Mechanical & Fluid Systems

Touchdown Ball-Bearing System for Magnetic Bearings

In the event of a touchdown, ball bearings provide full support. The torque-limited touchdown bearing system (TLTBS) is a backup mechanical- bearing system for a high-speed rotary machine in which the rotor shaft is supported by magnetic bearings in steady-state normal operation. The TLTBS provides ballbearing support to augment or supplant the magnetic bearings during startup, shutdown, or failure of the magnetic bearings. The TLTBS also provides support in the presence of conditions (in particular, rotational acceleration) that make it difficult or impossible to control the magnetic bearings or in which the magnetic bearings are not strong enough (e.g., when the side load against the rotor exceeds the available lateral magnetic force).

Posted in: Briefs, TSP

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Using ERF Devices To Control Deployments of Space Structures

A report proposes devices containing electrorheological fluids (ERFs) damper for controlling deployments of lightweight, flexible structures in outer space. The structures would include spring members that could be wound or compressed for compact stowage during transport. The ERF based damper would keep the structures compacted and/or regulate the speeds with which the structures would spring out for deployment. After deployment, ERF based dampening mechanism could be used to rigidize the structures or damp their vibrations. The report describes several potential variations on the basic concept of an ERF-controlled structural member, including compartmentalization of the interior volume to prevent total loss of the ERF in case of a leak and the use of multiple, individually addressable electrode pairs to enable more localized control.

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Increasing the Life of a Xenon-Ion Spacecraft Thruster

A short document summarizes the redesign of a xenon-ion spacecraft thruster to increase its operational lifetime beyond a limit heretofore imposed by nonuniform ion-impact erosion of an accelerator electrode grid. A peak in the ion current density on the centerline of the thruster causes increased erosion in the center of the grid. The ion-current density in the NSTAR thruster that was the subject of this investigation was characterized by peak-to-average ratio of 2:1 and a peak-to-edge ratio of greater than 10:1. The redesign was directed toward distributing the same beam current more evenly over the entire grid and involved several modifications of the magnetic-field topography in the thruster to obtain more nearly uniform ionization. The net result of the redesign was to reduce the peak ion current density by nearly a factor of two, thereby halving the peak erosion rate and doubling the life of the thruster. (Note: NSTAR stands for NASA SEP Technology Application Readiness; SEP stands for solar electric propulsion.)

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Adaptive Deadband Synchronization for a Spacecraft Formation

A paper discusses general problems in estimation and control of the states (positions, attitudes, and velocities) of spacecraft flying in formation, then addresses the particular formation-flying-control problem of synchronization of deadbands. The paper presents a deadband- synchronization algorithm for the case in which the spacecraft are equipped with pulse- width-modulated thrusters for maintaining their required states. The algorithm synchronizes thruster-firing times across all six degrees of freedom of all the spacecraft. The algorithm is scalable, inherently adapts to disturbances, and does not require knowledge of spacecraft masses and disturbance forces. In this algorithm, one degree of freedom of one spacecraft is designated the leader, and all other degrees of freedom of all spacecraft as followers. The Cassini adaptive optimum deadband drift controller is the subalgorithm for control in each degree of freedom, and the adaptation is run until each spacecraft achieves a specified drift period. The adaptation is critical because a different disturbance affects each different degree of freedom. Then the leader communicates its thruster-firing starting times to the followers. Then, for each follower, a deadband- synchronization subalgorithm determines the shift needed to synchronize its drift period with that of the leader. This work was done by Daniel Scharf, Fred Hadaegh, and Bryan Kang of Caltech for NASA’s Jet Propulsion Laboratory. The software used in this innovation is available for commercial licensing. Please contact Karina Edmonds of the California Institute of Technology at (626) 395-2322. Refer to NPO-43258.

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Picosatellite Design Incorporates Thin Aluminum Structure and Self-Clinching Fasteners

A reduced parts count and lower weight are attributable to less fastening-related hardware. The CubeSat standard (10 × 10 × 10 cm and weighing 1kg) has evolved into one of the most widely accepted families of picosatellite designs. A CubeSat can package a universe of payload possibilities for launch into space at a fraction of the cost of traditional multimillion- dollar satellites. The success of any CubeSat project will owe much to how closely participants adhere to unforgiving timelines linked to launches scheduled far in advance. Serving to jumpstart projects and keep them on track within the typical 24 months from inception to launch, Pumpkin, Inc. created an off-the-shelf CubeSat Kit™ offering all the advantages of a standardized assembly. Conforming fully to the recognized CubeSat specification, the kit’s structure, electronics, and software are intended to save time and money.

Posted in: Mechanics, Briefs

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Phase-Oriented Gear Systems

Larger mechanical advantages can be realized in smaller packages. Phase-oriented gear systems are differential planetary transmissions in which each planet gear has two sets of unequal numbers of teeth indexed at prescribed relative angles (phases). The figure illustrates an application of the phase-oriented gearing concept to a relatively simple speed-reducing differential planetary transmission that includes a sun gear, an idler gear, three identical planet gears, a ground internal ring gear, and an output internal ring gear. Typically, the ground internal ring gear and output internal ring gear have different numbers of teeth, giving rise to a progressive and periodic phase shift between the corresponding pairs of teeth engaged by each successive planet gear. To accommodate this phase shift, it is necessary to introduce a compensating phase shift between the groundgear- engaging and output-gear-engaging sections of each planet gear. This is done by individually orienting each planet gear.

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Free-to-Roll Testing of Airplane Models in Wind Tunnels

Causes of, and cures for, wing-drop/rock behavior can be evaluated. A free-to-roll (FTR) test technique and test rig make it possible to evaluate both the transonic performance and the wing-drop/rock behavior of a high-strength airplane model in a single wind-tunnel entry. The free-to-roll test technique is a single degree-of-motion method in which the model is free to roll about the longitudinal axis. The rolling motion is observed, recorded, and analyzed to gain insight into wing-drop/rock behavior.

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