# Glossary of aerospace engineering

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This glossary of aerospace engineering terms pertains specifically to aerospace engineering and its sub-disciplines. 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](§ 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.
• 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.
• 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.[39][40]
• 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.[41]
• 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.
• 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.[42] 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.
• 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.

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