The U.S. Navy is investigating the possibility of having its future nuclear attack submarines fitted with sails — the tower-like structures on their forward upper-hulls — that are inflatable, popping up when required, but otherwise concealed to preserve the sub’s speed, maneuverability, and acoustic stealth. The Inflatable Deployable Sail System (IDSS) is ultimately hoped to result in technology that will allow the Next-Generation Attack Submarine, or SSN(X), to be able to “operate submerged without the impediments of a sail.”
In a document published earlier this year on the website of the Navy’s Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) program, under which the service seeks to provide funding for start-ups and small businesses, Naval Sea Systems Command (NAVSEA) outlines its initial requirements for the IDSS initiative. The outlined objective: to “Develop technology that will advance traditional submarine design toward accommodating an Inflatable Deployable Sail System (IDSS) for future submarines.”
While the IDSS could represent potentially cutting-edge technology for the Navy, it’s also worth noting that China has unveiled a sail-less submarine design in recent years.
In 2019, in a surprise development, China unveiled a new submarine, roughly 150 feet long, with no apparent sail, and just a small fairing in its place. A photo of this unique submarine is provided at the top of this article. Initially, there was speculation this was an unmanned design, but the consensus is now that it’s likely a testbed for advanced new technologies likely to be found in future Chinese submarine designs. Clearly one of these concepts is a submarine without a sail.
In the United States, too, there has already been some developmental work on exotic new sail designs, albeit using sub-scale submarines. This has included work at the Large Scale Vehicle Range (LSVR), part of the Navy’s Acoustic Research Detachment (ARD), around 375 miles from the ocean at Bayview, Idaho.
The NAVSEA document notes that speed, maneuverability, and stealth are the primary drivers behind the IDSS concept but identifies the key challenge that will have to be overcome to ensure it will actually work — how to safely navigate a submarine without a sail. Traditionally the sail is used as an observation and conning platform when operating on the surface of the water, as well as providing one potential entry/exit route for the crew.
Projecting well clear of the water, a traditional sail can also be used to help conduct vertical replenishment (VERTREP) operations. It also provides a position from which to provide force protection, too, and has an important function of breaking through the ice and providing initial access outside of the sub when operating in polar regions.
Above all else, the sail is normally used to house important sensors like periscopes, communication antenna masts, as well as electromagnetic and radar sensor systems. In the past, when submarines spent much of their time surfaced when engaged in combat, the sail also incorporated a physical command center, but more modern designs primarily have their critical functions contained within the main hull. The mast can even be used as a storage locker of sorts for special operations forces.
Finally, the sail can sometimes serve as a location for diving planes, although this is becoming less of a common design element, evidenced by their relocation to the hull on the Los Angeles class SSNs starting with the improved 688i standard. Sail-mounted diving planes remain a characteristic of the Navy’s ballistic missile submarines, possibly including the forthcoming Columbia class.
Now, however, technological developments could mean that the Navy is able to make at least trial use of “Advances in inflatable structures [that] can provide the freeboard needed for surface transit with a temporary and reusable structure.”
Above all, the challenge will be to find a way of retaining something akin to the functionality of the sail's bridge area while the submarine is running on the surface, and allowing members of the crew to keep an eye out for maritime traffic or other potential hazards.
The IDSS is not expected to carry any sensor systems, which will instead have to be incorporated into the hull.
The specific requirements from NAVSEA for an inflatable sail include a minimum 16-foot freeboard (the distance from the waterline to the top of the sail), capacity for at least two crew at the front of the bridge hatch, and another two behind them, plus all the required power, lighting and communications, flip-up windshield, storage lockers, and more. Of course, there also needs to be a means to access or escape the main pressure hull, via a ladder system, and the entire bridge module should not weigh more than 4,000 pounds. The structure will have to be resilient, too, capable of operating in icy waters and providing ballistic protection against small arms fire.
A range of minimum operational constraints are outlined, including the requirement for 10,000 operational cycles, ability to maintain shape at periscope depth, in operations in Sea State 6, and at temperatures ranging from -60°F to 150°F.
“A generally soft or soft/rigid hybrid inflatable structure with a rigid or hybrid rigid/inflatable bridge,” the IDSS is expected to deploy or retract, in the space of just one minute, with pressurization achieved using seawater and/or air.
“The structural loadings, deployment/retrieval operations, and stability mechanisms required present significant design and material challenges for an inflatable and deployable sail,” NAVSEA notes.
The command suggests that industry might look at some of the kinds of inflatable soft structures now being used elsewhere in the Pentagon, as well as by NASA, which is looking at designs for inflatable space habitats, for example. In particular, these technologies include “inflatable control surfaces, deployable energy absorbers, and temporary on-demand structures.”
The document then goes on to note many of the advanced manmade fibers that are used in these kinds of soft inflatable structures, of which Kevlar is probably the most familiar but also including Vectran, DSP (dimensionally stable polyester), PEN (polyethylene napthalate), and Spectra, a type of ultra-high molecular weight polyethylene.
“The soft structures considered for use in developing the IDSS may include, but are not limited to, control volumes constructed of inflated membranes, 3-D woven preforms, flexible bladders, coated fabrics, and hybrid (soft/rigid) material systems, and hard goods-to-soft goods connections,” the document continues. “Hybrid inflatables may include inflatable elements with semi- or fully-rigid reinforcements serving as deployment shaping controls and abrasion-resistant contact surfaces.”
Ultimately, NAVSEA may call upon interested parties to produce full-scale IDSS prototypes of their virtual design concepts that will be able to test inflation/deflation, and resistance to wave slap and impact loadings.
That, of course, is still some way off and although the document doesn’t provide any kind of timeline and the Congressional Research Service’s latest report on SSN(X) doesn’t envisage a first example of the new submarine even being procured until Fiscal Year 2031. It’s also possible that the IDSS, if it does prove successful, might only be introduced on later blocks of the SSN(X) design.
The Navy has been examining whether SSN(X) should be a follow-on to the current Virginia class design, a design based on the Columbia class SSBN, or an all-new design. Last December, The War Zone reported that the vessel would be significantly wider than the current Virginia class, making it closer to the dimensions of the advanced Seawolf class, while also utilizing technology being developed for the Columbia class.
If IDSS is successful and is adopted as part of the SSN(X) program, a further phase of the development, as envisaged by NAVSEA, would see it adapted for other potential applications, namely “future underwater weapons, unmanned underwater vehicles (UUVs), unmanned surface vehicles (USVs), and commercial/industrial dual use.”
The latter category is notably broad, encompassing lighter-than-air (LTA) vehicles, space vehicle structures and habitats, “chemical/biological containment systems for internal use aboard aircraft and mass transit ground vehicles,” and others.
It’s clear that the U.S. Navy is now looking seriously at operating future nuclear attack submarines with inflatable constructions in place of their traditional sails. While this proposal sounds like it would offer considerable advantages, on paper, actually developing a reliable technology is likely to be a much more challenging proposition. After all, if the inflatable sail fails to properly retract, for example, it would render the submarine highly vulnerable and at the very least end its mission.
However, if NAVSEA’s initiative is a success, it could permanently change the way advanced attack submarines look while making them even more effective fighting machines.
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