Development of hot CMC structures for space reentry vehicles via flight experiments

Kurzfassung

Hot structures made of ceramic matrix composites (CMC) for space reentry vehicles play a key role regarding the feasibility of advanced and reusable future space transportation systems. Thus realization of applicable flight hardware concerning hot primary structures like a nose cap or body flaps and thermal protection systems (TPS) requires system competence w.r.t. sophisticated know how in material processing, manufacturing and qualification of structural components and in all aspects from process control, use of NDI techniques, arc jet and hot structure testing. However, the most important issue to realize hot primary structures for operational space reentry vehicles is validation of materials and design concepts via real flight testing due to limitations of any existing ground based testing facility or validation methodology. So this has been the strategy of DLR since one decade and multiple reentry flight tests with samples and structural components have been performed until now. Coated and uncoated C/C-SiC specimens were integrated into the ablative heat shield of Russian FOTON capsules and they were tested during two missions in 1992 and 1994. Following on, a hot structure experiment called CETEX which principally was a kind of a little nose cap had been developed and tested during the EXPRESS mission in 1995. These three flight tests were the first ones in Europe carried out with such a kind of material and hot structural concept and many lessons were learned w.r.t. material behaviour and structural design performance under the severe environmental conditions of ballistic capsule reentry. Within an ESA program called FESTIP we developed a new design concept for a rigid surface TPS based on CMC’s which should be adaptable to the outer side of a cryogenic tank structure of a future SSTO vehicle. Special TPS concept features are (flat) integrally stiffened CMC panels, hot CMC fasteners for outside attachment capability, thermal displacement compensation, sealing and insulation, provision of a purge gap etc. Based on this concept a flight experiment was realized featuring all principal design aspects mentioned before and adding three different anti oxidation coating systems. The experiment again will be integrated into the heat shield of a Russian FOTON capsule and the mission is planned for October 2002. On the longer run we started to follow a new approach based on the idea of using flat panels for a TPS instead of trying to realize continuous shaped surfaces which is today’s design. So the vehicle’s surface will look facetted and various questions w.r.t. design, manufacturing, heat loads at edges, change of the vehicle’s aerodynamic properties etc. arise. Just now a flight experiment called SHEFEX (SHarp Edge Flight EXperiment) is in the definition phase and flight is scheduled to 2004. The nose cap development for NASA’s X-38 would have been the most ambitious flight experiment until now and our flight hardware was finished and installed successfully to the flight vehicle V201 in October 2001. However as the X-38 project has been stopped just now and other publications already described the development of the nose cap system it will not be focused on within this paper. Highlighting the most important technical issues and achievements of the flight experiments performed so far and reporting about ongoing plannings finally will provide a good insight into DLR’s past and future development road map w.r.t. CMC hot structures and TPS for space reentry vehicles.