Development and ground tests & Models validation
Development and ground tests
Tests in representative environment are needed to increase the knowledge of the materials and structural assembly in order to perform the critical assessment and support the development of the proposed inflatable TPS solutions. Laboratory tests will be performed on the two building elements of inflatable heat shields: the flexible TPS membrane and the large inflatable structure. Successful testing will eventually increase the TRL of the inflatable heatshields technology to TRL 4 and the TRL of multi-layer flexible TPS (thermal blanket) to TRL 5.
Within the consortium, high expertise in laboratory testing is available, in particular:
- Critical to the development of the inflatable heat shield technology is the development of flexible material systems whose performance must be verified through testing, in order to assess its capability to withstand the thermal conditions experienced during entry. Samples of the flexible TPS membrane (FTPS) will be tested in the DLR's arc-heated facility LBK with its two test legs L2K and L3K in both Earth and Mars environments, and different configurations (e.g. stagnation point and shear testing, to simulate the fluxes acting on different regions of the heatshield during entry).
- The dynamics of the morphing process require validation to guarantee that desired shape is achieved and that the inflated system provides the desired structural performance. The full inflatable structure will be packed and inflated on ground at 1:1 scale in Arescosmo facilities. On-top of this inflation/deployment test, a static load test series will be conceived in order to evaluate the capability of the heatshield to withstand mechanical load without any permanent deformation. This will be done by means of distributed masses hanging on the IAD inflatable structure. These tests will aim at reaching TRL 4 for the structure element of the inflatable heatshield concept.
Models validation
Validation of the numerical models is another objective of the test campaign, and it is a critical aspect to increase robustness of the inflatable heat shield design process and the extrapolation of results to flight. Within EFESTO, three areas will be covered:
- Materials: thermal characterization of flexible TPS membranes is the starting point for the definition of a suitable solution. Aerothermodynamic (ATD) assessment of FTPS layups requires knowledge of the thermal properties of all materials, i.e. outer layer, insulation, gas barrier. The following thermal properties will be measured: (1) thermal conductivity, (2) density, (3) heat capacity. Measurements will be carried out according to common standards and cover the complete operational temperature range of the reference missions. In case of ablative materials, the measurements will include state changes caused by pyrolysis.
- Aerothermodynamics: aerothermodynamics will both for aeroshell and payload integrity define feasibility conditions for nominal flights for either Mars or Earth missions. Validation of wall temperature and heat flux distribution based on ground test results is a target for a thermal balance assessment. Indeed surface ablation, transient heat transfer in decomposing materials and other fluid-structure interactions have to be modelled and coupled to ATD simulations. Extrapolations to flight, based on ground tests validation, are a key point aiming at increasing the TRL of inflatable heat shields in Europe. Moreover, experience gained in Europe25 gave important lessons learned for payload design/housing improvement at the back of the aeroshell to prevent/mitigate any over-heating on payload involving wake flow Radiative Heat Transfer . Radiative heating effects on the payload due to the heatshield ATD behaviour shoud be correctly included in the numerical model and validated.
- Structural morphing process: validation of the inflation process can be achieved by an hybrid analysis-test method, which foresees the implementation of virtual (i.e. CAE/FEM based) models of the IAD, both for the relevant EDL application and for the ground test model. Basically, those two models will differ for what concerns the gravity environment, of course 0-g for the EDL model and 1-g for the ground model. In the case of the ground model, validation will be seek via correlation between test data and analytical data.