engineering eﬀort needed to design spacecraft with

Proceedings of the Space Nuclear Conference 2005San Diego, California, June 5-9, 2005

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Engineering Effort Needed to Design Spacecraft with Radiation

Constraints

Dr. Robert C. Singleterry Jr.

NASA Langley Research Center, MS 388, 6 East Reid St., Hampton, VA 23681Tel: (757) 864-1437, Email: [email protected]

Abstract – A roadmap is articulated that describes what is needed to allow designers, to include researchers,management, and engineers, to investigate, design, build, test, and fly spacecraft that meet the mission requirementsyet, be as low cost as possible. This roadmap describes seven levels of tool fidelity and application: 1) MissionSpeculation, 2) Management Overview, 3) Mission Design, 4) Detailed Design, 5) Simulation and Training, 6)Operations, and 7) Research. The interfaces and output are described in top-level detail along with the transportengines needed, and deficiencies are noted. This roadmap, if implemented, will allow Multidisciplinary Optimization(MDO) ideas to incorporate radiation concerns. Also, as NASA moves towards Simulation Based Acquisition (SBA),these tools will facilitate the appropriate spending of government money. Most of the tools needed to serve these levelsdo not exist or exist in pieces and need to be integrated to create the tool.

I IntroductionThe ability to design spacecraft optimized to mission

requirements at the lowest cost is the ultimate goal ofany mission and spacecraft design team (referred to as adesigner). As will be shown in this paper, a major ele-ment of spacecraft design is the radiation protection ele-ment. All other elements of spacecraft design are under-stood enough to be engineering problems and not scienceproblems; however, while we understand the propagationof radiation through materials, we currently do not havenearly enough ability to determine the risk of that radi-ation to humans, electronics, and other materials with amanageable uncertainty. We also do not yet have the abil-ity to predict all particle ejection events associated withthe sun. Therefore, the current design of spacecraft forradiation protection is based on conservative estimates ofradiation damage. Research is occurring in the probabil-ity risk assessment methodology, but a production qual-ity tool for boundary conditions or response functions isnot ready. Production quality computational tools incor-porating good engineering practices that a spacecraft de-signer could use to incorporate the radiation protectioncomponent in their design environment do not yet exist.Best estimate research tools do exist however. The cur-rent task is to convert these research tools to productiontools based on the latest physics and risk limits.

The purpose of this paper is to describe the type oftools that have been identified to date and that the NASA

Langley Research Center and others are designing to pro-vide to the designer and other researchers. There are afew items that must be explained for those who do notdeal with this subject every day in order to understandwhy this is an important subject and why every designershould have radiation protection as their second thoughtfor every decision they make. Also, there are no models,experimental results, or computational data in the paperbecause most of these tools do not exist or do not ex-ist in a coherent form to generate results as a designershould see them. However, without a written roadmapsuch as this, there is no organization to the effort to gen-erate these tools and make them useful to and utilized bythe design and research communities.

This paper is a culmination of ideas from many dif-ferent people in the space radiation, design, engineering,and other communities. It has very few original ideasfrom the author. However, much more work has to bedone to implement these ideas and put radiation protec-tion in a place where is impacts the design process theleast, but still produces a spacecraft that meets missionrequirements.

I.A What is a DesignerThere are many views of what a designer is suppose

to be. In this paper, terms are used that will identify ex-perts and non-experts. In the current design process, aradiation expert is needed to consult on designs, create


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models, and calculate values. Here, a radiation expertis a Ph.D. in physics or engineering and well versed inor the author of the code being used. It is inefficient tolet this type of staff member continue in this role. Thisperson would be better utilized in creating and upgrad-ing the codes. A more appropriate staff member to pre-form this role is a BS or MS degreed engineer who has agood understanding of the codes and the theory behindit. However, this person is not an expert as defined aboveand therefore, is called in this paper a non-expert. In re-ality, the non-expert is not alone in their role. An expertis available and part of the design process reviews, but isnot part of the day-to-day design tasks.

I.B Risk and Where It Comes FromFor human activity in low earth orbit (LEO), the ac-

ceptable risk is defined as 3% excess risk of fatal cancerover a lifetime. The exposure limits associated with thisrisk are primarily based on the atomic weapon data fromJapan, which is based on neutron and low LETa radia-tion: X and γ-rays.

For electronics, an empirical database and direct ir-radiation testing are used to define the functionality ofparts to radiation.1 That database is primarily based onlow LET X and γ-rays, electrons, and some higher LETproton and heavy ion data. There is a view that if anelectronic part can withstand a certain LET created withprotons then it should be able to withstand any parti-cle with the same LET. Figure 1 shows that at the sameLET, higher Z particles have a very different effect inmaterials. There are no regulatory limits associated withelectronics; however, loss of function is undesirable.

Materials have no record except that generated in thenuclear power industry – again, mainly low LET events;military results, if any exist, are classified and currentlyunavailable. These data do not represent the high LETradiation found in space. There currently are no regu-latory limits associated with materials; however, loss offunction is undesirable.

Space radiation is composed of numerous compo-nents. Galactic Cosmic Rays (GCR) are particles that arepresent everywhere in the solar system and are low inten-sity but are very high energy. They consist of fully-stripednuclei from hydrogen to uranium and are traveling nearthe speed of light; therefore, they have great penetratingpower. GCR generally create a chronic risk to humans,electronics, and materials because of their low intensity.Solar Particle Events (SPE) are high energy, high inten-sity ejections of material from the sun. They can createan acute risk to humans, electronics, and materials.

In LEO, and near other planetary bodies, variousmagnetic fields exist that trap radiation as shown in Fig-

ure 2 for LEO. This radiation is mainly protons and elec-trons, but it does contain neutrons splashed out of theatmosphere (if one exists) and other charged particles.These splash neutrons decay with a half-life of 10-12 min-utes into protons and electrons which populate the innerbelts as shown in Figure 3. The outer belts are populatedby infiltration of particles, mainly protons, from the solarwind and GCR. Depending on the orbit, they can pro-duce an acute risk and a chronic risk. The earths mag-netic field is depressed under the south Atlantic and al-lows more and higher energy particles at lower altitudes.This is the South Atlantic Anomaly (SAA) which is tra-versed by the normal Shuttle (STS) and Space Station(ISS) orbits. Other planetary bodies, like Jupiter, haveother type of belts with different intensities and energiesassociated with them. These data will have to be com-piled into a coherent model that can be programmed asa boundary condition to the transport engines for usewithin this roadmap.

Planetary bodies generate particle radiation. This ra-diation is generated within the soil and rock itself whenGCR or SPE penetrate into the soil or rock and frag-ment. The lighter particles scatter and some scatter outthrough the surface. This is a hazard for operations onplanetary bodies.

Figure 4 shows the components of space radiationwhile Figure 5 shows the GCR abundance and energyspectrum. This shows the depth of the nuclear and atomicphysics needed to solve this problem and the transportmethods needed to enable the proper response functions.The large energy spectrum that needs to be modeled andthe number of particles that need to be tracked stresscurrent technologies and methods. Larger, faster, andclustered computers only solve part of the problem. Newmethods and computing platforms are needed to completeproduction tools for the designer of spacecraft.

To achieve an acceptable risk to the humans, elec-tronics, and materials (referred to as occupants in thispaper), the shielding of these radiation components is es-sential. However, they are not shielded like terrestrialcomponents by utilizing mass, time, and distance. Theenergy of the particles is orders of magnitude higher thanany radiation shielded on the earth’s surface. Therefore,the technique of putting mass between the source andthe object is not appropriate. Also, the cost of launch-ing the amount of mass necessary is prohibitive. Theother legs of the shielding paradigm, time and distance,are not available strategies in the spacecraft design pro-cess. Therefore, other strategies are needed to shield theseradiations. While electromagnetic shielding is being re-searched, its potential is questionable. That leaves theutilization of materials in an intelligent manner to reduce

aLinear Energy Transfer in keV/µm for other units


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the damage caused by the radiation, and the protectionof the occupant by special means: radio-pharmaceuticalsfor humans as an example.

These particles are traveling at energies sufficient tofragment when interacting with nuclei of any materialsurrounding the occupant (including the occupant itself).These fragments are also at high energy and are a directfunction of the material in which they fragment. There-fore, the ability to shield these fragments and their pri-maries is a direct function of all the materials used tobuild the spacecraft. The materials also reduce the energyof charged particles and electromagnetic radiation (EMR)through a continuous interaction between the material’selectron charge density and the charge of the particle orthe EMR. Therefore, the larger the electron charge den-sity, the faster a charged particle loses energy, EMR losesenergy or is absorbed, or the particle is scattered awayfrom the interior of the spacecraft. The electron chargedensity is dependant on the material. For neutrons andother neutral particles, only nuclear interactions are avail-able, and therefore, the closer the target atoms are inmass to the projectile – say, a neutron, then the fasterthe projectile will lose energy and be absorbed or be scat-tered away from the interior of the spacecraft.

The last risk item that needs to be explained is theconcept of ALARA (As Low As Reasonably Achievable).It is an operational constraint in a radiation environment,but it highly impacts the design process. The ALARAconcept means that during operations with a radiationsource, the operator must do what is reasonable to re-duce the radiation exposure to its employees and equip-ment. This is not a check-off box for the designer. It isa real legal requirement and must be an integral part ofthe design process. It also places a constraint on the de-sign team to work with operators to achieve a provableALARA strategy that is reflected in the design.2

I.C Why is the Radiation Risk Important?This is an important question since there are many

risks associated with space flight. However, once the mis-sion is over, the radiation exposure is the major risk forthe astronaut. For electronics and materials, once themission is over, the radiation exposure for these items iszero and so is the risk to the mission. Therefore, the as-tronaut’s radiation exposure is a life-long risk and mustbe dealt with in the design phase of the mission along withthe operations phase to minimize mission costs, keep theastronaut safe during the mission, provide an acceptablerisk after the mission, and perform the mission require-ments.

A designer makes many choices about components,materials, placement, operation, etc., . . . in the designprocess. The primary function of a component usually

drives these choices; however, if it does not, then whatdoes? The assertion in this paper is that the radiationfunction of that component should then be the driver.

Why does every component have a radiation func-tion? As stated above, every bit of material acts as aparticle shield and a particle fragment generator. Asthe radiation travels through material, its characteristicschange. The designer needs to set up an overall path forthe particles so that the radiation’s characteristics changeto protect the occupant and the spacecraft still meetsthe mission requirements. Therefore, every non-primaryfunction choice made should be made to reduce the radi-ation effects to the occupant.

Examples are needed to illustrate these design limitsand their importance. For a typical mission with humans,a pressure vessel is needed. For nominal 7-14 day stayson the lunar surface, an aluminum structure for a habi-tat will meet dose requirements for the astronaut with-out an SPE. However, if the designer makes the choiceof aluminum as the structural component based on theirdesign comfort level, this is not ALARA. Other materialssuch as carbon composites should be considered and ana-lyzed through trade studies before a final material choiceis made. As another example, the placement of the astro-naut’s sleeping quarters may be made for esthetic quali-ties, and even though it still meets dose limits, ALARAdictates that the designer creates a place where dose canbe minimized to place the sleeping quarters. Of course,there are other constraints that must also be considered.For example, surrounding the sleeping quarters with ev-ery refrigerator on the spacecraft (or other heavy compo-nents) may meet dose limits and ALARA; however, thenoise associated with this arrangement may violate otherregulations that the designer must consider. This is thetest of reasonability inherent in ALARA.

Therefore, it is important for the designer to con-sider the primary function of the component being de-signed first, and the radiation risk associated with thatcomponent along with other life-long risks second. Otherregulatory constraints are then considered third.

II Verification, Validation, and Accreditation(VV&A) vs. Best Practices

Of course, design is carried out in virtual space andtherefore computational tools are needed to analyze thedesign to determine if design requirements are met andthe ALARA principle is considered. It is these designtools that are the crux of this paper. The designer isby definition not a radiation expert, nor should they be;however, they need the knowledge of an expert while an-alyzing their design. A well thought out tool can bridgethis gap between the non-expert and the expert. Thenon-expert designer is not designing in a vacuum. Ex-


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perts are available for consultation and are part of thereview process.

When computational tools are talked about, the lastthing on a designer’s mind is the applicability of this toolto the design question being asked. VV&A is a process3

to manage uncertainty in the quantities computed for thedesign. What the management of uncertainties means tothe designer is that they have full knowledge of the un-certainty during the design process and are therefore re-sponsible to manage that uncertainty. Currently, mostNASA designs are based on best practices (this is basedon anecdotal evidence when asked about current compu-tational practices at NASA Langley).4 This means thata scientist or engineer has made as good of an estimatebased on the relevant physics as is possible with the re-sources available. Therefore, when they pass the code tothe designer (or as usually occurs, passes the results ofthe code to the designer), the designer has no knowledgeof the uncertainty, cannot manage it, and cannot there-fore be responsible for it. This places the responsibility ofuncertainty management back with the scientist or engi-neer. The designer is designing without the informationnecessary to produce a good design.

Therefore, computational tools need to be more thanthe best estimate that can be made by the relevantphysics and resources. An understanding of the uncer-tainty in the tool is necessary, and it would be best ifa quantitative estimate of the uncertainty is part of thetool. This allows the designer to make choices based ona sound understanding of what is needed, what is under-stood, and how well it is understood.

For radiation tools, many parts are inherent in a de-sign. Some of these parts are common with other engi-neering analyses like geometry. However, there are uniquedata and methods where the uncertainty must be under-stood. Good VV&A practices dictate that each datasetand each method be isolated and uncertainties of each beascertained. Therefore, each method or dataset that de-termines cross sections or interaction probabilities, as anexample, must be able to be formatted for all transportmethodologies. In this manner, an understanding of theuncertainty for each component is directly related to thedata or method. In this way, a designer can understandthat when a particular tool is used, a particular uncer-tainty is associated with it, and the designer can thenmanage that uncertainty.

III Impact on the Design ProcessWith the design process highly coupled to radiation,

other engineering disciplines must also be tied into theprocess since their results impact the radiation risk to theoccupants. This is called Multi-Disciplinary Optimiza-tion (MDO). All disciplines that are needed to design the

spacecraft must work together to enable the designer tocreate the lowest cost spacecraft yet still meet all missionrequirements. The ALARA requirement is also part ofthis process. However, the designer asks different ques-tions that can take different tools to generate an answer.Therefore, the design process can be broken up into var-ious levels that take various fidelity tools. Again, a tradeoff occurs here. The lower the fidelity of the tool, thefaster it runs. Also, different tools generate different typesof answers. As an example, a low fidelity, fast tool maygenerate dose values somewhat accurately everywhere ina spacecraft, but a higher fidelity, slower tool may gen-erate accurate angular flux values everywhere, couple allthe particles together, and represent more physics thana low fidelity tool. The tool to use depends on the an-swer wanted. For an expert, it is (usually) no problem tochoose the proper tool. For a non-expert designer, choos-ing the proper tool is a problem. Therefore, even thoughthe computational engine might be the same, a differentuser interface is usually wanted to generate different out-puts and help guide the designer to the proper tool. Inthis way, different levels of tools are created.

There are at least seven (probably more) levels of pro-duction quality analysis tools needed: 1) Mission Specu-lation, 2) Management Overview, 3) Mission Design (orrequirements generation), 4) Detailed Design, 5) Simula-tion and Training, 6) Operations, and 7) Research. Whilethese are very broad topics, the ability of each tool level toanalyze the radiation element is important and the driv-ing factor to the overall spacecraft design. The limitationof this list is the lack of experimental data to backup therecommendation offered here; however, these recommen-dations are logical and obtainable.

The description of these levels does not describe anexact interface between the tool and the user. NASAStandard 3000 will define the output numbers that areneeded from a regulatory view and the physical situationwill dictate the information that will need to be input.However, the designer’s interface is not described in de-tail because the designer is integral to the design of theinterface. Currently, the designer has not been incorpo-rated into the tool interface process. Without designerapproval of the interface, the tool will never be used.

III.A Mission SpeculationMission speculation occurs when a designer: re-

searcher, technologist, manager, citizen, etc. . . , wants todetermine the major obstacles in performing a mission.Again, radiation is a major factor in any design, and if anew technology is being used to drive the mission, thenthat technology is going to have to be vetted against ra-diation concerns. A fast method of radiation risk deter-mination is needed. The fidelity needed here is still aquestion. Much debate will occur on this level and may


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include access to all of the tool levels discussed below.Either a table look-up method from a depth versus dosecurve or some other simple mechanism can be used togenerate the answer wanted.

III.B Management OverviewManagement overview is the analysis needed to

present management with the mission overview or to per-form a trade study or a paper study at the mission level.Usually, it is the science or technology that drives thechoice of the mission. This is where a “quick and dirty”analysis is needed to determine if current technology orTRL 7-9b technology can suffice to supply the mission.Table lookups or simple pre-run transport solutions witha web or simple interface can be used to supply this levelof information. At this level, especially for trade studiesor requirement studies, a particular tool may not be asnecessary to get the process started as a “rule of thumb”briefing to familiarize the design team with why radiationis important and how to mitigate this risk starting earlyin the design process. This is an important aspect of de-sign and has been a problem at NASA. Some well knownexamples are the Viking and Skylab Missions.

Viking was designed and mass limits reached when itwas determined that the solar cells were not large enoughbecause they degrade from radiation, dust, and other en-vironmental triggers. They were resized to accommodatethe need. However, mass limits were exceeded, thereforesecondary science experiments were eliminated. Otherparts of the craft needed more shielding and again, sincemore mass was added, more secondary science experi-ments were eliminated. For Skylab, it was deemed thatthe film vault would not protect the film from radiationfor the entire mission. Therefore, the vault was modifiedto meet requirements along with extra support structureto meet launch loads. The mission’s life was shortenedbecause the craft was heavier and its orbit decayed faster.

The moral to this story is that radiation concernsmust be incorporated into the design process as early aspossible. The problem to date is that the tools neededto incorporate these concerns did not exist or exist in aprimitive or inadequate state. While dose versus depthcurves for various materials do exist, these are not usu-ally enough to give a good estimate of what is needed togenerate and evaluate requirements.

III.C Mission DesignThe mission design level is where the mission plan-

ners, after management approval of course, design themission and lay-out the mission requirements. A rela-tively accurate analysis is needed, but it does not haveto be detailed. Trajectory, material classes, and mass

distributions are important. A web based package likeSIREST,5 based on the HZETRN code6 for high energycharged particles, is sufficient to satisfy this level assum-ing approximate mass distributions and material classesare known.

In NASA parlance, this level of tool is sufficientenough to get the designer through the Preliminary De-sign Review (PDR). Because of the nature of the differenttasks within this level, different interfaces may be neededto accommodate it. This can be anything from a simplematerial lay-up analysis to a full fledge mass distributionfrom a Computer Aided Design (CAD) model utilizingthousands of rays. The computational engine is the same,just the run time and the accuracy of the answer is dif-ferent. For intermediate analyses, a series of spheres canbe used where a simple upper and lower mass distributioncan be analyzed all the way to 16 or 32 different segments.

The ultimate goal is to design a spacecraft that canmeet the requirements for a PDR. Many changes will bemade to the design in this process. A fast tool is neces-sary to achieve this goal. A perturbation or variationalbased analysis method would be the method of choice;however, because of the number of particles that must betracked, this becomes slower than running a fast forwardtransport solver for each design. For example, SIRESThas a multi-material transport engine that will fulfill therequirements for this level. Different interfaces or levelsof fidelity of input data are needed, but these can be fullyaccommodated.

III.D Detailed DesignThe detailed design level is where the details of the

design that will be built and flown are to be optimizedwith all other technologies to be incorporated in the de-sign. The designer’s interface of choice is CAD. Mostother engineering disciplines utilize some version of a fi-nite element method of solution which is intimately com-patible with the CAD interface. There is only one codethat can utilize fully 3-D, CAD interface, tightly coupledparticle, finite element, space radiation capable trans-port: ATTILA,7 which is currently licensed by TranspireInc. A 1-D finite element analysis is available through theUniversity of New Mexico called LOBOTRN.8 This 1-Dmethod includes higher order phase space interpolationthan ATTILA performs. Therefore, it is able to transporta beam and other scenarios much more accurately. Bothof these codes have the Fokker-Planck operator9 imple-mented to allow the charged particle to diverge in a 3-Dmanner. While these methods have not solved numerousspace related problems to date, they show exceptionalpromise in solving these types of problems and need tobe cultivated.

bTechnology Readiness Level 7-9 are technologies that can be brought to flight readiness with a small investment


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The other 3-D solution method utilizes the MonteCarlo method; however, currently no program exists thatcan be utilized in the CAD environment without se-vere geometry approximations. All these Monte Carlocodes utilize combinatorial geometry methods and havea problem modeling objects like a simple incandescentlight bulb. Therefore, a second, Monte Carlo specificmodel must be created which adds another layer to theVV&A process and slows down the generation of an an-swer needed by the designer. It also introduces the needfor an expert to generate that model.

Current Monte Carlo codes that can be utilized in thespace environment, like HETC-HEDS,10 do not incorpo-rate variance reduction (biasing) techniques that reducethe number of particle histories, and hence runtime, togenerate the same uncertainty as in the unbiased model.The biasing technique of choice is weight windows. Thereare two flavors of weight windows: local and global. Forthe local variance reduction method,11 an adjoint deter-ministic solution can be used to steer more particles inthe direction of interest like a detector. However, withthis technique, the solution away from the detector con-verges at a much slower rate if at all. For the global vari-ance reduction method,12 a forward deterministic solu-tion can be used to accelerate convergence in all parts ofthe model. With these weight windowing variance reduc-tion techniques, a Monte Carlo calculation can be greatlyaccelerated. Preliminary results suggest that a 100 to1000 times speedup is theoretically possible.11, 12

The stochastic and deterministic methods are a nat-ural for a fidelity discussion. As the number of historiesin a Monte Carlo run tends to infinity, the solution tendstowards the correct solution for the physics modeled inthe interaction probabilities. As the space, angle, andenergy mesh size gets smaller for deterministic codes, thesolution converges to the correct solution for the physicsmodeled in the cross sections. However, we cannot waitfor an infinite number of histories or an infinite numberof phase space mesh elements to compute. Each solutionmethod has advantages and disadvantages. The secretis to know which one to use for the answer the designeris seeking. They can also work together to get a moreaccurate answer faster than either one alone: a hybridmethodology. The space radiation transport world needsto move in this direction to be able to be of use to thedesigner and enable an ALARA solution! Without thishybrid methodology, the designer will not be able to uti-lize either method because an expert will be needed tocreate an approximate model so that the solution time issomething that can be tolerated by the design process.This is an unwanted outcome.

An unnecessary tension exists between stochas-tic (MCNPX, HETC-HEDS, FLUKA, MARS, PHITS,

etc. . . ) and deterministic (ATTILA, LOBOTRN,HZETRN, GRNTRN, ONELD, ANISN, etc. . . ) campswithin the space radiation field. Each method has its ad-vantages and disadvantages. Fortunately, they are mostlyorthogonal in this respect. Where one has an advantage,the other has a disadvantage. In short, each is useful andwanted at the detail design level. Both types of codesneed to be brought to the same level of completeness forVV&A and utilization in the design process. It is always aprudent engineering strategy to have two different meth-ods solving the same model to ensure nothing has beenoverlooked. If they work together, a hybrid method, thena stronger tools exists that did not exist before. Thisshould be the goal of any transport method developmenteffort.

For a hybrid method, a natural fidelity argument canbe made with a combined tool. If the designer needs amedium fidelity answer, then the tool should run just aforward deterministic calculation. This is adjustable bythe number of phase elements used in the model. If a highfidelity answer is needed at a point, then a hybrid methodshould be employed, for example, an adjoint deterministicsolution accelerated Monte Carlo run. If a high fidelityglobal solution is needed, then another hybrid methodshould be employed, for example, a forward determinis-tic solution accelerated Monte Carlo run. Again, whena higher fidelity is used, it is assumed that the time tosolution is longer.

The main foreseen problem with this level of fidelityis the uncertainty in the cross sections or the interactionprobabilities. Currently, the uncertainty in these valuesis larger than the uncertainty in the higher fidelity trans-port methods. Therefore, if the uncertainty in the crosssection data is not decreased, then these higher fidelitymethods will not gain the designer any reduction in uncer-tainty for the answer being sought. This however shouldnot be a stumbling block philosophically or in reality.

III.D.1 Human Models Currently within NASA, theCAM and CAF (Computerized Anatomical Male and Fe-male) human models are the basis for dose equivalent(and associated quantities), and hence risk, for space cal-culations in SIREST and HZETRN. Currently, however,the CAM and CAF models cannot be used directly inany stochastic or deterministic model. These models canbe converted to CAD format and utilized in the finite el-ement deterministic codes with no further problems andcan be used in the stochastic codes once spline surfaceparticle tracking is implemented. Other efforts like thedigital astronaut are looking at this area, and the resultswill be very useful for the detailed design level. This in-cludes the voxel based human models that are being usedin various medical areas.


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Some problems do exist in the human response mod-els. First, the science is still being performed. There-fore, we have no engineering limits for deep space. Thecurrent 3% excess fatal cancer risk for LEO may changedue to new data being generated. Currently, we have norisk based response functions for humans in space, justexposure limits that represent the risk. The risk limitfor humans will be changing in the near future. Insteadof excess fatal cancers, the new limit will probably bebased on Risk of Exposure Induced Death (REID) andshould be between 3 to 5% with an uncertainty assess-ment for chronic and clinically significant risk for acuteexposure.13

III.D.2 Electronics and Materials Incomplete andcrude models for electronics and materials exist at thistime. Much research is needed here to bring these modelsto the same level of completeness as the human models.Other papers14 are beginning to address the electronicsproblem from a probabilistic methodology.

III.E Simulation and TrainingSimulation and training are another level of design

and take place after the Critical Design Review (CDR).This is where the design is tested, modified to as-builtand tested, operational scenarios are created and tested,and people are trained to operate and maintain the space-craft. Here, the interface to the computational tool is justas important as the computational tool. Virtual reality oran immersive environment is the best interface available.The user can be immersed in the environment and canbe shown the effects of the radiation as they traverse thespacecraft and the mission. This psychologically bringsthe invisible radiation environment to the user, which isimportant because of the lack of physiological queues fromradiation. The uses of this type of interface are numer-ous. Other interfaces, like those used in the detailed de-sign level are also useful, but for a different purpose. Ofcourse, if a major redesign is needed, then the process willrevert back to the detailed design level until the updatedCDR is completed.

How NASA uses the word “simulation” is a bit differ-ent than an engineer or scientist would use the word. Forthis application, the meaning is somewhat varied and atthis moment in time, not strictly defined. Accordingly, avague but descriptive definition will be given here. Simu-lation has combined two functions that were different inthe past: verification and validation of the design and cre-ation of operational and maintenance procedures. Train-ing is then the teaching to and modification of those pro-cedures with the users.

In the past, separate tools were created for all thetasks described in this section. However, with the ad-

vance of virtual environments, a single tool has the abil-ity to work with the data generated in the detailed designstep and provide all the necessary information assumingthe design does not change. If the design changes enoughto warrant the design data suspect, then a new set of datacan be generated using the tools from the detailed designlevel. With the advent of a finite element, multienergygroup representation of the transport process, an adjointsolution can be formulated for the spacecraft.

The adjoint solution represents the importance of aboundary condition to a particular function of interest in-side the spacecraft. If the point of interest, for example,were the human lens on an astronaut, then in the detaileddesign phase, an adjoint solution would be generated atthat point for all the relevant radiation encountered onthe trip. With this pre-calculated solution, as the bound-ary conditions change, to fully solve the transport equa-tion for the lens of the eye, the boundary condition isconvolved with the adjoint solution (simple matrix multi-plication) and the full transport solution (energy and an-gle) is realized. This is a quick and straight-forward wayto generate a very accurate solution with a simple tech-nique. Its down side is that if the design is constantlychanging, then computer time is wasted generating ad-joint solutions that are of limited value.

With this technique, the tasks of simulation andtraining can be completed in a virtual environment (or anon-virtual environment for that matter) with ease. Mostvirtual environments utilize the existing CAD drawingand project it into the interface: a wall, CAVE, or otherdevice. The adjoint solution can be directly projectedinto this environment and with the right combination ofcomputation devices, a near-real time “walk” through thespacecraft can be achieved. Users can be trained, designstested against design basis accident scenarios, operationand maintenance procedures created and tested, etc. . . .This technique is powerful if the design remains constant.

III.F OperationsThe last well-defined design level in this paper is op-

erations. This tool is for operators and users to appro-priately respond to radiation events during the mission.For this level, an adjoint model again is the best solu-tion available because it utilizes the full transport solutionfrom the as-built design. Once the boundary condition isapplied to the adjoint solution, a quick determination ofthe proper action to take (prepared and rehearsed in thesimulation and training section) is available to the oper-ator and users. This is essential for long lunar stays andtrips to Mars.

It is assumed that the appropriate infrastructure isavailable. Today, for LEO operations, NASA relies onsatellites like SOHO and GOES and other informationto help determine appropriate actions. In the Martian


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environment, some of the infrastructure, like a “GOES”satellite, does not yet exist. Either that infrastructureneeds to be built, or other methods to gather the neededoperation’s information must be created.

III.G ResearchThis can be considered a catch-all category, but in

reality, it has a specific function. This level of tool canvary from a low fidelity tool looking at a particular inte-grated phenomenon to a high fidelity tool tracing particletrajectories through a material. The interface can also bevaried. The crux of this tool is that it is a tool set thatall the other levels rely on to be a test bed and methodcheckout on top of answering questions that a designerdoes not ask, such as what are the detailed shielding char-acteristics material XYZ. Also, the response functions ofthe occupants are developed and tested here before de-ployment in the design level tools. This is an importantcategory of tool. The VV&A level of this tool set is alsonot as high as the designer level tools. Here, a best es-timate value could be the answer sought to allow an es-timate of uncertainty for a method being deployed. Ofcourse, as the research tool matures, its uncertainty isbetter understood and its VV&A level increases until itis ready to be deployed for use by designers.

IV Simulation-Based AcquisitionNASA is utilizing the Simulation-Based Acquisition

(SBA) model for procurement. This model utilizes simu-lation of the mission to develop requirements for the pro-curement process. Therefore, a medium or high fidelitytool is not needed, but a low fidelity tool is the righttool for this task. The tool will be able to perform tradestudies and other tasks needed to generate data that canbe used to create a request for proposal, perform tradestudies, and analyze a design. NASA will be using theNExIOM15 data dictionary to establish communicationbetween the various tools to prepare for an acquisition.It is unclear at this time how much MDO will be per-formed if any. This process is just now being clarifiedwithin NASA so details are scarce and changing. How-ever, the SBA model will be used to acquire all hardwarefor Project Constellation (the current mission for lunarreturn and Mars).

V ConclusionIn conclusion, much physics and engineering work is

needed to fully define and complete these levels of pro-duction codes in this roadmap. These levels do not havesharp distinctions. They bleed from one level to the next.Stochastic and deterministic codes along with the requi-site cross sections and interfaces are needed to complete atool set that can be used to generate an optimized space-

craft to meet mission requirements – at all levels and inall disciplines. The proper radiation analysis tools in thehands of designers and managers can solve the engineer-ing problems needed to allow the lowest possible cost andsafe access to space for the future. However, the design-ers will need to help by telling the tool programmers whatthe designers need to perform their design tasks. Thesetools also lay the groundwork for when the science of ra-diation damage is better understood and models createdto determine risk from the radiation fields that better re-flect the occupant.

References1 Radiation Effects and Analysis Website at

http://radhome.gsfc.nasa.gov/top.htm

2 NASA-STD-3000, Section 5.7 Radiation. This de-fines the mission requirements for the Crew Ex-ploration Vehicle (CEV) and can be found athttp://msis.jsc.nasa.gov/.

3 W.L. Oberkampf, T.G. Trucano, C. Hirsch, Verifica-tion, Validation, and Predictive Capability in Com-putational Engineering and Physics, Sandia ReportSAND2003-3769, Unlimited Release, February 2003.

4 M. Hemsch, et. al., Verification, Validation, and Ac-creditation Practices, Incubator Institute Project forthe NASA Langley Research Center, October 2005.

5 http://sirest.larc.nasa.gov/, a web based radiationtool that includes, STS, ISS, Mars, Jupiter, and otherradiation assessment tools.

6 J.W. Wilson, F.F. Badavi, F.A. Cucinotta,J.L. Shinn, G.D. Badhwar, R. Silberberg, C.H. Tsao,L.W. Townsend, R.K. Tripathi, HZETRN: De-scription of a Free-Space Ion and NucleonTransport and Shielding Computer Program,NASA Technical Paper 3495, May 1995.

7 http://www.radiative.com/.

8 F. Gleicher, A.K. Prinja, Advanced Finite ElementDiscretizations for High-Energy Ion Transport , Trans-actions of the American Nuclear Society, Vol. 84, 92,2001.

9 J.E. Morel, A.K. Prinja, J.M. McGhee, T.A. Ware-ing, B.C. Franke, A Discretization Scheme for the 3-DContinuous-Scattering Operator , Transactions of theAmerican Nuclear Society, Vol. 89, pp. 360-362, 2003.

10 T.M. Miller, L.W. Townsend, T.A. Gabriel, T. Han-dler, Fragment Fluence Predictions Compared withHigh-Energy Heavy Ion Beam Laboratory Data, Trans-actions of the American Nuclear Society, Vol. 90, 2004.


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Figure 1: Nuclear Emulsion Effects of Constant LET versus Z

11 A. Haghighat, J.C. Wagner, Monte Carlo VarianceReduction with Deterministic Importance Function,Progress in Nuclear Energy, Vol. 42, No. 1, pp. 25-53, 2003.

12 M.A. Cooper, E.W. Larsen, Automated Weight Win-dows for Global Monte Carlo Particle Transport Cal-culations, Nuclear Science and Engineering,Vol. 137, pp. 1-13, 2001.

13 F.A. Cucinotta, Radiation Limits for Space Explo-

ration, Seminar, February 24, 2005.

14 Z. Wei, B.D. Ganapol, Reliability Quantification ofElectronics in the Space Environment , Space NuclearConference 05, San Diego, CA, June 2005.

15 NASA Exploration Information Ontology Model (NEx-IOM) Primer and Vision, ESMD-RQ-0025, BaselineVersion, February 16, 2005.


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Figure 2: Magnetic Field for the Earth

Figure 3: Magnetic Field for the Earth


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Figure 4: Energy Spectrum of Various Space Radiation Types

Figure 5: Abundance by Z Number and Energy Spectrum of GCR

engineering eﬀort needed to design spacecraft with

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