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China has just built the world’s largest free-piston driven shock tunnel, potentially widening its lead over the US and its struggling hypersonic weapons program.

The South China Morning Post reported that its Sichuan-based facility could simulate extreme flight conditions up to Mach 33, or 2.5 to 11.5 kilometers per second.

The facility has a diameter of 80 centimeters, twice the size of the X3 Expansion Tube at the University of Queensland in Australia, which was until recently the largest facility of its kind.

The new facility can reportedly provide ground test support for developing hypersonic vehicles such as scramjet-powered aircraft by simulating the escape velocity from Earth’s gravitational field.

China’s new hypersonic wind tunnel is based on an Australian invention known as a Stalker tube, named after Australian scientist Raymond Stalker, who proposed the design during the Cold War.

Earlier hypersonic wind tunnels used expensive, explosive and challenging to store hot hydrogen gas, making construction and maintenance of these facilities costly and complex.

In contrast, Stalker’s design uses relatively cheap and inert high-pressure nitrogen gas to drive a piston to several hundred kilometers an hour. The design can compress air and burst through several strong membranes to generate extremely hot and fast shock waves as encountered by aircraft at hypersonic speeds.

The South China Morning Post article notes that the design was so successful that it enabled Australia to develop hypersonic-related technology, such as the scramjet, despite its limited resources and manpower.

It also notes that in 2020 the US signed an agreement with Australia for the joint development of a Mach 8 hypersonic glide vehicle in response to China and Russia’s advances in developing hypersonic weapons.

The South China Morning Post claims that China’s hypersonic wind tunnel marks substantial improvements over its Western-made counterparts. For one, it features a high-pressure nitrogen tank wrapped around the piston launching tube, which reduces vibrations that can affect test results’ accuracy.

It also reduces the size and complexity of the facility compared to standard Stalker tubes. The facility’s 840-kilogram piston also features a unique structure design and new materials, allowing full reusability and reducing operational costs.

However, the South China Morning Post mentions that Stalker tubes are limited by the short simulation length that lasts only a thousandth of a second, which is too brief for some experiments. The article states that China’s new facility will work with other types of wind tunnels to overcome this limitation.

The South China Morning Post reported in January that China had unveiled the world’s first wind tunnel capable of ground testing a full-sized hypersonic missile throughout its various stages of flight. This approach identifies critical design and engineering problems, avoiding the costly test failures that have dogged the US hypersonic weapons program.

Although details about the facility remain classified, it is known to allow for tests that cover the separation stage between a hypersonic weapon’s booster stage, which propels it to near-hypersonic speeds, and the activation of its scramjet engine, which drives it to hypersonic speeds.

As far as Chinese researchers are concerned, there are no other such facilities elsewhere worldwide, the same article said, noting that US hypersonic test facilities can only simulate a particular stage of flight due to technical limitations.

Moreover, China is also building a JF-22 wind tunnel, a potential game-changing facility that it claims will put it 20-30 years ahead of the US in the hypersonic arms race, as reported by the South China Morning Post in May last year. Upon completion this year, the JF-22 can simulate flights of up to 10 kilometers per second or up to Mach 30.

China claims the JF-22 is more powerful than any known US hypersonic testing facility, according to the same article. That includes LENS II, the US’ most advanced hypersonic weapons test facility, which has simulated flights of up to Mach 7, with the simulation lasting 30 milliseconds.

In contrast, the JF-22 can simulate Mach 30 flights of up to 130 milliseconds, with a much higher top speed.

Despite these advancements, the South China Morning Post notes that powerful wind tunnels consume enormous amounts of power, with the power grid in Sichuan experiencing blackouts when researchers started the machines. In addition, some wind tunnels cannot be connected to the local power grid but must rely on specialized generator facilities.

China and Russia’s advances in hypersonic weapons have given the US a sense of urgency to accelerate its testing program. The US Senate Armed Services Committee’s (SASC) 2023 proposed defense policy bill notes that the US can only maintain strategic nuclear deterrence through the rapid modernization of its legacy nuclear capabilities and accelerated development of ballistic, cruise and hypersonic missiles.

It also notes that the US Department of Defense (DOD) and the Secretary of Energy should leverage all available tools to reduce the risk of schedule delays in nuclear modernization and hypersonic missile programs.

Although the US maintains the most hypersonic weapons testing facilities, there are rising concerns that its testing capabilities are lagging behind its near-peer competitors. A July 2022 report by the US Congressional Research Service (CRS) notes that in 2014 the US had 48 critical hypersonic testing facilities needed to mature hypersonic defense systems for defense systems development through 2030.

Further, the SASC notes in the 2023 National Defense Authorization Act the DOD’s overdue investment in fielding hypersonic offensive and defensive capabilities and encourages additional funding for hypersonic weapons’ research to leapfrog near-peer competitors China and Russia.

The SASC also emphasizes that further investment in hypersonic test infrastructure is vital to rapidly fielding emerging hypersonic weapons technologies.

Asia Times has also reported on the US’ rush to accelerate its hypersonic weapons testing program, noting that America aims to enlist the aid of the private sector to increase its hypersonic weapons tests from a few tests a year to a test per week.

Asia Times and others have noted that the US’ aggressive pace of hypersonic weapons testing, coupled with overly complicated and poor weapon design, test planning and preflight testing deficiencies, have conspired to stunt America’s development of hypersonic weapons systems. Nevertheless, the US is seeking to expand its hypersonic weapons testing facilities.

This April, Defense News reported that although the Biden administration has not released detailed spending tables for the US 2023 defense budget, it does aim to upgrade hypersonic testing facilities at the Arnold Engineering Development Complex in Tullahoma, Tennessee, among other hypersonic-related proposals.

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