The most efficient way for a spacecraft to slow itself down in order to orbit or land on a celestial body is to rely on the presence of an atmosphere and "aerobrake" using an ablative heat shield. Ablation is the process whereby shield material transforms and vaporises under intense heat due to friction with the planetary atmosphere, sacrificing itself for the payload. Control of and communication with a spacecraft is limited during atmospheric entry and a shield that is too thin or one with the wrong kind of thermal properties would prove fatal. Since the 1950s we have known enough to protect relatively small, specialised unmanned capsules for entries into several planets and moons in the Solar System. But large safety factors are used to account for our inability to more precisely predict the optimal shielding, and as a result smaller payloads are carried. We need a way to repeatedly test our theories and computations on the ground, since rare flight test opportunities are so expensive. An expansion tube is the facility that comes closest to recreating the extreme flight conditions, and X3 at the University of Queensland is the largest such device in the world.
The most common ablative heat shield is the charring ablator. It releases volatiles, usually hydrocarbons, in a step called pyrolysis, leaving behind a porous "char" which provides an effective layer of thermal insulation for the payload but which itself can ablate when the local conditions are extreme enough. Under internal pressure, deeper layers of volatiles are injected up through the char. Their change in phase absorbs a great deal of energy which lowers the char temperature and blocks heat transfer from the shock layer. But what effect does this injected layer of gas have on the fluid dynamics of the outside flowfield? To what extent does it encourage instabilities that work against its cooling benefit by bringing super-hot shock layer gas into contact with the shield surface?
A novel scaled model has been designed which can be used to help address these and other questions. Placed inside the X3 expansion tube and exposed to air and nitrogen flows of approximately 8-9 [km][s]−1, the steel model injected hydrogen gas at room temperature through laser-drilled "nozzlets" prior to test gas arrival in order to represent, to the extent possible, injection of volatiles from a charring ablator. An expansion tube relies on fine control of the pressures in each section and the influence of pre-injected gas was largely unknown. Furthermore, since X3 is itself still an experimental facility, there was a need to better characterise the test gas conditions.
Computational Fluid Dynamics (CFD) simulations were used to address these problems, and in doing so helped outline a new, more detailed picture of test gas flow in an expansion tube. This showed, surprisingly, that the test gas moves faster than the shock wave in the laboratory frame of reference. Thermocouple sensors embedded in the model provided heat transfer traces which were compared with traditional empirical predictions and nonequilibrium CFD simulations, while optical images provided additional confirmation of the test conditions. Simulation was compared with experiments and showed that surface "blowing" cooled the surface only up to a certain level, before mixing of the injection and hot shock layer led to heating approaching no-injection levels.
Other injection methods can be more realistic, but this approach to ablation simulation arguably offers a valuable means of isolating phenomena. For example, mass flow rate can be metered and large numbers of laser drilled nozzlets can in theory improve the realism. The nature and role of turbulence in blunt body ablation is an outstanding problem in hypersonic aerothermodynamics. A major motivation for this work was to map a path out for tackling this very difficult question, even if it could not be addressed directly here.