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Technologies for Safe and Controlled Martian Entry

Final Report Summary - SACOMAR (Technologies for safe and controlled Martian entry)

Executive summary:

The SACOMAR project brought several European and Russian scientists in the field of Martian entry technologies together. Although the team was geographically distributed quite broad, the cooperation worked extremely well. There was an intense scientific exchange, which helped achieving all objectives of the project. In summary, the following goals were achieved:
- a project file transfer protocol (FTP) server and project portal on website base have been established;
- a reliable data base resulting from experimental and numerical studies for the design of future Mars missions has been created;
- main scientific achievements are the precise determination of the surface catalysis of materials in a broad range in the Martian atmosphere and improvement in transport properties and thermochemical modelling of computational fluid dynamics (CFD) codes;
- the knowledge on Martian entry has been extended and can be transferred for future interplanetary missions;
- the cooperation between European Commission (EC) and Russia worked very well and will help to intensity the collaboration in future space programmes;
- SACOMAR activities have been presented in several international conferences and published in journals.

Project context and objectives:

When entering a planetary atmosphere the high-speed deceleration of blunt bodies leads to a strong bow shock formation in front of a capsule and heating of the gas and subsequent heating of the vehicle. The internal structure of the vehicle is thermally protected using a thermal protection system (TPS). Most of the capsules use a TPS based on ablation materials. The modelling of complex chemical processes in the boundary layer flow around the ablation and inside the ablation materials is very challenging and needs reliable experimental data gained by means of sophisticated measurement techniques. In order to extract the influence of different parameters on the aerothermal heating during Martian entry, a study on selected and well defined materials is an important step.

The main objective of SACOMAR is the experimental and numerical study of gas-surface interaction phenomena in the high enthalpy flow field behind the bow shock in front of a model at Martian entry flow conditions. The improvement of physical modelling using experimental data and its implementation into numerical simulation codes is essential to understand and interpret the physical processes. At the end, the project will allow to estimate the aerothermal loads on the vehicle more accurately. The main activities of the project are the following:
- definition of requirements on experiments, modelling and CFD codes using realistic Mars mission profiles;
- experiments on the measurement of flow parameters in the free stream and behind the shock and stagnation point heat flux rate;
- improvement of existing physical models with respect to non-equilibrium effects, transport properties and gas-surface interaction chemistry;
- implementation of improved physical models into the CFD codes and simulation of experiments;
- synthesis of the data and reporting.

In the reporting period, the following goals were achieved:
- a project ftp server and project portal on website base have been established;
- a reliable data base resulting from experimental and numerical studies for the design of future Mars missions has been created;
- the knowledge on Martian entry has been extended and can be transferred for future interplanetary missions;
- the cooperation between EC and Russia worked very well and will help to intensity the collaboration in future space programmes;
- SACOMAR activities have been presented in several international conferences and published in journals.

Project results:

Work progress and achievements for the reporting period

WP1 - Project management project management
- Effective management and coordination of the project to ensure high technical and economic efficiency
- The overall legal, financial and administrative management
- Control of financial and budgetary means of the project and supply of all necessary deliverables
- Decision on the evolution of the project as it progresses with respect to its milestone achievements.

All milestones of the project in this reporting period have been achieved.

WP2 - Technical coordination
Each work package (WP) will provide bimonthly a short status report on the work progress obtained, inform about possible delays and indicate any problems. Critical points will be reported in particular those which endanger the objectives of the project or delay the input towards other WPs.

Depending on the progress made, dedicated teleconferences will be organised by the coordinator with the WP leaders, to discuss the possible problems, propose solutions for the sake of the project.

The good technical coordination of SACOMAR allowed having strong interaction between experimental, modelling and numerical activities. This allowed achieving partially unique results concerning the aerthermodynamic issues of Martian entry.

WP4 - Requirements and synthesis
Task 4.1: Requirements
This task has the following main objective:
- definition of the requirements for the modelling, simulation and testing activities to be conducted in the framework of the SACOMAR project.

According to the work plan, the following main activities have been performed by TAS-I:
- overview on the current physico-chemical modelling techniques for carbon dioxide (CO2) high-enthalpy flows, including transport properties, gas phase kinetics and surface chemistry, their status and chances for improvement;
- identification of type of CFD simulations needed to gather information about the implementation of the developed models and of a standard method for data exchange;
- definition of the requirements on the experimental activity to be performed to exploit the Martian atmosphere's relevant properties in a typical entry trajectory;
- definition of criteria for the validation of numerical simulations results over experimental data and of their use for the extrapolation to flight task.

Task 4.2: Synthesis
The objective of this task was the synthesis of achieved results with respect to their application for future missions. The synthesis of the experimental and numerical work has been carried out. Critical assessment of the code-to-code validation and experiment-numeric comparison identified main outcome of the study with respect to the transport properties, thermal non-equilibrium and surface catalysis at Martian entry conditions.

TAS-I synthesis showed that the comparison of the shock stand-off distance, which is strongly linked to the thermo-chemical behaviour of the gas in the shock layer showed partially remarkable differences in the data of different codes. It was also the case for the temperature along the stagnation line from free stream to the model surface. The strong deviation of the ITAM data is mainly related to the use of a direct simulation Monte Carlo (DSMC) code for rarefied gases in comparison to the Navier-Stokes (NS) computations for continuum flow. The difference is more significant for the TP2, which is definitely in the continuum flow regime and therefore not suitable for DSMC computation. Another remarkable difference compared to the majority of the results is the data of the TAS-I NS-code.

While the DLR code TAU uses on temperature model, the codes of CIRA and TsNIImash use a two temperature model. The DSMC code of ITAM considers three translational, rotational and vibrational temperatures. The code of TAS-I uses besides the translational temperature, which is equal to the rotational temperature, five vibrational temperatures for all major molecules in the flow.

The most outcome was the improvements in the thermochemical and transport properties of the TAS-I code, which is the design tool of the industry.

The results of the study have been reported in the deliverable 4.2 'Synthesis of main results of the modelling and simulation'.

WP5 - Aerothermal tests
Task 5.1: Test plan
This task has the following main objectives:
- review of experience and facility capabilities of DLR, TsNIImash, TsaGI and IPM with respect to CO2+ N2 high enthalpy flows;
- definition of test conditions in terms of total enthalpy and Pitot pressure.

After a detailed analysis of the performance of testing facilities and their comparison with the important trajectory data points of the ExoMars entry phase, a test plan has been defined. With reference to the ExoMars trajectory four test conditions were identified which enable direct facility-to-facility comparison.

Finally, a test matrix was defined specifying test conditions in terms of total enthalpy and Pitot pressure for each test facility. Based on experiences and capabilities of the facilities, measurement techniques were specified to be applied for heat flux measurements and flow characterisation. The test matrix and test models including the instrumentation have been described in the SACOMAR deliverable D5.1 'Test plan for experiments'.

Task 5.2 'Tests in HEG shock tunnel'
According to the description in the description of work (DoW), the test model has to be manufactured, instrumented and installed in the test section of the shock tunnel HEG. Tests have to be carried out at two test conditions as defined in the DoW. The free stream has to be calibrated experimentally before the tests. The main measured parameters are the shock stand-off distance using the Schlieren technique, pressure distribution and heat flux distribution in the free stream and on the model surface.

The instrumented test model was manufactured and installed into the HEG test section. All calibration and main tests with CO2 flow have been performed successfully. The comparison between computed and measured shock stand-off distance is quite good.

The results of this work have been described in the SACOMAR deliverable D5.2 'Results of experimental study in the high enthalpy shock tunnel Göttingen (HEG)'.

Task 5.3 'Tests in IT-2 facility'
This task has the following main objectives:
- model designing, manufacturing and equipping with heat flux and pressure gages;
- measurement of stagnation pressure and heat flux distributions in the test section;
- model tests in IT-2 wind tunnel in CO2 flow at M=12 using Schlieren visualisation according test plan.

As mentioned before, numerical rebuilding has been carried out for all experiments. This step should show the ability and shortcomings of these codes. We start with the relatively low enthalpy experiments in the IT-2 facility of TsAGI. The main outcome of this activity was the demonstration of the effect of the CO2 dissociation on the specific heat ratio, which leads to change in the shock stand-off distance. Such results are very rare in the literature. Effective values of specific heat ratio have been obtained by TsAGI for all test three flow test regimes based on the values of the bow shock stand-off distance. These bow shock stand-off distances were experimentally determined by evaluation of the shadowgraphs taken in IT-2 (averaging the distances over several test runs for each flow regime) and allow for a comparison to the shock stand-off distances determined by CFD for regime 1 to 3.

The results of this work have been described in the SACOMAR deliverable D5.3 'Results of experimental study in TsAGI IT-2 hot shot wind tunnel'.

List of websites: http://www.dlr.de/as/sacomar

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