A New Approach towards Tailored Thermal Protection Material for Hypersonic Boundary Layer Control
Alexander Wagner1(*), Divek Surujhlal1, Carolin Rauh2, Martin Frieß2, Fiona Kessel2, Hendrik Weihs2
1Institute of Aerodynamic and Flow Technology, German Aerospace Center (DLR), Göttingen, Germany
2Institute of Structures and Design, German Aerospace Center (DLR), Stuttgart, Germany
* Corresponding author: Alexander.Wagner@dlr.de
Keywords: passive transition delay, ultrasonically absorptive TPS, C/C-SiC, High Enthalpy Shock Tunnel Göttingen (HEG)
The presented study aims for a new class of porous thermal protection material (TPS) able to withstand the extremely demanding hypersonic flight environment and being applicable for active and passive hypersonic boundary layer control through tailored porosity.
The study addresses two of the main challenges relevant to hypersonic sustained flight: high surface heat fluxes and viscous drag in combination with the need to develop advanced high temperature materials.
The study explores different manufacturing techniques to develop a new temperature stable, C/C-SiC material with controllable porosity. The technique will reuse selected components of an existing process to manufacture C/C-SiC, which is a temperature stable and oxidation resistant material used as dense TPS on hypersonic test vehicles.
To generate the porosity a novel approach is chosen by using 3D reinforcement techniques such as Z-pinning, tufting and stitching. The design of the porous TPS will be guided by a methodology leading to optimal porosity parameters to control surface heat flux and vehicle drag by means of active and passive boundary layer control techniques such as transpiration cooling and boundary layer transition delay by means of absorption of ultrasonic instabilities.
Ultimately, the study aims for verification tests at hypersonic flight conditions in the High Enthalpy Shock Tunnel (HEG) at DLR Göttingen, HEG2018.
Figure 1 provides an overview on possible applications of a tailored porous thermal protection material around a hypersonic vehicle such as the X-43. These application scenarios include for instance transpiration cooling on leading edges and highly loaded surfaces, boundary layer transition delay by targeted absorption of boundary layer instabilities in specific frequency ranges, intentional boundary layer tripping upstream of combustors and fuel injection on compression ramps.
The successful development of the proposed TPS materials will provide the potential for:
1) lighter hypersonic vehicles due to decreased surface heat loads and therefore reduced TPS weight,
2) decreased combustion chamber/vehicle length due to increased supersonic combustion efficiencies through improved air/fuel mixing,
3) increased efficiency of control surfaces as well as increased robustness of intakes by locally controlling the boundary layer state.
The study explored different manufacturing techniques reusing selected components of an existing process to manufacture temperature stable and oxidation resistant C/C-SiC, already used as dense TPS on hypersonic vehicles.
To generate the directed porosity a novel approach is chosen by using 3D-reinforcement techniques such as Z-pinning, tufting and stitching (Dell’Anno, 2016; Mouritz, 1997).
As depicted in figure 2 on the left, these techniques insert an organic thread, perpendicular or at an angle through the fiber plane into the dry fabric preform.
In the course of the subsequent manufacturing process the thread will be decomposed thereby defining a cylindrical channel of approximately the former thread size, forming the material porosity. Compared to the existing C/CSiC, Dittert, 2018, this is a major change leading to a number of advantages.
Whereas, in C/C-SiC with random porosity, pores predominantly form between the fiber layer, the proposed techniques generate the porosity in the (x,y) fiber plane, as shown in Figure 2 (left).
The technique allows a greatly improved contouring flexibility of curved components and, at the same time, ensures the pore alignment to be perpendicular (or at a predefined angle) with respect to the surface. The porosity properties are mainly controlled by using appropriate organic thread diameters and techniques with adjustable stitching distances.
Figure 2 (right) shows a computer tomographic image of a C/C-SiC sample in the (z,y) fiber plane.
Figure 3 confirms the acoustic damping properties of the new material showing the reflection coefficient as function of pressure and frequency in ranges relevant to hypersonic applications as shown in Wagner, 2015.
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Deutsches Zentrum für Luft – und Raumfahrt (DLR). (2018). The High Enthalpy Shock Tunnel Göttingen of the German Aerospace Center (DLR). Journal of large-scale research facilities, 4, A133. http://dx.doi.org/10.17815/jlsrf–4–168
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P. Mouritz, K. H. Leong and I. Herszberg, “A review of the effect of stitching on the in-plane mechanical properties of fibrereinforced polymer composites,” Composites Part A: Applied Science and Manufacturing, vol. 28, pp. 979-991, 1997.
Wagner, V. Wartemann, M. Kuhn, C. Dittert and K. Hannemann. “The Potential of Ultrasonically Absorptive TPS Materials for Hypersonic Vehicles,” AIAA 2015-3576. 20th AIAA International Space Planes and Hypersonic Systems and Technologies Conference. July 2015.