NASA Facts the Next Step Large Space Structures

8/7/2019 NASA Facts the Next Step Large Space Structures

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Source of A cquisitionNAS A Washington, D. C.

An Educational Publicationof theNational Aeronauticsand Space AdministrationNF-129

TheNextStep: LargeSpaceS t r u c k ~

Figure 1 .Giant 100-meter diameter hoop-column or "maypole" antenna fully deployed in space.1992: They float high andsilen t in the cold blackness of communicate instantly, anywhere, anytime. And they arespace. They seem enormous to us, but then our human the sprawling, gangly platforms-power modules, tool

' scale does not count for much up here. Only a few people sheds, scientific workshops in space.have in fact ever seen them in the irfinal working shapes, Hard to believe there was a time when we could launcheven though all of us on Earth use them constantly, every only simple furniture-sized satellites. One of these giantday antennas down on Earth would easily shade the RomanThey are the wheel-like antennas with dishes 100 meters Coliseum. They are among the largest structures everacross and masts as tall as the Statue of Liberty-radio built, and they have mightily changed the world of therelays for a world drawn closer by the awesome ability to 1990s.


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barge Space StructuresNow that we have a Space Shuttle to move large andbulky cargoes routinely into Earth orbit, long-termplanners and researchers in government, industry,and universities are shaping the work for a new era-the building of very large and complicated structuresin space. Although enormous themselves, these newantennas and space platforms will actually serve toshrink the total costs of launching satellites for awide range of scienti fic and commercial uses, andwill dramatically improve our worldwide communi-cations network.The Shuttle Orbiter's closed cargo bay carriesup to 29,500 kilograms (32 tons, or three times theweight of a passenger bus) on each trip into space.Freight costs are charged both by weight and bylength, so it will be wise to design space hardwarethat is both light and short. Structures that wouldhave been too fragile to stand up under their ownweight on Earth wil l now be able to fold up in thecargo bay and deploy safely into their final shapesin the weightlessness of space.And the new capability tosupervise this deploymentand construction in orbit will be crucial-the Shuttlewill carry a work force of up to seven people intospace on each one of its frequent f lights, and wil lremain close at hand while the jobs are done.All of this creates exciting new possibilities for theengineering of space hardware, and poses a brandnew set of challenges. What are the strongest, light-est, and most stable materials to use in space con-struction? How do you load the Shuttle so as to buildthese colossal objects with the fewest trips intospace? What are the best ways to assemble themonce the materials are delivered to the orbiting"sites"? And the most obvious question: What kindsof structures will we build?Eventually we will want manned space stationsand huge solar power collectors. But for the timeframe of the late 1980s and 1990s, planners are con-centrating on two main classes: Large Space An-tennas and Multipurpose Space Platforms.Large Antennas: Bigger, Stronger, FewerIn 1974, listeners in remote regions of Appalachia,Alaska, and the Rockies heard a new voice fromabove, and in that same moment joined the modernworld. The reason was ATS-6, a nine-meter-widedish antenna that relayed TV signals down to smallreceivers in previously isolated areas of the UnitedStates.ATS-6 was the largest civilian communicationsdish launched in the pre-Shuttle era. Some of thenew super-antennas will be ten times that size, big-ger than a football f ield! This tremendous size meansa boost in transmitting power as well as an increasedsensitivi ty to weak signals from the ground. And so,instead of having massive dishes on Earth strainingto hear weak messages from space as we do today,the roles will be reversed. A few super-antennasplaced in high geostationary orbits to cover theglobe will replace countless smaller satellites inspace. And millions of inexpensive home rooftopdishes will receive the satellite signals now pickedup by only a few very large and powerful groundstations.The implications of this coming boom in antennaperformance for change in our daily lives are pro-

found, and not yet ful ly explored. Many ideas andtechnologies do already exist: working models ofpersonal "Dick Tracy" wrist radios; designs for elec-tronic mail systems with home delivery via satellite;300-channel TV sets tuned to stations all over theworld.And there will be non-communications applica-tions, like the remote sensing of mineral deposits,ocean resources, and soil moisture to improve agri-culture, navigation, fishing, and mining; radar for mil-itary surveillance or weather monitoring; trackingdeep space probes like Voyager or scanning distantgalaxies with orbiting radio antennas; even reflectinglight or solar power back to Earth from low-orbit.The task now facing engineers is to set this fan-tastic revolution in motion by designing, testing, andplacing these large structures in space.Deployable AntennasThe first large space antennas wil l be deployables.They will fold into compact containers on Earth, goup whole in one Shuttle trip, then deploy automati-cally in space in a single operation. The key, obvi-ously, is to have the largest possible dish unfoldingfrom the smallest and lightest possible package.One type, the hoop-column or "maypole" antenna,would open up in orbit much as an umbrella does. Acylinder no bigger than a school bus could be trans-formed within an hour into a gigantic antenna dish100 meters across (two acres in area).Depending on the length of thevarious strings thatstretch the fabric taut inside its stiff outer hoop, thistype of antenna can be designed for many shapes-that is, the bowl of the dish could be made flat, morehollowed out, or even made of four different sur-faces, each focusing a beam in its own different di-rection. Multi-beam feeds could also allow one an-tenna to do the work of several by pointing signalstoward different areas of the Earth's surface below.In another type, the offset wrap-rib type of anten-na, the dish fabric is attached to flexible ribs thatwrap around a central hub. The whole package is atfirst quite compact, but once in space another mirac-ulous transformation in size takes place. A long(about 150 meters for a 100-meter-wide dish) masttelescopes out from the core and turns a corner sothat the dish is offset and not blocked by the mast,an advantage in sensitive radar and radiometry mis-sions. Then, like a pinwheel coming to life, the ribsunfurl and straighten until they fully extend to stretchand support a round dish.Other designs have also been explored, li ke thewire-wheel antennas that resemble huge bicyclewheels, or the TRAC (Truss Radial-Rib Antenna Con-figuration) antenna that radiates spokes from a coreto support a dish in its final roundness.MaterialsWhatever their shape, these large space structureswill put great demands on the materials from whichthey are made. Even though they'll be free from theweight stresses imposed by Earth's gravity, there willbe other strains from their tight packaging and fromthe hostile radiation and frig id temperatures of space.Engineers will need to build with new materials fora new age-materials that are at the same time light,super-strong, flexible or rigid (depending on theuse), and thermally stable. Telescoping masts must


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be light, yet stay very stiff. Antenna ribs need to bestrong, but should be flexible enough to wrap aroundtheir hub.And everything needs to remain fixed in posit ionequally well in the hot Sun as in cold shadow, be-cause if a structure were to expand with heat it wouldruin the extremely precise shape of an antenna(some of which can be off no more than a few milli-meters in a total diameter of 100meters).One substance that meets these hard demandsquite well is the graphite-epoxy composite now usedin lightweight tennis rackets, golf clubs, airplaneparts, and in the Space Shuttle itself. A three-meter-long hollow tube of this material can be lifted withone finger, yet for its purpose is ten times strongerthan steel.Other materials suit specific jobs. The hundredsof threads that pull and stretch a hoop-column an-tenna into shape might be made of a quartz filament,because quartz is very stable. The dishes themselvesshould be made of fabrics that fold like cloth beforethey are deployed. These would be metal mesheswoven like nylon stockings or soft porch screeningand coated with gold for ref1ectivity.A finer mesh willbe used for dishes that deal in smaller wavelengths.For very small wavelengths there are ultra-thin mem-branes made of transparent films coated with metalsthat look and feel like sheets of Christmas tinsel.~latforases:Osrtp~lstsin%heSkySuppose that five different groups want to fly sci-entific or remote sensing instruments in Earth orbit,all at about the same height and inclination to theequator. Why not, instead of cluttering the sky withfive individual satellites, build a huge platform towhich all five can be attached? They could share the

Figure 3. Offset wrap-rib antenna deployed in space.

Figure2. Deployment sequence for 100meterdiameterhoop-column [maypole) antenna.


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cost of power and communications systems, stabilitycontrol, and cooling devices. Shuttle astronautswould need to make just one repair stop at a timeinstead of five, and could replace any of the originalfive devices with a new one as needed. It seems asound and economical idea.Such a good idea, in fact, that Multipurpose SpacePlatforms are now being designed for the late 1980s.The possible uses for these platforms are almost asvaried as their sizes and shapes. Any of these mightbe "plugged in" to a typical one: an astronomicaltelescope, a communications dish, a sensor to traceair pollution or search for minerals.As Figures 5 and 6 show, platforms will not be justsimple rectangular slabs. There are bird-like config-urations with cross arms to hold sensitive instru-ments apart from one another. There are modules,like large rafts, that would support several instru-ment pallets in a cluster. The only common element Figure4. Engineers at the NASA Langley Researchwill be the central "bus" that houses the platform's Center experiment with assembling possible structurespower generator (attached to wing-like solar collec- for space use out of lightweight graphite epoxy com-tors) and thermal and electronic systems. posite cones.Some of these platforms, especially those withcommunications antennas, wil l need to hover in geo-stationary orbits 35,900 kilometers above the Earthin order to look down on large sections of the globeor to stay fixed in one spot (as seen from the ground).So will many of the super-antennas. Since the Shut-tle orbiter itself flies no higher than a few hundredkilometers, rockets can be attached either to an un-deployed package right out of the cargo bay or toan already assembled structure to boost i t higher.Eventually, no matter how cleverly the platformsand antennas are packed, they will be too large tounfold in a single deployable unit. At that point wewill have to send up these "erectables" in separatepieces. Two such pieces (or a dozen, or even a hun-dred) can be loaded into the cargo bay on Earth,lifted into space, unfolded, and finally assembledinto a single gigantic structure in orbit.What kinds of building blocks will we use on thesefloating construction sites? Ideally they should be Figure5. A basic orbital science platform features abasic, simple, and adaptable to many different kinds power supply, communications and electronic equip-of structure. For example, twenty deployable boxes ment, and a heat radiator. Science experiments for thecould be latched together to form a simple flat plat- platform will be brought to orbit by the Space Shuttleform. Or several of the raft-like modules shown in and returned to Earth at their completion.Figure 8could be snapped together to form a stilllarger surface.All of these have their roots in common householdobjects-in collapsible cardboard boxes, foldingdeck chairs, telescoping car radio antennas, accor-dion baby gates-anything we have tried to makesmaller and more portable. Masts for dish antennaswill telescope into their full lengths from small cylin-ders. Latticed trusses will store as flat packages,unfold first into diamond shapes then finally intotetrahedrons.But in each case, no matter how flexible theirhinges when stored, the modules must hold stiffwhen deployed, as would the hexagonal pieces forlarge antennas. Looking a bit like mini-trampolineswhen unfolded, these hexagons will be attached pre-cisely and rigidly to form great reflecting surfacesmany city blocks in area.Not all of these potential building blocks will needto unfold. Some of them will store quite easily justas they are, like the l ight graphite-epoxy tubes that Figure6. Geostationary science platforms are placedwill stack inside one another like ice cream cones in orbits, high above Earth, that permit them to beand sit on racks in the cargo bay like arrows in a synchronized with Earth's rotation.


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quiver. These tubes would then be attached to formstruts-struts that can themselves be joined to buildlarger beams or trusses. Or they might be used toform a thin hoop for a space antenna. Highly adapt-able to many creative shapes, these struts will belike giant tinkertoys for the practical constructionprojects in space.

Deployable antennas will, in a sense, bui ld them-selves-they will unfold with the push of a button."Erectables" will not. Someone or something willhave to snap the separate pieces together.Ongoing assembly projects (some of these struc-tures will require several Shuttle trips to deliver allthe pieces) will therefore mean having the first con-struction sites in space, and so a new type of workfor the human race.At various NASA Centers, researchers are nowdetermining the most efficient ways to do the de-ployment and assembly jobs of the Shuttle era. Thetesting is done underwater in a cavernous, vaguelyeerie Neutral Buoyancy Tank that simulates to thebest of our ability the weightless conditions of space.Technicians here practice the mechanical tasks nec-essary for the construction of these gargantuanspace structures. Figure8. Deployment sequence of a space platform.

Figure 7. Segments of a modular antenna are removed from the cargo bay of the Space Shuttle by remote manipulatorarms and are unfolded before being attached to the main structure.

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Figure 9. Space-suited divers experiment with building techniques for future large space structures in the NeutralBouyancy Simulator at the NASA Marshall Space Flight Center. The giant water tank approximates the weightlesscondition of Earth orbit.Many factors are taken into account: safety andfatigue of the astronauts; speed in moving from oneplace to another; the requirement for simple toolsand the need to restrain them so they don't float

away; how much time the Space Shuttle loses linger-ing around the "site" when it could be returning toEarth for another load.In one method of assembly, astronauts tethered tothe Shuttle would simply move from beam to columnto module, manually snapping, locking, or latchingeverything together. Their travel time could beshortened by wearing the jet-packs called MannedManeuvering Units, models of which are used inNeutral Buoyancy Tank simulations.But it is not yet certain how we will combine man-power, machine operations, deployment, or assem-bly jobs to build these antennas and platforms. Forsome projects it might be more efficient to moveastronauts around on a scaffold in a Mobile WorkStation (Figure 10) instead of having them f ly all over.The scaffold rests on a frame in the cargo bay andmoves either up-down or right-left.As sections of thestructure are finished they are moved away from thestation so that the part to be bu ilt is always in reach.Astronauts might also stand in open cherry-pickersattached to the Shuttle's 15-meter Remote Manipu-lator Arm, and be moved from beam joint to beamjoint like telephone linemen working on high wires.Even more sophisticated would be the closed cherry-pickers where workers inside a comfortable chamberwould work with remote control arms.

Or, for repetitious or dangerous tasks, unmannedfree-flying teleoperators-essentially programmedrobots-could do the work with their own mechan-ical arms. There might also be assembler devices toform three-dimensional structures from struts by fol-lowing simple repeatable steps, and the Maneuver-able Television (MTV) units that would transmit pic-tures to technicians in the Shuttle control room sothat they could direct work by remote control.These devices will most likely be used later in theShuttle era. In the meantime, astronauts will have tolearn to erect structures the size of large stadiumsin the peculiar world of low gravity. Seemingly easytasks will become complicated-workers trying toturn ordinary bolts will be as likely to turn them-selves as the bolts, thanks to the lack of leveragethat comes with weightlessness.These are precisely the problems studied duringsimulated assembly jobs in the Neutral BuoyancyTank, and they in turn influence the choice of tech-nology, like using latches that snap firmly togetherwith one quick mot ion instead of a series of twistsand turns. The goal is to $tandardize hardware andassembly methods in order to get the jobs done asquickly and correctly as possible.Space Fabrication: The Automated Beam BuilderAfter the deployables, and after the erectables, thenext logical step is to build large structures ~om~ pl et efrom scratch by fabricating the building blocks in space.


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Figure 1 1 . Using Space Shuttles and advanced heavy lift launch vehicles to transport materials,equipment, and crew,kilometer-size structures such as this space solar power station may someday be erected.1992: These antennas andplatforms were only designedten years ago. Engineers used computers to whittle awayat the options and build their working models. The modelsthat seemed most promising were tested further-under-watel; in soundless chambers, and in great warehouseswhere they were jiggled, twisted, measured, and cali-brated. Finally, when all was right, came the launches.

Those giants have changed both worlds-above andbelow the atmosphere-and in building them we have

learned much. Now our sights are set on settling space.Manned space stations and great solar power collectorswill bridge us nicely into the next century, when Earth-orbit may become a permanent human domain, an ex-pansion of the planet's biosphere.

But before cities, before even "log cabins," came thelarge space structures, the antennas and platforms of theearly Shuttle era.

Classrooma~%ivstoes1. Research and compare communications satellitetechnology of the 1960s '70% and '80s.2. List objects that share the qualities of collapsi-bili ty and portabi lity with large space antennasand platforms. How did they come to be de-veloped? What other objects may be developedin a similar fashion in the future?3. Keeping in mind loading constraints on the SpaceShuttle, how antenna surfaces are shaped,crew limitations, etc., describe, step by step,how the structures pictured in this bookletmight be deployed and/or assembled in space.4. Compare the capabilities of 100-meter antennas

with the smaller space antennas now beingused. How are they different? Compile a bibli-ography on the possible applications of LargeSpace Antennas.5. Describe a typical working day in the life of anastronaut building a large structure in space.6. Build a space platform model from commerciallyavailable building sticks or blocks, from tooth-picks connected to styrofoam, etc. Take intoaccount its instrument load, need for electricalpower, and the stowing of component parts.7. Read a current science fiction novel or see ascience fiction film that involves large spacestructures and write a critique of the hardwarecreat$d for the story.

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