SAE TECHNICALPAPER SERIES 972485

Research Issuesfor an Interplanetary Habitat

Marc M. Cohen, ArchD., ArchitectNASA Ames Research Center

The Engs’7ee4ng Society— For Advancing Mobility

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27th International Conferenceon Environmental Systems

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972485

Design Research Issuesfor an Interplanetary Habitat

Marc M. Cohen, Arch.D., ArchitectNASA Ames Research Center

ABSTRACT

This paper presents an “inquiryby Design” approach to the problem ofarchitectural design for the crew habitat ofan interplanetary vehicle. This habitatmust meet a range of difficultrequirements to protect the crews healthand safety during the approximately 6 to12 month voyage each way. It mustprovide a habitable environment thataffords the crew privacy, groupactivities, recreation, exercise,communications, training facilities, andhealth care. It must incorporatecountermeasures against prolongedexposure,.to zero gravity and shieldingagainst radiation from solar flares andgalactic cosmic rays.

The design research involves theinvestigation of a prototypeinterplanetary habitat that incorporatessubstantial radiation shielding for thecrew quarters and a human powered,short-arm centrifuge for zero gravitycountermeasures, It includes privatecrew quarters, life support system,stowage and equipment volumes. Thisinquiry by design exercise explores thehabitat components subsystem bysubsystem to find the challenges thatemerge for integrating them. Thisexercise in designing an optimizedinterplanetary habitat reveals some keydesign and engineering issues for NASAto consider in developing a Mars vehicle.

INQUIRY BY DESiGN

The concept of ‘Inquiry byDesign” presents an approach toresearch whereby the designer seeks to

learn about a design problem bydeveloping design solutions to it. Toparaphrase John Zeisel’s approach, adesig! solution comprises a“hypothesis” about a problem, thatreveals the deeper character of thedesign problem [Zeisel, 1981]. Theapproach involves analyzing the designproblem to create a design problemdefinition that decomposes into a set ofdistinct design issues. The designresearcher creates a set of propositionsto test how to address each issue.These propositions add up to acompound “hypothesis” about thedesign problem and how to resolve it.

In this paper, I present theproblem first with its component issues,then the design solution with itspropositions, and an evaluation of whatthis “inquiry by design” approachteaches us. Another way to view inquiryby design is a ‘what if” approach. Forexample, what if it is necessary to takethe requirement for radiation protectionseriously? Or, what if it is necessary toprovide zero-gravity countermeasures?

DISTINCTION BETWEENPLANETARY ANDINTERPLANETARY HABITATS

Before delving into the designresearch issues and propositions for theinterplanetary habitat, it is important tounderstand what characteristics it shareswith the problem of the surface habitat,and, what is more important, whichissues make the two habitat applicationsdistinctly different from each other. Agreat deal depends upon whether thedesigner intends the habitat to serve

also as a planetary surface habitat inaddition to serving as a transit habitatbetween the Earth and a destinationplanet, which in most recent thinking isMars.

The central design researchproblem for both planetary surface andinterplanetary vehicle habitats is how tooptimize their design for the uniqueapplication for which the designerintends them [Cohen, July 1996; Cohen,Sept. 1996]. The specific operationaland functional requirements for surfaceand vehicular habitats are in fact radicallydifferent, It is vital to understand thosedifferences and their implications forchoice of habitat configuration, structure,materials, outfitting, operations and theoverall impact upon mission architecture.The key planning decision lies in theallocation of functions between theinterplanetary and surface habitats, andthe appropriate candidate architecturalplans, sections, elevations, adjacencies,connectivities, and functional separationsfor each habitat application. Forexample, in contrast to interplanetaryhabitats, surface habitat design mustinclude a broad range of site planningconsiderations.

This habitat system optimizationgoal means how best to incorporate thefunctional components within the differentgravity orientations, how to protectagainst the relevant environmentalhazards, and how to respond to theoperational imperatives for eachapplication. The two situations differprofoundly. The interplanetary vehiclehabitat must support the crew for twovoyages of at least six months in zerogravity, accommodate countermeasuresagainst zero gravity such as the humanpowered centrifuge, operate life supportin an essentially closed system, andprotect the crew from radiation hazardsincluding solar protons and galacticheavy ions. In contrast, the Marssurface habitat must perform in .38 G toaccommodate a major mass and volumedemand to support the sciencelaboratories, and crew quarters for asojourn of 600 days on the surface,afford dust protection, support frequentEVAs for scientific field work, and mayoperate with an open loop, in situresource utilization (ISRU) enhanced lifesupport system. However, there is noneed to impose any of these surface

features upon the interplanetary habitat,nor, conversely, is there a need toimpose the interplanetary features uponthe surface habitat. It will be vital tooptimize both habitats to minimize theextraneous burden of accommodatingfunctions unnecessary to each portion ofthe mission, and instead to maximize theaccommodation of the necessaryfunctions for each portion.

THE PROBLEM OF THEINTERPLANETARY HABITAT

The interplanetary habitat is aunique and fairly complex designproblem. For the purpose of this designresearch, the interplanetary habitat is justthat: part of an interplanetary vehicle thatcrews assemble in low Earth orbit (LEO)and then injects on its trajectory to Mars.Upon arrival at Mars, the vehicle goesinto a Mars orbit. The crew transfer to adescent/ascent vehicle and descend tothe Mars surface where they live in asurface base habitat that arrivedseparately from the interplanetary crewvehicle. When the crew finishes theirsojourn on the Mars surface, theyascend back to orbit in thedescent/ascent vehicle, rendezvous withthe orbiting interplanetary vehicle, andthen inject on the Earth return trajectory.Upon returning to Earth, theinterplanetary vehicle either goes intoLEO where it can rendezvous with ashuttle, space station or other vehicle orthe crew use Apollo or Soyuz typevehicles to make a ballistic entry to returndirectly to the Earth’s surface.

Given this scenario, theinterplanetary habitat faces the demandof supporting a Mars crew of typicallysix or eight members for six to eightmonths outbound to Mars, then goinginto a dormant state for up to about 600days, then supporting the returning crewfor up to a year on the return voyage.

DESIGN RESEARCH ISSUES

These considerations yield a setof nine leading design research issues:pressure vessel structure, radiationprotection, living quarters, zero-gravitycountermeasures, crew privacy & groupactivities, stowage, circulation, integration

into the interplanetary vehicle, andhabitat size. For each of these issues inthe design problem decomposition, Ipresent a corresponding proposition for adesign solution. Although these issuesmay resemble ‘requirements,’ they differfrom requirements because they posethe question “What is the nature of theproblem they embody and what are theconnections between them?”

1. PRESSURE VESSEL - Whatis the optimum shape and structure forthe Habitat pressure vessel? Becauseall space habitat architecture ispneumatic, the pressure vessel structureis often the first consideration in a design.In the case of the interplanetary habitat,the surface area to volume ratio isimportant for minimizing structural mass,minimizing surface requiring thermalinsulation, and minimizing the surface forradiation protection.

2, RADIATION PROTECTION *

Is it possible architecturally andgeometrically to protect the crew fromradiation exposure? Radiation exposureis one of the greatest potential hazardsto crews in transit through interplanetaryspace. This design effort must takeseriously the recommendations of theexperts on radiation protection, whoseem to converge on an agreement thatinterplanetary crews need substantialprotection on the order of 30gm/cm2shielding against galactic cosmic rays(GCRs) [Townsend, Nealy, Wilson &Simonson, Feb 1990, p.8, and Cohen,July 1996]. How to introduce thisshielding mass and volume emerges as amajor architectural and engineeringchallenge.

The radiation protection questionhas such far-reachin9implications that itrequires greater explication than the otherdesign issues.

Galactic Cosmic Rays (alsoknown as HZE particles) consist of thenuclei of atoms (primarily helium,manganese, iron, xenon, and argon)traveling across the galaxy from manystars. They are spatially isotropic sothat shielding a spacecraft against GCRsrequires an omnidirectional enclosurearound the crew [Simonson & Nealy,1991, Feb, p. 9]. GCRs are relativelyconstant compared to SPEs, although

the strength of their flux waxes andwanes with the solar cycle, so GCRstend to vary inversely with the incidenceof SPEs. At solar minimum (quiet part ofthe solar cycle) GCRs are strongest butat solar maximum (active part of the solarcycle) the GCRs are weakest as thesolar wind pushes them away.However, even the weaker GCRs atsolar maximum pose a significant hazard,The main cause for the concern aboutGCRs is that their radio biologicaleffectiveness in tumorigenesis is thirtytimes as great as a unit doseequivalent of solar particles, [Fry, 1987].For this reason, it is advantageous toprovide omnidirectional shielding to thecrew habitat on the interplanetaryvehicle, instead of providing just a solarstorm shelter.

Campbell & Harris [1992]calculated that the GCR exposure for anunshielded vehicle of 1 gm/cm2 is 62.85cSv per year in interplanetary space at 1astronomical unit from the sun duringSolar Minimum. For a 180 day (half year)transit to Mars, this unshielded dosewould be 31.4 cSv, perilously close tothe National Council on RadiationProtection (NCRP) annual limit,considering all the uncertainty factors.Townsend, Wilson, Nealy [1988, p. 7]indicate their uncertainty about the GCRspectrum at up to 50%, with aconsequent dose uncertainty of up to70%. They translate this radio biologicaluncertainty factor into a shieldingrequirement of 25g/cm2against GCRs.Fry shows that 30 gm/cm2serves as afairly effective barrier to GCRs.Campbell and Harris calculated that ininterplanetary space during solarminimum when the GCR flux is at itsgreatest, the shielding thickness must beso gm/cm2to limit the annual dose to31.4 cSv or 15.7 cSv for a 180 daytransit. Townsend, Nealy, Wilson &Simonsons [1990. Feb., p. 8J calculationfor GCRs during solar minimum indicatesthat 30g/cm2of aluminum reduces theannual dose to 35.2 cSv or 176 cSv fora 180 daytransit.

Prompt Effects from GCRs Inaddition to these findings within theliterature that NASA has published, thereare significant concerns about so-called“prompt’ effects of radiation, particularly

from GCRs (HZEs). The most recentsurvey pubilcation on radiation from theNational Research CouncWs SpaceStudies Board [1996, p. 26] explains:

“The concern about HZE particles isthat the energy deposition may besignificantly different from that ofradiation qualities for which we havesome radiobiologicat understanding.One particle of very high Z andenergy can traverse a number ofcontiguous cells. There is a verydense ionization in the inner part orcore of the particle track, withsecondary particles and delta raysextending to neighboring cells....”

The Space Studies Board [p. 26]expressed particular concern about theeffects of HZEs upon the central nervoussystem (CNS):

The effects of HZE particles on theCNS include (1) cellular effects,including biochemical changes, (2)functional changes; and (3) lateeffects, especially DNA repairdeficiencies.

In addition to these HZE effects,the Space Studies Board expressedconcern about a variety of effects from adiversity of radiation types. Specificcellular effects of possible concern thatthe Space Studies Board cites includeeffects to the brain, to the respiratory andcardiac nerve centers in the floor of thefourth ventricle. Functional effects ofpossible concern include impairment ofneural networks involved in motorperformance, with such effects asimpaired balance. Taste aversion is afunctionaVbehavioral effect. Othereffects from both low and high linearenergy transfer (LET) radiation includecataracts [p. 27], mutations or otherheritable effects [p. 281.

The Armed Forces RadiobiologyResearch Institute Scientific AdvisoryCommittee [1994] published theproceeding of a workshop in which theparticipants discussed a comparablerange of concerns. Dr. John Lett’scomments indicate [p. 5]:

“The study concluded that somephotoreceptor cells were killed anddegenerated rapidly. The data

suggest that there could be a loss ofphotoreceptors along the path of Fe[iron] nuclei, Consequently, lesionsfrom local tracks of Fe particles in thebrain of an astronaut could causedegeneration and loss of functionwithin the duration of a Marsmission.” [emphasis added]

Other items of concern in theArmed Forces Institute Workshop includecataracts [p. 6), “forebrain damage” [p,7], possible changes to thehippocampus gland [p. 9], and tasteaversion, [p. 13).

One of the most strikingexperimental results among the fewexperiments with HZEs on animals is theeffect upon the sprm-producing cells.Sapp, Philpott et al [1992] reported theirresults from irradiating mice from a varietyof sources, then examining their spermproducing tissues under an electronmicroscope to count the damaged cells.They found very substantial differencesbetween the effects of x-rays and thesmaller HZEs on the one hand and theeffects of Fe nuclei on the other hand [p.(2)181]:

“The loss of spermatogonia due toirradiation is readily apparent. . . . Itis evident that killed and damagedcells are rapidly removed post-irradiation since virtually all Type Bspermatogonia are absent;

At radiation levels of 0.10 Gy [1Gray = 100 RAD] and less,spermatogonial response is similar,for X-ray, Helium, Neon and Argonradiation, as reflected by survival inStage 6 tubules 72 hours post-irradiation. Approximately one-fourthof all spermatogonial cells are lost atthe 0.10 Gy dose level. Irradiationwith Iron, however, results in adramatic decrease in survivingspermatogonia. Almost one-half ofall spermatogonia are lost at thevery low 0,01 Gy exposure level,”[emphasis added]

In light of this accumulatingevidence and concern, it will be difficultfor space habitat designers to brush offhealth and safety concerns aboutradiation. This design hypothesis aboutan interplanetary habitat attempts to take

the radiation safety data seriously, andto respond with an appropriate standardof care.

3. LIVING QUARTERS - Is itfeasible to create habitable living spacewithin an interplanetary vehicle? Givena volume of living space shielded againstGCRs, creating habitable living quartersbecomes the next challenge. Thegeometry of this volume must make aworkable transition from the pressurevessel shell and the shielding mass to asuitable configuration in which to createthe living environment. The key featuresof this living environment include aperceptually consistent up-downorientation, appropriate scale and usabledimensions, attachment hard points formounting floors and partitions, utilities,and other such accommodations.

4. PRIVACY & GROUPACTIVITIES - Can the Habitataccommodate both private and commonactivity areas? Given a suitableenvelope in which to construct crewliving quarters, it is vital to provide bothindividual private and group activityareas. The private areas includeindividual crew cabins and hygienefacilities. The group areas include thewardroom (dining area) galley, exerciseand recreation areas, medical treatmentareas (which may also sharecommonality in part with private cabins)and work areas.

5. ZERO-GRAVITYCOUNTER-MEASURES - Is it practicalto incorporate countermeasures againstextended exposure to weightlessness?The prolonged journey in interplanetaryspace implies extended exposure toweightlessness, which can have severedetrimental effects upon crew health,including loss of bone mass, loss ofmuscle mass and cardiovasculardeconditionin9.It is necessary for thehabitat or vehicular design to incorporateeffective countermeasures against theseeffects of weightlessness [G rymes,Wade & Vernikosj.

6. STOWAGE - Is it feasible toprovide adequate and accessiblestowage for all the consumables,equipment and infrastructure the crew willrequire? Supporting the Mars crew for12 to 18 months in the interplanetary

vehicle will require a considerable volumeof stowage for consumables of all kindsand for a wide variety of equipment.The consumables may include food,water, make-up oxygen and othergases and clothes.

7. CIRCULATION - How canthe design provide ingress, egress, andaccess to all parts of the habitat withoutwasting precious volume? The crewmust be able to enter the habitat and toexit it for transfer to other vehiclesincluding the descent/ascent vehicle.The internal circulation must affordconvenient, non-interfering access to allportions of the habitat,

8. INTEGRATING HABITATINTO VEHICLE - How is it possible tointe9rate the crewed interplanetaryhabitat into the interplanetary vehicle?The habitat must be capable of beingintegrated into the interplanetary vehicleensemble, including an aerobrake,ascent/descent vehicle, fuel tanks,engines, and a structural system thatholds them all together.

9. HABITAT SIZE - What is theappropriate and necessary size for theinterplanetary habitat? All the abovedesign issues converge upon thequestion of what is the appropriate sizeand mass for the interplanetary habitat.Sizing this habitat has, in turn, aprofound ripple effect throughout theMars mission architecture.

THE CANDIDATE PROPOSITIONSAS DESIGN HYPOTHESIS

For each of the issues statedabove, the inquiry by design approachsuggests the following designpropositions. Taken together, the designpropositions constitute a designhypothesis in response to the issues inthe design problem decomposition.

1. SURFACE AREA TOVOLUME: SPHERICAL PRESSUREVESSEL - A spherical shell geometry isthe most efficient shape for minimizing theratio of surface area to volume.Implementing a spherical shell presentsthe benefit of minimizing the mass ofpressure vessel structure, the surface

area requiring insulation, and the livingenvelope needing radiation shielding.This design proposition advocates sucha spherical shell. FIGURE 1 shows thisspherical pressure vessel with twopressure ports for hatches at oppositepoles.

2. RADIATION PROTECTION:WATER SHIELD - The leading difficultyin installing a substantial quantity ofradiation protection within a habitat arisesin launching it to LEO. Since theradiation shielding is likely to weigh in therange of tens of metric tons of mass, itbecomes problematic to incorporate it intothe habitat pre-launch on the ground. Itis also problematic to install a large massof rigid shielding in the habitat while inLEO. The proposed solution is toemploy water shielding, with a thicknessof 30cm as recommended by Townsendet. al [Townsend, Nealy, Wilson &Simonson, Feb 1990]. This designproposition calls for installing water tanksin the habitat, configured to provideshielding, but to launch it to LEO dry.After achieving LEO, one or more“tanker” craft launches (could be bySpace Shuttle) will bring the water to thehabitat, to which it will pump the water,filling the tanks and providing shielding.Preliminary calculations indicate that themass of this water would beapproximately 42.3 metric tons with a 7moutside diameter [Cohen, July 1996, p.7, shows 46mT for a 7m inside diameterfor the water shield]. This mass andvolume of water shielding makes thegreatest impact upon the habitat designof any of the design propositions, and itcauses a profound ripple effectthroughout the entire system architecture,with large effects on the launch masses.

Truncated Octahedral Geometryfor Water Tanks The design of the watershielding tanks is pivotal in creating aworkable shielded interplanetary habitat.This design proposition suggests ageometry based upon the Archimedeansolid known as the regular truncatedoctahedron, which consists of eighthexagonal faces and six square faces.The edge of the square is the same asthe edge of the hexagon, The hexagonis triangulated into six triangular sub-faces. The square is triangulated intofour triangular sub-faces There wouldbe two shapes of modular tank,

corresponding to the two faces: eighthexagonal tanks and four square tanks.The tanks would be made of aluminum orstainless steel. The square faces wouldbe capable of incorporating an openingfor crew circulation, ventilation or to passutilities through the water shield wall.

FIGURE 2 shows each of thesetank geometries with filleted edges. Thisgeometry has the advantage of no acutecorners that would be difficult to weld andfabricate, and where corrosion ordegradation of the weidments would bemore likely to occur.

This array of tanks would enclosethe living quarters, affording the crewradiation shielding protection wheneverthey were inside, during the normalcourse of their daily routine. The insideclear dimension is 6.3 to 6.4m and theoutside dimension is 7.Om. FIGURE 2ashows the truncated octahedral geometryof the water tanks.

The next geometric alternative tothe truncated octahedron was adodecahedron with twelve pentagonalfaces, possibly with some truncation tocreate a set of smaller (hexagonal> faces,but the dodecahedral geometry placedthese vertices or truncations of them innon-orthogonal and difficult to manipulatelocations. The dodecahedron’spentagonal faces have the advantage ofall obtuse internal angles, but asecondary framework would requirerigidized vertices as the dodecahedron isnot naturally self-rigidizing.

A spherical shape may seem likea logical solution, but there would bemany difficulties associated with makinga single double-sided spherical shelltank, even with interior partitions. Aspherical trigonometric solution may work,but the spherical gores would be mostadvantageous as a spherical truncatedoctahedron.

Structural Struts to ConnectWater Tank Vertices to Pressure Vesseliill The key interface between thetruncated octahedral water tanks and theouter pressure vessel shell is a set ofstruts that will connect the vertices of thetruncated octahedron to hard points onthe pressure vessel structure. Thecorners of the water tanks meet at thesevertices, and it remains to be seenwhether they require a frameworkfollowing the edges of the truncatedoctahedron to hold them in place, or

whether the water tanks can providesufficient stiffness and strength withintheir own structure to allow a “framelessconstruction.” In either case — framed orframeless -- the truncated octahedronwould make its structural connection atthe vertices. These radial struts wouldrequire a considerable degree of linearelasticity -- like a shock absorber -- toallow the spherical pressure shell toexpand and contract without introducinghigh local stresses into it at theconnection points. FIGURE 2b showsthe array of structural struts radiating fromthe truncated octahedral vertices.

Eliminate “Solar Storm Shelter”The traditional concept of a “solar stormshelter” that could also provide GCRprotection has always been misguided.The typical envelope for a solar stormshelter affords four to eight m3 volume, towhich the crew would retreat during atwo to three day flare. However, GCRsoccur in a constant flux, and it iscompletely unrealistic to expect a crew ofsix to eight to live in a volume with amaximum of one m3 per person for ayear. Such an arrangement would surelymaximize the detrimental effects ofconfinement, lack of privacy, andinterpersonal stress. Also, the proposalthat the crew should simply sleep in thestorm shelter -- receiving protection forabout 1/3 of the voyage -- are almost asunrealistic because it negates privacy forthe crew members almost as completely.

Placing the complete livingquarters inside the water shieldeliminates the need for a separate “solarstorm shelter. Instead, the 6Am diameterwater shield will protect the crew duringtheir normal day from the steadybombardment of GCRs, and alsoprovide solar storm protection, whileaffording them a reasonable volume forthe living environment.

3. CREW QUARTERS INSIDETHE WATER SHIELD - Configuring thecrew living quarters inside the watershield leads to demands on severalattributes of this design proposition:installing a transverse floor deck to createusable volumes, providing crewpassage through the transverse deck,and the ability to utilize the deck as anequipment accommodation and supportsystem. The transverse deck would

connect to selected interior truncatedoctahedral vertices of the water tanksystem, and thereby transfer itsstructural loads to the radial struts that tieinto the structural pressure vessel.FIGURE 3 shows this transverse deckas described in the three paragraphsbelow.

Transverse Floor Deck CreatesUsable Volumes Implementing atransverse floor deck divides the crewliving environment volume into “upper’and ‘lower” levels. The upper levelwould contain the crew living quartersincluding the private cabins, thewardroom, galley, and perhaps somerecreation areas. The lower level wouldaccommodate the zero-gravitycountermeasures and some recreationactivities.

Crew Passage through the DeckTo afford the crew direct passage fromthe lower to upper levels, the transversedeck includes a through passage. Thelocation of this pass-through affects theusability of the floor area on the deckabove and influences the circulationpattern and the utilization of both upperand lower levels. The choice of locationis between the center or edge of thedeck. In this design proposition, thethrough passage appears at the edge ofthe transverse deck.

Deck as Equipment SupportSystem The transverse deck must besufficiently deep and robust toaccommodate equipment in two ways.First, it must accept the recessedinstallation of equipment, utilitydistribution and stowage. Second it mustsupport the surface mounting ofequipment and interior space-dividingpartitions. The floor deck structure maybe a rigid, flat space frame, capable ofdistributing the loads to its edge where itwould meet the ‘space truss” created bythe water tank system. The depth of thedeck appears here as .3m, but functionalstowage requirements may drive itsubstantially deeper -- perhaps as deepas im.

4, CREW HABITABILITYACCOMMODATIONS ON UPPERLEVEL - The installation of crew livingquarters on the upper level of the habitatmakes it possible to make maximum useof the largest available floor area. Theinside faces of the water shield create a

sloping “ceiling” that will help lend visualinterest and a dome-like effect over thislevel of the crew quarters, which mayenhance up-down orientation. FIGURE 4illustrates the features described in thethree paragraphs below.

Private Cabins Each crewmember will need an individual privatecabin (Connors, Harrison, & Akin, pp.82-90J. For married couples, it may bepossible to arrange adjacent cabins, witha common wall that is collapsible orremovable. This design propositionclusters the private cabins together,although there may be a very goodargument in sound isolation, fordistributing them around the habitat,separated as far apart as possible.

1-lygiene Facilities The Hygienefacilities seem to fall somewhat naturallyamong the private accommodations.However, if the upper level volume hastoo many demands upon it, it may bepossible to locate the hygiene facilitieselsewhere -- either in the lower levelvolume or outside the water shield in thestowage zone.

Group Activities. Wardroom &Galley The design of the group activitiesarea, which include the wardroom, galleyand associated recreational facilities,requires a great deal of care in planningand implementation as Nixon, Miller &Fauquet demonstrated in their study forthe Space Station Wardroom (Nixon,Miller & Fauquet, 1989). The wardroommust serve as a multi-purpose area foreating meals, holding meetings,conducting video conferences, hostingpress briefings, disassembling andrepairing equipment, and possiblyperforming medical treatment (Stuster,pp. 61-62).

5. ZERO-GRAVITYCOUNTER-MEASURE: HUMAN-POWERED CENTRIFUGE ONLOWER LEVEL - This designproposition argues that it is neitherpractical nor necessary to spin up anentire spacecraft to provide artificialgravity as a countermeasure toweightlessness. Instead, it adopts theHuman Powered Centrifuge (HPC)developed by the Life Science Divisionat NASA-Ames Research Center as asolution to proved short duration, highgravity centrifugation as acountermeasure. Please see F1GURE

5a for a photograph of the prototypeHPC at Ames. This design approachinstalls the centrifuge in the lower level ofthe living environment. Please seeFIGURE Sb for a view of the HPCinstalled in the lower level of theinterplanetary habitat. The HumanPowered Centrifuge provides bothcentrifugation and exercise for the crewmember who uses it in the supinebicycle ergometer position to pedal thedrive system. It can accommodate asecond subject who benefits from thecentrifugation but does not pedal thedrive system.

6. STOWAGE INSTALLATIONALONG INSIDE OF PRESSURESHELL - Since free volume inside thewater shield will be very precious, thisdesign proposition states that anythingthat does not have a very strongargument for being inside it should belocated outside the water shieldenvelope. The considerable volume ofstowage thus belongs outside the watershield. All the stored provisions for thelong voyage out to Mars and back toEarth would reside in a stowage systemalong the inside of the pressure vessel,across an open passage zone from thewater shield.

This stowage zone wouldaccommodate life support equipment,including physical chemical systems andbioregenerative systems such as plantgrowth chambers. Anything that needsonly infrequent crew attention could gointo this stowage volume. lt is possiblethat some or all of the hygiene facilitieswould best go in this stowage zone also.If the interplanetary crew have anextravehicular capability, the EVA spacesuits and their servicing supportsystems would reside in this stowagevolume. The mass of stowed suppliesand equipment will contribute addedshielding to protect the crew againstradiation. FIGURE 6 shows an isometricview of the stowage volume isolatedfrom the rest of the habitat configuration.

7, CIRCULATION - An efficientand not overly extensive circulationsystem is a key to effective crewaccess, ingress, and egress throughoutthe habitat. The key elements of thisdesign proposition for such a system arethe pressure hatches through the primary

pressure vessel structure, the pass-through in selected square faces of thewater shield tanks, and the appropriaterotation of the water shield relative to theother geometries to ensure noninterference between the orthogonallypositioned square faced tanks with passthroughs, and the transverse deck withinthe living environment. Initially thesquare faced-tanks were at 90 verticaland horizontal. By rotating the watershield on the order of 20 out of planewith the transverse deck, the circulationpattern avoids some interferences andcreates new and interesting relationshipsamong the elements. FIGURES 8 and 9afford a view of some aspects of thiscirculation system

Pressure Hatches The Habitatincludes two pressure hatches situatedat the opposite poles which appear hereat top and bottom of the habitat. Eachpressure port has a Space Station-likehatch door, 1 .25m square, with roundedcorners, that translates out of the way forthe crew to bring bulky equipmentthrough. These pressure hatches openinto the circulation zone between thewater shield and the stowage. It isintentional that the pressure hatches donot line up with the square face passthroughs in the water shield, which mayrequire supplemental water shieldingtanks in the stowage volume. Also, thestowage volume areas just inside thepressure hatches serve as a kind of antechamber for stowing and stagingequipment to be used outside the hatch.

Water Shield Pass-ThrghsThe water jacket pass-throughs thatoccur in the square faced water tanksshould be large enough to allowpassage of any equipment neededinside the water shield. The inside edgeis rounded to allow crew members to slipthrough easily. It would be possible toinstall a non-pressure hatch in thissquare opening if so desired.

Orthogonal Clear Passages Thedesign of the circulation system includesa system of clear passageways at rightangles to each other in which there is nostowage. The effective clear width alongthese meridians is 1 .4m to accommodatethe movement of large and bulky items.FIGURE 6 shows both the stowagevolume and these circulation passagezones.

8. HABITAT INTEGRATION INVEHICLE CONFIGURATION -

Integrating the habitat into the largervehicle ensemble stands out as the mostdifficult integration challenge. Theconsiderable unknowns that emerge asassumptions of “best guesses’ comprisea large part of this challenge. This designproposition for vehicular integration of theHabitat takes a conservative approachin the sense of relying on concepts thatare already fairly well described and wellunderstood, The design concept is toorder these elements from bow to stern ina clear sequence: aerobrake, habitat,descent/ascent vehicle, fuel tanks, andengines. FIGURE 7 shows a sketch ofthis approach, which will requiresubstantially more effort to achieveconfidence in it as a successful designsolution.

9. HABITAT SIZING ANDINTEGRATION - FIGURE 8 shows theinterplanetary habitat with the relevantdimensions. The outside diameter of thestructural pressure sphere is lOm.Several inhouse studies indicate that a1 Om diameter is within reason for aninterplanetary habitat. FIGURE 10presents a view of the integrated habitat,with all the systems discussed above intheir places

EVALUATION OF THE “DESIGNHYPOTHESIS”

The totality of this interplanetaryhabitat design constitutes a hypothesisabout how to provide a habitat thattakes seriously the leading safetyconstraint of effective radiation shielding,in addition to meeting all the othercommonly agreed requirements.

RESOLVED ISSUES - Thisdesign hypothesis succeeds in resolvingsome of the design issues raised in thedesign problem decomposition. Theseresolved issues include: the sphericalhabitat, water shielding architecture andgeometry, private cabin geometry,structural support of the water shield,and the installation of the HumanPowered Centrifuge.

Spherical Habitat is feasible andworkable This design hypothesisshows that a spherical habitat is feasible

from the viewpoint of interior utilization,even though the “shielde& volume doesnot extend out to the pressure vesselwalls.

Water Shielding is Architecturallyand Geometrically Feasible The biggestdesign challenge was to find a way topackage the water tankage that worksarchitectural, geometrically, andstructurally. The truncated octahedralgeometry provides a feasible solution,with the caveat that it would benecessary to place the tanks inside thepressure vessel before welding it closedalong its meridian or equatorial seams.The presence of such a large quantity ofwater may provide a resource to the lifesupport system,

Structural Support of WaterJacket is Feasible at Vertices As acorollary to the truncated octahedron, thegeometry lends itself to structural supportat the vertices. This consideration isessential for demonstrating structuralfeasibility. The caveat is that thesystem must avoid creating local high-stress concentrations in the outeraluminum pressure vessel structure.With empty tanks, the launch loads willbe low, but the aerocapture loads withthe filled tanks may be high.

Irregular Sleep Cabin is Not aProblem The rotation of the water shieldtankage about the living environmentcreates some irregular geometric faces,areas and volumes, which lead toirregular shapes and dimensions in thepartitions between the private crewcabins. Although this will pose achallenge to the craftsmen who installthese partitions, it is not a problem for theinterplanetary crew. The Skylab cabinsdesigned by Raymond Loewy andAssociates were all different, nonstandardized shapes, which gave eachcrew cabin a small element of intrinsicindividuality and character [Man-SystemIntegration Standard, p 8-18].

Powered Centrifuge Another importantfinding of this exercise is that it is feasibleto install the Human Powered Centrifugein such an interplanetary habitat (andprobably in almost any other one withsufficient volume. Essentially, what theHPC requires is a circular cross sectionalarea through the volume to accommodatethe 150” (3.84 rn) rotational diameter,plus surrounding structure.

UNRESOLVED ISSUES ANDSHORTCOMINGS - Although thisinquiry by design showed that it ispossible to resolve several of theleading design research issues, therewere many more -- albeit less prominentones -- that remain unresolved. Theseunresolved issues include the secondarywater shields, non-vertex structuralconnections, depth of stowage volume,irregular faces of the rotated truncatedoctahedron, stowage zone as a singleloaded circulation zone, usefulness of thelower level crew quarters, radial strutinterference in the drculation zone, lackof windows, and the integration of thehabitat into the interplanetary vehicle.

Provide Secondary Water ShieldPanel At Pass-Through Assuming that anon-pressurized hatch door in the waterjacket shield is largely unnecessary, andit there is one, it will not be a 30cm watershield, raises the question of whether toinstall supplemental shielding in thestowage volume opposite the pass-through.

Structural Connections to WaterShield Difficult if not at Vertices While itis good to have major structuralconnection points on the water shieldtankage at the vertices, because thesevertices will occur in very irregularlocations on the rotated water shield, itwould be useful and valuable to havestructural connection points elsewhere onthe truncated octahedron. However, thisdesign neither anticipates nor providesfor these secondary structural connectionpoints. The solution would appear tonecessitate a secondary structureoverlaying the tanks, with secondary,triaconation-like gussets or stiffenersrunning from the vertices to the center ofeach triangular face, creating an additionalstructural geometry.

Dimensions Of Stowage VoIumLinfficrent Depthi The designproposition offers a stowage volumedepth of ,4m, (15.625”), considerablyless than the standard closet depth of24” (.8m) in American homes, and evensmaller in comparison to the .9m (36”)deep Space Station racks. While thisdepth of stowage may be sufficient forsome items, it will not suffice for manyothers. Thus it may be necessary tomake some portions of the stowagevolume much deeper, expanding it into

the circulation zone between the watershield and the stowage. For somepurposes, like a hygiene facility or anavigation work station, it may beadvantageous to extend this expansionall the way across the 1 m circulationzone. It will be important to calculate thevolume of stowage required to supportthe human mission to Mars.

Rotated Water Shield Trunc-QctaCreates Irregular Faces Althoughrotating the water shield tankage doesnot create a problem in the private crewcabins, it may create difficulties almosteverywhere else. By introducing thisvariance into the system, it may makethe installation of equipment much moredifficult throughout the rest of the shieldedliving environment, except where it canattach to the transverse deck. It alsocreates the potential for visual andperceptual confusion about spatialorientation. The rotated trunc-octahedronwill conflict visually with the simpleup/down geometry of the transversefloor deck and the partitions mountedupon it. This perceptual questionrequires further attention.

Location of Radial StrutsInterferes with the Circulation Zone Thearray of structura’ struts that radiate fromthe trunc-octa water tank verticesintersect the stowage and moreimportantly the widened circulation zonein an arbitrary pattern, creating possibleinterferences with through circulation. Toavoid these interferences and nonstandard intersections of stowagevolumes, the designer must exercisegreat care in selecting the rotational anglefor the water shield.

Stowage Zone Is Inefficient as aSingJe-Loaded Corridor” In mostbuildings on Earth, it would beconsidered very inefficient to “load” acorridor with useful rooms on only oneside, unless there was a compellingreason to eave the other wall unused bycirculation connections. Such acompelling reason might be to installwindows, However, there is no suchpurpose in the stowage and circulationzones in this Habitat.

Lower Crew Quarters VolumeHas Limited Usefulness Although thelower level crew quarters servesadmirably to accommodate the HumanPowered Centrifuge, this volume is notvery useful for anything else. An

unfortunate aspect of this designproposition is that it rendered the rest ofthe spatial volume almost useless as didthe Human Powered Centrifuge wheninstalled in the lower level of theControlled Environment ResearchChamber at NASA-Ames. One possiblesolution is to install an isogrid floorbetween the centrifuge motion envelopeand the upper portion of the lowervolume, creating a third level, The sizeof the volume within the 7m outsidediameter water shield is too small to allowthis intermediate level, suggesting thateither the diameter should be bigger, orthe next version should insert a shortcylindrical section between the upperand lower 7m diameter hemispheres.

No. Possibility Of Windows ForWardroom Or Crew Quarters Perhapsthe greatest single shortcoming of thisdesign hypothesis for a habitat is that itprecludes the possibility of a window tothe outside in the crew living quarters. Invirtually every NASA program, the crewshave argued for the best windowspossible. On Skylab, the 18” (46cm)window in the Wardroom was a majorfocus of crew attention and activity, andNASA has elevated this success to astandard requirement [Man-SystemIntegration Standard, pp. 8-17, 8-37, 8-38]. In this Habitat, the windows mustbe located outside the shieTdedenvelope, in the outer pressure shell. Itwould be necessary to install windowwork stations in the unshielded volumealong the outer pressure shell, for use inrendezvous and docking with othervehicles. This installation of window-workstations would be feasible. It mightbe possible to strike a balance betweena large high quality (such as highdefinition TV) view screen and a smallporthole. This arrangement sets up apotentially paradoxical situation of askingthe crew to risk an exposure to radiationfor short periods to accomplish highpriority tasks at this window-workstation.

Integrating Habitat into thejntmjnta,Y&t’l Integrating theHabitat into the Interplanetary Vehicleremains the most problematic aspect ofthis design exercise. None of the designpropositions address it adequately.Instead, there needs to be a separatebut companion study of integrating theHabitat into the vehicie from the outsideinwards toward the Habitat.

Mass of Water Shielding Lastbut far from least, the mass of the watershield poses a very substantial concernfor adding cost and fuel requirements tothe human mission to Mars. Aconventional rule of thumb is that akilogram of payload to Mars will add fromthree to five kilograms of fuel to themission. In the case of a 42.3 mT massof water (plus about 3mT for the watertanks to lift it to LEO), the increase in fuelmight range from about 127 mT to 212mT of added fuel. The 45.3mT of waterand tankage requires 1.3 shuttle launchequivalents, and the additional fuel totake it to Mars requires 3.6 to 6.0 shuttlelaunch equivalents, for a total of 4.9 to7.3 shuttle launch equivalents. With aconservative cost estimate of $400million per shuttle flight to lift 3SmT toLEO, the price of this radiation protectionwill total from $1.96 billion to $2.92 billion.

If NASA is serious abouteffective radiation protection for the crew,it will need to confront these numbersforthrightly, analyze them honestly, andfind constructive ways to respond tothem. Ultimately, NASA will need tofactor this cost for radiation protection intothe total mission architecture.

CONCLUSION

This exercise in Inquiry byDesign was useful in discovering thecriticality of certain design issues and thefeasibility of design propositions toresolve them. This analysis shows thatthis interplanetary habitat ‘designhypothesis” is valid principally to theextent that it is architecturally andgeometrically feasible to install asubstantial water shield for radiationprotection. The analysis also confirmsthat it is feasible to install a HumanPowered Centrifuge for zero-gravitycountermeasures. The analysisghlights the challenge of integrating thehabitat into a larger interplanetary vehicleensemble.

ACKNOWLEDGMENTS

I wish to thank Andrew Gonzales andMarcus Murbach of the AdvancedProjects Branch, Space Projects Divisionat NASA-Ames for their careful reading of

this paper and their thoughtful commentsand suggestions.

REFERENCES

Campbell, Paul D., & Harris, J. D,,(1992, December) Crew HabitableElement Space Radiation Shieldingfor Exploration Missions. LESC30455, Houston, TX: LockheedEngineering and Sciences Company,Contract NAS9-1 7900.

Cohen, Marc M., (1996) Design of aPlanetary Habitat Versus anInterplanetary Habitat, SAE 961466,26th International Conference onEnvironmental Systems, Monterey,CA July 8-11, 1996) Warrendale, PA:Sodety of Automotive Engineers.

Cohen, Marc M,, (1996) Habitat Distinctions:Planetary Versus InterplanetaryArchitecture, AlAA-96-446, AIAASpace Programs and TechnologiesConference, Sept. 24-26, Huntsville,AL, Washington DC: American Instituteof Aeronautics and Astronautics.

Connors, M. M., Harrison, A. A., & Akins, F.R. (1985) LIVING ALOFT: HumanRequirements for ExtendedSpscefliyl-it, NASA SP-483, WashingtonDC: NASA Scientific and TechnicalInformation Division.

Fry, A. J. M., (1987), “RadiationProtection Guidelines for SpaceMissions,” in McCormack, Swenberg& Bucker, Eds, pp. 71 5-728.

Grymes, A. A., Wade, C. E., & Vernikos,J., (1996) Biomedical Issues in theExploration of Mars, MS-95-483,pp. 225-239 in Stoker & Emmart,eds, Strategies for Mars, San Diego,CA: Univelt, lnc.

Man-System Integration Standar Vol. 1,NASA STD-3000, Rev, 8, (1995, July)Houston TX: Johnson Space Center.

McCormack, Swenberg & Bucker, Eds,(1987) Terrestrial Space Radiationand Its Biological Effects, New York:Plenum Press, Proceedings of theNATO Advanced Study Institute,October 11-25, 1987, Corfu, Greece.

Nixon, David, Miller, Christopher R., &Fauquet, Regis, (1989, December)Space Station Wardroom Habitabilityand Equipment Study, NASA CR4246, Washington DC: NASA Scientificand Technical Information Division.

Sapp, W. J., Philpott, D. E., Williams, C.S., Williams, J. W., Kato, K., Miquel,J, M., & Serova, L, (1992)Comparative Study ofSpermatogonial Survival after X-RayExposure, High LET (HZE) Irradiationand Spaceflight, Advances in SpaceResearch, Vol. 12, No, 2-3, pp.(2)179-(2)189. UK: COSPAR.

Armed Forces Radiobiology ResearchInstitute - Scientific AdvisoryCommittee, (1994, Feb. 10-11) QRequirements Regarding particleCarcinogenisis in the SpaceRadiation Environment. Workshop onHeavy Energetic Charged ParticleRadiation and the Central NervousSystem, Bethesda, MD: ArmedForces Radiobiology ResearchInstitute.

Simonson, Lisa C., & Nealy, John E.,(1991, Feb) Radiation Protection forHuman Missions to the Moon andIy1r., NASA TP 3079, Washington,DC: NASA Scientific and TechnicalInformation Division.

Space Studies Board — NationalResearch Council (1996) RadiationHazards to Crews of InterplanetaryMissions: Biological Issues andResearch Strategies, WashingtonDC: National Academy Press.

Stuster, Jack W., (1986) Space StationHabitability RecommendationsBased on a Systematic ComparativeAnalysis of Analogous Conditions,NASA CR 3943, Washington DC:NASA Scientific and TechnicalInformation Division.

Townsend, L. W., Nealy, J. E,, Wilson,J. W,, & Simonson, L C., (1990,Feb) tirnjpfjlactCoicRay Shielding Requirements DuringSolar Minimum, NASA TM 4167,Washington DC: NASA Scientific andTechnical Information Division.

Townsend, Lawrence W., Nealy, JohnE., & Wilson, John W., (1988, May)Preliminary Estimates of RadiationExposures for Manned InterplanetaryMissions from Anamolously LargeSolar Flaro Events, NASA TM-100620, Hampton, VA: NASALangley Research Center.

Zeisel, John (1981), Inquiry by Desigii:Tools for Environment-BehaviorResearch, Monterey, CA:Brooks/Cole Publishing Co.

For further information or tocomment upon this paper, pleasecontact:

Marc M. Cohen, Arch.D., ArchitectAdvanced Projects BranchSpace Projects DivisionMail Stop 244-14NASA-Ames Research CenterMoffett Field, CA 94035-1000TEL (415) 604-0068FAX (415) 604-0673mcohen@mail.arc.nasa.gov

FIGURE 1 The spherical pressure vessel presents the minimum ratio of surfacearea to volume. The pressure vessel supports the water tank structural by means ofradial struts that connect to the vertices of the water tanks.

Structural berthing ring

Cross-section of sphericalpressure vessel shell

Radial strut from vertex ofiruncated octahedron watertanksSquare face of truncatedoctahedron water tank, with crewpass-through

Cross-section of truncatedootahedron water tankage

Triangulated hexagonal faceof truncated octahedron

Cross-section of square facewater tank with pass-through

jnteriOr vertex of truncatedoctahedron

FIGURE 2 “Exploded Perspective” of the three types of water tanks in thetruncated octahedron geometry. From left to right: the square face tank with a crew passthrough, the hexagonal face tank, and the plain square tank.

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FIGURE 2a The water tank geometry is based upon the Archimedean solid known as thetruncated octahedron. The square faces may include pass-throughs for crew circulation,ventilation or utilities. The hexagonal faces are triangulated.

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FIGU RE 2b The array of structural struts radiates from the truncated octahedral vertices tohard points on the pressure vessel shell.

FIGURE 3 The transverse deck within the water shield volume creates usableupper and lover volumes for the living environment.

FIGURE 4 The upper level crew quarters would include private cabins, hygienefacilities, the wardroom and galley.

FIGURE 5a Photograph of the Human Powered Centrifuge experimental facility inthe lower level of the Controlled Environment Research Chamber at NASA-Ames

Research Center.

FiGURE 5b CAD view of the Human Powered Centrifuge installed in the lowerlevel of the living quarters of the hypothetical interplanetary habitat.

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FIGURE 6 The stowage volume will occur along the inside face of the pressure vesselshell, providing both consumables and equipment volume, and also additional radiationshielding for the crew. A widened circulation zone occurs on the orthogonal meridians.

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