ESA – Aerothermodynamic Tools for Hypersonic Inflatable Decelerators

Vorticity was appointed by the European Space Agency to define a roadmap for the development of European capabilities in the field of Large Inflatable Entry Decelerators (LIEDS). Development strategies were defined for numerical simulation, ground facility/measurement techniques and flight capabilities; all of which may be undertaken by the Agency in the future.

The primary objectives of the programme were to:

  • Investigate the capability of ESA member states to develop LIEDs
  • Demonstrate the capability of ESA member states to test and characterise flexible Thermal Protection System (TPS) materials

The first activity in the programme was to define two candidate reference missions for LIEDs, which could be used to determine the key characteristics of LIEDs for potential missions and thus the facilities required to qualify them.

Reference missions, parameters and ESA member state capabilities

Two reference missions were selected. The first was delivery of a scientific payload to the Mars highlands and the second was the recovery of an ISS (International Space Station) resupply vehicle to the surface of the Earth after delivering its payload to the ISS. Following this, the aerothermodynamic requirements and other critical parameters for these reference missions were defined.

Measurement techniques for the parameters described in the first activity were evaluated. The measurement techniques were grouped into three categories: ground testing (including wind-tunnel testing and CMM measurement); free-flight testing (covering sounding rockets, balloon drop testing and ballistic range testing) and numerical methods (such as CFD/FSI and DSMC).

The capability of various facilities in ESA member states to perform these necessary measurements was then evaluated. Ground test, flight test and numerical methods were considered. Potential facilities within each domain were located, any deficiencies were identified and required improvements defined.

Aerothermal characterisation testing

In parallel, ESA member state capability to conduct aerothermal characterisation of flexible TPS material was developed. It was decided to conduct this testing because there was a relatively low level of knowledge within Europe of the aerothermal performance of flexible TPS. The aerothermal test points were defined based on the reference missions. Screening of possible materials was conducted and a TPS layup with an outer layer of Cerafib Tex99 and an insulation layer of Insulair NP 650 Plus was decided upon.

A custom graphite sample holder was designed and constructed for the testing. Four flow conditions were chosen which covered the two reference missions (Earth and Mars) and allowed determination of the influence on the material performance of peak heat flux, total heat load and atmosphere composition.  28 tests were conducted. The test campaign had two break points to allow tests later in the campaign to be agreed between the various stakeholders after examination of earlier test results. The Insulair and Cerafib materials were successfully characterised based on the test results gathered.

Development roadmap

A roadmap for developing the capabilities of facilities in ESA member states was then defined. The required measurements identified were compared to the existing facilities. One preferred method was identified for each measurement type, whether numerical, free-flight testing or ground-based testing and that method were often supported by other methods. For example, the preferred method of estimating the static continuum aerodynamic is through CFD/FSI software codes, but these codes will be validated via supersonic wind-tunnel testing and sounding rocket testing.

A roadmap was produced which presents a number of activities designed to improve European capabilities in the field of Large Inflatable Entry Devices to the point that mission design and implementation may be considered.