Christian Dittert

Thermal Testing of the Sharp Leading Edge of SHEFEXII

The octahedral sharp leading edge of SHEFEXII is made from C/C-SiC fiber reinforced ceramic. It is instrumented by three thermocouples and eight pressure ports. A duplicate was ground-tested in arc-heated wind tunnel L3K. Transient thermal response of the material to the load was also determined by the heat-balance method HEATS and compared to the measurement data. During ground-testing, the hermocouples measured 1179°C, whereas the maximum temperature recorded during flight was measured at 30km altitude with 848°C. The simulated data corresponds well to both thermocouple data and data from an external infrared camera. During ground-testing, the very tip of the leading edge reaches 1635°C. The nose was in good order after ground testing.

Flowfield and Pressure Decay Analysis of Porous Cones

A transpiration-cooled cone is investigated with respect to varying cooling mass flow along the surface caused by the orthotropic permeability of a ceramic composite material and varying wall thickness. The measurements comprise the dynamic pressure distribution at the cone surface, the reservoir pressure, and the overall mass flow rate. Material permeability depending on fiber orientation is measured on cylindrical samples. Furthermore, a numerical approach with the two-dimensional Darcy equation is presented to simulate the pressure loss across the orthotropic wall structure and the mass flow distribution along the surface. Finally, the results from the experiment and the simulation are compared and assessed with respect to the fiber orientation of the material. The resulting mass flow distributions demonstrate the flow dependence on the fiber orientation. Furthermore, the outflow behavior at the surface is investigated, focusing on the exhaust angle, and the results of the measurements coincide with th...

The Potential of Ultrasonically Absorptive TPS Materials for Hypersonic Vehicles

The potential of ultrasonically absorptive carbon fiber reinforced carbon (C/C) based materials for passive boundary layer transition control on hypersonic flight vehicles is investigated. Based on previous studies on C/C, optimized C/C based materials are developed and studied with respect to their temperature stability and ultrasonic absorption properties at hypersonic flight regimes of interest. Generic vehicle shapes providing an essentially two dimensional flow field allowing the second mode instability to be the dominant boundary layer instability are studied using an engineering approach. Furthermore, the study provides an overview on the test procedures established in DLR to assess the ultrasonic absorption properties of porous materials and to experimentally investigate the impact of potential candidates on the boundary layer transition process.

Experimental and numerical acoustic characterization of ultrasonically absorptive porous materials

The paper addresses the experimental and numerical acoustic characterization of ultrasonically absorptive porous materials with random microstructure such as carbon fiber reinforced carbon ceramic C/C or C/C-SiC. The present study builds upon previous efforts by the authors, improving and extending the established experimental method, complemented by a numerical analysis based on linear acoustics. The latter includes a blind-hole porosity approximation, only accounting for the larger cracks in the C/C with complex acoustic impedance given by the inverse Helmholtz Solver approach, and a highly parametrized homogeneous acoustic Absorber model, accounting for the complete volumetric structure of the porous absorber albeit with lower fidelity. The experimental approach is complemented by high-speed Schlieren visualization and Mach-Zehnder Interferometer measurements to qualitatively and quantitatively assess the interaction between an ultrasonic wave packet and a porous surface. It is found that neglecting the smaller pores and only accounting for the surface porosity, as done in the blind-hole porosity approximation, leads to the underestimation of the acoustic energy absorption coefficient. Phase shifts were found to be experimentally assessable, but remain to be corroborated by a numerical analysis. The comparisons carried out in this paper will pave the way for accurate determination of impedance boundary conditions to be applied in direct numerical simulations of hypersonic transition delay over C/C. The main emphasis of the paper is to assess the potential and the limitations of the experimental methods and the comparison of the experimental results to the numerically obtained absorption characteristics.

Characterization of the Flow Field of Anisotropic Porous Cones with Different Wall Thickness

In hypersonic flight, sharp body concepts have several advantages compared to the classic blunt approach. Conversely, the heat load on the sharp leading edge is significantly increased. Therefore, it is necessary to consider active cooling concepts for the sharp leading edge. In this paper a transpiration-cooled cone is investigated with focus on the varying cooling mass flow along the surface, caused by anisotropic permeability of the material, and increasing wall thickness. Two cones with different angles are investigated with respect to their cooling flow behavior in the DLR test facility AORTA, which was built to characterize porous material. The measurements include the dynamic pressure distribution at the surface, in addition to reservoir pressure and the total mass flow. Furthermore a numerical approach is presented to simulate the pressure loss over the wall thickness with the two-dimensional Darcy equation and the mass flow distribution at the surface. Finally, the results from the experiment and the simulation are presented and assessed with respect to the fiber orientation of the material. The results show clear trends of mass flow distribution along the circumference of the cone. These mass flow distributions can be explained by the flow dependence of the fiber orientation and the resulting permeability.