1,888 research outputs found
Vibration characteristics of a deployable controllable-geometry truss boom
An analytical study was made to evaluate changes in the fundamental frequency of a two dimensional cantilevered truss boom at various stages of deployment. The truss could be axially deployed or retracted and undergo a variety of controlled geometry changes by shortening or lengthening the telescoping diagonal members in each bay. Both untapered and tapered versions of the truss boom were modeled and analyzed by using the finite element method. Large reductions in fundamental frequency occurred for both the untapered and tapered trusses when they were uniformly retracted or maneuvered laterally from their fully deployed position. These frequency reductions can be minimized, however, if truss geometries are selected which maintain cantilever root stiffness during truss maneuvers
Tunneling between bilayer quantum Hall structures in a strong magnetic field
We calculate the tunneling current in a quantum Hall bilayer system in the
strong magnetic field limit. We model the bilayer electron system as two Wigner
crystals coupled through interlayer Coulomb interactions, treated in the
continuum limit. We generalized the Johansson and Kinaret (JK) model and were
able to study the effect of the low energy out-of-phase magnetophonon modes
produced as a result of tunneling events. We find the same scaling behavior of
the tunneling current peak with the magnetic field as found by JK but were able
to find the tunneling current scaling behavior with interlayer distance as
well.Comment: 4 pages, 1 figure, SemiMag16 conference paper to be published in
International Journal of Modern Physics
Vortex Lattice Structural Transitions: a Ginzburg-Landau Model Approach
We analyze the rhombic to square vortex lattice phase transition in
anisotropic superconductors using a variant of Ginzburg-Landau (GL) theory. The
mean-field phase diagram is determined to second order in the anisotropy
parameter, and shows a reorientation transition of the square vortex lattice
with respect to the crystal lattice. We then derive the long-wavelength elastic
moduli of the lattices, and use them to show that thermal fluctuations produce
a reentrant rhombic to square lattice transition line, similar to recent
studies which used a nonlocal London model.Comment: 4 pages, 3 figures, final version with various referee suggested
modifications, scheduled to appear in PR
Dynamic characteristics of a space-station solar wing array
A solar-wing-array concept is described which meets space-station requirements for minimum fundamental frequency (0.4 Hz), component modularity, and growth potential. The basic wing-array design parameters are varied, and the resulting effects on the array vibration frequencies and mode shapes are assessed. The transient response of a free-free space station (incorporating a solar-wing-array point design) to a load applied at the space-station center is studied. The use of the transient response studies in identifying critically loaded structural members is briefly discussed. The final 150-kW space-station configuration has a fundamental elastic frequency of 0.403 Hz
Structural performance of two aerobrake hexagonal heat shield panel concepts
Structural sizing and performance are presented for two structural concepts for an aerobrake hexagonal heat shield panel. One concept features a sandwich construction with an aluminum honeycomb core and thin quasi-isotropic graphite-epoxy face sheets. The other concept features a skin-rib isogrid construction with thin quasi-isotropic graphite-epoxy skins and graphite-epoxy ribs oriented at 0, +60, and -60 degs along the panel. Linear static, linear bifurcation buckling, and nonlinear static analyses were performed to compare the structural performance of the two panel concepts and assess their feasibility for a lunar transfer vehicle aerobrake application
An integrated in-space construction facility for the 21st century
Preliminary results are presented of studies being conducted by NASA on the construction of very large spacecraft. The various approaches are discussed for constructing spacecraft and their relative merits. It is observed that the Space Station Freedom has all of the basic design characteristics to permit its growth into an in-space construction facility for very large spacecraft. Also it is noted that if disturbances from construction operations are intolerable to other Space Station experiments, a co-orbiting construction facility could be built using previously developed Space Station truss hardware and systems. A discussion is also presented of a new PATHFINDER research initiative on on-orbit construction. This research effort is aimed at developing construction methods for very large spacecraft and includes the development of a 100 meter long space crane
Preliminary design of a large tetrahedral truss/hexagonal heatshield panel aerobrake
An aerobrake structural concept is introduced which consists of two primary components: (1) a lightweight erectable tetrahedral support truss; and (2) sandwich hexagonal heatshield panels which, when attached to the truss, form a continuous impermeable aerobraking surface. Generic finite element models and a general analysis procedure to design tetrahedral truss/hexagonal heatshield panel aerobrakes is developed, and values of the aerobrake design parameters which minimize mass and packaging volume for a 120-foot-diameter aerobrake are determined. Sensitivity of the aerobrake design to variations in design parameters is also assessed. The results show that a 120-foot-diameter aerobrake is viable using the concept presented (i.e., the aerobrake mass is less than or equal to 15 percent of the payload spacecraft mass). Minimizing the aerobrake mass (by increasing the number of rings in the support truss) however, leads to aerobrakes with the highest part count
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