Jeffrey H. Smith

Modeling muddles: Validation beyond the numbers

Abstract A nonnumerical methodology for estimating the validity of complex models is presented. The validity of such models was estimated qualitatively by identifying the arguments used by the interested parties associated with the model. The patterns of reasoning about the foundations of the model were identified through an analysis of the arguments defending and denying the validity of the model. The approach is illustrated by using the expected utility (EU) model as an example. The analysis revealed contrasting sets of pro-EU and con-EU model arguments. A fundamental difference in the forms of arguments was observed: The pro-EU model used broad arguments covering a variety of issues (top-down) and the con-EU model used specific empirical results to question the fundamental grounding of EU model theory (bottom-up). The approach holds promise for large complex models that are difficult, if not impossible, to validate quantitatively.

Preliminary system analysis of in situ resource utilization for Mars human exploration

We carried out a system analysis of processes for utilization of Mars resources to support human exploration of Mars by production of propellants and life-support consumables from indigenous resources. Seven ISRU processes were analyzed to determine mass, power and propellant storage volume requirements. The major elements of each process include CO/sub 2/ acquisition, chemical conversion, and storage of propellants. Based on two figures of merit [(a) the ratio of the mass of propellants that must be brought from Earth in a non-ISRU mission to the mass of the ISRU system, tanks and feedstocks that must be brought from Earth for a ISRU mission, and (b) the mission mass in LEO saved by use of ISRU] the most attractive process (by far) is one where indigenous Mars water is accessible and this is processed via Sabarier/electrolysis to methane and oxygen. These processes are technically relatively mature. Other processes with positive leverage involve reverse water gas shift and solid oxide electrolysis. These processes would be appropriate if accessible water is not available on Mars. However the technologies involved are still immature. Processes that require storage of large amounts of hydrogen were deemed infeasible because of power, volume and mass considerations. The critical interfacial issues with Mars are finding accessible sources of water and acquisition of CO/sub 2/ from the atmosphere. A technology development and demonstration program is proposed that hinges heavily on the search for accessible water. A roadmap summarizes the future steps needed to implement ISRU in the human mission architecture.

Comparison of Lunar Surface Mission Productivities as a Function of Mission Duration and Weighted Science Objectives

We formulate, model and analyze two proposed missions near the lunar south pole of different durations and scientific emphasis. Each mission is conducted by two teams of two astronauts riding in two pressurized rovers. Each team must remain within the same locality as the other team in case there is need for a rescue. Our analytical approach combines the best features of two analytic techniques, SciMax and HURON, to overcome difficulties of combinatorial explosiveness of the trade space, and to enable scheduling of “as much science as possible” rather than an inflexible set of experiments that may not make optimal use of available time. Weighting of individual science goals leads to different sets of scientific investigations and different amounts of time to be spent at the various sites. We demonstrate that increased rover driving speeds increases the amount of time available to spend at sites, and thus increases science productivity dramatically up to a point. Beyond that point, increasing driving speed improves productivity marginally due to the lack of availability of EVA time.

Methodology for Evaluating Modular Assembly of Large Space Platforms

This paper presents a methodology for analytically comparing approaches to modular assembly of large space platforms. The methodology combines a physical model of the modules, a life‐cycle cost model, and a risk model to capture influential trade‐offs. The physical model includes alternative module design characteristics, assembly time scenarios, alternative work systems (human, robotic), and infrastructures. A life‐cycle cost framework is defined to capture the benefits and costs of modular alternatives for single or multi‐mission (programmatic) applications. A probabilistic risk model to address launch and assembly risks is employed to capture uncertainties in launch, as well as in the assembly approaches and their complexity (number of assembly steps, module connections). An illustration of the tradeoffs between these models for a single mission is described using a 448kW Solar Electric Transport Vehicle (SETV) supporting a human lunar mission. The illustration was limited to the launch and assembly ph...

Reducing the Risk and Improving Mission Success for NASA's Human Mission to a Near-Earth Asteroid: How Many Robotic Surveyors?

NASAs recent attention and interest in sending a human mission to land on a Near-Earth asteroid raises the question of need for a robotic surveyor. This paper describes a Bayesian approach for comparing the productivity and cost-risk tradeoffs of sending (versus not sending) one or more robotic surveyor missions prior to a human mission to land on an asteroid. The probability of finding an asteroid suitable for landing was derived from an analysis of more than 1200 asteroids in order to define a quantitative estimate of suitability. The low cost of the surveyors relative to the human mission underlined the multi-surveyor strategy as relatively inexpensive insurance against the risks of encountering an unsuitable asteroid for landing on arrival by a human mission.

Reconciling Scientific Aspirations and Engineering Constraints for a Lunar Mission via Hyperdimensional Interpolation

With the goal of facilitating communication and cooperation between NASA’s scientific and engineering/design communities, a tool (in the form of a table) is developed which displays the majority of the architectural-parameter trade space under consideration for a given mission, enabling all interested parties to see which sets of constraints would be acceptable for a desired level of science or any other objective. A combination of computer optimization runs and interpolation is employed to enable calculation of the table within a reasonable period of time. Three alternative interpolation methods (radial basis function (RBF), kriging, and regression) are investigated. A hypothetical mission to the Moon’s Schrodinger crater serves as a test case. Kriging, with the lowest average error rate, is judged to be the best interpolation method for this case study, but the more consistent results of the regression method may be preferable in certain other circumstances.

Mobility productivity impacts on selection of lunar exploration architectures

The productivity of scientific exploration of the Moon and Mars has been significantly improved through the mobility of roving vehicles (rovers) since these vehicles allow scientists to conduct operations well beyond the immediate vicinity of the landing area. This paper reports on a quantitative approach developed to evaluate the productivity of alternative human and robot work-system alternatives for a lunar science mission. A graph-search approach for task planning was used for assigning human and robotic work systems to scientific tasks in order to evaluate the productivity of different mobility options. The results were used to identify the benefits and costs of alternative rover combinations in order to establish guidelines for the roles of the different vehicle types. Pressurized rovers displayed advantages over unpressurized rovers due to enhanced range and duration yielding more science productivity. Multiple pressurized rovers were found to be more productive than multiple unpressurized rovers.

Toward a systematic approach for selection of NASA technology portfolios

There is an important need for a consistent analytical foundation supporting the selection and monitoring of R&D tasks that support new system concepts that enable future NASA missions. This capability should be applicable at various degrees of abstraction, depending upon whether one is interested in formulation, development, or operations. It should also be applicable to a single project, a program comprised of a group of projects, an enterprise typically including multiple programs, and the overall agency itself. Emphasis here is on technology selection and new initiatives, but the same approach can be generalized to other applications, dealing, for example, with new system architectures, risk reduction, and task allocation among humans and machines. The purpose of this paper is to describe one such approach, which is in its early stages of implementation within NASA programs, and to discuss several illustrative examples.

The Space Station Freedom evolution-phase: Crew-EVA demand for robotic substitution by task primitive

Space Station Freedom represents a significant demand for automation and robotics services as substitutes for crew-performed extravehicular activities (crew-EVA). Results are reported from a study aimed at identifying this demand for crew-performed activities and the crew-task primitive distributions derived for input to future robotic substitution studies. Generic EVA tasks are developed from historical EVA mission timelines, and a set of 70 task primitives defined. The generic task activities are partitioned into task setup, kernel, and tear-down, with standardized task times and frequencies. These standardized times are coupled with inputs from numerous mission databases in a probablistic simulation to obtain estimates of total crew-EVA task time demand by crew task primitive. The use of probablistic model is found to be crucial for understanding, isolating, and addressing the large uncertainties in the EVA task kernels.<<ETX>>