Traditionally the use of heavy, heat resistant, rigid shields have been used to ballistically guide spacecraft back into the atmosphere. This expensive technology is restrictive and often results in spacecraft not returning to earth since it is cost ineffective.
Ultra lightweight ballutes (inflatable parachutes) provide excellent performance and packaging benefits, however the analysis and design of an inflatable structure for re-entry involves the need for many new technologies.
The concept of using inflatable’s for re-entry has existed since the 1960’s, but lack of faith in the technology prohibited any advancement to testing stages. However significant advances have been made in the areas of lightweight flexible materials, aero thermal analysis, trajectory control, aero elastic modelling, blow up simulation and hypersonic testing.
This has re-sparked much interest in, and advancement of, the testing of ballutes by several space agencies around the world. Existing ‘fore-body attached’ designs by ESA and NASA, as well as a hybrid of the two, have been analyzed at peak pressure and heating positions within their trajectory to evaluate the effect of varying design choices.
After modelling expected trajectories, peak heating rates and pressure loads were found to occur at altitudes ranging from 54 to 62 km and 62.7 to 67.7 km altitude respectively. These altitudes are much higher than traditional re-entry techniques where atmosphere density is much lower. This had the effect of keeping heating rates between 105 to 200 kW/ over the inflatable surface whilst exposed to pressures between 3.5 to 5 kPa (all significantly lower than traditional techniques).
A larger half cone angle (as used by NASA in their IRVE design) had the effect of much smoother and lower pressure and heating distribution over the windward face. However the larger diameter saw larger bending moments and shear stresses in the material where attached to the central payload. ESA’s multi-stage IRDT design, attached to a much wider payload which bears much more share of the acting pressure, as well as a lower aspect ratio, experienced much less stress in its materials however its smaller half cone angle caused a turbulent distribution of heating and pressure. It also has the added risk of a secondary inflation to slow the ballute to safe landing speeds once peak loads have been experienced.
Errors between theoretical and computational methods were kept between 1 and 16%, adding significance to the authors’ findings, however further experimental testing must be done, with consideration given to the effects of aero-elasticity and ablation, as well as construction techniques.