The Galileo probe’s 47.5 km/s entry into Jupiter on December 7th, 1995 was an engineering triumph. The probe survived it’s entry into the largest gas giant planet in our solar system, and the full scientific mission that it was built for. However, analysis of the actual ablation of the Galileo probe’s heat shield performed by Milos showed that the ablation on both the stagnation point and the frustum of the probe’s sphere-cone heat shield did not agree well with what had been predicted, and that only half of the heat shield actually ablated during the entry. This means that the heat shield, that made up half the mass of the whole probe, was larger than was required.
The Galileo probe’s heat shield was designed using two separate CFD codes, and three different types of experiments to validate the simulations and some parameters used in them. Laser irradiation experiments and arc-jet experiments in a 50%H2/50%He mixture were performed to test the heat shield materials and find their heat of ablation. Experiments were also performed in a ballistic range using air to study convective heat transfer, and argon to study radiative heat transfer as it had been found that argon closely pproximated the radiative heat transfer of 89%H2/11%He Jupiter entry mixtures. The ballistic range experiments were able to recreate the levels of radiative heat transfer that they expected for the actual flight.
Computational Fluid Dynamics (CFD) studies have aimed to properly recreate the ablation of the Galileo probe’s heat shield. These include a paper by Matsuyama et al in 2005 that was able to recreate the frustum ablation by using the injection induced turbulence model proposed by Park, the correct atmospheric composition at the entry point, and more complex coupled radiative energy transfer. Stagnation point recession was still overestimated. A paper by Park in 2009 was able to recreate the stagnation point recession “fairly closely” by improving on past methods and including radiation absorption in the vacuum ultraviolet region and the effect of spallation.
While the majority of gas giant entry research carried out in the last fifty years was either done to design the Galileo probe, or to analyse issues with it, the US National Research Council “Vision and Voyages for Planetary Science in the Decade 2013-2022” report identified probes to Uranus and Saturn as high priorities for future space missions giving new context to the study of gas giant entry in the coming years. While simulations have made up the majority of gas giant entry research in the two decades since the Galileo probe’s entry, it is worth considering how experiments can aid the development of the next generation of gas giant entry probes.
The expansion tube, a modified shock tube that uses an extra low pressure shock tube to accelerate the shocked test gas to superorbital planetary entry conditions (through an unsteady expansion), typically between 6–15 km/s, is potentially well suited for simulating entry for planned missions to Uranus and Saturn. Expansion tube experiments using the true gas composition will also allow the flow phenomena experienced during an actual gas giant entry to be studied, something that was not able to be done when the Galileo probe was designed, which involved experiments more focused on engineering design as opposed to how the fundamental physics affected the entry.
Due to the fact that simulating the lower end of gas giant entry at the true flight velocity (20 km/s) in an expansion tube is pushing the limits of what these facilities can do in their current configurations, a thorough understanding of how the facility functions is required so the maximum performance can be extracted from the conditions that are tested, and that is what this paper examines. In this paper a parametric study of the X2 expansion tube as it can currently be configured is carried out using an equilibrium analysis code developed by the authors to create a performance map of the facility, and this is then used to extract the proposed test conditions that maximise the performance of the facility. Initial experiments have been carried out with positive results seen.
The theoretical results and experiments shown in this paper indicate that it is pos sible to simulate 20 – 22 km/s Uranus entry in the X2 expansion tube at the University of Queensland with the current tunnel configuration. This compares well with the 22.3 km/s Uranus entry trajectory analysed by Palmer et al in 2014 based on an entry discussed in the aforementioned “Vision and Voyages for Planetary Science in the Decade 2013-2022” report.
