Experimental and theoretical investigation of fatigue life in reusable rocket thrust chambers

During a test program to investigate low-cycle thermal fatigue, 13 rocket combustion chambers were fabricated and cyclically test fired to failure. Six oxygen-free, high-conductivity (OFHC) copper and seven Amzirc chambers were tested. The failures in the OFHC copper chambers were not typical fatigue failures but are described as creep rupture enhanced by ratcheting. The coolant channels bulged toward the chamber centerline, resulting in progressive thinning of the wall during each cycle. The failures in the Amzirc alloy chambers were caused by low-cycle thermal fatigue. The zirconium in this alloy was not evenly distributed in the chamber materials. The life that was achieved was nominally the same as would have been predicted from OFHC copper isothermal test data

Using Electronegativity and Hardness to Test Density Functional Universality

Density functional theory (DFT) is used in thousands of papers each year, yet lack of universality reduces DFT's predictive capacity, and functionals may produce energy-density imbalances. The absolute electronegativity (\chi) and hardness (\eta) directly reflect the energy-density relationship via the chemical potential dE/dN and we thus hypothesized that they probe universality. We studied \chi and \eta for atoms Z = 1-36 using 50 diverse functionals covering all major classes. Very few functionals describe both \chi and \eta well. \eta benefits from error cancelation whereas \chi is marred by error propagation from IP and EA; thus almost all standard GGA and hybrid functionals display a plateau in the MAE at 0.2-0.3 eV for \eta. In contrast, variable performance for \chi indicates problems in describing the chemical potential by DFT. The accuracy and precision of a functional is far from linearly related, yet for a universal functional we expect linearity. Popular functionals such as B3LYP, PBE, and revPBE, perform poorly for both properties. Density sensitivity calculations indicate large density-derived errors as occupation of degenerate p- and d-orbitals causes "non-universality" and large dependency on exact exchange. Thus, we argue that performance for \chi for the same systems is a hallmark of universality by probing dE/dN. With this metric, B98, B97-1, PW6B95D3, APFD are the most "universal" tested functionals. B98 and B97-1 are accurate for very diverse metal-ligand bonds, supporting that a balanced description of dE/dN and dE2/dN2, via \chi and \eta, is probably a first simple probe of universality

Two-Loop Beta Functions Without Feynman Diagrams

Starting from a consistency requirement between T-duality symmetry and renormalization group flows, the two-loop metric beta function is found for a d=2 bosonic sigma model on a generic, torsionless background. The result is obtained without Feynman diagram calculations, and represents further evidence that duality symmetry severely constrains renormalization flows.Comment: 4 pp., REVTeX. Added discussion on scheme (in)dependence; final version to appear in Phys. Rev. Let

High accuracy short-term PWV operational forecast at the VLT and perspectives for sky background forecast

In this paper we present the first results ever obtained by applying the autoregressive (AR) technique to the precipitable water vapour (PWV). The study is performed at the Very Large Telescope. The AR technique has been recently proposed to provide forecasts of atmospheric and astroclimatic parameters at short time scales (up to a few hours) by achieving much better performances with respect to the 'standard forecasts' provided early afternoon for the coming night. The AR method uses the real-time measurements of the parameter of interest to improve the forecasts performed with atmospherical models. We used here measurements provided by LHATPRO, a radiometer measuring continuously the PWV at the VLT. When comparing the AR forecast at 1h to the standard forecast, we observe a gain factor of $\sim$ 8 (i.e. $\sim$ 800 per cent) in terms of forecast accuracy. In the PWV $\leq$ 1 mm range, which is extremely critical for infrared astronomical applications, the RMSE of the predictions is of the order of just a few hundredth of millimetres (0.04 mm). We proved therefore that the AR technique provides an important benefit to VLT science operations for all the instruments sensitive to the PWV. Besides, we show how such an ability in predicting the PWV can be useful also to predict the sky background in the infrared range (extremely appealing for METIS). We quantify such an ability by applying this method to the NEAR project (New Earth in the Alpha Cen region) supported by ESO and Breakthrough Initiatives