Dynamic Qualification of a New CMC Fastener

Abstract

A lot of engineering effort is currently being invested in the development of Reusable Launch Vehicles (RLV) to lower launch costs and provide economical access to space in such vehi-cles. Ceramic Matrix Composites (CMC) are very favorable materials for hot structures and thermal protection systems (TPS). In addition to the improvement of the material itself, the development of suitable joining techniques is very important. Due to the high application temperatures, joining methods based on high temperature resistant materials are required. Depending on the operation of space re-entry vehicles in some cases joining elements are needed which have on the one hand the capability to attach TPS elements onto the vehicle structure with only external access. On the other hand they must be able to withstand high temperatures up to about 1600 °C. This value is derived from a real scenario of the re-entry of NASA‘s X-38 experimental space vehicle. Besides, random vibration loads are introduced during the flight phases, strictly speaking the launch and the re-entry phase. In the past such kind of joining elements have been developed by DLR and were pre-qualified during a test campaign within the FESTIP-I program (Fig.1) and patented. Respec-tively a fastener (Fig.2) was designed for the FESTIP-Program which was locked by pulling in a cone. The fastener had a conical shank which is pressed together while inserting it into the conical nut. To provide sufficient deformation capability for the shank, the fastener with a shank diameter of 10 mm had a minimum length of 35 mm. In a continuation of the developments mentioned the performance under a fatigue load en-vironment had to be proven and in addition a shorter structure length was required. Therefore a new ceramic fastener (Fig.3), which combines the function of a screw and a rivet and which enables a shorter structure length, has been developed at DLR within a contract with ESTEC (Netherlands) and ERA (England). The test campaign was carried out in the DLR research center of Stuttgart at the Institute of Structures and Design.