2003-01- 2631 Clarifying Objectives and Results of Equivalent System Mass Analyses for Advanced Life Support Julie A. Levri NASA Ames Research Center Alan E. Drysdale Boeing Copyright 0 2003 SAE International ABSTRACT INTRODUCTION This paper discusses some of the analytical decisions The ALS ESM Guidelines document (Levri et a/., 2003) that an investigator must make during the course of a life was drafted to provide detailed instructional material for support system trade study. researchers who are performing ESM evaluations. The document was developed under the Systems Integration, Equivalent System Mass (ESM) is often applied to Modeling and Analysis (SIMA) Element of the ALS evaluate trade study options in the Advanced Life Program. It provides a definition of ESM, describes how Support (ALS) Program. ESM can be used to identify to calculate ESM, and discusses interpretation of ESM which of several options that meet all requirements are results. most likely to have lowest cost. It can also be used to identify which of the many interacting parts of a life ESM is used as a measure of transportation cost in ALS support system have the greatest impact and sensitivity trade studies. An ESM trade study may be performed to to assumptions. compare various technologies, hardware devices, hardware configurations andlor control approaches. This paper summarizes recommendations made in the Because the cost to transport a payload is proportional to newly developed ALS ESM Guidelines Document and the mass of that payload, a mass-based measure such expands on some of the issues relating to trade studies as ESM is often used to quantify the cost of the life that involve ESM. In particular, the following three points support system and associated infrastructure. An ESM are expounded: value is the sum of the life support system mass and appropriate fractions of supporting system masses, including pressurized volume, power generation, cooling, 1) The importance of objectives: Analysis objectives drive the approach to any trade study, including and crewtime, for maintaining a specified crew over the identification of assumptions, selection of characteristics duration of a specified mission. to compare in the analysis, and the most appropriate techniques for reflecting those characteristics. This paper does not discuss the ESM equation and parameter definition. (For the reader’s interest, the ESM 2) The importance of results inferprefafion: The accuracy equation and parameter definitions are provided in Levri desired in the results depends upon the analysis et al., 2003.) Rather, this paper addresses three issues objectives, whereas the realized accuracy is determined of interest that are considered in the ESM Guidelines by the data quality and degree of detail in analysis Document: the importance of objectives, the importance methods. of results interpretation, and the importance of analysis documentation. Clear objectives are necessary for 3) The importance of analysis documentation: identification of the questions of interest and related Documentation of assumptions and data modifications is assumptions, identification of system characteristics to critical for effective peer evaluation of any trade study. reflect in the analysis, and definition of the system to the ESM results are analysis-specific and should always be appropriate extent and level of detaii. Results reported in context, rather than as solitary values. For interpretation and analysis documentation is critical this reason, results reporting should be done with because the author of the study is the person in the best adequate rigor to allow for verification by other position to evaluate the results, and the only one in full researchers. knowledge of all the details. Althotigh these are * important issues for all trade studies, this paper Ideally, analysis objectives are defined at the inception of discusses that importance as it applies to trade studies the study, in an appropriate level of detail. However, the that involve ESM in some manner. need for further clarification of the objectives often arises during the course of the research. Consequently, a trade When it is feasible and appropriate, single metrics, such study can be iterative. With experience, an analyst can as ESM, can facilitate comparison. For ESM to be gain foresight into the proper level of detail that is applied appropriately, the trade study options must first required in defining the study objectives. As the details of meet some common prerequisites. Only options that trade options emerge during the study, the objectives of satisfy those prerequisites should be included in the the study may require re-clarification. study. After the trade options have been identified, some characteristics may require comparison, using ESM. Study objectives drive decisions on the mission of However, it may not be possible to normalize all interest, selecting characteristics to reflect in the parameters in a study to a single ESM value. For analysis, defining the appropriate system, and applying example, when buying a car, most considerations can be data appropriately, as discussed below. reduced to some metric like cost per passenger mile; however, issues such as aesthetic appearance and IDENTIFYING THE MISSION OF INTEREST AND safety may not be easily lumped into that same metric. RELATED ASSU M PTI0 NS Thus, some characteristics can either be examined as prerequisites, or compared in some other manner. The results of an ESM analysis depend on the assump?ions made about the operrting environment, the THE IMPORTANCE OF OBJECTIVES subsystem of interest, and the surrounding system. Consequently, an ESM analysis must be done with a Analysis objectives should drive all facets of the ESM particular mission and set of assumptions in mind. computation. Objectives should be clearly defined in order for the investigator to determine the mission of The ALS Reference Missions Document (Stafford ef a/., interest and system characteristics to capture in the 2001) is one source that analysts can use for selection of study, to define the appropriate system, and to a particular reference mission for consideration in a trade appropriately apply data. study. Indeed, if the mission of interest is addressed in the ALS Reference Missions Document (RMD), it is Analyses should address important questions. Important recommend that the RMD assumptions be used in the questions are generally ones that make a difference to study baseline. If not, these missions can be used as a decisions that need to be made in the near future. starting point and changes to that mission can be Currently, there is a lot of interest in technology selection, documented. thus that is an appropriate area to investigate. However, the technologies must be significantly different for the When making trade comparisons, the analyst must answer to be of much value. For example, if two consider the suitability of each trade study option in a technologies make a difference to the overall mission mission. For example, if comparing two technologies, the cost of I%it m,a kes little difference which one is functions that the technology performs should be selected. desirable for that mission scenario. As an explicit (and relatively obvious) example, it would be inappropriate to The quantity of interest in an ESM analysis is compare two soybean-processing devices in reference to comparison of the total system impacf of trade options, a mission in which food is entirely prepackaged and which is often more complicated than simple accounting ready to eat. (In such a scenario, a food-processing of hardware items. Determination of the total system device would never be needed.) However, the suitability impact involves defining the system of interest to the of trade options in particular missions is not always clear. appropriate extent and level of detail to comprehensively In fact, the objectives of a trade study may actually be to capture cost impacts of trade options. In order to capture identify the appropriateness of a trade option to a the cost impacts of the trade options on the entire life particular mission, based upon the total system impact of support system, the analyst must consider the important that option. For example, a valid study could consider interfaces to the system of interest. This concept applies the cost of growing soybeans to provide the crew with to all systems involved in a mission, even if some costs soybean-based fresh foods on a mission, compared to are traditionally considered to be out of the scope of the not growing soybeans and providing the crew with other life support system. food types. Although technically not illustrative of ESM, some Top-level assumptions related to the missions in the ALS computations other than the total system impact can be Reference Missions Document are documented in the interesting. For example, it may be useful to know that ALS Baseline Values and Assumptions Document the equipment mass of one trade study option is 30% (BVAD) (Hanford 2002). The BVAD also identifies heavier than an alternative, even if that computation possible assumptions for missions that are not included does not reflect the total system impact. in the RMD. Such assumptions include, for example, the . . number of crew members, number of visits to each site, For an example of prerequisite characteristics, consider mission duration, habitable volume, infrastructure costs, an analysis whose objective is to identify the lowest ESM crewmember body mass, and typical metabolic loads. technology that provides a certain level of moisture When deemed applicable by the analyst, the values content reduction (water activity) in wasted food provided in the BVAD may be applied to trade studies. materials. Thus, only technologies that can provide that (One motivation for developing the BVAD is to provide specific function, at the required level of moisture guidance to the ALS Community on reasonable removal (or greater) should be considered in the assumptions for various missions. Use of a common set an a lysis. of assumptions facilitates identification of the reasons for differences in trade study results.) Notice that the objective in the prior example strongly drives the analysis approach. For example, if the reason In particular, infrastructure costs, or “equivalency factors” for removing the water from the wasted food is to drive the relative impacts of mass, volume, power, stabilize the food materials before being stored as cooling and crewtime needs on the computed cost. As a wastes, then the important prerequisite is that the water result, the analyst should make efforts to use the most be removed down to a certain water activity from that appropriate and reliable information for the equivalency material. However, consider the case where water factors. With continued research, collection of more removal from the wasted food was intended to not only accurate data will hopefully result in equivalency factors stabilize the waste but also to recover water for later of greater accuracy in the BVAD. consumption by the crew. In that case, the objectives of the study may be to determine the lowest ESM option In addition to the top-level mission assumptions, notions that removes water to a certain level and at a certain about the details of system hardware, configuration and purity. Also notice that the objectives of the prior control are inherently made throughout a trade study. As example were to “identify the lowest ESM technology...”. the analyst gains knowledge of the details of operation of By declaring this in the objective, the study considers the various subsystems within the system of interest, power needs, logistics, crewtime, etc., not just moisture initial assumptions may require revision. All assumptions content removal, and may address issues such as (both top-level and more detailed) should be described alternative sources of water, such as availability of water (and well organized) throughout the trade study from local materials. documentation. As this can be quite arduous, effort should be focused on the issues that most affect the However, comparable function might not be the only results. required characteristic. For example, if the analysis is being done to evaluate technologies for water recovery SELECTING SYSTEM CHARACTERISTICS TO from wasted food for a low-Earth orbit mission, then any REFLECT IN THE ANALYSIS implementations’ of technologies that are incompatible with microgravity should be eliminated from the study. Based upon the analysis objectives, the analyst must Similar prerequisites may exist for performance, determine which characteristics should be captured in availability, safety, radiation susceptibility, noise levels the trade study. Characteristics of interest might be and other characteristics, depending on the analysis based upon function, availability, safety, gravity objectives and the investigator’s judgement. dependence, radiation susceptibility, noise levels, or a variety of other attributes. This determination will depend The prerequisites for some characteristics may be on the particular study, and often an initial trade study exceeded in some trade options. Generally, there is no needs to be made in order to develop a methodology and value in exceeding the performance requirements. identify what assumptions or data are needed. Thus, it However, depending on the objectives of the study, in can be necessary to iterate analysis steps to some cases, the benefits (or detriments) of exceeding appropriately represent the most significant those prerequisites should be quantified or qualified. In characteristics. the example in the previous paragraph, the water recovery efficiency (percentage of water recovered) may The investigator must then determine the methods by be greater in one trade option than in another. The which those characteristics will be reflected. difference (if in fact beneficial or detrimental) of Characteristics of interest may be considered exceeding this requirement might be reflected in an ESM prerequisites for inclusion in the study, and/or they may be compared (quantitatively and/or qualitatively) between 1 Most technologies are not microgravity sensitive. trade study options. Note that qualitative comparisons However, particular implementations of technologies are should only be used when there is inadequate data often microgravity sensitive. For example, a hardware available for a quantitative comparison. There is a device that uses gravity flow might be fitted with danger in making biased decisions based on qualitative appropriate pumps to make it microgravity compatible. comparisons of critical issues. Thus, it may be important for an analyst to consider alternative implementations of a techno!ogy in a trade study. analysis, by some other quantitative means, or in a computation, and define the system accordingly, so that qualitative manner in the study. the system can be sized appropriately. Care should be taken that a significant interface has not been neglected. Some characteristics that are not prerequisites in the trade study may also require comparison. For example, This section presents brief examples to illustrate system assume that an analyst decides that comparable definition for ESM purposes. For the purposes of functionality is the only prerequisite for including a Advanced Life Support, the system extent may range technology in an evaluation for a Mars surface mission. If from the entire life support system and interfaces to any the specific function required is water recovery from subset thereof. inedible plant materials, then many different technologies, including heat-drying, freeze-drying, The system should be defined to the appropriate extent. composting, and incineration may be included in the The analyst should consider any portion of the life analysis. However, the above-mentioned options may be support system that has a significant effect upon, or is very different in terms of availability, safety, radiation significantly affected by the trade options. For example, susceptibility, noise levels, waste stabilization consider a comparison between the total system impacts capabilities, and other characteristics. If such differences of growing 20m2 of salad crop versus growing 20m2 of are expected by the investigator to be important in potato crop. In addition to the differences in crop selection between trade options, then those differences production specifics, the evaluation should include should be compared in the study. differences outside of the biomass production system that have a significant impact on study results. A proper For any characteristics that will be compared, the analyst comparison of the two options, would involve must decide upon a means of comparison. For a general development of an entire dietary concept, and example, after considering the type and quality of specification of how all parts of the diet would be analysis data that is available, the analyst may conclude satisfied. The potato crop option may also be more that an ESM evaluation would reflect differences in effective in generating oxygen, removing carbon dioxide launch costs between two trade options, given functional and producing food energy than the salad crop option, and performance requirements. However, the analyst resulting in sizing differences in air revitalization might conclude that availability, safety and other equipment and prepackaged food stores. Additionally, differences should not or cannot be reflected in the ESM different quantities and compositions of nutrient solution evaluation. Such a decision may be made for reasons of may be needed for the different crop options, which may data inadequacy, uncertainty about flight requirements impact requirements for shipping or recycling of for those characteristics, or the availability of a more nutrients. Depending on the analyst’s assumptions about appropriate quantification method. the role and operation of the water recovery system (WRS), the WRS may require resizing between trade After determining the characteristics of interest and the options. The analyst’s decision of which subsystems to means by which to capture those characteristics in the include when defining the system for study depends on study, the system may be defined to the appropriate the analysis objectives and the investigator’s judgement. extent and level of detail. The system should be defined to the appropriate level of DEFINING THE SYSTEM TO THE APPROPRIATE detail. In the comparison of salad and potato crops, EXTENT AND LEVEL OF DETAIL issues such as palatability differences and differences in crew psychological benefits between the two options may The analyst should define the system to the extent and be difficult to reflect in the ESM computation. In that level of detail necessary to capture the total system case, if considered by the analyst to be important, such impact of the trade options. However, quantification of differences should be addressed, qualitatively and/or the “total” system impact often requires more resources quantitatively, elsewhere in the study. Some differences, (time and money) than are available for an analysis. In such as the sensitivity of the different crop types to such cases, the analyst must use judgement to system perturbations, may be, in theory, quantifiable in determine the most important characteristics to reflect, the ESM computation. However, the data necessary for by the most effective and appropriate means. Again, the such quantification may not be available. If inadequate decisions made by the analyst must be driven by the data exists for quantifying critical differences between specific study objectives. options, then those differences should be discussed qualitatively in some other manner in the study. In order to minimize the number of characteristics that require In general, the ESM for the entire life support system should be calculated. It may, however, only be investigation, a preliminary sensitivity analysis to identify necessary to calculate ESM for a portion of the life the change in outcome per change in parameter values can aid in identifying the most critical study parameters. support system, if the rest of the system remains identical, for all ESM-quantified characteristics, between trade study options. Thus, the analyst must determine Regardless of the system definition approach, a the characteristics that will be reflected by an ESM justification of the system extent and level of detail should be provided in the analysis report, so that the environment of the mission and any other anticipated analysis choices can be scrutinized. Such choices should mission requirements. be in harmony with the objectives of the analysis. For example, launch forces and the level of gravity APPROPRIATE APPLICATION OF DATA during different phases of a mission will affect equipment structural strength requirements, and may drive Raw data provided by researchers and technology modification of processes to accommodate differences in developers may require some modification in order to fit phase separation, convection and other characteristics. into the context of a particular ESM evaluation. Data As another example, for a Mars surface mission, modifications may include, but are not limited to, equipment exposed to the Mars surface may require adjustment for a different development state (notably modification for radiation conditions and operation during flight ready), adjustment for environment, and/or system dust storms. Trade option assumptions may also need to scaling. Although development state adjustment and accommodate constraints on the release of materials to system scaling are the most common types of data the Martian environment due to planetary protection modification in an ESM analysis, other types of concerns. Additionally, in order to satisfy a requirement modification may be necessary. All data modifications on the degree of risk to the crew equivalent to that of should be explained and quantified in the analysis report. Shuttle or International Space Station missions, equipment for a Mars surface (or any longer-term, more Adiustment for Development State distant mission) may require greater system reliability. Depending upon the objectives of the analysis, some Probably the most difficult issue in comparing flight ready data may require adjustment for the appropriate options with developmental options is the amount of development state. In ALS trade studies, the flight-ready detail available. Flight equipment is by definition development state is typically of greatest interest for complete, thus necessary data is available. ESM evaluations. As an alternative to modifying data for Developmental equipment, however, might have no data a flight-ready development state, an analyst may state on maintenance costs or crew time, and may not have all assumptions that flight equipment is similar to some the pieces necessary for safely operating the equipment earlier design stage Such assumptions are often in a space environment. For example, a bench-top necessary, since the data needed for development state electrolysis unit might not have all of the sensors and adjustments is often unavailable. safety equipment required to prevent - a hydrogen explosion. It might not have been run long enough to For trade study comparisons, all equipment data should know the lifetime of the consumables and spares. be representative of comparable development states. For example, equipment in the infancy of development The analyst must judge the importance of development should not be compared with equipment for a different* state adjustments in the reflection of trade study options technology at a flight-ready state. Adjusting data to a against the availability of data and the analysis resources future development state often requires that the needed to make such modifications. Requirements of investigator make assumptions in order to modify values future missions are (by definition) uncertain, and the or to estimate missing values. All such assumptions and information needed to make data adjustments is often their reasoning should be thoroughly documented so that unavailable. For example, if a failure modes and effects the analysis results can be understood in the appropriate analysis has not been conducted on a system, then the context. analyst will have little knowledge of the types of modifications needed to bring a system to a certain level For example, in determining a flight-ready development of reliability. The analyst may also not have knowledge of the degree of reliability/availability that will be required for state ESM, the analyst may predict that advances through R&TD will result in improved design future missions. As time progresses and future missions come closer in time, some mission requirements will be sophistication such as material types, automation, and developed. However, in the current state of mission processor efficiency, thereby reducing the flight-ready ESM. Similarly, other critical characteristics may uncertainty, an analyst must anticipate the requirements necessitate modification of the raw data to appropriately of future missions. Thus, an investigator may choose to capture those qualities in the analysis. qualitatively discuss necessary data alterations, rather than attempt to quantify the modifications. Regardless of the approach, all data modifications deemed appropriate To adjust raw data for a flight-ready development state, for the study should be clearly explained in the analysis in addition to improvements in the technology during documentation. development, the investigator should account for the System Sizinq and Data Scaling 2 However, for the purposes of forecasting changes in characteristics for a particular piece of equipment, it is The analyst must determine the proper hardware sizes valid to compare data for an early development state with for the mission scenario of interest for the system that data for a later development state. has been deemed appropriate for the study. Hardware is has been steady-state calculations of daily loads, under commonly sized according to a characteristic parameter. nominal operation. Lacking other information, throughput, or mass flowrate through the equipment, is often a good correlating Once the appropriate sizing parameter values have been parameter. For example, in sizing an oxygen generation determined, the investigator should scale the hardware device, a typical approach is to multiply the number of raw data to those values. For example, a technology crewmembers by the historical, individual, average developer may provide hardware raw data for a oxygen demand. laboratory prototype that processes only a fraction of the flow that would require processing in an actual mission The level of detail necessary for defining the system of or test bed. Thus the hardware data must be scaled up interest determines the level of detail necessary in the to accommodate the desired mass flowrate (the sizing system-sizing effort. In this respect, the investigator’s parameter). An industrial process, on the other hand, is judgement should be used to determine which material likely to be much larger than needed for space missions compounds should be included in the sizing effort. For and must be scaled down. If an objective of the analysis example, if the analyst deems it necessary to compare is to compare functionally similar processors for a mineral requirement differences in biological systems, specific mass throughput (or other sizing parameter), then the sizing effort must consider the minerals of raw data scaling is necessary. interest. However, if a material compound has a relatively minor effect on system sizing, then it may be A simple approach is to scale raw data linearly with appropriate to exclude that particular compound from the respect to the sizing parameter. However, more accurate sizing effort. scaling might be obtained by using scaling factors, which are not necessarily linear, provided by the technology Depending on the objectives of the analysis, the developer or by using component-specific scaling factors approach to system sizing may range from steady-state that are standard to the chemical industry. (The reader is to transient conditions. In other words, system sizing referred to Yeh, et. a/., 2001, for examples of scaling efforts may lie anywhere in between (or include) these factors used in the Chemical Engineering Industry.) two approaches. A steady-state mass balance requires less analysis effort but is less realistic than the more As a simplification, the entire piece of hardware can be effort-intensive transient mass balance. Steady-state scaled to the same sizing parameter. However, because mass balances can be easily implemented using there can be components of a piece of hardware (e.g. spreadsheet software, whereas transient mass balances controls logic) that are not dependent on a sizing are most easily performed through the use of transient parameter (such as mass flowrate), more accurate sizing simulation tools. If batch or semi-batch processors exist parameters should be used for those components, if in the system; an alternative between resource-intensive readily available. Whatever the approach, sizing transient simulation and (possibly) inaccurate steady- parameter values, scaling factors and associated state estimates may be necessary. In such a case, the assumptions should be adequately explained in the analyst might choose to modify steady-state flowrates to analysis documentation. agree with the batch or semi-batch nature of the processors. Thus, if a steady-state mass balance is used THE IMPORTANCE OF RESULTS but the analyst knows that an incinerator (for example) I NTERPR ETATION will actually be operating in semi-batch mode, then the analyst may modify the mass balance results for the To legitimately implement ESM results in a selection incinerator to reflect that knowledge. This may mean process, the results must be more accurate than the estimating modifications for the mass flowrate of degree of separation between option results. Thus, a materials into the semi-batch mode incinerator and for rough calculation may be adequate to rank two options if the material containment that is needed prior to the result values are grossly different from each other. In incineration. other cases, a high level of accuracy is required to make comparisons of options that have very similar ESM In this same vein, the investigator should decide whether values. If the results of an ESM comparison are too close to size the system for solely nominal (no fault) operation, to make a judgement, then both options may be equally or if the system should be sized for specific off-nominal good from the ESM perspective, and other issues (such events (faults). Thus, system sizing possibilities span a as TRL, robustness and/or simplicity/complexity) are space bounded by 1) steady-state sizing with nominal likely to drive technology selection. operation, 2) steady-state sizing with off-nominal events, 3) transient sizing with nominal operation, and 4) The desirable degree of results accuracy can depend transient sizing with off-nominal events. For each upon the analysis objectives. Because of the individual study, the analyst’s judgement should be used compounding nature of error in analyses, if study to determine the most appropriate sizing tactics. The objectives necessitate a very detailed analysis involving most common approach to date for system sizing efforts large amounts of data, then a high degree of accuracy in data and methods is necessary. However, if the J objectives necessitate an analysis of a low level of detail, ESM results are analysis-specific and should always be then the required degree of data and methods accuracy reported in context, rather than as solitary values. For might be reduced. this reason, results reporting should be done with adequate rigor to allow for verification by other Researchers that provide data for an ESM evaluation researchers. All input data, sources, analysis may execute various levels of rigor in the data collection approaches and critical assumptions made on the process. In addition, if data is obtained from hardware, configuration, and control approaches in the documentation, rather than directly from a researcher, system should be clearly stated in reporting the error propagation in data values can be a concern. On evaluation results, so that the results can be considered the other hand, data from sources with a very rigorous in context. Even if the ALS WAD is used to determine review process can be quite reliable. Consequently, an values for an analysis, the analysis documentation analyst may be faced with data from a range of sources should identify which values were applied and explain the and various degrees of accuracy. Because the accuracy implied assumptions. of data feeds into the confidence of ESM results, the investigator should have at least a basic impression of Some of the decisions that must be explained in the data quality. Confidence in results is also affected by results documentation include the following material, and decisions affecting the rigor of development state a//a ssociated assumpfions,a s a minimum: adjustments and scaling efforts. The analyst should balance the degree of effort needed for adequate results 1. Description of analysis objectives. confidence with resource availability during each step of 2. Explanation of characteristics deemed worthy of the ESM evaluation process. examination (e.g. critical characteristics). 3. Explanations of which (if any) critical characteristics The degree of results separation that is necessary to are captured by ESM and which are reflected by conclude a significant difference (Le. judge one option some other quantitative or qualitative means. preferable over another) depends upon the degree of 4. Justification of the system extent and level of detail confidence in the data used and the degree of accuracy chosen for the ESM evaluation and other in the analysis methods. Data confidence and analysis evaluations. accuracy is analysis-specific. Therefore, the degree of 5. Input data values, their sources, and a brief results separation that is required to declare one option explanation of those values implemented. preferable to another is also analysis-specific. The 6. Justification of development state adjustments, investigator’s judgement should be used to determine sizing methods, and scaling factors, including the analysis-specific necessary degree of results quantification and references. separation. 7. A discussion of the expected accuracy of ESM (and ~ other) results and associated interpretation. If data accuracy is uncertain, a reasonable assumption is that an order of magnitude of separation (Le. factor of CONCLUSION 10) between ESM results is adequate to conclude a significant difference between options. A difference of a This paper discussed the importance of some of the few percent is likely to be sensitive to assumptions. analytical decisions that an investigator must make However, the degree of confidence in technology data during the course of a trade study. When applied generally improves as the technology is further appropriately, an ESM analysis can be a useful tool for developed. Thus, ESM comparisons of trade options at making choices between trade study options. However, advanced development stages may require less of a in cases where no conclusive decision can be drawn degree of separation between results in order to declare from an ESM analysis, the results may be useful in a significant difference between options. determining the most important cost drivers (mass, volume, power, cooling or crewtime) for a trade study THE IMPORTANCE OF ANALYSIS option. The analysis effort can also draw attention to the DOCUMENTATION need for possibly critical data that needs to be col!ected and/or verified. All assumptions (both top-level and more detailed), input data, sources, and analysis methods should be ACKNOWLEDGMENTS described (and well organized) throughout the trade study documentation. As this can be quite arduous, The authors acknowledge the co-authors on the ALS effort should be focused on the issues that most affect ESM Guidelines Document for their insight to these the results and add value to the final report. issues. The authors of the ALS ESM Guidelines Document are Julie Levri, Alan Drysdale, Michael Ewert, Sometimes the greatest value of an analysis can be the John Fisher, Anthony Hanford, John Hogan, Harry identification of superior or inferior input data and their Jones, Jitendra Joshi, and David Vaccari. sources. In this same vein, unreferenced data values can make a calculation suspect. . REFERENCES on Environmental Systems, SAE Technical Paper #2001-01-2304. 1. Hanford, Anthony J. and Ewert, Michael K. (2002) Advanced Life Support Baseline Values and CONTACT Assumptions Document, CTSD-ADV-484, JSC 47804, NASA Johnson Space Center, Houston, TX. Julie A. Levri 2. Levri, Julie A.; Drysdale, Alan E.; Ewert, Michael K.; NASA Ames Research Center Fisher, John W.; Hanford, Anthony J.; Hogan, John Mail Stop 239-8 A,; Jones, Harry W.; Joshi, Jitendra A.; and David A. 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Yeh, H.Y.; Jeng, Frank; Brown, Cheryl; Lin, Chin; and Ewert, Michael (2001) “Advanced Life Support SIMA: Systems, Integration, Modeling and Analysis ’‘ Sizing Analysis Tool” 31 International Conference WRS: Water Recovery System