Design and manufacturing of protective structures for nano-satellites and for atmospheric re-entry, based on carbon/carbon with nano silica coating, subject to the effects of the space environment

In the last decade the Space Community, as researchers, Agencies and companies focused their attention on two main research fields , reusable Thermal Protection System for Atmospheric Re-Entry and Nano satellites. An important cost reduction is achieved by improving these two research fields. For thermal protection systems a reliable reusability guarantees a longer life of re-entry vehicles and, for nano-satellites, the design, realization and optimization of such small spacecraft can open new frontiers in the realization of constellations of cube-sats, that could lower the weight and thus the mission costs, without however lowering the quality of their purposes. Of course for nanosatellites, that now have an average life of one or two years, the challenge is to achieve reliable protection systems for longer missions, facing all the hazards that Space Environment presents. In this frame, the present work, conceived in collaboration with the Italian Space Agency in the frame of an evolution of the ASA Phase B2 Program, suggests a new technology capable to satisfy the two challenges, both TPS and Nanosatellites Space Environment protection systems. The study of a new nano-silica coating, applied on Carbon/Carbon Substrates, capable to resist to the extreme re-entry environment and to a long stay in LEO orbit is presented, with a particular emphasis given to the material selection and the Space Environmental tests. The complete test chain commonly used for the qualification of space products has been used for the spaceability evaluation of the materials proposed. In particular, outgassing tests, thermal expansion coefficient tests, Thermal Vacuum, Atomic Oxygen / UV ageing evaluation have been performed. A careful material selection was carried on in order to guarantee the reliability of both the SEPPS and TPS. The choice felt on Carbon / Carbon Composites for the substrates, on Pyro-Paint varnishes for the coating, in particular Al2O3, SiC and ZrO2 varnishes. The nano-inclusions adopted for filling the coating varnishes were Silica nano-particles. Two types of Carbon / Carbon composites, adopted for their excellent thermo-mechanical behavior at high temperatures, have been the subject of the present study: for the realization of the TPS a self-made Shell Structure were adopted, while for the realization of the SEPPS a commercial Mitsubishi plate was used. Being the TPS a sandwich structure, a numerical simulation was carried out with the aim of select the best core material, starting from four kind of materials, Reticulated Vitreous Carbon Foam, SiC Foam, Alumina gasket and Pyrogel. The results brought to the selection of SiC foam for its great insulation properties. A complete screening and qualification test campaign, performed at Aerospace Systems Laboratory, was carried out, in order to understand which one of the selected coating could fulfill the Space Assurance Standards and guarantee a reliable protection from Space Environment and re-entry environment. First of all outgassing test was carried out on simple coated and nano-charged coated samples with 0.5 and 1 % of nano Silica. All the materials analyzed showed off a very good behavior, remaining very under the limits imposed by the ECSS reference standard. Because of the best results given by the 1% charged samples, for the rest of the study this percentage was chosen. The test campaign continued evaluating the Thermal Expansion Coefficient of the materials under investigation in the range 25-1400°C. The best thermo-mechanical and structural stability was found in the Al2O3 nano-coated samples, which showed off a very good morphology from a SEM/EDX imaging campaign. The residual thermal stress was evaluated by mean of Thermal Cycling between the extreme temperatures of +90° C and -60°C, typical orbital thermal reference points. The coated samples showed very good insulation properties, being much more inert than not coated samples in a thermal-vacuum environment. The nano-filled samples showed off the best insulation behavior as the Silica lower the thermal conductivity of the samples. No modification in shape or color was found after the test, as well as no cracks on the coating. Atomic Oxygen / UV test campaign ended the Space Environment Characterization. Nano-coated samples with all the Pyro-paint varnishes were analyzed by mean of an Atomic Oxygen Radio Frequency Plasma Source, reaching a Fluence of 8.7 E20 atoms/cm2. The best result was from Al2O3 in terms of Erosion rate. A SEM/EDX imaging campaign was then performed on Al2O3 sample: the analysis showed off that the thin Al2O3 ceramic nano-coating, is able to protect the bottom core from the detrimental aging effects due to atomic oxygen and ultraviolet irradiation. The prototype built for TPS was successfully tested in Laboratory Environment by a torch test. For over 7 minutes the nano-coated leading edge was exposed to a temperature of over 1200°C. The insulation properties of the coating, the C/C shell and the SiC foam led to an average temperature difference of 580 °C between the front side and the inside of the carbon shell, and of 609 °C between the front side and the interface. The result obtained by this brand new type of coating is remarkable: a nano-Silica filled coating composed by Al2O3 is capable to protect the substrates both from Space Environment hazards, such as Atomic Oxygen, Ultraviolet Radiation and Thermal stresses, and from a high energy and temperature flux that simulate the Re-entry conditions. As a prosecution and a planned application of the present activity, The Space Environment Protection Plate System will be also tested both in laboratory for the evaluation of the thermal properties, in collaboration with the Moscow Aviation Institute that yet supported the research, and in orbit, mounted on a cube-sat, designed and realized in collaboration with the Aerospace Systems Laboratory, during next year, as a technological demonstrator. The design phase of the cube-sat is completed as well as the preliminary tests on electronics and Outgassing of solar panels. The results are encouraging to continue on this research path, as, at least for one of the materials proposed, the spaceability and the reliability for the protection from a long life stay in LEO environment was reached, as well as a very good resistance to a possible re-entry scenario