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- See double-dabble.
- Dalton = atomic
- 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.
- To suppress oscillations
- 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
- damped wave
- Any wave
decreases with time or whose total energy decreases by transfer to other
portions of the wave spectrum.
- The suppression of oscillations
or disturbances; the dissipation of energy with
time. See viscous
- 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
- 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
- dark trace tube
- A cathode-ray
tube, on which the face is bright, and signals are displayed as dark traces or dark
- dart configuration
- A configuration
of an aerodynamic
vehicle in which the control surfaces are at the tail of the vehicle.
- data-acquisition station
- A ground station at which various functions to control
satellite operations and to obtain data from the satellite are
- data link
- Any communications channel or
circuit used to transmit data from a sensor to a computer, a readout device,
or a storage
- data point
- A unit of fundamental information obtained through the processing of raw
- 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
- data smoothing
- The mathematical process of fitting a smooth curve to dispersed data
- 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
- 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
- In radar, a C-display in
which the blips extend
vertically to give a rough estimate of distance. Also called D-indicator,
- dead band
- An arrangement incorporated in a guidance
system which prevents an error from being corrected until that error exceeds a
- dead reckoning (abbr DR)
- In navigation, determination of position by
advancing a previous known position for courses and
- 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
- De Broglie wavelength
- For a particle of mass m and velocity v , the De Broglie wavelength,
= h/mv, where h is Planck
- 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
- 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
- deca = deka.
- 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
- 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
- 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.
-(N / t) = N
If No is the number of atoms present at time zero
- 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.
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.
- 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.
- To cause to move slower; to decrease speed.
- 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,
- December solstice = winter
- 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
- 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
- decibel per second
- A unit used to measure the rate of
decay of a sound.
- decimal coefficient of absorption
- See absorption
- decimal digit
- 1. One of the digits used in decimal notation, i.e., 1,2,3,4,5,6,7,8,9, or
- 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
- decimetric wave
- See frequency
- decimillimetric waves
- See frequency
- decision element
- In computer operations, any device which as the result of the input of data
issues one of two or more possible instructions.
- 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
- 1. A device for translating electrical signals into
- 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.
- Equipment for separation, demodulation,
or demultiplexing commutated signals. See commutator.
- The reduction of atmospheric pressure; particularly, various techniques
for preventing caisson
- 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.
- The act of removing chemical, biological, or radiological contamination
from, or neutralizing it on, a person, item, or area.
- Of circuits or devices, interconnected through any means which passes only
the static characteristics of a signal.
- 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
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
- The clarity, fidelity, sharpness, resolution and brilliancy of an image,
as a photographic image.
- A sudden or rapid burning, as opposed to a detonation
- deflecting force = coriolis
- deflection-modulated indicator = amplitude-modulated
- 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
- When measured at the earth's surface the deflection of the vertical is
equal to the angle between the geoid and the
- 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
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
- To remove gas from a material, usually by heating under high
vacuum. Compare get.
- The deliberate removal of gas from a material, usually by application of
heat under high
- Slang for demagnetize.
- 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
- 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
- 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
- 4. Of a mechanical system, the minimum number of independent generalized
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.
- 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
- 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.
- The time (or equivalent distance) displacement of some characteristic of a
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
emitted by excited nuclei in a radioactive process, so called because they are
emitted an appreciable time after the fission.
- delayed plan position indicator
- See plan
- delay element
- A device for causing time delay of a signal. See delay line.
- 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
- 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
- demand system
- An oxygen system in which oxygen flows to the user during inspiration
- The process of recovering the modulating
wave from a modulated carrier.
- An electronic device which operates on an input of a modulated carrier to
recover the modulating
wave as an output.
- 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
at high altitudes.
- 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
- density specific impulse
- The product of the specific
impulse of a propellant
combination and the average specific gravity of the propellants.
- departure = deviation,
- 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.
- Of a parachute, to release so as to let it fill out or to unfold and fill
- 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
- depth perception
- The ability to estimate depth or distance between points in the field of
- 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
- design gross weight
- The gross
weight at take-off that an aircraft, rocket, etc., is expected to have,
used in design calculations.
- The process of removing sorbed gas.
- 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
- detached shock = detached
- detached shock wave
- A shock
wave not in contact with the body which originates it. See bow wave. Also
called detached shock.
- 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.
- See recognition,
- 1. = sensor, sense 1.
- 2. An instrument employing a sensor, sense 1, to detect the pressure of
something in the surrounding environment.
- A rapid chemical reaction which propagates at a supersonic
- detonation wave
- A shock
wave in a combustible mixture, which originates as a combustion
- deuterium (symbol D, d)
- A heavy isotope of hydrogen having one proton and one neutron in the
- 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
- The nucleus of a deuterium
- 1. In statistics, the difference between two numbers. Also called
- 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
- 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
- dewpoint temperature = dewpoint.
- DF (abbr) = direction
- See radio
- 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.
- 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
- 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.
- A substance that contains few or no free charges and which can support
- 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
- 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
- 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
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
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
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.
- 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
- 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
- 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
- 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
- diffuse radiation
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 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
- A term frequently applied to the process by which solar radiation is
scattered by dust and other suspensoids in the atmosphere. See diffuse
- 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
- 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
- diffuse sky radiation
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.
- 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
- 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
- A measure of the rate of diffusion of
a substance, expressed as the diffusivity coefficient K. When K
is constant, the diffusion equation is
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,
- 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
length. The coefficient is the called then eddy diffusivity, and is
in general several orders of magnitude larger than the molecular diffusivity.
- The divergence
vector of adjacent streamlines.
The opposite of confluence.
- 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
- Using discrete
expressions to represent variables.
- digital computer
- A computer which
operates with information, numerical or otherwise, represented in a digital form.
- digital output
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.
- To express an analog measurement of a variable in discrete
- 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.
- 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.
- 1. = dip
- 2. = magnetic
- dip angle (symbol )
- The vertical angle between the true horizon and the apparent horizon.
- dip equator = aclinic
- 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
- 1. A system composed of two, separated, equal electric or magnetic charges
of opposite sign.
- 2. = dipole
- 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
- dig pole = magnetic
- 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
- direct-current discharge
- The conduction of direct current through two electrodes
immersed in a gas. See Townsend
- direct cycle = direct air
- directional antenna
- An antenna that
radiates or receives radio signals more efficiently in some directions than in
others. See Adcock
- A group of antennas arranged for this purpose is called an antenna
- directional array = antenna
- 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
- 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 of relative movement
- See relative
- 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
- direct product = scalar
- 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
- 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
- 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.
- A break in sequence or continuity of anything.
- Composed of distinct or discontinuous elements.
- discrete radio source
- A source of small angular extent of cosmic
- discrete spectrum
- A spectrum in
which the component wavelengths (and wave numbers and frequencies) constitute
sequence of values (finite or infinite in number) rather than a continuum of
values. See continuous
- discrete variable
- A quantity that may assume any one of a number of individually distinct or
- In general, a circuit in
which output depends upon the difference between an input signal and a
- A parabolic
reflector type of radio or radar antenna.
- 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
- 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.
- 1. In rocketry (a) deviation from a prescribed flight
path, (b) specifically, circular
- 2. A measure of the scatter of data points around a mean value or around a
- 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,
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
- dispersive medium
- A medium in which the phase
velocity of a wave, either electromagnetic
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
- 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.
- The graphic presentation of the output data of
any device or system.
- 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
- 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
- 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
- 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.
- Having a period of, occurring in, or related to a day.
- diurnal 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
- diurnal motion
- The apparent daily motion of a celestial
body as observed from a rotating body.
- 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
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
- The divergence theorem is used extensively in manipulating the
meteorological equations of motion and aerodynamic equations of motion.
- See ionosphere.
- DME (abbr) = distance
- 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.
- The act of coupling two or more orbiting objects; the operation of
mechanically connecting together, or in some manner bringing together, orbital
- Slang for a protuberance or blister that houses an instrument or
instruments on an otherwise smooth skin of a rocket.
- 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
- In transistors, the N-type semiconductor,
the electrode containing impurities which increase the number of available
electrons. Contrast acceptor.
- 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
- 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
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
- 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
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
- 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
- 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
- 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
- Dor, Dora
- International Astronomical Union abbreviations for Dorado. See constellation.
- Dorado (abbr Dor, Dora)
- See constellation.
- Doran (abbr) = Doppler
- Pertaining to the back.
- 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
- 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
- The unstable compounds used in a double-base propellant do not require
a separate oxidizer.
- 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
Thus, 111 (binary) = 7 (decimal)
10111 (binary) = 23
- 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
- double-integrating gyro
- A single-degree-of-freedom gyro having
essentially no restraint of its spin axis about the output axis. In this gyro
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
- 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
- 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.
- 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.
- A period during which equipment is not operating correctly because of
- DR (abbr) = dead
- 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
- 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
- drag parachute
- 1. = drogue
- 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.
- See ionosphere.
- 1. The lateral divergence from the prescribed flight path
of an aircraft, a rocket, or the like, due primarily to the effect of a
- 2. A slow movement in one direction of an instrument pointer or other
- 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
- 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
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.
- 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
- 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.
- A remotely controlled aircraft.
- Any discrete
variation in signal level during the reproduction of recorded data which
results in a data-reduction error.
- 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 =
Constantwhere 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
- 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
- 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
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
- dry friction damping = coulomb
- 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
- 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
- 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
- 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.
- 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
- ducted-fan engine
- An aircraft engine incorporating a fan or propeller
enclosed in a duct; especially,
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
- 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,
- 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
- 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
- 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.
- In computer operations, (a) to destroy intentionally or accidentally
stored information, (b) to transfer all or part of the contents of one section
into another section.
- 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
- duplex operation
- The operation of associated transmitting and receiving apparatus in which
the processes of transmission and reception are concurrent. Compare diplex
- 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
- 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
- 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 =
gdzwhere 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
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
- 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
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
- 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
- dynamic response = frequency
- dynamic scale
- The scale of the flow about a model relative to a flow about its
- 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
- 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
- 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
- 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
- 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
- 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
- A machine combining motor and generator action in a single magnetic
field, either with two armatures or with one armature having two separate
- 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.
- In an electron tube, an electrode
which performs a useful function by means of secondary emission.
- 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.
- Difficult or labored breathing.