Chinese scientists close in on laser propulsion for superfast, silent submarines

Scientists developing China’s next-generation nuclear submarine technology say they have found a way to significantly improve the efficiency of the laser propellers that could one day drive the underwater vessels.

The researchers said the new technology can produce nearly 70,000 newtons of thrust – almost the force of a commercial jet engine – using 2 megawatts of laser power emitted through the submarine’s coating of optical fibres, each thinner than a human hair – an efficiency previously thought impossible to achieve.

The laser pulses not only generate thrust but also vaporise seawater, creating bubbles all over the submarine’s surface in a phenomenon known as “supercavitation” which can significantly reduce water resistance.

Theoretically, the development could allow a submarine to travel faster than the speed of sound without producing the mechanical noise vibration that usually gives away its location, according to the researchers.

This science fiction-like technology – called “underwater fibre laser-induced plasma detonation wave propulsion” – could have “broad application prospects in areas such as stealth propulsion for submarines”, they said.

The project team is led by Ge Yang, associate professor with the school of mechanical and electronic engineering at Harbin Engineering University in Heilongjiang province, where China’s first submarine was developed.

The leap in technological progress of the PLA Navy’s weaponry and equipment in recent years is closely related to this vast institution, based in northeastern China’s heavy industrial manufacturing centre.

The US government has imposed severe sanctions and blockades on the university’s more than 30,000 students and scientists.

According to a peer-reviewed paper by Ge’s team, published in the Chinese academic journal Acta Optica Sinica last month, the technology pulses a large number of high-powered laser beams around the submarine from various angles.

“This method can also be applied to underwater weapons, causing a supercavitation phenomenon, thereby significantly increasing the underwater range of projectiles, underwater missiles, or torpedoes,” Ge and his colleagues said.

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The possibility of underwater laser propulsion was first proposed by Japanese scientists 20 years ago. The idea is to use lasers to generate plasma in water and then use the detonation wave formed by plasma expansion for propulsion.

Little progress was made as the scientists found it impossible to generate a driving force in a specific direction, because of the way the detonation wave spreads from a single point in all directions.

A number of countries, including China, funded extensive follow-up research, with one promising approach involving loading the force of the detonation wave on to tiny spherical particles made of metal or other materials.

When these particles, known as working media, leave at high speed in a specific direction, they exert an opposite force on the submarine, according to Newton’s law.

However, until now, all efforts have resulted in very low efficiency, with 1 watt of laser power generating only one millionth of a newton of thrust, which has no practical application value.

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Ge and his team said they have solved the problem, designing a laser engine that improves the efficiency of converting lasers into thrust by three to four orders of magnitude.

Contrary to the general view of the global research community that adding constraining devices would cause significant energy loss, the Chinese scientists added a device similar to a gun barrel to the ends of the fibres.

According to the paper, the researchers solved the problem of energy loss by adjusting the shape and internal structure of the barrel, smoothing the barrel-fibre interface into a U-shape.

They also used a pair of barrels to bombard the working medium particles and added carefully designed protruding structures inside the barrel to reduce the interaction and internal friction between shock waves, the paper said.

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Some of the technologies behind the breakthrough originated in the aerospace defence field, where China has developed advanced plasma electric propulsion engines as part of its significant investment in hypersonic weapons research.

According to the paper, this field of research, involving the physical mechanisms of detonation shock waves and propulsion media, provided valuable insights for the design and manufacture of underwater laser propellers.

While a nuclear reactor on a submarine generates more than 150 megawatts of thermal power – enough for the laser propulsion system – there are still many challenges to overcome before the technology can be applied to nuclear submarines, the team said.

These include heat dissipation of the optical fibres, durability in high-power and high-salinity environments, as well as matching optical fibre emission windows with the submarine’s surface anechoic tiles.

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The technology would also require significant changes to submarine steering and surfacing control methods, according to the paper.

The researchers said that despite these challenges, this disruptive technology aligns with the current global shift from mechanical transmission to pure electric propulsion in the new industrial revolution.

In addition to potential military applications, underwater laser propulsion could also be applied to improve the efficiency of civilian ships and achieve “green shipping”, Ge’s team said.

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