind blowing over fixed objects. Aerobraking – is a spaceflight maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis). The resulting drag slows the spacecraft. Aerobraking is used when a spacecraft requires a low orbit after arriving at a body with an atmosphere, and it requires less fuel than does the direct use of a rocket engine. Aerocapture – is an orbital transfer maneuver used to reduce the velocity of a spacecraft from a hyperbolic trajectory to an elliptical orbit around the targeted celestial body. Aerodynamics – is the study of the motion of air, particularly with respect to its interaction with a solid object, such as an airplane wing. Aerodynamics is a sub-field of gas dynamics, which in turn is a sub-field of fluid dynamics. Many aspects and principles of aerodynamics theory are common to these three fields. Aeroelasticity – is the branch of physics and engineering that studies the interactions between the inertial, elastic, and aerodynamic forces that occur when an elastic body is exposed to a fluid flow. Although historical studies have been focused on aeronautical applications, recent research has found applications in fields such as energy harvesting and understanding snoring. The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity, which deals with the static or steady response of an elastic body to a fluid flow; and dynamic aeroelasticity, which deals with the body's dynamic (typically vibrational) response. Aeroelasticity draws on the study of fluid mechanics, solid mechanics, structural dynamics and dynamical systems. The synthesis of aeroelasticity with thermodynamics is known as aerothermoelasticity, and its synthesis with control theory is known as aeroservoelasticity. Aeronautics – is the science or art involved with the study, design, and manufacturing of air flight capable machines, and the techniques of operating aircraft and rockets within the atmosphere. Aerospace architecture – is broadly defined to encompass architectural design of non-habitable and habitable structures and living and working environments in aerospace-related facilities, habitats, and vehicles. These environments include, but are not limited to: science platform aircraft and aircraft-deployable systems; space vehicles, space stations, habitats and lunar and planetary surface construction bases; and Earth-based control, experiment, launch, logistics, payload, simulation and test facilities. Earth analogs to space applications may include Antarctic, desert, high altitude, underground, undersea environments and closed ecological systems. Aerospace bearing – Aerospace bearings are the bearings installed in aircraft and aerospace systems including commercial, private, military, or space applications. Aerospace engineering – is the primary field of engineering concerned with the development of aircraft and spacecraft.Encyclopedia of Aerospace Engineering. John Wiley & Sons, 2010. . It has two major and overlapping branches: Aeronautical engineering and Astronautical Engineering. Avionics engineering is similar, but deals with the electronics side of aerospace engineering. Aerospace materials – are materials, frequently metal alloys, that have either been developed for, or have come to prominence through, their use for aerospace purposes. These uses often require exceptional performance, strength or heat resistance, even at the cost of considerable expense in their production or machining. Others are chosen for their long-term reliability in this safety-conscious field, particularly for their resistance to fatigue. Aerospike engine – is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes. It belongs to 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 need for thrust. Aerostat – is a lighter than air aircraft that gains its lift through the use of a buoyant gas. Aerostats include unpowered balloons and powered airships. Aerostructure – is a component of an aircraft's airframe. This may include all or part of the fuselage, wings, or flight control surfaces. Aft-crossing trajectory – is an alternate flight path for a rocket. The rocket's rotation (induced by the deployment from the aircraft) is slowed by a small parachute attached to its tail, then ignited once the carrier aircraft has passed it. It is ignited before it is pointing fully vertically, however it will turn to do so, and accelerates to pass behind the carrier aircraft. AGL – Above ground level Aileron – is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around the aircraft's longitudinal axis), which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'. Air-augmented rocket – Aircraft – is a machine that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. Common examples of aircraft include airplanes, helicopters, airships (including blimps), gliders, and hot air balloons. Aircraft flight control systems – A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed. Aircraft flight mechanics – Airfoil – An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of a wing, blade (of a propeller, rotor, or turbine), or sail (as seen in cross-section). Airlock – is a device which permits the passage of people and objects between a pressure vessel and its surroundings while minimizing the change of pressure in the vessel and loss of air from it. The lock consists of a small chamber with two airtight doors in series which do not open simultaneously. Airship – An airship or dirigible balloon is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from large gas bags filled with a lifting gas that is less dense than the surrounding air. Albedo – is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by an astronomical body (e.g. a planet like Earth). It is dimensionless and measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation). Anemometer – is a device used for measuring wind speed, and is also a common weather station instrument. The term is derived from the Greek word anemos, which means wind, and is used to describe any wind speed instrument used in meteorology. Angle of attack – In fluid dynamics, angle of attack (AOA, or ) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. Angular momentum – In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a system remains constant unless acted on by an external torque.