The pioneering Australian Space Scientist Professor Ray Stalker did something in the late 1960s that no-one had ever done before. He found a way to create true hypersonic flow in the laboratory. He invented the “free-piston shock tunnel”.
The shock tunnel uses a heavy piston that speeds up to almost 1000 km/hr to burst a thick steel plate. This creates a shock wave that travels down the shock tube to heat and compress the “test gas”, which is the air that we want to pass over and through our scramjet. Once the shock wave has given its energy to the air, the air is accelerated through a hypersonic nozzle and ploughs into the test section at enormous speed, creating the conditions we need to test scramjets on the ground.
When a scramjet is flying in the atmosphere, it is moving at hypersonic speed through still air. In the laboratory, the scramjet is stationary, while the air is moving at hypersonic speed. As far as the scramjet is concerned, this is the same thing. Its only the “relative” speed that matters.
Leaders in hypersonics
The University of Queensland in Brisbane is home to the most productive scramjet test facility in the world. It’s a free-piston shock tunnel called “T4” that was designed by Professor Stalker. T4 has been operating since 1989 and has performed over 12,000 hypersonic experiments!
Figure 2 shows a video of a scramjet test at the University of Queensland. When you watch the video you will see that the tests are quite violent! However, you will also notice that the experiment only last for a very short time. In fact, a typical test time for a scramjet test in T4 is three milliseconds! That is three-thousandths of a second. It is much shorter than anything we as humans can experience!
A typical test time for a scramjet test in T4 is three milliseconds! That is three-thousandths of a second
So how can we do a useful scramjet test in such a short time? Well it’s the tremendous speed of the air that makes it all possible. Some quick numbers tell the story. In a typical experiment the air is moving at roughly 2000 m/s. So if the experiment lasts for 3 milliseconds, then a hypersonic jet of air 6 meters long passes through the scramjet. A typical model that is tested in T4 has a length of one meter, so the jet of hypersonic air created by T4 is 6 times longer than the model. This is more than enough to do a scramjet test.
But how do we measure anything in such a short time?
Figure 3 shows an image of a scramjet model that has been opened up so we can see inside it In a typical experiment, hydrogen fuel is injected at the front of the scramjet and combustion occurs between the hydrogen and the oxygen in the air. In order to measure the effect of the combustion we need very specialised electronics that can respond quickly enough. In a typical experiment, we record data at 1 million samples per second. Nowadays, with the amazing electronics that we use every day, this is not that difficult.
To finish up I thought we would put a human face on all the technology. Figure 4 shows PhD student Ryan Whitside and Research Fellow Wilson Chan who are doing experiments in T4.