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This work presents a simple and useful project process model. The project model directly shows how a few basic parameters determine project duration and cost and how changes in these parameters can improve them. Project cost and duration can be traded-off by adjusting the work rate and staffing level. A project's duration and cost can be computed on the back of an envelope, with an engineering calculator, or in a computer spreadsheet. The project model can be simulated dynamically for further insight. The project model shows how and why projects can greatly exceed their expected duration and cost. Delays and rework requirements may create work feedback loops that increase cost and schedule in non-proportional and non-intuitive ways.

This paper considers system design and technology selection for the crew air and water recycling systems to be used in long duration human space exploration. The ultimate objective is to identify the air and water technologies likely to be used for the vision for space exploration and to suggest alternate technologies that should be developed. The approach is to conduct a preliminary systems engineering analysis, beginning with the Air and Water System (AWS) requirements and the system mass balance, and then to define the functional architecture, review the current International Space Station (ISS) technologies, and suggest alternate technologies.

The least expensive life support for brief human missions is direct supply of all water and oxygen from Earth without any recycling. The currently most advanced human life support system was designed for the International Space Station (ISS) and will use physicochemical systems to recycle water and oxygen. This paper compares physicochemical to direct supply air and water life support systems using Equivalent Mass (EM). EM breakeven dates and EM ratios show that physicochemical systems are more cost effective for longer mission durations.

Popular depictions of space exploration as well as government life support research programs have long assumed that future planetary bases would rely on small scale, closed ecological systems with crop plants producing food, water, and oxygen and with bioreactors recycling waste. In actuality, even the most advanced anticipated human life support systems will use physical/ chemical systems to recycle water and oxygen and will depend on food from Earth. This paper compares bioregenerative and physical/chemical life support systems using Equivalent System Mass (ESM), which gauges the relative cost of hardware based on its mass, volume, power, and cooling requirements. Bioregenerative systems are more feasible for longer missions, since they avoid the cost of continually supplying food.

This paper describes engineering rules of thumb for life support system design. One general design rule is that the longer the mission, the more the life support system should use regenerable technologies and recycling. A more specific rule is that, if plants supply more than about half the food, the plants will provide all the oxygen needed by the crew. There are many such design rules that can help in planning the analysis of life support systems or in assessing design concepts. These rules typically describe the results of steady state, “back of the envelope,” trade-off calculations. They are useful in suggesting plausible candidate life support system designs or approaches. Life support system engineers should consider the basic design rules and make quick steady state calculations as a guide before doing detailed design.

Future space life support systems may use crop plants to grow most of the crew’s food. A harvest failure can reduce the food available for future consumption. If the previously stored food is insufficient to last until the next harvest, the crew may go hungry. This paper considers how the overall food supply system should be designed to cope with food production failures. The food supply system for a mission will use grown food, or stored food, or both. The optimum food supply mix depends on the costs and failure probabilities of stored and grown food. A simple food system model assumes that either we obtain the nominal harvest or a failure occurs and no food is harvested. Given the probability that any particular harvest fails, it is easy to compute the expected number of failures and the total food shortfall over a mission.

The purpose of dynamic modeling and simulation of Advanced Life Support (ALS) systems is to help design them. Static steady state systems analysis provides basic information and is necessary to guide dynamic modeling, but static analysis is not sufficient to design and compare systems. ALS systems must respond to external input variations and internal off-nominal behavior. Buffer sizing, resupply scheduling, failure response, and control system design are aspects of dynamic system design. We develop two dynamic mass flow models and use them in simulations to evaluate systems issues, optimize designs, and make system design trades. One model is of nitrogen leakage in the space station, the other is of a waste processor failure in a regenerative life support system. Most systems analyses are concerned with optimizing the cost/benefit of a system at its nominal steady-state operating point. ALS analysis must go beyond the static steady state to include dynamic system design.

The decision to develop a particular life support technology or to select it for flight usually depends on the cost to develop and fly it. Other criteria such as performance, safety, reliability, crew time, and technical and schedule risk are considered, but cost is always an important factor. Because launch cost would account for much of the cost of a future planetary mission, and because launch cost is directly proportional to the mass launched, equivalent mass has been used instead of cost to select advanced life support technology. The equivalent mass of a life support system includes the estimated mass of the hardware and of the spacecraft pressurized volume, power supply, and cooling system that the hardware requires. The equivalent mass of a system is defined as the total payload launch mass needed to provide and support the system. An extension of equivalent mass, Equivalent System Mass (ESM), has been established for use in the Advanced Life Support project.

The Environmental Control and Life Support (ECLS) requirements to reach the International Space Station (ISS), the Moon, and Mars as part of the Vision for Space Exploration (VSE) are similar to the earlier ECLS requirements for Apollo, Space Shuttle, and ISS. It seems reasonable that the VSE life support designs will develop in the same way. The ECLS for spacecraft to reach ISS and the Moon can use the Shuttle and Apollo approaches. However, the long duration ECLS for the Moon base should be the same as for Mars, because the Moon will be the testbed for Mars. The ECLS for Mars could be similar to that of ISS, but it should be redesigned to incorporate lessons learned, to take advantage of twenty years technical progress, and to respond to the much more difficult launch mass and reliability requirements for Mars.

Exobiochemistry is the biochemistry of extraterrestrial life. It describes the potential energy and material basis of extraterrestrial life and is needed to guide the search for alien life. The diverse biochemistry of Earth indicates that a wide range of exobiochemistry is possible on other planets. Any exobiochemistry we discover will probably use the same energy sources as Earth's natural biochemistry - light, biological organic material, and more rarely abiotic chemicals. Extraterrestrial life will be based on familiar chemical principles and so will probably capture, store, and release energy using oxidation-reduction reactions similar to those found on Earth. Any extraterrestrial life must produce some chemical indication of its existence. Useful elements will be concentrated, stored, and recycled, altering their availability and isotopic composition.

Researchers in astrobiology should develop alternate concepts for the detection of extraterrestrial life. We should search for extraterrestrial ecology, exoecology, as well as for extraterrestrial biology, exobiology. Ecology describes the interactions of living things with their environment. All ecosystems are highly constrained by their environment and conform to well-known and inescapable system design principles. An ecology could exist wherever there is an energy source and living things can employ some method to capture, store, and use the available energy. Terrestrial ecosystems use energy sources including light, organic molecules, and, in thermal vents and elsewhere, simple inorganic molecules. Ecosystem behavior is controlled by matter and energy conservation laws and is described by dynamic systems theory. Typically in an ecosystem different molecules are not in chemical equilibrium and scarce materials are conserved, stored, or recycled.

This study introduces several new concepts for suited EVA astronaut ingress/egress (departure and return) from a pressurized planetary surface habitat, based on use of a rear-entry suit and a suit lock or suitport. We provide insight into key operational aspects and integration issues, as well as the results of a requirements analysis and risk assessment of the concepts. The risk assessment included hazard analysis, hazard mitigation techniques, failure mode assessment, and operational risk assessment. Also included are performance and mass estimates for the egress concepts, and concepts for integration of the egress concepts with potential planetary habitat designs.

How much integrated system level test should be performed on a spacecraft before it is launched? Although sometimes system test is minimized, experience shows that systems level testing should be thorough and complete. Reducing subsystem testing is a less dangerous way to save cost, since it risks finding problems later in system test, while cutting systems test risks finding them even later on orbit. Human-rated spacecraft test planning is informal, subjective, and inconsistent, and its extent is often determined by the decision maker's risk tolerance, decision-making style, and long-term or short-term view. Decisions on what to test should be guided by an overall mission cost-benefit analysis, similar to the risk analysis used to guide development efforts.

This report considers crewmembers’ life support needs for air, water, and food in a long duration lunar surface base. It also considers requirements for washing and clean-up water, waste recycling, and the crew's use of air, water, and food during Extravehicular Activity (EVA). The life support mass flow is described, including the needs of the statistical average crewmember, the expected variation between crewmembers, and the potential range of the total crew's average requirements. To develop the lowest cost, most reliable life support system that meets the crew needs, we must understand how the requirements impose design constraints and cost drivers and provide options and opportunities. We also must be aware of the degree of flexibility and potential change in requirements as their costs and implementation become defined.

This paper considers the design of a Mars Transfer Vehicle (MTV) water processor. The Constellation Program has begun to consider the first human mission to Mars, and the MTV water processor is of special interest. Mars transit system design is not affected by Extra-Vehicular Activity (EVA) or In-Situ Resource utilization (ISRU). The total duration of Mars transit and return is relatively fixed at about four hundred days, while Mars and lunar surface stays can vary from a few days to many years. The Mars transit water processor will operate in zero gravity, like the International Space Station (ISS) Water Recovery System (WRS), so the ISS WRS design can serve as a reference baseline for the Mars transit system. The paper develops the MTV water requirements and considers the suitability of the ISS WRS for Mars transit. The ISS WRS meets MTV requirements and requires less mass than direct resupply for Mars transfer, but it has excess capacity for the requirements.

This paper considers the design of a life support system for transit to Mars and return to Earth. Because of the extremely high cost of launching mass to Mars, the Mars transit life support system must minimize the amount of oxygen, water, and food transported. The three basic ways to provide life support are to directly supply all oxygen and water, or to recycle them using physicochemical equipment, or to produce them incidentally while growing food using crop plants. Comparing the costs of these three approaches shows that physicochemical recycling of oxygen and water is least costly for a Mars transit mission. The long mission duration also requires that the Mars transit life support system have high reliability and maintainability. Mars transit life support cannot make use of planetary resources or gravity. It should be tested in space on the International Space Station (ISS).

We modeled BIO-Plex designs with separate or combined atmospheres and then simulated controlling the atmosphere composition. The BIO-Plex is the Bioregenerative Planetary Life Support Systems Test Complex, a large regenerative life support test facility under development at NASA Johnson Space Center. Although plants grow better at above-normal carbon dioxide levels, humans can tolerate even higher carbon dioxide levels. Incinerator exhaust has very high levels of carbon dioxide. An elaborate BIO-Plex design would maintain different atmospheres in the crew and plant chambers and isolate the incinerator exhaust in the airlock. This design option easily controls the crew and plant carbon dioxide levels but it uses many gas processors, buffers, and controllers. If all the crew’s food is grown inside BIO-Plex, all the carbon dioxide required by the plants can be supplied by crew respiration and the incineration of plant and food waste.

To facilitate analysis, Advanced Life Support (ALS) systems are often assumed to be linear and time invariant, but they usually have important nonlinear and dynamic aspects. This paper reviews nonlinear models applicable to ALS. Nonlinear dynamic behavior can be caused by time varying inputs, changes in system parameters, nonlinear system functions, closed loop feedback delays, and limits on buffer storage or processing rates. Dynamic models are usually cataloged according to the number of state variables. The simplest dynamic models are linear, using only integration, multiplication, addition, and subtraction of the state variables. A general linear model with only two state variables can produce all the possible dynamic behavior of linear systems with many state variables, including stability, oscillation, or exponential growth and decay. Linear systems can be described using mathematical analysis.

This paper considers the cost and benefit of planetary surface ExtraVehicular Activity (EVA) on the Moon and Mars. The Exploration Systems Architecture Study (ESAS) scenarios are used as a basis. The benefits of surface EVA depend on the number of sites visited, the total duration of EVA, and the maximum distance of exploration. The costs of EVA are measured by the total emplaced mass required to support a sortie mission or to establish and support a long term base. The later lunar sorties described in the ESAS have longer duration and use rovers not provided earlier, so they are more cost-effective in surface exploration. The planned permanent lunar base provides one-sixth the cost per EVA hour and a thirty percent lower cost per kilometer of explorable distance, but exploration is limited to a single site. There is an important trade-off between the number of different sites explored and the total time spent in surface exploration.

Space power systems include power source, storage, and management subsystems. In current crewed spacecraft designs, solar cells are the power source, batteries provide storage, and the crew performs any required load scheduling. For future crewed planetary surface systems using Advanced Life Support, we assume that plants will be grown to produce much of the crew's food and that nuclear power will be employed. Battery storage is much more costly than nuclear power capacity and so is not likely to be provided. We investigate scheduling of power demands to reduce the required peak power generating capacity. The peak to average power ratio is a good measure of power capacity efficiency. We can easily schedule power demands to reduce the peak power below the potential maximum, but simple scheduling rules may not achieve the lowest possible peak to average power ratio.