Bøger af N a S a

Statistics is the science of the collection, organization, analysis, and interpretation of numerical data, especially the analysis of population characteristics by inference from sampling. In engineering work this includes such different tasks as predicting the reliability of space launch vehicles and subsystems, lifetime analysis of spacecraft system components, failure analysis, and tolerance limits. A common engineering definition of statistics states that statistics is the science of guiding decisions in the face of uncertainties. An earlier definition was statistics is the science of making decisions in the face of uncertainties, but the verb making has been moderated to guiding. Statistical procedures can vary from the drawing and assessment of a few simple graphs to carrying out very complex mathematical analysis with the use of computers; in any application, however, there is the essential underlying influence of "chance." Whether some natural phenomenon is being observed or a scientific experiment is being carried out, the analysis will be statistical if it is impossible to predict the data exactly with certainty. The theory of probability had, strangely enough, a clearly recognizable and rather definitive start. It occurred in France in 1654. The French nobleman Chevalier de Mere had reasoned falsely that the probability of getting at least one six with 4 throws of a single die was the same as the probability of getting at least one "double six" in 24 throws of a pair of dice. This misconception gave rise to a correspondence between the French mathematician Blaise Pascal (1623-1662) and his mathematician friend Pierre Fermat (1601-1665) to whom he wrote: "Monsieur le Chevalier de Mere is very bright, but he is not a mathematician, and that, as you know, is a very serious defect."CONTENTS: Introduction - Preliminary Remarks - Statistical Potpourri - Measurement Scales - Probability and Set TheoryProbabilityDefinitions of Probability - Combinatorial Analysis (Counting Techniques) - Basic Laws of Probability - Probability Distributions - Distribution (Population) Parameters - Chebyshev's Theorem - Special Discrete Probability Functions - Special Continuous Distributions - Joint Distribution Functions - Mathematical Expectation - Functions of Random Variables - Central Limit Theorem (Normal Convergence Theorem) - Simulation (Monte Carlo Methods)StatisticEstimation Theory - Point Estimation - Sampling Distributions - Interval Estimation - Tolerance Limits - Hypothesis/Significance Testing - Curve Fitting, Regression, and Correlation - Goodness-of-Fit Tests - Quality Control - Reliability and Life Testing - Error Propagation LawBibliography

It is a beginning. Over forty-five years have elapsed since the X-15 was conceived; 40 since it first flew. And 31 since the program ended. Although it is usually heralded as the most productive flight research program ever undertaken, no serious history has been con-assembled to capture its design, development, operations, and lessons. This monograph is the first step towards that history. Not that a great deal has not previously been written about the X-15, because it has. But most of it has been limited to specific aspects of the program; pilot's stories, experiments, lessons-learned, etc. But with the exception of Robert S. Houston's history published by the Wright Air Development Center in 1958, and later included in the Air Force History Office's Hypersonic Revolution, no one has attempted to tell the entire story. And the WADC history is taken entirely from the Air Force perspective, with small mention of the other contributors. In 1954 the X-1 series had just broken Mach 2.5. The aircraft that would become the X-15 was being designed to attain Mach 6, and to fly at the edges of space. It would be accomplished without the use of digital computers, video teleconferencing, the internet, or email. It would, however, come at a terrible financial cost-over 30 times the original estimate. The X-15 would ultimately exceed all of its original performance goals. Instead of Mach 6 and 250,000 feet, the program would record Mach 6.7 and 354,200 feet. And compared against other research (and even operational) aircraft of the era, the X-15 was remarkably safe. Several pilots would get banged up; Jack McKay seriously so, although he would return from his injuries to fly 22 more X-15 flights. Tragically, Major Michael J. Adams would be killed on Flight 191, the only fatality of the program. Unfortunately due to the absence of a subsequent hypersonic mission, aeronautical applications of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is unlikely that many of the actual hardware lessons are still applicable. That being said, the lessons learned from hypersonic modeling, simulation, and the insight gained by being able to evaluate actual X-15 flight research against wind tunnel and predicted results, greatly expanded the confidence of researchers. This allowed the development of Space Shuttle to proceed much smoother than would otherwise have been possible. In space, however, the X-15 contributed to both Apollo and Space Shuttle. It is interesting to note that when the X-15 was conceived, there were many that believed its space-oriented aspects should be removed from the program since human space travel was postulated to be many decades in the future. Perhaps the major contribution was the final elimination of a spray-on ablator as a possible thermal protection system for Space Shuttle. This would likely have happened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the ablators.

For the wide variety of structural types subject to significant dynamic loads, increasingly rigorous performance requirements dictate a derivative requirement for improvements in the technologies for controlling dynamic response. Aerospace structures, subject to stringent static as well as dynamic response requirements and characterized by complex behaviors including closely spaced and often coupled modes, provide one example of a class of structures requiring improved control technologies. Similarly, for many types of civil structures --e.g., cable-stayed and suspension bridges-- also characterized by stringent performance requirements and complex structural behaviors. Control of dynamic response dictates improved design (and retrofit) approaches. Also for many mechanical systems, e.g., medical devices -- performance is constrained by limits on the control of dynamic response. In designing for dynamic loads, structural and mechanical engineers have several techniques at their disposal, including passive damping, isolation, active and semi-active control. The study presented here focuses on a novel passive damping technology based on exploiting the unique properties of shape-memory materials (SMM). SMMs are a family of materials displaying a characteristic thermoelastic phase transformation which itself is the basis of two important mechanical hystereses -- shape-memory effect (SEE) and superelastic effect (SEE). As supported by this study, SME and SEE each provides an energy dissipation mechanism with extraordinarily attractive properties for damping applications. As elaborated below, the properties of SMM damping devices include:hysteretic damping with a diversity of distinct force/deflection hysteretic behaviorshighly reliable energy dissipation based on a precisely repeatable solid state phase transformationvery high damping per unit mass and per unit volume of SMM materialrelative insensitivity to temperature variation over wide range of operating temperaturesessentially zero creep over range of operating temperatures encountered in most space and all civil structureswide range of design operating temperaturesexcellent fatigue and corrosion resistancepure hysteretic damping --i.e., energy dissipation is frequency independent

This report grew out of a 10-week program in engineering systems design held at Stanford University and the Ames Research Center of the National Aeronautics and Space Administration during the summer of 1975. The project brought together nineteen professors of engineering, physical science, social science, and architecture, and two co-directors. This group worked for ten weeks to construct a convincing picture of how people might permanently sustain life in space on a large scale. The goal of the summer study was to design a system for the colonization of space. This report, like the design itself, is intended to be as technologically complete and sound as it could be made in ten weeks, but it is also meant for a readership beyond that of the aerospace community. Because the idea of colonizing space has awakened strong public interest, the report is written to be understood by the educated public and specialists in other fields. It also includes considerable background material.The technical director, Gerard K. O'Neill of Princeton University, made essential contributions by providing information based on his notes and calculations from six years of prior work on space colonization and by carefully reviewing the technical aspects of the study.