Advanced Ablation Characterization and Modelling

 

In the frame of FP7 the European Union specified innovative materials and simulation methodolo­gies as important space transport transportation technologies. This is in line with some on-going ESA TRP activities. The majority of the thermal protection materials in existence are old and are not in line with recent safety rules.  It can thus be seen that there is a European need for well characterised, accessible, environmentally friendly alternative thermal protection system materials, which can be designed and manufactured with acceptable costs.

During entry into a planetary atmosphere, a vehicle is subjected to severe heating caused by extremely high gas temperatures in the surrounding shock layer. Historically, an ablative thermal protection system has been used to protect the vehicle (Apollo, Galileo, Huygens, Viking and many others). In the last two decades space related material research activities in Europe almost completely shifted to reusable thermal protection systems. Since during that period most European missions were heading to low Earth orbit, there was no considerable space related research and development on ablative materials and the physico-chemical modelling of ablation processes. More recent as well as future missions more often on the exploration of other planets or on sample return to Earth, as e.g. the American Stardust mission or the European ExoMars missions. Therefore, a drift back to ablator materials has been observed.

A material is called ablative due to a certain thermophysical behaviour, not due to a certain microscopic structure. There are many different ablative materials with strongly varying mechanical and thermal properties. From the variety a certain ablative material can be chosen depending on the levels of heat flux and pressure that are expected for a certain space mission. The most important groups of ablative materials are listed in Table 1 together with the heat fluxes they might be exposed to.

 

Peak heat flux

Mission

Graphite / Carbon-Carbon

> 100 MW/m2

Jupiter, Saturn

Carbon phenolic

10 – 100 MW/m2

Earth return, Jupiter, Saturn, Venus

Silica phenolic

1 – 10 MW/m2

Titan

Cork phenolic

1 – 3 MW/m2

Mars – Launcher payload bays, Backshields

Silicone+internal charge

< 1 MW/m2

Launcher stages – LEO re-entry (leeward)

            Table 1: Ablative materials

 

 

The discontinuity in R&D activities on ablators becomes obvious in the level of complexity of models for the simulation of ablation processes. In many other physical and technical fields the level of complexity of simulation models has significantly increased during the last decades, e.g. in fluid dynamics or structural mechanics three-dimensional time-resolving simulation models have been developed and validated and are state-of-the-art models in technical application. Existing ablation models, however, are still equivalent to those of the late 1980s which are based on a one-dimensional treatment and simplifying considerations. Commonly the numerical core is very simple, the models neglect all aspects of fluid-structure interaction inside the porous charred ablator. Only conductive heat transfer and pyrolysis are modelled in detail inside the solid. No material anisotropy is considered, and the material is considered to exist in three states: virgin, char and reacting with a front between the zones (see figure on right side) For the virgin and char material data is required, and is interpolated for the reacting zone.  The validity of the existing models very strongly depends on the availability of mechanical and thermal propertie of both, virgin and charred material.

 

 

Neues Bild (23)

Principle of thermal protection by ablative cooling

 

 

                                                                                                                                                         

Objectives of ABLAMOD

Therefore the ABLAMOD project aims at providing a sound technological basis for the industrial introduction of advanced models for the description and characterization of ablation materials. On behalf of the European commission DLR works for these objectives in cooperation with European partners.

 

The most critical Research, Technology & Development associated building blocks to achieve this goal will be defined and all critical elements will be subject of detailed investigations by applying dedicated analytical, numerical and experimental tools.

The scientific and technological objectives of the project are:

  • to strengthen the space transportation capabilities and to enhance the capacities for space exploration by developing new phenomenological models to predict an ablator’s response in severe thermomechanical environments. Ablation materials are a key technology for current and future space exploration missions. By extending the prediction capabilities of ablation models safety margins for future exploration missions can be reduced providing additional possibilities for scientific and technological investigations.
  • to solidify physical understanding of material behaviour under extreme thermal loads. Currently, considerable geometrical and physical simplification is a common feature of existing models which include a detailed treatment only for conductive heat transfer and pyrolysis inside the ablation material. Processes arising from thermomechanical fluid-structure interaction, as e.g. the internal flow of the pyrolysis gas or internal radiation are either geometrically restricted to the outer surface or neglected completely. The main goal will be to identify all processes that have a significant impact on ablation and to develop phenomenological models for their description.
  • to apply novel sophisticated measurement techniques for the characterization of an ablator’s behaviour inside a severe thermo mechanical environment. The corresponding measurements will adjust the complexity level used for characterization to the complexity level of the advanced models. The necessary optical techniques will be identified and applied.
  • Spectroscopic techniques wlll be applied to characterize in detail the interaction between pyrolysis gas and hot environment in order to identify the physical states of all important gaseous components.
  • Highly resolved video techniques will be developed which allow for a continuous assessment of surface degration.
  • to use the most powerful European facilities for model validation: The validity of ablation models very strongly depends on the availability of mechanical and thermal properties of both, virgin and charred material which in addition must be provided for the complete temperature range. These properties typically are provided from laboratory measurements which can be performed at moderate thermal loads only and a validation in realistic thermomechanical environment is required. Dedicated validation experiments will be performed in the most powerful European arc heated facilities.
  • to increase the innovation capacity of future developments by proposing a new concept: Current models are tuned to a single material and cannot make extrapolations to different (even very similar) materials. By increasing the understanding of the physics, a step towards a predictive ablator modelling capability, which will improve the ablator design capability and allow tailoring of materials to specific mission, can be made.
  • to apply the developed modelling code for flight extrapolation: after its validation with experimental data the new modelling code will be used to estimate the performance of the ablative thermal protection system for realistic launcher and re-entry flight configurations

 

European value

The proposed developments will contribute to

-        improve the European capabilities to access planet surfaces, to collect and finally return samples to Earth in the frame of future space exploration activities,

-        enhance the reliability and cost efficiency of space transportation system by identifying the potential for reducing safety margins and enlarging the payload capacities,

-        strengthen the technological innovation and growth potential by improving physical understanding and providing new models with considerable potential for future industrial use.

enlarge the European industries’ competitive strength in use of tailored high performance materials not only for space application, but in any technological field with extreme thermal loading.

 

 


Participants:

German Aerospace Center, DLR

ASTRIUM  (Airbus Defence and Space), AST

Avio Propulsione Aerospaziale,  AVIO

Centro Italiano Ricerche Aerospaziali, CIRA

Fluid Gravity Engineering Ltd., FGE

University of Strathclyde, UNIV. of STR

Von Karman Institute, VKI

AMORIM, AM

Austrian Institute of Technology, AIT

Österreichisches Gießerei-Institut, OGI

 

 

 

Contact:

Dr.- Ing Ali Gülhan

German Aerospace Center

Institute of Aerodynamics and Flow Technology

Supersonic and Hypersonic Technology Department

Tel.: +49 2203 601 2363

Fax: +49 2203 601 2085

E-Mail: Ali.Guelhan@dlr.de

 

 

 

 

Meetings and conferences:

2013 01 23 Kick off

2013 04 26 Telecon

2013 07 03 Porto, 1. Project Meeting

2014 01 16 Vienna, 2. Project Meeting

Reports:

Requirements on material samples and characterisation

Cork based ablator samples

Carbon based ablator samples

Test plan for validation experiments

CFD simulation of the flow parameters in L3K and SCIROCCO

Characterization of ablator samples before testing