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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.
- 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
- 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
- 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 and
the elementary charge, e, F = = 9.64870
coulombs per mol. See physical
- 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
- 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
- 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
- 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
- feedback path
- In a feedback
control loop the transmission path from the loop output signal to
the loop feedback
- 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
- 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
- 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 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
- The accuracy to which an electrical system, such as a radio, reproduces at
the essential characteristics of its input
- 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
- 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
- field intensity = field
- field luminance = adaptation
- field strength
- 1. For any physical field, the flux
density, intensity, or
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
- 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
- filter = wave
- 1. The decomposition of a signal into its
- 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
- 2. A projecting flat plate or structure, as a cooling fin.
- final mass
- The mass of a rocket after its
- 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
- 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
- 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
- 2. The action or event of launching a rocket.
- firing chamber = combustion
- 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
du = cvdTwhere 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 + pdvUse of the equation of state yields
an alternative form,
dq = cpdT - dpwhere cp is the specific
heat at constant pressure.
For open systems the variation of total rather
than specific quantities is important:
dQ = dU + pdV - hdmwhere 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
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.
law of thermodynamics, third
law of thermodynamics, energy
- first point of Aries = vernal
- first point of Cancer = summer
- first point of Capricornus = winter
- first point of Libra = autumnal
- 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
- 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
- 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
- flame 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
- 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
- 2. In a captive test, an elbow in the exhaust conduit or flame
bucket that deflects the flame into the open.
- flame tube = inner
- Flamsteed number
- A number sometimes used with the possessive form of the Latin name of the
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
- 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
- A reversal of flame in a system, counter to the usual flow of the
- 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
- 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
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
- 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
- flight Mach number
- A free-stream
Mach number measured in flight as distinguished from one measured in a wind
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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.
- 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
- 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
density.) Compare luminous
- 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
- 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
- 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)
modulation of a carrier by
subcarrier(s) which is (are) frequency modulated by information.
- FM/PM (abbr)
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
- 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.
- 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
- 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
- footward acceleration
- See physiological
- 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
- 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
- forced wave
- See resonance.
- force function
- The negative of potential,
- The relative ease with which a metal can be shaped through plastic
- International Astronomical Union abbreviation for Fornax. See constellation.
- Fornax (abbr For, Forn)
- See constellation.
- forward acceleration
- See physiological
- forward scatter
- The scattering
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
- 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
- Fourier coefficients
- See Fourier
- Fourier integral
- The representation of a function f(x) for all values of x in terms of
infinite integrals in the form
- 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,
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.
(n = 1,2,...); the Fourier integral requires
a continuous spectrum.
- 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
- See Fourier
- 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
- See Fourier
- 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
- 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:
||Line due to
||iron and calcium
||iron (and 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
- 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
- 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
- free-flight trajectory
- The path of a body in free fall.
- free-flow area = void
- 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
- 2. A gyro
not provided with an erection system, i.e., a gyro free to move about its
- 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.
gas dynamics, note.
- free 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
- 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
- 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
- 2. = free
- 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
- 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
- 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
- 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:
Note that band N extends from
0.3*10 to 3*10 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.
||Metric subdivision waves
||Atlanctic City frequency subdivision
- 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
The C-band, 3.9 to 6.2 gigacycles,
overlaps the S- and X-bands. These letter designations have no official
Approximate frequency range, gigacycles
Approximate wavelength range, centimeters
||0.225 to 0.39
||140 to 76.9
||0.39 to 1.55
||76.9 to 19.3
||1.55 to 5.20
||19.3 to 5.77
||5.20 to 10.90
||5.77 to 2.75
||10.90 to 36.00
||2.75 to 0.834
||36.00 to 46.00
||0.834 to 0.652
||46.00 to 56.00
||0.652 to 0.536 |
- 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
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
- 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
- frequency deviation
- See frequency
- frequency equation
- An equation relating phase speed
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
waves of the system. See group
- frequency modulation (abbr FM)
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
- 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
- Fresnel region
- The region between the antenna and the
- 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
- 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
- friction layer = planetary
- 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
- 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
- frost point
- See dew
- 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 / lgwhere 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
- 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
- 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
- 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
- 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
- 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
- fundamental frequency
- 1. Of a periodic quantity, the lowest component frequency of
quantity which has the same period as the
- 2. Of an oscillating system, the lowest natural
- 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
- 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
- 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.