Nuclear propulsion was the last major innovation in submarine technology that in theory, enabled endurance limited only by food and consumables and crew stamina.
Technologies like air independent propulsion (AIP) or lithium batteries that extended the endurance of diesel submarines with major compromises in performance. Nuclear attack submarines are large, ocean going vessels in the >2,500 ton range with a US Navy attack submarine displacing over 6,000 tons.
The US Navy’s submarine fleet as proposed by the 2018 NDAA, will still be woefully short of demand.
In the 21st Century, there has been a virtual explosion in undersea infrastructure worldwide that will likely continue.
Communication cables, pipelines and installations for exploiting undersea resources, fisheries, power generation systems, sensors for both civilian and military purposes, are now found in every major ocean and continental shelf. Friendly infrastructure have to be defended, or alternatively, enemy installations are opportunities for disruption.
Along with this explosion in requirements to operate in littoral or continental shelf waters, is the spread of A2/AD systems that make it more and more difficult for surface vessels to operate in contested zones.
This suggests that the submarine’s unique attributes of stealthiness and ability to operate – whether gathering intelligence or projecting power – in strongly defended A2/AD zones will be in great demand.
Most nuclear submarines are ill suited to littoral undersea operations. Likewise, diesels, with or without AIP, cannot offer the combination of endurance and performance of a nuclear powered vessel.
S Navy Nuclear SSNs may not be the answer, for they are few in number, overbuilt and too big for littoral operations.
Submarine detection technology is rapidly advancing.
Limiting passive radiated noise, the historical trump card against detection, is rapidly giving way to expanded use of active sonar. In the past, active sonar is frowned on because it gave away the position of the hunter.
But with USVs and Undersea Unmanned Vehicles (UUVs), and active sonar arrays in a networked battlefield, that is no longer the case.
The widespread (and indiscriminate) use of active sonar risk obsoleting many existing submarine designs that prioritized limiting emitted noise.
The evolution of stealth fighter designs illustrate how critical it is to eliminate turbines or propellers and control surfaces from being visible to active radar. Pump jet propulsors with steerable jet streams that eliminate control surfaces and elimination of the sail is the next major innovation in stealthy submarines in a world of active sensors.
Presently, anechoic tiles and surface treatments are typically only applied to the top and side with little consideration for being “pinged” from below.
Future submarines need to take a page from stealth fighter designs where flat, slab like surfaces and coatings are used to limit signature from all aspects.
Minimization of magnetic, radar, infra-red and visible light signatures, or waves from both active and passive detection is a problem for the 21st Century submarine.
Technologies like conformal sonar arrays and digital signal processing are advancing rapidly, making possible active noise cancellation or echo reduction, or alternatively, spoofing the return signal, or the teasing out of signal from noise. Working together with a fleet of UUVs offer the prospect of creating a “phantom” fleet of signatures while the real submarine stays cloaked.
These techniques, initially developed for F-35 stealth fighters using AESA and EW suites, need to become navalized to enhance submarine survivability in an intense A2/AD undersea environment.
Navalization of stealth fighter technologies confront institutional hurdles.
Navies are, compared to the Air Force, a much more conservative organization. Traditions like “nuclear only” that dates from Admiral Hyman Rickover held up consideration of the threat from increasingly lethal non-nuclear submarines that compare poorly in performance and endurance to a nuclear powered vessel.
The silent service remain deeply committed to the existing paradigm of “affordable” nuclear fast attack submarines like the Virginia Class that still cost $2.7 billion a copy:
A bargain compared to the Seawolf Class, but is it affordable enough?
Is the US Navy heading into the same trap that resulted in too few Destroyers having to do duties they are overbuilt for, but the Littoral Combat Ships (LCS) can’t do? Or, not having enough inexpensive frigates.
The question is, whether it is time to consider a new class of moderate cost littoral nuclear attack submarines that is not optimized for “blue water”, but littoral or continental shelf waters?
The dominant paradigm for a modern nuclear attack submarine is for a steam generating reactor driving turbines that directly drive the propulsor or propeller.
The French Barracuda offers a limited hybrid design that enable electric propulsion for low speed cruising and turbo-mechanical drives at higher speeds. The Tullibee (SSN-597) and Glenard P. Lipscomb (SSN-685) were two nuclear-electric vessels that experimented with the technology. But it was not competitive against nuclear-steam turbo-mechanical for blue water operations.
At the other end of the spectrum, a “mini” 400 ton nuclear submarine NR-1 was used for many key deep-ocean, bottom-exploration tasks.
While an NR-2 was proposed and studied, it was never built.
Thus, the US Navy have to use fast attack submarines for tasks into increasingly well defended and contested undersea environments.
Los-Angeles and Virginia class fast attack submarines displacing 6-8,000 tons are cumbersome in littoral waters and their stealthiness become less and less of an advantage in littorals that will become well defended in the future like the South China or Yellow Sea.
A small nuclear attack submarine that is large enough to support a good sized crew and carry unmanned systems with many of the capabilities of NR-1 but considerably more endurance and performance would be ideal for the growing need to operate in these waters.
The dominant design for nuclear propulsion places a premium on sustained high speed even though most submarines cannot do so and maintain acceptable radiated noise levels. Sustained high speed, except for “getaway” or transit, or for stunts, is a capability that is rarely needed.
Because nuclear reactors react relatively slowly to throttling, it also means that nuclear energy generate a large infra-red signature that is a much bigger liability in littoral vs. blue waters.
Hybrids offer a novel solution.
A nuclear-steam-electric hybrid is a potentially attractive alternative to the dominant nuclear turbo mechanical drive. One or more modular reactors can be used to generate steam to drive a turbine generator.
By being able to completely shut down a reactor module and tightly match energy demand with supply, it reduces the amount of excess (waste) heat dumped that is detectable.
Machinery noise from the nuclear turbo-mechanical generator can be more readily controlled if the system is operated at (and optimized for a relatively narrow power band) with no requirements for rapid throttling as with a turbo-mechanical drive. Electricity generated can be stored in state-of-the-art lithium batteries. Reactor shielding, the bane of NR-1, can potentially make use of lead acid cells doing double duty.
Electric power from batteries driving propulsors offer the prospect of extremely low radiated noise and yet maintain a high degree of “throttlability” with only limited compromises in sustained high speed cruising that would be a function of the nuclear plant’s power ramp and maximum output.
Making the propulsors jets steerable (like the F-35B) and eliminating control fins is an additional benefit in minimizing active signature.
Building such a submarine within a small displacement (1,500 to 2,500 tons surfaced) with bottoming capabilities, 1,000 meters depth, 60 days endurance, transit speed of 20 knots, burst speeds above 30 knots, and state of the art signature management technologies and support for unmanned platforms would ideally require a “clean sheet” design like an enlarged NR-2.
Extended endurance and stealth on the seafloor is a unique capability that would give back the US Navy a major advantage rapidly being eroded by A2/AD systems.
Ideally, the new Littoral attack submarine (LSSN) design can be the product of a collaboration with (e.g.) Japan that have begun work on their next generation submarines, and can contribute certain technologies like lithium ion batteries that they excelled at while the US can contribute an existing nuclear power plant design.
With costs, affordability, and the need for large numbers quickly an issue, it may be expedient to begin with a known robust design like the Swedish Gotland, or the German Type 212. The use of an existing reactor design with a proven reliability record like multiple units of the NR-1 reactor / turbo-generator drive train would cut developmental cost and time.
Modification to add features like bottoming, stealth, pump-jet propulsors and increase depth capabilities will likely require major changes.
The question is, can such a new vessel be built in quantity (more than 20) for less than US$700 million a copy?
Type 212s presently are in the 300 to 400 million euros range, Gotlands are in the US$200 million range, while the Japanese Soyru is around US $550 million. None of these candidates, however, are built in quantity.
Keeping costs down will be a major challenge, but not impossible if the volume is there.
The 21st Century brings new challenges for the silent service.
LSSNs may be a lot more usable and valuable and can complement existing fast attack submarines.