China's Submarine Dream (And Nightmare for the U.S. Navy): 'Hunt for Red October' Subs
If China’s rim-driven pumpjet propulsion technology works, it would be a significant advance for the People’s Liberation Army Navy’s undersea force.
In a recent article that appeared in the South China Morning Post, Beijing claims to have developed such a silent propulsion system—which some have compared to the so-called caterpillar-drive in Tom Clancy’s Hunt for Red October.
With vastly improved acoustical performance, a new generation of advanced Chinese nuclear attack submarines (SSNs) could add another dimension to Beijing’s anti-access/area denial (A2/AD) capabilities.
Further, new Chinese ballistic missile submarines hiding inside their heavily defended ‘bastions’—like the Soviet boomer fleet before them—would be much more difficult to detect and eliminate, greatly enhancing Beijing’s strategic nuclear deterrence.
But that’s only if China can build an operationally relevant rim-driven pumpjet propulsor—American naval analysts are mostly convinced that the new Chinese silent propulsion system is a science project that may never make it to sea.
“If it is well-built, a rim-driven pump jet would be a quieter propulsion system than traditional propellers, and could be quieter than shaft-driven pump jets like those on some U.S. submarines,” Bryan Clark, a retired U.S. Navy undersea warfare officer and analyst the Center for Strategic and Budgetary Assessments, told The National Interest.
“The question is whether the Chinese can build one with the fine machining necessary to achieve the degree of quieting possible. The article doesn't address that. The basic technology is straightforward, but building a good one is hard. Manufacturing precision equipment like turbines has been a challenge for China’s shipbuilding industry.”
Retired U.S. Navy submariner Thomas Callender, a senior research fellow for defense programs at the Heritage Foundation and former director of capabilities at the Office of the Deputy Under Secretary of the Navy (Policy) agreed with Clark’s assessment.
“I agree that if engineers can develop a shaft-less rim-driven electric motor pump-jet, it would reduce the noise signature of the host submarine since without large traditional shaft with multiple bearings along its length (shaft must be long enough to connect propulsion motor inside the engine room to screw or pump-jet at the stern) and only having one bearing per pump-jet, the noise associated with the shaft would be reduced,” Callender told The National Interest in an email.
“In addition, since the propeller is not driven by a traditional steam propulsion turbine, but by an electric motor, there is no need for large reduction gear which reduces RPM of steam turbine (in 1000’s of RPM at higher speeds) to more efficient and quiet propeller speed (for submarine typically less than 200 rpm max).”
The improved quieting would also likely more than offset potential drawbacks such as a greater magnetic signature.
“A rim-driven pump jet would use an electric motor that is installed in the rim around the propulsor. Like any electric motor, it would generate a magnetic field. Because it’s outside the hull, it might be easier to detect with magnetic anomaly detection, but it could be designed to shield some of the field,” Clark said.
“It again comes down to how well they build the propulsion system. In any event, magnetic anomaly detection does not work at long ranges, and is not useful as a search capability. It is generally used to target a submarine once it has been located and tracked.”
While there are advantages to a rim-driven pumpjet, there also some serious potential drawbacks. One problem is that such motors may not be able to generate the horsepower to drive a massive nuclear submarine.
“If China can put a well-built rim-driven pump jet on a submarine, the next question is how much thrust it provides,” Clark said.
“With submarine propulsion, one of the tradeoffs is quietness versus speed. Most changes to the propulsion architecture that reduce noise also reduce sprint speed. One of the concerns I have heard from engineers is whether a rim-driven pump jet can deliver the horsepower needed to reach high sprint speeds for torpedo evasion or repositioning.”
Callender noted that a single rim-driven pumpjet would probably be insufficient. The U.S. Navy’s forthcoming Columbia-class SSBN design will incorporate a permanent magnet electric drive propulsion—eschewing the traditional steam-driven propulsion turbine. The new propulsion system will be much quieter, Callender said, but it will come at the price of being enormous.
“The electric drive motor with sufficient power to drive Columbia SSBN will be extremely large, partially contributing to its 43-foot hull diameter,” Callender said.
“For example, similar sized Ohio Class SSBN produced 60,000 shaft horsepower. Virginia SSN produces 40,000 shaft horsepower to power a submarine.”
Because of the sheer size and weight of the electrical motors, there are some size constraints that are inherent to a rim-mounted pumpjet.
“Bryan is correct and I agree that the most critical technical issue with the rimless electric motor pump-jet as the main propulsion for an SSN or SSBN is delivering sufficient power in size and weight limitations of a stern pump-jet,” Callender said.