Back to Table of Contents


Source edition 1965. Please read the Introduction to find out more about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
Greek symbols may not appear correctly in some browsers. For example a gamma may appear as γ.

Video display of a beacon's replies to interrogations from two or more nonsynchronized radars.
racon (From radar beacon)
A transponder for interrogation by a primary radar.
(From radio detection and ranging).
1. A method, system, or technique of using beamed, reflected, and timed radio waves for detecting, locating, or tracking objects (such as rockets), for measuring altitude, etc., in any of various activities, such as air traffic control or guidance.
2. The electronic equipment or apparatus used to generate, transmit, receive, and, usually, to display radio scanning or locating waves; a radar set.
The terms primary radar and secondary radar may be used when the return signals are, respectively, by reflection and by the transmission of a second signal as a result of triggering responder beacon by the incident signal.
radar altimeter = radio altimeter.
radar altitude
The altitude of an aircraft or spacecraft as determined by a radio altimeter; thus, the actual distance from the nearest terrain feature.
radar astronomy
The study of celestial bodies within the solar system by means of radiation originating on earth but reflected from the body under observation. See radio astronomy.
radar band
See frequency band.
radar beacon
A beacon transmitting a characteristic signal on radar frequency, permitting a craft to determine the bearing and sometimes the range of the beacon.
A racon returns a coded signal when triggered by the proper type of radar pulse; a ramark continuously transmits a signal which appears as a radial line on the plan position indicator.
radar beam
See beam.
radar cross section
The ratio of power returned in a radar echo to power received by the target reflecting the signal. Compare scattering cross section.
radar duct
See radio duct.
radar echo
See echo.
radar frequency
See frequency band.
radar horizon
The angle of elevation at which the beam from a radar antenna is intercepted by the earth's horizon. Compare radio horizon.
radar indicator = radarscope.
radar mile
A time unit of 10.75 microseconds duration; the time it takes for the signal emitted by a radar to travel from the radar to a target one mile distant and return to the radar.
radar range
1. The distance from a radar to a target as measured by the radar.
2. The maximum distance at which a radar set is effective in detecting targets.
Radar range depends upon variables such as weather conditions, type of target, etc. Radar range, sense 2, is sometimes given a specific definition, e.g., the range at which the set is effective one-half of the time.
radar range equation
The relation between the maximum range Rmax at which a point target is detectable and the properties of the radar and the target
Rmax = [(PA2λ2σ)/(4π)3Smin)]1/4
where P is the transmitted power of the radar; λ is its wavelength; σ is the scattering cross section of the target; A is the antenna gain; and Smin is the threshold signal.
radar reflectivity
In general, the measure of the efficiency of a radar target in intercepting and returning a radar signal. It depends upon the size, shape, aspect, and the dielectric properties at the surface of the target. It includes the effects of not only reflection (see reflectivity) but also scattering and diffraction.
radar reflector
A device capable of or intended for reflecting radar signals. See corner reflector, parabolic reflector.
radar scan
1. The searching motion of a radar beam in any of various path configurations; the pattern of the motion of a radar beam.
2. Radar scanning.
radar scanning
The action or process of moving or directing a searching radar beam. See circular scanning, conical scanning, helical scanning.
The cathode-ray tube or oscilloscope in a radar set, which displays the received signal in such a manner as to indicate range, bearing, etc. Sometimes called a radar indicator.
radar screen
1. The face of a cathode-ray oscilloscope used in a radar set.
2. A network of radar installations, or their emanations, serving, e.g., to detect strange aircraft.
radar set
An electronic apparatus consisting principally of a transmitter, antenna, receiver, and indicator for sending out scanning beams and receiving and displaying the reflected waves or the waves emitted by a radar beacon. See radar.
radar shadow
A condition in which radar frequency signals do not reach a region because of an intervening obstruction.
radar target
An object which reflects a sufficient amount of a radar signal to produce an echo signal on the radar screen.
radar volume
The volume in space that is irradiated by a given radar. For a continuous-wave radar it is equivalent to the antenna radiation pattern. For a pulse radar it is a function of the cross-section area of the beam of the antenna and the pulse length of the transmitted pulse.
radar wave
A transmitted or reflected radio wave used in radar; a radio wave in one of the frequency bands used for radar.
Motion along a radius.
radial motion
Motion along a radius, or a component in such a direction, particularly that component of space motion of a celestial body in the direction of the line of sight.
radial velocity
In radar, that vector component of the velocity of a moving target that is directed away from or toward the ground station.
The angle subtended at the center of a circle by an arc equal in length to a radius of the circle. It is equal to 360°/2π or approximately 57 degrees 17 minutes 44.8 seconds.
In radiometry, a measure of the intrinsic radiant intensity emitted by a radiator in a given direction. It is the irradiance (radiant flux density) produced by radiation from the source upon a unit surface area oriented normal to the line between source and receiver, divided by the solid angle subtended by the source at the receiving surface. It is assumed that the medium between the radiator and receiver is perfectly transparent; therefore, radiance is independent of attenuation between source and receiver.
If the radiant source is a perfectly diffuse radiator (that is, emits exactly according to Lambert law), then its radiance is equal to its emittance per unit solid angle. The radiance of a light source is termed luminance (formerly, brightness).
radiancy (symbol R, , W)
The rate of radiant-energy emission for a unit area of a source in all the radial directions of the overspreading hemisphere.
1. Pertaining to the emission or the measurement of electromagnetic radiation. Compare luminous.
2. In astronomy, the apparent location on the celestial sphere of the origin of the luminous trajectories of meteors seen during a meteor shower.
For convenience, the common meteor showers are named for the constellations of stars in which their radiants appear.
3. In describing auroras, a projected point of intersection of lines drawn coincident with auroral streamers; that is, the point from which the aurora seems to originate.
radiant density = radiant energy density.
radiant emittance
See emittance, sense 1.
radiant energy (symbol U )
1. The energy of any type of electromagnetic radiation. Also called radiation.
2. Infrequently, any energy that may be radiated, as, for example, acoustic energy.
radiant energy density (symbol u )
The instantaneous amount of radiant energy contained in a unit volume of propagating medium.
radiant energy thermometer
An instrument which determines the black body temperature of a substance by measuring its thermal radiation.
The substance need not be thermally black over the whole spectrum, since it is possible to limit the measurement to those frequencies where it is black.
radiant flux (symbol Φ)
The rate of flow of radiant energy.
radiant flux density = radiant flux per unit area
When applied to a source, it is called radiancy or radiant emittance (symbol W). When applied to a receiver, it is called irradiancy or irradiance (symbol H).
radiant heat
Infrared radiation.
This term, still used in certain engineering fields, is to be avoided since it confuses the distinct physical concepts of radiation and heat.
radiant intensity
Radiant flux per unit solid angle.
radiant temperature
The temperature obtained by use of a total radiation pyrometer when sighted upon a nonblack body.
This is always less than the true temperature.
radiating element
A basic subdivision of an antenna which in itself is capable of radiating or receiving radiofrequency energy.
1. The process by which electromagnetic energy is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space. This concept is to be distinguished from conduction and convection. A group of physical principles known as the radiation laws comprise, to a large extent, the current state of practical knowledge of the complex radiative processes.
2. The process by which energy is propagated through any medium by virtue of the wave motion of that medium, as in the propagation of sound wave through the atmosphere, or ocean waves along the water surface.
3. = radiant energy.
4. = electromagnetic radiation, specifically, high-energy radiation such as gamma rays and X-rays.
5. Corpuscular emissions, such as α or β-radiation.
6. = nuclear radiation.
7. = radioactivity.
radiation belt
An envelope of charged particles trapped in the magnetic field of a spatial body. See Van Allen belt.
radiation constants
Values used in Planck law and other radiation calculations. The first radiation constant (symbol c1) = 3.7415 erg centimeters squared per second. The second radiation constant (symbol c2) = 1.43879 centimeters degrees K. See physical constants.
radiation cooled
Of a structure, pertaining to the use of materials able to radiate heat at a rate such that the rate of increase of the temperature of the material is low.
radiation counter
An instrument used for detecting or measuring moving subatomic particles by a counting process.
radiation dose
The amount of radiation absorbed by a material, system, or tissue in a given amount of time; usually measured in one of the commonly accepted units as roentgen, roentgen-equivalent-man, roentgen-equivalent-physical, etc.
radiation laws
1. The four physical laws which, together, fundamentally describe the behavior of black-body radiation: (a) the Kirchhoff law is essentially a thermodynamic relationship between emission and absorption of any given wavelength at a given temperature; (b) the Planck law describes the variation of intensity of black-body radiation at a given temperature, as a function of wavelength; (c) the Stefan-Boltzmann law relates the time rate of radiant energy emission from a black body to its absolute temperature; (d) the Wien law relates the wavelength of maximum intensity emitted by a black body to its absolute temperature.
2. All the more inclusive assemblage of empirical and theoretical laws describing all manifestations of radiative phenomena; e.g., Bouguer law and Lambert law.
radiation lobe
A portion of the radiation pattern bounded by one or two cones of nulls.
radiation medicine
That branch of medicine dealing with the effect of radiation, specifically high-energy radiation such as X-rays, gamma rays, and energetic particles on the body and with the prevention or cure of physiological injuries resulting from such radiation.
radiation pattern
A graphical representation of the radiation of an antenna as a function of direction. Cross sections in which radiation patterns are frequently given are vertical planes and the horizontal plane, or the principal electric and magnetic polarization planes. Also called antenna pattern, lobe pattern, coverage diagram.
Two types of radiation patterns should be distinguished. They are: (a) the free-space radiation pattern which is the complete lobe pattern of the antenna and is a function of the wavelength, feed system, and reflector characteristics, and (b) the field radiation pattern which differs primarily from the free-space pattern by the formation of interference lobes whenever direct and reflected wave trains interfere with each other as is found in most surface-based radars. The envelope of these interference lobes has the same shape, but, for a perfectly reflecting surface, it has up to twice the amplitude of the free-space radiation pattern.
radiation pressure (symbol Pr)
Pressure exerted upon any material body by electromagnetic radiation incident upon it. See Poynting-Robertson effect.
This pressure is manifested whenever the electromagnetic momentum is a radiation field is changes, and is exactly twice as great when the radiation is reflected at normal incidence as it is when the radiation is entirely absorbed at normal incidence. The magnitude of any radiation-pressure effect is directly proportional to the intensity of the radiation, and is very small by most standards. On a perfectly reflecting surface Pr = u/3 where u is radiation density, the amount of radiative energy per unit volume in the space above the surface. Radiation pressure has a perceptible effect on the orbit of earth satellites, especially those with a large reflecting surface such as Echo.
radiation pyrometer
See pyrometer, note.
radiation shield
1. A device used on certain types of instruments to prevent unwanted radiation from biasing the measurement of a quantity.
2. A device used to protect human beings from the harmful effects of nuclear radiation, cosmic radiation, or the like.
3. = heat shield.
radiation sickness
A syndrome following intense acute exposure to ionizing radiations. It is characterized by nausea and vomiting a few hours after exposure. Further symptoms include bloody diarrhea, hemorrhage under the skin (and internally), epilation (hair falling), and a decrease in blood-cell level.
1. Any source of radiant energy, especially electromagnetic radiation.
2. A device that dissipates the heat from something, as from water or oil, not necessarily by radiation only.
Generally, the application of the terms radiator (in sense 2) or heat exchanger to a particular apparatus depends upon the point of view: If the emphasis is upon merely getting rid of heat, radiator is most often used, or sometimes cooler; if the emphasis is upon transferring heat, heat exchanger is used -- but these distinctions do not always hold true.
1. Communication by electromagnetic waves, without a connecting wire.
2. Pertaining to radiofrequency, as in radio wave.
Exhibiting or pertaining to radioactivity.
radioactive gas
1. In atmospheric electricity, any one of the three radioactive inert gases, radon, thoron, and actinon, which contribute to atmospheric ionization by virtue of the ionizing effect of the alpha particles which each emits on disintegration. These three gases are isotopic to each other, all having atomic number 86.
2. Any gaseous material containing radioactive atoms.
radioactive ionization gage
An ionization gage in which the ions are produced by radiations (usually alpha particles) emitted from a radioactive source.
1. Spontaneous disintegration of atomic nuclei with emission of corpuscular or electromagnetic radiations.
The principal types of radioactivity are alpha decay, beta decay, and isomeric transition. To be considered as radioactive a process must have a measurable lifetime between approximately 10E-10 second and approximately 10E17 years. Radiations emitted within a time too short for measurement are called prompt. Prompt radiations such as gamma rays and X-rays are often associated with radioactive disintegrations.
2. The number of spontaneous disintegrations per unit mass and per unit time of a given unstable (radioactive) element, usually measured in curies.
radio altimeter
A device that measures the altitude of a craft above the terrain by measuring the elapsed time between transmission of radio waves from the craft and the reception of the same waves reflected from the terrain. Also called radar altimeter.
radio astronomy
1. The study of celestial objects through observation of radiofrequency waves emitted or reflected by these objects.
In this sense radio astronomy includes both the use of radiation emitted by the celestial bodies and of radiation originating on earth and reflected by celestial bodies (radar astronomy).
2. Specifically, the study of celestial objects by measurement of the radiation emitted by them in the radiofrequency range of the electromagnetic spectrum.
Radio astronomy measurements are usually of the intensity of the received signal but often include polarization of the signal and angular size of the source.
radio beacon
Any radio transmitter, together with its associated equipment, that emits signals enabling the determination, by means of suitable receiving equipment, of direction, distance, or position with respect to the beacon.
radio beam
See beam.
The study of the effects produced on living organisms by radiation.
radio blackout = blackout, sense 1.
radio channel
A frequency band comprised of the emission bandwidth, the interference guard bands, and the frequency tolerance.
radio command
A radio signal to which a rocket, satellite, or the like responds.
radio control
1. Remote control of a pilotless airplane, a rocket, etc., by means of radio signals that activate controlling devices.
2. Any radio apparatus used for this kind of control.
radio direction finder
A radio-receiving set, together with its associated equipment, used to determine the direction from which a radio signal is transmitted.
radio duct
A rather shallow, almost horizontal layer in the atmosphere through which vertical temperature and moisture gradients are such as to produce an index of refraction lapse rate of greater than -48 N-units per 1000 feet. Strong temperature, or moisture inversions, or both are necessary for the formation of radio ducts. The resulting superstandard propagation is such as to cause the curvature of rays traveling through it to be greater than that of the earth. Radio energy which originates within the duct and leaves the antenna at angles near the horizontal may thus be trapped within the layer. See anomalous propagation, skip effect.
The effect is similar to that of a mirage (it is sometimes called radio mirage), and radar targets may be detected at phenomenally long ranges if both target and radar are in the duct. The greater the elevation angle between radar and target, the less the possibility of serious distortion due to transmission through ducts. Ducts may be surface based or elevated, with thickness ranging from a few tens of feet up to a maximum of 1000 feet. Elevated ducts are generally associated with subsidence or frontal inversions and are seldom found above 15,000 to 20,000 feet.
radio energy
Electromagnetic radiation of greater wavelength (lower frequency) than infrared radiation, that is, of wavelength greater than about 1000 microns (0.01 centimeter). The high-frequency end of the radio energy spectrum is known as microwave radiation. See frequency bands.
radio fadeout = fadeout.
radiofrequency (abbr RF)
1. A frequency at which coherent electromagnetic radiation of energy is useful for communication purposes.
Roughly, the radiofrequency of the electromagnetic spectrum lies between 10E4 and 10E12 cycles per second. See frequency bands.
2. Specifically, the frequency of a given radio carrier wave.
radiofrequency band
See frequency band.
radio goniometer = radio direction finder.
radio guidance system
A guidance system that uses radio signals to guide an aircraft or spacecraft in flight; the system includes both the flight-borne equipment and the guidance station equipment on the ground.
radio hole
Strong fading of the radio signal at some position in space along an air-to-air or air-to-ground radio path. The effect is caused by the abnormal refraction of radio waves.
radio horizon
The locus of points at which direct rays from a radio transmitter become tangential to the earth's surface. Assuming a smooth surface, the distance of the horizon is given approximately by the equation
where r is the distance, statute miles, and h is the height, feet, of the antenna above the surface. See effective radius of the earth, scatter propagation. Compare radar horizon.
The horizon extends beyond (below) the geometrical and visible horizons as the result of normal atmospheric refraction. It may be decreased or increased in particular cases as standard propagation is replaced by substandard propagation or superstandard propagation, respectively. Beyond the radio horizon, surface targets cannot be detected under normal atmospheric conditions although significant amounts of radio power have been detected in the diffraction zone below the horizon. It is now felt that this represents power scattered by turbulence-produced atmospheric inhomogeneities.
radio interferometer
An interferometer operating at radiofrequencies.
Radio interferometers are used in radio astronomy and in satellite tracking.
radio meteor
A meteor which has been detected by the reflection of a radio signal from the meteor trail of relatively high ion density ( ion column). See whistling meteor. Compare photographic meteor.
Such an ion column is left behind a meteoroid when it reaches the region of the upper atmosphere between about 80 and 120 kilometers, although occasionally radio meteors are detect at higher altitudes. The maximum reflection occurs when the column is perpendicular to the line to the transmitter-receiver.
An instrument for detecting and, usually, measuring radiant energy. Compare bolometer. See actinometer, photometer.
radiometer vacuum gage = Knudsen gage.
radiometric magnitude (symbol mrad)
The magnitude of a celestial body measured with reference to the total radiation observable through the atmosphere.
The science of measurement of radiant energy.
In practice, there is no clear distinction between radiometry and photometry although photometry usually refers to measurement in the visible and near-visible range.
radio mirage
See radio duct, note.
A radioactive nuclide; an atom which emits corpuscular or electromagnetic radiation.
radiophare = radio beacon.
This term is commonly used in international terminology.
An instrument, usually balloon-borne, for the simultaneous measurement and transmission of meteorological data while moving vertically through the atmosphere. See dropsonde.
The instrument consists of transducers for the measurement of pressure, temperature, and humidity; a modulator for the conversion of the output of the transducers to a quantity which controls a property of the radiofrequency signal; a selector switch which determines the sequence in which the parameters are to be transmitted; and a transmitter which generates the radiofrequency carrier.
The range of frequencies of electromagnetic radiation usable for radio communication.
The radiospectrum ranges from about 10 kilocycles per second to over 300,000 megacycles per second. Corresponding wavelengths are 30 kilometers to 1 millimeter. See frequency bands.
radio telescope
A device for receiving, amplifying, and measuring the intensity of radio waves originating outside the earth's atmosphere or reflected from a body outside the atmosphere.
A radio telescope usually includes a source of radiation of known power for calibration of the received signal. The term radio telescope is not restricted to devices incorporating a paraboloidal dish antenna. A radio telescope can use any antenna or combination of antennas which will accept the radiation being studied.
radio theodolite = radio direction finder.
radio waves
Waves produced by oscillation of an electric charge at a frequency useful for radio communication. Formerly called Hertzian waves. See frequency bands, electromagnetic radiation.
radius vector
A straight line connecting a fixed reference point or center with a second point, which may be moving; specifically, in astronomy, the straight line connecting the center of a celestial body with the center of a body which revolves around it, as the radius vector of the moon. See polar coordinates, spherical coordinates.
radix = base (of a number system).
radix point
The index which separates the digits associated with negative powers from those associated with the zero and positive powers of the base of the number system in which a quantity is represented. For example, binary point, decimal point.
(From radar dome. Pronounced raydome ). A dielectric housing for an antenna.
ram air
Air entering an airscoop or air inlet as a result of the high-speed forward movement of a vehicle.
A fixed radar frequency facility which continuously emits a signal so that a bearing indication appears on a radar display. See radar beacon.
ram drag
The drag produced by the momentum of air entering an airscoop or an air inlet of an aeronautical vehicle in flight.
ramjet = ramjet engine.
ramjet engine
A type of jet engine with no mechanical compressor consisting of a specially shaped tube or duct open at both ends, the air necessary for combustion being shoved into the duct and compressed by the forward motion of the engine, where the air passes through a diffuser and is mixed with fuel and burned, the exhaust gases issuing in a jet from the rear opening. The ramjet engine cannot operate under static conditions. Often called a ramjet. Also called Lorin tube.
Eluding precise prediction, completely irregular. Compare stochastic.
In connection with probability and statistics, the term random implies collective or long-run regularity; thus, a long record of the behavior of a random phenomenon presumably gives a fair indication of its general behavior in another long record, although the individual observations have no discernible system of progression.
random error
Errors that are not systematic, are not erratic, and are not mistakes.
Such random errors are caused by disturbed elements in the measuring instrument and usually are of an approximately normal or Gaussian distribution. Such random errors are sometimes called short-period errors.
random number
An expression formed by a set of digits selected from a sequence of digits in which each successive digit is equally likely to be any of the digits.
random noise
An oscillation whose instantaneous amplitude occur, as a function of time, according to a normal (Gaussian) curve. Also called Gaussian noise, random Gaussian noise.
random sample
A sample taken at random from a population.
random variable
A variable characterized by random behavior in assuming its different possible values. Mathematically, it is described by its probability distribution, which specifies the possible values of a random variables together with the probability associated (in an appropriate sense) with each value. A random variable is said to be continuous if its possible values extend over a continuum and discrete if its possible values are separated by finite intervals. Also called variate. See probability theory.
random vibration
An oscillation whose instantaneous magnitude is not specified for any given instant of time. The instantaneous magnitudes of a random oscillation are specified only by probability distribution functions giving the fraction of the total time that the magnitude, or some sequence of magnitudes, lies within a specified range.
A random vibration whose instantaneous magnitudes occur according to the Gaussian distribution is called Gaussian random vibration. Wide-band vibration amplitude is usually expressed as root-mean-square acceleration in gravitational units of acceleration g. The parameter used to specify the frequency distribution of a random vibration is power spectral density (g2 per cycle per second), sometimes called acceleration density or acceleration spectral density.
1. The difference between the maximum and minimum of a given set of numbers; in a periodic process it is twice the amplitude, i.e., the wave height.
2. The distance between two objects, usually an observation point and an object under observation. See slant range.
3. A maximum distance attributable to some process, as in visual range or the range of a rocket.
4. An area in and over which rockets are fired for testing, as Atlantic Missile Range.
5. = radar range.
range attenuation
In radar terminology, the decrease in power density (flux density) caused by the divergence of the flux lines with distance, this decrease being in accordance with the inverse-square law.
For one-way transmission, this attenuation is proportional to 1/RE2 where R is the range from the transmitter. For a radar and a point target, the range attenuation is proportional to 1/RE4, the transmission being two way.
range error
The error in radar range measurement due to the propagation of radio energy through a nonhomogeneous atmosphere. This error is due to the fact that the velocity of radio-wave propagation varies with the index of refraction and that ray travel is not in straight lines through actual atmospheres. The resulting range error is generally insignificant. Compare azimuth error.
range gating
The use of circuits in radar to suppress signals from all targets falling outside selected range limits.
range-height-indicator scope
(abbr RHI-scope). A type of radar indicator (radar-scope); an intensity-modulated indicator on which echoes are displayed in coordinates of slant range and elevation angle, simulating, thereby, a vertical cross section of the atmosphere along some azimuth from the radar.
The power of the signal returned from the target is used to modulate the intensity of the electron beam.
range marker
The index marks displayed on radar indicators to establish the scale or facilitate determination of the distance of a target from the radar. On the plan position indicator scope, for example, range markers take the form of concentric circles with the position of the radar at the center. See azimuth marker. Also called distance marker.
range only measurement of trajectory and recording (abbr Romotar)
A nonambiguous spherical and elliptical, long-baseline, range-only trajectory measuring system utilizing phase comparison techniques with range modulation frequencies.
The system consists of three of more receivers which track a transponder interrogated by a signal transmitter. The reference signal from the ground transmitter is also received by the ground receivers. Simultaneous range measurements are made by the ground receivers, which are correlated with base timing from which space position can be computed by triangulation. The system operates on 387 and 417 megacycles.
range rate
The rate at which the distance from the measuring equipment to the target or signal source being tracked is changing with respect to time. See radial velocity.
range ring
A circle on a plan position indicator, particularly one with an adjustable diameter, to indicate distance from the antenna. See distance marker.
range safety officer
An official on a rocket test range whose responsibility is to supervise the planning and execution of each test to insure the maximum safety of all personnel and property within the range boundaries.
range strobe
An index mark which may be displayed on some types of radar indicators to assist in the determination of the exact range of a target.
range sweep
See sweep, note.
range wind
The component of a ballistic wind which is parallel to the longitudinal axis of the range.
ranging pulse
In a radar system the pulse used to measure the range of the object being tracked.
ranging system
A radar system which measures range (distance).
Rankine cycle
An idealized thermodynamic cycle consisting of two constant-pressure processes and two isentropic processes.
Rankine temperature scale
(abbr degrees R). A temperature scale with the degree-interval of the Fahrenheit temperature scale and the zero point at absolute zero. The ice point is thus 491.69 degrees Rankine and the boiling point of water is 671.69 degrees Rankine.
Rankine vortex
A two-dimensional circular flow in which a circular region about the origin is in solid rotation:
V/R = Constant where V is the tangential speed and R is the distance from the origin; the region outside is free of vorticity, the speed being inversely proportional to the distance from the origin (as in the V-R vortex)
VR = Constant.
This vortex has occasionally been used as a model for the surface wind distribution in a hurricane. It is characteristic of a cylindrical vortex in a liquid with a free surface.
(From radiosonde observation). An observation of the vertical distribution of temperature, pressure, and relative humidity, obtained by means of a radiosonde.
rarefraction wave = expansion wave.
rarefied gas dynamics
The study of the phenomena related to the molecular or noncontinuum nature of gas flow at densities where
λ / l > 0.01
where λ is molecular mean free path and l is a characteristic dimension of the flow field.
Flow with λ / l > 0.01 is called molecular flow.
Flow with λ / l < 0.01 is called continuum flow.
Flow with λ / l of approximately 0.01 to 0.1 is called slip flow.
Flow with λ / l of approximately 0.1 to 10 is called transition flow.
Flow with λ / l > 10 is called free molecule flow.
Slip flow and transition flow are not always distinguished from each other. The value 1 is sometimes used instead of 10 as the boundary value for transition flow and free molecule flow.
rare gas = inert gas.
The pattern followed by the electron beam exploring element scanning the screen of a television transmitter or receiver.
raster line
One line of a raster, or scanning pattern.
rate gyro
A single-degree-of-freedom gyro having primarily elastic restraint of its spin axis about the output axis. In this gyro an output signal is produced by gimbal angular displacement, relative to the base, which is proportional to the angular rate of the base about the input axis.
rate integrating gyro
A single-degree-of-freedom gyro having primarily viscous restraint of its spin axis about the output axis. In this gyro an output signal is produced by gimbal angular displacement, relative to the base, which is proportional to the integral of the angular rate of the base about the input axis.
rate of decay
1. Of a sound, the time rate at which the sound pressure level (or other stated characteristic) decreases at a given point and at a given time. A commonly used unit is the decibel per second.
2. Of a radioactive nuclide, the number of nuclei of that nuclide changing (or disintegrating) per unit time. It is usually expressed as the instantaneous rate of decay by - dN/dt where N is the total number of the state nuclides present at the given time t.
rate of incidence = impingement rate.
ratio deviation
In a frequency modulation system, the ratio of the maximum frequency deviation to the maximum modulating frequency of the system. Also called modulation index.
rational horizon = celestial horizon.
See horizon.
RATO, Rato, or rato
(From rocket-assisted take-off).
1. A take-off in which a rocket or rockets, commonly of the solid-fuel type, are used to provide additional thrust. Hence, RATO bottle, Rato bottle, rato unit , etc., a rocket so used.
2. A RATO bottle or unit; the complete apparatus on an aircraft, comprising rockets, ignition system, etc., for assisted take-off. See JATO.
A magic tee modification for the acceptance of higher power; a circular loop of coaxial line closed upon itself and having four branching connections.
raw data
Data that is in a form ready for processing.
Different groups regard data in various forms as raw, dependent on their function. A photographic processing group may regard the latent image as raw data, a reading group may regard the photographic image as raw data, a computing group may regard certain digits data as raw data, and so on.
A measurement of wind direction and speed at altitude by radar tracking of a balloon-borne target.
A combination raob and rawin; an observation of temperature, pressure, relative humidity, and winds-aloft by means of radiosonde and radio direction finding equipment or radar tracking.
1. An elemental path of radiated energy; or the energy following this path. It is perpendicular to the phase fronts of the radiation. See incident ray, reflected ray, refracted ray.
2. One of a series of lines diverging from a common point, as radii from the center of a circle.
3. A long, narrow, light colored streak on the lunar surface originating from a crater. Rays range in length to over 150 kilometers and usually several radiate from the same crater, like spokes of a wheel.
Rayleigh atmosphere
An idealized atmosphere consisting of only those particles, such as molecules, that are smaller than about one-tenth the wavelength of all radiation incident upon that atmosphere. In such an atmosphere, simple Rayleigh scattering would prevail.
This model atmosphere is amenable to reasonably complete theoretical treatment, and hence has often served as a useful starting point in description of the optical properties of actual atmospheres. The polarization of skylight, for example, exhibits almost none of the complexities found in the real atmosphere.
Rayleigh formula
See aerodynamic force, drag, drag coefficient.
Rayleigh law
See Rayleigh scattering.
Rayleigh limit
See Rayleigh scattering.
Rayleigh number
The nondimensional ratio between the product of buoyancy forces and heat advection and the product of viscous forces and heat conduction in a fluid. It is written as
where g is the acceleration of gravity; ΔzT is a characteristic vertical temperature difference in the characteristic depth d ; α is the coefficient of expansion; v is the kinematic viscosity; and k the thermometric conductivity.
The Rayleigh number is equal to the product of the Grashof and Prandtl numbers, and is the critical parameter in the theory of thermal instability.
Rayleigh scattering
Any scattering process produced by spherical particles whose radii are smaller than about one-tenth the wavelength of the scattered radiation. Compare Mie scattering.
In Rayleigh scattering, the scattering coefficient varies inversely with the fourth power of the wavelength, a relation known as the Rayleigh law. The angular intensity polarization relationships for Rayleigh scattering are conveniently simple. For particles not larger than the Rayleigh limit, there is complete symmetry of scattering about a plane normal to the direction of the incident radiation, so that the forward scatter equals the backward scatter. The Rayleigh scattering coefficient ks is
where n is the number of scatters of diameter d; m is the index of refraction; and λ is the wavelength of the radiation.
Rayleigh wave
1. A two-dimensional barotropic disturbance in a fluid having one or more discontinuities in the vorticity profile.
2. A surface wave associated with the free boundary of a solid, such that a surface particle describes an ellipse whose major axis is normal to the surface and whose center is at the undisturbed surface. At maximum particle displacement away from the solid surface the motion of the particle is opposite to that of the wave.
The propagation velocity of a Rayleigh wave is slightly less than that of a shear wave in the solid; the wave amplitude of the Rayleigh wave diminishes exponentially with depth.
rays (abbr R)
See aurora.
ray tracing
A procedure used in the graphical determination of the path followed by a single ray of radiant energy as it travels through media of varying index of refraction.
reaction balance
A type of thrust meter using a balance to measure the static thrust of a rocket or jet engine.
reaction engine
An engine that develops thrust by its reaction to a substance ejected from it; specifically, such an engine that elects a jet or stream of gases created by the burning of fuel within the engine. Also called reaction motor.
A reaction engine operates in accordance with Newton third law of motion, i.e., to every action (force) there is an equal and opposite reaction. Both rocket engines and jet engines are reaction engines.
reaction motor = reaction engine.
reaction propulsion
Propulsion by reaction to a jet or jets ejected from one or more reaction engines.
reaction time
In human engineering, the interval between an input signal (physiological) or a stimulus (psychophysiological) and the response elicited by the signal.
reaction turbine
A type of turbine having rotor blades shaped so that they form a ring of nozzles, the turbine being rotated by the reaction of the fluid ejected from between the blades. Compare impulse turbine.
reactor = nuclear reactor.
reactor core
In a nuclear reactor the region containing the fissionable material.
In computer operations, to acquire information, usually from some form of storage. See write.
read in
In computer operations, to introduce information into storage.
1. The action of a radio transmitter transmitting data either instantaneously with the acquisition of the data or by playing of a magnetic tape upon which the data have been recorded. See instantaneous readout.
2. The data transmitted by the action described in sense 1.
3. In computer operations, to extract information from storage.
readout indicators
Any type of indicating instrument from which meaningful information and data can be directly obtained and used.
readout station
A recording or receiving radio station at which data are received from a transmitter in a probe, satellite, or other spacecraft.
real time
Time in which reporting on events or recording of events is simultaneous with the events.
For example, the real time of a satellite is that time in which it simultaneously reports its environment as it encounters it; the real time of a computer is that time during which it is accepting data.
real-time data
Data presented in usable form at essentially the same time the event occurs.
The delay in presenting the data must be small enough to allow a corrective action to be taken if required.
rearward acceleration
See physiological acceleration.
Reamer temperature scale
A temperature scale in which, under a pressure of 1 atmosphere, the ice point is 0 degrees and the boiling point of water is 80 degrees.
An oxygen system with a circuit closed to the atmosphere, to which oxygen is added to meet the user's needs; carbon dioxide and water vapor are removed from the expired gas.
received power
In radar, the power of a target signal received at the antenna. This power is normally of the order of microwatts as compared to the megawatts of transmitted power. Also called scattered power.
1. The initial component or sensing element of a measuring system. For example, the receiver of a thermoelectric thermometer is the measuring thermocouple.
2. An instrument used to detect the presence of and to determine the information carried by electromagnetic radiation. A receiver includes circuits designed to detect, amplify, rectify, and shape the incoming radiofrequency signals received at the antenna in such a manner that the information- containing component of this received energy can be delivered to the desired indicating or recording equipment.
A sensory nerve ending or organ in a living organism that is sensitive to physical or chemical stimuli.
1. A direction 180 degrees from a given direction.
2. The quotient of 1 divided by a given number.
reciprocal centimeter
See wave number, note.
reciprocating engine
An engine, especially an internal-combustion engine, in which a piston or pistons moving back and forth work upon a crankshaft or other device to create rotational movement.
See principle of reciprocity.
The psychological process in which an observer so interprets the visual stimuli he receives from a distant object that he forms a correct conclusion as to the exact nature of the object.
Recognition is a more subtle phenomenon than the antecedent step of detection, for the latter involves only the simpler process of interpreting visual stimuli to the extend of concluding that an object is present at some distance from the observer.
The process by which a positive and a negative ion join to form a neutral molecule or other neutral particle, also process by which radicals or dissociations species join to form molecules.
Recombination is applied both to the simple case of capture of free electrons by positive atomic or molecular ions, and also to the more complex case of neutralization of a positive small ion by a negative small ion or a similar (but much more rare) neutralization of large ions. Recombination is, in general, a process accompanied by emission of radiation. The light emitted from the channel of a lightning stroke is recombination radiation as is airglow. The much less concentrated recombinations steadily occurring in all parts of the atmosphere where ions are forming and disappearing does not yield observable radiation. The rate at which electrons, small ions, and large ions recombine is a function of their respective mobilities and of their concentration. The former dependence is expressed in terms of the recombination coefficient of the particular ion type.
recombination coefficient
A measure of the specific rate at which oppositely charged ions join to form neutral particles (a measure of ion recombination). Compare combination coefficient.
recombination energy
The energy released as heat or light when two oppositely charged ions join to form a neutral atom or molecule, or two dissociated atoms combine to form a stable molecule.
Of a rocket vehicle or one of its parts, so designed or equipped as to be located after flight and recovered with or without damage.
1. The procedure or action that obtains when the whole of a satellite, or a section, instrumentation package, or other part of a rocket vehicle is retrieved after a launch, as in recovery was counted upon to give added data.
2. The conversion of kinetic energy to potential energy such as in the deceleration of air in the duct of a ramjet engine. Also called ram recovery.
3. In flying, the action of a lifting vehicle returning to an equilibrium attitude after a nonequilibrium maneuver.
recovery capsule
A capsule designed to be recovered after reentry. See reentry vehicle.
recovery gear
The devices and equipment used to mark and locate a nose cone or other part of a rocket vehicle after impact.
recovery package
A package attached to a reentry or other body designed for recovery, containing devices intended to locate the body after impact.
This package may, for example, release a balloon that will buoy up a reentry body (if it impacts in water) and serve as a radio beacon or light.
recovery temperature
Short for adiabatic recovery temperature.
1. In metals, the change from one crystal structure to another, as occurs on heating or cooling through a critical temperature.
2. The formation of a new strain-free grain structure from that existing in cold-worked metal, usually accomplished by heating.
rectangular curvilinear coordinates
See curvilinear coordinates.
A static device having an asymmetrical conduction characteristic which is used to convert attending current into direct current.
A rotating device for this purpose is called a converter. Compare inverter.
1. In a countdown to stop the count and to return to an earlier point in the countdown, as in we have recycled, now at T minus 80 and counting. Compare hold.
2. To give a completely new checkout to a rocket or other object.
The condition occurring under negative g in which objects appear to have a red coloration due to uncertain causes, possibly venous congestion of engorged eyelids. Compare blackout, sense 3.
red shift
In astronomy, the displacement of observed spectral lines toward the longer wavelengths of the red end of the spectrum. Compare space reddening.
The term red shift is applied both to the Doppler effect caused by the relative speed of recession of the observed body and the gravitational or relativistic shift in which the frequency of light emitted by atoms in stellar atmosphere is decreased by a factor proportional to the mass-radius relationship of the star.
Red Spot Hollow
See Great Red Spot.
reduced frequency (symbol k)
The frequency of vibration of a body, or of the variation of the flow behind the body, expressed as the circular frequency times the representative length of the body divided by the velocity of the flow.
1. In information theory: of a source, the amount by which the logarithm of the number of symbols available at the source exceeds the average information content per symbol of the source.
The term redundancy has been used loosely in other senses. For example, a source whose output is normally transmitted over a given channel has been called redundant, if the channel utilization index is less than unity.
2. The existence of more than one means for accomplishing a given task, where all means must fail before there is an overall failure to the system.
Parallel redundancy applies to systems where both means are working at the same time to accomplish the task, and either of the system is capable of handling the job itself in case of failure of the other system. Standby redundancy applies to a system where there is an alternative means of accomplishing the task that is switched in by a malfunction sensing device when the primary system fails.
Reech number
The reciprocal lg/V2 , of the Froude number, where g is the acceleration of gravity; l is a characteristic length; and V is a characteristic speed.
The event occurring when a spacecraft or other object comes back into the sensible atmosphere after being rocketed to higher altitudes; the action involved in this event.
reentry body
That part of a space vehicle that reenters the atmosphere after flight above the sensible atmosphere.
reentry nose cone
A nose cone designed especially for reentry, consisting of one or more chambers protected by an outer shield. See heat sink.
reentry trajectory
That part of a rocket's trajectory that begins at reentry and ends at target or at the surface.
If the rocket is unguided at reentry, its reentry trajectory is ballistic in character.
reentry vehicle
Any payload carrying vehicle designed to leave the sensible atmosphere and then return through it to earth.
This term applies both to return vehicles from orbital or space payloads and to boostglide vehicles.
reference ellipsoid
An ellipsoid of revolution used as a datum for geodetic measurements. See geoid.
reference frame = coordinate system.
reference line = datum line.
reference plane = datum plane.
reference point = datum point.
reference signal
In telemetry, the signal against which data-carrying signals are compared to measure differences in time, phase, frequency, etc.
An assumed zero value of a quantity relative to which magnitudes of the quality are measured, or a structure having this zero value of the quantity; e.g., a voltage measured relative to the ground as a referent.
reflectance (symbol ρ)
The ratio of the radiant flux reflected by a body to that incident upon it. Also called reflection factor.
For an opaque body, the sum of the reflectance and the absorptance for the incident radiation is unity ρ + α = 1.
reflected code = cyclic code.
reflected ray
A ray extending outward from a point of reflection.
reflected wave
1. A shock wave, expansion wave, or compression wave reflected by another wave incident upon a wall or other boundary.
2. In electronics, a radio wave reflected from a surface or object.
reflecting telescope
A telescope which collects light by means of a concave mirror.
The process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached. See albedo, reflectivity, radar reflectivity.
For true reflection to occur there must be a real discontinuity of the index of refraction or at least it must change over an interfacial layer of thickness small compared to the wavelength of the radiation. If the change of refractive index is gradual (as may occur in a stratified medium) radiation may be returned by a process of continuous refraction, not to be confused with reflection. In radar, the term reflection is often applied to the return of radio energy from a volume of precipitation or cloud particles, where scattering is the important process. When the scale of the irregularities on the reflecting surface is small compared to the wavelength, regular or specular reflection (also called mirror reflection, regual reflection) results; if the irregularities are large compare of reflection is not affected by wavelength except as the relative scale of the irregularities of the surface change with wavelength. the fraction of the incident radiation reflected does depend on wavelength because of the selective nature of the absorptivity and transmissivity. The idealized white body is a total reflector; a black body reflects none of the incident radiation. The laws of specular reflection are: (first law) the reflected ray lies in the same plane as the incident ray and the normal to the surface at the point of incidence; and (second law) the angle of reflection equals the angle of incidence, both measured from the normal to the surface.
reflection coefficient
A measure of the quality of specular reflection produced by a given surface; defined as the ratio of the radiant energy reflected along the geometrical reflection path to the total that is incident upon the surface. By definition, a reflection coefficient of 1.0 implies perfect specular reflection. Compare reflectivity.
Reflection coefficients of less than 1.0 occur either as a result of energy loss by absorption at the reflection surface, or by scattering of the energy out of the geometrical reflection path due to the diffuse or irregular nature of the reflecting surface. Note that the reflection coefficient varies with wavelength since a surface which might appear to be rough at very short wavelengths is much smoother to longer wavelength radiation. It also varies with polarization.
1. A measure of the fraction of radiation reflected by a given surface; defined as the ratio of the radiant energy reflected to the total that is incident upon that surface. Compare reflection coefficient. See radar reflectivity.
The reflectivity of any given substance is, in general, a variable strongly dependent upon the wavelength of the radiation in question. The reflectivity of a given surface for a specified broad spectral range, such as the visible spectrum or the solar spectrum, is referred to as the albedo.
2. In thermal radiation, a property of a material, measured as the reflectance of a specimen of the material that is thick enough to be completely opaque and has an optically smooth surface.
1. In general, any object that reflects incident energy; usually it is a device designed for specific reflection characteristics. See retroreflector, corner reflector, parabolic reflector, radar reflector.
2. In an antenna, a parasitic element located in a direction other than the general direction of the major lobe of radiation.
3. A material of high scattering cross section that surrounds a reactor core to reduce the escape of neutrons, many of which are reflected back into the core.
4. A repeller.
refracted ray
A ray extending onward from the point of refraction.
refracted wave
A wave that has had its direction of motion changed by refraction.
refracting telescope
A telescope which collects light by means of a lens or system of lenses. Also called refractor.
The process in which the direction of energy propagation is changed as the result of a change in density within the propagating medium, or as the energy passes through the interface representing a density discontinuity between two media. In the first instance the rays undergo a smooth bending over a finite distance. In the second case the index of refraction changes through an interfacial layer that is thin compared to the wavelength of the radiation; thus, the refraction is abrupt, essentially discontinuous. See atmospheric refraction. Compare reflection, diffraction, scattering.
refraction error
See astronomical refraction error, terrestrial refraction error, curved-path error.
refraction index = index of refraction.
refractive index = index of refraction.
refractive modulus = modified index of refraction.
1. The algebraic difference between an index of refraction and unity.
For the atmosphere, refractivity may be more conveniently expressed in N-units: N = (n - 1) 10E6 The deviation of the refractivity at any altitude from the usual standard profile is expressed in B-units (for radiofrequencies up to 20 kilomegacycles):
B = N + 0.12h where h is altitude in feet.

The deviation of the refractivity at any altitude from the gradient at which the refraction curvature of a tangential ray will match the curvature of the earth may be expressed in M- units:
M = N + 0.048h where 0.048 is 10E6 divided by the radius of the earth in feet.

2. = index of refraction.
This usage should be discouraged.
An instrument for measuring the index of refraction of a liquid, gas or solid.
refractor = refracting telescope.
A material, usually ceramic, that resists the action of heat, does not fuse at high temperatures, and is very difficult to break down.
refractory metal
A metal with melting point above 4000 degrees F.
Usually refers to columbium, molybdenum, tantalum, or tungsten.
Capable of undergoing refraction.
1. = positive feedback.
2. In computer operations, the process of restoring a storage device, whose information storing state may deteriorate, to its latest undeteriorated state. See rewrite.
regenerative cooling
The cooling of a part of an engine by the fuel or propellant being delivered to the combustion chamber; specifically, the cooling of a rocket-engine combustion chamber or nozzle by circulating the fuel or oxidizer, or both, around the part to be cooled.
regenerative detector
A demodulator whose gain or conversion ratio is increased by the addition of positive feedback or regeneration at the carrier frequency.
The sensitivity, small-signal selectivity, and distortion are increased over those found in a detector without regeneration.
regenerative engine
A liquid propellant rocket engine cooled by regenerative cooling.
A device used in a thermodynamic process for capturing and returning to the process heat that would otherwise be lost. Also called a heat exchanger (which see).
A portion of the ionosphere usually characterized by a particular altitude or range of altitudes, in which concentration of free electron tend to form.
region of escape = exosphere.
A device capable of retaining information, often that contained in a small subset (e.g., one word) of the aggregate information in a digital computer. See storage.
The statistical counterpart or analog of the functional expression, in ordinary mathematics, on one variable in terms of others. Thus, regression curve, regression coefficient.
regression of the nodes
Precessional motion of a set of nodes. See precession.
The expression is used principally with respect to the moon, the nodes of which make a complete westerly revolution in approximately 18.6 years.
regular reflection = specular reflection.
regular reflector = specular reflector.
reheat = reheating (especially in sense 1).
1. The addition of heat to a working fluid in an engine after a partial expansion.
2. The retention of heat in a fluid, as after passing through a turbine stage, owing to the inefficiency of the stage.
Of angle measurements in navigation, measured from the heading of a craft, as relative bearing.
relative angular momentum
The moment of the relative momentum about a point. See angular momentum.
relative coordinate system
Any coordinate system which is moving with respect to an inertial coordinate system.
Referred to a relative system, various apparent forces arise in Newton laws owing to motion of the system. See, e.g., centrifugal force, coriolis force.
relative distance
See relative movement, note.
relative humidity (symbol U )
The (dimensionless) ratio of the actual vapor pressure of the air to the saturation vapor pressure. The corresponding ratios of specific humidity or of mixing ratio give approximations of sufficient accuracy for many purposes in meteorology. The relative humidity is usually express in percent. Also called humidity. See absolute humidity, dew point.
The ratio of mixing ratio to saturation mixing ratio is preferred as a definition of relative humidity by the International Meteorological Organization.
relative momentum
The product of the mass of a particle and its relative velocity; or, in the case of a fluid, the product of density and relative velocity. See momentum.
relative motion = apparent motion, relative movement. See motion.
relative movement
Motion of one object or body measured relative to another. Usually called apparent motion when applied to the change of position of a celestial body as observed from the earth. Also called relative motion.
The expression is usually used in connection with problems involving motion of one craft or vehicle relative to another, the direction of such motion being called direction of relative movement and the speed of such motion being called speed of relative movement or relative speed. Distance relative to a specified reference point, usually one in motion, is called relative distance.
relative position
A point defined with reference to another position, either fixed or moving.
The coordinates of such a point are usually bearing, true or relative, and distance from an identified reference point.
relative scattering function
See relative scatter intensity.
relative scatter intensity
For scattering of radiation under any given set of physical conditions: the ratio of the radiant intensity scattered in any given direction to the radiant intensity scattered in the direction of the incident beam.
The value of this ratio is a function of the angle between the direction in question and the directions of the incident beam. Thus, it may be symbolized as f(ρ), the relative scattering function. Compare scattering function. See scatter angle.
relative speed = speed of relative movement.
relative sunspot number
A measure of sunspot activity, computed from the formula R = k (10 g + f) where R is the relative sunspot number; f is the number of individual spots; g is the number of groups of spots; and k a factor that varies with the observer (his personal equation), the seeing, and the observatory (location and instrumentation). Also called sunspot number, sunspot relative number, Wolf number, Wolf-Wolfer number, Zurich number.
relative vorticity
See absolute vorticity.
In general, pertaining to material, as a particle, moving at speeds which are an appreciable fraction of the speed of light thus increasing the mass.
relativistic mass equation
The equation
m = m0 [1 - (v2/c2)]-1/2 = m0/(1 - β2)1/2
where β = v/c for the relativistic mass m of a particle or body of rest mass m 0 when its velocity is v. See relativistic velocity.
relativistic particle
A particle with a velocity so large that its relativistic mass exceeds its rest mass by an amount which is significant for the computation or other considerations at hand. See relativistic velocity.
relativistic red shift
See red shift, note.
relativistic velocity
A velocity sufficiently high that some properties of a particle of this velocity have values significantly different from those obtaining when the particle is at rest. See rest mass.
The property of most interest is the mass. For many purposes, the velocity is relativistic when it exceeds about one-tenth the velocity of light. See also Fitzgerald-Lorentz contraction.
A principle that postulates the equivalence of the description of the universe, in terms of physical laws, by various observers, or for various frames of reference. See relativistic mass equation, mass-energy equivalence.
relativity theory
See relativity.
relaxation time
1. In general, the time required for a system, object, or fluid to recover to a specified condition or value after disturbance.
2. Specifically, the time taken by an exponentially decaying quantity to decrease in amplitude by a factor of 1/ e = 0.3679.
Of a piece of equipment or a system, the probability of specified performance for a given period of time when used in the specified manner.
Abbreviation for roentgen-equivalent-man.
remaining body
That part of a rocket or vehicle that remains after the separation of a fallaway section or companion body.
In a multistage rocket, the remaining body diminishes in size successively as each section or part is cast away and successively becomes a different body.
remanence (symbol B)
The magnetic flux density which remains in a magnetic circuit after the removal of an applied magnetomotive force. Also called retentivity.
This should not be confused with residual flux density. If the magnetic circuit has an airgap, the remanence will be less than the residual flux density.
remote control
Control of an operation from a distance, especially by means of electricity or electronics; a controlling switch, lever, or other device used in this kind of control; as in remote-control armament, remote-control switch , etc.
remote indicating
Of an instrument, displaying indications at a point remote from its sensing element, often by electrical or electronic means.
remote velocity
The velocity of an object taken as a whole relative to the surrounding fluid at a point undisturbed by the moving object. Distinguished from the local velocity of any of the object's parts.
Pertaining to the kidneys.
1. The event of two or more objects meeting with zero relative velocity at a preconceived time and place.
2. The point in space at which such an event takes place, or is to take place.
A rendezvous would be involved, for example, in servicing or resupplying a space station.
Abbreviation for roentgen-equivalent-physical.
An electrode whose primary function is to reverse the direction of an electron stream. Also called reflector.
1. To restore a storage device to a prescribed state.
2. To place a binary cell in the initial or zero state. See clear.
In celestial mechanics and trajectory analysis, the deviation between an observed and a computed value, usually in the sense observed minus computed.
residual flux density (symbol Br)
The magnetic flux density at which the magnetizing force is zero when the material is in a symmetrically magnetized condition. See remanence.
residual air
The volume of air remaining in the lungs after a maximal expiration.
residual load
Of a vehicle, the sum of the payload, all items directly associated with the payload, and other relatively fixed weights of the overall vehicle; calculated as the difference between gross weight and the sum of propellant, tank, structure, and power-plant weights.
residual stress
In structures, any stress in an unloaded body. These stresses arise from local yielding of the material due to machining, welding, quenching, cold work, etc.
resistance (symbol R )
1. In electricity, the factor by which the square of the instantaneous conduction current must be multiplied to obtain the power lost by heat dissipation or other permanent radiation of energy away from the electrical current.
2. In mechanics, the opposition by frictional effects to forces tending to produce motion.
resistivity (symbol ρ)
In electricity, a characteristic proportionality factor equal to the resistance of a centimeter cube of a substance to the passage of an electric current perpendicular to two parallel faces. Also called specific resistance. R = ρ( l/A ) where R is the resistance of a uniform conductor, l is the length, A is its cross-sectional area, and ρ is its resistivity.
1. The ability of a film, a lens, a combination of both, or a vidicon system to render barely distinguishable a standard pattern of black and white lines.
When the resolution is said to be 10 lines per millimeter, it means that the pattern whose line plus space width is 0.1 millimeter is barely resolved, the finer patterns are not resolved, and the coarser patterns are more clearly resolved. In satellite television systems the limiting element is the television scanning pattern.
2. In radar, the minimum angular separation at the antenna at which two targets can be distinguished (a function of beamwidth); or the minimum range at which two targets at the same azimuth can be separated (equal to one-half the pulse length).
3. Of a gyro, a measure of response to small changes in input; the maximum value of the minimum input change that will cause a detectable change in the output for inputs greater than the threshold, expressed as a percent of one half the input range.
resolving power
1. = resolution, senses 1 and 2.
2. In a unidirectional antenna, the reciprocal of its beam width measured in degrees.
The resolution of a directional radio system can be different from the resolving power of its antenna, since the resolution is affected by other factors.
1. The phenomenon of amplification of a free wave or oscillation of a system by a forced wave or oscillation of exactly equal period. The forced wave may arise from an impressed force upon the system or from a boundary condition. The growth of the resonant amplitude is characteristically linear in time.
2. Of a system in forced oscillation, the condition which exists when any change, however small, in the frequency of excitation causes a decrease in the response of the system.
resonance frequency
A frequency at which resonance exists. Also called resonant frequency.
In case of possible confusion, the type of resonance must be indicated, as velocity resonance frequency.
resonant frequency = resonance frequency.
In radio and radar applications, a circuit which will resonate at a given frequency, or over a range of frequencies, when properly excited.
A very important type of resonator is the cavity resonator, a closed hollow volume having conducting walls. The frequency at which these cavities will resonate is a function of their volume and shape; thus, they are used for making accurate frequency comparisons and for generating radio frequencies, usually in the microwave region.
The interchange of gases of living organisms and the gases of the medium in which they live.
Respiration applies to the interchange by any channel as pulmonary respiration, cutaneous respiration, etc.
1. In general, an instrument that indicates reception of an electric or electromagnetic signal.
2. = transponder.
responder beacon = transponder beacon.
Of a device or system, the motion (or other output) resulting from an excitation under specified conditions.
Modifying phrases must be prefixed to the term response to indicate what kinds of input and output are being utilized. The response characteristic, often presented graphically, gives the response as a function of some independent variable such as frequency or direction. For such purposes it is customary to assume that other characteristics of the input (for example, voltage) are held constant.
A radio receiver which receives the reply from a transponder and produces an output suitable for feeding to a display system.
A responsor is usually combined in a single unit with an interrogator, which sends out the pulse that triggers a transponder, the combined unit being called an interrogator-responsor.
Specifically, the act of firing a stage of a rocket after a previous powered flight and a coast phase in a parking orbit.
rest mass
According to relativistic theory, the mass which a body has when it is at absolute rest. Mass increases when the body is in motion according to
where m is its mass in motion; m0 is its rest mass; v is the body's speed of motion; and c is the speed of light.
Newtonian physics, in contrast with relativistic physics, makes no distinction between rest mass and mass in general.
restricted propellant
A solid propellant having only a portion of its surface exposed for burning, the other surfaces being covered by an inhibitor.
In solid-propellant rockets, a layer of fuel containing no oxidizer, or of noncombustible material, adhered to the surface of the propellant so as to prevent burning in that region.
The sum of two or more vectors.
resultant wind
The vectorial average of all wind directions and speeds at a given place for a certain period.
Ret, Reti
International Astronomical Union abbreviations for Reticulum. See constellation.
retentivity = remanence.
International Astronomical Union abbreviation for Reticulum. See constellation
A system of lines, wires, etc., placed in the focal plane of an optical instrument to serve as a reference. Also called reticule.
A crosshair is a hair, thread, or wire constituting part of a reticle.
reticule = reticle.
Reticulum (abbr Ret, Reti)
See constellation.
See trace, note.
To ignite a retrorocket.
retroflector = retroreflector.
retrograde motion
1. Motion in an orbit opposite to the usual orbital direction of celestial bodies within a given system. Specifically, of a satellite, motion in a direction opposite to the direction of rotation of the primary.
2. The apparent motion of a planet westward among the stars. Also called retrogression.
retrogression = retrograde motion.
A rocket unit built into or strapped to a spacecraft that provides retrothrust.
Reflection wherein the reflected rays return along paths parallel to those of their corresponding incident rays. Also called retroflection.
Any instrument used to cause reflected rays to return along paths parallel to those of their corresponding incident rays. Also called retroflector.
One type of retroreflector, the corner reflector, is an efficient radar target.
(From retro acting.) A rocket fitted on or in a spacecraft, satellite, or the like to produce thrust opposed to forward motion.
Thrust used for a braking maneuver; reverse thrust.
The sequence of events preparatory to, and programmed to follow, the retrofiring for spacecraft reentry.
1. The persistence of sound in an enclosed space, as a result of multiple reflections after the sound source has stopped.
2. The sound that persists in an enclosed space, as a result of repeated reflection or scattering after the source of the sound has stopped.
reverberation time
In acoustics, the time required for the time average of the sound energy density, initially in a steady state, to decrease, after the source is stopped, to one-millionth of its initial value. The unit is the second.
reverse thrust
Thrust applied to a moving object in a direction to oppose the object's motion.
reversing layer
See photosphere, note.
A wall of concrete, earth, sandbags, or the like installed for protection, as against the blast of exploding fuel during a rocket abort.
1. Motion of a celestial body in its orbit; circular motion about an axis usually external to the body.
In some contexts, the terms revolution and rotation are used interchangeably but, with reference to the motions of a celestial body, revolution refers to motion in an orbit or about an axis external to the body, whereas rotation refers to motion about an axis within the body. Thus, the earth revolves about the sun annually and rotates about its axis daily.
2. One complete cycle of the movement of a celestial body in its orbit, or of a body about an external axis, as a revolution of the earth about the sun.
To move in a path about an axis, usually external to the body accomplishing the motion, as in the planets revolve about the sun. Hence revolution. See rotate.
In a storage device whose information storing state may be destroyed by reading, the process of restoring the device to its state prior to reading.
Reynolds number (symbol R, NRe)
(After Osborne Reynolds (1842-1912), English scientist.) A nondimensional parameter representing the ratio of the momentum forces to the viscous forces in fluid flow.
In aerodynamics, the Reynolds number of fluid flow about a body is often expressed as the fraction ρ = Vl/μ where ρ is the density of the fluid, V is its velocity, l is a characteristic dimension of the body, and μ is the coefficient of viscosity of the fluid. See critical Reynolds number, effective Reynolds number. As applied to the flow of gas through a circular tube the Reynolds number is a dimensionless quantity equal to the product of the gas density, ρ , in grams per cubic centimeter; times the flow velocity v, in centimeters per second; times the tube diameter, d, in centimeters; divided by the viscosity coefficient μ, in poises:
R = ρ vd/μ
Reynolds stresses
In the mathematical treatment of a viscous, incompressible, homogeneous fluid in turbulent motion, terms which represent the transfer of momentum due to turbulent fluctuations.
RF (abbr) = radiofrequency.
A satellite of Saturn orbiting at a mean distance of 527,000 kilometers.
rhombic antenna
An antenna composed of long-wire radiators comprising the sides of a rhombus. The antenna usually is terminated in an impedance. The sides of the rhombus, the angle between the sides, the elevation, and the termination are proportional to give the desired directivity.
rho-theta system
1. Any electronic navigation system in which position is defined in terms of distance, or radius ρ and bearing θ with respect to a transmitting station. Also called an R-theta system.
2. Specifically, a polar coordinate navigation system providing data with sufficient accuracy to permit the use of a computer which will provide arbitrary course lines anywhere within the coverage area of the system.
ribbon parachute
A type of parachute having a canopy consisting of an arrangement of closely spaced tapes. This parachute has high porosity with attendant stability and slight opening shock.
rice grains = granules.
Of a combustible mixture: having a relatively high proportion of fuel to oxidizer; more precisely, having a value greater than stoichiometric.
Richardson number (symbol NRi)
A nondimensional number arising in the study of shearing flows of a stratified fluid:
NRi = gβ/(u/z)2
where g is the acceleration of gravity; β is a representative vertical stability (commonly θ/θz, where θ is potential temperature); and u/z a characteristic vertical shear.
In Richardson's original interpretation, the Richardson number is a characteristic ratio of work done against gravitational stability to energy transferred from mean to turbulent motion. Theoretical studies have placed the critical Richardson number variously from 1/4 to 2, with instability for smaller values and stability for greater.
right ascension
Angular distance east of the vernal equinox; the arc of the celestial equator, or the angle at the celestial pole, between the hour circle of a point on the celestial sphere, measured eastward from the hour circle of the vernal equinox through 24 hours.
Angular distance west of the vernal equinox, through 360 degrees, is sidereal hour angle.
(from German rille meaning groove ). A deep, narrow, depression on the lunar surface which cuts across all other types of lunar topographic features.
rime icing
Rime aircraft icing is opaque, brittle, and granular. It is formed by the rapid freezing of small supercooled water droplets, allowing air to be trapped in. It is generally less hazardous than glaze icing, because it usually forms more slowly and is more conformal to the existing aerodynamic surface. It is the most frequent type, composing about 75% of icing reports.
ring around
Self-interrogation of a beacon due to insufficient isolation between receiver and transmitter, i.e., the beacon transmitter pulse passes through the receiver and retriggers the transmitter.
ring counter chain
A series of bistable elements triggered in sequence.
An open chain is reset by an externally applied reset pulse; a closed chain, by feedback from the last element in the chain.
See lunar crater.
riometer = relative ionospheric opacity meter.
Rirti (abbr)
Recording infrared tracking instrument.
rise time
The time required for the leading edge of a pulse to rise from one-tenth of its final value to nine-tenths of its final value. Rise time is proportional to time constant. See decay time.
See tektite.
Robitzsch actinograph
A pyranometer developed by M. Robitzsch. Its design utilizes three bimetallic strips which are exposed horizontally at the center of a hemispherical glass bowl. The outer strips are white reflectors, and the center strip is a blackened absorber. The bimetals are joined in such a manner that the pen of the instrument deflects in proportion to the difference in temperature between the black and white strips.
A high-altitude sounding system consisting of a small solid-propellant research rocket carried aloft by an aircraft. The rocket is fired while the aircraft is in vertical ascent.
1. A projectile, pyrotechnic device, or flying vehicle propelled by a rocket engine.
2. A rocket engine; any one of the combustion chambers or tubes of a multichambered rocket engine.
rocket airplane
An airplane using a rocket or rockets for its chief or only propulsion.
rocket-assisted take-off
The full term for RATO.
rocket booster
A booster, senses 2 and 3.
rocket engine
A reaction engine that contains within itself, or carries along with itself, all the substances necessary for its operation or for the consumption or combustion of its fuel, not requiring intake of any outside substance and hence capable of operation in outer space. Also called rocket motor.
Chemical rocket engines contain or carry along their own fuel and oxidizer, usually in either liquid or solid form, and range from simple motors consisting only of a combustion chamber and exhaust nozzle to engines of some complexity incorporating, in addition, fuel and oxygen lines, pumps, cooling system, etc., and sometimes having two or more combustion chambers. Experimental rocket motors have used neutral gas, ionized gas, and plasma as propellants. See liquid-propellant rocket engine, solid-propellant rocket engine, ion rocket, plasma rocket.
rocket fuel
A fuel, either liquid or solid, developed for, or used by, a rocket.
rocket launcher
A device for launching a rocket. See launcher.
Rocket launchers are wheel mounted, motorized, or fixed for use on the ground; or they are mounted on aircraft, as under the wings; or they are installed below or on the decks of ships.
rocket motor = rocket engine.
rocket nozzle
The exhaust nozzle of a rocket.
rocket plane
An airplane powered by rocket engines.
rocket propellant (abbr RP)
1. Any agent used for consumption or combustion in a rocket and from which the rocket derives its thrust, such as a fuel, oxidizer, additive, catalyst, or any compound or mixture of these.
2. The ejected fluid in a nuclear rocket.
rocket propulsion
Reaction propulsion by a rocket engine.
rocket ramjet
A ramjet engine having a rocket mounted within the ramjet duct, the rocket being used to bring the ramjet up to the necessary operating speed. Sometimes called a ducted rocket.
The science or study of rockets, including theory, research, development, experimentation, and application; the art or science of using rockets.
rocket ship
An aircraft, space-air vehicle, or spacecraft using rocket propulsion.
rocket sled
A sled that runs on a rail or rails and is accelerated to high velocities by a rocket engine.
This sled is used in determining g-tolerances and for developing crash survival techniques. Rocket sleds are at Edwards Air Force Base, Holloman Air Force Base, and the Naval Ordnance Test Station. See snort track.
rocketsonde = meteorological rocket.
rocket thrust
The thrust of a rocket engine usually expressed in pounds.
On a test stand, rocket thrust may be measured by use of strain gages, thrust-balancing pistons, dynamometers, or spring scales, each calibrated in pounds to represent the static weight moved by the engine.
rocket thrust chamber
That part of a rocket engine comprised of the combustion chamber and the diverging section of the nozzle.
rocket vehicle
A vehicle propelled by a rocket engine, used to place a satellite in orbit, place a missile upon target, carry a passenger over a rail as on a rocket sled, etc.
A high-altitude sounding system consisting of a small solid-propellant research rocket carried aloft by a large plastic balloon.
The rocket is fired near the maximum altitude of the balloon flight. It is a relatively mobile sounding system and has been used extensively on shipboard.
A type of photoreceptive cell in the retina of the mammalian eye. Rods are involved in detection of movement and scotopic vision (night vision).
rod threshold
The dimmest illumination in which the rods of the retina can function.
A unit of radiation, that quantity of X-rays or gamma rays which will produce, as a consequence of ionization, 1 electrostatic unit of electricity in 1 cubic centimeter of dry air measured at 0 degrees C and standard atmospheric pressure.
(abbr rem) A unit of radiation which when absorbed by a human being, produces the same effect as the absorption of 1 roentgen of high-voltage X-rays.
(abbr rep) A unit measuring a purely physical effect of radiation by the number of ion pairs produced per unit volume of target material per time unit. One rep is equivalent to the absorption of 93 erfs per gram of tissue.
Roentgen ray = X-ray.
1. The act of rolling; rotational or oscillatory movement of an aircraft or similar body about a longitudinal axis through the body - called roll for any degree of such rotation.
2. The amount of this movement, i.e., the angle of roll.
roll axis
A longitudinal axis through an aircraft, rocket, or similar body, about which the body rolls.
A roll axis may be a body, wind, or stability axis, or any other lengthwise axis.
rolling axis = roll axis.
rolling moment
A moment that tends to rotate an aircraft, a rocket, etc., about a longitudinal axis. This moment is considered positive when it tends to depress the starboard side of the body.
roll out
In computer terminology, to read out of a storage device by simultaneously increasing by one the value of the digit in each column and repeating this r times (where r is the radix) and, at the instant the representation changes from ( r - 1) to zero: generating a particular signal, or terminating a sequence of signals, or originating a sequence of signals.
Romotar (abbr) = range-only measurement of trajectory and recording.
root chord
In aerodynamics, the chord of a lifting surface at the intersection of that surface with its supporting body, e.g., wing root chord.
root-mean-square error (symbol σ)
In statistics, the square root of the arithmetic mean of the squares of the deviations of the various items from the arithmetic mean of the whole. Also termed standard deviation.
root-mean-square sound pressure = effective sound pressure.
Code name for window, sense 2.
To turn about an internal axis. Said especially of celestial bodies. Hence rotation. Compare revolve.
rotating cylinder gage
A type of molecular drag gage.
rotating disk gage
A type of molecular drag gage.
rotating Reynolds number
A nondimensional number arising in problems of a rotating viscous fluid. Also called rotation Reynolds number.
It may appear either as Ωh2/v in which case it equals one-half the square root of the Taylor number, or as Ωr2/v where r is a suitable radius, h is a representative depth, &Omega is the absolute angular speed, and v is the kinematic viscosity.
1. Turning of a body about an axis within the body, as the daily rotation of the earth. See revolution.
2. One turn of a body about an internal axis, as a rotation of the earth.
rotational speed (symbol n)
Revolutions per unit time.
rotational wave = shear wave.
rotation Reynolds number = rotating Reynolds number.
Roti (abbr)
Recording optical tracking instrument.
See gyro.
rotor angular momentum
(symbol H) Of a gyro, the product of spin angular velocity and rotor moment of inertia, usually expressed in gram centimeters squared per second. It is a measure of the ability of a gyrorotor to maintain the spin axis fixed in space.
rotor moment of inertia
The moment of inertia of a gyro rotor about its spin axis.
rounding error
In computations, the error resulting from deleting the less significant digits of a quantity and applying some rule of correction to the part retained. Also called round-off error.
round off
To delete less significant digits from a number and possible apply some rule of correction to the part retained.
round-off error = rounding error.
A set of instructions arranged in proper sequence to cause a computer to perform a desired operation, such as the solution of a mathematical problem.
RP (abbr) = rocket propellant.
Used with a number in designations of different propellants, as in RP-1 .
A rocket fuel consisting essentially of kerosene.
R-theta system = rho-theta system.
R-T unit
The receiver-transmitter portion of a radar beacon system.
rubber-base propellant
A solid propellant mixture in which the oxygen supply is obtained from a perchlorate and the fuel is provided by a synthetic rubber latex.
A form of combustion instability, especially in a liquid-propellant rocket engine, characterized by a low-pitched, low-frequency rumbling noise; the noise made in this kind of combustion.
rupture disk = burst disk.
Back to Table of Contents