The world’s first wind tunnel capable of testing a full-sized hypersonic missile through the critical stages of flight has been operating in China and helped prevent expensive test failures such as setbacks seen in the United States, according to scientists involved in the Chinese project.
The facility’s name, location and top Mach rating remained classified, but it had played an important role in China’s hypersonic weapons programme, allowing ground tests that could expose critical engineering and technological issues before missiles go up for a test flight, the researchers said.
A hypersonic missile can fly five times the speed of sound, or faster, for thousands of kilometres in the atmosphere, relying on a small rocket booster to push it to near-hypersonic speed.
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Then the missile dumps the booster, removing a cover on an air inlet, and starts an air-breathing engine known as a scramjet to reach higher speed.
The newly revealed Chinese wind tunnel is not only large enough to house a full hypersonic missile, but can create environments to simulate the different stages of flight throughout the test, according to researcher Cui Jie with the Beijing Power Machinery Institute in a conference paper released on the China Academic Journal Network Publishing Database last week. Secrecy surrounds how long the Chinese facility has operated.
Cui and his colleagues said a full simulation was important because there was potential for problems during the transition from one stage of flight to another, especially when the rocket booster separated and the scramjet engine started.
As far as the researchers are aware, there are no other similar facilities overseas, including the United States.
The US has rapidly increased its funding for hypersonic research in recent years, but the Pentagon does not have an operational hypersonic weapon because of repeated test failures.
In July, the engine of a hypersonic air-launched rapid response weapon failed to start after its launch from a B-52 bomber, according to the US Air Force. In October, the US Navy and Army suffered a similar setback because of a rocket booster failure. In December, another air force hypersonic missile test failed at the launch stage.
The US was a leader in hypersonic flight technology and had some of the world’s best wind tunnels, according to Cui and his colleagues. But because of technical limits, existing American test facilities could only simulate a certain stage of flight.
Nasa’s 8-Foot High-Temperature Tunnel, for instance, was similar to the Chinese facility in size but could not simulate the booster separation, according to the Chinese researchers.
They said simulating the full sequence of hypersonic flight on the ground had many challenges. In a typical hypersonic missile, the rocket booster is built inside the scramjet engine’s combustion chamber. When it separates, the booster could shoot out of the missile and destroy the wind tunnel.
In the front of the missile there is a lid covering the mouth of the air inlet which would also come off when the missile reaches a certain speed. But a free-flying lid could hit the wall of the tunnel or even damage the missile itself.
Cui’s team arrived at unconventional solutions to these problems, including inventing a trap-like device to catch the discarded booster and absorb its kinetic energy, and a robotic arm to secure the lid of the engine air duct.
During a test, the large wind tunnel would produce extremely hot air – more than 1,700 degrees Celsius (3,092 Fahrenheit) – and accelerate it to extremely high speeds against the live missile.
Nobody knew if the new facility – with more mechanical devices and complex structures than any other wind tunnel on the planet – could survive the harsh environment and produce good simulation results.
The booster trap was a major concern for Cui’s team. Sitting immediately behind the missile, the device not only had to withstand the scorching flames of the rocket but also use a unique aerodynamic design to avoid blocking the exhaust when the scramjet engine ignited.
And the robotic arm they used to remove and secure the air inlet cover had to withstand extremely hot and fast air streams, and its movement had to be completed in a split second or the scramjet engine would be choked.
Despite the challenges, the sensitive components prevailed, according to the researchers.
Powerful wind tunnels have been difficult to build and operating them has consumed an enormous amount of energy. The local power grid in Sichuan province, for instance, experienced shocks, and even blackouts, when such machines started, according to a Chinese researcher informed about these incidents.
But the Chinese government and military continue to invest in building the costly test facilities.
The JF-22, the world’s most powerful hypersonic shock tunnel capable of simulating flight at Mach 30, will be completed in Beijing this year.
The new facility will help China maintain a lead in hypersonic technology for decades to come, according to the project scientists.
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