Performance of a 2-layer radial hole effective media lens
The outer polyethylene layer of a 8λ0 radius 2-layer polyethylene/cross-linked polystyrene lens was implemented as a radial hole effective media that allows the entire lens to be milled from a single billet of cross-linked polystyrene, simplifying construction by avoiding layer fitting problems. Full wave simulations in a commercially available simulator showed that increasing the number of radial holes to 3000 gave peak directivity within 0.3dB of the original cross-linked polystyrene/polyethylene lens. Both Particle Swarm Optimization and icosahedron based methods were trialed for distributing the holes evenly over the sphere. The latter method produced symmetrical meshes allowing symmetry planes to be used to minimize computational burden, but was bound to the numbers of holes produced by successive triangular face subdivision iterations.
ADVANCED LIGHTWEIGHT ELECTRONICALLY STEERED ANTENNAS FOR RESPONSIVE SPACE PAYLOADS
Due to the mix of missions that Responsive Space payloads will be called upon to execute, and the available launcher technology, it appears likely that initial implementations will consist of constellations of small LEO satellites. It is likely that most Responsive Space payloads will need to maintain multiple simultaneous data links to the battlefield, even if the spacecraft's primary mission is something other than providing tactical communications. In order to maintain broadband communications with multiple fixed or mobile terminals distributed across a theater of interest, these LEO satellites will require agile high gain communications antennas. Providing such an antenna within the size, mass, and cost constraints of a Responsive Space payload presents a number of technical challenges. This paper presents some of the design trades involved in selecting an antenna technology for this application. It also gives examples of some recently developed antenna and beam forming technologies that show promise for addressing Responsive Space payload requirements. The devices shown have adequate performance for a wide variety of missions, were developed for space or military airborne applications, and can be qualified for the Responsive Space mission with low risk * .
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME