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Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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In electrical communications, the process, or the result of the process, by which fixed graphic material including pictures or images is scanned and the information converted into signals which are used either locally or remotely to produce in record form a likeness (facsimile) of the subject copy.
Large patches of bright material forming a veined network in the vicinity of sunspots. They appear to be more permanent than sunspots and are probably due to elevated clouds of luminous gas.
Of a radiant energy signal, to decrease, often temporarily, in received signal strength without a change of receiver controls. The opposite is build.
A type of fading in which the received signal strength is reduced to a value below the noise level of the receiver. The most common cause of fadeout is a disturbed ionosphere. Also called radio fadeout, Dellinger effect, Mögel-Dellinger effect. See blackout.
The variation of radio field strength caused by changes in the transmission medium with time.
Fahrenheit temperature scale (abbr F)
A temperature scale with the ice point at 32° and the boiling point of water at 212°.
Conversion with the Celsius (centigrade) temperature scale (abbr C) is by the formula
F = 9/5 C + 32
fail safe system
A system used to minimize risk in case of a malfunction.
Of a spacecraft or spatial body, to drop toward another spatial body under the influence of the latter's gravity.
fallaway section
A section of a rocket vehicle that is cast off and separates from the vehicle during flight, especially such a section that falls back to the earth.
false horizon
See horizon, note.
1. (a) Any vaned rotary device for producing a current or stream of air. (b) Specifically, a multivaned wheel or rotor used to take in air in a bypass engine or ducted-fan engine. It may be either a mere blower of a low-pressure compressor. See ducted fan.
2. A propeller, especially when the emphasis is upon its function of moving air rather than propelling.
fanned-beam antenna
A unidirectional antenna so designed that transverse cross sections of the major lobe are approximately elliptical.
fanning beam
A radiant energy beam, as a radar beam, which sweeps back and forth over a limited arc.
farad (abbr f)
The unit of electrical capacitance, the capacitance of a condenser between the plates of which there is a difference of potential of 1 volt when it is charged by a quantity of electricity equal to 1 coulomb.
Faraday constant (symbol F)
The product of the Avogadro constant NA and the elementary charge, e, F =NAe = 9.64870 coulombs per mol. See physical constants, tables.
fast ion = small ion.
fast neutron
A neutron of 100,000 electronvolts or greater energy.
fast reactor
A reactor containing no moderator, so that all the fissions take place at energies on the order of 100,000 electron-volts or higher.
1. A weakening or deterioration of metal or other material occurring under load, especially under repeated cyclic, or continued loading.
Self-explanatory compounds include: fatigue crack, fatigue failure, fatigue load, fatigue resistance, fatigue test.
2. State of the human organism after exposure to any type of physical or psychological stress (e.g., pilot fatigue).
fatigue strength
The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated. Also called fatigue limit.
F-corona = Fraunhofer corona.
In radar, a rectangular display in which a target appears as a centralized blip when the radar antenna is aimed at it. Horizontal and vertical aiming errors are respectively indicated by the horizontal and vertical displacement of the blip. Also called F-scan, F-scope, F-indicator.
1. To provide a signal.
2. The point at which a signal enters a circuit or device, as antenna feed.
3. The signal entering a circuit or device; input.
1. The return of a portion of the output of a device to the input; positive feedback adds to the input, negative feedback subtracts for the input.
2. Information, as to progress, results, etc., returned to an originating source.
3. In aeronautics, the transmittal of forces initiated by aerodynamic action on control surfaces or rotor blades to the cockpit controls; the forces so transmitted.
feedback control loop
A closed transmission path (loop), which includes an active transducer and which consists of a forward path, a feedback path, and one or more mixing points arranged to maintain a prescribed relationship between the loop input signal and the loop output signal.
feedback control system
A control system, comprising one or more feedback control loops, which combines functions of the controlled signals with functions of the commands to tend to maintain prescribed relationships between the commands and the controlled signals.
feedback path
In a feedback control loop the transmission path from the loop output signal to the loop feedback signal.
The sensation or impression that a pilot has or receives as to his, or his craft's, attitude, orientation, speed, direction of movement or acceleration, or proximity to nearby objects, or, as most often used, as to the aircraft's stability and responsiveness to control. See control feel.
femto (abbr f)
A prefix meaning multiplied by 10-15.
1. A line of readout or tracking stations for pickup of signals from an orbiting satellite.
2. A line or network of radar or radio stations for detection of a satellite in orbit.
3. A stationary plate or vane projecting from the upper surface of an airfoil, substantially parallel to the airflow, used to prevent spanwise flow.
Fermat principle
The principle which states that the path along which electromagnetic radiation travels between any two points will be that path for which the elapsed time for the travel is a minimum. See multipath transmission.
fermi (abbr f)
A unit of length equal to 10-13 centimeters.
In cartography, pertaining to or measured from an arbitrary reference line as in fictitious equator, fictitious latitude, fictitious longitude.
Transverse, oblique, and grid map projections are examples of fictitious projections.
fictitious year
The period between successive returns of the sun to a sidereal hour angle of 80° (about January 1). Also called Besselian year, Bessel fictitious year.
The length of the fictitious year is the same as that of the tropical year, since both are based upon the position of the sun with respect to the vernal equinox.
The accuracy to which an electrical system, such as a radio, reproduces at its output the essential characteristics of its input signal.
fiducial mark
An internally generated identification mark on a film; two or more of these are generally used for orienting a film for reading, and for determining the geometric center of the film.
The L-shaped corner marks and the + mark near the picture center, which are on the focal plane of the Tiros vidicon camera are fiducial marks. Their appearance on the image permits various calibrations such as determination of the degree of enlargement needed to fit the picture to the rectification grids, etc.
A region of space within which each point has a definite value of a given physical or mathematical quantity has some definite value.
One may speak of a gravitation field, magnetic field, electric field, pressure field, temperature field, etc. If the quantity specified at each point is a vector quantity, the field is said to be a vector field.
field brightness = adaptation luminance.
field intensity = field strength.
field luminance = adaptation luminance.
field strength
1. For any physical field, the flux density, intensity, or gradient of the field at the point in question. Also called field intensity.
Although field intensity is commonly used, it should be noted that this does not follow the strict radiometric definition of intensity, i.e., flux per unit solid angle.
2. = signal strength, in radar.
3. = electric field strength.
filamentary structure
A shell or membrane structure constructed of woven or layered filaments embedded in a suitable matrix.
film cooling
The cooling of a body or surface, such as the inner surface of a rocket combustion chamber, by maintaining a thin fluid layer over the affected area. Compare transpiration cooling.
filter = wave filter.
1. The decomposition of a signal into its harmonic components.
2. The separation of a wanted component of a time series from any unwanted residue (noise).
1. A fixed or adjustable airfoil or vane attached longitudinally to an aircraft, rocket, or similar body to provide a stabilizing effect.
2. A projecting flat plate or structure, as a cooling fin.
final mass
The mass of a rocket after its propellants are consumed.
F-indicator = F-display.
fine data channel
The channel of a trajectory-measuring system delivering accurate but ambiguous data as opposed to the coarse channel needed to resolve the ambiguity.
fine pressure = inlet pressure.
fineness ratio
The ratio of the length of a body to its maximum diameter, or, sometimes, to some equivalent dimension - said especially of a body such as an airship hull or rocket.
1. To ignite a rocket engine.
Usage is sometimes restricted to period of main chamber burning when small igniter chambers are used, especially with igniter idle provisions where the igniter may burn for some significant period prior to main chamber fire.
2. To launch a rocket.
A bright meteor with luminosity which equals or exceeds that of the brightest planets.
fire point
The temperature at which a substance, as lubricating oil, will give off a vapor that will burn continuously after ignition. Compare flashpoint.
1. The action or event of igniting a rocket engine.
2. The action or event of launching a rocket.
firing chamber = combustion chamber.
first law of thermodynamics
A statement of the conservation of energy for thermodynamic systems (not necessarily in equilibrium). The fundamental form requires that the heat absorbed by the system serve either to raise the internal energy of the system or to do work on the environment:
dq = du + dw
where dq is the heat added per unit mass; du is the increment of specific internal energy; and dw is the specific work done by the system on the environment. Although dq and dw are not perfect differentials, their difference, du, is always a perfect differential. Example of the application of this equation: in an adiabatic free expansion of gas into a vacuum, all three terms are zero.
For reversible processes the mechanical work is equal to the expansion against the pressure forces, i.e.,
dw = pdv
where p is the pressure and v is the specific volume. For a perfect gas, the internal energy change is proportional to the temperature change,
du = cvdT
where cv is the specific heat at constant volume and T is the Kelvin temperature. Therefore, the form of the first law usually used in meteorological applications is
dq = cvdT + pdv
Use of the equation of state yields an alternative form,
dq = cpdT - dp
where cp is the specific heat at constant pressure.
For open systems the variation of total rather than specific quantities is important:
dQ = dU + pdV - hdm
where Q is the total heat; U is the total internal energy; V is the volume; m is the mass of the system; and h is the specific enthalpy.
If a system contains the possibility of nonmechanical work, such as work done against an electric field, this work must be included in the first law.
See second law of thermodynamics, third law of thermodynamics, energy equations.
first point of Aries = vernal equinox.
first point of Cancer = summer solstice.
first point of Capricornus = winter solstice.
first point of Libra = autumnal equinox.
first quarter
The phase of the moon when it is near east quadrature, when the western half of it is visible to an observer on the earth. See phases of the moon.
fishbone antenna
An antenna consisting of a series of coplanar elements arranged in colinear pairs, loosely coupled to a balanced transmission line.
The splitting of an atomic nucleus into two more-or-less equal fragments.
Fission may occur spontaneously or may be induced by capture of bombarding particles. In addition to the fission fragments, neutrons and gamma rays are usually produced during fission.
Having the property of certain atomic nuclei, such as some isotopes of uranium and plutonium, of capturing neutrons and thereupon splitting into two particles with great kinetic energy.
The term properly is applicable to nuclei that undergo fission by neutrons of thermal energies; but it sometimes is applied loosely to cases where the neutron must be of high energy, an in U238 is fissionable by fast neutrons.
Fitzgerald-Lorentz contraction
A hypothesis that all measuring rods contract in the direction of motion in the ratio
where u is the speed of motion and c is the speed of light.
In navigation, a relatively accurate position determined without reference to any former position. It may be classed as visual, sonic, celestial, electronic, radio, hyperbolic, loran, radar, etc., depending upon the means of establishing it.
fixed-area exhaust nozzle
On a jet engine, an exhaust nozzle exit opening which remains constant in area. Compare variable-area exhaust nozzle.
fixed point
1. Positional notation in which corresponding places in different quantities are occupied by coefficients of the same power of the base. Contrast to floating point.
2. A notation in which the base point is assumed to remain fixed with respect to one end of the numeric expressions.
fixed satellite
A satellite that orbits the earth from west to east at such a speed as to remain fixed over a given place on the earth's equator at approximately 35,900 kilometers altitude. See stationary orbit, 24-hour satellite, synchronous satellite.
flame attenuation
Attenuation of a radio signal by the ionization produced in the rocket exhaust.
flame bucket
A deep cavelike construction built beneath a launcher, open at the top to receive the hot gases of the rocket positioned above it, and open on one or three sides below, with a thick metal fourth side bent toward the open sides so as to deflect the exhausting gases. See flame deflector.
flame deflector
1. In a vertical launch, any of variously designed obstructions that intercept the hot gases of the rocket engine so as to deflect them away from the ground or from a structure.
The flame deflector may be a relatively small device fixed to the top surface of the pad surrounded by the framework of the launcher, or it may be a heavily constructed piece of metal mounted as a side and bottom of a flame bucket. In the latter case, the deflector may be perforated with numerous holes connected with a source of water, bending at an angle of about 45° into the line of the exhaust stream. During thrust buildup and the beginning of the launch, a deluge of water pours from the holes in such a deflector to keep it from melting. See deluge collection pond.
2. In a captive test, an elbow in the exhaust conduit or flame bucket that deflects the flame into the open.
flame tube = inner liner.
Flamsteed number
A number sometimes used with the possessive form of the Latin name of the constellation to identify a star, as 72 Ophiuchi.
The Flamsteed number is used for stars numbered in Flamsteed's British Catalogue of 1725. For stars which do not appear in Flamsteed's catalog, numbers from other catalogs are used. See Bayer letter.
1. A bright eruption form the sun's chromosphere. Compare prominence.
Flares may appear within minutes and fade within an hour. They cover a wide range of intensity and size, and they tend to occur between sunspots or over their penumbrae.
Flares are related to radio fadeouts and terrestrial magnetic disturbances.
Flares eject high energy protons which present a serious hazard to men in unshielded spacecraft.
2. Pyrotechnic devices used for signaling or to provide illumination.
3. An expansion at the end of a cylindrical body as at the base of a rocket.
A reversal of flame in a system, counter to the usual flow of the combustible mixture.
The temperature at which a substance, as fuel oil, will give off a vapor that will flash or burn momentarily when ignited. Compare fire point.
Of the earth, the ratio of the difference between the equatorial radius (major semiaxis) and the polar radius (minor semiaxis) of the earth to the equatorial radius. Also called compression. See astronomical constants.
The flattening of the earth is the ellipticity of the spheroid and equals the ellipticity of the ellipse forming a meridional section the spheroid. If a and b represent the major and minor semiaxes of the spheroid, and f is the flattening of the earth,
f = (a - b) / a
The magnitude of the flattening is sometimes expressed by stating the numerical value of the reciprocal of the flattening, a/(a - b).
See ionosphere.
flicker control
Control of an aircraft, rocket, etc. in which the control surfaces are deflected to their fullest degree with any motion of the remote control. Compare proportional control. See bang-bang control.
1. The movement of an object through the atmosphere or through space, sustained by aerodynamic, aerostatic, or reaction forces, or by orbital speed; especially, the movement of a man-operated or man-controlled device, such as a rocket, a space probe, a space vehicle, or aircraft.
2. An instance of such a movement.
flight attitude
The attitude of an aircraft, rocket, etc., in flight; specifically, the attitude of an aircraft with respect to the relative wind.
flight characteristic
A characteristic exhibited by an aircraft, rocket, or the like in flight, such as a tendency to stall or to yaw, an ability to remain stable at certain speeds, etc.
flight control system = vehicle control system.
flight Mach number
A free-stream Mach number measured in flight as distinguished from one measured in a wind tunnel.
The path made or followed in the air or in space by an aircraft, rocket, etc.; the continuous series of positions occupied by a flying body; more strictly, the path of the center of gravity of the flying body, referred to the earth or other fixed reference.
flightpath angle
The angle between the horizontal and a tangent to the flightpath at a point.
flight profile
A graphic portrayal or plot of the flight path of an aeronautical vehicle in the vertical plane.
flight simulator
A training device or apparatus that simulates certain conditions of actual flight or of flight operations.
flight space
The space above and beyond the earth's surface now used, or potentially to be used, for flight of aircraft, spacecraft, or rockets.
flight test
1. A test by means of actual or attempted flight to see how an aircraft, spacecraft, space-air vehicle, or missile flies.
2. A test of a component part of a flying vehicle, or of an object carried in such a vehicle, to determine its suitability or reliability in terms of its intended function by making it endure actual flight.
flight test vehicle
A test vehicle for the conduct of flight tests, either to test its own capabilities or to carry equipment requiring flight test.
1. A device having two stable states and two input terminals (or types of input signals) each of which corresponds with one of the two states. The circuit remains in either state until caused to change to the other state by application of the corresponding signal.
2. A similar bistable device with an input which allows it to act as a single-stage binary counter.
floating point
In computer operations, a positional notation in which corresponding places in different quantities are not necessarily occupied by coefficient of the same power of the base. Compare fixed point.
Floating point corresponds to multiplication using powers of 10; for example, 186,000 can be represented as 1.86 * 105. By shifting the point so that the number of significant digits in any quantity does not exceed machine capacity, widely varying quantities can be handled. The scale factor may be fixed for each problem, or indicated along with the digits and sign for each quantity.
Patches of relatively dense, dark or bright clouds in the sun's atmosphere. They appear in photographs taken with the spectroheliograph.
The emission spectra usually studied are those of calcium and hydrogen; e.g., bright calcium flocculi, and dark or bright hydrogen flocculi. Measures of the extent of these three kinds of flocculi are tabulated in the quarterly Bulletin of Character Figures of Solar Phenomena, Zurich, Int. Astron. Union.
flotation gear
1. Gear or apparatus, commonly inflatable bags, vest, rafts, and the like, carried aboard a vehicle to support the vehicle or persons if downed in water.
2. A buoyant landing gear, usually called floats.
A stream or movement of air or other fluid, or the rate of fluid movement, in the open or in a duct, pipe, or passage; specifically, an airflow.
flow chart
A graphical representation of a sequence of operations using symbols to represent the operations.
A flow chart is a more detailed representation than a diagram.
fluctuation velocity = eddy velocity.
A substance which, when in static equilibrium, cannot sustain a shear stress; a liquid or a gas.
This concept is only approximated by actual liquids and gases.
Reciprocal of viscosity.
fluid parcel
In any fluid, an imaginary portion of that fluid which for theoretical studies may be considered to have all the basic dynamic and thermodynamic properties of the fluid but which is small enough so that its motion with respect to the surrounding fluid does not induce marked compensatory movements. Also called parcel.
The size of the fluid parcel cannot be given precise numerical definition but it must be large enough to contain a great number of molecules and small enough so that the properties assigned to it are approximately uniform within it.
Emission of light or other radiant energy as a result of and only during absorption of radiation of a different wavelength from some other source. Also called photoluminescence. See luminescence. Compare phosphorescence.
An aeroelastic self-excited vibration in which the external source of energy is the airstream and which depends on the elastic, inertial and dissipative forces of the system in addition to the aerodynamic forces.
1. The rate of flow of some quantity, often used in reference to the flow of some form of energy. Also called transport. See power.
2. In nuclear physics generally, the number of radioactive particles per unit volume times their mean velocity.
flux density
The flux (rate of flow) of any quantity, usually a form of energy, through a unit area of specified surface. (Note that this is not a volumetric density like radiant density.) Compare luminous density.
In radar, flux density commonly is referred to as power density. It is essential to understand that the flux density of radiation is in no sense a vector quantity, because it is the sum of the flux corresponding to all ray directions incident upon one side of the unit area.
flux-density threshold = threshold illuminance.
An interplanetary mission in which the vehicle passes close to the target planet but does not impact it or go into orbit around it.
flying spot
A rapidly moving spot of light, usually generated by a cathode-ray tube and used to scan a surface containing visual information.
flying test bed
An aircraft, rocket, or other flying vehicle used to carry objects or devices being flight tested.
FM (abbr) = frequency modulation.
FM/AM (abbr)
1. Amplitude modulation of a carrier by subcarrier(s) which is (are) frequency modulated by information.
2. Alternate FM or AM operations.
FM/FM (abbr)
Frequency modulation of a carrier by subcarrier(s) which is (are) frequency modulated by information.
FM/PM (abbr)
Phase modulation of a carrier by subcarrier(s) which is (are) frequency modulated by information.
foamed plastics
Plastic materials, used primarily for insulation, in which a foaming agent is used to provide minute voids to improve insulating qualities-often foamed in place within the structure.
focal length
The distance between the optical center of a lens, or the surface of a mirror, and its focus.
focal plane
A plane parallel to the plane of a lens or mirror and passing through the focus.
focal point = focus, in optics.
focus (plural focuses)
1. That point at which parallel rays of light meet after being refracted by a lens or reflected by a mirror. Also called focal point.
2. A point having specific significance relative to a geometrical figure. See ellipse, hyperbola, parabola.
folded dipole antenna
An antenna composed of two parallel, closely space dipole antennas connected at their ends with one of the dipole antennas fed at its center.
folding fin
A fin hinged as its base to lie flat, especially a fin on a rocket that lies flat until the rocket is in flight.
Any object, group of objects, technique, or procedure considered to be a second or subsequent generation in the development of the object, group of objects, technique, or procedure. See generation.
foot (abbr ft)
The foot (international) is exactly 0.3048 meter.
The American Survey foot is 0.3048006 meter.
The old U.S. foot, used prior to July 1, 1959, was 0.3048006 meter.
foot-candle (abbr ft-c)
A unit of illuminance, incident light, or illumination equal to 1 lumen per square foot. This is the illuminance provided by a light source of one candle at a distance of 1 foot, hence the name. Compare lux, phot.
Full sunlight with zenith sun produces an illuminance of the order of 10,000 foot-candles on a horizontal surface at the earth's surface. Full moonlight provides an illuminance of only about 0.02 foot-candle also at earth's surface. Adequate illumination for steady reading is taken to be about 10 foot-candles; that for close machine work is about 30 to 40 foot-candles.
foot-lambert (abbr ft-l)
A unit of luminance (or brightness) equal to 1 / candle per square foot, or 1 lumen per square foot.
In Great Britain this is also called the equivalent foot-candle.
foot-to-head acceleration
See physiological acceleration.
footward acceleration
See physiological acceleration.
For, Forn
International Astronomical Union abbreviations for Fornax. See constellation.
forbidden line
A line in a spectrum resulting from a transition from a metastable state within an atom. Forbidden lines are not found in ordinary sources, but may be conspicuous in very large bodies of rarefied gas where the time interval between collisions of atoms is long.
Forbidden lines of oxygen appear, for example, in the aurora.
Forbush decrease
The observed decrease in cosmic ray activity in the earth's atmosphere about a day after a solar flare.
The Forbush decrease is believed to be caused by a shielding effect of the magnetic fields contained in the plasma cloud emitted from the sun at the time of the flare.
force (symbol F)
The cause of the acceleration of material bodies measured by the rate of change of momentum produced on a free body.
force balance transducer
A transducer in which the output from the sensing member is amplified and fed back to an element which causes the force-summing member to return to its rest position.
forced oscillation
An oscillation of a system in which the response is imposed by the excitation. If the excitation is periodic and continuing, the oscillation is steady state. Also called force vibration.
forced vibration = forced oscillation.
forced wave
See resonance.
force function
The negative of potential, sense 1.
The relative ease with which a metal can be shaped through plastic deformation.
International Astronomical Union abbreviation for Fornax. See constellation.
Fornax (abbr For, Forn)
See constellation.
forward acceleration
See physiological acceleration.
forward scatter
The scattering of radiant energy into the hemisphere of space bounded by a plane normal to the direction of the incident radiation and lying on the side toward which the incident radiation was advancing; the opposite of backward scatter.
In Rayleigh scattering, forward scatter accounts for half of the total. As the particle size increases above the Rayleigh limit, an increasing fraction of the total scattering is forward scattering.
Fourier analysis
The representation of physical or mathematical data by the use of the Fourier series or Fourier integral.
Fourier coefficients
See Fourier series.
Fourier integral
The representation of a function f(x) for all values of x in terms of infinite integrals in the form
See Fourier transform, Fourier series.
Fourier series
The representation of a function f(x) in an interval (-L, L) by a series consisting of sines and cosines with a common period 2L, in the form,
where the Fourier coefficients are defined as


When f(x) is an even function, only the cosine terms appear; when f(x) is odd, only the sine terms appear.
The conditions on f(x) guaranteeing convergence of the series are quite general, and the series may serve as a root-mean-square approximation even when it does not converge.
If the function is defined on an infinite interval and is not periodic, it is represented by the Fourier integral. By either representation, the function is decomposed into periodic components whose frequencies constitute the spectrum of the function. The Fourier series employs a discrete spectrum of wavelengths 2 L / n(n = 1,2,...); the Fourier integral requires a continuous spectrum.
See Fourier transform.
Fourier transform
An analytical transformation of a function f(x) obtained (if it exists) by multiplying the function by e-iux and integrating over all x,
where u is the new variable of the transform F(u) and i2 = -1. If the Fourier transform of a function is known, the function itself may be recovered by use of the inversion formula:
The Fourier transform has the same uses as the Fourier series: for example, the integrand F(u) exp (iux) is a solution of a given linear differential equation, so that the integral sum of these solutions is the most general solution of the equation.
When the variable u is complex, the Fourier transform is equivalent to the Laplace transform.
See Fourier integral, spectral function.
fourth state of matter = plasma.
The central part of the retina, which contains a high concentration of color-sensitive receptors known as cones. See foveal vision.
foveal vision
Vision in which the eye is so oriented toward the pertinent light source as to have the light fall upon that central portion of the retina called the fovea. Also called photopic vision.
Foveal vision permits much higher resolution than does parafoveal vision and is the normal mode of seeing under daytime conditions.
Fraunhofer corona
That portion of the radiation from the corona consisting of the Fraunhofer spectrum scattered by interplanetary particles. Compare K-corona, L-corona.
Fraunhofer lines
Dark lines in the absorption spectrum of solar radiation due to absorption by gases in the outer portions of the sun and in the earth's atmosphere.
Fraunhofer lines are designated by letters, as the K-line, or by wavelength, as the 4046-angstrom line of iron.
The major Fraunhofer lines are:
angstroms (nm) Line due to
A 7594 Telluric oxygen
B 6867 Telluric oxygen
C 6563 hydrogen, H
D1 5896 sodium
D2 5890 sodium
D3 5876 helium
E 5270 iron and calcium
b1 5184 magnesium
F 4861 hydrogen, H
G 4308 iron (and calcium)
H 3968 calcium
K 3934 calcium
Fraunhofer region
That region of the field in which the energy flow from an antenna proceeds essentially as though coming from a point source located in the vicinity of the antenna.
If the antenna has a well-defined aperture a in a given aspect, the Fraunhofer region in that aspect is commonly taken to exist at distances greater than 2a2 / from the aperture, being the wavelength.
Fraunhofer spectrum
The visible solar spectrum.
free air = free atmosphere.
free atmosphere
That portion of the earth's atmosphere, above the planetary boundary layer, in which the effect of the earth's surface friction on the air motion is negligible, and in which the air is usually treated (dynamically) as an ideal fluid. The base of the free atmosphere is usually taken as the geostrophic wind level. Also called free air.
free electron
An electron which is not bound to an atom.
free fall
1. The fall or drop of a body, such as a rocket, not guided, not under thrust, and not retarded by a parachute or other braking device.
2. The free and unhampered motion of a body along a Keplerian trajectory, in which the force of gravity is counterbalanced by the force of inertia. See weightlessness.
free field
An isotropic, homogeneous, sound field free from bounding surfaces. Also called free sound field.
free flight
Unconstrained or unassisted flight, as: (a) the flight of a rocket after consumption of its propellant or after motor shutoff; (b) the flight of an unguided projectile; (c) the flight in certain kinds of wind tunnel of an unmounted model.
free-flight angle
The angle between the horizontal and a line in the direction of motion of a flying body, especially a rocket, at the beginning of free flight.
free-flight trajectory
The path of a body in free fall.
free-flow area = void fraction.
free gyro
1. A two-degree-of-freedom gyro whose spin axis may be oriented in any specified attitude.
In a free gyro, output signals are produced by gimbal angular displacements which correspond to components of the angular displacement of the base.
2. A gyro not provided with an erection system, i.e., a gyro free to move about its axes.
free jet
A fluid jet without solid boundaries, such as a jet discharging into the open.
free molecule flow
1. A flow regime in aerodynamics in which molecules emitted from an object, as it passes through a resistive medium, do not affect the flow of oncoming molecules by scattering interactions, i.e., the mean free path of the emitted molecules in much longer than a characteristic linear dimension of an object.
2. Flow about a body in which the number of collisions between the molecules of the fluid is negligible compared with the collisions between these molecules and the body. Also called free molecular flow. See rarefied gas dynamics, note.
free oscillation
Oscillation of a system in the absence of external forces.
free progressive wave
A wave in a medium free from boundary effects. A free wave in a steady state can only be approximated in practice. Also called free wave.
free radical
An atom or group of atoms broken away from a stable compound by application of external energy, and, although containing unpaired electrons, remaining free for transitory or longer periods.
Interest centers on three radicals, atomic hydrogen (H), atomic nitrogen (N), and the amine radical (NH). In their free state, they are highly active, combining with each other or with other substances to form other stable molecules, and yielding in the process energies well in excess of those available from conventional chemical fuels. Their use in propulsive systems, depends upon their being isolated and available in bulk, either in pure form or dissolved in a desired concentration in another fuel. See aeroduct.
free sound field = free field.
free space
An ideal, perfectly homogeneous medium possessing a dielectric constant of unity and in which there is nothing to reflect, refract or absorb energy. A perfect vacuum possesses these qualities. Compare homogeneous atmosphere.
Radio signal strength measurements are often expressed in terms of decibels above or below free-space values at a given distance from the transmitter. A free-space radiation pattern would show only the minor and major lobes of the antenna and not the interference pattern normally produced by reflection from the earth's surface.
free stream
1. The stream of fluid outside the region affected by a body in the fluid.
2. Pertaining to the free stream, sense 1, as in free-stream dynamic pressure, free-stream flow, free-stream Mach number, free-stream static pressure, free-stream temperature, free-stream turbulence, free-stream velocity. See remote velocity.
free-stream capture area
The cross-sectional area of a column of air swallowed by a ramjet engine.
free streamline
A streamline separating fluid in motion from fluid at rest.
Both pressure and speed are constant along a free streamline.
free turbine
In a turbine engine, a turbine wheel that drives the output shaft and is not connected to the shaft driving the compressor.
free-vortex compressor
An axial-flow compressor designed so as to impart to the fluid tangential velocities that are inversely proportional to the distance from the axis of rotation, as in a vortex.
free wave
1. Any wave not acted upon by any external force except for the initial force that created it.
2. = free progressive wave.
freezeout method
A method for controlling humidity by passing moist air over a cold surface, thus condensing and freezing out water vapor and possibly carbon dioxide.
The general region of the ionosphere in which the F1-layer and F2-layer tend to form. See ionosphere.
frequency (symbol f)
Of a function periodic in time, the reciprocal of the primitive period. The unit is the cycle per unit time and must be specified.
In the International System the cycle per second is called the hertz (Hz).
frequency assignment
The specific frequency or frequencies authorized by competent authority; expressed for each radio channel by: (a) the authorized carrier frequency, the frequency tolerance, and the authorized emission-bandwidth, (b) the authorized emission-bandwidth in reference to a specific assigned frequency (when a carrier does exist), or (c) the authorized frequency band (when a carrier does not exist).
frequency band
A continuous range of frequencies extending between two limiting frequencies.
Specific frequency bands used in radio and radar are often designated by names, numbers, or letters. The band designations as decided upon by the Atlantic City Radio Convention of 1947 and later modified by Comite Consultatif International Radio (CCIR) Recommendation No. 142 in 1953 were:
Band number Frequency range Metric subdivision waves Atlanctic City frequency subdivision
4 3- 30 Myriametric Very-low VLF
5 30- 300 Kilometric Low LF
6 300- 3,000 Hectometric Medium MF
7 3,000- 30,000 Decametric High HF
8 30- 300 Metric Very-high VHF
9 300- 3,000 Decimetric Ultra-high UHF
10 3,000- 30,000 Centimetric Super-high SHF
11 30,000- 300,000 Millimetric Extremely high EHF
12 300,000- 3,000,000 Decimillimetric -- --
Note that band N extends from 0.3*10N to 3*10N cycles; thus band 4 designates the frequency range 0.3*104 to 3*104 cycles. The upper limit is included in each band; the lower limit is excluded.
Description of bands by means of adjectives is arbitrary and the CCIR recommend that it be discontinued.
The designation ELF, extremely low frequency, has recently been proposed for the band extending from 3 kilocycles down to 1 cycle per second. These frequencies have been used for years in the study of lightning and associated phenomena and may be useful in communicating with spacecraft.
The frequency bands used by radar (radar frequency bands) were first designated by letters for military secrecy. Those designations were:
Frequency band
Approximate frequency range, gigacycles
Approximate wavelength range, centimeters
P-band 0.225 to 0.39 140 to 76.9
L-band 0.39 to 1.55 76.9 to 19.3
S-band 1.55 to 5.20 19.3 to 5.77
X-band 5.20 to 10.90 5.77 to 2.75
K-band 10.90 to 36.00 2.75 to 0.834
Q-band 36.00 to 46.00 0.834 to 0.652
V-band 46.00 to 56.00 0.652 to 0.536
The C-band, 3.9 to 6.2 gigacycles, overlaps the S- and X-bands. These letter designations have no official sanction.
frequency bias
A constant frequency purposely added to the frequency of a signal to prevent the signal frequency from going to zero.
frequency channel
1. The band of frequencies which must be handled by a carrier system to transmit a specific quantity of information.
2. A band of radio frequencies within which a station must maintain its modulated carrier frequency to prevent interference with stations on adjacent channels.
3. Any circuit over which telephone, telegraph, or other signals may be sent by an electric current.
frequency departure
The amount of variation of a carrier frequency or center frequency from its assigned value.
The use of the term frequency deviation in this sense should be discouraged.
frequency deviation
See frequency departure, note.
frequency equation
An equation relating phase speed to wavelength and to the physical parameters of the system in a linear oscillation. Also called dispersion equation.
Mathematically, the frequency equation is the result of substituting a simple harmonic solution in the homogeneous differential equations of motion and the homogeneous boundary conditions. The frequency equation thus describes the free waves of the system. See group velocity.
frequency modulation (abbr FM)
Angle modulation of a sine-wave carrier in which the instantaneous frequency of the modulated wave differs from the carrier frequency by an amount proportional to the instantaneous value of the modulating wave. Compare pulse modulation, amplitude modulation, phase modulation, intensity modulation.
Combinations of phase and frequency modulation are commonly referred to as frequency modulation.
frequency offset transponder
A transponder which changes the signal frequency by a fixed amount before retransmission.
frequency response
1. The portion of the frequency spectrum which can be sensed by a device within specified limits of amplitude error.
2. Response of a system as a function of the frequency of excitation.
frequency-shift keying (abbr FSK)
That form of frequency modulation in which the modulating wave shifts the output frequency between predetermined values, and the output wave is coherent with no phase discontinuity.
frequency swing
In frequency modulation, the peak difference between the maximum and minimum values of the instantaneous frequency.
frequency tolerance
The extent to which a carrier frequency (or when a carrier is not present, a frequency coinciding with the center of an emission bandwidth) is permitted to depart, solely because of frequency instability, from the authorized carrier frequency (or when a carrier is not present from the assigned frequency).
Fresnel region
The region between the antenna and the Fraunhofer region.
If the antenna has a well-defined aperture a in a given aspect, the Fresnel region is that aspect is commonly taken to extend a distance 2aa / in that aspect, being the wavelength.
Fresnel zone
Any one of the array of concentric surfaces in space between transmitter and receiver (or between radar antenna and target) over which the increase in distance over the straight line path is equal to some multiple of one-half wavelength. Also called half-period zone. See interference region.
Outside of rather unusual multipath transmission of radio energy in the free atmosphere, Fresnel zones are of importance primarily in studying the interference lobes produced by the interaction of a direct and a surface-reflected wave. Thus, for a given path, reflected radio energy arriving at the receiver from any point along any of the surface Fresnel zones will be some multiple of 180° out of phase with the direct wave, thereby producing destructive or constructive interference as the multiple is odd or even, respectively.
friction layer = planetary boundary layer.
fringe region
The upper portion of the exosphere, where the cone of escape equals or exceeds 180°. In this region the individual atoms have so little chance of collision that they essentially travel in free orbits, subject to the earth's gravitation, at speeds imparted by the last collision. Also called spray region. See escape velocity.
A powdered ceramic prepared by fusing a physical mixture of oxides into a uniform melt, which is then quenched and milled into a fine, homogeneous powder.
frost point
See dew point, note.
Froude number (symbol NFr)
The nondimensional ratio of the inertial force to the force of gravity for a given fluid flow; the reciprocal of the Reech number. It may be given as
NFr = v2 / lg
where v is a characteristic velocity; l a characteristic length; and g is the acceleration of gravity, or it may be given as the square root of this number.
frozen flow
Flow in which gas composition is invariant throughout the flow field.
frozen-in field
1. A magnetohydrodynamic field in a medium of negligible electrical resistance, in which the motion of the material is along the lines of magnetic force, which are thereby constant or frozen.
2. The entrapment of magnetic field lines by a perfectly conducting fluid.
If a magnetic field is somehow established in a fluid with infinite conductivity, a motion of the fluid will carry the field lines and hence the field energy with it. If the fluid is compressed, the field will be compressed and if turbulence occurs in the fluid, the field will become badly twisted.
frustration threshold
The point at which an individual feels or shows frustration over inability to achieve an objective.
F-scan = F-display.
F-scope = F-display.
FSK (abbr) = frequency-shift keying .
Also called Appleton layer. See ionosphere.
Any substance used to produce heat, either by chemical or nuclear reaction, as used, e.g., in a heat engine. See rocket propellant.
With a liquid-propellant rocket engine, fuel is ordinarily distinguished from oxidizer where these are separate.
fuel cell
1. A fuel tank, especially one of a number of fuel tanks, as in airplane's wing; also, a compartment within a fuel tank.
2. A device which converts chemical energy directly into electrical energy but differing from a storage battery in that the reacting chemicals are supplied continuously as needed to meet output requirements.
fuel consumption
The using of fuel by an engine or power plant; the rate of this consumption, measured, e.g., in gallons or pounds per minute.
fuel cooled
Cooled by fuel. Said of a rocket engine, an oil cooler, etc. See regenerative cooling.
fuel shutoff
The action of shutting off the flow of liquid fuel into a combustion chamber or of stopping the combustion of a solid fuel; the event or time marking this action. Compare cutoff.
fugacity (symbol f)
In thermodynamics, a measure of the tendency of a substance to escape by some chemical process from the phase in which it exists.
full moon
The moon at opposition, with a phase angle of 0°, when it appears as a round disk to an observer on the earth because the illuminated side is toward him. See phases of the moon.
full pressure suit
A suit which completely encloses the body and in which a gas pressure sufficiently above ambient pressure for maintenance of function, may be sustained.
fully ionized plasma
The state of a plasma where all the neutral particles have lost at least one electron.
In the case of hydrogen atoms which have only one electron, no further ionization or excitation is possible when the plasma is fully ionized. For helium and all the heavier gas atoms, there are many bound electrons, each succeeding one requiring more energy to be stripped away from its atom. Therefore, in a fully ionized plasma of these gases, the ions can be further excited and multiply in an ionized state.
A magnitude so related to another magnitude that for any value of one there is corresponding value of the other.
For instance, the area of a circle is a function of its radius. The radius is also a function of the area.
functional reserves
The ability of the body to accomplish additional muscular or other activity and useful work beyond the normal level of activity of an individual.
function table
1. A mathematical chart which lists the values of a dependent quantity in relation to independent variables.
2. A routine by which a computer can determine the value of a dependent quantity from the values of independent variables.
3. A device or circuit which translates information from one representation to another.
fundamental circle = primary great circle.
fundamental frequency
1. Of a periodic quantity, the lowest component frequency of a sinusoidal quantity which has the same period as the periodic quantity.
2. Of an oscillating system, the lowest natural frequency.
The normal mode of vibration associated with this frequency is known as the fundamental mode.
3. The reciprocal of the period of a wave.
fundamental mode of vibration
Of a mechanical system, the mode having the lowest natural frequency.
fundamental star places
The apparent right ascensions and declinations of 1535 standard comparison stars obtained by leading observatories and published annually under the auspices of the International Astronomical Union.
fused ceramic
A ceramic body or coating prepared by heating ceramic powders above the melting point, then cooling to form a coherent mass or film.
An igniter squib for a rocket.
The combining of atoms and consequent release of energy.
fusion power density
The power generated per unit volume in a controlled thermonuclear plasma.
Using a deuterium reaction at a density of 1016 particles per cubic centimeter and a temperature of 60,000 volts, the power density is about 1000 watts per cubic centimeter. The energy comes off as kinetic energy of the reaction products.
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