Clavius: Bibliography - glossary

CLAVIUS

BIBLIOGRAPHY
glossary

ablative: Able to be eroded or flaked away. An ablative heat shield is one in which the shield material itself vaporizes and takes the heat away with it as it goes. The Apollo command module used an ablative heat shield made of a resinous material held in an aluminum honeycomb.
accelerometer: A sensor used to detect accelerations (changes in velocity). These are widely used in guidance and control systems to detect motion of the spacecraft.
Aerozine 50: The fuel component in many hypergolic engines used in the Apollo program, especially the descent and ascent engines in the lunar module. It is composed of hydrazine and unsymmetrical dimethylhydrazine in approximately 50-50 proportions by weight. The associated oxidizer is nitrogen tetroxide.
AGS: Abort Guidance System, pronounced "aggs". A simple guidance system in the lunar module to allow it to rendezvous with the command module in case the landing had to be aborted or in case the primary guidance system failed.
ALSEP: Apollo Lunar Surface Experiments Package. Prepackaged equipment used to carry out long-term experiments on the lunar surface. The typical ALSEP consisted of sensor modules connected by cables to a transmission device for relaying data back to earth.
APS: 1. Ascent Propulsion System. Pronounced "apps". The rocket engine in the upper part of the lunar module. It provides the thrust to lift the lunar module ascent stage from the lunar surface to rendezvous with the command module.
2. Auxiliary Propulsion System. A motor on the S-IVB stage.
AS: Apollo-Saturn. The NASA mission designator for Apollo project missions launched using one of the Saturn family of rocket boosters. NASA's convention in the 1960s and 1970s for designating a mission used one letter to represent the responsible project and one letter to describe the launch vehicle used. Apollo 11 is appropriately designated AS11, or also AS-506 indicating the sixth mission launched with the Saturn V (5xx) rocket.
attitude: A description of a spacecraft's orientation in space relative to some frame of reference. The Apollo spacecraft used gyroscopic equipment to establish a fixed frame of reference for their attitude. This equipment was periodically corrected using distant stars as a fixed reference.
azimuth: The horizontal angle used in combination with elevation to describe the location of an object in the sky. Compass directions such as north and south are azimuths.
Beta cloth: A fabric made from Teflon-coated glass fibers and used extensively in the Apollo program. It was considered tough and reflective, and most importantly it was fireproof.
catadioptric: Referring to an optical system which uses both reflection (mirrors) and refraction (lenses). This also describes the unwanted effect of lens elements reflecting images inside a compound camera lens. Undesirable catadioptric effects include ghosting and lens flares.
CDR: Commander. This abbreviation is used in the flight plans and checklists. The commander sits in the left seat of the command module during launch and cruise, and occupies the left station at the lunar module controls.
cislunar: Pertaining to the space between earth and the moon.
closed loop: A method of designing a control or guidance system in which all the elements (sensors, computers, actuators, etc.) are linked in such a way as to operate continuously with feedback between the elements. The guidance systems for Apollo spacecraft were closed loop systems.
CMP: Command Module Pilot. The CMP is the second in command of the mission and remains on board the command module while the commander and lunar module pilot descend to the lunar surface in the LM. He occupies the center seat in the command module during launch and cruise.
cross-sun: An orientation for photography in which the sun is shining at right angles to the direction of photography. See also down-sun and up-sun
CSM: Command / Service Module. The designation for the combination of command module and service module, which is the normal configuration until just prior to re-entry.
deboost: An orbital maneuver in which retrograde delta-v is applied in order to lower the altitude of the orbit. The orbiting object will eventually gain velocity as a result of moving to a lower orbit.
declination: The angular measurement of a point according to the celestial sphere, corresponding to an angle above or below the celestial equator. Equivalent to latitude in terrestrial measurements. The complement to right ascension.
delta-v: Engineering notation for a change in velocity, written as the Greek letter delta ("change in") and a lowercase v (standard notation for velocity). This is commonly used to specify an engine firing maneuver. For example, a spacecraft moving at 35,000 fps that wanted to go 38,000 fps would need a delta-v of 3,000 fps achieved by firing its engine for a certain length of time.
deorbit: A deboost which lowers the altitude of the orbit such as to bring about a landing.
downlink: Communication from a spacecraft to its ground control station. The opposite of uplink.
down-sun: An orientation for photography in which the sun is shining from behind the photographer and strongly illuminating the subject. See also cross-sun and up-sun.
DPS: Descent Propulsion System. The rocket engine in the lower part of the lunar module. It provides the thrust for the lunar module to leave orbit and descend to the lunar surface, and to control the descent for a soft touchdown.
elevation: The vertical angle used in combination with azimuth to describe the location of an object in the sky. An object on the horizon has an elevation of zero. An object at the zenith has an elevation of 90°, at which azimuth becomes meaningless.
EMU: Extravehicular Mobility Unit. An astronaut's space suit consisting of the garment itself and the PLSS and OPS backpack.
EVA: Extra-Vehicular Activity. An operation during a space flight where the astronaut ventures outside the spacecraft. Moonwalks are an EVA, as are the activities of space shuttle astronauts working in the cargo bay. On later Apollo missions the CMP performed an EVA to retrieve the film from cameras in the service module's SIM bay.
fiducial: The technical term for the crosshairs that appear in photographs taken on the lunar surface. These are part of a reseau grid that can be used for photogrammetric analysis.
fps: Feet per second. Apollo spacecraft velocities were read out in feet per second. This measure is still used as a measure of spacecraft velocity today, although because NASA must cooperate with engineers from many countries where the SI system prevails, it is becoming increasing common to use meters per second.
GET: Ground Elapsed Time. A time reference used in transcripts and flight plans to give the time at which events during a mission occur, measured in hours, minutes, and seconds since liftoff.
hydrazine: In the narrow sense, a hypergolic rocket fuel used with nitrogen tetroxide. It has the chemical formula N₂H₄ and is highly toxic and corrosive. In the broader sense it refers to a number of chemically related hypergolic fuels including monomethyl hydrazine and unsymmetric dimethylhydrazine.
hypergolic: Describes two substances that spontaneously combust on contact with each other. Hypergolic propellants are useful on spacecraft rocket motors because no ignition system is required. All motors used on the lunar module were hypergolic.
lbf: Pounds-force. In the English measurement system, the unit of force as opposed to pounds-mass, lbm, the unit of mass. It corresponds to the Newton (N) in the SI system. The English Gravitational System uses the slug as the primary unit of mass.
lbm: Pounds-mass. In the English measurement system, the unit of mass as opposed to pounds-force, lbf, the unit of force. The corresponding SI unit is the kilogram (kg). The English Gravitational System uses the slug as the primary unit of mass.
LM: Lunar Module. Pronounced "lem" because the original abbreviation had been LEM for Lunar Excursion Module. The spacecraft used to land on the lunar surface. It was composed of a descent stage and an ascent stage and could carry two explorers plus their equipment. It was produced by Grumman Aerospace in Bethpage, Maryland.
LMP: Lunar Module Pilot. Something of a misnomer since the commander actually controls the lunar module. The LMP's responsibility was to operate the lunar module computer and pass information to the commander. The LMP is seated the right seat in the command module and occupies the right station at the lunar module controls.
LOI: Lunar Orbit Insertion. A retrograde engine burn that slows the spacecraft down enough to be captured into an orbit around the moon. Normally this is done as a combination of two maneuvers by the CSM SPS engine, LOI-1 and LOI-2. The first slows the spacecraft and the second establishes a circular orbit.
LRV: Lunar Roving Vehicle, the lunar rover. Designed and manufactured by Boeing and used by Apollos 15, 16, and 17.
MCC: 1. Mid-Course Correction. A maneuver to correct a trajectory undertaken at a point halfway to the destination. In theory this is the optimum point for trajectory correction because trajectory errors will be most apparent and simultaneously action taken to correct them will be the most efficient. In practice a space flight profile will include the possibility of MCCs at several points during the flight.
2. Mission Control Center. The ground station in Houston, Texas from which the Apollo missions were monitored.
MESA: Modular Equipment Stowage Assembly. A storage bay attached to the right forward panel of the lunar module descent stage. This tray was lowered in drawbridge fashion by the astronauts once they had egressed the lunar module. For Apollo 11 it was deployed by Armstrong using a lanyard accessible to the forward hatch so that the television camera could record his first step on the lunar surface. The MESA carried equipment and supplies for the lunar surface operations such as cameras, film magazines, sample return containers, spare batteries for the backpacks, and tools.
MSFN: Manned Space Flight Network. Pronounced "misfin". The communication system linking control centers for launch and mission direction with the spacecraft during flight. Antenna stations around the world, ships, and aircraft received signals from space and relayed them through radio transmission and land lines to the Mission Control Center.
nitrogen tetroxide: A common hypergolic oxidizer, chemical formula N₂O₄, that combusts spontaneously with members of the hydrazine family. It also reacts spontaneously with air.
OPS: Oxygen Purge System. The upper portion of an astronaut's space suit backpack containing emergency oxygen tanks and plumbing necessary to service the backpack between uses.
periapsis: In orbital mechanics the point at which an orbiting object is closest to the object being orbited. This is the general term for the concept behind more specialized terms such as perigee and perihelion.
pericynthion: The point in a lunar orbit at which an object launched from someplace other than the moon passes closest to the moon's surface.
perigee: The point at which an object in earth orbit passes closest to the earth's surface.
perilune: The point in a lunar orbit at which an object launched from the moon passes closest to the lunar surface.
photogrammetry: The science of determining the dimensions of objects from photographs taken of them using special cameras. The Hasselblad 70mm cameras used on the lunar surface were equipped to provide photographs suitable for photogrammetry.
PLSS: Portable Life Support System. Pronounced "pliss". The lower portion of an astronaut's space suit backpack containing breathing oxygen, the cooling system, communications gear, and batteries to power the space suit.
PTC: Passive Thermal Control. A technique for controlling the radiant heating of a spacecraft by slowly rotating it so that the sun does not always shine on the same side. Also informally called a "barbecue roll".
pyro: A pyrotechnical device, such as an explosive bolt.
RCS: Reaction Control System. The set of steering jets located at various points on the exterior of a spacecraft used to control the attitude of a spacecraft and effect small changes in velocity. Also called "steering jets" or "attitude control thrusters." The term "quad" is often used to describe a cluster of four RCS jets arranged orthogonally.
reseau: A grid of points or marks placed in a photograph by a transparent plate very carefully and precisely aligned with the camera's optics. This provides a stable and reliable basis for measuring objects in a photograph and for detecting and correcting distortions in the photograph. The Hasselblad 70mm cameras used on the moon were fitted with reseau plates.
reticle: A fiducial.
retrograde: Contrary to the direction of motion or rotation. A retrograde orbit is one that goes in the opposite direction of the rotation of the body being orbited. A retrograde maneuver is one carried out in the opposite of the direction of travel.
rev: A revolution, or once around in an orbit.
right ascension: An angular measurement of a point on the celestial sphere, corresponding to an angle before or after the vernal equinox line. Roughly equivalent to longitude in terrestrial measures, except that right ascension is given in hours, minutes, and seconds rather than in degrees, minutes, and seconds. Complement to declination.
S-band: The radio band spanning frequencies from 2 to 8 gigahertz. The Apollo spacecraft used the S-band for long-range communications via the MSFN.
S-IVB: The second stage of the Saturn 1B launch vehicle, or the third stage of the Saturn V launch vehicle. The variant produced for the Saturn V featured a restartable engine since its engine was to be used for TLI. The S-IVB stage traveled on a separate trajectory. In some missions it impacted the lunar surface, and on others it entered solar orbit.
slug: The unit of mass in the English Gravitational System, approximately 32.2 pounds-mass. Because "pound" is ambiguous and can refer either to mass or force, the slug is used exclusively as a unit of mass. Earth's gravity exerts a force of 32.2 pounds-force on a mass of 1 slug.
specific impulse: A description of the propulsive capability per unit mass of a particular propellent in a specific engine. Units are seconds.
SPS: Service Propulsion System. The rocket engine on the service module. It was fueled by hypergolic propellants, hydrazine and nitrogen tetroxide.
sublimation: The conversion of a substance from its solid form directly to its gaseous form without the intervening liquid form. Dry ice (frozen carbon dioxide) sublimates at normal room temperature. The principle of sublimation allows water to be used as a coolant in a vacuum.
sublimator: A device used in a space suit or a space craft that provides a method of cooling in space. Water is allowed to seep through the pores in a porous nickel plate into the surrounding vacuum where it freezes into an ice cake covering nearby coils of tube. Coolant pumped through the coils is cooled by the ice, which then sublimates into the vacuum carrying away the heat with it. The sublimated ice is replaced by more water through the plate.
TEI: Trans-Earth Injection. The posigrade maneuver by which the spacecraft leaves lunar orbit and begins the return to earth. TEI for the Apollo missions was accomplished by the SPS engine.
TLI: Trans-Lunar Injection. The maneuver by which a spacecraft leaves earth orbit and enters a trajectory intended to take it to the moon. TLI for the Apollo missions was accomplished using the S-IVB engine.
ullage: The volume in a closed container which is not occupied by the stored material. Also a maneuver in which weightless liquid propellants in a tank are forced into a fuel pump prior to engine ignition. This maneuver is performed either by RCS jets or by ullage rockets specifically intended for this purpose. The maneuver causes the propellants to settle to the bottom of the tank.
up-sun: An orientation for photography in which the sun is in front of the photographer and the subject is backlit. This frequently results in strong glare on the photograph. See also cross-sun and down-sun.
vidicon: In early television cameras, the component which converts light (i.e., an image projected by a lens onto its front surface) into an electrical signal. Modern cameras use a charge-coupled device (CCD). The vidicon resembles a very small television picture tube. It can be damaged by too much light (cf. Apollo 12). The decay of the photoelectric response sometimes produces "smeared" afterimages. The tube registers only intensity, not color. Filters must be used to record color information.