nasa facts manned space flight projects mercury and gemini

8/7/2019 NASA Facts Manned Space Flight Projects Mercury and Gemini

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Vol. II, No . 8

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NASAAn Educational Services Publication of the

National Aeronautics and Space Administration oJ-MANNED SPACE FLIGHT C -51PROJECTS MERCURY AND GEMIN-I N6 5 1696

Astronauts Virgil I. Grissom (left) and John W. Young (right) in pressure suits climb into the Gemini ProcedurTrainer for a flight test profile and training session in preparation fo r the first manned Gemini flight.


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MANNED SPACE FLIGHTA major goal of the United States space pro

gram is manned space flight to the moon, andsafe return to earth of the astronauts, before theend of this decade.

NASA's manned space flight program hasbeen divided into three big steps, or projectsMercury, Gemini, and Apollo.

Already accomplished, Project Mercury pavedthe way by using experimental one-man vehiclesand proving that men could be sent into spaceand returned safely to earth.

Project Gemini's two-man spacecraft will fulfill a two-part plan: first, extending orbit missionsup to 2 weeks at a time; second, developing thetechnique of rendezvous and docking, duringwhich two space vehicles are maneuvered closetogether and finally joined, or "docked."

That same technique of orbit rendezvous-butaround th e moon instead of earth-will enableastronauts in the three-man Project Apollo spacecraft to achieve lunar landings.

PROJECT MERCURYProject Mercury became an official program of

NASA on November 26, 1958. A Space TaskGroup to initiate this new venture was formedat Langley Field, Virginia. This group evolvedinto the Manned Spacecraft Center, Houston,Texas and after a nationwide call fo r je t pilotvolunteers, seven astronauts were chosen inApril 1959.

The one-man Mercury spacecraft was designedand built with a maximum orbiting weight ofabout 3,200 pounds. Shaped somewhat like abell (truncated cone), the craft is 74.5 incheswide across the bottom and about 9 feet tall.The astronaut escape tower on top adds another17 feet for an overall length of approximately26 feet at launch. Two boosters were chosenthe Army's Redstone (78,000 Ibs. thrust) and AirForce's Atlas (360,000 Ibs. thrust)-for suborbitaland orbital flights, respectively . Before themanned flights began, Ham, th e chimp, success-

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Mercury one-man spacecraft alongside mock-up oftwo-man Gemini.

fully achieved a suborbital Mercury-Redstone 2(MR-2) flight on January 31,1961.

Then all was ready for the historic MR-3 flightof May 5, 1961, as Astronaut Alan B. Shepard,Jr., made the first U.S. manned space flight. Hissuborbital mission of 19 minutes took his Freedom7 spacecraft 116 miles high into space.

After another countdown for MR-4 on July 21 ,1961, the Redstone booster hurled AstronautVirgil I. "Gus" Grissom through the second ballistic (suborbital) flight in the Liberty Bell 7.

This ended the Redstone non-orbital tests asthe Mercury-Atlas series of flights advanced toorbitol missions. Again preceding the firstmanned attempt the chimp Enos made an orbitalflight (MA-5) on November 29, 1 ~ 6 1 . An MA-6 space milestone, on February 20,1962, made Astronaut John H. Glenn, Jr., thefirst American in orbit, completing three circuitsin Friendship 7.

On the MA-7 mission of May 24, 1962, Astronaut M. Scott Carpenter in Aurora 7 completed another three-orbit flight.

MA-8 of October 3, 1962, doubled the flighttime in space as Astronaut Walter M. Schirra, Jr.,orbited six times, landing Sigma 7 in the Pacificrecovery area, instead of the Atlantic.

Finally, on May 15-16, 1963, AstronautL. Gordon Cooper, Jr.'s Faith 7 completed a 22-orbit mission of 34 112 hours, triumphantly concluding the flight phase of Projec t Mercury. Thetechnical report, "Mercury Project Summary" waspublished in October 1963, five years after theproject began.


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Vol. II, No.8Originally, Project Mercury was assigned only

two broad missions by NASA-f irst , to investigateman's ability to survive and perform in the spaceenvironment; and second, to develop the basicspace technology and hardware fo r mannedspace flight programs to come.

Beyond these basic goals, Mercury accomplished the following: developed a NASA man-agement system to carryon more advancedmanned space flight ventures; explored the fundamentals of spacecraft reentry; started a familyof launch vehicles from existing rockets that ledto new booster designs; expanded the aerospaceindustry by NASA contracting; set up an earthgirdling tracking system; trained a pool ofastronauts easily augmented for future spaceexploration programs.

PROJECT GEMINIProject Gemini was named after the twin stars

Castor and Pollux in th e constellation Gemini.NASA decided to fol low the Mercury's basic

"capsule" design for Gemini spacecraft, savingtime and engineering efforts. But the two-mancraft is wider (7.5 feet), taller (11.5 feet), andmore than twice as heavy (7700 Ibs.). Thesedimensions give 50 percent more cabin space,making room for much new equipment and withit far greater performance flexibility.

Since Mercury's Redstone and Atlas boosterslacked the power to orbit the heavier two-man

ADAPTER SECTION

Pacraft, a modified version of the military Titabecame the Gemini Launch Vehicle (GLV), a total thrust of 530,000 pounds (first sta430,000 pounds). The hypergolic (self-ignitpropellants used are non-explosive, an astronsafety factor.

Chosen for Gemini's prime mission of orbrendezvous and docking was the Agena-D "get" vehicle, a new version of the reliaAgena-B second-stage that, with Thor or Aboosters, had orbited many satelliteslaunched Mariner and Ranger probes. Agen"stop-and-restart" rocket engine, capablecutoff and reignition at least four times, isportant for r e n d ~ z v o u s maneuvers with GemThe hypergolic propellants are hydrazinenitrogen tetroxide.

Agena-D is 32 feet long and 5 feet in dieter, is shaped like a cylinder and tapered atfront end to a blunt point.

FIRST GEMINI-TITAN FLIGHT (GT-l)The first Gemini-Titan test fl ight (GT-1)

April 8, 1964, was an almost perfect lift-off,Titan boosted the unmanned Gemini craft iorbit from Launch Complex 19 at Cape KenneFla.Gemini's apogee (high point of orbit) was 2miles, perigee (low point) 99.6 miles, andrevolution period around the earth was 89.minutes.

EQUIPMENTMODULE REENTRY MODULE

AGEHADOCKING

STRUCTURE

Separated segments of Gemini mock-up. At right is docking structure of Agena whichcontains apparatus for link-up of spacecraft and rocket.


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at Cape Kennedy in April 1964.

Vol. II, No. 8In orbit 4 days, the craft came down as cal

culated on April 12. There was no attempt atrecovery since the fl ight was mainly a structuralintegrity test for th e booster / spacecraft combination, an d fo r its guidance system.

GEMINI EXPERIMENTSFollowing GT - 1, NASA scheduled a series of

scientific, biological, and technological experiments fo r subsequent Gemini flights. Separatebioastronautics (space medicine) experiments involve the physiological reactions of the astronautsin space.

In "extravehicular activities " experiments, theGemini astronauts, each in turn, will leave theirspacecraft and practice getting around and working in empty space. It is planned that thisactivity will include tests of specially designedspace tools that a weightless astronaut can usewithout twisting effect.

Capability to leave the craft is afforded bylife-support features of the Gemini space suit.While outside of th e spacecraft, the astronaut,in effect, will be a " human satellite. "

GEMINI SPACECRAFTThe Gemini craft is designed to be piloted by

its two-man crew. After an automated launch,the Gemini spacemen take over: turning, changin g speed, even shifting orbits .

The spacecraft cons ists of two major port ions-the reentry module (package) and theadapter module. The latter, in turn, also hastw o separate sections, so that Gemini, aslaunched, is actually a three-part structure.

Only the reentry module returns to earth .This module contains the " living " section wherethe two astronauts ride .

The l ife-supporting cabin is double-walledwith an inner shell around the crew ' s pressurizedcompartment and an outer shell as the craft' sexternal hull.

Between these shells is a storage space fo relectronic gear an d other apparatus-a technological improvement over the Mercury craft,whose components (parts) were " stacked" upon


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-- - -----Vol. II, No.8one another inside the crowded pilot's compartment. In contrast, much Gemini equipment isin the double-walled storage area, where it canbe easily checked, adjusted, replaced, even whenGemini is in place atop Titan on the launch pad.

The aft (rear) section contains more equipment, an d bottommost is the ablative (meltingan d evaporating) heat shield that protects thereentry module from air-friction heat on earthreturn.

One purpose of the two-part adapter module,which tapers and flares out from 7 112 feet to 10feet in diameter, is to "adapt " (fit) the narrowGemini to the Titan booster's broad top. Secondly, the adapter's 90-inch deep volume isanother housing area for equipment.

The adapter section adjacent to the crew 'sreentry module is the retrograde assemblage,holding two sets of engines -retrorockets (forreducing speed) and space-maneuvering thrusters.

The adapter's aft part, the equipment section,holds fuel cells, attitude controls, radio units,and a l iquid-coolant radiator to dissipate internalspacecraft heat away into open space.

The lower en d of this two-part adapter ismated , by means of a metal collar, to the to p ofthe Titan launch rocket .

EJECTION SEATBased on a jetplane technique, Gemini ' s light

weight ejection seats can , in emergency, catapultth e astronauts out of two large hinged hatchesthat open mechanically .

Unlike Mercury ' s automatic ejection sensors,Gemini's system relies entirely upon the astronaut crew ' s quick reflexes, because Titan ' s nonexplosive propellants merely burn and allow timefor human reactions.

COMPUTERAmong Gemini innovations is a " shoebox "

compute r , weighing only 57.6 Ibs. and occupying a mere cub ic foot of cabin space, yet able tomake the computations fo r the intricate rendezvous and docking maneuvers with th e orbitingAgena-D.

PageFUEL CELL

Another " space first " is Gemin i' s fuel cegenerat ing electrical power by chemical meanMuch l ighter than the equivalent batteries threplace, two groups of fuel cells provide 1,00watts each, supplying th e spacecraft' s totelectrical needs.

GUIDANCE SYSTEMAboard Gemini is an ingenious inertial gui

ance system which records and totals every bof progress forward, backward, and sidewayfrom the earth-launch starting point to the spacdesti nati on .

Linked into the guidance system during orbitmaneuvers are other units-compute r, radaelectronic controls, attitude thrusters, propulsiounits-so that the astronauts ' master conrols caaccurately achieve rendezvous and docking witAgena .

RENDEZVOUS RADARGemini ' s high-definition radar gives the rang

(distance), bearing (direction and angle of approach), and closing speeds of the chase antarget vehicles, w ith data starting when they ar250 miles apart.

Later, the high-intens ity flashes of Agenal ight beacon become visible to the astronauts, aa maximum range of 50 miles . These opticobservations, plus radar t racking, are then combined, as manual controls gu ide Gemini towarrendezvous w ith Agena .

Undoubtedly, the Gem ini astronauts will evetually blend their own intuitive skills wi th computer data, and ga in the know-how to swing thtwo vehicles around in space with the routinease of parking cars on earth .

COMMUNICATIONS SYSTEMSThree major communications systems are ca

ried aboard Gemini: the voice system, which includes an intercom connection between thastronauts; the rece iver of command signals anupdated orbit information from Manned FligControl; data collection tapes and their relatransmitters for automatic transmission of repo rto earth .


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Gemini spacecraft is raised to top of gantry for mating to Titan launch vehicle.


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around and goes backwards. Its fore endthus in docking position as the later-launcGemini arrives nose first.

ENVIRONMENTAL CONTROL SYSTEM (ECS)The Gemini Environmental Control System in

the reentry module's cabin compartment sustainsthe astronaut pilots and permits them to carryout their duties.

Though the Gemini l i fe-support system is simi lar to the Mercury ECS, major engineeringchanges have gone into it. Each astronaut hastwo parallel suit circuits for oxygen, plus the option of using the cabin's habitable (purified) atmosphere while being partially unsuited fo r morefreedom of action.

Mercury's bottled gas has been replaced inGemini by a liquid oxygen supply, requiring lessstorage volume fo r the maximum 14-day supply.

The third major ECS change is an improvedmethod of dissipating unwanted heat into space.

FOOD AND WATERBesides being a body fuel, food is important

to man as a psychological "uplif t ."A basic diet of 2,550 calories per man will be

fulfilled by freeze-dried foods including meats,soups, desserts, and fruits. Water restoresthem to their original form.

Water will be recovered from the atmospherewithin the spacecraft, also will be derived as afuel cell byproduct.

GEMINI LAUNCH VEHICLEThe Gemini launch vehicle is a modification

of the military Titan II. Fueled by stable andstorable propellants, it is 1a feet wide and 89feet long (flrst stage booster 70 feet). The combined Gemini-Titan stands 108 feet high.

An important new Gemini launch vehicle device is the Malfunction Detection System, whoseelectronic monitors watch the vehicle's perfqrmance during launch for possible booster trouble.Warning signals allow ample time for the astronauts to abort (cut short) th e mission, if necessary, by using their ejection seats.

AGENA-D TARGET VEHICLEIn the rendezvous of Agena-D and Gemini,

the tw o spacecraft will be joined nose to nose.Hence, after first being orbited, the Agena turns

Bringing the front ends of two spacecraft together, while whirling around earth at near18,000 mph, is much more difficult than mid -arefueling between a jetplane and a tankeHence, the mid-space meeting of rocket vehiclemust feature the quickest and most reliable contact via easy-fitting docking devices.

At the Agena's forward end, the target dockin g adapter (TDA) is a cylindrical collar, withiwhich is a docking cone illuminated by two approach lights. Self-adjusting mechanisms locfirmly to the inserted nose of Gemini and mooth e tw o vehicles.

Prior to docking, an invaluable aid to thastronauts is the l ighted status display panemounted outside Agena's lOA. Its visual monitors reveal the Agena equipment ' s operationreadiness by means of nine lights and thredials. Typical are green lights fo r "OKdock," "PPS TIME" (primary propulsion systemwhich clocks the amount of burning time left fothe main rockets before fuel depletion.

STANDARDIZED SPACELAUNCH VEHICLE

Historically famous as the booster fo r sihighly sucessful Mercury space flights, the Atlawill also serve as the Agena launch rocket iGemini's rendezvous experiments.

Known as the Standardized Space launcVehicle (SSLV), this 62-foot booster is powereby five conventional liquid rocket engines. Atlapower alone cannot orbit the Agena, and abooster burnout, pyrotechnic devices (explosivbolts) at the SSlV-adapter's forward end releasthe Agena, whose own engine fires to gaiorbital velocity.

LAUNCH CHECKOUTSPrior to a scheduled Gemini rendezvous mis

sion, all the hardware of two complete launchvehicles and tw o spacecraft is meticulouslchecked and rechecked many times over. Because the failure of the tiniest relay or switch


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Page 8can ruin an entire multimillion dollar space venture-and also risk tw o human l ives- this tediously painstaking checkout procedure is vitallynecessary.

All the major vehicles arrive at Cape Kennedybroken down into modules and sections. Afterdozens of checkouts until al l modules are matedinto full vehicles, 7 days remain before lift-off.In that week, the final tests include: radio andradar circuits; simulated f l ight via computer; simulated launch with the tw o astronauts aboard thespacecraft; servicing and fueling of Titan withcheckout of engine systems; complete check oftotal Gemini Titan equipment; 300-minute dressrehearsal countdown; final 2-day servicing andcheckout.

Only after this long parade of cross-checkcheck-outs does the real countdown start.

Sharing in all main tests, the tw o Gemini astronauts gain full confidence that the Titan boosterand Gemini spacecraft will successfully bear theminto orbit, and safely return them to earth.

The Mission Control Center of MSC, at CapeKennedy, will monitor early Gemini flights. Inlater missions, the f l ight phase will be monitoredby a ne w Mission Control Center at the MannedSpacecraft Center, Houston, Tex.; the launchphase will still be conducted by the Cape Kennedy team.

TRACKING NETWORKExpanded from the former Mercury network,

the Gemini tracking system comprises 13 landstations an d two tracking ships, the latter fillinglandless gaps in the Pacific Ocean.

Besides tracking equipment, al l stations havetwo-way communications with the Gemini spacecraft . Some have additional telemetry equipment or command signal transmitters.

GEMINI RENDEZVOUS MISSIONA Gemini rendezvous mission calls fo r send

in g up the Agena target vehicle from CapeKennedy's Launch Complex 14 approximately24 hours prior to the Gemini chase vehicle's liftoff from Launch Complex 19.

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Vol. II, No.8The Agena is propelled into a circular orbit

185 miles up, after which precise velocity andtrajectory elements are calcu lated. About 24hours later Gemini blasts off, within the specified"launch window"- the time interval during whichlaunch will produce an orbit permitting the tw ocraft to meet.

Gemini ' s orbit must be in the same plane(slant toward equator) as Agena ' s. This restrictsth e Gemini launch window to about 2 hours ineach 24, for the 5-day period the target vehiclecan wait for rendezvous. Certain orbit-correctingmaneuvers by the Agena, before Gemini launch,can double the launch window .

The basic plan is to maneuver the Gemini intoa circular orbit whose altitude is approximately23 miles less than the Agena.

TARGET CAPTUREWhen the distance between vehicles IS 250

miles, radar is switched on . As the gap closesto 50 miles, the Gemin i astronauts pick up theAgena ' s flashing beacon and take over themanual controls.

Aiding the astronauts is the status displaypanel outwardly mounted on the Agena-D, g iving visual data on Agena fuel reserves, electricalpower, attitude position, al l of which is processedthrough the computer along with the Geminicraft's own movements.

During these maneuvers the relative speeddifference between the vehicles has been cut toless than a 2 mph dri ft rate so that their nosestouch gently.

If the first docking maneuver is imperfect, theAgena is merely bumped away and no harm isdone. The astronauts simply back off fo r another tr.y. In a successful contact, the Gem ini 'snarrow end enters the Agena's target dockingadapter, whose latches clamp shut to preventthe tw o vehicles from slipping apart. Then amotorized Agena unit pulls the Gemini noseconeinward all the way.

Once the two craft are tightly moored, matchin g electrical contacts meet and give the Geminiastronauts direct control of Agena's onboardequipment-guidance, propulsion, attitude control, relay switches, and the rest.


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Vol. II, No.8 PageREENTRY AND RECOVERYUnion of the two vehicles results in a larger

Gemini / Agena spacecraft almost 50 feet long,much more powerful and versatile than either ofthe tw o original craft. With the main Geminirockets in back and the Agena thrusters in thefront, the two-part Gemini-Agena spacecraft canmove either forward or backward without havingto turn around. The doubled rocket power alsoallows fo r unique space performances throughthe series of manned Gemini missions.

Their rendezvous maneuver completed, thastronauts unhook their Gemini craft from thAgena (left in orbit) and prepare for earth-returnThey first ascertain that upon firing th e retrorockets, their downward trajectory will land themin a designated recovery area . This may require orbit correct ions by means of theipropulsion units.

PA INTING !Y DAV IS MELTZER, RESEARCH BY GEORGE w , BEATTY C> N .G

Scanning the dark sky, Gemini's radar locates Agena a t a distance of 250 miles and holdsthe quarry in view (far left). Gemini 's computer determines the approach speed and pointa t which the tw o craft will meet . An 8-foot boom antenna on the target craft receivesradio signals from the astronauts, who order Agena to assume a stabil ized posit ion. Guidedby flashing beacons, th e astronauts speed up their vehicle and maneuver it toward Agena'scone -shaped docking collar. Though the tw o craft t rave l 17,500 miles an hour, theirdifference in speed is only a little more than one mile an hour a t final closure. Crewmenmaneuver Gemini's nose into a V-slot in Agena's collar; then mechanical latching fingerssnap the craft together. The space-mated couple turn around, placing Agena's restartableengine in position to propel the vehicle into a new orbital path so tha t the astronauts canprobe deeper into space. They will detach Agena before re-entering earth 's atmosphere.


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Page 10Because of Gemin i 's slight " lift " (gliding abil-

ity), there is a capability for " aiming" toward thedesired landing site . However, provision ismade for th e possibil ity of a random landing;equipment an d training for Gemini astronaut sur-vival are more elaborate than they were in Proj-ect Mercury.

A survival pack for each Gemini crewman isstowed in a rectangular-shaped cavity in the backof his spacecraft seat. If he leaves the seat, hispersonal parachute and survival pack-bothbeing strapped to him-are at the same timepulled along .

Gemini spacecraft will have large spacecraftparachutes fo r water .Iandings, as in ProjectMercury.

If reentry is unavoidably over land, the astronauts will eject themselves in the air and parachute down by themselves. The craft's impacton hard ground is too violent fo r the astronautsto risk, but low enough to prevent serious vehicledamage.

ASTRONAUT SELECTIONThe National Aeronautics and Space Admin

is tration started with the seven Mercury astro-nauts in April 1959. For the Gemini andApollo missions, it chose 9 others in September1962, an d 14 more in October 1963.

There now are 28 astronauts (o f the 30 se-lected) comprising the pool from which the twoman Gemini crews are chosen.

NASA ASTRONAUT TRAININGAll astronauts undergo training to reach peak

mental and physical efficiency fo r Gemini flights.Basic sc ience studies have been expanded to in-clude such courses as computer fundamentals,guidance technology, an d astrogation (spacenavigation).

To prepare fo r space missions, astronautspractice earth-simulated flights in trainer devices.The space pilot-room simulators teach astronautsthe elements of rendezvous and docking bymeans of electronic dials and dummy controls.

Vo l. II, No . 8One Gemini f l ight trainer is located at the

Manned Spacecraft Cen ter's new Clear Lake siteat Houston, Tex ., wh e re th e Mission ControlCenter is also stationed . Another simulator in-stallation is at Cape Kennedy, Fla .

Centrifuges at Johnsville , Pa. and at AmesResearch Center in Californ ia provide high-gloads to match the stresses of powered launches,training astronauts to handle controls desp iteacceleration strains .

BIOMEDICAL ANDBIOASTRONAUTICS EXPERIMENTS

Gemini f l ight plans call fo r various spacemedicine investigations conceived by NASA'sbioastronautics authorities.

In general l ife-support (bioastronautics) studies,important information to be gained during Gem inimissions includes: ways to improve space suitsfor greater comfor t and health; more efficientbody waste management; methods of exercisingto keep the astronauts in good muscle tone .

A major problem is that the crew of theGemini, unlike that of th e Mercury, will have longperiods of zero-g inactivity . These periods mightbe physically detrimental, unless su itable musGleexercises are developed for the weightless sta te,plus work-rest cycles that keep spacemen mentallyalert.

NASA scientists also face " human engineering " problems involved in space suit design . Toalleviate discomfort during confinement for 14-day Gemini trips, parts of the astronaut's suitwill be removed fo r comfort an d more freemovement.

The GT -3 an d GT -4 Gemini crews werechosen early in 1964. From GT -5 on , otherpaired teams of astronauts take over theass ignments.

From among the Gemini astronaut roster, willundoubtedly emerge th e three spacemen-unknown as yet-destined to be the first u.S. citizensto fly an Apollo spacecraft to the moon, near theend of this decade.

Those three " Iunarnauts" will have an eternalplace in history as Columbuses of space wholanded on a new world.


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Gemini spacecraft mockup being checked ou t by Astronaut John W. Young.

DEFINITIONSAPOGEE: In an orbit about the earth, the point at whichthe satellite is farthest from the earth; the highest alt itu dereached by a sounding rocket.ASTRONAUTICS: The art, skil l, or activity of operatingspace vehicles. In a broader sense, the science of spaceflight .ATTITUDE: The position or orientation of an aircraft,spacecraft, etc., either in motion or at rest, as determinedby the relat ionship between its axes and some referencel ine or plane such as the horizon .BALLISTIC TRAJECTORY: The trajectory followed by abody being acted upon only by gravi tat ional forces andthe resistance of the medium through which it passes.

BOOSTER ROCKET: A rocket engine, either solid orl iquid fuel, that ass ists th e normal propulsive system orsustainer engine of a rocket or aeronaut ical vehicle inso me phase of it s fl i ght. A rocket used to set a missilevehicle in motion before another engine takes over.DOCKING: The process of bringing tw o spacecraft together while in space .HYPERGOLlC: Propellants, fuel and oxidizer, which ign it espontaneously upon contact; hydrazine and nitrogentetroxide are examples.MODULE: A self-contained unit of a launch vehicle orspacecraft which serves as a building block for the overallstructure. The module is usually designated by its primary

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Astronaut Tom Stafford takes his turn in the Gemini Docking Simulator.

DEFINITIONS- (Continued from page 11 )function as " command module " , lunar landing module, etc .A one-package assembly of funct ionally associated electronic parts ; usually a plug-in unit.P E R I G E ~ : That orbital po in t nearest the earth when theearth is the center of attraction.PITCH: The movement of an aircraft or spacecraft abautits lateral (nose going up or down) axis.PROPELLANT: Any agent used fo r consumption or combustion in a rocket and from which the rocket derives itsthrust, such as a fuel, oxidizer, addit ive, catalyst , or anycompound or mixture of these .REACTION, ENGINE: An engine that deve lo ps thrust byits reaction to ejection of a substance from it; specifically,such an engine that ejects a je t or stream of gases createdby the burning of fuel within the engine .REENTRY: The event occurring when a spacecraft orother object comes back into the sensible atmosphere afterbeing rocketed to altitudes above the sensible atmosphere;the action involved in this event .RETROROCKET: A rocket fitted on or in a spacecraft, satellite, or the like to produce thrust opposed to forwardmotion .

NASA FACTS fo r mot is designed for bulletin-boord displayuncut, or for 8 x 10)1, l oose leaf no tebook insertion whencu t olong dotted l ines ond folded a long solid lines. Fornotebook ring insertion, punch a t solid dots in th e margins.

ROLL: The rotational or oscil latory movement of an aircraft or similar body which takes place about a longitudinalaxis through the b o d y ~ c a l l e d " rol l " fo r any amount ofsu ch rotati on .SENSOR: The component of on instrument that converts oninput signal into a quantity which is measured by anotherport of the i nstrument . Also cal led " sensing element. "SUBORBITAL: Non-orbit ing or ball istic f l ight trajectoryfrom launch point to target point .TRAJECTORY: In general, the path traced by any body,as a rocket , moving as a result of externally appl ied forces .WEIGHTLESSNESS: A condition in which no acceleration ,whether of gravity or other force can be detected by anobserver within the system in question . A condition inwhich gravitat ional an d other external forces act ing on abody produce no stress, either internal or external, in thebody .YAW : The lateral rotational or osc i l latory movement ofon aircraft, rocket, or th e l ike about a transverse axis .The amount of this movement , i .e ., the angle of yaw .

NASA FACTS is an educat ional publication of NASA's Educational Programs and Services Office. It will be mailed toaddressees who request it from: NASA, Educational Publicat ions Distribution Center, AFEE-l, Washington, D.C., 20546 .

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