Mechanical & Fluid Systems

On Release of Microbe-Laden Particles From Mars Landers

A paper presents a study in which rates of release of small particles from Mars lander spacecraft into the Martian atmosphere were estimated from first principles. Because such particles can consist of, or be laden with, terrestrial microbes, the study was undertaken to understand their potential for biological contamination of Mars. The study included taking account of forces and energies involved in adhesion of particles and of three mechanisms of dislodgement of particles from the surface of a Mars lander: wind shear, wind-driven impingement of suspended dust, and impingement of wind-driven local saltating sand particles. Wind shear was determined to be effective in dislodging only particles larger than about 10 microns and would probably be of limited interest because such large particles could be removed by preflight cleaning of the spacecraft and their number on the launched spacecraft would thus be relatively small. Dislodgement by wind-driven dust was found to be characterized by an adhesion half-life of the order of 10,000 years — judged to be too long to be of concern. Dislodgement by saltating sand particles, including skirts of dust devils, was found to be of potential importance, depending on the sizes of the spacecraft-attached particles and characteristics of both Mars sand-particle and spacecraft surfaces.

Posted in: Briefs, TSP, Mechanical Components, Mechanics

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Stable Satellite Orbits for Global Coverage of the Moon

A document proposes a constellation of spacecraft to be placed in orbit around the Moon to provide navigation and communication services with global coverage required for exploration of the Moon. There would be six spacecraft in inclined elliptical orbits: three in each of two orthogonal orbital planes, suggestive of a linked-chain configuration. The orbits have been chosen to (1) provide 99.999-percent global coverage for ten years and (2) to be stable under perturbation by Earth gravitation and solar-radiation pressure, so that no deterministic firing of thrusters would be needed to maintain the orbits. However, a minor amount of orbit control might be needed to correct for such unmodeled effects as outgassing of the spacecraft.

Posted in: Briefs, TSP, Mechanical Components, Mechanics

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Improved Gear Shapes for Face Worm Gear Drives

These shapes offer potential for increasing precision and reducing vibration and noise. Shapes different from the traditional ones have been proposed for face worm gears and for conical and cylindrical worms that mesh with them. The proposed shapes are based on the concept of generating a face worm gear surface by use of a tilted head cutter instead of by the traditional use of a hob. (As used here, “head cutter” is also meant to signify, alternatively, a head grinding tool.)The gear-surface-generation equipment would be similar to that used for generation of spiral bevel and hypoid gears. In comparison with the corresponding traditional hob, a tilted head cutter according to the proposal would be larger, could be fabricated with greater precision, and would enable the generation of gear surfaces with greater precision and greater productivity.

Posted in: Briefs, TSP, Mechanical Components, Mechanics

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Two High-Temperature Foil Journal Bearings

These are prototypes of foil bearings for aircraft gas turbine engines. An enlarged, high-temperature -compliant foil bearing has been built and tested to demonstrate the feasibility of such bearings for use in aircraft gas turbine engines. At 150 mm in diameter, this is the largest foil bearing known to date. This bearing is a scaled-up version of a patented 100-mm-diameter foil bearing, augmented by coating the foil with a proprietary high-temperature material. In a companion development, a foil bearing as described above has been combined with a 150-mm-diameter active magnetic bearing to make a hybrid foil magnetic bearing.

Posted in: Briefs, TSP, Mechanical Components, Mechanics

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Using Plates To Represent Fillets in Finite-Element Modeling

Structural deflections are approximated by use of simplified computational submodels of filletsA method that involves the use of fictitious plate elements denoted bridge plates has been developed for representing the stiffnesses of fillets in finiteelement calculations of deflections, stresses, and strains in structures. In the absence of this method, it would be necessary to either neglect the effects of fillets to minimize the computational burden or else incur a large computational burden by using complex computational models to represent the fillets accurately. In effect, the bridge plates of the present method are reduced-order models of fillets that do not yield accurate stresses within fillets but do make it possible to accurately calculate the dynamic characteristics of the structure and to approximate the effects of fillets on stresses and strains elsewhere in a structure that contains the fillets. Such approximations are accurate enough for final modal analysis and preliminary stress analyses.

Posted in: Briefs, Mechanical Components, Mechanics

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Stochastic Representation of Chaos Using Terminal Attractors

Fictitious control forces stabilize what would otherwise be unstable computed trajectories.A nonlinear version of the Liouville equation based on terminal attractors is part of a mathematical formalism for describing postinstability motions of dynamical systems characterized by exponential divergences of trajectories leading to chaos (including turbulence as a form of chaos). The formalism can be applied to both conservative systems (e.g., multibody systems in celestial mechanics) and dissipative systems (e.g., viscous fluids).

Posted in: Briefs, TSP, Mechanical Components, Mechanics

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Lunar Constellation of Frozen Elliptical Inclined Orbits

A document discusses the design of orbits of spacecraft for relaying communications between Earth stations and robotic and human explorers in craters in one of the polar regions on the Moon. In simplest terms, the basic problem is to design a constellation of orbits to provide continuous and preferably redundant communication coverage of one of the poles with a minimal number of spacecraft and little or no controlled maneuvering of the spacecraft to maintain the orbits. The design method involves the use of analytical techniques for initial selection of orbits, followed by a numerical procedure for tuning the coverage of the constellation to obtain a design. In an example application, the method leads to a constellation of three spacecraft having elliptical, inclined orbits, the apoapsides of which would remain in the hemisphere (North or South) containing the pole of interest. The orbits would be stable and would maintain the required spacecraft formation for at least 10 years, without need for controlled maneuvering if gravitation is the only force considered to affect the orbits. A small amount of controlled maneuvering would be needed to counteract effects of solar-radiation pressure and other perturbations.

Posted in: Briefs, TSP, Mechanical Components, Mechanics

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