Speaker
Description
There are a number of design-for-demise methodologies which can be employed to increase the demisability of spacecraft components, which reduces the ground risk from re-entry. One of the less used concepts is to increase the heating to a component. Active methods, such as the use of energetic materials, and passive methods such as the use of lattice structures have been attempted with varying degrees of success. Recent high enthalpy demise ground testing has shown that heating to objects is significantly affected by length scales and flow paths. This suggests that simple geometric features such as holes, grooves and steps have the potential to increase the heat flux received by a spacecraft component during re-entry, with minimal change to the component.
In order to test this hypothesis, a set of simple shape adaptations have been tested in the DLR H2K cold hypersonic wind tunnel. Four concepts were selected based on sparse literature data on the likely effectiveness; holes, facets, steps and grooves. The test involved the use of a PEEK model, with infra-red thermography used to measure the temperature change at all locations on the model. As PEEK is a well characterised material in terms of emissivity and conductivity, the heat flux can be inferred from this data. A one-dimensional semi-infinite wall assumption was used, which will have some errors at sharp corners, but the methodology provides a good estimate of the heating to the features tested.
The grooves, steps and facets concepts were tested by adapting a cylinder to include these features. Tests were performed with the cylinder axis normal to the incoming flow, and at 45 degrees to the incoming flow. All three concepts showed a significant increase in the peak heat flux, and both steps and grooves produced a significant increase in the overall heat flux. The facets were less effective, resulting in a similar integrated heating over the surface. Grooves were the most effective of these concepts, both in terms of the integrated heating, but also in terms of the effectiveness at essentially any angle of attack. This is due to the grooves continually providing new leading edges such that the boundary layer growth is limited. Preliminary indications are that heat flux increases of 50% could be possible, which is significant in terms of the demisability potential of materials such as steel.
Holes were tested on a sphere cap. Three different hole sizes were used, and significant heating is observed downstream of the hole. Clusters of holes have also been shown to be highly beneficial, with smaller gaps between the holes increasing the effectiveness. Closed holes were also tested, and showed some benefit, but significantly less than through holes.
Two of these concepts, holes and grooves, will be tested further to demonstrate their effectiveness in a plasma wind tunnel.