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D

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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dabble
See double-dabble.
Dalton = atomic mass unit.
Dalton law
The empirical generalization that for many so-called perfect gases, a mixture of these gases will have a pressure equal to the sum of the partial pressures that each of the gases would have as sole component with the same volume and temperature, provided there is no chemical interaction.
Dalton law of partial pressure = Dalton law.
damp
To suppress oscillations or disturbances.
damped natural frequency
The frequency of free vibration of a damped linear system.
The oscillation of a damped system may be considered periodic in the limited sense that the time interval between zero crossings in the same direction is constant if the system is linear, even though successive amplitudes decrease progressively. The frequency of the oscillation is the reciprocal of this time interval. The damped natural frequency decreases as the damping increases, and approaches zero as the damping approaches critical damping.
damped wave
Any wave whose amplitude decreases with time or whose total energy decreases by transfer to other portions of the wave spectrum.
damping
The suppression of oscillations or disturbances; the dissipation of energy with time. See viscous damping.
damping factor
The ratio of the amplitude of any one of a series of damped oscillations to that of the following one at the same phase.
damping ratio
The ratio of actual damping to critical damping.
It may be expressed as the ratio of output under static conditions to twice the output at the lowest frequency where a 90° phase shift is observed.
dark adaptation
The process by which the iris and retina of the eye adjust to allow maximum vision in dim illumination, following exposure of the eye to a relatively brighter illumination.
dark blips
See dark trace tube.
dark trace tube
A cathode-ray tube, on which the face is bright, and signals are displayed as dark traces or dark blips.
dart configuration
A configuration of an aerodynamic vehicle in which the control surfaces are at the tail of the vehicle. Contrast canard.
data-acquisition station
A ground station at which various functions to control satellite operations and to obtain data from the satellite are performed.
data link
Any communications channel or circuit used to transmit data from a sensor to a computer, a readout device, or a storage device.
data point
A unit of fundamental information obtained through the processing of raw data.
data processing
Application of procedures, mechanical, electrical, computational, or other, whereby data are changed from one form into another.
data processor
A machine for handling information in a sequence of reasonable operations.
data reduction
Transformation of observed values into useful, ordered, or simplified information.
data smoothing
The mathematical process of fitting a smooth curve to dispersed data points.
datum
Any numerical or geometrical quantity or set of such quantities which can serve as a reference or a base for measurement of other quantities.
For a group of statistical references, the plural form is data; as geographic data for a list of latitudes and longitudes. Where the concept is geometrical the plural form is datums; as in two geodetic datums have been used.
datum line
Any line which can serve as a reference or base for the measurement of other quantities.
datum plane
A plane from which angular or linear measurements are reckoned. Also called reference plane.
datum point
Any point which can serve as a reference or base for the measurement of other quantities.
daughter, daughter element = decay product.
day
1. The duration of one rotation of the earth, or another celestial body, on its axis.
A day is measured by successive transits of a reference point on the celestial sphere over the meridian, and each type takes its name from the reference used. Thus, for a solar day the reference is the sun; a mean solar day if the mean sun; and an apparent solar day if the apparent sun. For a lunar day the reference is the moon; for sidereal day the vernal equinox; for a constituent day an astre fictif or fictitious star. The expression lunar day refers also to the duration of one rotation of the moon with respect to the sun. A Julian day is the consecutive number of each day, beginning with January 1, 4713 BC.
2. A period of 24 hours beginning at a specified time, as the civil day beginning at midnight, or the astronomical day beginning at noon.
daylight saving time
See time.
daytime visual range = visual range.
D-display
In radar, a C-display in which the blips extend vertically to give a rough estimate of distance. Also called D-indicator, D-scan, D-scope.
dead band
An arrangement incorporated in a guidance system which prevents an error from being corrected until that error exceeds a specified magnitude.
dead reckoning (abbr DR)
In navigation, determination of position by advancing a previous known position for courses and distances.
dead spot
In a control system, a region centered about the neutral control position where small movements of the actuator do not produce any response in the system.
dead time
In a radiation counter, the time interval, after the start of a count, during which the counter is insensitive to further ionizing events.
De Broglie wavelength
For a particle of mass m and velocity v , the De Broglie wavelength, = h/mv, where h is Planck constant.
debug
1. To isolate and remove malfunctions from a device, or mistakes from a routine or program.
2. Specifically, in electronic manufacturing, to operate equipment under specified environmental and test conditions in order to eliminate early failures and to stabilize equipment prior to actual use. Also called burn-in.
Debye length
A theoretical length which describes the maximum separation at which a given electron will be influenced by the electric field of a given positive ion. Sometimes referred to as the Debye shielding distance or plasma length.
It is well known that charged particles interact through their own electric fields. In addition, Debye has shown that the attractive force between an electron and ion which would otherwise exist for very large separations is indeed cut off for a critical separation due to the presence of other positive and negative charges in between. This critical separation or Debye length decreases for increased plasma density.
Debye shielding distance = Debye length.
deca = deka.
decade
1. The interval between any two quantities having the ratio of 10:1.
2. A group of series of 10.
decade counter
A counter that counts to 10 in one column of decimal notation; a scale of 10 counter.
decametric wave
See frequency bands.
decay
Decrease of a radioactive substance because of nuclear emission of alpha or beta particles, positrons, or gamma rays. See radioactivity. In beta decay, for example, the emission of a -particle, i.e., an electron, causes radioactive change into a daughter element of the same atomic weight as the parent element but of atomic number higher by 1.
decay constant
1. = attenuation constant.
2. (symbol ) A constant relating the instant rate of radioactive decay of a radioactive species to the number of atoms N present at a given time t. Thus,

-(N / t) = N

If No is the number of atoms present at time zero then

decay product
A nuclide resulting from the radioactive disintegration of a radionuclide, being formed either directly or as the result of successive transformations in a radioactive series. Also called daughter, daughter element.
A decay product may be either radioactive or stable.
decay time
1. In computer operations, the time required for a pulse to fall to one-tenth of its peak value. See rise time.
2. In charge-storage tubes, the time interval during which the magnitude of the stored charge decreases to a stated fraction of its initial value.
The fraction is usually 1/e where e is the base of natural logarithms.
3. Approximately the lifetime of an orbiting object in a nonstable orbit.
Decay time is usually applied only to objects with short orbit lifetimes caused by atmospheric drag.
decayed object
An object once, but no longer, in orbit.
Decca
A long-range, ambiguous, two-dimensional navigation system using continuous-wave transmission to provide hyperbolic lines of position through the radio frequency phase comparison techniques from four transmitters.
Frequency band, 68 to 150 kilocycles.
decelerate
To cause to move slower; to decrease speed.
deceleration
The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.
deceleration parachute
A parachute attached to a craft and deployed to slow the craft, especially during landing. Also called a brake parachute , drogue parachute, parabrake.
December solstice = winter solstice.
deci (abbr d)
A prefix meaning multiplied by 10-1; one-tenth.
decibel (abbr db)
1. A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P1/P2.
2. One-tenth of a bel.
The power P2 may be some reference power; in electricity, the reference power is sometimes taken as 1 milliwatt (abbr dbm); in acoustics, the decibel is often taken as 20 times the common logarithm of the sound pressure ratio, with the reference pressure as 0.0002 dyne per square centimeter.
decibel per second
A unit used to measure the rate of decay of a sound.
decimal coefficient of absorption
See absorption coefficient.
decimal digit
1. One of the digits used in decimal notation, i.e., 1,2,3,4,5,6,7,8,9, or 0.
2. One of 10 possible conditions.
decimal notation
A mathematical system in which each digit is the coefficient of some power of 10.
decimal point
The base point in decimal notation.
decimal-to-binary conversion
The mathematical process of converting a quantity from decimal notation to the equivalent binary notation. For example: 1 = 1; 7 = 111; 23 = 10111, etc. See binary notation.
decimetric wave
See frequency bands.
decimillimetric waves
See frequency bands.
decision element
In computer operations, any device which as the result of the input of data issues one of two or more possible instructions.
declination
1. (symbol ) Angular distance north or south of the celestial equator; the arc of an hour circle between the celestial equator and a point on the celestial sphere, measured northward or southward from the celestial equator through 90 degrees, and labeled N or S to indicate the direction of measurement.
2. (symbol D) Magnetic declination. See equatorial system.
decoder
1. A device for translating electrical signals into predetermined functions.
2. In computer operation, a network or device in which one of two or more possible outputs results from a prescribed combination of inputs. Also called many-to-few matrix.
decommutator
Equipment for separation, demodulation, or demultiplexing commutated signals. See commutator.
decompression
The reduction of atmospheric pressure; particularly, various techniques for preventing caisson disease.
decompression sickness
A disorder experienced by deep sea divers and aviators caused by reduced atmospheric pressure and evolved gas bubbles in the body, marked by pain in the extremities, pain in the chest (chokes), occasionally leading to serve central nervous symptoms and neurocirculatory collapse. See bends, dysbarism.
decontamination
The act of removing chemical, biological, or radiological contamination from, or neutralizing it on, a person, item, or area.
decoupled
Of circuits or devices, interconnected through any means which passes only the static characteristics of a signal.
decrement
A decrease in the value of a variable. See increment.
decrement gage
A gage in which pressure is measured by the rate of decay in amplitude of the oscillations of an element suspended in the gas and set into motion by external controls. Also known as decrement viscosity gage or viscosity manometer.
Various types of decrement gage are distinguished according to the design of the oscillating element.
decrement viscosity gage = decrement gage.
definition
The clarity, fidelity, sharpness, resolution and brilliancy of an image, as a photographic image.
deflagration
A sudden or rapid burning, as opposed to a detonation or explosion.
deflecting force = coriolis force.
deflection-modulated indicator = amplitude-modulated indicator.
deflection of the vertical
The angular difference, at any place, between the direction of plumb line (the vertical) and the perpendicular (the normal) to the reference spheroid. This difference seldom exceeds 30 seconds of arc. Also called station error.
When measured at the earth's surface the deflection of the vertical is equal to the angle between the geoid and the reference spheroid.
deflector
A plate, baffle, or the like that diverts something in its movement or flow; as: (a) a plate that projects into the airstream on the underside of an airfoil to divert the airflow, as into a slot-sometimes distinguished from a spoiler; (b) a conelike device placed or fastened beneath a rocket launched from the vertical position, to deflect the exhaust gases to the sides; (c) any of several different devices used on jet engines to reverse or divert the exhaust gases; (d) a baffle or the like to deflect and mingle fluids prior to combustion, as in certain jet engines.
degas
To remove gas from a material, usually by heating under high vacuum. Compare get.
degassing
The deliberate removal of gas from a material, usually by application of heat under high vacuum.
degauss
Slang for demagnetize.
degradation
Gradual deterioration in performance.
degree of freedom
1. A mode of motion, either angular or linear, with respect to a coordinate system, independent of any other mode.
A body in motion has six possible degrees of freedom, three linear and three angular.
2. Specifically, of a gyro the number of orthogonal axes about which the spin axis is free to rotate.
3. In an unconstrained dynamic or other system, the number of independent variables required to specify completely the state of the system at a given moment.
If the system has constraints, i.e., kinematic or geometric relations between the variables, each such relation reduces by one the number of degrees of freedom of the system. In a continuous medium with given boundary conditions, the number of degrees of freedom is the number of normal modes of oscillation.
4. Of a mechanical system, the minimum number of independent generalized coordinates required to define completely the positions of all parts of the system at any instant of time.
In general, the number of degrees of freedom equals the number of independent generalized displacements that are possible.
Deimos
A satellite of Mars orbiting at a mean distance of 23,500 kilometers.
deka (abbr da)
A prefix meaning multiplied by 10. Sometimes spelled deca.
Del, Dlph
International Astronomical Union abbreviations for Delphinus. See constellation.
de Laval nozzle
[After Dr. Carl Gustaf Patrik de Laval (1845-1913), Swedish engineer.] A converging-diverging nozzle used in certain rockets. Also called Laval nozzle.
delay
The time (or equivalent distance) displacement of some characteristic of a wave relative to the same characteristic of a reference wave; that is, the difference in phase between the two waves. Compare lag.
In one-way radio propagation, for instance, the phase delay of the reflected wave over the direct wave is a measure of the extra distance traveled by the reflected wave in reaching the same receiver.
delayed neutrons
Neutrons emitted by excited nuclei in a radioactive process, so called because they are emitted an appreciable time after the fission. Compare prompt neutrons.
delayed plan position indicator
See plan position indicator.
delay element
A device for causing time delay of a signal. See delay line.
delayer
A substance mixed in with solid rocket propellants to decrease the rate of combustion.
delay line
In electronic computers, any device for producing a time delay of a signal.
delay-line storage
A storage or memory device consisting of a delay line and means for regenerating and reinserting information into the delay line.
Dellinger effect = fadeout.
del-operator (symbol )
The operator used in vector calculus and defined in Cartesian coordinates as
Delphinus (abbr Del, Dlph)
See constellation.
delta ray
1. An electron ejected by recoil when a rapidly moving alpha particle or other charged particle passes through matter.
2. By extension any secondary ionizing particle ejected by recoil when a primary particle passes through matter.
delta wing
A triangularly shaped wing of an aircraft.
deluge collection pond
A facility at a launch site into which used to cool the flame deflector is flushed as the rocket begins its ascent. Also called a skimmer basin.
demand oxygen system = demand system.
demand system
An oxygen system in which oxygen flows to the user during inspiration only.
demodulation
The process of recovering the modulating wave from a modulated carrier.
demodulator
An electronic device which operates on an input of a modulated carrier to recover the modulating wave as an output.
denitrogenation
The removal of nitrogen dissolved in the blood and body tissues, usually by breathing of pure oxygen for an extended period of time in order to prevent aeroembolism at high altitudes.
densitometer
An instrument for the measurement of optical density (photographic transmission, photographic reflection, visual transmission, etc.) of a material, generally of a photographic image.
density function
The number of particles per unit volume. See distribution function.
density specific impulse
The product of the specific impulse of a propellant combination and the average specific gravity of the propellants.
departure = deviation, sense 1.
dependent variable
Any variable considered as a function of other variables, the latter being called independent. Compare parameter.
Whether a given quantity is best treated as a dependent or independent variable depends upon the particular problem.
depletion layer
In a semiconductor, a region in which the mobile carrier charge density is insufficient to neutralize the net fixed charge density of donors and acceptors. Also called barrier.
deploy
Of a parachute, to release so as to let it fill out or to unfold and fill out.
depressed pole
The celestial pole below the horizon, of opposite name to the latitude.
The celestial pole above the horizon is called the elevated pole.
depression angle = angle of depression.
depth perception
The ability to estimate depth or distance between points in the field of vision.
derivative data
Data which have been derived from other data by mathematical techniques.
descending node
That point at which a planet, planetoid, or comet crosses to the south side of the ecliptic; that point at which a satellite crosses to the south side of the equatorial plane of its primary. Also called southbound node. The opposite is ascending node or northbound node.
design gross weight
The gross weight at take-off that an aircraft, rocket, etc., is expected to have, used in design calculations.
desorption
The process of removing sorbed gas.
destruct
The deliberate action of destroying a rocket vehicle after it has been launched, but before it has completed its course.
Destructs are executed when the rocket gets off its plotted course or functions in a way so as to become a hazard.
destruct line
On a rocket test range, a boundary line on each side of the downrange course beyond which a rocket cannot fly without being destroyed under destruct procedures, or a line beyond which the impact point cannot pass. See impact line, command destruct.
detached shock = detached shock wave.
detached shock wave
A shock wave not in contact with the body which originates it. See bow wave. Also called detached shock.
detachment
A particular state of isolation in which man is separated or detached from his accustomed behavioral environment by inordinate physical and psychological distances. This condition may compromise his performance.
detection
See recognition, note.
detector
1. = sensor, sense 1.
2. An instrument employing a sensor, sense 1, to detect the pressure of something in the surrounding environment.
detonation
A rapid chemical reaction which propagates at a supersonic velocity.
detonation wave
A shock wave in a combustible mixture, which originates as a combustion wave.
deuterium (symbol D, d)
A heavy isotope of hydrogen having one proton and one neutron in the nucleus.
The symbol D is often used to designate deuterium in compounds, as HDO for molecules of that composition. Official chemical nomenclature uses the designation d with a number which designates the carbon atom to which deuterium is bound; e.g. 2-d propane designates CH3CHDCH3.
deuteron
The nucleus of a deuterium atom.
deviation
1. In statistics, the difference between two numbers. Also called departure.
It is commonly applied to the difference of a variable from its mean, or to the difference of an observed value from a theoretical value.
2. = magnetic deviation.
3. In radio transmission, the apparent variation of frequency above and below the unmodulated or center frequency.
dew point
The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur; the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Any further cooling usually results in the formation of dew or frost. Also called dewpoint temperature.
When this temperature is below 0° C, it is sometimes called the frost point.
dewpoint temperature = dewpoint.
DF (abbr) = direction finder.
See radio direction finder.
diabatic process
A process in a thermodynamic system in which there is a transfer of heat across the boundaries of the system.
Diabatic process is preferred to nonadiabatic process.
diamonds
The pattern of shock waves often visible in a rocket exhaust which resembles a series of diamond shapes placed end to end.
diaphragm manometer
A displacement manometer employing a flexible diaphragm as the movable partition.
diastolic blood pressure
The pressure exerted by the blood during periods between cardiac contraction.
dichotomy
In astronomy, a configuration of three bodies so that they form a right triangle; specifically, such a configuration in the solar system with the sun at the apex of the 90° angle.
dielectric
A substance that contains few or no free charges and which can support electrostatic stresses.
In an electromagnetic field, the centers of the nonpolar molecules of a dielectric are displaced, and the polar molecules become oriented close to the field. The net effect is the appearance of charges at the boundaries of the dielectric. The frictional work done in orientation absorbs energy from the field which appears as heat. When the field is removed the orientation is lost by thermal agitation and so the energy is not regained. If free-charge carriers are present they too can absorb energy.
A good dielectric is one in which the absorption is a minimum. A vacuum is the only perfect dielectric. The quality of an imperfect dielectric is its dielectric strength; and the accumulation of charges within an imperfect dielectric is termed dielectric absorption.
dielectric absorption
See dielectric.
dielectric constant (symbol )
For a given substance, the ratio of the capacity of a condenser with that substance as dielectric to the capacity of the same condenser with a vacuum for dielectric. It is a measure, therefore, of the amount of electrical charge a given substance can withstand at a given electric field strength; it should not be confused with dielectric strength.
The dielectric constant is a function of temperature and frequency and is written as a complex quantity
where ' is the part that determines the displacement current and '' the dielectric absorption (see dielectric). For a nonabsorbing, nonmagnetic material ' is equal to the square of the index of refraction and the relation holds only at the particular frequency where these conditions apply.
dielectric gradient
The spatial variation of the dielectric constant in a substance or medium.
This term is used frequently in reference to the propagation of radio energy. The magnitude of these gradients and the distance over which they occur, relative to the wavelength of the incident radiation, determine the extent to which targets will reflect radar energy. Sufficiently intense dielectric gradients (or index of refraction gradients) are believed to be the cause of certain echoes known as angels.
dielectric strength
A measure of the resistance of a dielectric to electrical breakdown under the influence of strong electric fields; usually expressed in volts per centimeter. Sometimes called breakdown potential.
difference of latitude
The shorter arc of any meridian between the parallels of two places, expressed in angular measure.
difference of longitude
The smaller angle at the pole or the shorter arc of a parallel between the meridians of two places, expressed in angular measure.
differential analyzer
An analog computer designed and used primarily for solving differential equations.
differential correction
In celestial mechanics, a method for finding from the observed residuals minus the computed residuals (O - C) small corrections which, when applied to the orbital elements or constants, will reduce the deviations from the observed motion to a minimum.
differential manometer
A manometer which indicates the pressure difference across two ports.
differential pressure
The pressure difference between two systems or volumes.
differential thermal analysis
The technique of detecting endothermic and exothermic phase changes and other processes within a heated material by the corresponding temperature changes.
differential transducer
A device which is capable of measuring simultaneously two separate stimulus sources and which provides an output proportional to the difference between the stimuli. See transducer.
differentiator
1. In computer operations, a device whose output is proportional to the derivative of an input signal.
2. In electronics, a transducer whose output waveform is the time derivative of its input waveform.
diffracted wave
A wave whose front has been changed in direction by an obstacle or other nonhomogeneity in a medium, other than by reflection or refraction.
diffraction
The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.
Diffraction is an optical edge effect.
Reference to Huygens' principle is a common means of explaining diffraction. Analysis of the interference between individual Huygens wavelets which originate in the vicinity of the edge of an irradiated body reveals that detectable amounts of radiant energy must invade the nominal shadow zone of the object, and there, by interference, set up characteristic energy distributions known as diffraction patterns. The amount of diffractive bending experienced by a ray is a function of wavelength; thus dispersion occurs, although dispersion is in the opposite sense to that produced by refraction.
diffraction propagation
Wave propagation around objects, or over the horizon, by diffraction.
Diffraction is due to the fact that from every point in a wave front a spherical front is generated which falls off in intensity away from the forward direction. A continuous series of such actions carries radiation around objects, or around the curvature of the earth, but with rapidly diminishing intensity.
diffuser
A specially designed duct, chamber, or section, sometimes equipped with guide vanes, that decreases the velocity of a fluid, as air, and increases its pressure, as in a jet engine, a wind tunnel, etc. See supersonic diffuser.
diffuse radiation
Radiant energy propagating in many different directions through a given small volume of space; to be contrasted with parallel radiation.
The ideal form of diffuse radiation is isotropic radiation. Careful distinction should be made between this concept and that of a perfectly diffuse radiator.
diffuse reflection
Any reflection process in which the reflected radiation is sent out in many directions usually bearing no simple relationship to the angle of incidence; the opposite of specular reflection. See diffuse reflector, perfectly diffuse reflector.
A term frequently applied to the process by which solar radiation is scattered by dust and other suspensoids in the atmosphere. See diffuse sky radiation.
diffuse reflector
Any surface which reflects incident rays in a mutliplicity of directions, either because of irregularities in the surface or because the material is optically inhomogeneous, as a paint, although optically smooth; the opposite of a specular reflector. See perfectly diffuse reflector.
Ordinary writing papers are good examples of diffuse reflectors, where as mirrors or highly polished metal plates are examples of specular reflectors. Almost all terrestrial surfaces (except calm water) act as diffuse reflectors of incident solar radiation.
diffuse skylight = diffuse sky radiation.
diffuse sky radiation
Solar radiation reaching the earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. Also called skylight, diffuse skylight, sky radiation.
Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25 percent of the incident radiation), about two-thirds ultimately reaches the earth as diffuse sky radiation.
diffuse sound
Sound energy for which energy is uniform in the region considered and when all directions of energy flux at all parts of the region are equally probable.
diffusion
In an atmosphere, or in any gaseous system, the exchange of fluid parcels between regions, in apparently random motions of a scale too small to be treated by the equations of motion.
2. In materials, the movement of atoms of one material into the crystal lattice of an adjoining material, e.g.,penetration of the atoms in a ceramic coating into the lattice of the protected metal.
3. In ion engines, the migration of neutral atoms through a porous structure incident to ionization at the emitting surface.
diffusion coefficient
The absolute value of the ratio of the molecular flux per unit area to the concentration gradient of a gas diffusing through a gas or a porous medium where the molecular flux is evaluated across a surface perpendicular to the direction of the concentration gradient. See diffusion, coefficient of mutual diffusion.
diffusion equation
See diffusivity.
diffusion velocity
1. The relative mean molecular velocity of a selected gas undergoing diffusion in a gaseous atmosphere, commonly taken as a nitrogen (N2) atmosphere.
The diffusion velocity is a molecular phenomenon and depends upon the gaseous concentration as well as upon the pressure and temperature gradients present.
2. The velocity or speed with which a turbulent diffusion process proceeds as evidenced by the motion of individual eddies.
diffusive equilibrium
The steady state resulting from the diffusion process, primarily of interest when external forces or sources and sinks exist within the field. See isothermal equilibrium.
diffusivity
A measure of the rate of diffusion of a substance, expressed as the diffusivity coefficient K. When K is constant, the diffusion equation is
where q is the substance diffused; 2 is the Laplacian operator; and t is time. The diffusivity has dimensions of a length times a velocity; it varies with the property diffused, and for any given property it may be considered a constant or a function of temperature, space, etc., depending on the context. Also called coefficient of diffusion. See conductivity, kinematic viscosity, exchange coefficients.
In the case of molecular diffusion the length dimension is the mean free path of the molecules. By analogy, in eddy diffusion, length becomes the mixing length. The coefficient is the called then eddy diffusivity, and is in general several orders of magnitude larger than the molecular diffusivity.
difluence
The divergence vector of adjacent streamlines. The opposite of confluence.
digit
1. A single symbol or character representing an integral quantity.
2. Any one of the symbols used in positional notation as coefficients of each power, or order, of the base. See binary digit, decimal digit.
digital
Using discrete expressions to represent variables.
digital computer
A computer which operates with information, numerical or otherwise, represented in a digital form.
digital output
Transducer output that represents the magnitude of the stimulus in the form of a series of discrete quantities coded to represent digits in a system of notation. Compare analog output.
digitize
To express an analog measurement of a variable in discrete units.
digitizer
A device which converts analog data into numbers expressed in digits in a system of notation. Also called analog-to-digital converter.
dihedral angle
The acute angle between two intersecting planes or between lines representative of planes.
D-indicator = D-display.
Dione
A satellite of Saturn orbiting at a mean distance of 378,000 kilometers.
dioptric light
A light concentrated into a collimated beam by means of refracting lenses or prisms.
One collimated by means of a reflector is a catoptric light.
dip
1. = dip angle.
2. = magnetic dip.
dip angle (symbol )
The vertical angle between the true horizon and the apparent horizon.
dip equator = aclinic line.
diplexer
A device permitting an antenna system to be used simultaneously or separated by two transmitters. Compare duplexer.
diplex transmission
The simultaneous transmission of two signals using a common carrier wave. Compare duplex operation, multiplexing.
dipole
1. A system composed of two, separated, equal electric or magnetic charges of opposite sign.
2. = dipole antenna.
dipole antenna
A straight radiator, usually fed in the center, and producing a maximum of radiation in the plane normal to its axis. The length specified is the overall length.
Common usage in microwave antennas considers a dipole to be a metal radiating structure which supports a line current distribution similar to that of a thin straight wire, a half wavelength long, so energized that the current has two nodes, one at each of the far ends.
dig pole = magnetic pole.
direct air cycle
A thermodynamic propulsion cycle involving a nuclear reactor and gas turbine or ramjet engine, in which air is the working fluid. Also called direct cycle.
Air is successively compressed in the compressor section, heated in the nuclear reactor, and expelled through the turbine-tailpipe section to obtain thrust.
direct-current discharge
The conduction of direct current through two electrodes immersed in a gas. See Townsend discharge, glow discharge, arc discharge.
direct cycle = direct air cycle.
directional antenna
An antenna that radiates or receives radio signals more efficiently in some directions than in others. See Adcock antenna, loop antenna, sense antenna.
A group of antennas arranged for this purpose is called an antenna array.
directional array = antenna array.
directional emittance
Emittance in a stated direction from a surface. The direction is usually specified as angle from the normal.
directional gyro
1. A two-degree-of-freedom gyro with a provision for maintaining its spin axis approximately horizontal.
2. A flight instrument incorporating a gyro that holds its position in azimuth and thus can be used as a directional reference.
directional properties
1. Of metals, properties whose magnitude varies depending on the relation of the test axis to a specific direction within the metal. The variation results from preferred orientation or from fibering of constituents or inclusions.
2. For thermal radiation properties, in a specified direction from the surface, usually measured as the angle from the normal.
directional stability
The property of an aircraft, rocket, etc., enabling it to restore itself from a yawing or sideslipping condition. Also called weathercock stability.
direction angle
In tracking, the angle between the antenna baseline and an imaginary line connecting the center of the baseline with the target.
direction cosine
1. The cosine of the angle formed by the intersection of a line, as a line of sight to an orbiting body, with an axis of a rectangular coordinate system with the origin on the line.
Every line has three direction angles and three direction cosines: 1, m, n corresponding to , , , the direction angles with the x, y, and z axes.
2. Specifically, in tracking, the cosine of the angle between a baseline and the line connecting the center of the baseline with the target.
direction finder (abbr DF) = radio direction finder.
direction of relative movement
See relative movement.
directivity
The ability of an antenna to radiate or receive more energy in some directions than in others. See beam.
The directivity of an antenna implies a maximum value, and it is equal to the ratio of the maximum field intensity to the average field intensity at a given distance.
direct motion
Eastward or counterclockwise motion of a planet or other object as seen from the North Pole (motion in the direction of increasing right ascension).
direct product = scalar product.
directrix
An auxiliary line used in the geometrical construction of a conic.
direct solar radiation
In actinometry, that portion of the radiant energy received at the instrument direct from the sun, as distinguished from diffuse sky radiation, effective terrestrial radiation, or radiation from any other source. See global radiation.
Direct solar radiation is measured by pyrheliometers.
direct wave
A radio wave which travels directly from the transmitting antenna to the receiving antenna, in contrast with an indirect wave, which undergoes an abrupt change of direction by refraction or reflection.
discharge correction factor (symbol d)
Of a rocket nozzle, the ratio of the mass flow rate in the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressure.
discharge tube
A form of cold-cathode ionization gage in which the color and form of a cold-cathode discharge, without the presence of a magnetic field, gives an indication of the pressure and the nature of the gas.
discone antenna
An antenna formed of a disk and a cone whose apex approaches and becomes common with the outer conductor or the coaxial feed at its extremity.
The center conductor terminates at the center of the disk which is perpendicular to the axis of the cone.
discontinuity
A break in sequence or continuity of anything.
discrete
Composed of distinct or discontinuous elements.
discrete radio source
A source of small angular extent of cosmic radio waves.
discrete spectrum
A spectrum in which the component wavelengths (and wave numbers and frequencies) constitute a discrete sequence of values (finite or infinite in number) rather than a continuum of values. See continuous spectrum.
discrete variable
A quantity that may assume any one of a number of individually distinct or separate values.
discriminator
In general, a circuit in which output depends upon the difference between an input signal and a reference signal.
dish
A parabolic reflector type of radio or radar antenna.
dislocation
In crystallography, a type of lattice imperfection whose existence in metals is postulated in order to account for the phenomenon of crystal growth and of slip, particularly for the low value of shear stress required to initiate slip.
One section of the crystal adjacent to the slip plane is assumed to contain one more atomic plane that the section on the opposite side of the slip plane. Motion of the dislocation results in displacement of one of the sections with respect to another.
dispersion
1. In rocketry (a) deviation from a prescribed flight path, (b) specifically, circular dispersion.
2. A measure of the scatter of data points around a mean value or around a regression curve.
Usually expressed as a standard-deviation estimate, or as a standard error of estimate. Note that the scatter is not centered around the true value unless systematic errors are zero.
3. The process in which radiation is separated into its component wavelengths.
Dispersion results when an optical process, such as diffraction, refraction, or scattering, varies according to wavelength.
4. In spectroscopy, a measure of the resolving power of a spectroscope or spectrograph, usually expressed in angstroms per millimeter.
5. As applied to materials, a scattering of very fine particles (e.g., ceramics) within the body of a metallic material usually resulting in overall strengthening of the composite material.
dispersion equation = frequency equation.
dispersive medium
A medium in which the phase velocity of a wave, either electromagnetic or hydromagnetic, is a function of the frequency.
A plasma is a dispersive medium whereas free space is not, since waves of all frequencies travel in free space with the velocity of light.
displacement
A vector quantity that specifies the change of position of a body or particle usually measured from the mean position or position of rest.
Displacement can be represented by a rotation vector or translation vector or both.
displacement manometer
A differential manometer which indicates the pressure difference, if any, across a solid or liquid partition which can be displaced against a restoring force.
display
The graphic presentation of the output data of any device or system.
dissociation
The separation of a complex molecule into constituents by collision with a second body, or by absorption of a photon.
The product of dissociation of a molecule is two ions, one positively charged and one negatively charged.
distance marker
A reference marker indicating distance, particularly such a marker on a radar indicator, to indicate distance of a target from the radar antenna. On a plan position indicator it is usually one of a series of concentric circles. Also called range marker. See range ring.
distance measuring equipment (abbr DME)
A radio aid to navigation which provides distance information by measuring total round-trip time of transmission from an interrogator to a transponder and return.
distorted-angle fabric
Material of a special, often basketlike weave suitable for pressure suits. Such fabrics permit a certain amount of flexibility when the suit is pressurized.
distortion
1. An undesired change in waveform.
Noise and certain desired changes in waveform, such as those resulting from modulation or detection, are not usually classed as distortion.
2. In a system used for transmission or reproduction of sound, a failure by the system to transmit or reproduce a received waveform with exactness.
3. An undesired change in the dimensions or shape of a structure as, distortion of a fuel tank due to abnormal stresses or extreme temperature gradients.
distribution function
The density function or number of particles per unit volume of phase space. The distribution function is a function of the three space coordinates and the three velocity coordinates.
A point in phase space represents a given position in ordinary space and a given velocity in velocity space. Therefore, the distribution function evaluated at such a point is the number or average density of particles per cubic length and cubic velocity that have the position and the velocity which is represented by the point. Distribution function represents the average density over a reasonably long time, or the most probable distribution of particles at a particular time.
diurnal
Having a period of, occurring in, or related to a day.
diurnal aberration
Aberration caused by the rotation of the earth. The value of diurnal aberration varies with the latitude of the observer and ranges from zero at the poles to 0.31 second of arc.
diurnal circle
The apparent daily path of a celestial body, approximating a parallel of declination.
diurnal motion
The apparent daily motion of a celestial body as observed from a rotating body.
divergence
1. The expansion or spreading out of a vector field; also a precise measure thereof.
In mathematical discussion divergence is considered to include convergence, i.e., negative divergence.
2. A static instability of a lifting surface or of a body on a vehicle wherein the aerodynamic loads tending to deform the surface or body are greater than the elastic restoring forces.
divergence theorem
The statement that the volume integral of the divergence of a vector, such as the velocity U, over the volume V is equal to the surface integral of the normal component of U over the surface s of the volume, often called the export through the closed surface, provided U and its derivatives are continuous and single-valued throughout V and s. This may be written
where n is a unit vector normal to the element of surface ds, and the symbol indicates that the integration is to be carried out over a closed surface. This theorem is sometimes called Green's theorem in the plane for the case of two-dimensional flow, and Green's theorem in space for the three-dimensional case described above. Also called Gauss theorem.
The divergence theorem is used extensively in manipulating the meteorological equations of motion and aerodynamic equations of motion.
D-layer
See ionosphere.
DME (abbr) = distance measuring equipment.
Dobson spectrophotometer
A photoelectric spectrophotometer which is used in the determination of the ozone content of the atmosphere. The instrument compares the solar energy at two wavelengths in the absorption band of ozone by permitting the two radiations to fall alternately upon a photocell. The stronger radiation is then attenuated by an optical wedge until the photoelectric system of the photometer indicates equality of incident radiation. The ratio of radiation intensity is obtained by this process and the ozone content of the atmosphere is computed from the ratio.
docking
The act of coupling two or more orbiting objects; the operation of mechanically connecting together, or in some manner bringing together, orbital payloads.
doghouse
Slang for a protuberance or blister that houses an instrument or instruments on an otherwise smooth skin of a rocket.
dogleg
A directional turn made in the launch trajectory to produce a more favorable orbit inclination, as in Echo I was launched on a dogleg to achieve an orbit inclined 47° to the equator.
donor
In transistors, the N-type semiconductor, the electrode containing impurities which increase the number of available electrons. Contrast acceptor.
doping
Addition of impurities to a semiconductor or production of a deviation from stoichiometric composition to achieve a desired characteristic.
Doppler broadening
The broadening of either an emission line or an absorption line due to random motions of molecules of the gas that is emitting or absorbing the radiant energy. See pressure broadening.
In the case of an emitting gas, for example, those molecules which are approaching the observer as they emit quanta of radiant energy will, because of the Doppler effect, appear to send out a train of waves of slightly shorter wavelength than that characteristic of a stationary molecule, while receding molecules will appear to emit slightly longer wavelengths. The net effect, averaged over many molecules, is to superimpose, on the natural line width, a bell-shaped broadening that is proportional to the square root of the absolute temperature of the gas.
Doppler effect
The change in frequency with which energy reaches a receiver when the receiver and the energy source are in motion relative to each other. Also called Doppler shift.
In the case of sound, or any other wave motion where a real medium of propagation exists (excepting, therefore, light and other electromagnetic radiations) one must distinguish two principal cases: If the source is in motion with speed v relative to a medium which propagates the waves in question at speed c, then the resting observer receives waves emitted, with actual frequency f as if they had a frequency f' given by the Doppler equation
f ' = f/[1 ± (v/c)]
where the positive sign refers to the case of the source receding from the observer, and the vice versa for the negative sign. If, on the other hand, the source is at rest relative to the propagating medium while the observer moves with speed v relative to the source,
f ' = f '[1± (v/c)]
where the positive sign now refers to the case of observer approaching the source. For electromagnetic radiation, f/f ' = [1 (v/c)]/[1 ± (v/c)] where the top signs represent the source receding from the observer and the bottom signs, approaching the observer.
Doppler error
In using Doppler radar, the error in the measurement of target radial velocities due to atmospheric refraction. Compare range error, azimuth error.
These errors result from (a) the assumption of a constant wave velocity for a nonhomogeneous atmosphere and (b) the refraction or bending of the rays such that the ray path does not coincide with the geometrical straight line between the target and the radar. Errors due to (a) are of no practical importance, and, as in the case of elevation-angle error, the effects due to (b) are negligible except for elevation angles near the horizontal.
Doppler-Fizeau effect
The Doppler effect applied to a source of light. When the distance between the observer and the source of light is diminishing, the lines of the spectrum are displaced toward the violet, and, when the distance is increasing, they are displace toward the red, the displacement being proportional to the relative velocity of approach or recession.
Doppler navigation
Dead reckoning performed automatically by a device which gives a continuous indication of position by integrating the speed derived from measurement of the Doppler effect of echoes from directed beams of radiant energy transmitted from the craft. See Doppler radar.
Doppler radar
A radar which detects and interprets the Doppler effect in terms of the radial velocity of a target.
Doppler ranging (abbr Doran)
A continuous-wave trajectory measuring system which utilizes the Doppler effect to measure the distances between a transmitter, a rocket transponder, and several receiving stations.
From these measurements trajectory data are computed. In contrast to less sophisticated systems, Doran obviates the necessity of continuously recording the Doppler signal by making simultaneous distance measurements with four different frequencies.
Doppler shift
1. = Doppler effect.
2. The magnitude of the Doppler effect, measured in cycles per second.
Doppler system
In radar, any system utilizing the Doppler effect for obtaining information.
Doppler, velocity and position (abbr Dovap)
A continuous-wave trajectory measuring system using the Doppler effect caused by a target moving relative to a ground transmitter and receiving stations.
The transmitter interrogates a frequency doubling transponder and the output is received at three or more receiver sites for comparison with the interrogation frequency. The intersection of ellipsoids formed by the transmitter and each receiver site provides the spatial position of the target.
Dor, Dora
International Astronomical Union abbreviations for Dorado. See constellation.
Dorado (abbr Dor, Dora)
See constellation.
Doran (abbr) = Doppler ranging.
dorsal
Pertaining to the back.
dosimeter
1. An instrument for measuring the ultraviolet in solar and sky radiation. Compare actinometer.
2. A device, worn by persons working around radioactive material, which indicates the dose of radiation to which they have been exposed.
dot product = scalar product.
double amplitude
In vibration terminology, the total, or peak-to-peak, dimensional displacement of a vibrating structure.
double-base propellant
A solid rocket propellant using two unstable compounds, such as nitrocellulose and nitroglycerin.
The unstable compounds used in a double-base propellant do not require a separate oxidizer.
double-dabble
A technique for binary to decimal conversion. Starting with the most significant bit, proceed, bit-by-bit, as follows: If the next bit is 0, double what you have (double); if the next bit is 1, double what you have and add 1 (dabble).
Thus, 111 (binary) = 7 (decimal)
10111 (binary) = 23 (decimal).
double-entry compressor
A centrifugal compressor that takes in air or fluid on both sides of the impeller, with vanes on each side to accelerate the fluid into the diffuser. The double-entry compressor is not a multistage compressor.
double-integrating gyro
A single-degree-of-freedom gyro having essentially no restraint of its spin axis about the output axis. In this gyro an output signal is produced by gimbal angular displacement, relative to the base, which is proportional to the double integral of the angular rate of the base about the input axis.
double local oscillator
An oscillator mixing system which generates two radio-frequency signals accurately spaced a few hundred cycles apart and mixes these signals to give the difference frequency which is used as the reference.
This equipment is used in an interferometer system to obtain a detectable signal containing the phase information of an antenna pair and the reference signal to allow removal of the phase data for use.
double precision
Of a computer, capable of processing twice as many columns as the number of digits in the quantities usually processed. See precision.
double sheath
See plasma sheath.
double stars
Stars which appear as single points of light to the eye but which can be resolved into two point by a telescope.
A double star is not necessarily a binary, a two-star system revolving about a common center, but may be an optical double, two unconnected stars in the same line of sight.
doublet
1. In fluid mechanics, a source and a sink of equal strength whose distance apart is zero.
2. In spectroscopy, two lines resulting from transitions from the same state.
Dovap (abbr) = Doppler, velocity and position.
Dovap elsse
An elsse which utilizes the Dovap transponder as a signal source. Also called beat-beat Dovap.
down range
The airspace extending downstream on a given rocket test range.
downtime
A period during which equipment is not operating correctly because of machine failure.
DR (abbr) = dead reckoning.
Dra, Drac
International Astronomical Union abbreviation for Draco. See constellation.
Draco (abbr Dra, Drac)
See constellation.
dracontic month
The average period of revolution of the moon about the earth with respect to the moon's ascending node, a period of 27 days 5 hours 5 minutes 35.8 seconds, or approximately 27¼ days. Also called nodical month.
drag (symbol D)
A retarding force acting upon a body in motion through a fluid, parallel to the direction of motion of the body. It is a component of the total fluid forces acting on the body. See aerodynamic force.
drag coefficient (symbol CD)
A coefficient representing the drag on a given airfoil or other body, or a coefficient representing a particular element of drag. See Rayleigh formula.
drag parachute
1. = drogue parachute.
2. Any of various types of parachutes attached to high-performance aircraft that can be deployed, usually during landings, to decrease speed and also, under certain flight conditions, to control and stabilize the aircraft.
draperies (abbr D)
See aurora.
D-region
See ionosphere.
drift
1. The lateral divergence from the prescribed flight path of an aircraft, a rocket, or the like, due primarily to the effect of a crosswind.
2. A slow movement in one direction of an instrument pointer or other marker.
3. A slow change in frequency of a radio transmitter.
4. The angular deviation of the spin axis of a gyro from a fixed reference in space.
5. In semiconductors, the movement of carriers in an electric field.
drift mobility
In a semiconductor, the average drift velocity of carriers per unit electric field.
In general, the mobilities of holes and electrons are different.
drift rate
The amount of drift, in any of its several senses, per unit time.
Drift rate has many specific meanings in different fields. The type of drift rate should always be specified.
drift velocity
The average velocity of a charged particle in a plasma in response to an applied electric field.
The motion of an individual particle is quite erratic as it bounces off other particles and has its direction continually changed. On the average, however, a particle will slowly work its way in the direction of the applied electric force. This velocity is usually much smaller than the random velocity of the particle between collisions.
drogue
1. A device, usually shaped like a funnel or cone, dragged or towed behind something and used, e.g., as a sea anchor.
2. A funnel-shaped part at the end of the hose of a tanker aircraft, used in air refueling to drag the hose out and stabilize it and to receive the probe of the receiving aircraft.
3. = drogue parachute.
drogue parachute
1. A type of parachute attached to a body, used to slow it down; also called deceleration parachute or drag parachute.
2. A parachute used specifically to pull something, usually a larger parachute, out of stowage, as, a drogue parachute deploys a drag parachute.
drogue recovery
A type of recovery system for space vehicles or space capsules after initial reentry into the atmosphere using deployment of on ore more small parachutes to diminish speed, to reduce aerodynamic heating, and to stabilize the vehicle so that larger recovery parachutes can be safely deployed at lower altitudes without too great an opening shock.
drone
A remotely controlled aircraft.
dropout
Any discrete variation in signal level during the reproduction of recorded data which results in a data-reduction error.
dropsonde
A radiosonde equipped with a parachute, dropped from an aircraft to transmit measurements of atmospheric conditions as it descends.
dry adiabat
A line of constant potential temperature on a thermodynamic diagram. In terms of pressure p , and specific volume v , the equation for a dry adiabat may be written
pvcp /cv = Constant
where cp and cv are the specific heats of dry air at constant pressure and volume, respectively.
Meteorologically, the dry adiabat is intended to represent the lifting of dry air in a dry-adiabatic process. Since this is also an isentropic process, a dry adiabat is an isentrope.
dry-adiabatic atmosphere = adiabatic atmosphere.
dry-adiabatic lapse rate
The rate of decrease of temperature with height of a parcel of dry air lifted adiabatically through the earth's atmosphere in hydrostatic equilibrium.
This lapse rate is g/cp, where g is the acceleration of gravity and cp is the specific heat of dry air at constant pressure; numerically equal to 9.767° C per kilometer or about 5.4° F per thousand feet.
Potential temperature is constant with height in an atmosphere with this lapse rate.
dry emplacement
A launch emplacement that has no provision for water cooling during launch. Compare wet emplacement.
dry friction damping = coulomb damping.
dry weight
The weight of a rocket vehicle without its fuel. Compare take-off weight.
This term, appropriate especially for liquid rockets, is sometimes considered to include the payload.
D-scan = D-display.
D-scope = D-display.
DSIF (abbr)
Deep Space Instrumentation Facility. A worldwide network of tracking stations operated for the NASA by the Jet Propulsion Laboratory.
dual modulation
The process of modulating a single carrier wave or subcarrier by two different types of modulation (e.g., amplitude and frequency-modulation) each conveying separate information.
dual thrust
A rocket thrust derived from two propellant grains using the same propulsion section of a missile.
The dual-thrust technique is considered to provide what is in effect a two-stage propulsion system without the disadvantages of jettisoning the booster unit and with the advantages of lower weight and shorter length.
dual-thrust motor
A solid-propellant rocket engine built to obtain dual thrust.
In a single chamber unit the booster propellant grain may be bonded to the sustainer grain, with the thrust level regulated by mechanically changing the nozzle throat area or by using different grain compositions or configurations. In a dual-chamber unit, the separate chambers may be arranged in tandem, or concentrically.
duct
Specifically, a tube or passage that confines and conducts a fluid, as a passage for the flow of air to the compressor of a gas-turbine engine, a pipe leading air to a supercharger, etc.
ducted fan
1. A fan enclosed in a duct.
2. = ducted-fan engine.
ducted-fan engine
An aircraft engine incorporating a fan or propeller enclosed in a duct; especially, a jet engine in which an enclosed fan or propellant is used to ingest ambient air to augment the gases of combustion in the jetstream.
The air may be taken in at the front of the engine and passed around the combustion section, or it may be taken in aft of the combustion chamber. In the former case the ducted fan may be considered a type of bypass engine.
ducted rocket = rocket ramjet.
ducting
The trapping of an electromagnetic wave, in a waveguide action, between two layers of the atmosphere, or between a layer of the atmosphere and the earth's surface. See refractivity, total refraction.
Ducting is likely to occur where the gradient of the index of refraction exceeds 48 N-units per 1000 feet of altitude.
duct propulsion
A means of propelling a vehicle by ducting a surrounding fluid through an engine, adding momentum by mechanical or thermal means, and ejecting the fluid to obtain a reactive force. Compare rocket propulsion.
dummy
1. In computer operations, an artificial and intrinsically useless unit of information inserted solely to fulfill certain prescribed conditions such as word length or block length.
2. In rocketry, an inert stage, i.e., no propellant.
dummy antenna
A device which has the necessary impedance characteristics of an antenna and the necessary power-handling capabilities, but which does not radiate or receive radio waves. Also called artificial antenna.
In receiver practice, that portion of the impedance not included in the signal generator is often called dummy antenna.
dump
In computer operations, (a) to destroy intentionally or accidentally stored information, (b) to transfer all or part of the contents of one section of storage into another section.
duplexer
A device which permits a single antenna system to be used for both transmitting and receiving.
Duplexer should not be confused with diplexer, a device permitting an antenna system to be used simultaneously or separately by two transmitters.
duplex operation
The operation of associated transmitting and receiving apparatus in which the processes of transmission and reception are concurrent. Compare diplex transmission.
dust
In meteor terminology, finely divided solid matter, with particle sizes in general smaller than micrometeorites, as meteoric dust, meteoritic dust.
duty factor
1. In computer operations, the ratio of active time to total time.
2. In a pulse carrier composed of pulses that recur at regular intervals, the product of the pulse duration and the pulse repetition frequency.
duty ratio
In a pulse radar or similar system the ratio of average to peak pulse power.
dye marker
A substance which, when placed in water, spreads out and colors the water immediately surrounding so as to make a spot readily visible from the air.
dynamical friction
1. The drag force between electrons and ions drifting with respect to each other.
In a fully ionized plasma, collisions between electrons and ions are small angle Coulomb collisions and they produce a velocity-dependent drag force which is called dynamical friction.
2. Sliding friction, in contrast to static friction.
dynamical mean sun
A fictitious sun conceived to move eastward along the ecliptic at the average rate of the apparent sun. See mean sun.
The dynamical mean sun and the apparent sun occupy the same position in January, when the earth is at perihelion.
dynamic balance
The condition which exists in a rotating body when the axis about which it is forced to rotate, or to which reference is made, is parallel with a principal axis of inertia. No products of inertia about the center of gravity of the body exist in relation to the selected rotational axis.
Dynamic unbalance may be expressed in terms of slug-foot squared (or equivalent weight * length squared units) or in terms of inclination of the principal axis from the reference axis.
dynamic height
The height of a point in the atmosphere expressed in a unit proportional to the geopotential at that point. Since the geopotential at altitude z is numerically equal to the work done when a particle of unit mass is lifted from sea level up to this height, the dimensions of dynamic height are those of potential energy per unit mass. Also called geodynamic height.
The standard unit of dynamic height Hd is the dynamic meter (or geodynamic meter), defined as 10 meters per second squared; it is related to the geopotential , the geometric height z in meters, and the geopotential height Z in geopotential meters by
d= 10d Hd = 9.8dZ = gdz
where g is the acceleration of gravity in meters per second squared. (Some sources prefer to give the constants 10 and 9.8 the units of meters per second squared so that the units of and Z would be the same as those of the geometric height.) The dynamic meter is about 2 percent longer than the geometric meter and the geopotential meter. One of the practical advantages of the dynamic height over the geometric height is that when the former is introduced into the hydrostatic equation the variable acceleration of gravity is eliminated. In meteorological height calculations, geopotential height is more often used than dynamic height.
dynamic load
A load imposed by dynamic action, as distinguished from a static load. Specifically, with respect to aircraft, rockets, or spacecraft, a load due to an acceleration of craft, as imposed by gusts, by maneuvering, by landing, by firing rockets, etc.
dynamic meteorology
The study of atmospheric motions as solutions of the fundamental equations of hydrodynamics or other systems of equation appropriate to special situations.
dynamic meter
The unit of measurement of dynamic height. Also called geodynamic meter.
dynamic model
A model of an aircraft or other object having its linear dimensions and its weight and moments of inertia reproduced in scale in proportion to the original.
dynamic parallax
A value for the parallax of a binary star computed from the observations of the period and angular dimensions of the orbit by assuming a value for the mass of the binary system. Also called hypothetical parallax.
dynamic pressure (symbol q)
The pressure of a fluid resulting from its motion, equal to one-half the fluid density times the fluid velocity squared (1/2pV2). In incompressible flow, dynamic pressure is the difference between total pressure and static pressure. Also called kinetic pressure. Compare impact pressure.
dynamic response = frequency response.
dynamic scale
The scale of the flow about a model relative to a flow about its prototype.
If two such flows have the same Reynolds number, both flows are said to be at the same dynamic scale.
dynamic similarity
The relationship existing between a model and its prototype when, by virtue of similarity between their geometric dimensions and mass distributions or elastic characteristics, the motion of the model in some respect (such as linear velocity, acceleration, vibration, flutter, etc.) is similar to the motion of the prototype; also, the similarity between the fluid flow about a scale model and its prototype when the flows have the same Reynolds number.
dynamic stability
The characteristics of a body, such as an aircraft or rocket, that causes it, when disturbed from an original state of steady flight or motion, to damp the oscillations set up by restoring moments and gradually return to its original state; specifically, the aerodynamic characteristics.
dynamic storage
In computer operations, information storage in which the information is continuously changing position, as, for example, delay-line storage, or magnetic-drum storage.
In computer programming, memory allocated and deallocated as needed on the fly, also dynamic memory allocation.
dynamic viscosity
Of a fluid, the ratio of the shearing stress to the shear of the motion. It is independent of the velocity distribution, the dimensions of the system, etc., and for a gas it is independent of pressure except at very low pressures. Also called coefficient of molecular viscosity, coefficient of viscosity.
For the dynamic viscosity µ of a perfect gas, the kinetic theory of gases gives
where is the gas density, c is the average speed of the random heat motion of the gas molecules and is proportional to the square root of the temperature, and is the mean free path. For dry air at 0° C, the dynamic viscosity is about 1.7 * 10-4 gram per centimeter per second.
Whereas the dynamic viscosity of most gases increases with increasing temperature, that of most liquids, including water, decreases rapidly with increasing temperature.
dynamometer
An instrument for measuring power or force; specifically, an instrument for measuring the power, torque, or thrust of an aircraft engine or rocket. See thrust meter.
dynamo theory
The hypothesis, first proposed by Balfour Stewart, which explains the regular daily variations in the earth's magnetic field in terms of electrical currents in the lower ionosphere, generated by tidal motions of ionized air across the earth's magnetic field.
dynamotor
A machine combining motor and generator action in a single magnetic field, either with two armatures or with one armature having two separate windings.
dyne
That unbalanced force which acting for 1 second on a body of 1 gram mass produces a velocity change of 1 centimeter per second.
The dyne is the unit of force in the CGS system.
dynode
In an electron tube, an electrode which performs a useful function by means of secondary emission.
dysbarism
A condition of the body resulting from the existence of a pressure differential between the total ambient pressure and the total pressure of dissolved and free gases within the body tissues, fluids, and cavities.
Characteristic symptoms, other than hypoxia, caused by decreased barometric pressure are bends and abdominal gas pains at altitudes above 25,000 to 30,000 feet. Increased barometric pressure, as in descent from high altitude is characterized by painful distention of the ear drums.
dyspnea
Difficult or labored breathing.
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