The biggest free-piston-powered shock tunnel in the world is already operational in southwest China. It enables low-cost, high-quality wind tunnel tests for hypersonic research.

The facility can simulate severe flight conditions at speeds ranging from 2.5 to 11.5 kilometers per second (1.55-7.14 miles per second), or more than 33 times the speed of sound, according to a report published by South China Morning Post (SCMP) on Thursday.

“(We) have built the world's largest free-piston driven expansion tube wind tunnel with high enthalpy,” said Lyu Zhiguo, the project team leader of the Hypervelocity Aerodynamics Institute under the China Aerodynamics Research and Development Centre in Mianyang. Enthalpy is a measure of the amount of heat in the air.

The cutting-edge facility, designed by an Australian scientist, will eventually aid in a number of missions, including sending Chinese astronauts to the moon and developing a hypersonic aircraft that could travel anywhere in the world in an hour, according to the researchers.

The testing tube is roughly twice as large as comparable facilities in the West, with a diameter of 80cm (2.6 ft), claimed the researchers.

“It can be used for national-level engineering projects, such as the return capsule of the lunar landing project, entry into an alien atmosphere with an interstellar exploration aircraft, and the development of hypersonic vehicles, such as scramjet-powered aircraft, by providing ground test support to simulate the environment at the 'second cosmic speed',” claimed researchers, referring to the escape velocity from the so-called Earth's gravitational field.

According to Lyu's team, the piston endured numerous demanding, repeated tests without suffering any damage because of a special construction design and the usage of novel materials.

They claimed that the piston's “full reusability” would drastically lower the wind tunnel's operational costs.

However, the 840kg (1,852lb) piston, which is the heaviest piston ever utilized in a wind tunnel, presents a difficulty. The piston must withstand crushing pressure from the compressed air equivalent to 10,000 times the force of gravity while traveling at a speed of up to 540 km/h.

According to Lyu's team, the piston endured numerous demanding, repeated tests without suffering any damage because of a special construction design and the usage of novel materials.

Raymond Stalker, an Australian space engineer, created the free-piston-driven tunnel in the 1960s, which is why it is called a Stalker tube.

To research flying at hypervelocity, or speeds more than Mach 5 (five times the speed of sound), the Soviet Union and the United States constructed a huge number of wind tunnels during the Cold War.

In these wind tunnels, extreme flight conditions were typically simulated using hot hydrogen gas. However, because hydrogen is expensive, difficult to store, and highly explosive, its development, use, and maintenance were complex and expensive.

High-pressure nitrogen, an inexpensive inert gas that forms up 80% of the atmosphere, was suggested as the new design by Stalker.

This nitrogen would be used to propel a piston. When accelerated to several hundred kph, the hefty piston may compress air and pierce multiple layers of solid membranes to produce hot, rapid shock waves comparable to those produced by an aircraft traveling at hypersonic speeds.

The cutting-edge hypersonic flight technology, like the scramjet, has been developed by Australia, a modest player in the space race, with little investment and workforce thanks to the piston-driven wind tunnels established by Stalker and his colleagues in the 1980s.

Because of the Stalker tubes' performance, the U.S. and Australia inked a contract in 2020 to work together to create a Mach 8 hypersonic glide vehicle to slow down China's and Russia's rapid technological advancement.

According to Lyu's team, the new Chinese facility is more than just an enlarged version of the Australian tunnel.

They claimed that the X3 tunnel's connection to a high-pressure nitrogen tank, which vibrates together with the tube as the piston moves, could compromise the precision of ground test results.

By covering the piston launching tube with the nitrogen storage component, the Chinese team claimed to have found a solution.

According to their investigation, the novel design not only decreased the size and complexity of the entire facility, but also cut the vibrations to a small portion of those of conventional Stalker tubes.

Over the past few decades, other piston-driven wind tunnels have been constructed worldwide, but the University of Queensland's X3 Expansion Tube, with a diameter of around 40 centimeters, was until recently the largest.