Absolute humidity – describes the water content of air and is expressed in either grams per cubic meter[2] or grams per kilogram.[3]
Absolute value – In
mathematics, the absolute value or modulus|x| of a
real numberx is the
non-negative value of x without regard to its
sign. Namely, |x| = x for a
positivex, |x| = −x for a
negativex (in which case −x is positive), and |0| = 0. 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.
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.[9] 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.[9]
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.
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[10] and understanding
snoring.[11] 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.[12]
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.[13] 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.
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.
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'.
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
airtightdoors 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.[16] Aerostats gain their lift from large gas bags filled with a
lifting gas that is less dense than the surrounding air.
Anemometer – is a device used for measuring
wind speed, and is also a common
weather station instrument.[17] 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.[18] 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.
Angular velocity – In
physics, the angular velocity of a particle is the rate at which it rotates around a chosen center point: that is, the time rate of change of its
angular displacement relative to the origin (i.e. in layman's terms: how quickly an object goes around something over a period of time – e.g. how fast the earth orbits the sun). It is measured in angle per unit time,
radians per second in
SI units, and is usually represented by the symbol
omega (ω, sometimes Ω). By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.
Anticyclone – An anticyclone (that is, opposite to a
cyclone) is a
weatherphenomenon defined by the United States
National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere".[19]
Antimatter rocket – is a proposed class of
rockets that use
antimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the
rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher
energy density and
specific impulse than any other proposed class of rocket.
Apsis – is an extreme point in the
orbit of an
object. The word comes via Latin from Greek and is
cognate with apse.[20] For
elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- (from περί (peri) 'near') and ap-/apo- (from ἀπ(ό) (ap(ó)) 'away from') added to a reference to the
body being orbited.
Arcjet rocket – or arcjet thruster is a form of
electrically powered spacecraft propulsion, in which an electrical discharge (arc) is created in a flow of propellant[21][22] (typically
hydrazine or
ammonia). This imparts additional energy to the propellant, so that one can extract more work out of each kilogram of propellant, at the expense of increased power consumption and (usually) higher cost. Also, the
thrust levels available from typically used arcjet engines are very low compared with chemical engines.
Areal velocity – In
classical mechanics, areal velocity (also called sector velocity or sectorial velocity) is the rate at which
area is swept out by a particle as it moves along a
curve.
Argument of periapsis – (also called argument of perifocus or argument of pericenter), symbolized as ω, is one of the
orbital elements of an
orbiting body. Parametrically, ω is the angle from the body's
ascending node to its
periapsis, measured in the direction of motion.
Aspect ratio (aeronautics) – In
aeronautics, the aspect ratio of a
wing is the ratio of its
span to its mean
chord. It is equal to the square of the wingspan divided by the wing area. Thus, a long, narrow wing has a high aspect ratio, whereas a short, wide wing has a low aspect ratio.[23] Aspect ratio and other features of the
planform are often used to predict the aerodynamic efficiency of a wing because the
lift-to-drag ratio increases with aspect ratio, improving
fuel economy in aircraft.
Asteroid – Asteroids are
minor planets, especially of the
inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the
Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active
comet such as a tail. As
minor planets in the outer Solar System were discovered they were typically found to have
volatile-rich surfaces similar to comets. As a result, they were often distinguished from objects found in the main
asteroid belt.[24]
Atmospheric entry – is the movement of an object from
outer space into and through the gases of an
atmosphere of a
planet,
dwarf planet or
natural satellite. There are two main types of atmospheric entry: uncontrolled entry, such as the entry of
astronomical objects,
space debris or
bolides; and controlled entry (or reentry) of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing the controlled atmospheric entry, descent and landing of spacecraft are collectively termed as EDL.
Attitude control – is controlling the orientation of an object with respect to an
inertial frame of reference or another entity like the
celestial sphere, certain fields, and nearby objects, etc. Controlling vehicle attitude requires
sensors to measure vehicle orientation,
actuators to apply the torques needed to re-orient the vehicle to a desired attitude, and
algorithms to command the actuators based on (1) sensor measurements of the current attitude and (2) specification of a desired attitude. The integrated field that studies the combination of sensors, actuators and algorithms is called "Guidance, Navigation and Control" (GNC).
Automatic direction finder – (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station.[25][26]
Avionics – are the
electronic systems used on aircraft,
artificial satellites, and
spacecraft. Avionic systems include communications, navigation, the display and management of multiple systems, and the hundreds of systems that are fitted to aircraft to perform individual functions.
Axial stress – a normal stress parallel to the axis of cylindrical symmetry.
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.[27]
This stabilizes the ballute as it decelerates through different flow regimes (from supersonic to subsonic).
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.[29] Bearings can be measured in
mils or degrees.
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 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.[32]
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.[33]
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.
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
aircraftwing the boundary layer is the part of the flow close to the wing, where
viscousforces 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.[36] 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,[37] 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-loopcable ties.
Camber – the asymmetric curves on the top and bottom, or front and back, of an aerofoil
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 compressor – Centrifugal compressors, sometimes called radial compressors, are a sub-class of dynamic axisymmetric work-absorbing
turbomachinery.[41] 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.[42][43]
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.
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[45] or isothermal compressibility[46]) 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 may be expressed as
, 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.[47] 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.
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).
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.[49] 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.[50]
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.[51]
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).
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
propulsivemaneuvers 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.
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 StatesAir 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.
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.[55]
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.[56] 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.[57][58] 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.[59] 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: , or ) 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.[60]
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:
is the drag
force, which is by definition the force component in the direction of the flow velocity,
Drop test – is a method of testing the in-flight characteristics of
prototype or
experimentalaircraft and
spacecraft by raising the test vehicle to a specific altitude and then releasing it. Test flights involving powered aircraft, particularly
rocket-powered aircraft, may be referred to as drop launches due to the
launch of the aircraft's rockets after release from its carrier aircraft.
Ductility – is a measure of a material's ability to undergo significant plastic deformation before rupture, which may be expressed as percent elongation or percent area reduction from a tensile test.
Eccentricity vector – In
celestial mechanics, the eccentricity vector of a
Kepler orbit is the
dimensionlessvector with direction pointing from
apoapsis to
periapsis and with magnitude equal to the orbit's scalar
eccentricity. For Kepler orbits the eccentricity vector is a constant of motion. Its main use is in the analysis of almost circular orbits, as perturbing (non-Keplerian) forces on an actual orbit will cause the
osculating eccentricity vector to change continuously. For the eccentricity and
argument of periapsis parameters, eccentricity zero (circular orbit) corresponds to a singularity. The magnitude of the eccentricity vector represents the eccentricity of the orbit. Note that the velocity and position vectors need to be relative to the inertial frame of the central body.
Eigenvector slew – In aerospace engineering, especially those areas dealing with
spacecraft, the eigenvector slew is a method to calculate a steering correction (called a slew) by rotating the spacecraft around one fixed axis, or a
gimbal. This corresponds in general to the fastest and most efficient way to reach the desired target orientation as there is only one acceleration phase and one braking phase for the angular rate. If this fixed axis is not a
principal axis a time varying torque must be applied to force the spacecraft to rotate as desired, though. Also the
gyroscopic effect of
momentum wheels must be compensated for.
Empennage – The empennage (/ˌɑːmpɪˈnɑːʒ/ or /ˈɛmpɪnɪdʒ/), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an
arrow.[63][64][65] The term derives from the
French language verb empenner which means "
to feather an arrow".[66] Most aircraft feature an empennage incorporating vertical and horizontal stabilising surfaces which stabilise the
flight dynamics of
yaw and
pitch,[63][64] as well as housing
control surfaces.
Enstrophy – In
fluid dynamics, the enstrophy E can be interpreted as another type of
potential density; or, more concretely, the quantity directly related to the
kinetic energy in the flow model that corresponds to
dissipation effects in the fluid. It is particularly useful in the study of
turbulent flows, and is often identified in the study of
thrusters as well as the field of
combustion theory.
Given a domain and a once-weakly differentiable vector field which represents a fluid flow, such as a solution to the
Navier-Stokes equations, its enstrophy is given by:[67]
Where . This is quantity is the same as the squared
seminormof the solution in the
Sobolev space ::::.
In the case that the flow is
incompressible, or equivalently that , the enstrophy can be described as the integral of the square of the
vorticity,[68]
In the context of the incompressible Navier-Stokes equations, enstrophy appears in the following useful result[20]
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 times the enstrophy.
Equations of motion – In
physics, equations of motion are
equations that describe the behavior of a
physical system in terms of its
motion as a
function of time.[69] More specifically, the equations of motion describe the behavior of a physical system as a set of mathematical functions in terms of dynamic variables. These variables are usually spatial coordinates and time, but may include
momentum components. The most general choice are
generalized coordinates which can be any convenient variables characteristic of the physical system.[70] The functions are defined in a
Euclidean space in
classical mechanics, but are replaced by
curved spaces in
relativity. If the
dynamics of a system is known, the equations are the solutions for the
differential equations describing the motion of the dynamics.
Expander cycle (rocket) – is a power cycle of a
bipropellant rocketengine. 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.
F
Fatigue – In
materials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum
stress values that cause such damage may be much less than the strength of the material typically quoted as the
ultimate tensile stress limit, or the
yield stress limit.
Flap – is a
high-lift device used to reduce the
stalling speed of an
aircraftwing at a given weight. Flaps are usually mounted on the wing trailing edges of a
fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in
drag so they are retracted when not needed.
Flight control system (helicopter) – A
helicopterpilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic
flight.[72] Changes to the
aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a deliberate way. To tilt forward and back (pitch) or sideways (roll) requires that the controls alter the
angle of attack of the main rotor blades cyclically during rotation, creating differing amounts of
lift (force) at different points in the cycle. To increase or decrease overall lift requires that the controls alter the
angle of attack for all blades collectively by equal amounts at the same time, resulting in ascent, descent, acceleration and deceleration.
Flight dynamics – is the study of the performance, stability, and control of vehicles
flying through the air or in
outer space.[73] It is concerned with how forces acting on the vehicle determine its
velocity and
attitude with respect to time. For a
fixed-wing aircraft, its changing
orientation with respect to the local air flow is represented by two critical angles, the angle of attack of the wing ("alpha") and the angle of attack of the vertical tail, known as the
sideslip angle ("beta"). A sideslip angle will arise if an aircraft yaws about its centre of gravity and if the aircraft sideslips bodily, i.e. the centre of gravity moves sideways.[74] These angles are important because they are the principal source of changes in the aerodynamic forces and moments applied to the aircraft. Spacecraft flight dynamics involve three main forces: propulsive (rocket engine), gravitational, and atmospheric resistance.[75] Propulsive force and atmospheric resistance have significantly less influence over a given spacecraft compared to gravitational forces.
Flight management system – A flight management system (FMS) is a fundamental component of a modern airliner's
avionics. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that modern civilian aircraft no longer carry
flight engineers or
navigators. A primary function is in-flight management of the flight plan. Using various sensors (such as
GPS and
INS often backed up by
radio navigation) to determine the aircraft's position, the FMS can guide the aircraft along the flight plan. From the cockpit, the FMS is normally controlled through a
Control Display Unit (CDU) which incorporates a small screen and keyboard or touchscreen. The FMS sends the flight plan for display to the
Electronic Flight Instrument System (EFIS), Navigation Display (ND), or Multifunction Display (MFD). The FMS can be summarised as being a dual system consisting of the Flight Management Computer (FMC), CDU and a cross talk bus.
Floatstick – is a device to measure
fuel levels in modern large
aircraft. It consists of a closed tube rising from the bottom of the fuel tank. Surrounding the tube is a ring-shaped float, and inside it is a graduated rod indicating fuel capacity. The float and the top of the rod contain
magnets. The rod is withdrawn from the bottom of the
wing until the magnets stick, the distance it is withdrawn indicating the level of the fuel. When not in use, the stick is secured within the tube.
Fluid statics – or hydrostatics, is the branch of
fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "
fluids at
hydrostatic equilibrium[78] and the pressure in a fluid, or exerted by a fluid, on an immersed body".[79]
Force – In
physics, a force is any influence that, when unopposed, will change the
motion of an
object. A force can cause an object with
mass to change its
velocity (which includes to begin moving from a
state of rest), i.e., to
accelerate. Force can also be described intuitively as a push or a pull. A force has both
magnitude and
direction, making it a
vector quantity. It is measured in the
SI unit of
newton (N). Force is represented by the symbol F (formerly P).
Freefall – In
Newtonian physics, free fall is any motion of a
body where
gravity is the only
force acting upon it. In the context of
general relativity, where gravitation is reduced to a
space-time curvature, a body in free fall has no force acting on it. An object in the technical sense of the term "free fall" may not necessarily be falling down in the usual sense of the term. An object moving upwards might not normally be considered to be falling, but if it is subject to only the force of gravity, it is said to be in free fall. The
Moon is thus in free fall around the
Earth, though its
orbital speed keeps it in
very far orbit from the
Earth's surface. In a roughly uniform
gravitational field, in the absence of any other forces, gravitation acts on each part of the body roughly equally. When there is no
normal force exerted between a body (e.g. an
astronaut in orbit) and its surrounding objects, it will result in the sensation of
weightlessness, a condition that also occurs when the gravitational field is weak (such as when far away from any source of gravity).
Fuselage – In
aeronautics, the fuselage (/ˈfjuːzəlɑːʒ/; from the
Frenchfuselé "spindle-shaped") is an
aircraft's main body section. It holds
crew, passengers, or
cargo. In single-engine aircraft, it will usually contain an
engine, as well, although in some
amphibious aircraft the single engine is mounted on a
pylon attached to the fuselage, which in turn is used as a floating
hull. The fuselage also serves to position the
control and
stabilization surfaces in specific relationships to
lifting surfaces, which is required for aircraft stability and maneuverability.
Future Air Navigation System – (FANS), is an
avionics system which provides direct data link communication between the pilot and the
air traffic controller. The communications include air traffic control clearances, pilot requests and position reporting.[80]
Gas-generator cycle (rocket) – is a power cycle of a pumped liquid
bipropellant rocketengine. Part of the unburned propellant is burned in a
gas generator (or preburner) and the resulting hot gas is used to power the propellant pumps before being exhausted overboard, and lost. Because of this loss, this type of engine is termed open cycle.
Geostationary orbit – also referred to as a geosynchronous equatorial orbit[a] (GEO), is a
circulargeosynchronous orbit 35,786 kilometres (22,236 miles) in altitude above Earth's
equator (42,164 kilometers in radius from Earth's center) and following the
direction of
Earth's rotation. An object in such an orbit has an
orbital period equal to the Earth's rotational period, one
sidereal day, and so to ground observers it appears motionless, in a fixed position in the sky.
Geosynchronous orbit – (sometimes abbreviated GSO) is an Earth-centered
orbit with an
orbital period that matches
Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one
sidereal day). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky may remain still or trace out a path,
typically in a figure-8 form, whose precise characteristics depend on the orbit's
inclination and
eccentricity. A circular geosynchronous orbit has a constant altitude of 35,786 km (22,236 mi), and all geosynchronous orbits share that semi-major axis. A special case of geosynchronous orbit is the
geostationary orbit, which is a circular geosynchronous orbit in Earth's
equatorial plane. A satellite in a geostationary orbit remains in the same position in the sky to observers on the surface.
Glide ratio – As the aircraft
fuselage and control surfaces will also add drag and possibly some lift, it is fair to consider the lift-to-drag ratio (or L/D ratio) of the aircraft as a whole. As it turns out, the
glide ratio, which is the ratio of an (unpowered) aircraft's forward motion to its descent, is (when flown at constant speed) numerically equal to the aircraft's L/D. This is especially of interest in the design and operation of high performance
sailplanes, which can have glide ratios almost 60 to 1 (60 units of distance forward for each unit of descent) in the best cases, but with 30:1 being considered good performance for general recreational use. Achieving a glider's best L/D in practice requires precise control of airspeed and smooth and restrained operation of the controls to reduce drag from deflected control surfaces. In zero wind conditions, L/D will equal distance traveled divided by altitude lost. Achieving the maximum distance for altitude lost in wind conditions requires further modification of the best airspeed, as does alternating cruising and thermaling. To achieve high speed across country, glider pilots anticipating strong thermals often load their gliders (sailplanes) with
water ballast: the increased
wing loading means optimum glide ratio at greater airspeed, but at the cost of climbing more slowly in thermals. The maximum L/D is not dependent on weight or wing loading, but with greater wing loading the maximum L/D occurs at a faster airspeed. Also, the faster airspeed means the aircraft will fly at greater
Reynolds number and this will usually bring about a lower
zero-lift drag coefficient.
Glider – is a
fixed-wing aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose
free flight does not depend on an engine.[87] Most gliders do not have an engine, although
motor-gliders have small engines for extending their flight when necessary by sustaining the altitude (normally a sailplane relies on rising air to maintain altitude) with some being powerful enough to take off
self-launch.
Gravity – (from
Latin gravitas 'weight'[95]), or gravitation, is a
natural phenomenon by which all things with
mass or
energy—including
planets,
stars,
galaxies, and even
light[96]—are attracted to (or gravitate toward) one another.
On Earth, gravity gives
weight to
physical objects, and the
Moon's gravity causes the
tides of the oceans. The gravitational attraction of the original gaseous matter present in the
Universe caused it to begin
coalescing and
forming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become weaker as objects get further away.
H
Hall effect thruster – In
spacecraft propulsion, a Hall-effect thruster (HET) is a type of
ion thruster in which the
propellant is accelerated by an
electric field. Hall-effect thrusters (based on the discovery by
Edwin Hall) are sometimes referred to as Hall thrusters or Hall-current thrusters.
Hall-effect thrusters use a
magnetic field to limit the electrons' axial motion and then use them to ionize propellant, efficiently accelerate the
ions to produce
thrust, and neutralize the ions in the plume. The Hall-effect thruster is classed as a moderate
specific impulse (1,600s) space propulsion technology and has benefited from considerable theoretical and experimental research since the 1960s.[97]
Heat shield – A heat shield is designed to protect an object from overheating by dissipating, reflecting, absorbing heat, or simply gradually burn and fall away from the aircraft, pulling the excess heat with it. The term is most often used in reference to
exhaust heat management and to systems for dissipation of heat due to friction.
Helicopter – is a type of
rotorcraft in which
lift and
thrust are supplied by horizontally-spinning
rotors. This allows the helicopter to take off and land vertically, to
hover, and to fly forward, backward and laterally. These attributes allow helicopters to be used in congested or isolated areas where
fixed-wing aircraft and many forms of
VTOL (Vertical TakeOff and Landing) aircraft cannot perform.
Hydrostatics – Fluid statics or hydrostatics is the branch of
fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "
fluids at
hydrostatic equilibrium[78] and the pressure in a fluid, or exerted by a fluid, on an immersed body".[79] It encompasses the study of the conditions under which fluids are at rest in
stable equilibrium as opposed to
fluid dynamics, the study of fluids in motion. Hydrostatics is a subcategory of fluid statics, which is the study of all fluids, both compressible or incompressible, at rest.
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 1⁄2 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]
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]
The orbit of a planet is an
ellipse with the Sun at one of the two foci.
A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
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.
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
massm traveling at a
speedv is . In
relativistic mechanics, this is a good approximation only when v is much less than the
speed of light.
Kite – is a
tetheredheavier-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]
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 that 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 into 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
Lander –
spacecraft 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 defines a mechanical system to be a pair of a
configuration space and a smooth function called Lagrangian. By convention, where and are the
kinetic and
potential energy of the system, respectively. Here and is the velocity vector at is tangential to (For those familiar with
tangent bundles, and
Given the time instants and Lagrangian mechanics postulates that a smooth path describes the time evolution of the given system if and only if is a stationary point of the
action functional
If is an open subset of and are finite, then the smooth path is a stationary point of if all its directional derivatives at vanish, i.e., for every smooth
The function on the right-hand side is called perturbation or virtual displacement. The directional derivative 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.
Lagrangian point – In
celestial mechanics, the Lagrange points /ləˈɡrɑːndʒ/ (also Lagrangian points, L-points, or libration points) are points near two large
orbiting bodies. Normally, the two objects exert an unbalanced gravitational force at a point, altering the orbit of whatever is at that point. At the Lagrange points, the
gravitational forces of the two large bodies and the
centrifugal force balance each other.[124] This can make Lagrange points an excellent location for satellites, as few
orbit corrections are needed to maintain the desired orbit. Small objects placed in orbit at Lagrange points are in equilibrium in at least two directions relative to the
center of mass of the large bodies.
Laser broom – is a proposed ground-based
laserbeam-powered propulsion system whose purpose is to sweep
space debris out of the path of other
artificial satellites such as the
International Space Station. It would heat one side of an object enough to change its orbit and make it hit the atmosphere sooner. Space researchers have proposed that a laser broom may help mitigate
Kessler syndrome, a theoretical runaway cascade of collision events between orbiting objects.[125] Space-based laser broom systems using a laser mounted on a satellite or space station have also been proposed.[126][127][128][129]
Laser Camera System – (LCS), is short-range, high precision autosynchronous triangulation scanner. The camera uses a laser to measure the distance between itself and points on a target and is able to create a three-dimensional representation of the area it has scanned.
Latus rectum – is the
chord parallel to the directrix and passing through a focus; its half-length is the semi-latus rectum (ℓ).
Launch window – In the context of
spaceflight, launch period is the collection of days and launch window is the time period on a given day during which a particular
rocket must be launched in order to reach its intended target.[130][131] If the rocket is not launched within a given window, it has to wait for the window on the next day of the period.[132] Launch periods and launch windows are very dependent on both the rocket's capability and the orbit to which it is going.[133][134]
Leading edge – The leading edge of an
airfoil surface such as a
wing is its foremost edge and is therefore the part which first meets the oncoming air.[135][136]
Liquid fuel – Liquid fuels are combustible or energy-generating molecules that can be harnessed to create
mechanical energy, usually producing
kinetic energy; they also must take the shape of their container. It is the fumes of liquid fuels that are flammable instead of the fluid. Most liquid fuels in widespread use are derived from
fossil fuels; however, there are several types, such as
hydrogen fuel (for
automotive uses), ethanol, and
biodiesel, which are also categorized as a liquid fuel. Many liquid fuels play a primary role in transportation and the economy. Liquid fuels are contrasted with
solid fuels and
gaseous fuels.
Liquid-propellant rocket – or liquid rocket, utilizes a
rocket engine that uses
liquid propellants. Liquids are desirable because they have a reasonably high density and high
specific impulse (Isp). This allows the volume of the propellant tanks to be relatively low. It is also possible to use lightweight centrifugal
turbopumps to pump the
rocket propellant from the tanks into the combustion chamber, which means that the propellants can be kept under low pressure. This permits the use of low-mass propellant tanks that do not need to resist the high pressures needed to store significant amounts of gases, resulting in a low
mass ratio for the rocket.[citation needed]
Lithobraking – is a landing technique used by uncrewed space vehicles to safely reach the surface of a celestial body while reducing landing speed by impact with the body's surface.
Loiter – In
aeronautics and
aviation, loiter is the phase of flight consisting of flying over some small region.
Lunar Module – The Apollo Lunar Module, or simply Lunar Module (LM/ˈlɛm/), originally designated the Lunar Excursion Module (LEM), was the
Lunar landerspacecraft that was flown between
lunar orbit and the Moon's surface during the
United States'Apollo program. It was the first crewed spacecraft to operate exclusively in the airless vacuum of space, and remains the only crewed vehicle to land anywhere beyond Earth.
Lunar space elevator – or lunar spacelift, is a proposed transportation system for moving a mechanical climbing vehicle up and down a ribbon-shaped tethered cable that is set between the surface of the
Moon "at the bottom" and a docking port suspended tens of thousands of kilometers above in space at the top.
Magnetoplasmadynamic thruster – A magnetoplasmadynamic (MPD) thruster (MPDT) is a form of
electrically powered spacecraft propulsion which uses the
Lorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet.
Mass – is both a
property of a
physical body and a
measure of its
resistance to
acceleration (rate of change of
velocity with respect to time) when a
net force is applied.[152] An object's mass also determines the
strength of its
gravitational attraction to other bodies. The
SI base unit of mass is the
kilogram (kg). In
physics, mass is not the same as
weight, even though mass is often determined by measuring the object's weight using a
spring scale, rather than
balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Mass driver – or electromagnetic catapult, is a proposed method of
non-rocket spacelaunch which would use a
linear motor to
accelerate and catapult
payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make
electromagnets. Sequential firing of a row of electromagnets accelerates the payload along a path. After leaving the path, the payload continues to move due to
momentum.
Moment of inertia – otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a
rigid body is a quantity that determines the
torque needed for a desired
angular acceleration about a rotational axis, akin to how
mass determines the
force needed for a desired
acceleration. It depends on the body's mass distribution and the axis chosen, with larger moments requiring more torque to change the body's rate of rotation.
Momentum – In
Newtonian mechanics, linear momentum, translational momentum, or simply momentum is the product of the
mass and
velocity of an object. It is a
vector quantity, possessing a magnitude and a direction. If m is an object's mass and v is its velocity (also a vector quantity), then the object's momentum p is
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
rocketstages, 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
NACA –
United States National Advisory Committee for Aeronautics, replaced by
NASA in 1958.
In today's language, the law states that every
pointmass 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:
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
aerodynamicdesign 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.
Orbit phasing – In astrodynamics, orbit phasing is the adjustment of the time-position of spacecraft along its orbit, usually described as adjusting the orbiting spacecraft's true anomaly.[166] Orbital phasing is primarily used in scenarios where a spacecraft in a given orbit must be moved to a different location within the same orbit. The change in position within the orbit is usually defined as the phase angle, ϕ, and is the change in true anomaly required between the spacecraft's current position to the final position.
Orbital elements – are the
parameters required to uniquely identify a specific
orbit. In
celestial mechanics these elements are considered in
two-body systems using a
Kepler orbit. There are many different ways to mathematically describe the same orbit, but certain schemes, each consisting of a set of six parameters, are commonly used in
astronomy and
orbital mechanics. A real orbit and its elements change over time due to gravitational
perturbations by other objects and the effects of
general relativity. A Kepler orbit is an idealized, mathematical approximation of the orbit at a particular time.
Orbital inclination change – is an
orbital maneuver aimed at changing the
inclination of an orbiting body's
orbit. This maneuver is also known as an orbital plane change as the plane of the orbit is tipped. This maneuver requires a change in the orbital velocity vector (
delta v) at the
orbital nodes (i.e. the point where the initial and desired orbits intersect, the line of orbital nodes is defined by the intersection of the two orbital planes).
Orbiter Boom Sensor System – (OBSS), was a 50-foot (15.24 m) boom carried on board NASA's Space Shuttles. The boom was grappled by the Canadarm and served as an extension of the arm, doubling its length to a combined total of 100 feet (30 m).[1] At the far end of the boom was an instrumentation package of cameras and lasers used to scan the leading edges of the wings, the nose cap, and the crew compartment after each lift-off and before each landing. If flight engineers suspected potential damage to other areas, as evidenced in imagery captured during lift-off or the rendezvous pitch maneuver, then additional regions could be scanned.
Parasitic drag – also known as profile drag,[170]: 254 [171]: 256 is a type of
aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is a combination of form drag and
skin friction drag.[172][170]: 641–642 It affects all objects regardless of whether they are capable of generating
lift. Total drag on an aircraft is made up of parasitic drag and
lift-induced drag. Parasitic drag is so named because it is not useful, whereas lift-induced drag is the result of an
airfoil generating lift. Parasitic drag comprises all types of drag except lift-induced drag.[173]
Perpendicular axes theorem – states that the
moment of inertia of a
planar lamina (i.e. 2-D body) about an axis perpendicular to the
plane of the lamina is equal to the sum of the moments of inertia of the lamina about the two axes at right angles to each other, in its own plane intersecting each other at the point where the perpendicular axis passes through it.
Define perpendicular axes , , and (which meet at origin ) so that the body lies in the plane, and the 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]
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 and are equal,[175]
then the perpendicular axes theorem provides the useful relationship:
Plug nozzle – is a type of
nozzle which includes a centerbody or plug around which the working fluid flows. Plug nozzles have applications in aircraft, rockets, and numerous other fluid flow devices.
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R
Radar – system using the reflection from transmitted
electromagnetic waves to detect the distance and rough shape of an object, working even in
outer space, unlike
sonar
Rogallo wing – is a flexible type of
wing. In 1948,
Francis Rogallo, a
NASA engineer, and his wife
Gertrude Rogallo, invented a self-inflating flexible wing they called the Parawing, also known after them as the "Rogallo Wing" and flexible wing.[182] NASA considered Rogallo's flexible wing as an alternative recovery system for the
Mercury and
Geminispace capsules, and for possible use in other spacecraft landings, but the idea was dropped from Gemini in 1964 in favor of
conventional parachutes.
Terminal velocity – is the maximum velocity (speed) attainable by an object as it falls through a
fluid (
air is the most common example). It occurs when the sum of the
drag force (Fd) and the
buoyancy is equal to the downward force of
gravity (FG) acting on the object. Since the
net force on the object is zero, the object has zero
acceleration.[183]
is the object's final
velocity along the x axis on which the acceleration is constant.
is the object's initial velocity along the x axis.
is the object's
acceleration along the x axis, which is given as a constant.
is the object's change in position along the x axis, also called
displacement.
This equation is valid along any axis on which the acceleration is constant.
Total air temperature – In aviation,
stagnation temperature is known as total air temperature and is measured by a
temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest,
kinetic energy is converted to
internal energy. The air is compressed and experiences an
adiabatic increase in temperature. Therefore, total
air temperature is higher than the static (or ambient) air temperature. Total air temperature is an essential input to an
air data computer in order to enable the computation of static air temperature and hence
true airspeed.
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.
Velocity – The velocity of an object is the
rate of change of its
position with respect to a
frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's
speed and direction of
motion (e.g. 60
km/h to the north). Velocity is a fundamental concept in
kinematics, the branch of
classical mechanics that describes the motion of bodies.
Velocity is a physical
vectorquantity; both magnitude and direction are needed to define it. The
scalarabsolute 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.
Viscometer – (also called viscosimeter) is an instrument used to measure the
viscosity of a
fluid. For liquids with viscosities which vary with
flow conditions, an instrument called a
rheometer is used. Thus, a rheometer can be considered as a special type of viscometer.[189] Viscometers only measure under one flow condition.
Viscosity – The viscosity of a
fluid is a measure of its
resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example,
syrup has a higher viscosity than
water.[190]
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"[199] and "meta-calculus".[200]
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.
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.
X
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Y
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^Wallner, Lewis E. and Czika, Joseph, Jr, ARC-Jet Thrustor for Space Propulsion, NASA Technical note TN D-2868, NASA Lewis Research Center, June 1965 (accessed September 8, 2014)
^Kermode, A.C. (1972), Mechanics of Flight, Chapter 3, (p.103, eighth edition), Pitman Publishing Limited, London
ISBN0-273-31623-0
^"Asteroids". NASA – Jet Propulsion Laboratory. Retrieved 13 September 2010.
^Wu, H.-C. (2005). Continuum Mechanics and Plasticity. CRC Press.
ISBN1-58488-363-4.
^Keys, C. N.; Stepniewski, W. Z. (1984). Rotary-wing aerodynamics. New York: Dover Publications. p. 3.
ISBN0-486-64647-5. It is interesting to note that there has always been a strong intuitive association of rotary-wing aircraft with low disc loading which is reflected in the commonly accepted name of rotor given to their lifting airscrews.
^Annex 10 to the Convention on International Civil Aviation, Volume I – Radio Navigation Aids; International Civil Aviation Organization; International Standards and Recommended Practices.
^Doering, C. R. and Gibbon, J. D. (1995). Applied Analysis of the Navier-Stokes Equations, p. 11, Cambridge University Press, Cambridge.
ISBN052144568-X.
^Encyclopaedia of Physics (second Edition),
R.G. Lerner, G.L. Trigg, VHC Publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1 (VHC Inc.) 0-89573-752-3
^Flightwise - Volume 2 - Aircraft Stability And Control, Chris Carpenter 1997, Airlife Publishing Ltd.,
ISBN1 85310 870 7, p.145
^Depending on the vehicle's mass distribution, the effects of gravitational force may also be affected by attitude (and vice versa), but to a much lesser extent.
^Crane, Dale: Dictionary of Aeronautical Terms, third edition, p. 224. Aviation Supplies & Academics, 1997.
ISBN1-56027-287-2.
^Sparke, L. S.; Gallagher, J. S. III (2000). Galaxies in the Universe: An Introduction. Cambridge University Press. ISBN 978-0-521-59740-1. Archived from the original on March 24, 2021. Retrieved July 25, 2018.
^Hupp, E.; Roy, S.; Watzke, M. (August 12, 2006). "NASA Finds Direct Proof of Dark Matter". NASA. Archived from the original on March 28, 2020. Retrieved April 17, 2007.
^Uson, J. M.; Boughn, S. P.; Kuhn, J. R. (1990). "The central galaxy in Abell 2029 – An old supergiant". Science. 250 (4980): 539–540. Bibcode:1990Sci...250..539U. doi:10.1126/science.250.4980.539. PMID 17751483. S2CID 23362384.
^Hoover, A. (June 16, 2003). "UF Astronomers: Universe Slightly Simpler Than Expected". Hubble News Desk. Archived from the original on July 20, 2011. Retrieved March 4, 2011.
Based upon: Graham, A. W.; Guzman, R. (2003). "HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies". The Astronomical Journal. 125 (6): 2936–2950. arXiv:astro-ph/0303391. Bibcode:2003AJ....125.2936G. doi:10.1086/374992. S2CID 13284968.
^Jarrett, T. H. "Near-Infrared Galaxy Morphology Atlas". California Institute of Technology. Archived from the original on August 2, 2012. Retrieved January 9, 2007.
^Hofer, Richard R. (June 2004). Development and Characterization of High-Efficiency, High-Specific Impulse Xenon Hall Thrusters. NASA/CR—2004-21309 (Report). NASA STI Program.
hdl:
2060/20040084644.
^"GIRD-09". Encyclopedia Astronautix. Archived from
the original on December 21, 2016. Retrieved June 25, 2017.
^Das, K. K., Honnutagi, R., Mullur, L., Reddy, R. C., Das, S., Majid, D. S. A., & Biradar, M. S. (2019). "Heavy metals and low-oxygen microenvironment – its impact on liver metabolism and dietary supplementation". In Dietary Interventions in Liver Disease. pp. 315–32. Academic Press.
^
abKessler, Donald J.; Cour-Palais, Burton G. (1978). "Collision Frequency of Artificial Satellites: The Creation of a Debris Belt". Journal of Geophysical Research. 83 (A6): 2637–2646.
Bibcode:
1978JGR....83.2637K.
doi:
10.1029/JA083iA06p02637.
^Eden, Maxwell (2002). The Magnificent Book of Kites: Explorations in Design, Construction, Enjoyment & Flight. New York: Sterling Publishing Company, Inc. p. 18.
ISBN9781402700941.
^A.M. Kuethe and J.D. Schetzer (1959) Foundations of Aerodynamics, 2nd edition, John Wiley & Sons ISBN 0-471-50952-3
^Anderson, J. D. Jr. (1989). "Pressure, Temperature, and Density Altitudes". Introduction to Flight (3rd ed.). New York: McGraw-Hill. pp. 100–103.
ISBN0-07-001641-0.
^Dr Claude Phipps (2011). "Removing Orbital Debris with Lasers". Advances in Space Research. 49 (9): 1283–1300. arXiv:1110.3835. Bibcode:2012AdSpR..49.1283P. doi:10.1016/j.asr.2012.02.003.
^Young, Donald F.; Bruce R. Munson; Theodore H. Okiishi; Wade W. Huebsch (2010). A Brief Introduction to Fluid Mechanics (5 ed.). John Wiley & Sons. p. 95.
ISBN978-0-470-59679-1.
^McLean, Doug (2012).
"Continuum Fluid Mechanics and the Navier-Stokes Equations". Understanding Aerodynamics: Arguing from the Real Physics. John Wiley & Sons. pp. 13–78.
ISBN9781119967514. The main relationships comprising the NS equations are the basic conservation laws for mass, momentum, and energy. To have a complete equation set we also need an equation of state relating temperature, pressure, and density...
^Isaac Newton: "In [experimental] philosophy particular propositions are inferred from the phenomena and afterwards rendered general by induction": "
Principia", Book 3, General Scholium, at p.392 in Volume 2 of Andrew Motte's English translation published 1729.
^"Wright Brothers". Smithsonian National Air and Space Museum. Retrieved 29 September 2021.
^Geostationary orbit and Geosynchronous (equatorial) orbit are used somewhat interchangeably in sources.
^"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 tensorGμν vs. the
metric tensorgμν, 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).
^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.
^CNSA (China),
ESA (most of Europe), ISRO, (India),
JAXA (Japan),
NASA (United States) and
Roscosmos (Russia) are space agencies with full launch capabilities.