ABC News’ David Muir just recently got a chance to go aboard the USS Florida, providing a behind the scenes look at the interior of one of the converted SSGNs in its present configuration, which you can see in the video below.

Tubes one and two on each of the four SSGNs would be completely replaced with lockout chambers so combat divers and Navy SEALs could enter and exit the submarine underwater. Personnel could also install a Dry Deck Shelter (DDS) to the top of the hull linked to either one of these modified tubes, or both if required, which could accommodate swimmer delivery vehicle (SDV) mini-submarines. As the name suggests, the DDS provides a fully enclosed, dry space to work in on the submarine's deck, even while it is underwater.

A schematic showing the elements of the SSGN conversion plan the Navy settled on for the four Ohio class submarines.

The Navy had considered a number of potential configuration options for the new SSGNs. The concept that the service finally settled on retained 22 of the 24 missile tubes found on Ohio SSBNs, but modified them so that they were unable to fire Trident D5 nuclear-tipped submarine-launched ballistic missiles (SLBMs). Instead, each one would be able to launch up to seven BGM-109 Tomahawks using a Multiple All-Up-Round Canister (MAC) adapter. The SLBM fire control systems were similarly replaced with ones for the Tomahawk.

Ohio, when she was an SSBN, marking the completion of her 50th nuclear deterrent patrol in 1998. Members of her crew are seen standing on the deck spelling out "fifty" for the occasion.

The decision to covert Ohio class SSBNs into SSGNs originated with the 1994 Nuclear Posture Review, which determined that only 14 of the 18 Ohio class boats were necessary to meet the United States’ nuclear deterrence needs. Eight years later, the Navy began actually converting the four oldest Ohio class submarines – USS Florida, USS Georgia, USS Michigan, and USS Ohio – into the new configuration.

A decade and a half ago, the U.S. Navy was testing incredible new capabilities that it would subsequently integrate into its four yet to be converted SSGNs, including one highly elaborate, but obscure proof of concept exercise that solidified the SSGN concept for the seagoing service. Here is the story of how these vessels came to be and the highly unique, if not exotic capabilities, from drone mothership to command and control center, they possess.

Today, the U.S. Navy's quartet of converted Ohio class nuclear-powered guided-missile submarines, or SSGNs, are among America's most powerful, in-demand, and flexible weapons. These giant and secretive submarines are known for their ability to carry up to 154 Tomahawk land-attack cruise missiles and dozens of special operations frogmen into contested territory to ply their quiet trade, but really, they are much, much more than that.

The abortive Advanced SEAL Delivery System (ASDS) was supposed to have been able to directly dock with either one of these lockout chambers, as well. The Navy canceled the ASDS program in 2009 after cost overruns and other major setbacks, including a fire that had destroyed the original prototype the year before. With a DDS installed, a number of additional tubes on the SSGNs would also be blocked off, so the Navy decided to make tubes three through 10 reconfigurable into storage space, if necessary. A dedicated berthing area for a typical contingent of 66 special operators, with a surge capacity of up 102 personnel, was added in the reconfigured missile compartment, as well.

USN A US Navy diver sits in the DDS on the USS Ohio as the submarine sits pierside in Busan, South Korea in 2008. A Mk 8 SEAL Delivery Vehicle (SDV) mini-submarine is visible behind him on a docking platform that extends out of the rear of the shelter.

More recent reporting has indicated that a typical load for these submarines is around 100 Tomahawks. This most likely represents between 14 and 16 fully loaded tubes, which would equate to between 98 and 112 missiles in total. This would leave between six and eight tubes available for storage or other purposes, something we will come back to later on in the story. Beyond that, the SSGN configuration had an all-new a dedicated special operations mission control center and associated mission planning spaces. It also included additional and improved sensor and communications antenna masts on the sail. Other modifications that would allow these submarines to better operate in shallower waters closer to shore, were also likely involved with the conversion. A rich history of special mission submarines The Navy had substantial past experience with employing submarines as special operations motherships and in the tactical strike role, to say nothing of using them as specialized covert intelligence gathering platforms, when it had crafted the requirements for the Ohio SSGNs. The ability of a submarine, in general, to transport personnel and materiel, as well as launch raiding parties ashore, while using its inherent capabilities to help avoid detection, was well established by the end of World War II. Between the mid-1950s and early 1960s, the Navy, in cooperation with the Central Intelligence Agency (CIA) and U.S. Air Force, had even used submarines to secretly launch radar-reflecting balloons to probe hostile air defense capabilities. You can read more about these operations in this past War Zone story. By the Vietnam War, the Navy was using specially configured submarines to support special operations. These included Gato class USS Tunny and the first-in-class USS Grayback, both of which were diesel-electric submarines that had previously been configured to fire the Regulus nuclear-armed cruise missile.

USN The USS Grayback in 1982.

The “hangars” on the decks of these submarines for the airplane-sized Regulus were well suited to modification into lockout chambers for swimmers and shelters for mini-submarines, just like the Ohio’s Trident tubes. In 1968, the Navy went so far as to designate them LPSSs, or amphibious transport submarines. These boats supported special operations along the coast of North Vietnam and also helped gather intelligence. Grayback was notably involved in Operation Thunderhead in 1972, an attempt to rescue American aviators that the U.S. military believed had escaped from North Vietnam’s infamous Hanoi Hilton prison. Bad weather and other factors eventually led the Navy to abort the mission and SEALs and Underwater Demolition Team (UDT) members never made contact with any escapees. One SEAL, U.S. Navy Lieutenant Melvin Spence Dry, died during the mission. The U.S. military only acknowledged the operation in 2008, at which time Dry received a posthumous Bronze Star.

USN SEALs on board Grayback ahead of Operation Thunderhead. Lieutenant Melvin Spence Dry is seated at center with papers in his hands.

In the decades after Vietnam, a number of Sturgeon class nuclear-powered attack submarines also served in similar special operations support roles. In something of prelude to the Ohio SSGNs, as part of the Strategic Arms Limitation Talks, or SALT I agreement between the United States and the Soviet Union in 1981, the Navy disabled the SLBM capabilities on a number of SSBNs, reclassifying them officially as attack submarines. USS Sam Houston, USS John Marshall, USS Kamehameha, and USS James K. Polk – the first two belonging to the Ethan Allen class and the latter pair being from the Benjamin Franklin class – received further modifications that added DDSs to the top of the hull and dedicated spaces to carry embarked SEAL teams. These submarines continued sailing into the 1990s and Kamehameha was the last to leave service, with the Navy only decommissioning her in 2002.

USN USS Kamehameha, with twin dry deck shelters installed, in Guam in 2001.

A new kind of submarine mothership Still, while the Navy had decades of experience with using submarines to support tactical operations, including special operations, at sea and onshore, the Ohio SSGNs aimed to be far more robust and flexible multi-mission platforms than any of these previous conversions. As of 2004, the service was still very much fleshing out the specifics of the SSGN conversion and “writing the manual” on how to then employ these submarines. Georgia had become the main testbed for what was still very much an evolving concept, receiving a number of interim modifications including reconfigured internal mission spaces and additional data links and communications equipment. At that time, none of the four chosen Ohios had gone through the full conversion process and they were still years away from actually entering service in their new configuration. “Two years from now, when we open the wrapping paper to see USS Georgia, a brand-spanking-new SSGN, we are going to need an instruction manual,” U.S. Navy Commodore Robert Shuetz, then-commander of Submarine Squadron 17, said at a change-of-command ceremony for the submarine in December 2004. “A manual that hasn't been written yet; a manual that will describe in excruciating detail how this new 'toy' will be operated.” “This is where the crew of Georgia has excelled,” Shuetz continued. “They have written the first instruction manual for how this ship and her three sisters, the 'toys' in demand by every combat commander, will be operated.” Silent Hammer Two months earlier, off the coast of San Diego, California, Georgia, even without anything near the full suite of capabilities outlined in the conversion plan, had demonstrated just what the SSGN configuration might be capable of as part of an experiment nicknamed Silent Hammer. To enhance the realism of the scenario, the Navy inserted this test into a larger exercise, called Trident Warrior, that involved an array of other submarines, ships, aircraft, drones, and special operations forces (SOF). The Silent Hammer scenario, which lasted a little over a week, involved a joint task force with Georgia in the lead locating and neutralizing mock terrorists on land and at sea. The “red team” occupied various sites on San Clemente Island, situated some 80 miles west of San Diego, which the U.S. military routinely uses for exercise and other test purposes. The contractor-operated offshore support vessel, the R/V Acoustic Explorer, also served as a simulated maritime threat. The overall objective of the exercise for the “blue team” was to find and fix these faux militants using a variety of intelligence sources and then neutralize them with simulated Tomahawk strikes.

MIT Lincoln Lab Imagery taken by various platforms of the R/V Acoustic Explorer (AX) and the Weapons of Mass Effect (WME) and Global System for Mobile Communications (GSM) facilities on San Clemente Island, all of which were under the control of "terrorists" during the Silent Hammer exercise.

During the experiment, at least publicly, the focus was far more on the submarine’s ability to act as an intelligence-collection platform, as well as a broader “clandestine sea-base” that would provide a “headquarters node from which command and control operations and logistic support were conducted,” including for special operators ashore. “Our converted Tridents will generate their own intelligence, which allows onboard commanders to make decisions about what’s needed and determine what additional organic sensors should be deployed in virtually any scenario,” by-then-retired U.S. Navy Admiral Frank “Skip” Bowman wrote, referring to the Ohios collectively by the Trident submarine-launched ballistic missiles that the SSBN versions carry, said in the Winter 2005 edition of Undersea Warfare magazine, the official publication of the U.S. Navy’s Submarine Force. Bowman’s last position in the service had been as Director of Naval Nuclear Propulsion. “Silent Hammer demonstrated how a networked force, including sea-based SOF from an SSGN, can fill joint gaps – Intelligence, Surveillance, Reconnaissance (ISR) and Time Sensitive Strike – by conducting large-scale clandestine operations, supported by advanced unmanned systems, to reduce risk and increase capability,” U.S. Navy Captain J.S. Davidson, who headed up the Silent Hammer experiment, had explained in another interview for another story in that same issue of Undersea Warfare. An intelligence nerve center It's hard to overstate how significant the intelligence fusion capabilities demonstrated during Silent Hammer were. For the experiment, Georgia had an embarked joint service command team onboard, who used modified spaces in the submarine to run a forward operations center that controlled other assets under the waves, riding on the surface, in the air, and on land. This was intended to reflect the capabilities that the submarine would have after going through the SSGN conversion, which would create new, more robust mission spaces for command and control elements and intelligence gathering personnel, among others. This was the first time the Navy had ever done this as part of the development of the SSGN concept of operations and it put the operational commanders right in the thick of things in a whole new way. Unlike traditional surface command ships, such as the USS Blue Ridge, the Georgia was allowing these officers and their staff to direct forward operations while sailing concealed below the surface of the ocean. The submarine’s command center was linked to rear command centers, and their intelligence networks, via satellite. It also had direct data-link feeds from a number of other sources.

MIT Lincoln Lab A diagram showing the extensive intelligence network employed during Silent Hammer.

In the air, these included the Pelican, a highly modified, pilot-optional Cessna 337 propeller-driven aircraft, and a specially configured Sabreliner twin-engine business jet. The Pelican belonged to the U.S. Naval Postgraduate School’s Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) and was configured at the time in a way that matched the capabilities of the MQ-1 Predator drone. The Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory operated the Sabreliner as a surrogate for smaller, lower-altitude unmanned aircraft.

USN A briefing slide giving an overview of the CIRPAS Pelican.

The Lincoln Lab also had their heavily modified Boeing 707 airliner, nicknamed Hannah, a well-known cutting-edge communications and sensor testbed, in the air playing the role of a airborne radar with synthetic aperture and ground-moving-target indicator capabilities. This effectively made it, in part, a surrogate for a U.S. Air Force E-8C Joint Surveillance Target Attack Radar System (JSTARS) battlefield management command and control aircraft.

Massachusetts Air And Space Museum The Lincoln Lab's modified Boeing 707 sensor testbed, nicknamed Hannah.

MIT Lincoln Lab Examples of the imagery that the MIT Lincoln Lab's platforms produced during Silent Hammer.

Navy EA-6B Prowler electronic warfare planes and EP-3E Aries II intelligence, surveillance, and reconnaissance aircraft also took part in Trident Warrior and fed information into this network of information sources. Down below, Georgia was networked together with other vessels taking part in Trident Warrior, including two Los Angeles class fast attack submarines, the USS La Jolla and USS Pittsburgh. In addition, members of the Silent Hammer experiment team were on board the first in class amphibious assault ship USS Tarawa and the Wasp class USS Bonhomme Richard, which were also taking part in the larger exercise. Ashore, U.S. Navy SEALs, along with other unspecified attached special operators, likely including U.S. Air Force Joint Tactical Air Controllers (JTAC), were in direct contact with Georgia. They emplaced their own “unattended” sensors to monitor for potential hostile activity and otherwise fed even more data back to the submarine. We also know that the Defense Advanced Research Projects Agency (DARPA) supplied unspecified payloads, as well as sensor systems for the exercise. Georgia itself demonstrated how she might launch unmanned aircraft and an unmanned underwater vehicles (UUV) during the exercise to support intelligence collection efforts. We will talk more about these shadowy developments later on. Data fusion pioneers The amount of intelligence information collected during the exercise was staggering. The supporting aircraft, ground sensors, and other offboard sensors collected more than 21,000 individual images during the exercise. In total, the task force created nearly 11 gigabytes of data, including thousands of textual alerts and nearly 3,000 actual intelligence “products,” such as PowerPoint presentations distilling various pieces of information, according to an article in a 2007 edition of the Lincoln Laboratory Journal. Unfortunately, this wealth of information also risked being overwhelming. So, the Navy and the Lincoln Lab had also developed a computerized and heavily automated network system, state-of-the-art for the time, that allowed the command center onboard Georgia to rapidly parse through the mountains of available information for the most relevant data and only download what they needed in full. Being able to avoid downloading unnecessary information was particularly important given the bandwidth limitations in the data links available between the submarine and its various offboard information sources, especially 15 years ago.

MIT Lincoln Lab A breakdown of the information flow between Georgia and the various intelligence gathering assets during Silent Hammer.

MIT Lincoln Lab What the actual search interface looked like to operators on Georgia.

Silent Hammer planners, as well as the Lincoln Lab, had been acutely aware of data sharing issues based on lessons learned from a smaller SSGN developmental experiment in 2003, nicknamed Giant Shadow, which involved the USS Florida and took place in and around the secretive Atlantic Undersea Test and Evaluation Center, or AUTEC, off the coast of Andros Island in the Bahamas. Similar to the Silent Hammer scenario, Giant Shadow centered on an operation to destroy a chemical weapons plant that mock terrorists were operating on shore. “We can get this [imagery] real-time down to the submarine," U.S. Navy Captain William Toti, then commander of the Florida, said in an interview at the time with "60 Minutes" on CBS News. "The SEALs can look at it real-time as they’re planning their missions, and have a better sense of what’s going on.” The problem in that exercise, as it turned out, had been that there quickly became too much information for personnel on the submarine to sift through and process in real-time. "The providers, not the consumers, decided what information to transmit and when, which created a situation whereby analysts were overloaded with processing extraneous information, yet still had insufficient information for decision support,” according to the 2017 Lincoln Laboratory Journal article. The flow of information during Silent Hammer was better, but still showed room for improvement. The vast quantities of data meant that it was still easy for intelligence officers to miss important new developments as they did their best to prioritize the efforts. Of the more than 21,000 images that various platforms collected during the exercise, less than 7,000 made their way into the networked database and “blue team” personnel only ever looked at 361 of them at any resolution, downloading just 45 of them in full for more extensive analysis. Still, the task force that Georgia led was ultimately able to find all of the simulated threats and successfully carry out the mock strikes to neutralize them.

MIT Lincoln Lab A full breakdown of intelligence collected during Silent Hammer.

MIT Lincoln Lab A flowchart showing how the intelligence from various sources eventually produced targeting grade information for simulated strikes on San Clemente Island during Silent hammer.

Secretive payloads For how much is known about Georgia’s participation in Silent Hammer, as well as the overall scope and scale of the intelligence gathering and networking systems employed during the exercise, there is little information about the testing of the submarine’s capabilities to launch underwater unmanned vehicles (UUVs) and unmanned aircraft. It’s not clear what type or types of UUVs participated in Silent Hammer, or if Georgia deployed any of them herself. However, during the earlier Giant Shadow exercise, Florida had become the first Navy submarine to launch and recover the Seahorse Autonomous Undersea Vehicle (AUV) via a modified missile tube. It is very possible that this undersea drone took part in Silent Hammer, as well.

USN The Seahorse AUV.

The Applied Research Laboratory (ARL) at the Pennsylvania State University had begun development of Seahorse in 1999 under contract to the Naval Oceanographic Office, or NAVOCEANO. At 28 and a half feet long and weighing 10,800 pounds, this underwater drone was more than 10 feet longer than a Mk 48 heavyweight torpedo and just over 7,100 pounds heavier. Its main job was undersea mapping using a variety of sensors, including multi-beam bathymetric and synthetic aperture sonars, an Acoustic Doppler Current Profiler (ADCP) and a Conductivity, Temperature and Depth (CTD) sensor. Those same sensors could be used to scout out mines and other potential underwater hazards and, in the decades since the Navy took delivery of Seahorse, the service has acquired and fielded a large number of increasingly more capable torpedo-shaped UUVs of various sizes for mapping and mine clearance missions, among others. The Flexible Payload Module Georgia didn’t actually launch any unmanned aircraft during Silent Hammer, according to the Navy, but did release two Stealthy Affordable Capsule System (SACS) canisters, each containing an “inert test shape simulating a UAV,” from a Flexible Payload Module (FPM) installed in one of the submarine’s missile tubes. Since the 1990s, the Navy had been very interested in the idea of pairing unmanned aircraft with submarines to expand the ability of the boats to scout ahead and collect intelligence. Drones working with subs could also act as communications and data relays, probe and collect information on enemy defenses, and potentially even strike targets themselves. For example, in March 1996, the Los Angeles class attack submarine USS Chicago took part in a demonstration in which it tested its ability to both communicate with and actively control an early example of what was then known as the RQ-1 Predator. Development of the FPM dates back to at least 2000, when the Navy tasked two separate industry consortiums with crafting concepts for future submarines designs, as well as payloads and sensors for them, with an eye toward technologies that could be operational in the years to come. The Navy and DARPA managed this project, aptly named Submarine Payloads and Sensors, cooperatively.

General Dynamics Electric Boat

Northrop Grumman, a member of Team 2020, one of the consortiums, which Lockheed Martin headed up, developed the FPM. General Dynamics Electric Boat, the United States’ premier submarine builder, which had built the Ohios, among others, and was involved in the development of the Virginia class attack submarine at the time, was also part of Team 2020. The FPM was effectively an insert that would slot into a large diameter ballistic missile tube on a submarine, but could be adapted to hold multiple payloads, including numerous unmanned aircraft, that the crew could then launch independently. General Dynamics Electric Boat described it as a “plug and fight” system.

General Dynamics Electric Boat

Northrop Grumman designed the first iteration, which had 10 14-inch tubes and a pair of larger 20-inch ones, specifically around the dimensions of the Ohio’s missile tubes. The second FPM prototype, which Georgia carried during Silent Hammer, had only three tubes of an unknown diameter. Each one of those could accommodate a payload inside a SACS, another Northrop Grumman development.

General Dynamics Electric Boat

“The FPM and SACS comprise an encapsulation system that facilitates the launch of non-marinized payloads and weapons from a submarine,” according to the article on Silent Hammer from the Winter 2005 issue of Undersea Warfare. “This allows the use of Navy air- or surface-launched payloads – plus those from other services – without the need to redesign them for launching in an undersea environment.” SACS was “adaptable for long-term storage, variable release depths, launching under broaching or surface-loitering conditions, and the ability to encapsulate small or large payloads,” according that same article. "In the case of the SUAV [submarine-launched unmanned air vehicle], SACS rises buoyantly to the surface, a sensor in the capsule detects broach, the SACS end-cap is blown away, and the SUAV booster ignites to clear the water and build vertical speed," notes from a presentation that Steve Weinstein and William McGannon gave at the National Defense Industry Association’s (NDIA) 2002 Joint Undersea Warfare Technology Spring Conference explains. "At the proper moment, the SUAV wings are extended from alongside its long slender body to the horizontal position, the flight control software tilts the SUAV over to the horizontal flight position and once in stable flight, the SUAV turns and climbs to the pre-planned altitude to begin its mission." At the time, Weinstein and McGannon were employed with the Naval Sea Systems Command’s (NAVSEA) Submarine Sensor Systems division. The other industry collective that had taken part in the Submarine Payloads and Sensors program, called Forward Payloads And Sensors for Submarines (Forward PASS), had developed a similar system, known as the Broaching Universal Buoyant Launcher (BUBL), that worked in much the same manner. However, BUBL’s design was meant to work with a variety of existing launcher options on submarines, including torpedo tubes and countermeasures launchers, or even be carried externally. Of course, the external carriage option could have created performance problems or increased the sub’s acoustic signature, making it more vulnerable.

USN A briefing slide from 2002 offering basic information and graphics regarding FPM, SACS, and BUBL.

Raytheon was the team leader for Forward PASS, which also included Boeing and Pennsylvania State’s Applied Research Laboratory, among others. General Dynamics Electric Boat was part of both teams in order to provide its extensive knowledge base to help with submarine development and integration questions. There is no mention of Georgia employing BUBL during Silent Hammer. Submarine-launched drones While we don’t know what drones Georgia was supposed to have been simulating the launch of from the FPM specifically, Northrop Grumman had also already developed at least one submarine-launched drone known as Sea Ferret in the 1990s. This was an evolution of Ferret, which the company had originally developed for the U.S. Army. The Sundstrand TJ50 turbojet-powered Ferrets and Sea Ferrets are what we would call loitering munitions today. The approximately 145-pound drones carried both electro-optical sensor packages and 20-pound warheads and could fly out to a maximum range of around 370 nautical miles and a top speed of 300 knots and still be able to orbit around a target area for around two hours.

Public Domain A very low-quality image of Ferret.

In December 1996, the USS Asheville, another Los Angeles class attack submarine, simulated launching the Sea Ferret during a technology demonstration. A Cessna 206 light aircraft carried one of the drones under its wing to then simulate the unmanned aircraft in flight. Northrop Grumman had intended the final system, which the Navy did not ultimately adopt, to be torpedo tube-launched using a modified canister for a UGM-84 submarine-launched Harpoon anti-ship cruise missile. Still, the 1996 test “successfully simulated organic and inorganic UAV operations & SOF support,” according to Weinstein and McGannon 2002 NDIA presentation. It is certainly possible that Northrop Grumman could have developed a follow-on of some sort to Sea Ferret at the time of Silent Hammer. We also know that the Navy had been holding workshops and other defense industry engagement events to gauge options for submarine-launched unmanned aircraft starting in 2000, around the same time as the Submarine Payloads and Sensors initiative. A slide from a General Dynamics Electric Boat briefing at the 2006 NDIA Systems Engineer Conference, which also touches on the Flexible Payload Module (FPM) development, shows concept art for at least five different potential submarine-launched drone designs.

General Dynamics Electric Boat A briefing slide from 2006 showing various submarine-launched drone concepts, as well as other potential payloads that could work with a future universal adapter.

By 2002, a team that included General Dynamics, Lockheed Martin, AeroVironment, and Kollmorgen, had also demonstrated a modified Universal Modular Mast that could shoot small unmanned aircraft into the sky from periscope depth. An artist’s conception of the system shows a drone design virtually identical to the Blackwing, which AeroVironment officially began developing four years later for the Navy as a submarine-launched system.

USN