High Response Diamond Based Heat Transfer Gauge Development

Humans' exploration of solar system brings one of the most challenging engineering feats, the design of spacecraft which can survive extreme heat loads while re-entering Earth's atmosphere, such as recent asteroid sample return mission Hyabusa, and to enter other planets atmospheres such as the Mars Science Laboratory. Currently, large uncertainties exist in the expected flight heat loads leading to spacecraft heat shields which are much heavier than necessary when every gram is critical. This projects aims to build a novel high response diamond based heat transfer sensor which can be used to accurately measure the heat transfer on subscale spacecraft models in high speed wind tunnels.

To slow a spacecraft to either enter into planetary orbit or land on the surface, aerodynamic braking in the planet's atmosphere causes a conversion of the vehicles kinetic energy to thermal energy, generating gas temperatures hotter than the surface of the sun (6000 degrees C) in front of the vehicle. This results in enormous heat transfer to the vehicle, in excess of 100's MW/m2. This requires the vehicle to use thermal protection systems (TPS), which apply advanced ablating materials which limits the heat conduction to the spacecraft's structure. As the TPS adds significantly to the lift-off mass, any reduction in TPS will result in an increase in available payload to achieve mission objectives or reduce launch costs while increasing reliability.

To directly replicate the flow conditions experienced in spacecraft entry in a ground based wind tunnel is extremely difficult due to the high energy and pressures involved. This has led to engineers developing impulsive wind tunnels, which generate appropriate flow conditions for periods on the order of milliseconds by a cascade of high energy processes. Current heat transfer measurement techniques for impulsive high speed wind tunnels have been shown to be inadequate in measuring the heat fluxes in the front of subscale models in these tunnels due to slow time response (thermocouples, IR) or are damaged by particulates and have interference from the ionised flow field (thin film heat transfer and atomic layer thermopiles).

Diamond is an incredible material, and has unique combination of thermal properties allow it to be used as a very fast acting calorimeter up to 1 MHz. The gauge measures the temperature rise of a thin piece of diamond (50-500 micrometres), and by measuring the temperature rise on the rear side of the diamond using thin film gauges, this protects the electronics from particle debris and the ionised gas, and also high accuracy. For the high heat transfer rates seen on spacecraft, the temperature rise achieved would be appropriate for reliable measurement using current thin film resistive gauge technology.

In summary, this project will develop and test diamond based heat transfer gauges for application in accurately measuring heat transfer rates on spacecraft models. This aligns with the EPSRC research areas of both Sensor development and Fluid Dynamics research, with the proposed research both having direct impact, as well as facilitating research into the future.

UK based companies such as Lockheed Martin and space organizations such as ESA, could also benefit from the development of the diamond based calorimeter gauges. Data produced from the application of these gauges can be used to better inform the design of thermal protection systems (TPS) for spacecraft. This offers several benefits: missions can offer reduced lift-off cost due to lower mass; further mission capability is provided by increasing the available payload mass; destinations, such as Jupiter, that are currently impossible to reach due to the TPS taking all available mass on the spacecraft, now become viable.

Although the specific aim of this project is to develop heat flux instrumentation for use in ground testing of spacecraft, there are spin-off opportunities for the application of diamond based calorimeters to other heat transfer problems. Motivation to use the gauges is likely to be driven by niche qualities, such as the robustness of diamonds, electrical insulation capability or high response of the gauges. This would attract funding to UK industry, feeding directly into UK competitiveness in many fields, boosting R&D investment and improving quality of life for all.

The development of diamond based heat transfer gauges will have a positive impact upon UK based diamond manufacturer E6. This is through the feeding back of valuable data on the diamonds performance, which could allow for application in new markets. For example, diamond has a massive potential for LEO satellite cooling applications, however, doubts on its ability to survive high-speed particle impact has limited any development for this application. Thus, data produced on the toughness of artificial diamond when experiencing high-speed debris impact will provide valuable feedback to UK diamond manufacturers and expose new markets.

The gauge development employs innovative technology and recent advancement in artificial diamond production. The UK is a world leader in diamond manufacture, with companies such as E6 competing on the world market with their high quality artificial diamond based products. Through the promotion of a unique application of artificial diamonds in aerospace engineering design and development, the project will have a positive economic and social benefit on the UK based diamond manufacturing community. Publicity of the application of UK produced diamonds to spacecraft development, which evokes intrigue from the general public, will have positive implications for the industry.

If the potential of the gauges is fully realised, the gauges themselves will have significant commercial value due to the bespoke design, access to diamonds and expensive manufacturing equipment. Hypersonic experimentalists currently use heat flux gauges that are either produced by SME's (eg. Danec) are developed in-house at individual research institutes, like the Osney Lab, before being sold as bespoke gauges. The price of these gauges ranges from £500 to £2500 per gauge. There is the option for some of the diamond calorimeter technology to be patented and sold to other research institutes for application; however, this option is currently not under consideration. Any commercial activity would create further exposure in the community, which has the potential to attract R&D support for upcoming ESA funding calls related to spacecraft deceleration.

Development of heat transfer gauges in the UK that can be directly applied to the development of both UK and international spacecraft has an associated positive effect on the general population's quality of life. Recent announcements by Chancellor George Osborne on the UK's lead role in Mars missions, and the crowd-sourced Lunar mission show the UK government is keen to encourage public engagement with further space exploration, which offers a viable means of securing UK participation in any global space race, thus raising the prestige of UK industry and benefiting the UK workforce directly.