Aerospace Science and Technonlogy Dictionary T Section

T-time: Any specific time, minus or plus as referenced to zero or launch time, during a countdown sequence that is intended to result in the firing of a rocket propulsion unit that launches a rocket vehicle.
T-wave: In an electrocardiogram, the deflection which represents repolarization on the ventricles. It is normally upright, varying rather widely in amplitude and duration.
tables (data): (1) An array of data, each item of which may be unambiguously identified by means of one or more arguments. (2) A collection of data in which each item is uniquely identified by a label, by its position relative to the other items, or by some other means.
Tacan: A two dimensional navigation system which provides azimuth and distance to a fixed ground station for navigation in piloted aircraft. Used for tactical air navigation.
Tacan (abbr): = tactical air navigation.
tachycardia: Very rapid beating of the heart.
tactical air: (abbr Tacan)
A two-dimensional navigation system which provides azimuth and distance to a fixed ground station for navigation in piloted aircraft. Distance is determined by pulse interrogation of the ground station with time comparison to the returned pulse. Azimuth is provided by comparison of a navigation continuous-wave audiosignal from pulse amplitudes to reference pulses.
tail assemblies: The rear part of a body, as of an aircraft or a rocket. The tail surfaces of an aircraft or rocket. Used for empennage, tail mountings, tails (assemblies), and vertical tails.
tail fin: A fin at the rear of a rocket or other body.
tail lobes: The two bundles of nearly-parallel magnetic field lines which stretch into the magnetotail, on opposite sides of the plasma sheet. The northern lobe contains field lines entering the north polar region of Earth, while the southern lobe contains lines emerging from the southern polar region.
tailward acceleration: See physiological acceleration.
take-off: (takeoff) 1. The action of a rocket vehicle departing from its launch pad. See lift-off.
2. The action of an aircraft as it becomes airborne.
3. To perform the action of a take-off. Said of a rocket vehicle or aircraft.
tandem launch: The launching of two or more satellites using a single launch vehicle.
tandem wing aircraft: An aircraft congiguration having two wings of similar span, mounted in tandem.
tangent ogive: An ogive whose circular-arc contours have their centers on a line normal to the axis at the base of the ogive, the arcs thus being tangent to the surface of the cylindrical body behind the ogive. See ogive.
tangential acceleration: The acceleration acting at the periphery of a system rotating about an axis.
tangential wavepath: For a direct radio wave, that path of propagation tangential to the surface of the earth. This path is curved slightly by atmospheric refraction.
tank: 1. A container incorporated into the structure of a liquid propellant rocket from which a liquid propellant or propellants are fed into the firing chamber or chambers.
2. A container for storage of liquid oxygen, liquid fuel, or other liquid propellant until transferred to the rocket's tanks or some other receptacle.
3. In computers, a container of mercury, or other liquid, and associated components used as delay-line storage.
tankage: Of a liquid propellant rocket, the aggregate of the tanks carried by the rocket.
tape blocks: An aggregate or group of characters, words, records, or information considered as a single unit and recorded on magnetic tape to adjacent physical locations. Blocking is done for convenience of data handling and particularly for ease in error recovery.
Tare (abbr): = telemetry automatic reduction system.
target: 1. Any object, point, etc., toward which something is directed.
2. An object which reflects a sufficient amount of a radiated signal to produce an echo signal on detection equipment. See radar target.
target acquisition: The process of optically, manually, mechanically, or electronically orienting tracking system in direction and range to lock on a target.
target board: A board usually painted in a distinctive pattern, having a known geometrical relationship to a camera, and used for determining the orientation of that camera.
target discrimination: Resolution of a radar.
target glint: = scintillation.
target masking: Technique used in vision contrast discrimination testing involving the ratio of the luminance of a target (object) to the luminance of the background, especially when light and dark adaptation are factors.
target signal: The radar energy returned to a radar by a target. Also called echo signal, video signal. The amount of this energy is termed received power.
TAU: Thousand AU Mission.
Tau, Taur: International Astronomical Union abbreviations for Taurus. See constellation.
Taurus: (abbr Tau, Taur)
See constellation.
Taylor number: (symbol N_Ta )
A nondimensional number arising in problems of a rotating viscous fluid. It may be written

$N$ _Ta = f² h² / v²
where f is the coriolis parameter (or, for a cylindrical system, twice the rate of rotation of the system); h represents the depth of the fluid; and v is the kinematic viscosity. the square root of the Taylor number is a rotating Reynolds number, and the fourth root is proportional to the ratio of the depth h to the depth of the Ekman layer.
Taylor series: See Taylor theorem.
Taylor theorem: 1. If all the derivatives of a function f(x) are continuous in the vicinity of x = a , then f(x) can be expressed in an infinite series (the Taylor series):

The case a = 0 is called a Maclaurin series.
2. The theorem of G.I. Taylor in the statistical theory of atmospheric turbulence:

where x is the distance traveled by a particle in the time interval T ; u is the fluctuation or eddy velocity of the particle; and R ( ) is the Lagrangian correlation coefficient between the particle's velocity at time t and t + .
TCM: Trajectory Correction Maneuver, spacecraft propulsive maneuver.
TDM: Time-division multiplexing.
TDR satellites: Orbiting communications satellites, developed by NASA to relay data from satellite sensors to groundstations and to track the satellites in orbit.
teardrop balloon: A sounding balloon which, when operationally inflated, resembles an inverted teardrop. This shape was determined primarily by aerodynamic considerations of the problem obtaining maximum stable rates of balloon ascension.
tearing mode: (Tearing Instability) A resistive MHD instability which is spatially localized near a rational surface and which grows at a rate slower than the MHD Alfvén rate, but faster than the resistive skin diffusion rate. The instability "tears" magnetic field lines and reconnects them into a new state of lower magnetic energy. (see other entries for more information about the terms used above.)
tearing modes (plasmas): Explosive reconnections of energetic particle accelerations at high voltages in the magnetosphere during substorms.
technical photography: The recording of photographic images for information relevant to some engineering phenomena of a qualitative nature.
technical sequential photography: Slow or rapid sequence photography serving the study of event occurrence, or duration, by the adjunct of time coordinates recorded on the film simultaneously with the event.
technology transfer: The process of converting scientific findings from research laboratories and governmental activities into commercially viable products.
tectonics: A branch of geology dealing with the broad architecture of the upper part of the Earth's crust, that is, the regional assembling of structural or deformational features, a study of their mutual relations, their origin, and their historical evolution. Used for tectonic movement.
tectonism: The processes of faulting, folding or other deformation of the lithosphere of a planetary body, often the result of large-scale internal movements below the lithosphere.
tektite: Small glassy bodies containing no crystals, composed of at least 65 percent silicon dioxide, bearing no relation to the geological formations in which they occur, and believed to be of extraterrestrial origin. Tektites are found in certain large areas called strewn fields. They are named, as are minerals, with the suffix ite , as australite , found in Australia, billitonite, indochinite , and rizalite , found in Southeast Asia, bediasite from Texas, and moldavite from Bohemia and Moravia.
Tel, Tele: International Astronomical Union abbreviations for Telescopium. See constellation.
telecommunication: Any transmission, emission, or receptin of signs, signals, writings, images, sounds, or information of any nature by wire, radio, visual, or electromagnetic systems.
teleconnections (meteorology): Statistically significant temporal correlations between meteorological parameters at widely separated points.
telefork: A ground support vehicle that serves as a tractor, a forklift, and a crane.
telemedicine: The use of telecommunication technologies as a medium for providing medical services (diagnostic, therapeutic, education, and research) by health care professionals to sites that are remote from the provider.
telemeter: 1. To measure at a distance. See telemetering, telemetry.
2. The electronic unit which transmits the signal in a telemetering system.
telemetering: 1. A measurement accomplished with the aid of intermediate means which allows perception, recording, or interpretation of data at a distance from a primary sensor. The most widely employed interpretation of telemetering restricts its significance to data transmitted by means of electromagnetic propagation.
2. Automatic radio communication intended to indicate or record a measurable variable quantity at a distance.
telemetry: The science of measuring a quantity or quantities, transmitting the results to a distant station, and there interpreting, indicating, and/or recording the quantities measured.
telemetry elsse: An elsse which utilizes the telemetry transmitter as a signal source.
teleoperators: Remotely controlled mobility modules which incorporate sensory and manipulative subsystems for the purpose of extending the human operators skills and cognitive capabilities into hostile or remote environments.
telephotometer: A photometer that measures the received intensity of a distant light source. When specifically used to measure the transmissivity of the intervening atmosphere (or other medium), it is usually termed a transmissometer. See visibility meter.
telephotometry: The body of principles and techniques concerned with measuring atmospheric extinction using various types of telephotometer.
telescopic observations: Monitoring meteor activity by a telescope or binoculars. This technique is used to determine radiant positions of major and minor showers, to study meteors much fainter than those seen in visual observations.
Telescopium: (abbr Tel, Tele)
See constellation.
Telesto: A natural satellite of Saturn orbiting at a mean distance of 294,660 kilometers.
telluric currents: Large scale surges of electric charges within the Earth's crust, associated with disturbances of the ionosphere. Used for earth currents.
telluric lines: Absorption lines in a solar spectrum produced by constituents of the atmosphere of the earth itself rather than by gases in the outer solar atmosphere such as those responsible for the Fraunhofer lines. The terrestrial nature of the absorption processes responsible for telluric lines is revealed by their intensity variation with solar zenith angle and by their freedom from any Doppler broadening due to their solar rotation. Water vapor produces the strongest of the telluric lines in the visible spectrum.
Tempel 2 comet: A comet for which a spacecraft rendezvous had been planned for 1988 because of its accessible orbit. It has been replaced by a planned spacecraft rendezvous with the Wild 2 comet in 1994.
temperature: 1. In general, the intensity of heat as measured on some definite temperature scale by means of any of various types of thermometers.
2. In statistical mechanics, a measure of translational molecular kinetic energy (with three degrees of freedom).
3. In thermodynamics, the integrating factor of the differential equation referred to as the first law of thermodynamics.
temperature dependence: The characteristic of a material which is dependent on changes in the ambient temperature.
temporal distribution: The statistical distribution based on time of phenomena, occurrences or events.
temporal logic: A logic that can express properties involving the seqencing of events in time.
temporal resolution: The precision with which an optical instrument or a system differentiates between time intervals. Used for multitemporal analysis.
Tenma satellite: The second Japanese X-ray mission, also known as Astro-B.
tensile strength: The property of solid material that indicates its ability to withstand a uniaxial tensile load.
tensile stress: Normal stress tending to lengthen the body in the direction in which it acts.
tensor: An array of functions which obeys certain laws of transformation. A one-row or one-column tensor array is a vector. The motivation for the use of tensors in some branches of physics is that they are invariants, not depending on the particular coordinate system employed.
tensors: Arrays of functions which obey certain laws of transformation. A one row or one column tensor array is a vector. Used for tensor fields and transformation tensors.
tephra: Tephra, the Greek word for ash, is used to describe any material that is ejected into the atmosphere. Tephra includes dense blocks and bombs, and lighter materials such as scoria, pumice, and ash. As one moves away from a volcano, the tephra deposits become finer grained (the particles are smaller) and thinner. This is because small particles stay aloft longer and stay within the eruption cloud for a greater distance from the volcano.
tera: (abbr T)
A prefix meaning multiplied by 10E12.
teracycle: = 1000 gigacycles.
tercentesimal thermometric scale: = approximate absolute temperature scale.
terminal: 1. A point at which any element in a circuit may be directly connected to one or more other elements.
2. Pertaining to a final condition or the last division of something, as terminal ballistics.
terminal area energy management: Automated guidance and landing system for the Space Shuttle orbiter.
terminal ballistics: That branch of ballistics dealing with the motion and behavior of projectiles at the termination of their flight, or in striking and penetrating a target.
Terminal Configured Vehicle Program: NASA Program for determining configurations for short haul transport aircraft, including V/STOL and VTOL aircraft. Used for TCV program.
terminal guidance: Guidance from an arbitrary point, at which midcourse guidance ends, to the destination.
terminal velocity: The maximum velocity attainable, especially by a freely falling body, under the given conditions.
terminator: The line of sunrise or sunset on a planet or its satellite.
terminator: The line separating illuminated and dark portions of a celestial body, as the moon, which is not self luminous.
ternary notation: A system of positional notation using 3 as a base.
terpenes: A class of unsaturated organic compounds having the empirical formula C10H15 occurring in most essential oils and oleoresinous plants. Structurally the important terepenes and their derivatives are classified as monocyclic (dipentene), bicylic (pinene), and acyclyic (myrcene).
Terra spacecraft: First in a series of EOS (Earth Observing System) spacecraft developed to advance the understanding of the ways that the Earth`s lands, oceans, air, ice, and life function as a total environmental system. The spacecraft carries five high-resolution instruments: the Advanced Spaceborne Thermal Emission Radiometer (ASTER), the Clouds and the Earth Radiant Energy System (CERES), the Multi-Angle Imaging Spectroradiometer (MISR), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Measurements of Pollution in the Troposphere (MOPITT) instrument.
terrain: The surface features of an area of land.
terrain following: The flight path of an aircraft, helicopter, or missile at a constant altitude above the terrain or highest obstacle. The altitude constantly changes to conform to the varying height of the terrain and/or obstacles.
terrestrial: The inner solar system planets: Mercury, Venus, Earth, Mars, composed of rocky materials and iron, with a density between 4.0 and 5.5 grams per cubic centimeter.
terrestrial: Of or pertaining to the earth.
terrestrial coordinates: = geographical coordinates.
terrestrial equator: = astronomical equator.
terrestrial latitude: Latitude on the earth; angular distance from the equator. See coordinate, table VI. Terrestrial latitude is named for the datum used to measure angular distance from the equator. Astronomical latitude is the angular distance between the direction of gravity and the plane of the equator. Geodetic or topographical latitude is the angular distance between the plane of the equator and a normal to the spheroid. Geodetic and sometimes astronomical latitude are also called geographic latitude. Geocentric latitude is the angle between a line to the center of the earth and the plane of the equator. Geodetic latitude is used for charts.
terrestrial longitude: Longitude on the earth; the arc of a parallel, or the angle at the pole, between the prime meridian and the meridian of a point on the earth. See coordinate, table VI. Terrestrial longitude is named for the datum used to measure it. Astronomical longitude is the angle between the plane of the reference meridian and the plane of the celestial meridian. Geodetic longitude is the angle between the plane of the reference meridian and the plane through the polar axis and the normal to the spheroid. Geodetic and sometimes astronomical longitude are also called geographic longitude. Geodetic longitude is used for charts.
terrestrial magnetism: The magnetism of the earth. Also called geomagnetism.
terrestrial meridian: = astronomical meridian.
terrestrial planets: The four small planets nearest the sun (Earth, Mercury, Venus, and Mars).
terrestrial pole: One of the poles of the earth. See geographical pole, geomagnetic pole, magnetic pole.
terrestrial radiation: The total infrared radiation emitted from the earth's surface; to be carefully distinguished from effective terrestrial radiation, atmospheric radiation (which is sometimes erroneously used as a synonym for terrestrial radiation), and insolation. Also called earth radiation, eradiation.
terrestrial refraction: 1. Any refraction phenomenon observed in the light originating from a source lying within the earth's atmosphere; as contrasted to astronomical refraction, sense 2. This is applied only to refraction caused by inhomogeneities of the atmosphere itself, not, for example, to that caused by ice crystals suspended in the atmosphere. Terrestrial refraction is responsible for such phenomena of meteorological optics as looming, sinking, stooping, towering, mirages, and terrestrial scintillation.
2. = terrestrial refraction error.
terrestrial refraction error: The angular refraction error, for a ground observer, of an object in the vicinity of the earth. Also called terrestrial refraction.
terrestrial scintillation: Generic term for scintillation phenomena observed in light that reaches the eye from sources lying within the earth's atmosphere; to be differentiated from astronomical scintillation which is observed in light from extraterrestrial sources such as stars. Also called atmospheric boil, atmospheric shimmer, shimmer, optical haze. Terrestrial scintillation is produced by irregular refraction effects due to passage, across the line of sight, of air parcels (schlieren) whose densities differ slightly from that of their surroundings. Density irregularities with dimensions of the order of centimeters, or at most decimeters, are responsible for most such scintillatory effects.
terrestrial triangle: See navigational triangle.
terrestrial-reference guidance: See guidance, note.
tesla: (abbr T)
The unit of magnetic flux density, one weber per square meter.
test: 1. A procedure or action taken to determine under real or simulated conditions the capabilities, limitations, characteristics, effectiveness, reliability, or suitability of a material, device, system, or method.
2. A similar procedure or action taken to determine the reactions, limitations, abilities, or skills of a person, other animal, or organism.
test bed: 1. A base, mount, or frame within or upon which a piece of equipment, especially an engine, is secured for testing.
2. A flying test bed.
test chamber: A place, section, or room having special characteristics where a person or object is subjected to experiment, as an altitude chamber ; specifically, the test section of a wind tunnel.
test firing: The firing of a rocket engine, either live or static, with the purpose of making controlled observations of the engine or of an engine component.
test flight: A flight to make controlled observations of the operation or performance of an aircraft or rocket, of an aircraft or rocket component, of a system, etc.
test pattern generators: Image-processing software.
test section: The section of a wind tunnel where objects are tested to determine their aerodynamic characteristics. Also called a test chamber.
test stand: A stationary platform or table, together with any testing apparatus attached thereto, for testing or proving engines, instruments, etc. See proving stand. Compare launch stand.
tethered satellites: Concept for scientific payloads suspended at altitudes of 120 km from Space Shuttle orbiters flying at 200-km altitude; control system would permit deployment and retrieval of the tethered satellites.
Tethys: A satellite of Saturn orbiting at a mean distance of 295,000 kilometers.
tetraethyl orthosilicate: An oxidation inhibiting coating used on the wing leading edges and nose cap of the Space Shuttle.
tetrahydrofuran: In organic chemistry, an intermediate and a solvent for polyvinyl chloride. Used for butylene oxides.
textures: The structural qualities of surfaces determined by the interrelation of their elements.
Thebe: A natural satellite of Jupiter orbiting at a mean distance of 221,900 kilometers.
thematic data: Thematic data layers in a data set are layers of information that deal with a particular theme. These layers are typically related information that logically go together. Examples of thematic data would include a data layer whose contents are roads, railways, and river navigation routes.
thematic mappers (LANDSAT): Landsat multispectral scanners designed to acquire data to catagorize the Earth's surface. Particular emphasis was placed on agricultural applications and land use.
theodolite: An optical instrument which consists of a sighting telescope, mounted so that it is free to rotate around horizontal and vertical axes, and graduated scales so that the angle of rotation may be measured. The telescope is usually fitted with a right-angle prism so that the observer continues to look horizontally into the eyepiece, whatever the variation of the elevation angle.
theoretical rocket: = ideal rocket.
thermal: 1. Of or pertaining to heat or temperature.
2. A vertical air current caused by differential heating of the terrain.
thermal accommodation coefficient: = accommodation coefficient.
thermal analysis: A general term covering a group of related techniques whereby the dependence of the parameters of any physical property of a substance or temperature is measured. Used for differential thermal analysis and DTA (analysis).
thermal barrier: (plasma control) In magnetic mirror devices, this is a depression of electrostatic potential formed by enhancing ion loss in the region between the central cell and the positive potential plug. The thermal barrier significantly reduces the density requirements in the plug and lowers the overall power required to sustain the solenoidal plugging by thermally decoupling central cell electrons from the end plugs.
thermal barrier: A popular term for speed limitations within an atmosphere imposed by aerodynamic heating. Also called the heat barrier.
thermal comfort: That condition which expresses satisfaction with the thermal environment and which is measured by such factors as air temperature, relative humidity, air velocity, etc.
thermal conductivity: Degree to which a substance transmits heat.
thermal conductivity: Time rate of unidirectional heat transfer per unit area, in the steady-state, between parallel planes separated by unit distance, per unit difference of temperature of the planes.
thermal conductivity: An intrinsic physical property of a substance, describing its ability to conduct heat as a consequence of molecular motion. The thermal conductivity bears the same relation to the conduction of heat as the dynamic viscosity does to the transfer of momentum. It can be defined by reference to the Newton law of cooling:

where k is the thermal conductivity; H the rate of heat conduction across a surface per unit area and per unit time; and the temperature gradient normal to the surface. Also called heat conductivity, coefficient of thermal conduction, coefficient of heat conduction.
thermal conductivity vacuum gage: A vacuum gage containing two surfaces at different temperatures between which heat can be transported by the gas molecules so that changes in the temperature (or in the heating power required to maintain constant temperature) of one of the surfaces can be correlated with the gas pressure by calibration against a McLeod gage. Various types of thermal conductivity gages are distinguished according to the method of indicating the temperature change. The common type are: Pirani gage; thermocouple gage; thermistor gage; bimetallic strip gage.
thermal decomposition: The breaking apart of complex molecules into simpler units by the application of heat.
thermal degradation: Impairment of properties caused by exposure to heat.
thermal diffusivity: The ratio of thermal conductivity of a substance to the product of its density and specific heat. Common units for this property are sq cm/s or sq ft/h.
thermal efficiency: 1. The efficiency with which a heat engine transforms the potential heat of its fuel into work or output, expressed as the ratio of the useful work done by the engine in a given time interval to the total heat energy contained in the fuel burned during the same time interval, both work and heat being expressed in the same units.
2. = thermodynamic efficiency.
thermal emission: The process by which a body emits electromagnetic radiation as a consequence of its temperature only.
thermal emissive power: The rate of thermal emission of radiant energy per unit area of emitting surface. Also called emissive power.
thermal excitation: In a gas, the translation energy.
thermal expansion: The increase in the dimensions or the volume of a body due to change in temperature.
thermal fatigue: In metals, fracture resulting from the presence of temperature gradients which vary with time in such a manner as to produce cyclic stresses in a structure.
thermal gas: A collection of particles that collide with each other and exchange energy frequently, giving a distribution of particle energies that can be characterized by a single temperature.
thermal infrared radiation: The middle wavelength ranges in the infrared portion of the electromagnetic spectrum. Ranging between 3 microns and 20 microns, most remote sensing applications utilize the 8- to 13-micron range. This is emitted energy whereas other infrared (near infrared) is reflected energy.
thermal instability: The conditions of temperature gradient, thermal conductivity, and viscosity which lead to the onset of convection in a fluid. Such gross phenomena as atmospheric winds are an example of this type of instability. In general, if the fluid is conducting, as a plasma, the applications of a magnetic field tends to reduce these thermal instabilities.
thermal insulation: A material applied to reduce the flow of heat.
thermal jet engine: A jet engine that utilizes heat to expand gases for rearward ejection. This is the usual form of aircraft jet engine.
thermal motions: See Doppler broadening.
thermal neutrons: Neutrons in thermal equilibrium with the medium in which they exist. Used for slow neutrons.
thermal noise: The noise at radio frequencies caused by thermal agitation in a dissipative body. Also called Johnson noise.
thermal pollution: Environmental temperature rise due to waste heat disposal.
thermal radiation: The electromagnetic radiation emitted by any substance as the result of the thermal excitation of its molecules. Thermal radiation ranges in wavelength from the longest infrared radiation to the shortest ultraviolet radiation.
thermal resistance: The extent to which a material retains useful properties as measured during exposure of the material to a specified temperature and environment for a specified time. Used for heat resistance.
thermal shock: The development of a steep temperature gradient and accompanying high stresses within a structure.
thermal stability: Resistance to permanent changes in property caused soley by heat.
thermal stresses: Stresses in metal, resulting from nonuniform temperature distribution.
thermal tide: A variation in atmospheric pressure due to the daily differential heating of the atmosphere by the sun; so-called in analogy to the conventional gravitational tide. See solar atmospheric tide.
thermal transpiration: The passage of gas through a connection between two vessels at different temperatures resulting in a pressure gradient when equilibrium is reached. Under conditions of molecular flow the equilibrium condition is expressed by

where p_a and T_aare the pressure and absolute temperature, respectively, in one vessel and p_b and T_b are the pressure and absolute temperature, respectively, in the other.
thermal velocity: average speed (magnitude of velocity) of a particle at a given temperature (in a Maxwellian velocity distribution).
thermalization (energy absorption): Also known as slowing-down, this is the process (generally arising from collisions) by which fast (superthermal) particles give up energy to the plasma and slow down to thermal speeds.
thermionic: Of or pertaining to the emission of electrons by heat.
thermionic cathode: = hot cathode.
thermionic conversion: The process whereby electrons released by thermionic emission are collected and utilized as electric current. The simplest example of this is provided by a vacuum tube, in which the electrons released from a heated anode are collected at the cathode or plate. Used as a method of producing electrical power for spacecraft.
thermionic emission: Direct ejection of electrons as the result of heating and material, which raises electron energy beyond the binding energy that holds the electron in the material.
thermionic tube: An electron tube in which one or more of the electrodes is heated to cause electron or ion emission.
thermionics: The study of the emission of electrons by heat.
thermistor: An electron device employing the temperature- dependent change of resistivity of a semiconductor.
thermites: Fire-hazardous mixtures of ferric oxide and powdered aluminum; upon ignition with a magnesium ribbon, the mixtures reach temperatures up to 4000 degrees F (sufficient to soften steel).
thermocapillary migration: Phenomenon where droplets (or bubbles) in a host fluid with a uniform temperature gradient migrate to the hot end of the host fluid because of the temperature dependence of the interfacial energy of the droplets.
thermochemical: Pertaining to a chemical change induced by heat.
thermochemistry: A branch of chemistry that treats of the relations of heat and chemical changes.
thermocouple: A device which converts thermal energy directly into electrical energy. In its basic form it consists of two dissimilar metallic electrical conductors connected in closed loop. Each junction forms a thermocouple. See thermopile. If the junctions are at different temperatures, an electrical potentials proportional to the temperature difference will exist in the circuit; the value of the potential generated is different for various combinations of materials. For meteorological purposes couples of copper and constantan are frequently used; these generate approximately 40 microvolts per degree C of couple temperature difference.
thermocouple gage: A thermal conductivity vacuum gage in which pressure change is sensed by a thermocouple in thermal contact with a heated filament which cools as pressure rises. Compare with Pirani gage.
thermocouples: Devices which convert thermal energy directly into electrical energy. In its basic form it consists of two dissimilar metallic electrical conductors connected in a closed loop. Each junction forms a thermocouple.
thermodynamic: Pertaining to the flow of heat or to thermodynamics.
thermodynamic efficiency: In thermodynamics, the ratio of the work done by a heat engine to the total heat supplied by the heat source. Also called thermal efficiency, Carnot efficiency.
thermodynamic energy equation: The mathematical statement of the concept of conservation of energy embodied in the first law of thermodynamics. For reversible processes of a perfect gas, it may be written in the form

where pQ is the rate of energy addition per unit volume by heating (including the effects of radiation, molecular conduction, condensation water vapor, and the generation of heat by friction); T is the Kelvin temperature; c_v is the specific heat at constant volume; p is pressure; and is density. See energy equation.
thermodynamic equilibrium: A very general result from statistical mechanics which states that if a system is in equilibrium, all processes which can exchange energy must be exactly balanced by the reverse process so that there is no net exchange of energy. For instance, ionization must be balanced by recombination, bremsstrahlung by absorption, etc. If a plasma complies with this statement, the distribution function of particle energies and excited energy levels of the atoms can be obtained from the Maxwell-Boltzmann distribution which is a function only of the temperature.
thermodynamic function of state: Any of the quantities defining the thermodynamic state of substance in thermodynamic equilibrium. Also called thermodynamic variable, state variable, state parameter. For a perfect gas, pressure, temperature, and density are the fundamental thermodynamic variables, any two of which are, by the equation of state, sufficient to specify the state. Quantities defined in terms of these, such as specific volume, potential temperature, etc., may also be used as thermodynamic functions of state. If the composition of the gas varies, this must be specified. Thus, some measure of water vapor is a thermodynamic function of state of the atmosphere.
thermodynamic potential: = Gibbs function.
thermodynamic probability: Under specified conditions, the number of equally likely states in which a substance may exist. The thermodynamic probability P is related to the entropy S by S = k ln P where k is Boltzmann constant. See third law of thermodynamics.
thermodynamic temperature scale: The Kelvin temperature scale or the Rankine temperature scale.
thermodynamic variable: = thermodynamic function of state.
thermodynamics: Study and application of principles describing the relation of heat transfer to various forms of energy, and the behaviors of physical systems where temperature is a significant feature. Used for heat equations, thermomechanics, and thermophysics.
thermodynamics: The study of the flow of heat.
thermoelasticity: Dependence of the stress distribution of an elastic solid on its thermal state, or of its thermal conductivity on the stress distribution.
thermography: Technique employing heat transfer transients.
thermogravimetric analysis: The technique of studying materials by observing weight changes caused by chemical reactions that occur when heat is applied.
thermomechanical treatment: Combination of material-forming processes with heat treatments in order to obtain specific material properties.
thermometer: A device for measuring temperature.
thermomigration: A technique for doping semiconductors in which exact amounts of known impurities are made to migrate from the cool side of a wafer of pure semiconductor material to the hotter side when the wafer is heated in an oven.
thermonuclear: Pertaining to a nuclear reaction which is triggered by particles of high thermal energy.
thermonuclear fusion: The combination of atomic nuclei at high temperatures to form more massive nuclei with the simultaneous release of energy. Thermonuclear fusion is the power source at the core of the Sun. Controlled thermonuclear fusion reactors, when successfully implemented, could become an attractive source of power on the Earth.
thermophoresis: A process in which particles migrate in a gas under the influence of forces created by a temperature gradient.
thermophotovoltaic conversion: A technique for efficiently converting heat or solar energy to electrical energy using heated surfaces coated with a selective emitter matetrial whose photon emission peaks sharply in a narrow wavelength.
thermopile: 1. A transducer for converting thermal energy directly into electrical energy, composed of pairs of thermocouples which are connected either in series or in parallel. See Moll thermopile, Eppley pyrheliometer.
2. A battery of thermocouples connected in series to form a single compact unit. The output voltage of N pairs of series-connected thermocouples is N time the voltage developed by a single pair, whereas the current developed by N pairs of parallel-connected thermocouples is N times the current developed by a single pair. Thermopiles are used in thermoelectric radiation instruments when the output of a single pair of thermocouples is not large enough.
thermoplastic films: Materials with a linear macromolecular structure that will repeatedly soften when heated and harden when cooled.
thermoregulation: A mechanism by which mammals and birds balance heat gain and loss in order to maintain a constant body temperature. Used for body temperature regulation.
thermosphere: See atmospheric shell.
thermoswitch: A temperature-activated switch.
thermotropic model: A model atmosphere used in numerical forecasting in which the parameters to be forecast are the height of one constant-pressure surface (usually 500 millibars) and one temperature (usually the mean temperature between 1000 and 500 millibars). Thus, a surface prognostic chart can also be constructed. The quasi-geostrophic approximation is employed and the thermal wind is assumed constant with height. See equivalent barotropic model.
theta pinch: A fast-pulsed pinch device in which the external current imposed goes in the azimuthal/circumferential direction (generally in a solenoid) around a cylindrical plasma. Use of a fast-rising solenoidal current causes a rapidly increasing axial magnetic field, which compresses and heats the plasma.
thick plates: Plates of steel or other material that are over two inches thick. The exact definition of dimensions that constitute thickness varies.
thin films: Films having a thickness much smaller than any lateral dimension, formed by deposition of a material or by a thinning process.
thindown: The expenditure of heavy primary cosmic ray energy in ionizing the substance, normally air, through which it passes. See bremsstrahlung.
third law of thermodynamics: The statement that every substance has a finite positive entropy, and that the entropy of a crystalline substance is zero at the temperature of absolute zero. See thermodynamic probability. Modern quantum theory has shown that the entropy of crystals at 0 degrees absolute is not necessarily zero. If the crystal has any asymmetry, it may exist in more than one state; and there is, in addition, an entropy residue deriving from nuclear spin.
thixotropy: The property of material that enables it to stiffen in a relatively short time on standing, but upon agitation or manipulation to change to a very soft consistency or to a fluid of high viscosity, the process being completely reversible.
Thomson scattering: Scattering of photons by electrons, in the classical (low photon energy) limit. Laser light passed through a plasma will experience Thomson scattering; the spread of the scattered light in energy provides a very good measurement of the electron temperature of the plasma.
threat evaluation: The evaluation of the potential harm of an approaching aircraft or other objects.
three axis stabilization: Maintenance of a stable platform in a desired 3-axis orientation in inertial space by utilizing gyros and accelerometers and which is independent of vehicle motion.
three-body problem: That problem in classical celestial mechanics which treats the motion of a small body, usually of negligible mass, relative to and under the gravitational influence of two other finite point masses.
three-way: Coherent communications mode wherein a DSS receives a downlink whose frequency is based upon the frequency of an uplink provided by another DSS.
threshold: Generally, the minimum value of a signal that can be detected by the system or sensor under consideration.
threshold contrast: The smallest contrast of luminance (or brightness) that is perceptible to the human eye under specified conditions of adaptation luminance and target visual angle. Also called contrast threshold, liminal contrast. Compare threshold illuminance. Psychophysically, the existence of a threshold contrast is merely a special case of the general rule that for every sensory process there is a corresponding lowest detectable intensity of stimulus, i.e., a limen.
threshold illuminance: The lowest value of illuminance which the eye is capable of detecting under specified conditions of background luminance and degree of dark adaptation of the eye. Also called flux-density threshold. Compare threshold contrast. See Allard law. This threshold, which controls the visibility of point light sources, especially at night, cannot be assigned any universal value, but nonflashing lights can generally be seen by a fully dark-adapted eye when the lights yield an illuminance of the order of 0.1 lumen per square kilometer at the eye.
threshold of audibility: For a specified signal, the minimum effective sound pressure level of the signal that is capable of evoking an auditory sensation in a specified fraction of the trials. The characteristics of the signal, the manner in which it is presented to the listener, and the point at which the sound pressure level is measured must be specified. Also called threshold of detectability. Unless otherwise indicated, the ambient noise reaching the ears is assumed to be negligible. The threshold is usually given as a sound pressure level in decibels, relative to 0.0002 microbar. Instead of the method of constant stimuli, which is implied by the phrase a specified fraction of the trials , another psychophysical method (which should be specified) may be employed.
threshold of detectability: = threshold of audibility.
threshold of discomfort: In acoustics, for a specified signal, the minimum effective sound pressure level of that signal which, in a specified fraction of the trials, will stimulate the ear to a point at which the sensation of feeling becomes uncomfortable. The term applies similarly for other senses.
threshold of feeling: In acoustics, for a specified signal, the minimum sound pressure level at the entrance to the external auditory canal which, in a specified fraction of the trials, will stimulate the ear to a point at which there is a sensation of feeling that is different from the sensation of hearing. Also called tickle.
threshold of pain: In acoustics, for a specified signal, the minimum effective sound pressure level of that signal which, in a specified fraction of the trials, will stimulate the ear to a point at which the discomfort gives way to definite pain that is distinct from mere non-noxious feeling of discomfort. The term applies similarly for other senses.
threshold sensitivity: Of a transducer, the lowest level of the input signal which produces desired response at the output. The term applies equally to psychophysics.
threshold voltage: The threshold energy necessary to remove an electron from the bound position to the conduction band in solid state devices.
throat: The narrowest portion of a constricted duct, as in a diffuser, a venturi tube, etc., specifically, a nozzle throat.
throat velocity: = critical velocity.
throatable: Of a nozzle: designed so as to allow a change in the velocity of the exhaust stream through changing the size and shape of the throat of the nozzle.
throttling: The varying of the thrust of a rocket engine during powered flight by some technique. Tightening of fuel lines, changing of thrust chamber pressure, pulsed thrust, and variation of nozzle expansion are methods to achieve throttling.
throughput: In vacuum technology, the quantity of gas in pressure-volume units at a specified temperature flowing per unit time across a specified open section of a pump or pipeline. The specified temperature may be the actual temperature of the gas or a standard reference temperature. It is recommended that throughput be referred to standard room temperature. The recommended unit of throughput is the torr liter per second at 20 degrees C. Other units of throughput in common use are micron liters per second at 25 degrees C and micron cubic feet per minute at 68 degrees F. Under conditions of steady-state conservative flow the throughput across the entrance to a pipe is equal to the throughput at the exit. In this case throughput can be defined as the quantity of gas flowing through a pipe in pressure-volume units per unit time at room temperature.
thrust: 1. The pushing or pulling force developed by an aircraft engine or a rocket engine.
2. The force exerted in any direction by a fluid jet or by a powered screw, as, the thrust of an antitorque rotor.
3. (symbol F). Specifically, in rocketry, F = mv where m is propellant mass flow and v is exhaust velocity relative to the vehicle. Also called momentum thrust.
thrust augmentation: The increasing of the thrust of an engine or power plant, especially of a jet engine and usually for a short period of time, over the thrust normally developed. The principal methods of thrust augmentation are the introduction of additional air into the induction system, liquid injection, and afterburning. With a piston engine, thrust augmentation usually refers to the direction of exhaust gases so as to give additional thrust.
thrust augmenter: Any device used to increase the thrust of a piston, jet, or rocket engine, such as an afterburner. See augmenter tube.
thrust axis: A line or axis through an aircraft, rocket, etc., along which the thrust acts; an axis through the longitudinal center of a jet or rocket engine along which the thrust of the engine acts; a center of thrust. Also called axis of thrust.
thrust chamber: = firing chamber.
thrust coefficient: = nozzle thrust coefficient.
thrust distribution: The location of areas of upward thrust (lift) on wings, airfoils, etc.
thrust horsepower: 1. The force-velocity equivalent of the thrust developed by a jet or rocket engine.
2. The thrust of an engine-propeller combination expressed in horsepower. It differs from the shaft horsepower of the engine by the amount the propeller efficiency varies from 100 percent.
thrust loading: The weight-thrust ratio of a jet or rocket-propelled aircraft or other vehicle expressed as gross weight in pounds divided by thrust in pounds. See power loading.
thrust meter: An instrument for measuring static thrust, especially of a jet engine or rocket. See reaction balance.
thrust power: The power usefully expended on thrust, equal to the thrust (or net thrust) times airspeed.
thrust reverser: A device or apparatus for reversing thrust, especially of a jet engine. See reverse thrust.
thrust section: 1. A section in a rocket vehicle that houses or incorporates the combustion chamber or chambers and nozzles.
2. In loose usage, a propulsion system.
thrust terminator: A device for ending the thrust in a rocket engine, either through propellant cutoff (in the case of a liquid) or through diverting the flow of gases from the nozzle.
thrust-weight ratio: A quantity used to evaluate engine performance, obtained by dividing the thrust output by the engine weight less fuel. If the pound is used as the unit of measure for thrust and weight, the result is pounds of thrust per pound of engine.
thunderstorms: Local storms resulting from warm humid air rising in an unstable environment.
THz: Terahertz (10¹² Hz).
tick: A short audible sound or beat, as that of a clock. A time signal in the form of one or more ticks is called a time tick.
tickle: = threshold of feeling.
tidal day: = lunar day.
tide: The periodic rising and falling of the earth's oceans and atmosphere. It results from the gravitational forces of the moon and sun acting upon the rotating earth. The disturbance actually propagates as a wave through the atmosphere and along the surface of the waters of the earth. Atmospheric tides are always so designated, whereas the term tide alone commonly implies the oceanic variety.
tie control points: Points that have been registered and/or rectified on an image or a planimetric surface with respect to some horizontal coordinate system and/or vertical datum.
TIGRIS: The Topologically Integrated Geographic and Resource Information System. The TIGRIS is a geographic information system that is used to capture and analyze map features (nodes, lines, and areas).
tiles: Ceramic surfacing units, usually relatively thin in relation to facial area, made from clay or a mixture of clay and other ceramic materials, called the body of the tile having either a "glazed" or "unglazed" face and fired above red heat in the course of manufacture to a temperature sufficiently high to produce specific physical properties and characteristics.
tilt rotor aircraft: A type of convertible aircraft which takes off, hovers, and lands as a helicopter but is converted into a fixed wing aircraft by the 90-degree tilting of its rotor or rotors for use as a propeller for forward flight.
tilt table: A device used to calibrate linear accelerometers with rated ranges of, or below, +/- 1.0 g. It allows the accelerometer to be positioned at different angles in reference to a surface perpendicular to the direction of the earth's gravity, so that the applied values of acceleration are equal to the cosine of the angle between the reference surface and the direction of the earth's gravity.
tiltmeters: Instruments used to measure small changes in the tilt of the Earth's surface usually in relation to a liquid-level surface or to the rest position of a pendulum.
timbre: That attribute of auditory sensation by which a listener discriminates between two sounds of similar loudness and pitch, but of different tonal quality. Timbre depends primarily upon the spectrum of the stimulus, but it also depends upon the waveform, the sound pressure, the frequency location of the spectrum, and the temporal characteristics of the stimulus.
time: (symbol t or )
The hour of the day reckoned by the position of a celestial reference point relative to a reference celestial meridian. Time may be designated solar, lunar , or sidereal as the reference is the sun, moon, or vernal equinox, respectively. Solar time may be further classified as mean or astronomical if the mean sun is the reference, or as apparent if the apparent sun is the reference. Time may also be designated according to the reference meridian, either the local or Greenwich meridian or, additionally, in the case of mean solar time, a designated zone meridian. Standard and daylight saving time are variations of zone time. Time may also be designated according to the timepiece, as chronometer time or watch time , the time indicated by these instruments.
time constant: Generally, the time required for an instrument to indicate a given percentage of the final reading resulting from an input signal; the relaxation time of an instrument. In the case of instruments such as thermometers, whose response to step changes in an applied signal is exponential in character, the time constant is equal to the time required for the instrument to indicate 63.2 percent of the total change, that is, when the transient error is reduced to 1/e of the original signal change. Also called lag coefficient. See lag. Compare rise time, time lag.
time dilation: Stretching of time produced by relativity. Time dilation is a predicted effect of the cosmological paradigm.
time division multiple access: Radio transmission method in which each station of a satellite communication network is assigned a time schedule for transmission (in lieu of frequency division); a multi-element antenna with an adaptive null steering array eliminates interference. Used for TDMA.
time division multiplex: A system for the transmission of information about two or more quantities ( measurands) over a common channel by dividing available time intervals among the measurands to form a composite pulse train. Information may be transmitted by variation of pulse duration, pulse amplitude, pulse position, or by a pulse code. (Abbreviations used are PDM, PAM, PPM, and PCM, respectively.)
time lag: The total time between the application of a signal to a measuring instrument and the full indication of that signal within the uncertainty of the instrument.
time marching: Techniques for solving a problem with partial differential equations that have a time derivation.
time of useful consciousness: The period between loss of oxygen supply (at altitude) and the inability of the individual to function efficiently.
time series: The values of a variable generated successively in time. A continuous barograph trace is an example of a continuous time series , whereas a sequence of hourly pressures is an example of a discrete time series. Graphically, a time series is usually plotted with time as the abscissa and the values of the function as the ordinate.
time sharing: A mode of operation that permits two or more users to execute computer programs concurrently on the same computer system by interleving the execution of theprogram. Time sharing may be implemented by time slicing, priority-based interrupts, or other scheduling methods.
time signal: 1. An accurate signal marking a specified time or time interval. It is used primarily for determining errors of timepieces. Such signals are usually sent from an observatory by radio or telegraph.
2. In photography, a time indication registered on the film to serve as a time reference for interpretation of the date recorded on the film.
time tic: Markings on telemetry records to indicate time intervals.
time tick: A time signal consisting of one or more short audible sounds or beats.
time to unconsciousness: The period between loss of oxygen supply (at altitude) and the onset of unconsciousness.
time zone: See zone time.
timing parallax: The distance on a film between a frame and a time signal which were simultaneously exposed.
timing pulse: In telemetry, a pulse used as a time reference.
Timoshenko beams: Simple structural units used by Stephen Timoshenko as models in developing analysis equations for deflections and deformations of beams and columns under load.
tip vanes: Wing mounted rotor tips with their spans oriented approximately parallel to the local free stream to increase the capture area and power output of the rotor.
TIROS N series satellites: A new term for the family of satellites designed to prototype Tiros N.
TIROS satellites: A series of NASA and NOAA satellites launched to monitor the Earth's weather. Those funded by NASA are called TIROS, and those funded by NOAA are called ESSA.
Titan: A satellite of Saturn orbiting at a mean distance of 1,222,000 kilometers.
Titan Centaur launch vehicle: A Titan III rocket augmented with a Centaur rocket for launching spacecraft requiring high-velocity escape trajectories.
Titania: A satellite of Uranus orbiting at a mean distance of 438,000 kilometers.
titration: The determination of the reactive capacity, usually of a solution, especially, the analytical process of successively adding measured amounts of a reagent (as a standard solution) to a known volume or weight of a sample or sample solution until a desired end point is reached.
tokamak devices: Experimental torroidal magnetic confinement devices where torroidal current runs through the plasma in order to produce fusion reactor like plasma conditions. The name is a Russian acronym for torroidal magnetic current.
tolerance: (mechanics) A group of prescribed limits for specific properties of a particular material.
tolerance: The allowable variation in measurements within which the dimensions of an item are judged acceptable.
tomography: Technique of making radiographs of plane sections of a body or an object; its purpose is to show detail in a predetermined plane of the body, while blurring the images of structures in other planes. Used for planigraphy.
TOPEX: The NASA Ocean Surface Topography Experiment, a proposed mission to utilize satellite altimetry to map the surface topography of the ocean from which the ocean currents are derived.
topocentric: Refers to a reference system centered at some location on the Earth (for example, the geographic location of an observatory). The topocentric distance to an object is the distance between the object and the topocenter (for example, the observer).
topocentric: Of measurements or coordinates, referred to the position of the observer on the earth as the origin.
topographic maps: Map that presents the horizontal and vertical positions of the features represented; distinguished from a planimetric map by the addition of relief in measurable form.
topography: The general configuration of a land surface.
topple: Of a gyro, the vertical component of precession or wander, or the algebraic sum of the two.
topple axis: That horizontal axis, perpendicular to the (horizontal) spin axis of a gyroscope, around which topple occurs.
tor: = torr.
torching: The degassing of a vacuum system by application of a gas burner flame to the walls during the pumping process.
tornado: A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.
toroidal wheels: Doughnut-shaped wheels designed particularly for vehicles used in soft, granular soil (planetary surfaces). Used for doughnut shape wheels.
torque: (symbol T)
About an axis, the product of a force and the distance of its line of action from the axis.
torque converters: Devices for changing the torque speed or mechanical advantage between an input shaft and an output shaft.
torquer: In a gyro, a device which produces torque about an axis of freedom in response to a signal input.
torquing: In a gyro, the application of torque to a gimbal about an axis of freedom for the following purposes: precessing, capturing, slaving, caging, or slewing.
torr: Provisional international standard term to replace the English term millimeter of mercury and its abbreviation mm of Hg (or the French mm de Hg ). The torr is defined as 1/760 of a standard atmosphere or 1,013,250/760 dynes per square centimeter. This is equivalent to defining the torr as 1333.22 microbars and differs by only one part in 7 million form the International Standard millimeter of mercury. The prefixes milli and micro are attached without hyphenation.
toruses: Geometric shapes involving a single (usually circular) loop with a hole through the center, such as a donut, a bicycle tire, a ring,
TOS: Transfer Orbit Stage, upper stage.
total conductivity: In atmospheric electricity, the sum of the electrical conductivities of the positive and negative ions found in a given portion of the atmosphere.
total curvature: The change in direction of a ray between object and observer.
total eclipse: An eclipse in which the entire source of light is obscured.
total emissive power: See emittance.
total emissivity: (symbol )
See emissivity.
total emittance: = total emissive power. See emittance.
total energy equation: In meteorology, an expression relating all forms of energy obtained by combining the thermodynamic energy equation with the mechanical energy equation. This equation expresses the fact that the combined internal, kinetic, and potential energy in a given volume of the atmosphere can vary only as a result of: (a) the transport of these forms of energy across the boundaries of the volume; (b) the work done by pressure forces on the boundary; (c) the addition or removal of heat; and (d) the dissipational effect of friction.
total energy systems: Energy systems which supply both electrical and heat requirements.
total head: = total pressure, sense 3.
total impulse: (symbol $I$ _t)
The integral of the thrust F over an interval of time t :

Total impulse is related to specific impulse I_sp by $I$ _t = I_sp dt, where = propellant flow weight rate.
total potential energy: See internal energy, note.
total pressure: 1. = stagnation pressure.
2. = impact pressure.
3. The pressure a moving fluid would have if it were brought to rest without losses.
4. The pressure determined by all the molecular species crossing the imaginary surface.
total radiation: Radiation over the entire spectrum of emitted wavelengths.
total refraction: The return of waves out of a medium or layer, due to refraction. Total refraction occurs most readily at low elevation angles. For any suitable layer in the atmosphere, there is a critical beam elevation angle below which total refraction can occur. In the ionosphere this angle is frequency dependent.
total scattering coefficient: = scattering coefficient.
total scattering cross section: = scattering power; see scattering cross section.
total solar eclipse: See solar eclipse.
total-pressure tube: A tube for measuring the stagnation pressure of a fluid; i.e., a pitot tube.
toughness: That property of a material by virtue of which it can absorb work.
toughness: The ability of a metal to absorb energy and deform plastically before fracturing. Toughness is usually measured by the energy absorbed in a notch impact test, but the area under the stress-strain curve in tensile testing is also a measure of toughness.
towering: A refraction phenomenon; a special case of looming in which the downward curvature of the light rays due to atmospheric refraction increased with elevation so that the visual image of a distant object appears to be stretched in the vertical direction. The opposite of towering is stooping.
towering cumulus: A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a cumulonimbus.
Townsend discharge: A type of direct-current discharge between two electrodes immersed in a gas and requiring electron emission from the cathode.
Townsend ionization coefficient: The average number of ionizing collisions an electron will make in drifting a unit distance in the direction of the applied electric force. These coefficients were first measured by Townsend and at present they are well tabulated for most gases. A typical value for the coefficient is one pair per centimeter at a pressure of 1 millimeter of mercury and a field strength of 100 volts per centimeter.
Tr Au: International Astronomical Union abbreviations for Triangulum Australe. See constellation.
TrA, Tr Au: International Astronomical Union abbreviations for Triangulum Australe. See constellation.
trace: The line appearing on the face of a cathode-ray tube when the visible dot repeatedly sweeps across the face of the tube as a result of deflections of the electron beam. See sweep. The path of the dot from the end of one sweep to the start of the next sweep is called a retrace. If more than one trace is shown on the same scope, the traces may be called A-trace, B-trace , etc.
track: 1. The path or actual line of movement on an aircraft, rocket, etc., over the surface of the earth. It is the projection of the flightpath on the surface.
2. To observe or plot the path of something moving, such as an aircraft or rocket, by one means or another, as by telescope or by radar - said of persons or of the electronic equipment, as, the observer, or the radar, tracked the satellite.
3. To follow a desired track.
tracked vehicles: Land vehicles equipped with continuous roller belts over cogged wheels for moving over rough terrain.
tracking: 1. The process of following the movements of an object. This may be done by keeping the reticle of an optical system or a radar beam on the object, by plotting its bearing and distance at frequent intervals, or by a combination of the two.
2. A motion given to the major lobe of an antenna so that a preassigned moving target in space remains in the lobe's field as long as it is within viewing range.
tracking antenna: A directional antenna system which changes in position, or characteristics, automatically or manually to follow the motions of a moving signal source.
tracking filter: An electric device for attenuating unwanted signals while passing desired signals, by means of phase-lock techniques which reduce the effective bandwidth of the circuit and eliminate amplitude variations.
tracking offset error: The angular error, in magnitude and direction, between an object being tracked and the center of reference established for the tracking instrument.
tracking problem: The problem of controlling a system so that the output follows a given path.
tracking radar: A radar used for following a target.
tracking rate: The rate at which an operator or a system follows a target. Usually expressed in terms of the rate of change of the parameter being measured.
tracking station: A station set up to track an object moving through the atmosphere or space, usually by means of radar or radio. See minitrack.
traffic control: Control of vehicular traffic such as priority highway lanes, stoplight control, rapid-transit train control, or air traffic control.
trailing hemisphere: The hemisphere that faces backwards, away from the direction of motion of a satellite that keeps the same face toward the planet.
train: Anything, such as luminous gas of ionized particles, left along the trajectory of a meteor after the head of the meteor has passed.
training analysis: Evaluation of all facets of instruction -- presentation methods, instructors, effectiveness of training, and testing.
training evaluation: Procedures for determining the effectiveness of instruction.
trajectories: In general, paths traced by bodies moving as a result of an externally applied force, considered in three dimensions.
trajectory: In general, the path traced by any body moving as a result of an externally applied force, considered in three dimensions. Trajectory is sometimes used to mean flight path or orbit , but orbit usually means a closed path and trajectory , a path which is not closed.
trajectory measuring system: A system used to provide information on the spatial position of an object at discrete time intervals throughout a portion of the trajectory or flightpath.
transceiver: A combination transmitter and receiver in a single housing, with some components being used by both units. See transponder.
transconductance: The real part of the transadmittance. Note: Transconductance is, as most commonly used, the interelectrode transconductance between the control grid and the plate. At low frequencies, transconductance is the slope of the control-grid-to-plate transfer characteristic.
transducer gain: The ratio of the power that a transducer delivers to a specified load under specified operating conditions to the available power of a specified source. If the input and/or output power consist of more than one component, such as multifrequency signal or noise, then the particular components used and their weighting must be specified. This gain is usually expressed in decibels.
transducers: Devices capable of being actuated by energy from one or more transmission systems or media and of supplying related energy to one or more other transmission systems or media, as microphones, thermocouples, etc. The energy in input and output may be of the same or different types (e.g., electric, mechanical, or acoustic).
transfer ellipse: = transfer orbit.
transfer orbit: In interplanetary travel, an elliptical trajectory tangent to the