Speaker
Description
The X2 expansion tube at the University of Queensland (UQ) is a long-running, high-enthalpy facility for planetary entry simulation, capable of reproducing conditions across all major planetary atmospheres at speeds up to 20 km/s. To meet increasing demands for higher fidelity and broader condition coverage, X2 is currently undergoing a comprehensive upgrade program aimed at expanding capability and improving experimental fidelity which will be discussed in this work.
X2 is capable of testing test models of up to around 100 mm diameter for around 100 microseconds. The ability to perform experiments with test models means that X2 is well equipped to study important planetary entry phenomena such as convective and radiative heat flux, thermochemical relaxation, shock standoff and novel phenomena such as magnetohydrodynamic aerobraking where magnets are used to influence the ionised post-shock planetary entry flow. X2 is also equipped with comprehensive physical and optical experimental hardware such as high-speed heat flux gauges, ultra-high-speed cameras, and an emission spectroscopy system ranging from the vacuum ultraviolet (VUV) down to 120 nm and the mid wave infrared (MWIR) up to 5 micron which allows us to comprehensively study the majority of important planetary entry phenomena.
At this point, X2 has been operational for 30 years. Recently we have engaged in a sustained effort to enhance and upgrade X2 to support its next 30 years and prepare it to take the high-fidelity experimental data required to continue improving our understanding of planetary entry phenomena. These upgrades target three key advances: (1) continuous expansion of the achievable velocity envelope, (2) improved reconstruction and diagnostics of test conditions, and (3) higher fidelity through reduced contamination and improved facility characterisation. This will be achieved through a multi-pronged approach of developing new facility driver conditions to increase the facility’s performance envelope at both the low and the high end, performing more detailed diagnostic measurements to improve test condition characterisation, and developing a high-vacuum system for X2’s shock tube to minimise contamination in our experiments.
Currently, with its powerful free-piston driver, X2 can achieve test conditions between roughly 6 and 15 km/s in air, which increases to up to 20 km/s for lighter giant planet entry hydrogen/helium test gases. Conditions around 3 km/s can be achieved using a fixed volume ‘cold driver’, but currently a gap exists between the capabilities of the cold driver and the current minimum performance free-piston driver condition. Recent work has been focused on designing new low compression ratio free-piston driver conditions to fill this gap. New conditions have been designed, preliminary ‘blanked off’ tests have been performed to test the driver conditions, new diaphragms have been procured, and in the coming months these new driver conditions will be tested on the facility.
In a similar manner, new driver conditions at the higher end have also been designed recently, allowing us to target even faster giant planet entry conditions above 20 km/s. Preliminary experiments have been performed which confirmed the higher performance of these conditions, and we are currently designing a heavier piston to better allow these conditions to be performed routinely on the facility.
Being a free-piston driven test facility, X2 is powered by a free-piston which compresses the facility’s driver gas up until a steel diaphragm ruptures, and then continues to compress the driver gas after diaphragm rupture to maintain an approximately constant driver pressure throughout the experiment. To better understand the driver and to aid in designing the new driver conditions and to understand the effect that the driver may have on X2 test conditions, we are in the process of installing an ‘instrumented pressure plate’ at the end of X2’s driver which will allow us to measure driver pressure, pressure next to the primary diaphragm, and hopefully piston position with a microwave system for every experiment. We are also in the process of installing a series of pressure ports down the length of X2’s driver to both track the piston and to measure the pressure behind it as travels down the tube. The pressure behind the piston is an important metric for understanding the pressure losses through the facility’s launcher which holds the piston before the experiment and through which compressed air from the facility’s high pressure reservoir flows through to propel the piston during its stroke.
In an expansion tube, like any shock tube based facility, measurements of facility shock speeds down the length of the facility’s driven tubes are necessary to aid reconstruction of the generated test conditions. Recently, there has been an acknowledgement that the shock speed history down the whole length of a facility’s driven tubes is important for properly understanding the final generated test condition. To address this, we have developed a microwave interferometry system for X2 which provides high-resolution shock speed measurements approximately every tube diameter down the full length of the facility for most test conditions. We are currently in the process of setting this system up so it will be used for all future X2 experiments and improving data analysis methods to increase measurement resolution even further for where it is needed.
In some cases the microwave interferometry system is not able to be used. For these cases, we have developed a new shock speed measurement system using ionisation gauges which discharge a capacitor when the flow passes a probe by placing two closely spaced electrodes in the flow at the surface of the probe. This system has the benefit of working well for low density conditions, which may give a very small voltage response when traditional high-speed pressure transducers or ‘PCBs’ are used for shock speed measurements. It also has the added benefit that any number of probes can be ‘daisy chained’ on a single instrumentation channel, making it a cheap and efficient way to perform medium resolution shock speed measurements for low density conditions. Currently we have been able to optimise the circuit to achieve a signal to noise ratio at shock arrival of 400 for these probes compared to 60 for a pressure transducer for an 18 km/s giant planet entry condition with an acceleration tube fill pressure of 0.5 Pa and a post-shock pressure of the order of a kilopascal. In the future, we are planning to further optimise the circuit to hopefully increase this signal to noise ratio further. We currently have two probes installed at the end of X2’s acceleration tube across from our standard PCB pressure transducer mounts. In the future, we have plans to install another 10 probes down the full length of X2’s acceleration tube after the circuit has been fully optimised.
While many X2 experiments are performed with air test gases, being a facility focused on the study of planetary entry, we also perform experiments using giant planet (hydrogen/helium), Venus and Mars (carbon dioxide and nitrogen), and Titan entry test gases (nitrogen and methane). When using non-air test gases especially, any residual air in the shock tube before the test gas is filled or water vapour which outgasses out of the facility’s walls when the shock tube is held at vacuum before the experiment, causes error which would not be present in the real planetary entry scenario. Currently, we use a standard rotary vane pump to evacuate X2’s shock tube, reaching ultimate pressures of the order of 10 Pa. This is not ideal for ensuring that all of the air is removed from the shock tube and that outgassing is maximised before the shock tube is filled, ensuring a high purity test gas. The tube is flushed with test gas several times to try to get all of the air out, but a better solution is needed. For this reason we are designing an automated system with a conformal poppet valve and a turbo pump to reach much lower base pressures and greatly increase the purity of the test gas for all of our experiments.
This presentation will discuss the current progress of these upgrades and how they are setting the X2 facility up for the future. Together, these upgrades position X2 for a new generation of high-fidelity planetary entry experiments, with improved coverage of the velocity space, enhanced diagnostic capability, and significantly reduced uncertainty in test condition reconstruction.
Summary
The X2 expansion tube at the University of Queensland (UQ) is a long-running, high-enthalpy facility for planetary entry simulation, capable of reproducing conditions across all major planetary atmospheres at speeds up to 20 km/s. To meet increasing demands for higher fidelity and broader condition coverage, X2 is currently undergoing a comprehensive upgrade program aimed at expanding capability and improving experimental fidelity which will be discussed in this work.