america in space the first decade - nasa spacecraft

8/8/2019 America in Space the First Decade - NASA Spacecraft

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NASASPACECFIAFrby W illiam R. Corliss

O:

National Aeronautics and Space Adm inistration, Washington, D.C. 205 46,


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I n t r o d u c t i o n

It is ten years since the N ational Aeronau tics andSpace A dministration was created to explore spaceand to continue the American efforts that hadalready begun with the launch of Explorer 1on January 31, 1958. Many changes have occurredsince that tumbling, 31 -pound cylinder went into an

Earth orbit. "NASA Spacecraft" represents one ofthe broad avenues selected by NASA as an approachto its objective of making widely known theprogress that has taken place in its program ofspace exploration. This report is a vividillustration of the changes that have occurredand the complexities that have developed. He reone finds descriptions of the present family of

spacecraftsom e small, some large; some spin-oriented, some accurately attitude-controlled; somemanned, some automated; some in low orbits,some in trajectories to the Moon and the planets;some free in space until they expire, otherscommanded to return to the Earth or to land 011

the Moon.

Oran W. NicksDeputy A ssociate A dministrator forSpace Science and A pplications


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TableOf

Contents

Spaceships and SpacecraftHow Spacecraft Work 2

01 Systems and Subsystems 2

The Power Supply Subsystem 3The Onboard Propulsion Subsystem 5

The Communication Subsystem 5The Attitude Control SubsystepThe Environment Control Subsystem 7

The Guidance and Control Subsystem 8The Computer Subsystem 9

The Structure Subsystem 0The Eng ineering Instrument Subsystem 1NASA S pacecraft Families 1


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1 Satellites stay in orbit because the centrifugal effectdue to their horizontal velocity just cancels thegravitational force trying to pull them back to Earth.Sounding rocket and ballistic trajectories are segmentsof ellipses. Space probes leave the Earth'sgravitational field completely.

NASASpacecraftSpaceshipsAnd Spacecraft

To Jules Verne, a spacecraft was a huge aluminumbullet fired toward the Moon from a giganticcannon buried in Florida soil, not too far fromCape Kennedy. In 1865, when V erne's "De laTerre a la Lune" appeared, spacecraft were con-ceived as well-appointed extensions of the drawingroom, and the gentlemen who traveled in themlikely as not wore top hats and formal attire. Thisromantic view of the spacecraft persisted well intothe Twentieth Century. To distinguish these visionsof fiction from today's complex space machines, wecall the former spaceships and the latter spacecraft.

A spacecraft is any vehicle that operates abovethe sensible atmosphere; that is, above the altitudesattainable by research balloons and aircraftapproximately 100 ,00 0 feet of altitude.

In the first ten years of space flight, over 60 0satellites have circled the globe. Even so, thesatellite has been g reatly outnumbered by thesounding rocket, a spacecraft that breaks throughthe atmosphere into space for only a few m inutes.Although sounding rockets do not linger long at highaltitudes, they have made major discoveries inspace science, such as the existence of X-ray stars.

A satellite is a spacecraft that has been givensufficient velocity by its launch rocket to be placedin orbit. Ultimately the trace of atmosphere still

present at satellite altitudes will slow the satellitedown and g ravity will pull it back to Earth. Thedistinction between a satellite and a long range rocketis that the satellite makes one or more completecircuits of the Earth.

In 1955, when the U nited States decided to launcha small research satellite during the InternationalGeophysical Year, the engineers assigned to the

task did not think of Jules Verne's elegant,well-padded projectile; they thought about extendingsounding rocket technology. Fo r a while, the first


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U.S. satellite project was called the LPR, the LongPlaying Rocket. Jules Verne may have the lastlaugh, however, because the U nited States andCanada have seriously considered launching smallsatellites with the help of a rebored 16-inch naval

cannon.

Spacecraft that are shot deep into space and escapethe gravitational pull of the Earth completely arecalled space probes. Depending on the target;they are called lunar, planetary, and deep spaceprobes. Deep space probes are placed in orbit aroundthe Sun to study the solar wind and interplanetarymagnetic field. In essence they are artificial planets.

One of three things may happen to a probe launchedtoward the M oon or one of the planets: ( 1 ) a nearmiss or fly-by, (2) injection into orbit around thebody, or (3) impact on the surface, with either a hardor soft landing. Fly-by probes usually go into orbitaround the Sun after planetary encounter. Lunarprobes may swing around the M oon and settle downto become Earth satellites.

Spacecraft may also be classified as manned orunmanned; as recoverable or unrecoverable.All manned spacecraft are made recoverable; so aremost sounding rockets. Those unmanned satellitesthat carry film packs or biological specimens are maderecoverable by adding retrorockets that force thespacecraft to reenter upon command from theground.

Spacecraft may be either active or passive. A passivesatellite transmits no radio signals to Earth but mayreflect them back. NA SA's Echo balloon satellitesare good e xamples of passive satellites. They arebig enoug h to see visually, and by their motionreveal the air density where they orbit. Activesatellites emit radio signals to make tracking easierand to transmit data from their instruments toground stations. When a satellite radio signal fadesnaturally or is intentionally cut off by a killer timer,the satellite becomes inactive or dark.

Satellites are also classified by their orbits. Apolar satellite orbits over the Earth's polar regions.A synchronous satellite orbits the Earth in thesame length of time it takes the Earth to make onerevolution on its axis. If the synchron ous satellite

is also an equatorial satellite, it will seem to rem ainin the same position in the sky at all times. It isthen a stationary or geostationary satellite.

The final taxonomic breakdown depends uponthe functions or uses of satellites. NASA d ividesits satellites into three categories:

1. Scientific satellites, which carry instruments tomeasure magnetic fields, space radiation, the Sun,

and so on. Examples: NASA's Explorer series,the OGO series.2. Applications satellites, which have utilitari anpurposes. They help forecast the weather, extendEarth communications, survey the Earth, findmineral deposits, test equipment, etc. Examples:the TIROS weather satellites, the Syncomcomm unication satellites.3. Manned satellites, which are designed to

check out equipment and man himself inpreparation for the m anned lunar landing andother manned space missions.

Spacecraft obviously differ greatly in size, shape,complexity, and purpose. Nevertheless, most requirepower supplies, radio transmitters, a means fororientation in space, as well as other equipment.Spacecraft can be described in general terms first,by showing how they work; second, by describingthe hundreds of N ASA satellites, probes, andsounding rockets launched since the Ag ency'sfounding in 1958. Fortunately, they can be collectedinto handy families. For exam ple, a TIROSfamily of weathe r satellites exists; so does a Gem iniclass of manned spacecraft.

Ho wSpacecraftWork

Of Systems and SubsystemsAutomobiles and spacecraft are both man-machinesystems. In the auto, a motor turns the wheels, asteering wheel changes its direction, a heater keepsthe occupant warm in the winter. Spacecraft havesubsystems that help ma n attain his objectives.

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Modern spacecraft are drivenlike the car simple, passive balloon satellite may haveeither by an astronaut or by a hum an controller on ess than 100 parts and require only the structurethe ground connected to the spacecraft by a radio link. subsystem. A soft lunar lander may contain

20 ,00 0 parts and use all nine subsystems.Of all the subsystems that make a spacecraft workproperly, nine are critical: Spacecraft are really extensions of man that enable

him to explore the cosmos and make use of theEarth's resources. Spacecraft can extend m an'shands to the Moon and planets, as they havealready done with the Surveyor surface sam plers.Man and a radio-linked spacecraft make aremarkable and useful man-machine partnership.

The Pow er Supply SubsystemOnly a few watts bring a spacecraft to life andmake it a useful extension of man. Satellites con-suming less than 10 watts of power discovered theVan A lien belts and the solar wind. The bigg estsatellites and space probes require only a fewhundred watts for their operation. A kilowatt or twowill keep man alive and in touch with the Earth.While spacecraft may not be power guzzlers of thesame order as the A merican home, there are nogas pumps or electric power lines out in space tokeep them running. Fuel must be carried alongor energy must be extracted from sunlight.

Sounding rockets have found batteries adequatefor their brief forays above the atmosphere. Theearly satellites also carried batteries into space,but they lasted only a few weeks. To improvesatellite longevity, the Vanguard 1 satellite carriedthe first solar cells aloft in 1958.

Since then, almostall satellites and space probes have their complementsof the little silicon wafers. The sho rt-mission mannedsatellites are the major exceptions.

Solar cells were invented at the Bell TelephoneLaboratories in 1954. They are only an inch long,a half inch wide, and a few hundredths of an inchthickabout the size of a razor blade. When sunlight


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2 Closeup view of the Applications Technology Satellite(ATS) showing the thousands of solar cells bonded tothe satellite's cylindrical surface.

strikes a solar cell, roughly 1 0 % of its energy is

converted into electrical energy; the remainder(90%) is reflected or turned into heat. The electricalcurrent flows between the layers of electron-rich(n-type) and electron-poor (p-type) silicon layersthat make up the thin solar-cell sandwich. Althougha single solar cell generates only a fraction of a watt,hundreds and thousands are commonly hookedtogether to provide the spacecraft with the power itneeds. To gather enough sunlight, solar cells are

fastened on the body of the spacecraft, or onextendable paddles.

Solar cells do not eliminate batteries. Because manyEarth satellites spend much of their life in theEarth's shadow, solar cells must charge up batteriesduring the satellite day so that power will be availableduring satellite night. In low orbits, the satelliteday-night cycle lasts only an hour an d a half.Consquently, the solar cell-battery combinationcharges and discharges several thousand times a year.

Why weren't solar cells used 011 the Mercury andGemini manned missions? They would have takentoo much roomroughly a hundred square feetper kilowatt. Batteries alone are too heavy formissions lasting more than a few days. TheMercury spacecraft did employ batteries, but withthe Gemini series NASA switched to fuel cells.

fuel cell is really a continuously fueled battery. fuel, such as gaseous hydrogen is made to react

with oxygen on a high-surface-area electrode.Electricity and a combustion productwater in thiscaseresult. There is 110 flame during fuel cellcombustion, just as there is none in a flashlight drycell, although a little heat is generated becauseenergy conversion is not 100% efficient. As fueland oxidizer are consum ed in the fuel cell, theyare replenished from external tanks. When mannedmissions are longer than a few days, the fuel cell

plus fuel and oxidizer tanks are lighter thanbatteries.

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Summarizing, batteries are used on most soundingrockets; solar cell-battery combinations on mostother unmanned spacecraft; and fuel cells 011 mostmanned spacecraft.

The Onboard Propuls ion SubsystemThe only practical way to bring an astronaut downout of orbit is to slow the spacecraft with asmall rocket and cause it to reenter the Earth'satmosphere. The Surveyor lunar probe also requireda rocket engine to slow it from several thousand milesper hour to a feather-like touchdown that didnot hurt its instruments. Geo stationary satellites,such as the Syncoms, must apply bursts of thrustto maintain their orbits (station keeping).

Onboard rockets are many times smaller than theirhuge counterparts that we see on the launch pad, butthey are equally sophisticated. For onething, the propellants must be storable; that is,they must survive in the space environment for daysor weeks before they are burned. For this reason,the usual launch roc ket fuels (kerosene o r liquidhydrogen) and oxidizer (liquid oxygen) arereplaced b y a solid fuel, a monopropellant (likehydrogen peroxide), or a storable bipropellant.

If liquids are selected for the onboard rocket, theremay b e no gravitational force to pull them into theengine when they are needed; say, in orbit orhalfway to Mars. Consequently, NASA hasdeveloped squeezable bladders, pistons, and otherpositive expulsion containers that force fluids intothe engine without assistance from gravity.

Onboard rockets present one more difficultrequirement: they must fire and shut off upon

command from E arth or the spacecraft's internalmemory. Maneuvers in space can be very touchy-- and delicate, particularly when the target is a

planet 100,000,000 miles away or a specific crateron the Moon.

The C om m unicat ion SubsystemRadios link spacecraft with the Earth-based dataacquisition system that ultimately connects the

spacecraft to man. Radios seem rather prosaic thesedays, but spacecraft comm unication is not quite

as easy as turning on channel 4 011 the TV. First,there is the problem of distancehundreds ofmiles in the case of deep-space probes. Second,some Earth satellites are prodigious data gatherersand must transmit huge quantities of information.Both distance and data volume can be achieved if thespacecraft has a big voice or the Earth receivingstation has big ears. A big spacecraft voice infersa high transmitter power; b ut power is a scarcecommodity in space and the big-ear approach isfavored. Thus NASA's data acquisition stationspoint large 85-foot and 2 10 -foot paraboloidalantennas at passing satellites and space probes outin the depths of space.

Very high frequencies have to be used for spacecraftcommunication. They have to be high enough topenetrate the Earth's ionosphere (over 20 MHz*)and low enough so that the signals are not absorbedby atoms and molecules in the atmosphere (under300 MHz). Terrestrial radio noise created by manand thunderstorms interfere at the lower frequencies.As a result, most space c ommunication systemsoperate between 100 and 3000 MHz.

Spacecraft do not send radio messages back toEarth continuously, although tracking beacons oftentransmit all the time. M ost spacecra ft transmittersare activated upon the receipt of a radio command

from Earth. Much of the time spacecraft are notwithin range of Earth-based receiving stations, andit would be wasteful of power and valuable datato send signals all the time. Instead, when aspacecraft comes over the horizon toward a dataacquisition station, it is commanded to read outits memoryusually a tape recorder. The spacecraftthen dumps these data in a burst to the radio earswaiting below.

*1 M Hz=one megahertz= 1,000,000 cyc les pe r sec ond .

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(1)

0>

0.

00Cl)

(1)

3 A continuously varying instrument reading canbe approximated by a series of digital binary datawords. The more bits used per word, the more acc uratethe analog-digital conversion. In this illustration, forexample, a three bit word limitation means that sensorreadings 1.8, 1.6 an d 1.7 all translate into binarylanguage as 010.

Data ecimal hree-bitword ensor ata ulse trainsno . eading ord

1 .3 102 .1 00

3 .8 10 ____fl4 .6 10

5 .7 10

Collectively, NASA ground stations often acquiremore than 50 miles of data on magnetic tapein a day. When an OGO satellite dumps itsmemory, it transmits the equivalent 01 a coupleof novels in a few minutes. In 1958 the problem was

getting any data at all from space; today, NASAmust cope effectively with vast quantities of it.Computer data processing on the ground is part ofthe answer. The tendency is for spacecraft to talkdirectly to ground-based computers in computerlanguage; that is, the binary number system, whichis based upon the number 2 instead of 10.' Thecomputer reduces the data and even draws graphsfor the experimenters.

The At t itude Control Subsys temOn Earth we find it easy to keep our verticalorientation by pushing against the ground with ourfeet. In space, though, there is little to pushagainst, and there are a surprising number of forcesthat tend to disturb the orientation (attitude) 01a spacecraft.

The two main tasks of a spacecraft's attitude controlsubsystem are stabilization and pointing. Stabilizationmeans keeping the spacecraft oriented in a specificdirection despite natural forces to the contrary.Spacecraft stabilization can be com pared to ke epinga light car on the road in the presence 0 1 strongwind gusts. Some spacecraft need to be pointed atselected targets as well as being stabilized. TheOrbiting Solar Observatories, for example,

*j the binary language: 001=1, 0102, 011_z3,100=4, 101=5, and so on.

Digital word number

must lock on to the Sun and follow it withtheir instruments. Weather satellites have to keeptheir cameras trained on the Earth. Bothstabilization and pointing require that the attitudecontrol subsystem generate turning forces or

torques on the spacecraft.

NASA has studied the natural forces existing inspace with an eye to overcoming them and evenharnessing them for attitude control. Sunlight, forexam ple, exerts a small but significant pressure on allspacecraft surfaces it hits. Unless a counter-torqueis created by the attitude control subsystem, thispressure can twist the spacecraft into undesired

orientations. Occasionally, solar pressure can beput to positive use; some 01 NASA's Marinerplanetary probes carried special vanes that appliedsolar pressure to attitude control the same way thatsails make use of the wind in propelling sailboats.

Gravity, too, is pervasive and persuasive; it alwaystries to pull the long axis of a satellite around sothat it points at the Earth. Our Moon is

gravitationally stabilized in this way by the Earth,always keeping one face toward us. Many NASAsatellites take advantage of this naturallystabilizing force by paying out long booms orpendulums. The Earth's field then swings these andthe satellite instruments around so they face theEarth. This is termed gravity-gradient stabilization.

The Earth's magnetic field is also useful. Bybuilding coils 01 wire in satellites and connectingthem to the space craft power supply, the satellites

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can be made into electromagnets. When electricalcurrent is applied, the satellite's magnetic fieldinteracts with that of the E arth, and the satellitewill turn like the armature of a motor. ManyNASA satellites are magnetically stabilized.

The m ajority of satellites are spin-stabilized. When

they are propelled into orbit, the final stage ofthe launch rocket gives them a twist that spinsthem at, say, 30 rpm. This spin, or its angularmomentum, stabilizes the spacecraft against theinfluences of solar pressure, meteoroid impacts, andother disturbing torques in the same way the spinof a rifle bullet keeps it from tumbling.

Spacecraft like the Surveyors and Orb iting

Geophysical Observatories need something morepowerful than gravitational and magnetic attitudecontrol schemes. Strong, controllable torques canbe created by pairs of small rocket engines mounted011 the periphery of the spacecraft. The rockets maybe small versions of the onboard engines justdescribed, or they may be bottles of compressedgas with electrically controlled valves. For verytiny, precisely measured bursts of thrust, NASA

engineers have developed thrusters that shootlittle bursts of gas. All attitude control schemesdepending 011 the rocket principle expel mass.Since mass is limited 011 spacecraft, so is theamount of pointing and stabilization achievable bymass expulsion.

Whenever a motor in a spacecraft starts up, theentire spacecraft experiences a twist in a directionopposite from that on the motor shaft. The Lawof Conservation of A ngular Momentum dem ands this.Again, a potential destabilizer can be turned to positiveuses. Gyroscopes and inertia wheels are just motorswith heavy rotors. When their speeds of rotation arechanged, they exert torques011 the spacecraft. Aset of three gyros mo unted w ith shafts parallel toeach of the spacecraft's three degrees of freedomcan con trol all aspects of spac ecraft orientation.Gyros and inertia wheels, of course, do not expel

valuable mass when they change spacecraftattitude, but they are limited in the sense that theycan spin only so fast without damaging themselves.

Suppose a meteoroid hits a solar-cell panel andstarts the spacecraft spinning. The gyro controllingthat spin axis will increase its speed of rotation,trying to build up enough torque to stop thespacecraft spin. How ever, if the disturbance was toogreat, the gyro may reach its maximum speedwithout stabilizing the spac ecraft. In this case, thegyro is said to be "saturated." By firing an onboardattitude control rocket or gas jet, the gyro can b e

desaturated.

The Enviromnent Control Subsystem

Controlling the environment means insuring thatman and his instruments can survive in space. Forman, this means taking an atmosphere along andenclosing him in a capsule that keeps out theharsh environment of outer space, in particularthe vacuum, the Sun's ultraviolet rays, and the

searing heat of atmospheric reentry.

Other environmental threats are: micrometeoroiddamage, radiation damage from the Van Allenbelts, and the high-g forces 01 rocket launch andspacecraft landing.

A unique problem faces the engineers who designNASA's planetary probes. These craft, if they areto enter other planets' atmospheres, must beable to withstand the high temperatures ofbiological sterilization. The purpose of spacecraftsterilization is the avoidance o f contaminating theother planets of the solar system. Were organismsof Earth origin inadvertently introduced into theecology of another planet, the opportunity tostudy an extraterrestrial life system in the naturalstate would be forever lost. If life has arisen else-where, the reverse problem existsthat of excluding

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extraterrestrial organisms from manned spacecraftand from Earth itself upon the return of astronauts.

Thermal control is a serious problem for the vastmajority of spacecraft. If instruments get too hot orcold they are apt to malfunction. Offhand, the

problem would seem to be one of heating spacecraftin the cold of outer space. It is true that an objectplaced far out in interstellar space will soon radiateaway mo st of its sensible heat and attain a temperaturea few degrees ab ove absolute zero. But in Earth orbit,the Sun and the sunlit side of the Earth bothradiate considerable heat to a satellite. In fact,the average temperature of an Earth satellite willnot be too different from room temperature. The

problem lies in that word average.When a satellite enters the Earth's shadow, itimmediately begins radiating its sensible heat tocold, starry space, and to the night side of Earth,which fills almost half the sky. Some hea t isreceived back from Earth, since even the nightside averages well above freezing. Earthshine isusually not enough, particularly if the satellite is in a

high orbit that keeps it in the Earth's shadow forseveral hours. Some heat comes from satelliteelectrical equipment; special heaters can be addedat cold-sensitive spots. Ultimately, the heat energycomes from the pow er supply.

The Surveyor lunar landers faced severe thermalproblems during the two-week, cold lunar nights.NASA made no attempt to maintain normal

operating temperatures in the Surveyors duringthese periods; missions were suspended until thenext lunar day. In some c ases, the heat of theSun was able to revive the frozen spacecraft.

Spacecraft that are in the Sun all of the time mustdeal with overheating. All space probes, somehigh-orbit satellites, and those satellites that keepone face oriented toward the Sun (the OSOs, for

example) must get rid of excess heat by radiatingit to empty space. Special metallic conduits (heatpipes) are sometimes installed to conduct internalheat out to the spacecraft surface or from the hotside to the cold side. The amount 01 heat escapingfrom the side of the spacecraft facing cold spacecan be autom atically adjusted by thermostaticallycontrolled louvers; which are Venetian-blind-likevanes that expose the hot internals of the spacecraft.

Astronauts must be kept cool, too, but a breathableatmosphere is even more critical. On shortmanned m issions, such as the Gemini satellite flightsand the Apollo Moon voyage, bottled atmospherecan be carried along from E arth. For missionsexceeding a month or so, this approach would costtoo much in terms of weight. The alternative is aspacecraft atmosphere that is continuously regeneratedor renewed.

The Guidance and Control Subsystem

Steering spacecraft safely to their targets is the taskof the guidance and control subsystem. Consider aspacecraft trying for a soft landing on the Moon.The attitude control subsystem has already turnedthe spacecraft around so that its retrorocket andlanding radar are pointed at the Moon. Radarsignals tell the spacecraft guidance and controlsubsystem the speed 0 1 descent and the distance fromthe surface. Built into the spacecraft memory isinformation telling the spacecraft when to fire itsretrorocket on the basis of radar data. Theguidance and control subsystem continuallycompares the real radar signals with the signals

its memory says it should be receiving. When theproper spacecraft velocity and distance from the

4 The thermal control louvers of OGO. These open likea Venetian blind to allow the hot spacecraft interiorto cool by radiation to dark space.


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Moon are reached, a command to fire retrorocketsis dispatched to the onboard propulsion subsystem.The rocket fires until radar data indicate thattouchdown w ill occur at near-zero velocity. Thisis the essence of control: comparison of realperformance with desired performance and thecommanding of the spacecraft to eliminate anydiscrepancy between the tw o.

Most spacecraft control functions are much simplerthan soft landing 011 the Moon. A good example isthe cutoff of a satellite transmitter to free afrequency channel for new spacecraft. This simpleact is accom plished by a killer timer that disconne ctsthe power supply after six months or a year ofoperation.

The lunar lander and killer timer examplesillustrate what engineers call closed and open-loopcontrol. The lunar landing requires continuousfeedback of information telling the guidance andcontrol subsystem how well it is doing its job.In such closed-loop-control situations the opportunityfor corrective action exists. With the killer timer,action is irrevocable.

A modern spacecraft possesses dozens of sensorsthat tell the guidance and control subsystem (whichoften includes the human controller 011 the groundvia the comm unication link) the status of thespacecraft. Thermom eters take spacecraft

temperature; gyros, star trackers, and Sun sensorsmeasure spacecraft attitude; and voltmeters andammeters relate how the power supply is performing.Less obvious are the signals that tell the humancontroller the positions of critical switches that fixthe spacecraft's mode of operation. For example,it is important to know which experiments are 01 1and which are off. Collectively, such status signalsare termed housekeeping data.

The Computer Subsystem

Computers are generally thought of as largemachines that belong on the ground rather than onspacecraft. Several spacecraft functions, however,have combined to make computers integral partsof large, modern spacecraft.

One of the simplest jobs for a spacecraft computeris analog-digital or A D conversion. Many spacecraftinstruments generate continuously varying or analogsignals. But spacecraft usually communicate with theground in digital language. To translate instrumentreadings into the lingua franca of space, small ADconverters are attached to analog-speakingequipment. In reality, AD c onverters are little,special-purpose computers.

Sometimes com putations are required011 highlyautomatic spacecraft, such as the Orb itingAstronomical Observatory (OAO). The OAO startracker readings, for example, must be transformedinto the proper geometric coordinates if the OAOattitude control subsystem is to know where to pointthe spacecraft telescope. Such transformationsinvolve a great deal of trigonometry; something alittle onboard computer does very nicely. Smallcomputers hav e also been installed011 some of themanned spacecraft to help the astronauts withguidance and navigation computations.

In principle, all comp utations could be ca rried outby sending the problem to ground-based computers

through the medium of the communicationsubsystem. Answers would be returned the sameway. NASA did use this approach with some ofthe earlier satellites, but as spacecraft became morecomplex it turned out to be too much of a burden0 11 the communication link. Now ground-based andonboard computers share the load.

The Structure Subsystem

The structure subsystem forms the backbone of thespacecraft. It supports, unites, and protects the


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5 The Orbiting Astronomical Observatory (OAO) startracker is programmed to search for and followcertain guide stars. Once the telescope has lockedonto a guide star, the star tracker sends its coordinatesto the guidance and control subsystem. With severalsuch fixes, the OAQ can compute its attitude inspace and then change the orientation of itsinstruments to pick up any given stellar target.

other subsystems. Basically, a spacecraft's structurecan be divided into two parts: the central core orskeleton and the deployable appendages (antennas,booms, solar cell paddles) that unfold once thespacecraft is out in space.

Almost all spacecraft are symmetrical about at leastone axis. There are two good reasons for this: (1)many spacecraft are spin-stabilized and needsymmetry around the spin axis to prevent wobbling;and (2) launch accelerations are powerful and arebest resisted by an axial thrust structure. These arethe reasons why so many spacecraft are cylinders,regular prisms, or spheres.

Many exceptions exist.In fact, spacecraft are ageome ter's delight. One finds cubes, parallelopipeds,polyhedrons, and cones. The balloon satellites area class by themselves. So are the manned spacecapsules w ith their blunt reentry shields.

Lightness is a premium comm odity011 spacecraft.For this reason NAS A has developed a wholespectrum of aluminum, m agnesium, and plasticstructures that weigh little but resist the stresses

of space use. The steels are restricted in use becauseof their high densities and also because they aremagnetican undesirable property when one is tryingto measure the extremely low magnetic fields inspace. One of the major structures0 11 manned spacecraftis the protective thermal shield needed during reentrythrough the atmosphere. NASA generally makesits reentry shields from a plastic containing emb eddedglass fibers. As the air in front of the reentering

spacecraft is heated to incandescence, the shieldablates, that is, it begins to decom pose and erod e.As the shield's surface deteriorates, a layer of gasis evolved continuously. This layer of gas insulatesthe spacecraft and carries away the excess heat.

Once a spacecraft with deployable parts attainsairless space, it undergoes an insectlike metamo rphosis.Freed from their cocoon when the launch shroud

has been b lown off, the solar cell paddles, radioantennas, and instrument booms unfold and unreel.As they deploy, they cause the spacecraft spin-rateto decrease. This despinning is the reverse of theskater's spinup maneuver caused by drawing in thearms and legs.

Solar cell paddles and other. spacecraft appendagescompete for "look angle." The solar cells mustintercept sunlight; instruments need a c lear viewof space phenomena; and antennas must not beobstructed. The fair partition of the solid anglearound each spacecraft is an important task inspacecraft design.

The Engineering Instrum ent SubsystemHere w e have the total of all housekeepingsensors. These sensors are the voltmeters, ammeters,thermometers, and other instruments that determinethe status of the spacecraft.

NASASpacecraftFamilies

Below are listed the major NASA sounding rocket,satellite, and space probe families launched sinceNASA was created in October 1958.

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6 One of O GO's ex tendable booms being tested.Long boom s help isolate sensitive instruments fromthe electromagnetic interference created in thespacecraft.

Apollo: The third series of U.S. manned space-craft, and the spacecraft designed for mannedflight to the Moon. Spacecraft weights have

ranged from 33,800 to 93,885 pounds.Apollo Saturn 201 2-26-66 Unmanned sub-

orbital test.

Apollo Saturn 203 7-5- 66 Unmannedorbital flight.

Apollo Saturn 202 8-25-66 Unmanned sub-orbital flight; flight evalua-tion of heat shield.

Apollo 1V 1-9-67 Unmannedorbital flight. First launchof Saturn vehicle todemonstrate launch vehiclecapability and spacecraftdevelopment.

Apollo V 1-22-68 First flight testof Apollo Lunar Moduleverified ascent and descentstages propulsion systems.

Apollo VI -4-68 Launch vehicledevelopment m ission.

Apollo Vl 10-11-68 First mannedflight; 10.8 days duration.Astronauts Schirra, Eiseleand Cunningham.

Argo: A family of relatively large sounding rocketsused by NA SA (and others) for ionosphere andradio astronomy experiments. Launch weightsvary between 10,000 and 14,000 pounds.

Ariel: A fam ily of two scientific satellites builtand launched by NASA, carrying experimentsprovided by Great Britain. Experiments focused

on ionosphere and atmosphere research.Satellites were named after the "airy spirit" inShakespeare's "The Tem pest." Ariel 1 waslaunched on April 26 , 1962 ; Ariel 11, on March27, 1964. They weighed 132 and 150 pounds,respectively.

Aerobee: A fam ily of sm all sounding rockets usedby NA SA for experiments in the upperatmosphere. Also used for zero-g tests and rocketastronomy. The Aerobee-150 weighs about1900 pounds at launch. There are several models.

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M i c r o m e t e o r it e

delayed readout

Micrometeorite

instantaneous readout

Inertia

G S F C 'electronics stack

Galactic noisereeling mechanism

Broad bandozone detector

O z o n e

scannereRF antenna

/

UK electronicsstack

Galactic noisedipole antenna

Ferrite rodantenna

Batterypack

Solar cellpaddles

Astrobee: A series of large sounding rockets. TheAstrobee-1500 , used occasionally by NAS A fortests, weighs about 11,500 pounds at launch.

ATS: NASA 's family of multipurpose A pplicationsTechnology Satellites. The ATS satellites areintended to test new space instruments and

satellite components, particularly those employedin synchronou s orbit satellites.

ATS 1 2-7-66 Tests 0 1 communication andmeteorological instruments.

ATS 11 -6-67 Gravity-gradient experimentand more sensor experiments. Didnot attain synchronous orbit.

ATS 111 11-5-67 Meteorological, communication,and navigation experiments.

Biosateffite:Family of recoverable scientific satellitesemployed for testing the effects 01 weightlessness,radiation, and lack of the Earth's 24-hour rhythmon biolog ical specimens. The first two B iosatelliteseach weighed about950 pounds. B iosatellite 1was launched on December 14 , 1966 , but itsretrorocket did not fire, making recoveryimpossible. Biosatellite 11, launched September 7,196 7, was recovered successfully.

Echo: A family of two passive NASA communicationsatellites. Once in orbit, these satellites wereautomatically inflated by a gas generator tobecome large spherical balloons 100 and 135feet in diameter, respectively. These large metallized

7 Ariel 11. A satellite with cylindrical symmetry andnumerous appendages.

8 Model of Biosatellite, showing primate experiment.Biosatel/ite is recoverable.

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balloons reflected radio signals between groundstations. Precision tracking of the Echo satellitesalso provided information about the densityof the upper atmosphere. Echo 1 w as launchedAugust 12, 1960; Echo 11, January 25, 1964.Both b ecame w rinkled and lost their sphericalshapes. Echo 1 reentered and burned in May 1968.

Explorer: A long series of scientific satellites thatbegan before NASA was created. The ArmyExplorer 1 was the first U.S. satellite to belaunched, on January 31, 1958. Explorers 11,111, and IV w ere also Army satellites; the rest werelaunched by NASA. Explorers 11 and V werefailures. As the following tabulation shows, the

Explorer satellites have varied widely in designand purpose.

Explorer VI -7-59 Launched intohighly elliptic orbit to exploremagnetosphere. First use ofsolar paddles. Weight: 14 2pounds.

Explorer VII 0-13-59 Solar physics

studies and the measurementof space radiation. Weight:92 pounds.

Explorer VIlI 1-3-60 First of NASA'sdirect measurementssatellites. Madesignificant measurements inionosphere and upperatmosphere. Weight: 90pounds.

satellite about 12 feet indiameter for measurementsof air density. Weight: 80pounds.

Explorer X -25-61 Launched intohighly elliptical orbit tomeasure interplanetaryphenomena. Returned onlytwo days of data. Weight:79 pounds.

Explorer XI -27-61 Designed to measurethe distribution of cosmicgamma rays. Weight: 82pounds.

Explorer XII -16-61 First of four NASAEnergetic Particles Explorers,designed to measure theradiation belts, cosmic rays,solar wind, and magneticfields. Weight: 83 pounds.

Explorer XIH -25-61 First of three NASAMicrometeoroid E xplorers.Satellite was actually fourthstage of Scout rocket coveredwith micrometeoroiddetectors. Orbit w as too low;reentered in three days.Weight: 187 pounds.

Explorer XIV 10-2-62 Second EnergeticParticles Explorer. Weight:

89 pounds.

Explorer IX -16-61 Small balloon

9 Explorer Vill being tested on a vibration table beforelaunch. Instruments for atmospheric and ionosphericresearch are located around the waist.

13

10 E xplorer XXI, an IM P, is shown nested inside itslaunch shroud atop the Delta launch vehicle at Cape

Explorer XV 0-27-62 Third EnergeticP ti l E l P


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launch shroud atop the Delta launch vehicle at CapeKennedy. Solar cell paddles are retracted. Sphere onboom holds magnetometer.11 The polyhedral Explorer XXV was built by theState University of Iowa for measurements in the V anAllen belt and polar regions. Solar cells are mountedon faces. 0

1 1

Particles Explorer. Purposewas to study artificialradiation belt. Weight: 100pounds.

Explorer XVI 2-16-62 Second Micro-meteoroid Explorer. Weight:

159 pounds.Explorer XVII -3-63 An Atmosphere

Explorer, intended for themeasurement of density,pressure, composition, andtemperature directly. Weight:405 pounds.

Explorer XVIII 1-27-63 First of the IMPS

(Interplanetary MonitoringPlatforms). Placed in ahighly eccentric orbit tomeasure magnetic fields,plasma, and radiation.Weight: 138 pounds.

Explorer XIX 2-19-6 3 A balloon-typeAir Density Explorer. Weight:

94 pounds.Explorer XX -25-64 An Ionosphere

Explorer. This satellite wasa topside sounder that sentradio pulses down into theionosphere and listened forechos. Weight: 98 pounds.

Explorer XXI 0-4-6 4 Second IMP. Weight:

135 pounds.Explorer XXH 0-10-64 First of NASA's

Beacon Explorers. Primarilya radio beacon for studyingradio propagation in theionosphere. Weight: 116pounds.

Explorer XXIII 1-6-64 Third MicrometeoroidExplorer. Weight: 295 pounds

Explorer XXIV 1-21-64 Another balloon-type Air Density Explorer.Weight: 19 pounds.

Explorer XXV 1-21-64 A satellite tomonitor the radiation belt.One of the Injun series builtby the State University of

I L h d ith


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Explorer XXXII

Explorer XXXIII

Iowa. Launched withExplorer XXIV for NASA'sfirst dual launch. Weight: 90pounds.

12-21-64 Last of fourEnergetic Particles Explorers.Weight: 101 pounds.

4-29-65 NASA 's secondBeacon E xplorer. Weight:134 pounds.

5-29-65 Third IMP. Weight:130 pounds.

11-6-65 A G eodetic

Explorer, called Geos forshort. Carried radiobeacons and flashing lightto enhance tracking. Weight:385 pounds.

11-19-6 5 A Solar Explorerfor monitoring solarradiation during theInternational Year of theQuiet Sun (IQSY). Weight:125 pounds.

11-29-65 A DirectMeasurement E xplorer forionospheric studies. Weight:218 pounds.

5-25-66 An AtmosphereExplorer similar toExplorer XVII. Weight:490 pounds.

7-1-66 An Anchored IMPwith a possibility of achievinga lunar orbit. Did not attainlunar orbit; in orbit around theEarth. Weight: 20 7 pounds.

5-24-6 7 Fifth IMP. Weight:163 pounds.

7-19-6 7 First AnchoredIMP to achieve orbit aroundthe Moon. Weight: 230pounds.

1-11-68 Second GeodeticExplorer.

3-5-68 Second SolarExplorer.

Explorer XXVI

Explorer XXVH

Explorer XXVffl

Explorer XXIX

Explorer XXX

Explorer XXXI

Explorer XXXVI

Explorer XXXVII

Explorer XXXIV

Explorer XXXV

12 Explorer XXX during ground tests. Solar cells aremounted on the flat faces.

15

Gemini: The second series of manned NASA Earth


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satellites. Two astronauts occupied each G eminicapsule, hence the series name. The prime purposeof the Gemini flights was to check out equipmentand techniques to be used during the Apollomissions to the Moon. During the Gemini Program,NASA performed the first rendezvousexperiments and the first extravehicular activities(walks in space). G emini capsule weights variedbetween 7000 and 14,000 pounds.

Gem ini 1 -8-64 Unmanned orbital testflight.

Gem ini 11 -19-65 Unmanned suborbitaltest flight.

Gemini 111 -23-65 Three-revolution flight.Astronauts: Grissom and Y oung.

Gemini IV -3-65 62 revolutions, roughlyfour days. First U.S. walk in space.Astronauts: McDivitt and W hite.

Gemini V 8-21-65 120 revolutions, roughlyeight days. First extended U.S.manned space flight. Rendezvous

maneuvers. Astronauts:Cooper and Conrad.

Gemini VII 2-4-6 5 20 6 revolutions, roughlytwo weeks. Rendezvoused withGemini VI-A. Astronauts:Borman and L ovell.

Gem ini VI-A 2-15-65 15 revolutions, a littleover one day. Rendezvoused withGemini Vil. (Gemini VI wascancelled 10-25-65 when targetvehicle failed to attain orb it.)Astronauts: Schirra and Stafford.

Gemini Vill -16-66 6.5 revolutions, 10 .7hours. Reentered early becauseof m alfunctioning spacecraftthruster. Astronauts:

Armstrong and Scott. Firstdocking in space.

Gemini 1XA 6-3-66 45 revolutions, about threedays. Rendezvous tests withunmanned target. Walk in space.Astronauts: Stafford and Cernan.(Gemini IX was cancelled May17, 196 6 when target vehiclefailed to orbit).

Gemini X -18-66 43 revolutions, roughly

three days. More rendezvousexperiments. Astronauts:Young and Collins.

Gem ini XI -12-66 44 revolutions, about threedays. High apogee flight to 853miles. Rendezvous, docking,extravehicular ac tivity, tetherevaluation. Astronauts: Conradand Gordon.

Gem ini XII 1-11-66 59 revolutions, aboutfour days. Demonstrated automaticreentry. Further docking,rendezvous, and extravehicularexperiments. Astronauts:Lovell and Aldrin.

Iris: Family of small sounding rockets used for upperatmosphere experiments. Launch weight: about1300 pounds.

Javelin: Family of sounding rockets based on Argorocket.

Journeyman: Large NASA sounding rocket. Launchweight: about 14,000 pounds.

Lunar Orbiter: Series of five lunar probes placedin orbit around the Moon. Purpose: reconno iter

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13 One G emini spacecraft is seen through the windowof another during the flights of Gemini V I-A and G eminiVil in December 1965.

Mariner: A family of planetary probes designed tofly by Mars and Venus, making scientificmeasurements in interplanetary space on the way.In the vicinity of a planet, the Mariner prob esscanned the planet with instruments and carriedout radio occultation experiments.

Mariner 1 -22-62 Venus probe. A launchfailure, destroyed by rangesafety officer.

possible landing sites for Apollo astronauts. Tooklarge number of high quality pictures of the lunarsurface. Weights: 850-86 0 pounds.

Lunar Orbiter 1 -10-6 6 F irst U.S. space-craft in lunar orbit.Returned 207 frames ofpictures (sets of two each).

Mariner 11 -27-6 2 First flyby of Venus.Made magnetic and atmosphericstudies near planet. EncounteredVenus December 14, 1962.Weight 449 pounds.

Mariner 111 1-5-64 M ars probe. Launchshroud failed to eject.

Lunar Orbiter V

11-6-66 Returned 211frames of pictures.

2-5-67 Returned 182frames of pictures.

5-4-67 Returned 163frames of pictures.Eighty percent of far sidephotographed by Orbiters1 to IV.

8-1-67 Covered five Apollolanding sites and 36scientific interest sites;completed far side highaltitude coverage; fullview of E arth in full phase.Returned 212 frames of

pictures.

Mariner IV 1-28-64 First flyby of Mars.Took first close-up pictures ofMars' cratered surface.Encountered Mars July 14, 1965.Weight: 575 pounds.

Mariner V -14-67 Second U.S. flyby ofVenus. Made measurements ofVenus' ionosphere, magnetosphere,and atmosphere. EncounteredVenus October 19, 1967. Weight542 pounds.

Lunar Orbiter 11

Lunar Orbiter 1 1 1

Lunar Orbiter IV

17

ercury-Atlas 11 -21-61 Unmanned subor-bital flight.


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Mercury: The first U.S. manned spacecraft. TheMercury capsule supported only one astronautin comparison to two in G emini. The primarypurpose of the flights was to demonstrate thatman could not only survive in space but couldperform useful tasks. Capsule weights ranged from2300 to 3033 pounds.

Me rcury-Atlas 1 -29-60 Unmanned test ofstructure and reentryheat protection shield.

Mercury-Redstone 1 1-21-60 Unmanned test.Launch abort.

Mercury-Redstone IA 12-19-60 Unmanned 235-mile test flight.

Mercury-Redstone 11 -31-61 Suborbital flightwith primate aboard.

14 The Lunar Orbiter spacecraft.

g

Mercury-Redstone 1115-5-61 M anned suborbital(Freedom 7 ) light. Astronaut:

Shepard.

Mercury-Redstone IV 7-21-61 Manned suborbital

(Liberty Bell 7)light. Astronaut:

Grissom.

Mercury-Atlas IV -13-61 Un manned, single-orbit flight, about1. 5hours long.

Mercury-Atlas V 1-29-61 Three-orbitflight, about 4.5 hourslong, with chimp Enos

aboard.Mercury-Atlas VI -20-62 Three-orbit,

(Friendship 7) anned flight, roughlyfive hours long. First U.S.orbital flight. Astronaut:Glenn.

ROCKET ENGINE

SOLARPANEL

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15 Model of OG O showing solar panels, numerousappendages, and boxlike body w ith cover door open.

Mercury-Atlas Vil -24-6 2 Three orbits,(Aurora 7) oughly five hours.

Astronaut: Carpenter.

Mercury-Atlas Vill 0-3-6 2 Six orbits, a little(Sigma 7) ver nine hours.

Astronaut: Schirra.

Mercury-Atlas IX -15-63 22 orbits, lasting(Faith 7) 4 hours and 20 minutes.

Astronaut: Cooper.

Nike: A large family of sounding rockets, includingthe Nike-Asp, the Nike-Cajun, and the N ike-Tomahawk. These areall small rockets with launchweights generally under a ton. NASA has firedhundreds of Nike-class sounding rockets during itsionosphere and upper atmosphere researchprograms.

Nimbus: A family of large, research-and-development,meteorological satellites. The Nimbus

satellites have tested a number of cameras andinfrared instruments for operational weathersatellite programs. Nimbus 1 was launchedAugust 28, 1964 and weighed 830 pounds. Thissatellite provided many thousands of highquality cloud-cover pictures. It was the firstweather satellite with three-axis stabilization.Nimbus 11, weighing 912 pounds, was launchedMay 1 5, 1966 . Besides weather pictures, it returnedinfrared, night-cloud-cover pictures.

OAO : NASA 's Orbiting Astronomical Observatory.The OAOs are large sophisticated satellitesfor studying the stars in the ultraviolet region ofthe spectrum and carrying ou t associatedexperiments in space science. OAO 1 waslaunched April 8, 1966 and w eighed 3900 pounds.Spacecraft systems anomalies developed on the

second day and no scientific results were obtained.

19

OGO: NASA has built a family of OrbitingGeophysical Observatories Each of these large

OSO ff l -8-67 Carried a solar monochromatorand spectrometer


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Geophysical Observatories. Each of these largesatellites can carry twenty or more experimentsin the fields of geophysics, space physics,and astronomy. Like the other Observatories,000s are large and relatively sophisticated,weighing between 1000 and 1300 pounds. Some

of the OGOs are injected into polar orbits andare called POGOs. Those in high eccentric orbitsare called EGOs.

OGO 111 -7-66 In eccentric orbit. MaintainedEarth-stabilization for more than

six weeks.OGO IV -28-67 In polar orbit. Eighteen

experiments returning data.OGO V -4-68 T wenty-four experiments

operating.

OSO: The Orbiting Solar Observatories are thesmallest of the Observatory class and carry fewerexperiments. Weights vary between 450 and 650

pounds. Some experiments are located in thespinning wheel section; others are in the sail, whichis kept pointed at the Sun.

osoI -7-62 Carried a dozen solar physicsexperiments, including an ultra-violet spectrometer aimed at the Sun.

OSO 11 -3-65 Instruments included acoronagraph and ultraviolet

spectroheliograph.

and spectrometer.

OSO IV 0-18-6 7 Continuation and expansionof previous experiments.

Pageos: A balloon-type, passive geodetic satellite.Observed w ith optical instruments. Pageos

was launched on June 24, 1966 . Weight:244 pounds.

Pegasus: The three Pegasus satellites have some-times been called Micrometeoroid Explorers, butthey are much larger than any Explorer-classsatellite, weighing about 23,000 pounds each.These satellites were orbited as byproducts 01 theSaturn 1 launch tests. Each was a Saturn S-IVupper stage which carried a large folded array ofcapacitor-type micrometeoroid detectors, deployedonce orbit was attained.

Pegasus 1 -16-65

Pegasus 11 -25-65

Pegasus 111 -30-65

Pioneer: The first five Pioneer probes were aimed

to fly in the general direction of the Moon,but not to h it it. They either fell back to E arth, orcontinued on into solar orbit. The second series,beginning with Pioneer VI, is aimed at deep-spaceexploration. These later Pioneers carry magn etom-eters, solar wind instrumen tation, radiationcounters, etc.

Pioneer 1 0-1 1-58 Reached altitude of70,000 miles. Returned data onVan A lien belts. Weight: about 84pounds.

Pioneer 11 1-8-58 A launch failure.

Pioneer 111 2-6-58 Reached an altitude ofover 63,500 miles. Helped mapthe magnetosphere. Weight: 13pounds.

Pioneer IV -3-59 In solar orbit; passedMoon at the distance of 37,300miles. Weight: 13.4 pounds.

OGO 1 -5-64 In eccentric orbit. Twobooms failed to deploy, blocking ahorizon sensor. Consequently, OGO 1could not stabilize facing the Earth.Many experiments still returned good

data.OGO 11 0-14-65 This OGO was placed in a

polar orbit. The attitude controlgas supply was exhausted prematurelydue to a sensor problem. Mostexperiments were successful.

Pioneer V

Pioneer VI

3-11-60 In solar orbit. Returneddata out to 22,000,000 miles.Weight: 95 pounds.

12-16 -65 First of the new series.In solar orbit. Weight: 140

pounds.

20

Pioneer VIl -17-66 In solar orbit. Weight:140 pounds

Surveyor: A series of seven soft-landing lunar probesb ilt t it th M ' f i


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140 pounds.

Pioneer Vffl 12-13-67 Weight: 14 5 pounds

Rangers: The first two Rangers were intended to testout techniques to be used on lunar and p lantaryspacecraft as well as measure the particles andfields present in interplanetary space. They wereto be aimed in the genera l direction of the Moon.The next three Rangers were to "rough land" aseismometer package011 the lunar surface. Thefinal four Rangers w ere designed to take close-uppictures of the lunar surface before crashlanding 011 it, providing data for planningthe lunar landing of Apollo astronauts. TheRangers weighed between 675 and 810pounds.

Ranger 1 -23-61 Launched into Earthorbit.

Ranger 11 1-18-61 Same as Ranger 1.

Ranger ff l -26 -62 In solar orbit; passedMoon at about 22,862 miles.

Ranger IV -23-62 Hit Moon. Timer failureprevented experiment operation.

Ranger V 0-18-6 2 In solar orbit; missedMoon by 450 miles.

Ranger VI -30-64 Hit Moon; but camerafailed.

Ranger VU -28-64 Hit Moon; returned 4316pictures.

Ranger Vill -17-65 Hit Moon; took 7137pictures.

Ranger 1X -21-65 Hit Moon; took 5814pictures.

Relay: A family of two active experimentalcommunication satellites. Relay 1, launchedDecember 13, 1962, was able to handle twelve

simultaneous two-way telephone conversations orone television channel. Relay 11 was an improvedversion, launched011 January 21, 1964. Theyweighed 17 2 and 1 83 pounds, respectively.

built to reconnoiter the Moon's surface inpreparation for the Apollo manned landing.The first Surveyors were primarily picture-takingspacecraft. Later m odels added experiments insoil mechanics and analyzed the surfacecomposition. Weights varied between 596 and630 pounds at lunar landing.

Surveyor 1 -30-6 6 Soft-landed on Moon.Sent back over 11,338 photos.

Surveyor 11 -20-66 Hit Moon, but one of thethree retrorockets failed duringmid-course maneuver; soft landingnot possible.

Surveyor 111 4-17-67 Successful soft landing.Took 6,315 photos. Soil samplerexperiment showed lunar surfacesimilar to damp sand in strength.

Surveyor IV -14-67 H it Moon; butcommunications lost 2.5 minutesbefore touchdown.

Surveyor V -8-67 Sent over 19,00 0 picturesof lunar surface. Alpha-scatteringexperiment showed surfacecomp osition similar to basalt.

Surveyor VI 1-7-67 Sent more than 30,000photos. Alpha-scatteringexperiment again showed basalt.

Surveyor Vil 1-7-68 Landed in lunar highlands.Took some 21,00 0 pictures.Alpha-scattering experimentshowed highlands to differappreciably from maria incomposition.

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16 The Survey or soft lunar lander. Retrorocket slowedthe descent of the spacecraft before touchdown. No tesolar cell panels on mast.

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Syncom: Family of three experimen tal, active,synchronous-orbit communication satellites. TheSyncoms were first injected into highly ellipticorbits. Near apogee, a rocket fired and placedthem in equatorial synchronous (stationary)orbits. An onboard gas-jet propulsion unit wasrequired to ma intain their orbital positions.

Weight: about 85 pounds each.Syncom 1 -14-6 3 In nearly synchronous

orbit; but communications failedjust after onboard rocket fired.

TIROS: A long, successful series of experimentalweather satellites. The TIROS series demonstratedconclusively that satellite cloud-cover andinfrared pictures would be useful in improvingthe accuracy of weather forecasting. The TIROSsuccesses formed the basis for the TIROSOperational Satellite (TOS), which the

Environmental Science Services Administrationcalls Environmental Survey Satellites or E SSAs.The T IROS satellites are hat-box-shaped andweigh between 260 and 300 pounds. All TIROSsatellites were spin-stabilized.

TIROS 1

TIROS 11

TIROS 111

TIROS IV

TIROS V

TIROS VI

TIROS Vil

Syncom 11 -26-63 First satellite placed insynchronous orbit. Many verysuccessful intercontinentalcommunication experiments.

Syncom 111 -19-64 First stationary Earthsatellite. Dem onstrated thepracticality and effectiveness ofstationary, active communicationsatellites.

4-1-60 23,000 weather pictures.

11-23-60 36,00 0 weather pictures.

7-12-61 Over 35,000 weatherpictures.




6-19-63 Still operating. Over125,000 weather pictures so far.

TIROS VIlI 2-21-6 3 Still active. Over 100 ,000weather pictures so far. FirstAPT (Automatic PictureTransmission) equipment.

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TIROS IX -22-65 Established feasibility ofcartwheel configuration foroperational weather satellites.

TIROS X -2-65 Placed in Sun-synchronousorbit.

Vanguard: A family of three scientific satellites.Begun in 1955, the Vanguard satellite programplanned to launch at least one small artificialsatellite during the International GeophysicalYear. Vanguard 1 was launched on March 17 ,1958, under the auspices of the U.S. Navy.The Project was transferred to NASA inOctober 1958.

Vanguard 11 -17-59 Launched to televisecloud cover, but excessive wobbledegraded data. Weight: 22

pounds.Vanguard 111 9-18-59 Carried micrometeoroid

detectors, radiation detectors, amagnetometer, and solar X-raydetectors. Weight: 100 pounds.

17 Syncom , a synchronous comm unication satellite;weight approximately 55 pounds. Solar cells aremounted on the circumference of the cylinder.

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18 Vanguard /11. The conical boom on top isolated themagnetometer from the rest of the spacecraft.

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DATE DUE

f i l l

AdditionalReading

For titles of books and teaching aids related to thesubjects discussed in this booklet, see NASA'seducational publication EP-48, Aerospace Bibliogra-phy, Fourth Edition.

Information concerning other educational pub licationsof the National Aeronautics and Space Adm inistrationmay be obtained from the Educational ProgramsDivision, Code FE , Office 01 P ublic Affairs, NASA,Washington, D. C., 20546 .

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TL 796.5 .U5 C6 1969

C or l i s s, Wi l l i a m R.

NASA spacecraf t

For sale by the Superintendent of Documents, U.S. Government Printing OfficeWashington, D.C. 20402 - Price 50 cents* U.S. GOVERNMENT PRINTING OFFICE 969 0-328-873


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