S

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

sabot

A device fitted around or in back of a projectile in a gun barrel or launching tube to support or protect the projectile or to prevent the escape of gas ahead of it.

The sabot separates from the projectile after launching.

Sagitta (abbr Sge, Sgte)

See constellation.

Sagittarius (abbr Sgr, Sgtr)

See constellation.

salvo launch

Act of launching two or more rockets simultaneously.

sample

In statistics, a group of observations selected from a statistical population by a set procedure. See random sample.

Samples may be taken at random or systematically. The sample is taken in an attempt to estimate the population.

sandwich

Of sandwich construction, as in sandwich panel, sandwich skin , etc.

sandwich construction

A type of construction in which two sheets, sides, or plates are separated by a core of stiffening material, generally lightweight. See honeycomb core.

Sarah

(From search and rescue and homing). A radio homing device originally designed for personnel rescue and now used in spacecraft recovery operations at sea.

saros

The eclipse cycle of about 18 years, almost the same length as 223 synodical months. See lunar cycle.

At the end of each saros the sun, moon, and line of nodes return to approximately the same relative positions and another series of eclipses begins, closely resembling the series just completed.

satellite

1. An attendant body that revolves about another body, the primary; especially in the solar system, a secondary body, or moon, that revolves about a planet. See table XIII for a list of satellites of the solar system.

2. A manmade object that revolves about a spatial body, such as Explorer I orbiting about the earth. See spacecraft, Table XIV.

3. Such a body intended and designed for orbiting, as distinguished from a companion body that may incidentally also orbit, as in the observer actually saw the orbiting rocket rather than the satellite.

4. An object not yet placed in orbit, but designed or expected to be launched into an orbit.

satelloid

A vehicle that revolves about the earth or other body, but at such altitudes as to require sustaining thrust to balance drag.

saturation-adiabatic lapse rate

A special case of process lapse rate, defined as the rate of decrease of temperature with height of an air parcel lifted in a saturation-adiabatic process through an atmosphere in hydrostatic equilibrium. Also called moist-adiabatic lapse rate.

Owing to the release of latent heat, this lapse rate is less than the dry-adiabatic lapse rate, and the differential equation representing the process must be integrated numerically. Wet-bulb potential temperature is constant with height in an atmosphere with this lapse rate.

saturation vapor pressure

1. The vapor pressure of a system, at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of the pure liquid or solid phase of that substance; that is, the vapor pressure of a system that has attained saturation but not supersaturation. Compare equilibrium vapor pressure, vapor tension.

The saturation vapor pressure of any pure substance, with respect to a specified parent phase, is an intrinsic property of that substance and is a function of temperature alone (see Clapeyron-Clausius equation).

2. = equilibrium vapor pressure.

Saturn

See planet, table.

saturnographic

Referring to positions on Saturn measured in latitude from Saturn's equator and in longitude from a reference meridian.

S-band

A frequency band used in radar extending approximately from 1.55 to 5.2 kilomegacycles per second.

scalar

Any physical quantity whose field can be described by a single numerical value at each point in space.

A scalar quantity is distinguished from a vector quantity by the fact that a scalar quantity possesses only magnitude, whereas a vector quantity possesses both magnitude and direction.

scalar acceleration

The square root of the sum of the squares of three orthogonal components of an acceleration.

scalar product

A scalar equal to the product of the magnitudes of any two vectors and the cosine of the angle θ between their positive directions. Also called dot product, direct product, inner product. See vector product.

For two vectors A and B, the scalar product is most commonly written A ^. B, read A dot B, and occasionally as (AB). If the vectors A and B have the components A_x, B_x, A_y, B_y, and A_z, B_z along rectangular Cartesian, x, y, and z axes, respectively, then A ^. B = A_xB_x + A_yB_y + A_zB_z = |A||B|cosθ = AB cosθ If a scalar product is zero, one of the vectors is zero or else the two are perpendicular.

scalar velocity

The square root of the sum of the squares of three orthogonal components of a velocity.

scale effect

Any variation in the nature of the flow and in the force coefficients associated with a change in value of the Reynolds number, i.e., caused by change is size without change in shape.

scale height (symbol h, h_s)

A measure of the relationship between density and temperature of any point in an atmosphere; the thickness of a homogeneous atmosphere which would give the observed temperature: h = kT/mg = R*T/Mg where k is the Boltzmann constant; T is the absolute temperature; m and M are the mean molecular mass and weight, respectively, of the layer; g is the acceleration of gravity; and R* is the universal gas constant. Compare virtual height.

scale model

A model of a different size from its prototype and having dimensions in some constant ratio to the dimensions of the prototype, especially such a model of smaller size than its prototype.

scale of 10 counter = decade counter.

scaler

A device that produces an output pulse whenever a prescribed number of input pulses have been received. Also called scaling circuit.

The number of input pulses per output pulse of a scaler is termed the scaling factor. A binary scaler is a scaler whose scaling factor is 2. A decade scaler is a scaler whose scaling factor is 10.

scaling circuit = scaler.

scaling factor

See scalar, note.

scanner

A radar mechanism incorporating a rotatable antenna, or radiator, motor drives, mounting, etc., for directing a searching radar beam through space and imparting target information to an indicator. See parabolic reflector.

scanning

In radar, the motion of the radar antenna assembly when searching for targets.

Scanning usually follows a systematic pattern involving one or more of the following: (1) In horizontal scanning (or searchlighting), the antenna is continuously rotated in azimuth around the horizon or in a sector (sector scanning); used to generate plan-position-indicator-scope displays. (b) Vertical scanning is accomplished by holding the azimuth constant but varying the elevation angle of the antenna; used in height-finding radars to generate the relative-height-indicator-scope display. (c) For conical scanning, a somewhat offcenter radiating element is rotated while its parabolic reflectors fixed in position so that the radiated beam generates a concially shaped volume with the antenna at the apex; used to determine accurate bearing and elevation angle of targets and employed in automatic tracking radars. (d) In helical scanning (or spiral scanning) the azimuth and elevation angle of the antenna are constantly varied so that at a given distance from the radar the radiated beam generates the surface of a hemisphere; used for radio direction finding, in certain types of search radars, and in tracking radars to search areas for targets.

scaphandre = full pressure suit.

scatter

1. = scattering.

2. The relative dispersion of points on a graph, especially with respect to a mean value, or any curve used to represent the points. See dispersion.

3. To accomplish scattering.

scatter angle

The angle between any given ray of scattered radiation and the incident ray. See relative scatter intensity, scattering.

Convention varies as to whether this angle is measured with respect to the direction in which the incident radiation was advancing or with respect to the direction from scatterer to radiation source.

scatter communication

See scatter propagation, note.

scattered power = received power.

scatterer = scattering particle.

scattering

The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. In scattering, no energy transformation results, only a change in the spatial distribution of the radiation. Also called scatter.

Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about one-tenth, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply.

scattering area coefficient

The dimensionless ratio of the scattering cross section to the geometric cross section of a scattering particle. Also called scattering area ratio.

scattering area ratio = scattering area coefficient.

scattering coefficient

A measure of the attenuation due to scattering of radiation as it traverses a medium containing scattering particles. Also called total scattering coefficient.

scattering cross section

The hypothetical area normal to the incident radiation that would geometrically intercept the total amount of radiation actually scattered by a scattering particle. It is also defined, equivalently, as the cross-section area of an isotropic scatterer (a sphere) which would scatter the same amount of radiation as the actual amount. Also called extinction cross section, effective area.

scattering function

The intensity of scattered radiation in a given direction per lumen of flux incident upon the scattering material.

scattering gage = scattering-type pressure gage.

scattering loss

That part of the transmission loss which is due to scattering within the medium or due to roughness of the reflecting surface.

scattering particle

The small particles responsible for scattering.

scattering power

In radar terminology, the ratio of the total power scattered by a target to the power in the incident wave, independent of the direction of scattering. The scattering power measures the loss of energy by absorption in the scatterers. Also called total scattering cross section. Compare radar reflectivity.

scattering-type pressure gage

An ionization gage in which measurement is made of the electrons scattered by collision of the gas molecules with the electrons from a p-particle emitter.

scatter propagation

Specifically, the long-range propagation of radio signals by scattering due to index of refraction inhomogeneities in the lower atmosphere. Also called tropospheric propagation.

Recognition of this process and the development of specialized equipment (basically, more powerful transmitters and sensitive receivers) has greatly increased the range of VHF and UHF communication. The over-all technique is known as scatter communication.

schlieren

(German, streaks, striae).

1. Regions of different density in a fluid, especially as shown by special apparatus.

2. Pertaining to a method or apparatus for visualizing or photographing regions of varying density in a field of flow. See schlieren photography, scintillation.

Used in compounds, such as schlieren lens, schlieren method, schlieren photograph, etc.

schlieren method

See schlieren.

schlieren photography

A method of photography for flow patterns that take advantage of the fact that light passing through a density gradient in a gas is refracted as though it were passing through a prism. Compare shadowgraph.

Schneider index

A composite weighted index of pulse and blood-pressure response to exercise, utilized as a test of physical efficiency.

Schuler pendulum

A hypothetical pendulum with a period of 84 minutes.

A simulated Schuler pendulum carried in a vehicle moving in the earth's gravitational field would always indicate the true vertical.

Schuler tuning

Adjusting a system performing the function of a pendulum so that is has a period of 84 minutes. See Schuler pendulum.

Schumann-Runge bands

See absorption band.

Schumann-Runge continuum

See absorption band.

scintillating counter = scintillation counter.

scintillation

1. Generic term for rapid variations in apparent position, brightness, or color of a distant luminous object viewed through the atmosphere.

If the object lies outside the earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; if the luminous source lies within the atmosphere, the phenomenon is termed terrestrial scintillation. As one of the three principal factors governing astronomical seeing, scintillation is defined as variations in luminance only. It is clearly established that almost all scintillation effects are caused by anomalous refraction occurring in rather small parcels or strata of air, schlieren, whose temperatures and hence densities differ slightly from those of their surroundings. Normal wind motions transporting such schlieren across the observer's line of sight produce the irregular fluctuations characteristic of scintillation. Scintillation effects are always much more pronounced near the horizon than near the zenith. Parcels of the order of only centimeters to decimeters are believed to produce most of the scintillatory irregularities in the atmosphere.

2. A flash of light produced in a phosphor by an ionizing event. See scintillation counter. 3. On a radar display, a rapid apparent displacement of the target from its mean position. Also called target glint or wander.

This includes but is not limited to shift of effective reflection point on the target.

scintillation counter

The combination of phosphor, photomultiplier tube, and associated circuits for counting scintillations, sense 2. Also called scintillating counter.

scintillation meter = scintillometer.

scintillometer

A type of photoelectric photometer used in a method of determining high altitude winds on the assumption that stellar scintillation is caused by atmospheric inhomogeneities ( schlieren) being carried along by the wind near tropopause level. Also called scintillation meter.

Scl, Scul

International Astronomical Union abbreviations for Sculptor. See constellation.

Sco, Scor

International Astronomical Union abbreviations for Scorpius. See constellation.

scope

The general abbreviation for an instrument of viewing, such as telescope, microscope, and oscilloscope. In radar installations, the cathode-ray oscilloscope indicators are commonly referred to as scopes or radarscopes.

Because of possible ambiguity this term should be avoided in formal reports.

Scor

International Astronomical Union abbreviation for Scorpius. See constellation.

Scorpius (abbr Sco, Scor)

See constellation.

scotopic vision

Vision associated with levels of illumination below approximately 0.01 foot-lambert, effective primarily in the detection of movement and low luminous intensities. Compare photopic vision. Also called parafoveal vision.

Scotopic vision is associated with rod function.

screaming

A form of combustion instability, especially in a liquid-propellant rocket engine, of relatively high frequency and characterized by a high-pitched noise.

screeching

A form of combustion instability, especially in an afterburner, of relatively high frequency and characterized by a harsh, shrill noise.

screen

1. A device to shield or separate one part of an apparatus from other parts, or to separate the effects of one part on others.

2. A surface on which images are displayed, as the face of a cathode-ray tube.

scrub

To cancel a scheduled firing, either before or during countdown.

Sct, Scut

International Astronomical Union abbreviations for Scutum. See constellation.

Scul

International Astronomical Union abbreviations for Sculptor. See constellation.

Sculptor (abbr Scl, Scul)

See constellation.

Scutum (abbr Sct, Scut)

See constellation.

sea clutter

See ground return.

sealed cabin

The occupied space of an aircraft or spacecraft characterized by walls which do not allow any gaseous exchange between the inner atmosphere and its surrounding atmosphere and containing its own mechanisms for maintenance of the inside atmosphere.

sea level = mean sea level.

sea-level pressure

The atmospheric pressure at mean sea level, either directly measured or, most commonly, empirically determined from the observed station pressure.

searchlighting

Horizontal scanning, in which the antenna beam is continuously rotated in azimuth.

search radar

A radar designed for the approximate location of (usually airborne) objects. Search radar beams are usually wide, wider in the vertical than in the horizontal, making it possible to scan large volumes of space quickly. Compare tracking radar.

sea return

See ground return.

seat belt = lap belt.

seat-to-head acceleration

See physiological acceleration.

second (abbr s)

See

The unit of time, the second, was defined originally as the fraction 1/86 400 of the mean solar day. The exact definition of the "mean solar day" was left to astronomers, but their measurements have shown that on account of irregularities in the rotation of the Earth, the mean solar day does not guarantee the desired accuracy. In order to define the unit of time more precisely, the 11th CGPM (1960) adopted a definition given by the International Astronomical Union which was based on the tropical year [see ephemeris second]. Experimental work had, however, already shown that an atomic standard of time-interval, based on a transition between two energy levels of an atom or a molecule, could be realized and reproduced much more accurately. Considering that a very precise definition of the unit of time of the International System, the second, is indispensable for the needs of advanced metrology, the 13th CGPM (1967) decided to replaced the definition of the second by the following:

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom (13th CGPM (1967), Resolution 1).

The previous is an excerpt (with the exception of the reference to "ephemeris second") from WWW version of the National Institute of Standards and Technology: Physics Laboratory's International System of Units (SI)

secondary

1. = secondary great circle.

2. A celestial body revolving around another body, its primary.

3. A particle emitted in secondary emission.

secondary circle = secondary great circle.

secondary cosmic radiation = secondary cosmic ray.

secondary cosmic rays

Secondary emission in the atmosphere stimulated by primary cosmic rays. See air shower.

secondary electron emission

The release of electrons from a surface which is bombarded by energetic electrons.

The yield or ratio of secondary to primary electrons is a function of the primary electron energy.

secondary emission

Emission of subatomic particles or photons stimulated by primary radiation; for example, cosmic rays impinging on other particles and causing them, by disruption of their electron configurations or even of their nuclei, to emit particles and photons or both in turn.

secondary great circle

A great circle perpendicular to a primary great circle, as a meridian. Also called secondary circle, secondary.

secondary instrument

An instrument whose calibration is determined by comparison with an absolute instrument.

secondary radar

See radar, note.

secondary radiation

Electromagnetic or particulate radiation resulting from absorption of other radiation in matter.

secondary scattering

See multiple scattering, scattering.

second law of thermodynamics

An inequality asserting that it is impossible to transfer heat from a colder to a warmer system without the occurrence of other simultaneous changes in the two systems or in the environment.

It follows from this law that during an adiabatic process, entropy cannot decrease. For reversible adiabatic processes entropy remains constant, and for irreversible adiabatic processes it increases. Another equivalent formulation of the law is that it is impossible to convert the heat of a system into work without the occurrence of other simultaneous changes in the system or its environment. This version, which requires an engine to have a cold source as well as a heat source, is particularly useful in engineering applications. See first law of thermodynamics.

Secor (abbr) = sequential collation of range.

Secor/DME

(Sequential collation of range/distance measuring equipment). A distance measuring system used in rocket tracking.

section

One of the cross-section parts that a rocket vehicle is divided into, each adjoining another at one or both of its end. Usually described by a designating word, as in nose section, aft section, center section, tail section, thrust section, tank section , etc.

sectionalized vertical antenna

A vertical antenna which is insulated at one or more points along its length. The insertion of suitable reactances or applications of a driving voltage across the insulated points results in a modified current distribution giving a more desired radiation pattern in the vertical plane.

sector scanning

See scanning.

secular

Pertaining to long periods of time on the order of a century, as secular perturbations, secular terms.

secular perturbations

Changes in the orbit of a planet or satellite that operate in extremely long cycles; long term perturbations.

secular terms

In the mathematical expression of an orbit, terms for very long period perturbations, in contrast to periodic terms , terms of short period.

Seebeck effect

The establishment of an electric potential difference tending to produce a flow of current in a circuit of two dissimilar metals the junctions of which are at different temperatures.

seeding

1. The introduction of atoms, such as sodium, with a low ionization potential into a hot gas for the purpose of increasing the electrical conductivity.

2. = cloud seeding.

seeing

A blanket term long used by astronomers for the disturbing effects produced by the atmosphere upon the image quality of an observed celestial body. Also called astronomical seeing.

Recent studies show that seeing is a combination of three principal and distinct effects that the human eye is not capable of distinguishing: (a) scintillation, i.e., fluctuations in brightness; (b) transverse displacements of the image; and (c) variations of the radius of curvature of the wavefront rendering the image in an out of focus.

seismic mass

The element in an accelerometer which is intended to serve as the force-summing member for applied accelerations, gravitational forces, or both.

selective absorption

Absorption which varies with the wavelength of radiation incident upon the absorbing substance. See absorption spectrum.

A substance which absorbs in such fashion is called a selective absorber and is to be contrasted with an ideal black body, white body, or gray body. In reality, all substances are selective absorbers when due regard is paid to their interaction with all wavelengths of the complete electromagnetic spectrum.

selective scattering

Scattering which varies with the wavelengths of radiation incident upon the scattering particles.

In general, the largest and most complex degree of selectivity is found for wavelengths nearly equal to the diameter of the scattering particles.

selectivity

The degree of falling off in response of a resonant device with departure from resonance.

selenocentric

Relating to the center of the moon; referring to the moon as a center.

selenographic

1. Of or pertaining to the physical geography of the moon.

2. Specifically, referring to positions on the moon measured in latitude from the moon's equator and in longitude from a reference meridian.

selenoid

A satellite of the earth's moon. (No such satellites are known).

selenology

That branch of astronomy that treats of the moon, its magnitude, motion, constitution, and the like. Selene is Greek for moon.

self-adaptive control system

A particular type of stability augmentation system which changes the response of a given control input by constantly sampling response and adjusting its gain, rather than having a fixed or selective gain system.

self-balancing potentiometer

See potentiometer.

self-excited vibration = self-induced vibration.

self-induced vibration

Vibration of a mechanical system resulting from conversion, within the system, of nonoscillatory excitation to oscillatory excitation. Also called self-excited vibration.

self-information = information content.

selsyn

(A trade name, from self-synchronous; often capitalized). An electrical remote indicating instrument operating on direct current, in which the angular position of the transmitter shaft, carrying a contact arm moving on a resistance strip, controls the pointer on the indicator dial.

semiactive homing guidance

Guidance in which a craft or vehicle is directed toward a destination by means of information received from the destination in response to transmissions from a source other than the craft.

In active homing guidance the information received is in response to transmissions from the craft. In passive homing guidance natural radiations from the destination are utilized.

semiactive tracking system

A trajectory measuring system which tracks a signal source normally aboard the target for other purposes, or a system that illuminates the target by use of a ground transmitter but requires no special electronics on board the missile, e.g., telemetry elsse, Dovap elsse, Cotat, Cotar, VHF/ ADF, pulse radar (skin track).

semicircular canals

Structures of the inner ear, the primary function of which is to register movement of the body in space. They respond to change in the rate of movement.

semiconductor

An electronic conductor, with resistivity in the range between metals and insulators, in which the electrical charge carrier concentration increases with increasing temperature over some temperature range. Certain semiconductors possess two types of carriers, namely, negative electrons and positive holes.

semiconductor device

An electron device in which the characteristic distinguishing electronic conduction takes place within a semiconductor.

semidiameter

1. The radius of a closed figure.

2. Half the angle at the observer subtended by the visible disk of a celestial body.

semidiameter correction

A correction due to semidiamter, particularly that sextant altitude correction resulting from observation of the upper or lower limb of a celestial body, rather than the center of that body.

semidiurnal

Having a period of, occurring in, or related to approximately half a day.

semimajor axis (symbol a )

One-half the longest diameter of an ellipse.

semiminor axis (symbol b )

One-half the shortest diameter of an ellipse.

semimonocoque

A structural concept in which longitudinal members as well as formers reinforce the skin and help carry the stresses. Compare with monocoque.

semitransparent photocathode

A photocathode in which radiant flux incident on one side produces photoelectric emission from the opposite side. See phototube.

sensation level

The level of psychophysiologic stimulation above the threshold.

sense antenna

An antenna used to resolve a 180 degrees ambiguity in a directional antenna.

sense-reversing reflectivity

The characteristic of a reflector that reverses the sense of a circularly polarized incident ray. See polarization.

For example, a perfect corner reflector is invisible to a circularly polarized radar because it reverses the sense.

sensibility

In measurements, the smallest change that is reliably detectable.

sensible atmosphere

That part of the atmosphere that offers resistance to a body passing through it.

sensible horizon

See horizon, note.

sensible temperature

The temperature at which average indoor air of moderate humidity would induce, in a lightly clothed person, the same sensation of comfort as that induced by the actual environment. Compare effective temperature.

Sensible temperature depends on the air temperature; radiation from the sun, sky, and surrounding objects; relative humidity; and air motion. The wet-bulb temperature is often taken as an approximate measure.

sensing element = sensor.

sensitivity

1. The ability of electronic equipment to amplify a signal, measured by the minimum strength of signal input capable of causing a desired value of output. The lower the input signal for a given output, the higher the sensitivity.

2. In measurements, the derivative representing the change in instrument indication produced by a change in the variable being measured.

sensitometry

The measurement of the light response characteristics of photographic film under specified conditions of exposure and development.

sensor

1. The component of an instrument that converts an input signal into a quantity which is measured by another part of the instrument. Also called sensing element.

2. The nerve endings or sense organs which receive information from the environment, from the organism, or from both.

separation

1. The action of a fallaway section or companion body as it casts off from the remaining body of a vehicle, or the action of the remaining body as it leaves a fallaway section behind it.

2. The moment of this action.

separation velocity

The velocity at which a space vehicle is moving when some part or section is separated from it; specifically, the velocity of a space probe or satellite at the time of separation from the launch vehicle.

September equinox = autumnal equinox.

sequencer

A mechanical or electronic device that may be set to initiate a series of events and to make the events follow in a given sequence. See program.

sequential collation of range

(abbr Secor) A spherical, long-baseline, phase-comparison trajectory measuring system utilizing three or more ground stations, time sharing a single transponder, to provide nonambiguous range measurements to determine the instantaneous position of a vehicle in flight.

sequential control

Control by completion of a series of one or more events.

Ser, Serp

International Astronomical Union abbreviations for Serpens (Cap. and Caud.). See constellation.

Serpens (Cap. and Caud.) (abbr Ser, Serp)

See constellation.

servo

1. = servomechanism.

2 Pertaining to or incorporating a servomechanism.

servomechanism

A control system incorporating feedback in which one or more of the system signals represent mechanical motion.

It should be noted that servomechanism and regulator are not mutually exclusive terms; their application to a particular system will depend on the method of operation of that system.

set

1. To place a storage device in a prescribed state.

2. To place a binary cell in the one state.

Sex, Sext

International Astronomical Union abbreviations for Sextans. See constellation.

sexidemical notation

A positional notation based on the integer sixteen.

Sext

International Astronomical Union abbreviation for Sextans. See constellation.

Sextans (abbr Sex, Sext)

See constellation.

sextant

A double-reflecting instrument for measuring angles, primarily altitudes of celestial bodies.

As originally used, the term applied only to instruments having an arc of 60 degrees, a sixth of a circle, from which the instrument derived its name. Such an instrument had a range of 120 degrees. In modern practice the term applies to a similar instrument, regardless of its range, very few modern instruments being sextants in the original sense.

sextant altitude

The altitude of a celestial body as actually measured by a sextant. See altitude difference.

sferics

1. (Also spelled spherics ). The study of atmospherics, especially from a meteorological point of view. This involves techniques of locating and tracking atmospherics sources and evaluating received signals (waveform, frequency, etc.) in terms of source.

2. = atmospherics.

sferics fix

The estimated location of a source of atmospherics, presumably a lightning discharge.

sferics observation

An evaluation, from one or more sferics receivers, of the location of weather conditions with which lightning is associated.

Such observations are more commonly obtained from networks of two or three widely spaced stations. Simultaneous observations of the azimuth of the discharge are made at all stations and the location of the storm is determined by triangulation.

sferics receiver

An instrument which measures, electronically, the direction of arrival, intensity, and rate of occurrence of atmospherics. In its simplest form the instrument consists of two orthogonally crossed antennas. Their output signals are connected to an oscillograph so that one loop measures the north-south component whereas the other measures the east-west component. These are combined vertically to give the azimuth. Also called lightning recorder.

Sge, Sgte

International Astronomical Union abbreviation for Sagitta. See constellation.

Sgr, Sgtr

International Astronomical Union abbreviation for Sagittarius. See constellation.

Sgte

International Astronomical Union abbreviation for Sagitta. See constellation.

Sgtr

International Astronomical Union abbreviation for Sagittarius. See constellation.

shadow

Darkness in a region, caused by an obstruction between the source of light and the region.

By extension, the term is applied to a similar condition when any form of radiant energy is cut off by an obstruction, as a radar shadow. The darkest part of a shadow in which light is completely cut off is called the umbra; a lighter part surrounding the umbra, in which the light is only partly cut off, is called the penumbra.

shadowgraph

1. A picture or image in which steep density gradients in the flow about a body are made visible, the body itself being presented in silhouette.

2. The optical method or technique by which this is done. A shadowgraph differs from a schlieren photograph in that the schlieren method depends on the first derivative of the refractive index while the shadow method depends on the second derivative. Interference measurements give the refractive index directly.

shadow shield

A shield that is interposed between a radiation source and a specific area to be protected.

Useful in space, a shadow shield is less effective in the earth's atmosphere because air scattering deflects radiation around it.

shaker

An electromagnetic device capable of imparting known vibratory acceleration to a given object.

shake-table test

A laboratory test for vibration tolerance, in which the device to be tested is place in a vibrator.

shaped-beam antenna

A unidirectional antenna whose major lobe differs materially from that obtainable from an aperture of uniform phase. Also called phase-shaped antenna.

shear strength

In materials, the stress required to produce fracture in the plane of cross section, the conditions of loading being such that the directions of force and of resistance are parallel and opposite although their paths are offset a specified minimum amount.

shear wave

A wave in an elastic medium which causes an element of the medium to change its shape without a change of volume. Mathematically, a shear wave is one whose velocity field has zero divergence. Also called rotational wave.

A shear plane wave in an isotropic medium is called a transverse wave.

sheath = plasma sheath.

shell

A body one of whose dimensions is small compared with the others.

shield

A body of material used to prevent or reduce the passage of particles or radiation.

A shield may be designated according to what it is intended to absorb, as a gamma-ray shield or neutron shield, or according to the kind of protection it is intended to give, as a background, biological, or thermal shield. The shield of a nuclear reactor is a body of material designed to prevent the escape of neutrons and radiation into a protected area, which frequently is the entire space external to the reactor. It may be required for the safety of personnel or to reduce radiation sufficiently to allow use of counting instruments.

shielding

The arrangement of shields used for any particular circumstances; the use of shields.

shimmer = terrestrial scintillation.

shock

1. = shock wave.

2. A blow, impact, collision, or violent jar.

3. A sudden agitation of the mental or emotional state or an event causing it.

4. The sudden stimulation caused by an electrical discharge on the animal or human organism (e.g., electric shock).

shock absorber

A device for the dissipation of energy used to modify the response of a mechanical system to applied shock.

shock front

1. A shock wave regarded as the forward surface of a fluid region having characteristics different from those of the region ahead of the wave.

2. The front side of a shock wave.

shock isolator

A resilient support that tends to isolate a system from applied shock. Also called shock mount.

shock mount = shock isolator.

shock spectrum

A plot of the maximum acceleration experienced by a single-degree-of-freedom system as a function of its own natural frequency in response to an applied shock.

shock tube

A relatively long tube or pipe in which very brief high-speed gas flows are produced by the sudden release of gas at very high pressure into a low-pressure portion of the tube; the high-speed flow moves into the region of low pressure behind a shock wave.

shock tunnel

A shock tube used as a wind tunnel.

shock wave

A surface or sheet of discontinuity (i.e., of abrupt changes in conditions) set up in a supersonic field or flow, through which the fluid undergoes a finite decrease in velocity accompanied by a marked increase in pressure, density, temperature, and entropy, as occurs, e.g., in a supersonic flow about a body. Sometimes called a shock. See attached shock wave, bow wave, condensation shock wave, detached shock wave, Mach wave, normal shock wave, oblique shock wave.

Shodop (abbr) = short-range Doppler.

shooting star = meteor.

shoran

(From short-range navigation). A precision electronic position fixing system using a pulse transmitter and receiver and two transponder beacons at fixed points. High- precision shoran is called hiran.

short-baseline system

A trajectory measuring system using a baseline the length of which is very small compared with the distance of the object being tracked.

short-period error = random error.

short-range Doppler (abbr Shodop)

A short range trajectory measuring system using the intersections of the ellipsoids of Dovap and the hyperboloids of Dovap elsse or telemetry elsse during a rocket launch.

short-range navigation = shoran.

short-wave radiation

In meteorology, a term used loosely to distinguish radiation in the visible and near-visible portions of the electromagnetic spectrum (roughly 0.4 to 1.0 micron in wavelength) from long-wave radiation ( infrared radiation).

shot

1. An act or instance of firing a rocket, especially from the earth's surface, as, the shot carried the rocket 200 miles.

2. The flight of a rocket, as, the rocket made a 200-mile shot.

shoulder harness

A harness that fastens over a person's shoulders to prevent his being thrown forward in his seat. See lap belt.

shower = air shower (cosmic rays).

shutdown

The process of decreasing engine thrust to zero.

shutoff = fuel shutoff.

SI (abbr) = International System of Units.

SID (abbr) = sudden ionospheric disturbance.

sideband

1. Either of the two frequency bands on both sides of the carrier frequency within which fall the frequencies of the wave produced by the process of modulation.

2. The wave components lying within such a band.

side lobe

See lobe.

sidereal

Of or pertaining to the stars.

Although sidereal generally refers to the stars and tropical to the vernal equinox, sidereal time and the sidereal day are based upon the position of the vernal equinox relative to the meridian. The sidereal year is based upon the stars.

sidereal day

The duration of one rotation of the earth on its axis, with respect to the vernal equinox. It is measured by successive transits of the vernal equinox over the upper branch of a meridian.

Because of the precession of the equinoxes, the sidereal day thus defined is slightly less than the period of rotation with respect to the stars, but the difference is less than 0.01 second. The length of the mean sidereal day is 24 hours of sidereal time or 23 hours 56 minutes 4.09054 seconds of mean solar time.

sidereal hour angle

(abbr SHA). Angular distance west of the vernal equinox; the arc of the celestial equator, or the angle at the celestial pole, between the hour circle of the vernal equinox and the hour circle of a point on the celestial sphere, measured westward from the hour circle of the vernal equinox through 360 degrees.

Angular distance east of the vernal equinox, through 24 hours, is right ascension.

sidereal month

The average period of revolution of the moon with respect to the stars, a period of 27 days 7 hours 43 minutes 11.5 seconds, or approximately 27 1/3 days.

sidereal period

1. The time taken by a planet or satellite to complete one revolution about its primary as seen from the primary and as referred to a fixed star.

2. Specifically, the interval between two successive returns of an earth satellite in orbit to the same geocentric right ascension.

sidereal time

Time based upon the rotation of the earth relative to the vernal equinox.

Sidereal time may be designated as local or Greenwich as the local or Greenwich meridian is used as the reference. When adjusted for nutation, to eliminate slight irregularities in the rate, it is called mean sidereal time.

sidereal year

The period of one apparent revolution of the earth around the sun, with respect to the stars, averaging 365 days 6 hours 9 minutes 9.55 seconds in 1955, and increasing at the rate of 0.000095 second annually.

Because of the precession of the equinoxes this is about 20 minutes longer than a tropical year.

sight = celestial observation.

sigma = standard deviation.

signal

1. A visible, audible, or other, indication used to convey information.

2. The information to be conveyed over a communication system.

3. Any carrier of information; opposed to noise.

signal strength

In radio, a measure of the received radiofrequency power, generally expressed in decibels relative to some standard value, normally either 1 milliwatt or that power which would have resulted at the same distance under free-space transmission. Also called field strength.

signal-to-noise ratio

(abbr SNR or S/N). A ratio which measures the comprehensibility of a data source or transmission link, usually expressed as the root-mean-square signal amplitude divided by the root-mean-square noise amplitude.

The higher the S/N ratio, the less the interference with reception.

signal transmission level

In a transmission system, the signal level, of a kind to be specified, at a designated position in the system.

The signal level at some specified position near the source may be taken as the zero reference level. In an acoustic system the signal level is often in the form of a sound pressure level; either the reference sound pressure or the reference sound pressure level must be specified.

signal velocity

See velocity of propagation, note.

signs

See zodiac.

silver-cell battery

A type of short-duration, high-power-density battery of light weight used for single-time, high-power applications in vehicles where weight is critical.

silver-disk pyrheliometer

An instrument used for the measurement of direct solar radiation. See pyrheliometer.

It is constructed in the following manner. A silver disk located at the lower end of a diaphragmed tube serves as the radiation receiver for a calorimeter. Radiation falling on the silver disk is periodically intercepted by means of a shutter located in the tube, causing temperature fluctuations of the calorimeter which are proportional to the intensity of the radiation. The instrument is normally used as a secondary instrument and is calibrated against the water-flow pyrheliometer. It is used by the U.S. Weather Bureau as a standard instrument.

simple average

See arithmetic mean, sense 2.

simple harmonic motion

A motion such that the displacement is a sinusoidal function of time.

simple harmonic quantity

A periodic quantity that is a sinusoidal function of the independent variable. Thus, γ = A sin ( ωx + φ) where y is the simple harmonic quantity; A is the amplitude; ω is the angular frequency; x is the independent variable; and φ is the phase of the oscillation.

The maximum value of the simple harmonic quantity is the amplitude A; it is sometimes called, for emphasis, the single amplitude to distinguish it from double amplitude which for a simple harmonic quantity is the same as the total excursion or peak-to-peak value. When a simple harmonic quantity is expressed as a complex quantity, the term amplitude must be used with caution in view of possible confusion with the alternate meaning of amplitude as the angle or argument of a complex quantity.

simple reflection = specular reflection.

simple reflector = specular reflector.

simple standard deviation

See standard deviation.

sine wave

A wave which can be expressed as the sine of a linear function of time, or space, or both.

single-degree-of-freedom system

A mechanical system for which only one coordinate is required to define completely the configuration of the system at any instant. See degree of freedom.

single-entry compressor

A centrifugal compressor that takes in air or fluid on only one side of the impeller, the impeller being faced with vanes only on that side.

single sheath

See plasma sheath, note.

single-sideband modulation

Modulation whereby the spectrum of the modulating wave is translated in frequency by a specified amount either with or without inversion.

single-sideband transmission

That method of operation in which one sideband is transmitted and the other sideband is suppressed. The carrier wave may be either transmitted or suppressed.

single-stage compressor

A centrifugal compressor having a single impeller wheel, with vanes either on one or on both sides of the wheel; also, an axial flow compressor with one row of rotor blades and one row of stator blades. Axial-flow compressors are normally multistage.

single-stage rocket

A rocket vehicle provided with a single rocket propulsion system. See stage.

sink

1. In the mathematical representation of fluid flow, a hypothetical point or place at which the fluid is absorbed.

2. A heat sink. See source.

sinking

In atmospheric optics, a refraction phenomenon, the opposite of looming, in which an object on or slightly above the geographic horizon apparently sinks below it. Compare inferior mirage, stooping.

Sinking occurs whenever the rate of density decreases with height through the atmosphere is of smaller magnitude than normal or, in extreme cases, where the density actually increases with height.

sintered ceramic

A ceramic body or coating prepared by heating a ceramic powder below its melting point but at a sufficiently high temperature to cause interdiffusion of ions between contacting particles and subsequent adherence at the points of contact.

sintering

The bonding of adjacent surfaces of particles in a mass of powders, usually metal, by heating.

sinus

A hollow or cavity; a recess or pocket. Specifically, sinuses: air cavities lined by mucous membrane which communicate with the nasal cavity; the ethmoidal, frontal, sphenoidal, and maxillary sinuses.

sinus barotrauma = aerosinusitis.

sinusoidal

Having the form of a sine wave.

skiatron

1. A dark trace oscilloscope tube. See dark trace tube.

2. A display employing an optical system with a dark trace tube.

skimmer basin = deluge collection pond.

skin

The covering of a body, of whatever material, such as the covering of a fuselage, of a wing, of a hull, of an entire aircraft, etc.; a body shell, as of a rocket; the surface of a body.

skin temperature

The outer surface temperature of a body.

skin tracking

The tracking of an object by means of radar without using a beacon or other signal device on board the object being tracked.

skip effect

A phenomenon in which sound or radio energy may be detected only at various distance intervals from the energy source as the result of the presence of an energy reflecting or refracting layer in the atmosphere. See radio duct.

For long radio waves, the ionosphere acts as the reflecting layer. For shorter wavelengths, the effect may be produced by strong superstandard propagation in elevated layers of the troposphere. Skip effects make it possible on occasion to detect targets at distances far greater than the normal radio horizon, while closer targets remain undetected.

skirt

The lower outer part of a rocket vehicle; specifically, the half-stage of an Atlas.

skirt fog

The cloud of steam and water that surrounds the engines of a rocket being launched from a wet emplacement.

skyhook balloon

(Originally a code name for a U.S. Navy Project.) A large free balloon having a plastic envelope, used especially for constant-level meteorological observations at very high altitudes.

sky light = diffuse sky radiation.

sky radiation = diffuse sky radiation.

sky screen

An optical device used to detect the departure of a rocket from its intended trajectory.

sky wave

In radio, radio energy that is received after having been reflected by the ionosphere. Compare wave.

slant range

The line-of-sight range of a radar or radio. See range.

slave

1. = slave station.

2. Device that follows an order given by a master through remote control.

slave antenna

A directional antenna that is positioned in azimuth and elevation by a servo system. The information controlling the servosystem is supplied by a tracking or positioning system.

slave station

In a hyperbolic navigation system, a station whose transmissions are controlled by a master station. Often shortened to slave. See hyperbolic navigation.

slaving

Of a gyro, the use of a torquer to maintain the orientation of the spin axis relative to an external reference such as a pendulum or magnetic compass.

sleeve-dipole antenna

A dipole antenna surrounded in its central portion by a coaxial sleeve.

slenderness ratio

A dimensionless number expressing the ratio of a rocket vehicle length to its diameter.

slew

To change the position of an antenna or range gear assembly by injecting a synthetic error signal into the positioning servo amplifier.

slewing

1. Of a gyro, the rotation of the spin axis caused by applying torque about the axis of rotation.

2. In radar, changing the scale on the display.

slip

A sprayable slurry comprising a frit suspended in a liquid carrier (sometimes also used for dip and brush coating).

slip flow

See rarefied gas dynamics, note.

slope angle

The angle in the vertical plane between the flightpath and the horizontal.

sloshing

The back-and-forth movement of a liquid fuel in its tank, creating problems of stability and control in the vehicle.

slow ion = large ion.

slug

A unit of mass; the mass of a free body which if acted upon by a force of 1 pound would experience an acceleration of 1 foot per square second; thus approximately 32.16 pounds.

slurry

A suspension of fine solid particles in a liquid.

slurry fuel

A fuel consisting of a suspension of fine solid particles in a liquid.

small calorie (abbr cal)

See calorie.

small circle

The intersection of a sphere and a plane which does not pass through the center of the sphere, as a parallel of latitude.

small ion

An atmospheric ion, apparently a singly charged atmospheric molecule (or, rarely, an atom) about which a few other neutral molecules are held by the electrical attraction of the central ionized molecule. Estimates of the number of satellite molecules range as high as 12. Also called light ion, fast ion.

Small ions may disappear either by direct recombination with oppositely charged small ions or by combination with neutral Aitken nuclei to form new large ions, or by combination with large ions of opposite sign. The small ion, collectively, is the principal agent of atmospheric conduction.

small perturbation

A disturbance imposed on a system in steady state, with amplitude assumed small, i.e., the square of the amplitude is negligible in comparison with the amplitude, and the derivatives of the perturbation are assumed to be of the same order of magnitude as the perturbation. See perturbation, method of small perturbations.

Snell law

See refraction, index of refraction.

Snort track

A rail track on which a supersonic rocket sled is driven, located at the Naval Ordnance Test Station.

snow = grass.

snubber

A device used to increase the stiffness of an elastic system, usually by a large factor, whenever the displacement becomes larger than a specified amount.

sofar

(From sound fixing and ranging). A system of navigation providing hyperbolic lines of position determined by shore listening stations which receive sound signals produced by depth charges dropped at sea and exploding in a sound channel which is at a considerable depth in most areas.

This system is used in Project Mercury for locating spacecraft down at sea.

softening range

An arbitrarily defined temperature range below the crystal melting point where a ceramic becomes soft and noticeably viscous; a softening range rather than a sharp melting point occurs in ceramics containing a glass base.

soft landing

The act of landing on the surface of a planet without damage to any portion of the vehicle or payload except possibly the landing gear.

soft radiation

Radiation absorbable by an absorber equivalent to 10 centimeters of lead or less.

Radiation which can penetrate more than 10 centimeters of lead is termed hard radiation.

solar

1. Of or pertaining to the sun or caused by the sun, as solar radiation, solar atmospheric tide.

2. Relative to the sun as a datum or reference , as solar time.

solar activity

Any type of variation in the appearance or energy output of the sun. See faculae, flare, flocculi, granules, prominence, spicules, sunspot.

solar air mass

The optical air mass penetrated by light from the sun for any given position of the sun in the sky.

solar antapex

See solar apex.

solar apex

The point on the celestial sphere toward which the sun is traveling. Also called apex of the sun's way.

The solar apex is at approximately right ascension 270 degrees declination 34 degrees N. The point diametrically opposite the solar apex on the celestial sphere is the solar antapex, right ascension 90 degrees declination 34 degrees S.

solar atmospheric tide

An atmospheric tide due to the thermal or gravitational action of the sun.

Six and eight hour components of small amplitude have been observed. They are primarily thermal in origin. The 12-hour component has by many times the greatest amplitude of any atmospheric tidal component, about 1.5 millibars at the equator and 0.5 millibar in middle latitudes. This relatively large amplitude is often explained as a resonance effect. The 24-component is a thermal tide with great local variability.

solar cell

A photovoltaic cell that converts sunlight into electrical energy.

solar constant

The rate at which solar radiation is received outside the earth's atmosphere on a surface normal to the incident radiation and at the earth's mean distance from the sun.

Measurements of solar radiation at the earth's surface by the Smithsonian Institution for several decades give a best value for the solar constant of 1.934 calories per square centimeter per minute. Measurements from rockets of the intensity of the ultraviolet end of the spectrum have corrected this value to 2.00 calories per square centimeter per minute with a probable error of +/- 2 percent.

solar corona

See corona.

solar corpuscular rays

Cosmic radiation supposedly originating in the sun. See corpuscular cosmic ray.

solar cosmic rays

Cosmic rays supposedly originating in the sun.

solar cycle

The periodic increase and decrease in the number of sunspots. The cycle has a period of about 11 years.

solar day

1. The duration of one rotation of the earth on its axis, with respect to the sun.

This may be either a mean solar day, or an apparent solar day, as the reference is the mean or apparent sun, respectively.

2. The duration of one rotation of the sun on its axis.

solar eclipse

The obscuration of the light of the sun by the moon.

A solar eclipse is partial if the sun is partly obscured, total if the entire surface is obscured, or annular if a thin ring of the sun's surface appears around the obscuring body.

solar flare

See flare.

solarimeter

1. = pyranometer.

2. Specifically, a pyranometer consisting of a Moll thermopile covered by a bell glass.

solar parallax

The angle at the sun subtended by the equatorial diameter of the earth. See parallax.

The adopted value of the solar parallax in the system of astronomical constants is 8.80 seconds of arc.

solar prominence = prominence.

solar protons

Protons emitted by the sun, especially during solar flares.

solar radiation

The total electromagnetic radiation emitted by the sun. See insolation, direct solar radiation, diffuse sky radiation, global radiation, extraterrestrial radiation, solar constant.

To a first approximation, the sun radiates as a black body at a temperature of about 5700 degrees K; hence about 99.9 percent of its energy output falls within the wavelength interval from 0.15 micron to 4.0 microns, with peak intensity near 0.47 micron. About one-half of the total energy in the solar beam is contained within the visible spectrum from 0.4 to 0.7 micron, and most of the other half lies in the near infrared, a small additional portion lying in the ultraviolet.

solar-radiation observation

An evaluation of the radiation from the sun that reaches the observation point. The observing instrument is usually a pyrheliometer or pyranometer.

Two types of such observation are taken. The more common consists of measurements of the radiation reaching a horizontal surface, consisting of both radiation from the sun (direct solar radiation) and that reaching the instrument indirectly by scattering in the atmosphere (diffuse sky radiation). The other type of observation involves the use of an equatorial mount that keeps the instrument pointed directly at the sun at all times. The sensitive surface of the instrument is normal to the path of the radiation and is shielded from indirect radiation from the sky.

solar radio burst

A sudden increase in the flux from the sun at radio frequencies.

solar radio waves

Radiation at radio frequencies originating in the sun or its corona.

solar simulator

A device which produces thermal energy, equivalent in intensity and spectral distribution to that from the sun, used in testing materials and space vehicles.

solar system

The sun and other celestial bodies within its gravitational influence, including planets, asteroids, satellites, comets, and meteors.

solar tide

See solar atmospheric tide.

solar time

Time based upon the rotation of the earth relative to the sun.

Solar time may be designated as mean or astronomical if the mean sun is the reference, or apparent if the apparent sun is the reference. The difference between mean and apparent time is called equation of time. Solar time may be further designated according to the reference meridian, either the local or Greenwich meridian or additionally in the case of mean time, a designated zone meridian. Standard or daylight-saving 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.

solar wind

Streams of plasma flowing approximately radially outward from the sun.

solar year = tropical year.

solenoid

A tube formed in space by the intersection of unit-interval isotimic surfaces of two scalar quantities.

Solenoids formed by the intersection of surfaces of equal pressure and density are frequently referred to in meteorology. A barotropic atmosphere implies the absence of solenoids of this type, since surfaces of equal pressure and density coincide.

solid angle (symbol ω)

A portion of the whole of space about a given point, bounded by a conical surface with its vertex at that point and measured by the area cut by the bounding surface from the surface of a sphere of unit radius centered at that point. See steradian.

solid propellant

Specifically, a rocket propellant in solid form, usually containing both fuel and oxidizer combined or mixed, and formed into a monolithic (not powered or granulated) grain.

solid-propellant engine = solid-propellant rocket engine.

solid-propellant rocket engine

A rocket engine fueled with a solid propellant. Such motors consist essentially of a combustion chamber containing the propellant, and a nozzle for the exhaust jet, although they often contain other components, as grids, liners, etc.

solid rocket

A rocket that uses a solid propellant.

solid rocket fuel

A solid propellant.

solid rotation

The rotation of a system as though is were a solid or rigid body rotating about a fixed axis, all points within the body having the same angular velocity.

solid-state devices

Devices which utilize the electric, magnetic, and photic properties of solid materials, e.g., binary magnetic cores, transistors, etc.

solstice

1. One of the two points of the ecliptic farthest from the celestial equator; one of the two points on the celestial sphere occupied by the sun at maximum declination.

That in the northern hemisphere is called the summer solstice and that in the southern hemisphere the winter solstice. Also called solstitial point.

2. That instant at which the sun reaches one of the solstices, about June 21 (summer solstice) or December 22 (winter solstice).

solstitial colure

That great circle of the celestial sphere through the celestial poles and the solstices.

solstitial point = solstice.

sonar

(From sound, navigation, and ranging.) A method or system, analogous to radar used under water, in which high-frequency sound waves are emitted so as to be reflected back from objects, and used to detect the objects of interest. Called asdic by the British.

sonar capsule

A device designed to reflect high-frequency sound waves. See sonar.

The sonar capsule, if attached to a reentry body, may be used to locate the reentry body in case of a water landing.

sone

A unit of loudness. A simple tone of frequency 1000 cycles per second, 40 decibels above a listener's threshold, produces a loudness of 1 sone.

The loudness of any sound that is judged by the listener to be n times that of the 1-sone tone is n sones. A millisone is equal to 0.001 sone. The loudness scale is a relation between loudness and level above threshold for a particular listener. In presenting data relating loudness in sones to sound pressure level, or in averaging the loudness scales of several listeners, the thresholds (measured or assumed) should be specified.

sonic

1. In aerodynamics, of or pertaining to the speed of sound; that which moves at acoustic velocity as in sonic flow ; designed to operate or perform at the speed of sound, as in sonic leading edge.

2. Of or pertaining to sound, as in sonic amplifier.

In sense 2, acoustic is preferred to sonic.

sonic agglomeration

The union of small particles suspended in a fluid medium into larger aggregates by the action of sound waves.

sonic barrier

A popular term for the large increase in drag that acts upon an aircraft approaching acoustic velocity; the point at which the speed of sound is attained and existing subsonic and supersonic flow theories are rather indefinite. Also called sound barrier.

sonic boom

A noise caused by a shock wave that emanates from an aircraft or other object traveling at or above sonic velocity .

A shock wave is a pressure disturbance and is received by the ear as a noise or clap.

sonic delay line = acoustic delay line.

sonic drilling

The process of cutting or shaping materials with an abrasive slurry driven by a reciprocating tool attached to an electromechanical transducer operating at ultrasonic frequencies.

sonic frequency = audiofrequency.

sonics

The technology of sound in processing and analysis. Sonics includes the use of sound in any noncommunication process.

sonic soldering

The method of joining metals by metallic bonding alloys through the use of mechanical vibrations to break up the surface oxides.

sonic speed

Acoustic velocity; by extension, the speed of a body traveling at a Mach number of 1.

sonic wave = sound wave.

sophisticated

Complex and intricate; making use of advanced art; requiring special skills to operate.

sorb

To take up gas by sorption.

sorbate

Gas taken up by a sorbent.

sorbent

The material which takes up gas by sorption.

sorption

The taking up of gas by absorption, adsorption, chemisorption, or any combination of these process. See absorption.

sound

1. An oscillation in pressure, stress, particle displacement, particle velocity, etc., in a medium with internal forces (e.g., elastic, viscous), or the superposition of such propagated oscillations.

2. A sensation evoked by the oscillation described above in the human ear.

In case of possible confusion, the term sound wave or elastic wave may be used for concept 1 and the term sound sensation for concept 2. Not all sound wave can evoke an auditory sensation, e.g., ultrasound. The medium in which the sound exists is often indicated by an appropriate adjective, e.g., airborne, water borne, structure borne.

sound absorption

Sound absorption is the change of sound energy into some other form, usually heat, in passing through a medium oron striking a surface.

sound barrier = sonic barrier.

sound energy

The energy which sound waves contribute to a particular medium.

sound energy density

At a point in a sound field, the sound energy contained in a given infinitesimal part of the medium divided by the volume of that part of the medium.

The terms instantaneous energy density, maximum energy density, and peak energy density have meanings analogous to the related terms used for sound pressure. In speaking of average energy density in general, it is necessary to distinguish between the space average (at a given instant) and the time average (at a given point).

sound energy flux

The average rate of flow of sound energy for one period through any specified area.

In a medium of density ρ for a plane or spherical free wave having a velocity of propagation c, the sound energy flux through the area S corresponding to an effective sound pressure p is J = ( p²S / ρc ) cos θ where θ = the angle between the direction of propagation of the sound and the normal to the area S.

sound energy flux density = sound intensity.

sound field

A region containing sound waves. See near field, far field.

sounding

1. In geophysics, any penetration of the natural environment for scientific observation.

2. In meteorology, same as upper air observation. However, a common connotation is that of a single complete radiosonde observation.

3. = air sounding.

sounding rocket

A rocket that carries aloft equipment for making observations of or from the upper atmosphere. See air sounding. Compare probe, sense 3.

Usually a sounding rocket has a near vertical trajectory.

sound intensity

In a specified direction at a point, the average rate of sound energy transmitted in the specified direction through a unit area normal to this direction at the point considered. Also called sound energy flux density, sound power density.

sound level

Specifically, a weighted sound pressure level, obtained by the use of metering characteristics and the weightings A, B, or C specified in American Standard Publication Z24.3-1944: Sound Level Meters for Measurement of Noise and Other Sounds. The weighting employed pressure is 0.0002 microbar.

A suitable method of stating the weighting is, for example, The A-sound level was 43 decibels.

sound power

Of a source, the total sound energy radiated by the source per unit of time.

sound power density = sound intensity.

sound pressure

At a point, the total instantaneous pressure at that point in the presence of a sound wave minus the static pressure at that point. See effective sound pressure.

sound pressure level

In decibels, 20 times the logarithm to the base 10 of the ratio of the sound pressure to the reference pressure. The reference pressure must be explicitly stated.

The following reference pressures are in common use: (a) 2 X 10E-4 microbar, (b) 1 microbar. Reference pressure (a) is in general use for measurements concerned with hearing and with sound in air and liquids, whereas (b) has gained widespread acceptance for calibration of transducers and various kinds of sound measurements in liquids. Unless otherwise explicitly stated, it is to be understood that the sound pressure is the effective (root-mean-square) sound pressure. It is to be noted that in many sound fields the sound pressure ratios are not the square roots of the corresponding power ratios.

sound probe

A device that responds to some characteristic of an acoustic wave (e.g., sound pressure, particle velocity) and that can be used to explore and determine this characteristic in a sound field without appreciably altering that field.

sound wave

A mechanical disturbance advancing with infinite velocity through an elastic medium and consisting of longitudinal displacements of the medium, i.e., consisting of compressional and rarefactional displacements parallel to the direction of advance of the disturbance; a longitudinal wave. Sound waves are small-amplitude adiabatic oscillations. The wave equation governing the motion of sound waves has the form ² = (1/c²)(²/t²) where ² is the Laplace operator, is the velocity potential, c is the speed of sound, and t is the time; the density variations and velocities are small. As so defined, this includes waves outside the frequency limits of human hearing, which limits customarily define sound. Also called acoustic wave, sonic wave. See ultrasonic, infrasonic, pressure wave.

Gases, liquids, and solids transmit sound waves, and the propagation velocity is characteristic of the nature and physical state of each of these media. In those cases where a steadily vibrating sound generator acts as a source of waves, one may speak of a uniform wave train; but in other cases (explosions, lightning discharges) a violent initial disturbance sends out a principal wave, followed by waves of more or less rapidly diminishing amplitude.

source

1. The location or device from which energy emanates as a sound source, heat source , etc.

2. Specifically, in the mathematical representation of fluid flow, a hypothetical point or place from which fluid emanates.

The strength of a source; the rate of mass flow of unit density across a curve enclosing the source is given by Q = 2π r v_r where r is the distance from the source and v_r is the radial speed.

3. Specifically, the device which supplies signal power to a transducer.

southbound node = descending node.

South Tropical Disturbance

An elongated dark band in the cloud surface of Jupiter at about the latitude of the Great Red Spot. It was first seen in 1901 as a dark spot which then spread rapidly. It has at times exceeded 180 degrees of longitude in length and, like the Red Spot, appears and disappears intermittently.

SP (abbr) = solid propellant.

space

1. Specifically, the part of the universe lying outside the limits of the earth's atmosphere.

2. More generally, the volume in which all celestial bodies, including the earth, move.

space-air vehicle

A vehicle operable either within or above the sensible atmosphere. Also called aerospace vehicle.

space biology = bioastronautics.

space capsule

A container used for carrying out an experiment or operation in space.

A capsule is usually assumed to carry an organism or equipment.

space charge

1. The electric charge carried by a cloud or stream of electrons or ions in a vacuum or a region of low gas pressure when the charge is sufficient to produce local changes in the potential distribution.

2. The net electric charge within a given volume.

space coordinates

A three-dimensional system of Cartesian coordinates by which a point is located by three magnitudes indicating distance from three planes which intersect at a point.

spacecraft

Devices, manned and unmanned, which are designed to be placed into an orbit about the earth or into a trajectory to another celestial body.

From 1957 through 1962 spacecraft were designated by the year and a Greek letter assigned in the order of launching, as 1958α for the first satellite of 1958. When more than one object was put in orbit by a single launch vehicle, each object was numbered, as 1961o2. (Space probes were not included in this system until 1960). Beginning January 1, 1963, arabic numerals supplanted Greek letters in the scientific designations of all spacecraft with a lifetime of more than 90 minutes. Thus, the first satellite launched in 1963 was 1963-1, the last was 1963-55. When more than one component is put in orbit, alphabetical suffixes are added to the designations, as 1963-4A. The letter A usually designates the component carrying the principle scientific payload; B, C, etc., are used as needed for any subsidiary payloads and then for inert components in order of maximum brightness. The designation system was promulgated formally in the COSPAR Guide to Rocket and Satellite Information and Data Exchange. The Guide has been published in full in COSPAR Information Bulletin No. 9, July 1962, and in IGY Bulletin No. 61, July 1962. Table XIV is a listing of scientific satellites and space probes launched through 1964 and is reprinted from the IG Bulletin (International Geophysics Bulletin) published by the National Academy of Sciences.

space equivalent

A condition within the earth's atmosphere that is virtually identical, in terms of a particular function, with a condition in outer space.

For example, at 50,000 feet, the drop in air pressure and the scarcity of oxygen creates a condition, so far as respiration is concerned, that is equivalent to a condition in outer space where no appreciable oxygen is present; thus, a physiological space equivalent is present in the atmosphere.

space medicine

A branch of aerospace medicine concerned specifically with the health of persons who make, or expect to make, flights into space beyond the sensible atmosphere.

space modulation

The combining of signals outside of an electronic device or conductor to form a signal of desired characteristics. See modulation.

space motion

Motion of a celestial body through space.

That component perpendicular to the line of sight is termed proper motion and that component in the direction of the line of sight, radial motion.

space polar coordinates

A system of coordinates by which a point on the surface of a sphere is located in three dimensions by (a) its distance from a fixed point at the center, called the pole; (b) the colatitude or angle between the polar axis (a reference line through the pole) and the radius vector (a straight line connecting the pole and the point); and (c) the longitude or angle between a reference plane through the polar axis and a plane through the radius vector and polar axis. See polar coordinates, spherical coordinates, cylindrical coordinates.

space probe

See probe, note, and spacecraft, note and Table XIV.

space reddening

The observed reddening, or absorption of shorter wavelengths, of the light from distant celestial bodies due to scattering by small particles in interstellar space. Compare red shift.

space simulator

1. Any device used to simulate one or more parameters of the space environment used for testing space systems or components.

2. Specifically, a closed chamber capable of approximately the vacuum and normal environments of space.

space suit

A pressure suit for wear in space or at very low ambient pressures within the atmosphere, designed to permit the wearer to leave the protection of a pressurized cabin.

span

1. The dimension of a craft measured between lateral extremities; the measure of this dimension.

2. Specifically, the dimension of an airfoil from tip to tip measured in a straight line.

Span is not usually applied to vertical airfoils.

spark discharge

That type of gaseous electrical discharge in which the charge transfer occurs intermittently along a relatively constricted path of high ion density, resulting in high luminosity. It is of short duration and to be contrasted with the nonluminous point discharge, with the diffuse corona discharge, and also with the continuous arc discharge.

The exact meaning to be attached to the term spark discharge varies somewhat in the literature. It is frequently applied to just the transient phase of the establishment of any arc discharge. A lightning discharge is a large-scale spark discharge, though its very length introduces certain details not found in laboratory short-spark processes.

spark spectrum

The spectrum of an ion. The degree of ionization, or order of the spectrum, is indicated by a Roman numeral following the symbol for the element. The first spark spectrum is indicated by II, the second by III, and so on. Thus Fe IV indicates the spectrum of an iron atom which has lost three electrons. See arc spectrum.

spatial

Pertaining to space.

spatio

A combining form meaning space.

special perturbations

A method of orbit determination by numerical integration which takes into account the perturbing forces which are causing the orbit to depart from the orbit as calculated by Kepler laws.

specific

A modifier generally implying per unit mass.

specific heat

The ratio of the heat absorbed (or released) by unit mass of a system to the corresponding temperature rise (or fall). If this ratio varies with temperature, it must be defined as a differential quotient dQ/dT, where dQ is the infinitesimal increment of heat per unit mass and dT is the infinitesimal increment of temperature.

For gases the thermodynamic process must be specified; two specific heats are defined, one being the specific heat in a constant-pressure process c_p = (dQ / dT)_pand the other, the specific heat in a constant-volume process c_v = (dQ / dT)_v In a perfect gas these are, by definition, constants with respect to temperature, and the difference of the specific heat at constant pressure and the specific heat at constant volume is equal to the gas constant: R = c_p - c_v

specific humidity

In a system of moist air, the (dimensionless) ratio of the mass of water vapor to the total mass of the system. The specific humidity may be approximated by the mixing ratio for many purposes: q = w /(1 + w ) where q is the specific humidity and w is the mixing ratio. See absolute humidity, relative humidity, dew point.

specific impulse (symbol I_sp)

A performance parameter of a rocket propellant, expressed in seconds, equal to the thrust F in pounds divided by the weight flow rate in pounds per second: I_sp = F /

Specific impulse is also equivalent to the effective exhaust velocity divided by the gravitational acceleration.

specific power

The energy delivered per pound of fuel in a reactor or in a radioisotope power source.

specific propellant consumption

The reciprocal of the specific impulse, i.e., the required propellant flow to produce one pound of thrust in an equivalent rocket.

specific speed

Of a pump, a parameter used to predict pump performance.

specific thrust = specific impulse.

specific volume (symbol v )

Volume per unit mass of a substance. The reciprocal of density.

spectra

Plural of spectrum.

spectral

1. Of or pertaining to a spectrum.

2. Referring to thermal radiation properties, for ratios such as emittance, reflectance, and transmittance, at a specified wavelength; for powers, such as emissive power, within a narrow wavelength band centered on a specified wavelength.

spectral absorptance

See absorptance, note.

spectral emissivity

See emissivity, note.

spectral function

The Fourier representation of a given function; that is, the Fourier transform if the given function is aperiodic or the set of coefficients of the Fourier series if the given function is periodic. Also called spectrum. See continuous spectrum, discrete spectrum.

spectral line

A bright, or dark, line found in the spectrum of some radiant source. See absorption line, emission line.

Bright lines indicate emission, dark lines indicate absorption.

spectrograph

See spectroscope.

spectroheliogram

See spectroheliograph.

spectroheliograph

An instrument for taking photographs (spectroheliograms) of the image of the sun monochromatic light. The wavelength of light chosen for this purpose corresponds to one of the Fraunhofer lines, usually the light of hydrogen or ionized calcium. A similar instrument used for visual, instead of photographic, observations in a spectrohelioscope.

spectrohelioscope

See spectroheliograph, note.

spectrophotometer

A photometer which measures the intensity of radiation as a function of the frequency (or wavelength) of the radiation. Also called spectroradiometer. See Dobson spectrophotometer.

In one design, radiation enters the spectrophotometer through a slit and is dispersed by means of a prism. A bolometer having a fixed aperture scans the dispersed radiation so that the intensity over a narrow wave band is obtained as a function of frequency.

spectropyrheliometer

An instrument which measures the spectral distribution of the intensity of direct solar radiation. See pyrheliometer, spectrophotometer.

spectroscope

An apparatus to effect dispersion of radiation and visual display of the spectrum obtained.

A spectroscope with a photographic recording device is called a spectrograph.

spectroscopic binaries

See binary star, note.

spectrum

1. In physics, any series of energies arranged according to wavelength (or frequency).

2. The series of images produced when a beam of radiant energy is subject to dispersion.

3. Short for electromagnetic spectrum or for any part of it used for a specific purpose as the radio spectrum (10 kilocycles to 300,000 megacycles).

4. In mathematics, = function.

5. In acoustics, the distribution of effective sound pressures or intensities measured as a function of frequency in specified frequency bands.

specular reflection

Reflection in which the reflected radiation is not diffused; reflection as from a mirror.