David Gildfind

Expansion Tubes in Australia

Expansion tubes are hypersonic impulse facilities which are able to produce chemically correct, high enthalpy test flows. They are ideally suited to reproducing super-orbital flight through different atmospheres, and provide an increasingly important tool for aerothermodynamic studies of planetary entry vehicles. Furthermore, their uniquely high total pressure capability makes them the only class of facility currently able to reproduce the free-stream total pressures associated with high Mach number scramjet powered access-to-space. In 1987 The University of Queensland was the first research group to use a free-piston driver to power an expansion tube, and presently operates two such facilities, X2 and X3. While the free-piston driver can maximise the performance of the expansion tube, this mode of operation relies on complex flow processes, provides short test duration, and raises many other challenges in terms of test flow characterisation, instrumentation, and so forth. In the process of developing its own facilities, The University of Queensland has identified and addressed many of these challenges, and in X2 and X3 it has established high performance and reliable capabilities for routine ground testing at hypersonic flight conditions.

Design and Commissioning of a New Lightweight Piston for the X3 Expansion Tube

The University of Queensland’s (UQ) X3 facility (Figure 1) is the world’s largest free-piston driven expansion tube. It is used to generate hypersonic test flows such as simulation of planetary entry (6-15 km/s) or scramjet flight (3-5 km/s).

T6: The Oxford University Stalker Tunnel

The University of Oxford has embarked on developing the UKs fastest wind tunnel, T6, in collaboration with the University of Queensland (Australia) using technology pioneered by the late Professor Ray Stalker. The T6 facility couples the ex-Australian National University T3 free piston driver to the barrels, nozzles and test section of the Oxford gun tunnel. The facility can operate in three different modes; as a shock tube, a reected shock tunnel or as an expansion tunnel. The T6 facility will be unique to Europe allowing for hypervelocity ground testing not possible in any current EU facilities, whilst having the exibility to conduct tests across a large range of speeds and binary scaling products (pL) of interest.

On the current limits of simulating gas giant entry flows in an expansion tube

Atmospheric entry to the Gas Giants involves entry velocities from 20 - 50 km/s, which is mostly beyond the capabilities of current ground testing facilities. This paper details an investigation exploring the operating limits of the X2 superorbital expansion tube at the University of Queensland (UQ) for the simulation of radiating gas giant entry flow condi- tions. Theoretical calculations show that the X2 expansion tube can simulate a proposed Uranus entry at 22.3 km/s but falls below the necessary 26.9 km/s velocity to simulate a proposed Saturn probe mission. An experimental analysis was able to confirm theoretical results for a 19 km/s gas giant entry flow condition, but not for two proposed conditions above 22.3 km/s at this time. Further theoretical analysis investigated the possibility of using an established blunt body test gas substitution to allow Uranus and Saturn entry shock layers to be simulated at achievable test flow velocities. This analysis showed that with an increased amount of helium diluent in the test gas (or a change to neon, a heavier diluent), Uranus and Saturn entry shock layers can be simulated in the X2 expansion tube. Due to the current interest in sending atmospheric entry probes to Uranus and Saturn, this is a useful conclusion, demonstrating that there are experimental facilities capable of producing aerodynamic test flows for simulating gas giant entry conditions.

Experimentally simulating gas giant entry in an expansion tube

In 2010, planetary entry probe missions to Uranus and Saturn were proposed. This paper details an investigation exploring the operating limits of the X2 superorbital expansion tube at the University of Queensland for the simulation of test conditions related to these proposed entries. Theoretical calculations showed that X2 could recreate the stagnation enthalpy of the proposed 22.3 km/s Uranus entry but not the stagnation enthalpy of the proposed 26.9 km/s Saturn entry. Experiments were able to confirm the theoretical performance calculations. However, losses caused some of the experimental shock speeds to be up to 10% slower than predicted, and due to the high velocity nature of the experiments, the shock speed errors were large making it difficult to properly quantify the test conditions. Further theoretical analysis investigated the possibility of using a more powerful free piston driver to simulate the Saturn entry conditions, and the analysis showed that with a slightly more powerful driver than X2’s current most powerful configuration, the stagnation enthalpy of the proposed Saturn entry or other slightly faster entries proposed in the literature could be simulated. However, a very powerful driver would be required to recreate the stagnation enthalpies of these entries with the excess enthalpy needed to perform binary scaled experiments for an X2 sized facility.

Working towards Simulating Gas Giant Entry Radiation in an Expansion Tube

Further exploration of the four gas giants in our solar system, Jupiter, Saturn,Uranus, and Neptune, is important for many reasons. The gas giants contain matter produced during the formation of the solar system that is thought to hold valuable clues about the origins of life [9]; Saturn’s moon Titan is the only moon in our solar system with its own atmosphere (which the Huygens probe entered in 2005), and Jupiter’s four Galilean moons, Io, Europa, Ganymede, and Callisto, are all worthy of exploration.

High Mach Number Scramjet Test Flows in the X3 Expansion Tube

The University of Queensland (UQ) has two free-piston driven expansion tube facilities; X2 has a total length of 23 m and was originally commissioned in 1995 [1]; X3 is much longer at 62 m, and was commissioned in 2001 [2].

Magnetohydrodynamic Drag Force Measurements in an Expansion Tube

The use of a magnetic field to manipulate the shock layer for a re-entry vehicle has been proposed as a possible method for increasing drag of planetary entry vehicles. Whilst the field of magnetohydrodynamics (MHD) is well established, the application of this field to planetary entry vehicles is not well understood. The current state of the literature uses analytical, numerical, and experimental means to investigate the feasibility of this technology. However, the validity of the analytical and numerical methods used thus far have not been well validated due to limited experimental data in realistic flow regimes. For this reason, the current paper presents some of the first measurements in this field done in an expansion tube-a type of facility which seems uniquely suited for this type of study. The unique advantage of the expansion tube is that it can generate high enthalpy flows representative of true flight conditions with a non-ionised freestream. This is one of the critical areas for uncertainty in existing experimental data for magnetohydrodynamic aerobraking, which have been predominantly done in arc jet tunnels. Therefore, the focus of the current paper has been on proving that the magnetohydrodynamic force is present and can be measured in an expansion tube. Two different methods have been investigated to achieve this aim. These are accelerometer measurements and strain measurements. Drag force measurements have been taken with several conditions with the accelerometer technique, however issues have arisen with the strain measurements which have prevented data to have been taken. Issues encountered for both methods have been discussed here, and possible solutions are provided. The acceleration measurements have shown that an MHD force can be measured, but further analysis is required to understand these measurements.

Expansion Tube Magnetohydrodynamic Experiments with Argon Test Gas

In this study an expansion tube is used to generate an experiment to study and evaluate the concept of magnetohydrodynamic aerobraking for planetary entry spacecraft. An expansion tube can theoretically generate the required hypersonic flowfield within which ionisation is confined to the shock layer; this is an important characteristic of the true flight case which previous experiments with arcjets have failed to achieve. The first part of this paper explores the operating envelope for The University of Queensland’s X2 expansion tube facility, to identify flow conditions which should produce a significant interaction between the shock layer and an applied magnetic field. Argon test gas was selected for initial experiments due to its simple chemistry and low ionisation enthalpy, and equilibrium chemistry was used to compute the expected properties of the shock layer around a blunt body model. A candidate flow condition was identified for further analysis with finite rate reacting argon CFD, which indicated that relatively short shock layer length scales for the X2 expansion tube would limit the degree of argon ionisation that was generated, however, sufficient ionisation was still expected. Three sphere models were tested: a steel ball, a plain magnet, and a magnet coated with an electrically insulating ceramic paint. It was found that shock stand-off is unchanged between the steel ball and the plain magnet, but increases by approximately 10% for the coated magnet. Spectroscopy revealed that radiance along the stagnation streamline for some, but not all, of the relevant argon wavelengths approximately doubles when a magnet is used, either coated or uncoated. This increase in radiance does not appear to be associated with changes to shock stand-off. Further work is now required to explain the observed phenomena, and also to address some key experimental challenges, which include accurate measurement of shock stand-off and reducing the significant shot-to-shot variability observed for the coated magnet experiments.

High Mach Number and Total Pressure Flow Conditions for Scramjet Testing

Scramjet-powered access to space is expected to entail flight between Mach 5 and 15, along a dynamic pressure ascent trajectory of up to 2,000 psf (96 kPa) [1]. Scramjet engines typically need to be tested at sub-scale, even in the largest expansion tube facilities, and in these cases pressure-length (p-L) scaling is applied to maintain similarity for many flight parameters, including Reynolds number and binary reaction rates [2].