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Stephen Chen

At a test facility in China, a hypersonic engine that reshapes its internal airflow channel – much like a throat tightening and relaxing – has operated continuously from a relatively modest Mach 1.8 (nearly twice the speed of sound) all the way to Mach 6 without failing.
And the material that kept superheated gases from escaping? Essentially the same black mineral found inside a pencil: graphite.
For years, engines of this type could not ignite until the aircraft had already reached Mach 4 – meaning a separate rocket booster was needed to get up to speed first, adding cost and complexity.
Now, researchers from Northwestern Polytechnical University and the Beijing Power Machinery Institute say they have solved a problem that has stumped engineers for decades.
They ran the variable-geometry ramjet – a type of air-breathing jet engine with no moving compressor – at a ground-based facility that simulates high-speed flight conditions.
The engine’s combustion chamber throat – a moving metal component that tightens and relaxes to manage airflow at different speeds – adjusted itself in one third of a second while inhaling gases at 1,650 degrees Celsius, according to a paper published in the Journal of Propulsion Technology on May 28.
And it did not leak.

For hypersonic engineers, sealing those moving parts had long been an unsolved problem.
A ramjet that can alter its internal shape is considered the field’s holy grail: it saves weight, burns fuel more efficiently, and opens the door to reusable hypersonic vehicles.
The catch is that every moving joint must form an airtight seal against gases rushing in at extreme heat and speed.
Previous designs leaked hot gas so severely that most countries abandoned the approach.
The Chinese team cracked the problem with a two-layer seal: a flexible ceramic-fibre ring that absorbs the initial heat shock, backed by a ring of graphite packing.
And graphite did the heavy lifting. On its own, the ceramic seal leaked like a sieve. With graphite packing tucked behind a small buffer chamber, leakage plunged to just 1.9 per cent of what it had been.
China produces nearly 80 per cent of the world’s graphite, including the high-purity grades required for aerospace applications. Starting in late 2024, Beijing restricted graphite exports to the United States, citing national security concerns.

The US defence industry, already years behind China in the hypersonic race, now faces vulnerabilities in its graphite supply chains – material critical for missile nose tips, rocket nozzles, stealth coatings, and nuclear reactor components.
Washington has invoked the Defence Production Act to fund domestic graphite mining and processing across the country. It is also financing projects in Canada, Mozambique and Australia.
Europe is also working to reduce its dependence on Chinese graphite under its Critical Raw Materials Act. Synthetic graphite made from petroleum coke is one alternative being explored, but it is costly, energy-intensive, and often insufficiently pure for the most demanding aerospace applications.
Building a new graphite supply chain from scratch would take a decade or more, according to industry estimates.


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Science

China conducts ground test of hypersonic ramjet that can change shape in flight

A Northwestern Polytechnical University hypersonic vehicle lifts off in northwestern China last year. The university’s researchers have reported a new breakthrough in graphite-sealed engine technology with implications for the US-China hypersonic race. Photo: Handout

In a ramjet, air is compressed, fuel ignited and exhaust expelled. Chinese researchers have now sealed a variable-geometry version with graphite, enabling it to run from Mach 1.8 to Mach 6 without leaking superheated gas. Photo: Shutterstock

Graphite, the mineral found inside every pencil, also seals hypersonic engines. China controls about 80 per cent of global supply, including the high-purity grades the aerospace industry depends on. Photo: Shutterstock

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The LineShine supercomputer at the National Supercomputing Centre in Shenzhen. Photo: National Supercomputing Centre in Shenzhen

TOP500 co-founder Jack Dongarra (second from left) and LineShine chief designer Lu Yutong (third from left) attend the TOP500 award ceremony at the International Supercomputing Conference in Hamburg, Germany, on Tuesday. Photo: National Supercomputing Centre in Shenzhen

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Chinese semiconductor researcher Wang Danhao died at the University of Michigan in March, shortly after being questioned by US federal law enforcement. Photo: Handout

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China

The graphite seal used in the Chinese breakthrough is the same one the US defence industry desperately needs


The graphite seal used in the Chinese breakthrough is the same one the US defence industry desperately needs.


China conducts ground test of hypersonic ramjet that can change shape in flight

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}The graphite seal used in the Chinese breakthrough is the same one the US defence industry desperately needs

The graphite seal used in the Chinese breakthrough is the same one the US defence industry desperately needs