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.
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.
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.
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.
= h/mv, where h is Planck
constant. In navigation, determination of position by advancing a previous known position for courses and distances.
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.
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.
2. A group of series of 10.
-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.
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
A decay product may be either radioactive or stable.
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.
Frequency band, 68 to 150 kilocycles.
A prefix meaning multiplied by 10-1; one-tenth.
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.
2. One of 10 possible conditions.
)
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.
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.
Various types of decrement gage are distinguished according to the design of the oscillating element.
When measured at the earth's surface the deflection of the vertical is equal to the angle between the geoid and the reference spheroid.
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.
A prefix meaning multiplied by 10. Sometimes spelled deca.
) The operator used in vector calculus and defined in Cartesian coordinates as
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.
See constellation.
2. By extension any secondary ionizing particle ejected by recoil when a primary particle passes through matter.
Whether a given quantity is best treated as a dependent or independent variable depends upon the particular problem.
The celestial pole above the horizon is called the elevated pole.
Destructs are executed when the rocket gets off its plotted course or functions in a way so as to become a hazard.
2. An instrument employing a sensor, sense 1, to detect the pressure of something in the surrounding environment.
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 CHCHDCH.
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.
When this temperature is below 0° C, it is sometimes called the frost point.
See radio direction finder.
Diabatic process is preferred to nonadiabatic process.
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.
) 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
' 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. 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.
2. In electronics, a transducer whose output waveform is the time derivative of its input waveform.
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 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.
The ideal form of diffuse radiation is isotropic radiation. Careful distinction should be made between this concept and that of a perfectly diffuse radiator.
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.
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.
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.
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.
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.
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.
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.
One collimated by means of a reflector is a catoptric light.
2. = magnetic dip.
) The vertical angle between the true horizon and the apparent horizon.
2. = dipole antenna.
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.
Air is successively compressed in the compressor section, heated in the nuclear reactor, and expelled through the turbine-tailpipe section to obtain thrust.
Direct solar radiation is measured by pyrheliometers.
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 base line and the line connecting the center of the baseline with the target.
A group of antennas arranged for this purpose is called an antenna array.
2. A flight instrument incorporating a gyro that holds its position in azimuth and thus can be used as a directional reference.
2. For thermal radiation properties, in a specified direction from the surface, usually measured as the angle from the normal.
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.
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.
The center conductor terminates at the center of the disk which is perpendicular to the axis of the cone.
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.
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.
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 can be represented by a rotation vector or translation vector or both.
The product of dissociation of a molecule is two ions, one positively charged and one negatively charged.
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.
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.
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.
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.
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.
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.
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
(v/c)]/[1 ± (v/c)] where the top signs represent the source receding
from the observer and the bottom signs, approaching the observer. 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.
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.
2. The magnitude of the Doppler effect, measured in cycles per second.
See constellation.
2. A device, worn by persons working around radioactive material, which indicates the dose of radiation to which they have been exposed.
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.
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.
The unstable compounds used in a double-base propellant do not require a separate oxidizer.
Thus, 111 (binary) = 7 (decimal)
10111 (binary) = 23 (decimal).
2. In spectroscopy, two lines resulting from transitions from the same state.
See constellation.
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.
A coefficient representing the drag on a given airfoil or other body, or a coefficient representing a particular element of drag. See Rayleigh formula.
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.
See aurora.
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.
In general, the mobilities of holes and electrons are different.
Drift rate has many specific meanings in different fields. The type of drift rate should always be specified.
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.
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.
2. A parachute used specifically to pull something, usually a larger parachute, out of stowage, as, a drogue parachute deploys a drag parachute.
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.
This term, appropriate especially for liquid rockets, is sometimes considered to include the payload.
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.
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.
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.
2. = ducted-fan engine.
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.
Ducting is likely to occur where the gradient of the index of refraction exceeds 48 N-units per 1000 feet of altitude.
2. In rocketry, an inert stage, i.e., no propellant.
In receiver practice, that portion of the impedance not included in the signal generator is often called dummy antenna.
Duplexer should not be confused with diplexer, a device permitting an antenna system to be used simultaneously or separately by two transmitters.
2. In a pulse carrier composed of pulses that recur at regular intervals, the product of the pulse duration and the pulse repetition frequency.
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.
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
= 10d Hd
= 9.8dZ = gdz
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. 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.
If two such flows have the same Reynolds number, both flows are said to be at the same dynamic scale.
In computer programming, memory allocated and deallocated as needed on the fly, also dynamic memory allocation.
For the dynamic viscosity µ of a perfect gas, the kinetic theory of gases gives
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.
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.
The dynamical mean sun and the apparent sun occupy the same position in January, when the earth is at perihelion.
The dyne is the unit of force in the CGS system.
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.