Towards Demisable Fiber Reinforced Plastics

For avoiding the Kessler syndrome, satellites in low Earth orbits must be removed in due time after the end of their service life by re-entering into the Earth's atmosphere. Active, controlled re-entry requires availability of thrusters and propellant at the end of the mission. Thus, the active re-entry comes at a high cost and reduces the available payload mass of a mission. It is accordingly desirable to leave the satellite to itself at the end of its life and allow it to re-enter in an uncontrolled way. Unfortunately, parts of such satellites can survive the destructive re-entry flight and may impact on the ground. These debris pose a risk to public safety if they go down over an inhabited area. Rules that limit the acceptable ground risk per mission to a specific threshold are therefore in place in many countries. The typical accepted risk is one incident in ten thousand missions (1:10 000). Fiber reinforced plastics (FRPs) have been considered easily demisable because of their organic matrices and the fact that they would be able to burn at ground conditions. However, wind tunnel tests have revealed that FRPs have a very high demise resistance and can act as an ablative heat shield. This results in FRPs generally being a problematic material in the ground risk assessment. The good news is that there have been material level tests that showed a desirable demise behavior, which resulted in higher demise rates. The goal of the COMP2DEM project is identifying the microscopic and macroscopic parameters that determine the demise process and phenomena and explaining the differences in the observed behavior. Based on this knowledge, new fiber-reinforced plastics with increased demisability shall be formulated. The presentation covers the first phase of the COMP2DEM project: the screening of different FRP variants by thermophysical material characterization and demise simulation in the arc heated wind tunnels of DLR. New FRPs with increased demisability are currently under development and an outlook on the developments in the project is given

DEVELOPING A FLEXIBLE THERMAL PROTECTION SYSTEM FOR MARS ENTRY: SYSTEMS ENGINEERING, MECHANICAL DESIGN AND MANUFACTURING PROCESSES

A flexible thermal protection system (FTPS) is needed to enable the use of deployable and inflatable hypersonic decelerators. These decelerators could increase entry vehicle drag area beyond that of a conventional rigid heatshield, enabling Mars missions with greater landed masses and higher-elevation landing sites than can be currently achieved. An FTPS is essential to protect the hypersonic decelerator and payload from atmospheric entry aerothermal loads; conventional rigid heatshields are constrained by the available space within the launcher fairing. An ESA technology development is ongoing to raise the European FTPS technology readiness level from 2 to 3 and to define an FTPS that may be integrated with a Mars-entry inflatable hypersonic decelerator. This paper presents the FTPS requirements, material selection, mechanical characterisation and manufacturing technique development

DEVELOPING A FLEXIBLE THERMAL PROTECTION SYSTEM FOR MARS ENTRY: THERMAL DESIGN AND TESTING

Flexible Thermal Protection Systems (FTPS) are a key technology needed to enable novel inflatable and deployable aerodynamic decelerators. A development campaign is underway to raise the European FTPS technology readiness level from 2 to 3, advancing design and test capability. An FTPS suitable for a reference Mars landing mission is being designed. The FTPS has three functional layers: outer layers of Nextel 440 BF-20 fabric; insulation layers of SIGRATHERM GFA5 graphite felt and Pyrogel XTE aerogel; and a silicone-coated Kevlar fabric gas barrier. The density, specific heat capacity and thermal diffusivity of candidate materials was measured. Results were then used in thermal simulations to define a baseline layup. The layup thermal conductance was assessed in thermocouple-instrumented layup tests. Layups including joints were also tested and found not to have significantly different conductance. Layup test thermal simulations showed good agreement with the experimental data. Future work will include arc-jet tests and thermal model optimisation

Electrical resistivity measured by millisecond pulse-heating in comparison to thermal conductivity of the stainless steel AISI 316 L at elevated temperature

Electrical resistivity of stainless steel AISI 316 L was measured by millisecond pulse-heating in the temperature range from room temperature to the melting point at approximately 1400°C. The measurement results of electrical resistivity as a function of specific enthalpy were combined with results of specific heat capacity measurements by differential-scanning calorimetry to obtain the relation between resistivity and temperature. Additionally to electrical resistivity and specific heat capacity, thermal diffusivity, density at room temperature, and thermal expansion were measured. From these results, thermal conductivity was calculated. Using the results of thermal conductivity and electrical resistivity, a Smith-Palmer-plot was drawn. It shows a significant deviation from the Wiedemann-Franz law with the Sommerfeld value due to the lattice component, electron scattering by solute atoms, and other smaller contributions

Thermal diffusivity and conductivity of ruthenium in the temperature range 200 to 1670 K

This work presents experimental results of thermal diffusivity and computed values of thermal conductivity of pure polycrystalline ruthenium specimens in the temperature range 200 to 1670 K for diffusivity and 250 to 1650 K for conductivity. The results of thermal diffusivity were obtained by an interlaboratory comparison using the laser flash method. A brief description of the two measuring systems applied is given. Specimens were disk shaped, 2 and 3 mm in thickness and 10 and 12.5 mm in diameter. Literature data are used to correct for thermal expansion of the specimens. All the values obtained from the individual laboratories as well as a polynomial fit to the results over the entire temperature range are presented and compared with results found in literature. By using the thermal diffusivity data and previously measured results of specific heat capacity of different pure polycrystalline ruthenium specimens, the values of ruthenium thermal conductivity are estimated and presented together with related literature data.11th International Workshops on Subsecond Thermophysics (IWSSTPs), Jun 21-24, 2016, Polan

Thermal diffusivity and conductivity of ruthenium in the temperature range 200 to 1670 K

This work presents experimental results of thermal diffusivity and computed values of thermal conductivity of pure polycrystalline ruthenium specimens in the temperature range 200 to 1670 K for diffusivity and 250 to 1650 K for conductivity. The results of thermal diffusivity were obtained by an interlaboratory comparison using the laser flash method. A brief description of the two measuring systems applied is given. Specimens were disk shaped, 2 and 3 mm in thickness and 10 and 12.5 mm in diameter. Literature data are used to correct for thermal expansion of the specimens. All the values obtained from the individual laboratories as well as a polynomial fit to the results over the entire temperature range are presented and compared with results found in literature. By using the thermal diffusivity data and previously measured results of specific heat capacity of different pure polycrystalline ruthenium specimens, the values of ruthenium thermal conductivity are estimated and presented together with related literature data.11th International Workshops on Subsecond Thermophysics (IWSSTPs), Jun 21-24, 2016, Polan

Intercomparison of thermal diffusivity measurements on CuCrZr and PMMA

The results of an inter laboratory comparison of thermal diffusivity measurements on two different materials, namely a copper alloy (CuCrZr) and a polymer (PMMA), are presented here. Both materials were selected with respect to their different thermal conductivity, since the copper alloy belongs to the family of good metallic conductors whereas the polymer is characterized by a low thermal conductivity. The measurements of the thermal diffusivity have been performed within a temperature range from RT to 500°C for the copper alloy and from RT to 100°C for the PMMA, respectively

Demisability assessment of space materials

Since March 2014, all ESA satellites and launcher upper stages which will be disposed of by atmospheric re-entry at the end of their operational life must demonstrate that the risk from fragments surviving the re-entry and causing casualties on ground is less than 1 in 10,000. This casualty risk is calculated by re-entry tools simulating the uncontrolled re-entry event using a computer aided design model of the spacecraft. The uncertainties on several parameters such as the aerothermodynamics fluxes model, the structural interfaces model, the materials model, and the level of detail of the spacecraft architecture will have an impact on the re-entry event simulation and the associated casualty risk calculations. To better understand the uncertainties associated to material modelling, five materials often used on space missions were tested in Plasma Wind Tunnels, mimicking atmospheric re-entry environment. Thermo-physical properties, thermo-optical properties and mechanical properties at high temperature were also characterized. Analysis of the samples after plasma wind tunnel tests was performed. A database compiling the materials properties measured and the plasma wind tunnel test results was created. The material properties characterised and generated during the activities will serve as inputs for the re-entry simulation events at equipment and system level