by Capt. Karl Hasslinger, USN (Ret), and Mr. John Pavlos
Since the demise of the Soviet Union, the U.S. military has been in transition, responding to an altered geostrategic landscape and the proliferation of advanced military systems. At the low end of the spectrum, U.S. forces face the challenges of humanitarian assistance and irregular warfare. At the high end, China is emerging as a new peer competitor with a different philosophy than its Soviet predecessor. Instead of building a large sea-going fleet, China is seeking to counter U.S. aircraft carriers and other sea-based power projection forces with an anti-access/area denial (A2AD) strategy based primarily on aerospace power, especially anti-ship cruise and ballistic missiles.
It's all about payload
Submarines are well equipped to confront the full spectrum of emerging challenges—including A2AD systems, to which they are largely immune—but only if they can carry the required payloads. Warships never have enough offensive payload capacity, as any planner can attest. This is especially true in the Pacific, where the distance to reload ports is most challenging. Surface forces find it particularly difficult to carry enough offensive firepower when they are within range of A2AD systems, which force them to devote a large part of their payload to defending themselves. In contrast, submarines, which rely on stealth for their defense, can dedicate their entire payload to offensive operations.
A Nuclear Posture Review (NPR) recommendation approved in 1994 provided an unprecedented opportunity to radically increase the Submarine Force's aggregate payload capacity at very reasonable cost. The NPR recommended reducing the number of strategic ballistic missile submarines (SSBNs) by four boats, which the Navy promptly converted to guided-missile submarines (SSGNs) for conventional strike and special operations. With 24 vertical tubes originally designed to hold large strategic ballistic missiles, each SSGN added tremendous payload volume. In addition, the large-diameter muzzle hatches of their missile tubes provided a much more flexible ocean interface than the traditional small hull openings for 21-inch torpedoes and Tomahawk missiles.
In 2011, USS Florida (SSGN 728) dramatically confirmed the value of the SSGN concept by shooting 93 of the 122 Tomahawk cruise missiles launched from submarines against targets in Libya during Operation Odyssey Dawn. Most SSGN payload tubes currently hold canisters containing seven Tomahawk land-attack missiles, but the tubes' size and operational flexibility also give them the potential to accommodate many other payloads, including recoverable undersea vehicles. However, the Navy will lose the SSGNs' tremendous payload volume and flexibility as these ships begin to retire in 2026, and that has driven a search for other means of launching high-volume conventional strikes from beneath the sea as well as supporting alternative payloads.
The Virginia Payload Module (VPM) concept
The 2009 Quadrennial Defense Review specifically directed the Navy to study methods to increase submarines' conventional strike capacity. The resulting study identified several options, including converting more SSBNs into SSGNs if a future Nuclear Posture Review made additional SSBNs available, building new SSGNs from scratch, and inserting a new payload module during the construction of future Virginia-class attack submarines (SSNs). Further analysis concluded that the concept of a Virginia Payload Module represented a viable alternative to additional SSGN conversions (which would be nearing the end of their service life) or an unaffordable new SSGN class.
The Integrated Undersea Future Strategy (IUFS) introduced by Submarine Force leadership in the spring of 2011 includes a goal to add the VPM concept to Virginia-class submarines beginning with Block V. With the first Block V boat scheduled to start construction in 2019, VPMs would begin joining the fleet just as the SSGNs are leaving it. Of course, it would take some time to build up this new strike force, and it is unlikely the Navy could—or should—replace all 88 payload tubes currently provided by SSGNs. Whereas the SSGNs' striking power is concentrated in only four ships, the VPM concept would spread it across many more platforms, which has significant operational advantages.
The current Block III Virginias already have two tubes in the bow with the same diameter as an SSGN's, but the bow location limits their length. Located amidships, the VPM's large-diameter tubes could be somewhat taller—albeit not as tall as SSGN tubes due to the smaller diameter of the Virginia hull. Their greatest advantage, however, would be accessibility. Whereas the bow tubes are outside the pressure hull and therefore inaccessible while the submarine is at sea, the VPM concept would provide access from inside the pressure hull. Block V Virginias would be the first SSNs with this capability, which is essential for carrying any payload that requires inspection, servicing or manipulation while underway.
In addition to its payload tubes, a VPM could provide reconfigurable internal volume for payload support, mission planning, and/or additional berthing. Although a payload module would of course lengthen the ship and increase its displacement, those changes would not preclude performing any SSN missions.
|Illustration of the Virginia Payload Module concept. Graphic courtesy of General Dynamics Electric Boat.
Developing the VPM concept
Adding modules—or "hull plugs," as shipbuilders call them—to attack submarines is nothing new. The Navy installed hull plugs during construction of the last ten boats of the Sturgeon (SSN 637) class in order to experiment with new propulsion systems and increase mission capabilities. More recently, General Dynamics Electric Boat installed a 100-foot module called the Multi-Mission Platform during construction of USS Jimmy Carter (SSN 23) in 2005. These modifications demonstrated that stretching submarine hulls is feasible, and stretching the Virginia class would involve less risk because it is the first submarine class specifically designed to accept hull modules during construction.
In developing the VPM concept, Electric Boat engineers adhered to the following tenets:
• Minimize the impact on the operating characteristics of the baseline ship.
• Minimize changes to the baseline ship's internal and external systems.
• Minimize the cost and risk associated with adding a VPM to future hulls.
• Avoid designing a point solution so a VPM could adapt readily to the widest range of future national security challenges.
• Provide a large ocean interface so that a VPM could share payloads with the SSGNs.
• Ensure that payloads can interface with the ship in a manner similar to the SSGNs.
With these tenets in mind, the engineers broke the concept down into three parts. The first was the basic hull module—the "truck" that would provide additional capacity to house today's payloads and the flexibility to accommodate different payloads in the future. The second was payload "middleware," a generic term for common electronic and physical interfaces with the ship that are designed to support the widest possible variety of payloads. The third part was the payloads themselves, including potential alternative payloads.
Configuring the VPM
Early versions of the VPM concept included up to eight large payload tubes, with the tubes lined up two abreast. However, placing the tubes outboard separated by a centerline passage made their muzzle doors protrude beyond the ship's molded lines. That necessitated a turtleback structure, with attendant hydrodynamic and potential acoustic problems, especially at the higher speeds SSNs are capable of. Since the payload tubes and their muzzle hatches are heavy, a large number of tubes also meant the hull plug had to be longer in order to float the ship.
The current concept, with four payload tubes located on the ship's centerline, strikes a good balance of ship length, impact on auxiliary systems (e.g., air conditioning, ventilation, trim and drain systems) and operational capability. It allows the tubes to be as tall as possible without protruding beyond the molded line of the ship, thereby maximizing not only their internal volume, but also the length of the weapons and other payloads they might hold. Along with the two vertical tubes in the bow (which hold six missiles each), four VPM tubes would increase the ship's maximum Tomahawk load-out to 40 missiles.
The pressure hull in the VPM concept tapers in from 34 feet fore and aft to a 26-foot wasp waist around the missile tubes. The wasp waist provides space outside the pressure hull for the bulky, large-diameter hatch mechanisms. An outer, non-pressure hull maintains the ship's 34-foot beam and encloses the hatch mechanisms and the bottom of the tubes within the molded line of the ship. The volume between the inner and outer hulls also houses additional ballast tanks and could potentially accommodate future external payloads as well.
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