Chinese scientists want to use lasers to power ultrafast, stealthy submarines. A laser expert says there's a major flaw in their plan.

• The US and China are in a race to develop the most advanced submarines.  
• Chinese researchers say they have made a breakthrough in laser propulsion technology that could one day be used on submarines and missiles.
• A laser propulsion expert at McGill University told Business Insider they see flaws in the claims. 

Submarines have long been known as the silent hunters of the deep, lurking undetected under the surface.

But as sonar systems have improved, minimizing sound on all on board components has become crucial to avoiding detection — not least the mechanical noises of propellers, generators, and nuclear reactor coolant systems that help propel submarines forward.

A team of engineers at China's Harbin University claim to have taken a step forward in developing technology that could make submarines almost silent and ultrafast, the South China Morning Post reported.

Rather than relying on nuclear or battery power, the scientists say they have found a way to use lasers to propel submarines — known as underwater laser propulsion.

Theoretically, submarines could be coated in a web of minuscule optical fibers that emit laser pulses, the scientists wrote in the journal Acta Optica Sinica.

The small laser pulses emitted by the fibers would vaporize seawater and generate plasma, the substance created when gas is heated.

In turn, the plasma would expand, creating a detonation wave that would act as an opposite force to the submarine, propelling it forward.

A large number of high-powered laser beams positioned around the submarine at various angles could produce nearly 70,000 newtons of thrust and would drive it forward at speeds almost as fast as a commercial jet, according to SCMP.

Ge Yang, the leading researcher on the project, said the technology could also be used on underwater missiles or torpedoes, "significantly increasing the underwater range," the outlet reported.

Stealth and speed

The team of scientists say this process would deliver not just speed but enhanced stealth as well.

Using lasers may also induce a process known as "supercavitation" — a coating of bubbles forms around an object in water, reducing drag resistance and making it travel faster.

This technology has already been used in Russian Shkval torpedoes since the 1970s, using rocket exhaust rather than laser power.

But Professor Andrew Higgins, who leads a team at McGill University developing laser propulsion technology for space travel, doesn't believe that lasers could be powerful enough to induce this "bubble" effect around torpedos, let alone submarines.

"Cavitation can be induced on the front of the torpedo by a spike or forward-facing rocket exhaust. I am doubtful a torpedo can carry enough of a power supply to power a laser that would continuously perform this operation," he told Business Insider.

And for a laser to produce enough thrust to propel an entire submarine?

"I am even more skeptical," Higgins said. "The average overall thrust is low and the jet power cannot exceed the power supply of the laser."

"Even with the high efficiencies of today's lasers, this approach would never be as efficient as a propeller, so there is no net propulsive gain."

As the SCMP notes, Japanese researchers first proposed this sort of laser propulsion methodology 20 years ago, but for decades the technology has only been able to produce thrust powers of one-millionth of a newton thrust.

Controlling the direction of the plasma detonation wave to steer the submarine is another issue.

The Chinese researchers say the unique design of the microcavities in the optical fibers has increased the efficiency of thrust and channels the direction of force, according to SCMP.

However, they admit there are many challenges to overcome before the technology can be applied to submarines.

Higgins says they also avoid a key issue in their claim that the technology would reduce noise — cavitation is an "acoustic dead giveaway."

"It would not be feasible or desirable to propel a nuclear submarine in this way."

"For nuclear submarines, acoustic stealth is the entire purpose, and great effort is expended in avoiding cavitation."

Race to advance military capabilities

While laser propulsion technologies for submarines remain in the theoretical stage for now, the US is locked in a race with China to develop advanced military capabilities.

Harbin University, where the researchers are based, is a hub of Chinese military development and is listed as "very high risk/top secret" in Australia's China Defense Universities Tracker.

According to the tracker, 52% of the university's total research budget was spent on defense research in 2018.

China currently operates 60 submarines, compared to the 67 operated by the US. But while US numbers are stagnating, China's submarine count is expected to rise to 80 by 2035.

China's navy is numerically the largest in the world, but analysts say it is not as technologically advanced and lacks experience compared to the US.