Aeroconseil (France)

Si3N4-SiC whisker composites were fabricated using several routes (i.e., pressure filtration or CIP) followed by HP or HIP. The fracture strength ranges from 650 MPa to 750 MPa on account of the whiskers orientation. Compared to the Si3N4 matrix, the toughness is increased. A strong R-curve effect can be obtained, suggesting that, to be efficient, the whisker diameter must exceed a critical size. 13 refs.

We analyze dust features present in the mid-infrared (Spitzer) and recently published L-band (UKIRT) spectra of the infrared galaxy IRAS 08572+3915. The line of sight toward the AGN nucleus crosses a high column density of carbonaceous dust whose characteristic absorption features appear clearly. They provide a real insight into the chemical environment of the diffuse interstellar medium. Thanks to the moderate redshift of IRAS 08572+3915, the wavelength of the aromatic CH stretching mode is free of major telluric lines, and a strong observational constraint of H sp2 /H sp3 ≤0.08 has been determined. This limit clearly shows that the bonding of hydrogen atoms in interstellar hydrogenated amorphous carbon is highly aliphatic. The presence of a broad absorption feature centered at 6.2 μm, probably arising from olefinic/aromatic structures, corresponds to the backbone of this carbonaceous material, which is the major carbon-containing component of the interstellar medium along this line of sight.

Main Focus is the design of an air breathing TSTO reusable launch vehicle using in -flight LO X-collection. Integration of such a device in a winged space transport system can have potentials to reduce weight and hence costs for access to space. Plant architecture, influencing parameters, mode of operation and optimum working conditions of the coll ection device will be discussed. This will lead to a suitable design point, field of operation and requirements for application in the first stage of a reusable space transport. Preliminary investigations have shown that a turbojet propelled Mach -4 vehicle is most suitable for application of the extraction system. Based on this a complete design of the TSTO for ACES application is carried out in terms of performance, feasibility and safety reasons as well as reductions of complexity and development effort. The evaluation of potentials and risks of a technology is only possible by its examination as integral part of the entire system.

Abstract This paper investigates the occurrence of phase alignment of the tropical stratospheric quasi-biennial oscillation (QBO) with the annual cycle. First, updating previous studies, observational results are shown for NCEP reanalysis data and Singapore radiosondes: both datasets show strong phase alignment of the QBO at 24.5 km. Phase alignment is investigated in a 3D mechanistic stratospheric model including explicit large-scale planetary waves, forced by a lower boundary geopotential anomaly, and a simple equatorial wave parameterization. The model simulates a QBO-like oscillation, with the period depending on the lower boundary momentum flux of the parameterized waves. Phase alignment is manifested in two different ways. First, simulated oscillations of both integer and noninteger year periods are shown to lock on to a certain phase of the annual cycle. Second, when the magnitude of the lower boundary momentum flux is varied about a range implying oscillation period close to 2 yr, the period of the resulting oscillation is exactly 2 yr for a finite range of such magnitude. Analysis of the 3D model results suggest that the the phase alignment is due largely to the annual cycle in tropical upwelling. This hypothesis is supported by simulations with a 1D equatorial model including both parameterized waves and seasonally varying upwelling. The oscillations in this model show significant phase alignment when the upwelling parameters are tuned to correspond to the 3D model, although the phase alignment is weaker than that seen in the 3D model.

A device simulator is used to analyze the heavy ion induced failure mechanism in insulated gate bipolar transistors (IGBTs) and to investigate hardening solutions. Single event latchup was already identified as the failure mechanism. Lateral and vertical modifications of the P/sup +/ plug are proposed to reduce the efficiency of the parasitic thyristor, responsible for the latchup, and validated by 2D-simulations on a N-channel IGBT cell structure.

The use of simplified equations of motion ‘is the reason why nearly circular orbit transfers are singular for both impulsive optimization and optimal control since the eccentricity is a factor in the equation governing the location of the burns. A more detailed analysis of the change of the costate during the burns and during the coasting phase enables us to define .a preliminary analytic guess of the initial costate for coplanar orbit transfer with free final time. A classical trajectory optimization tool is used to improve the estimate ofthe costate and to find the solution of the optimal transfer. The presentation is restricted to minimum fuel criterion and constant thrust case. The results obtained in the last section .demonstrate the performance of the preliminary algorithm and propose some further improvements..

The effects of input bias on the sensitivity to cumulative dose effects of 144 commercial CMOS inverters have been studied. The "worst case" irradiation conditions, as well as the most favorable case, have been inferred from these data. A saturation of the switching voltage shift had been measured for a null input bias. This effect appears to be due to the saturation of the NMOS and PMOS transistors threshold voltage shift for a null gate-to-source voltage.