Application of High Temperature Optical Fibers in a Reentry Vehicle

One of the most demanding environment is the one encountered by reentry vehicles at the impact with the atmosphere. The heat produced can reach few MW/m2 and can raise gas temperature above dissociation and even ionization energy. As a consequence the reentry structure is exposed not only to very high temperature but also to chemical attack by the atmospheric species such as atomic oxygen and nitrogen. Another concern is the surface catalysis. The gaseous species are adsorbed by the solid material that can favor recombination of nitrogen with atomic oxygen. This chemical reaction release a further amount of heat right at the surface of the material. Proper choice of material for the Thermal Protection Systems (TPS) can reduce this last type of reaction. To this aim our group at the Univ. of Rome “La Sapienza” in cooperation with prof. Currie at the Univ. of Maryland is proposing an experiment in order to measure catalytic recombination on several samples of classical and newly proposed materials to use as TPS for reentry vehicles. This experiment should fly on the European Experimental Reentry Testbed (EXPERT). EXPERT is an experimental capsule financed by the European Space Agency (ESA) and lead by the European Space Research and Technology Center (ESTEC). The aim of this capsule is to offer researchers from all over Europe the possibility of performing tests on the reentry environment focusing on topics such as hypersonic fluid dynamics and surface catalysis. Three suborbital flights are planned with the capsule which is designed to be reusable. The Volna decommissioned ballistic missile will be used for launch. Presently more than ten experiments are planned in the EXPERT experiment. In order to select the proper material at least with respect to the catalysis phenomenon one has to observe the species concentrations right close to the surface. To this aim prof. Bruno’s group at the Department of Aeronautical and Mechanical Engineering at “La Sapienza” University and our group at the Department of Aerospace and Astronautical Engineering of the same University are proposing an experiment that will observe those species by using one or more sapphire optical fibers. In fact light collected from the different samples of TPS materials that will be attached at the capsule surface will be analyzed inside the unmanned capsule during reentry. Sapphire has the right characteristics to conduct light also in the ultraviolet region (where some spectral lines of interest are located) as well as very high melting points. During reentry, light coming out from the TPS specimen material will carry information on temperature as well as on the species concentration in the plasma. Presently we are selecting the most appropriate spectrograph, the baseline solution at the moment is a Multi-Channel Spectrograph that will allow the observation of more samples at the same time. The spectrograph will analyze the light captured by the sapphire fibers. Data will be stored on board at a rate of few samples per second per channel. A selection of data is also transferred real time on the ground station in case the soft landing of the capsule will fail. Another area of interest for optical fiber in a spacecraft is the structural health monitoring for reusable launch vehicles. Soon after the Apollo program, based on expendable rockets and capsules, NASA started a new approach that was intended to reduce cost of access to space to manned missions. That lead in about ten years to the space shuttle whose reliability was proven to be not as high as expected. After the second disaster one has to recognize that the Russian Soyuz launch vehicle is more reliable and cost effective than the US Space Transportation System (STS). That is not surprising since, in spite of the not optimized launch vehicle design (Russian Soyuz is more than 30 years old) it has a very well proven activity (more than 1000 successful launches). However before the Columbia disaster NASA was already thinking to a future generation of reusable launch vehicles that could lead to a reduction of cost a factor of 10 in the mid term and a factor of 100 in the long term. To contribute to achieve such a goal the reduction of personnel used for maintenance and operation is required and consequently several health monitoring systems are envisaged. The most credited hypothesis for the Columbia disaster is the failure on a Thermal Protection System (TPS) on the leading edge of the wing of the Shuttle which is made of Carbon/Carbon. A health monitoring system on the TPS could have prevented such a disaster or at least could have given the opportunity to attempt a rescue of the crew while in orbit, if the failure was detected right at the beginning of the mission. Recently our group in cooperation with a group at the Department of Chemical Engineering, Materials, Row Materials and Metallurgy has been working on preliminary tests aimed to verify the feasibility of such a system. In Ref. [1] we have used telecommunication optical fibers with polyimide coating in an embedding test with a thermal spraying technique called HP/HVOF. CERMET particles (NiCr-WC based powder, and NiCrBFeSi) were partly melted and projected at high speed towards the target constituted by an aluminium plate with the fiber blocked on its surface by a lamination procedure. The embedding process was proven feasible even if in this preliminary study optical transmission tests were only partially successful since, some fibers were damaged. In Ref. [2] first tests with sapphire fibers were performed. Specifically a copper alloy was cast over the special fiber and an interferometric set-up was prepared using the embedded fiber. Only the speckle pattern has been observed at that time while the actual interferometric tests were left for future applications. Also in the paper was presented a test concerning the embedding of a sapphire fiber into very high temperature ceramics using the Self-propagating High-temperature Synthesis (SHS) technique. The method involves self-propagating reaction fronts that are induced by a laser pulse. The wavefront moves at a relatively high speed and at temperatures that can be higher than 3000 C. In the paper a small cylindrical sample of ZrB2 was prepared. The results were promising but not satisfactory, the fiber was almost destroyed. In Fig. 1 is reported a SEM micrograph showing the fiber decomposed into several filaments oriented in different directions. We realized that the fiber was subjected to a very severe environment in terms of temperature and mechanical stresses. For that reason a further step was required: to protect the fiber with a thick metallic coating capable of providing a high heat capacity and a higher strength. Electrowinning technique is the best candidate for providing such a protection. In order to use this technique a thin coating of metallic material has to be previously deposited using Physical Vapor Deposition (PVD). When used on a fiber laying on a metallic substrate, electrowinning provided an excellent means to block the fiber in place [3], before a plasma spray technique could be used [4]. Currently we are experimenting the embedding of sapphire optical fibers into superalloy. Specifically the alloy used is an Inconel with nominal composition: 74.92% Ni, 15.50% Cr, 8.00% Fe, 1.00%Al, 0.50% Ti and 0.08% C. A centrifugal induction furnace with graphite crucible was used to cast the abovementioned alloy over the fiber. The casting has been performed in a protective argon atmosphere. In Fig. 2 one can recognize the sample of superalloy with the embedded optical fiber. The first attempts were not successful because the flow of the molten metal broke the fiber. For that reason the fiber has been protected with a Ni-Co alloy using electrowinning as performed previously in Ref. [5]. In Fig. 2 one can easily recognize the presence of the coating on the fiber. In Fig. 3 is reported the SEM micrograph of a longitudinal section of a specimen manufactured previously with the same technique. The fiber metal interface is quite satisfactory. Optical transmission tests on the embedded fiber were conducted on a transversal section of a polished metallographic specimen. In spite of the poor optical quality of the fiber used the result was quite satisfactory