Spacecraft Heat Shield Material Testing in Plasma Flows

Context and potential impact:

This project is motivated by the engineering problems encountered during the high-speed atmospheric entry phase of spacecraft flight. Whether attempting to reach the surface of another planet or returning to the surface of Earth after a mission, a spacecraft must first travel through the atmosphere, typically at velocities on the order of 10 km/s. These high speeds generate high enthalpy plasma flows around the entry vehicle, which in turn impose extreme heat loads on the vehicle.

This phase of atmospheric flight therefore provides a major obstacle for advances in space exploration capabilities.

Thermal protection systems (also known as heat shields) are used to protect the vehicle from the high heat loads of atmospheric entry. This is typically achieved by using thermally insulating materials to provide a physical barrier between the hot surface flows and the payload. The heat resistance of the shield can also be enhanced by exploiting various physical and thermo-chemical effects.

For example, carbon-phenolic materials are commonly used in heat shield design; the vaporisation of the phenolic resin and subsequent interaction of the exposed carbon fibres with the flow provide effective mechanisms for heat absorption. However, since these reactions all occur and interact with each other simultaneously, they are difficult to predict, leading to high uncertainties in the aero-thermal response of these materials.

This necessitates large safety factors in the design process, which results in heavy vehicles, thereby resulting in increased launch costs and limited payload weights. Therefore, research aimed at improving our understanding of these materials is crucial for expanding the capabilities of space exploration technology.

This project falls within the EPSRC Engineering/Fluid Dynamics and Aerodynamics research areas. Aims and objectives:

The proposed project will aim to generate high-quality data to improve our understanding of the complex interactions between plasmas and heat shield materials. Experimental test campaigns will be conducted using the new Cold Expansion Tube (CXT) and Osney Plasma Generator (OPG) facilities at the University of Oxford. When used together, this allows for the testing of plasma pre-heated models at aerodynamic conditions representative of those encountered during real entry scenarios. The combined CXT-OPG facility will be the first of its kind in Europe once commissioned.

Novelty of research methodology:

Three different material types will be tested: solid graphite, a porous carbon-fibre pre-form (Calcarb) and a cork-based phenolic material (P50). This testing sequence first minimises the complexity of any interactions with the flow, before gradually introducing the complexities of porosity and then internal chemical reactions, allowing for individual mechanisms to be investigated in isolation.

The proposed test campaign will be accompanied by the development of new simulation tools, which will be validated and calibrated by the collected experimental data. The aim is that, once developed, these tools can be used alongside experimental campaigns to better understand existing heat shield materials, and aid in the development of new technologies.

Companies/collaborators involved: The project is funded in part by the European Space Agency (ESA) and Fluid Gravity Engineering Ltd.