Submarine systems do not develop and evolve in a vacuum. In most cases advances in the technology of one system impacts on each of the other systems in the boat. The submarine is an integrated weapons platform that depends on the capabilities of all its technologies. As sonars became more powerful and were able to analyze a wider spectrum of noise, for example, the requirement for submarine silencing grew. Equally, as the capabilities of the onboard computer systems grew it became possible to analyze noise faster and more accurately. It became possible to compute the paths that noise took through the water. It was found that there were sound channels in which noise traveled relatively unchanged. There were also areas where sound paths curved deep and came back up to the surface, were reflected, curved deep, and reappeared (these became known as convergence zones). Predicting these paths and zones in existing ocean conditions became an important part of the sonar’s task.
The increase in computer power also drove advances in Electronic Surveillance Measures (ESM) sensors as they became more able to analyze a wider electronic spectrum with greater speed and detail. As electronics devices became smaller and more capable, antennae and masts could have combined functions. For example, as far back as World War II simple radar antennae were mounted in the upper head of periscopes. The periscopes have evolved to take on more and more functions and an increasing number of antennae are mounted in their upper ends. Weapons also drove fire-control system and sonar development. As the fire-control systems became more capable, the torpedo in turn became “smarter.” The early Mk 14 torpedo could be compared with an air-launched “dumb bomb” or “iron bomb.” The Mk 37, however, had a hydrophone in its nose and could seek out its target as it got closer, making it more likely to achieve hits. In addition, it was wire guided. As computers and sonar systems became smaller, a computer could be put into the torpedo so that through a wire-guidance system the onboard fire-control computer could talk directly to the torpedo. There was a spool of thin wire connecting the torpedo with the torpedo tube, and the wire was used to send a signal to the torpedo once it was launched. The course of the torpedo was altered as needed if updated information showed the existing fire-control solution was in error or the target had made some radical maneuver. In addition, the torpedo could be programmed with the acoustic signature of the selected target and become a “fire and forget” weapon – the Mk 48 ADCAP (advanced capability) is such a torpedo. Therefore, in the discussion of the technologies below, one should view the items evolving as an entire group rather than individual systems.
Sonar consoles on USS Thresher. This set of active/passive sonar inboard electronics comprised the BQS-6/BQA-3 sonar. The active sonar console is in the middle with the passive on the left and the BQA-3 graphic indicator on the right. The man in the photo, Sonarman First Class R. E. Steinel, was aboard Thresher when it was lost.
When submerged a submarine must rely on sensors to navigate, locate targets, prosecute an attack, communicate, perform surveillance and accomplish any other assigned tasks. For convenience these sensors may be grouped into the following categories: sonar, electronic surveillance, radar, navigation, communications, and optical.
The electrical signal output of the hydrophone can be processed in several ways. The simplest is to look at the signal with respect to time, as through an oscilloscope, a process called time series analysis. It is, however, more useful to look at the intensity level at a particular frequency. To do this the signal is passed through a set of filters that only allow through the frequency of interest. This method allows the operator to listen for a particular noise – for example, the known frequency of a particular piece of machinery in an enemy ship. The selection of filters to select noise signals is used in an analog system. A digital system can sample a noise signal many times per second and convert each sample into a number which can be plugged into a set of equations to derive more information from the signal faster and more accurately than an analog system. The level of the signal is stored as a number that can be processed by a computer. The mathematical process generally used to perform the analysis is a Fourier Transform, and early computers performed this function using software and what is called a Fast Fourier Transform (FFT). As computers evolved, the processing also evolved into being hardwired in integrated circuits called digital signal processors (DSP).
The hydrophone is a simple device that listens “all around.” When multiple hydrophones are placed together and linked with electrical devices called delay lines, the array can discriminate in the direction it listens, something generally called “beamforming.” US Navy sonar systems are designated by a multiple letter number system. The first letter is the installation type (“B” for submarine), the second is the equipment type (“Q” for sonars), and the third is the equipment’s purpose (“R” for receiving, meaning in the case of sonar the passive elements, and “S” for sending or the active portions). Finally there is a number to designate the specific equipment. Thus BQR-7 is a submarine passive sonar. The following is a very general description of the sonar types as they evolved through the various fast attack submarines.
The BQR-7 system of sonar consoles.
The Nautilus up through the Skipjacks had a form of the BQR-4 and BQS-7 system. This was a system that used electronic beamforming through switches and analog filters to transmit active and receive passive information. The information was displayed on electronic screens and on scrolling paper plots. Tracking information was fed directly to the fire-control system in the form of bearing to the target. In the Permit Class several sonar arrays were combined and the system was generally designated the BQQ-1, the second “Q” meaning “combined” or “special.” The advent of computers capable of performing the digital filtering of the noise signal meant the signal could be analyzed faster and with more resolution. The Sturgeons received the BQQ-3 (which was backfitted to the Permits) – it was essentially a sonar system that not only used all-digital tracking and filtering, but featured the BQR-20 spectrum analyzer, which could display the noise signal in an X-Y graph display, the noise level on the vertical axis and the frequencies on the horizontal axis. This system was coupled with a library of existing spectral plots of known signals of interest (read this to mean known target signatures).
The BQQ-5 integrated console. This console, along with several other identical ones, constitute the entire sonar suite, and was standard equipment on Los Angeles Class and was backfitted onto some Sturgeon Class. (Author’s photograph: Submarine Force Library and Museum)
In the early days sonar depended on the ears and knowledge of the operator to discriminate the noise of a target from all the other noise in the ocean. The advances in sonar systems were generally directed at both extracting more information from the noise and quantifying the noise so it was not so dependent on the operator. The ability to analyze the level (loudness) of the noise at each of many discrete frequencies meant that the sound produced by an external source (such as a target ship) could be quantified as a specific noise signature. As the system became more accurate and faster, new analytical methods were introduced, such as Doppler analysis. Doppler is the frequency shift in a moving object – higher if it is closing, lower if it is moving away. As the hydrophone arrays became larger and the technology of hydrophones advanced, the accuracy of the bearing to the target improved. The result was an increase in the accuracy of Target Motion Analysis (TMA). TMA is vital to the “all passive sonar” approach to the fire-control problem and allows a more accurate fire-control solution. All the systems described improve the ability of the submarine to detect a target early and at long range, accurately identify the target, separate its signature from those contacts that may not be hostile, and determine what the target is doing in real time.
Some of the sonar arrays are:
Spherical Array – This array is located inside the bow sonar dome. It consists of a steel sphere about 16ft in diameter in the Permit Class to 24ft in the Virginia Class. On the outside of the sphere is a closely packed collection of hydrophones, each pointed out from the center so each hydrophone listens in a different direction.
Conformal Array – Here a set of relatively widely spaced hydrophones are mounted inside the bow dome and generally conform to the shape of the dome curve. It is designed for detecting low-frequency noise. Wide Aperture Array – Located in three places along each side of the submarine are planar arrays. These are used to achieve rapid passive ranging to a target.
Towed Array Sonar – A set of hydrophones in a linear array is towed on the end of a cable, which can be unreeled to a length of over 1,000ft. This array is therefore behind the turbulent area created by the submarine’s hull and propulsion. There are two types of towed array in use. The TB-29 is called the thin-line towed array and the TB-16 the thick-line towed array.
Spot Hydrophones – Single hydrophones are sited at various places around the hull in free flood spaces and ballast tanks. These monitor the noise created by machinery inside the hull and are used to determine the ambient noise level around the ship.
Sail Front Array – In this case there is an active and passive sonar array set (BQS-15 and Mine and Ice Detection Active Sonar, MIDAS) mounted on the front of the sail and under the forefoot. These arrays are used for looking upward and for under-ice and minefield navigation.
Threat Detection Array – At various points on the ship small arrays are mounted, specifically tuned to detect the noise of threats such as torpedoes. These are collectively known as the WLR-1 through -9 and the WLY-1 acoustic intercept systems
One of the most useful tasks performed by fast attack submarines during the Cold War was that of electronic surveillance. This consisted of using a variety of mast-mounted antennae coupled with a system that could analyze the electronic signals being received. The ESM looked at the electromagnetic spectrum in much the same way a sonar system looked at the noise field in the water. Early systems used analog filters to discriminate frequencies. Some antennae were moved into the tops of periscopes. As digital systems became standard, computer-driven analysis systems took over. The capabilities of each of the systems, however, are closely held secrets.
Fast attack submarines carry a radar system not unlike that carried by many commercial vessels of a similar size. It is generally used for navigation only, although it can be used for fire control and to provide a warning of airborne and surface threats under certain circumstances. However, radar is not a passive system, and a submarine that uses it broadcasts its position. Today the standard radar system is the mast mounted BPS-15.
Several systems are used to determine the ship’s position accurately. These have changed during the evolution of the nuclear fast attack submarine. On board Nautilus, navigation was by star sight, sun lines, Omni, Loran (both using shore-based radio stations), and dead reckoning – good enough for the time, but not good enough for the tasks ahead. The Ships Inertial Navigation System (SINS) was developed for the accurate position keeping needed for ballistic missile submarines, and its technology was modified to fit in fast attacks. The first satellite navigation system used the time signal of passing navigation satellites. The signal could be quickly received by sticking up an antenna. SINS was further downsized by using laser ring gyros, and increased computer power allowed many of the large cabinets to be eliminated. Today, the navigation systems consist of specialized inertial navigation coupled with the Global Positioning System (GPS). The submarine force is changing to the Electronic Chart Display Information System (ECDIS), which will eliminate the need for paper chart navigation.
Part of the Mk 113 fire-control system. This hybrid analog/digital system replaced the Mk 101 system that was the mainstay during the latter part of World War II and into the late 1950s. The system provided solutions for the torpedo shoot and was also used to fire the SubRoc missile. (Author’s photograph: Submarine Force Library and Museum)
Secure radio communication is vital for a fast attack. Its technology can be inferred, but security classification makes it seriously difficult to discuss accurately, and justly so. The systems have evolved from high-speed encrypted teletype transmissions to digital bursts to comms using orbiting satellites. For a long time, the Extremely Low Frequency (ELF) communication from giant antennae worked very well. The submarine could receive this data at a relatively shallow depth – a buoyant wire was trailed behind the sub, and this system might still be in use. The Virginia Class can receive information from many sources, even the internet.
The Virginia Class has, as standard equipment, the non-penetrating periscope, which is solely electronic in its image acquisition. The periscope does not penetrate the pressure hull, thus simplifying interior design of the operating spaces because the location of the control room is no longer defined by the scopes. This new device can acquire image data in a wider range of frequencies than the human eye and sends this data to computer memory for review.
A submarine fire-control system is a grouping of equipment that is tasked or designed to solve a complex relative motion problem. The idea is to fire a torpedo or missile that will intersect with the path of a target at the exact time the target reaches a particular spot. The problem is mathematical in nature, with a set of known variables and a set of unknown variables. The problem must be solved in real time. The variables can be further divided by three influences – those belonging to your ship (called “own ship”), those belonging to the weapon, in most cases a torpedo, and those belonging to the target.
Own ship variables are all known and consist of position, course and speed (which come from the onboard gyroscope and pitometer log), and depth (from onboard depth gauges). The weapon’s variables are also known and consist of the weapon’s speed, any turns that may be programmed into the weapon, and its running depth. The variables belonging to the target are mostly unknown and must be determined by various means. The method most familiar to the public – the use of a periscope to spot and track a target – went by the wayside after World War II. Thereafter, sonar became the primary means of supplying the target information that would enable the fire-control system to make the target’s unknowns into knowns. A fire-control system generally works by making a prediction and correction solution. The own ship’s information and that of the torpedo are continuously fed into the fire-control computer, as is the information from the sonar about the target’s bearing. The target’s speed, range, and depth are approximated from information such as the target’s capabilities and its screw speed. The fire-control computer then predicts what the bearing will be at some future time, correcting that prediction with new information. The prediction is computed over and over as fast as possible until the predicted bearing matches the actual bearing. At this point the fire-control system alerts the operators that a solution had been reached, and the weapon can be launched.
Loading a SubRoc Missile. The missile is being slid athwartships to align it with a torpedo tube on a Permit Class submarine. Note the restricted space in which to handle and conduct maintenance on the weapons. SubRoc was a submarine-launched rocket that was capable of carrying a nuclear warhead. Its use was limited and it was phased out and is no longer on submarines.
As the nuclear fast attack submarine evolved in engineering and technology, so did the fire-control system. Nautilus used a system not unlike that used on World War II fleet submarines. It was an electro-mechanical system that applied a complex system of syncros, servos, gearing, and cams to solve the relative motion problem. This system was relatively slow and had difficulty keeping up with the increased speed of the submarine and its supposed targets. Through the early 1960s the systems became more electrical and less mechanical, with increased use of hardwired analog computers. As advances in the fire-control system occurred, the new systems were installed on submarines being built, and backfitted where possible onto older boats.
The SubRoc was the first and only submarine-launched long-range nuclear-armed antisubmarine missile ever deployed by the US Navy. An enemy submarine had virtually no chance of escaping a SubRoc, especially since its sonar could not detect the missile in the air.
The Mk 113 was a hybrid system that had not only the analog elements but also a hardwired digital computer. Ballistic missile submarines made use of the Mk 84 digital computer for missile firing, and that system’s capabilities were carried over to the fast attack submarine’s Mk 113 system. As the power of programmable digital computers increased while their size decreased, they took over more of the mathematics from the electro-mechanical and analog systems. By the time the Los Angeles Class was into its full-up build cycle, the new Mk 117, an all-digital programmable fire-control system, was introduced and first installed on the USS Dallas (SSN-700). It was backfitted on most existing submarines and became the standard system until nearly the end of the Los Angeles Class’ build cycle. The Mk 117 system underwent a significant upgrade with the adoption of the Over The Horizon Targeting (OTH-T) that would be used for the Harpoon antiship missile and the Tomahawk cruise missile. The OTH-T substituted radio flash messages for onboard sensors in supplying information on the target to the fire-control system. About the same time the ship’s sonar systems changed from analog to digital computing with the installation of the BQQ-5 sonar system. Because the sonar and fire-control systems were now both fully digital there was a design effort to combine the systems into an integrated sonar/fire-control system.
The weapons suite for fast attack submarines is listed in the table below:
| Weapon designator | Type | Range | Speed (kts) | Warhead | Applicability (Class) | Remarks |
| Mk 14 Mod 3 | Torpedo | 4,500–9,000yd | 35–50 | 668lb TPX | Nautilus to Skipjack | WWII mainstay torpedo, in service until late 1970s |
| Mk 16 Mod 1–8 | Torpedo | 11,000yd | 40 | 960lb TPX | Nautilus to Skipjack | Used NAVOL fuel (hydrogen peroxide), withdrawn 1978 |
| Mk 27 | Torpedo | 6,000yd | 40 | 900lb TPX | Nautilus to Permit | Electric (storage battery) propulsion, replaced by Mk 37 |
| Mk 37 Mods 1, 3 | Torpedo | 8,000 – 18,000yd | Various | 330lb HBX-3 | Nautilus to Los Angeles | Homing and wire guided. Also in NT Mk37E |
| Mk 45 | Torpedo | 30,000 – 40,000yd | Various | Nuclear Capable | Skate to Sturgeon | In service 1957 to 1976, called ASTOR |
| Mk 48 | Torpedo | 30,000 – 40,000yd | Various | 800lb HBX-3 | Permit to Los Angeles | Wire guided, homing and pattern running, Uses OTTO monopropellant |
| Mk 48 ADCAP | Torpedo | 30,000 – 40,000yd | Various | 800lb HBX-3 | Permit to Virginia | Wire guided, homing and pattern running, Uses OTTO monopropellant, built to counter the deep diving Soviet submarines. |
| SubRoc UUM-44 | Rocket | 30nm | N/A | Nuclear Capable | Permit to Los Angeles | ASW weapon prior to advent of Mk-48 torpedo |
| Harpoon (UGM-84A/C) | Antiship Missile | 75nm | 600 | 488lb WDU-18 | Sturgeon to Virginia | Submarine version of a versatile standoff antiship weapon. |
| Tomahawk BGM-109B/C | Cruise Missile | 250 –1,350nm | 500 | 700– 1,000lb | Sturgeon to Virginia | Modern multipurpose weapon. Speed and range is warhead dependent |
| Special operations | SEAL Teams | N/A | N/A | N/A | Sturgeon to Virginia | Descendent from UDT teams of WWII. |
PLATE E
The nuclear fast attack submarine is fitted with a variety of weapons from torpedoes to missiles. At the top is a Mark 37 Mod 3 torpedo, which was an antisubmarine and anti-escort weapon. It was wire guided and had an acoustic homing feature. It evolved into the Mark 48 ADCAP that has become the mainstay torpedo in the US submarine fleet. Not only wire guided and fast, it has enough computing power aboard and enough of a sophisticated acoustic homing (passive and active) feature to be a “fire and forget” weapon. The white SubRoc between the two was an interim weapon to counter the threat of large Soviet submarine numbers. It was nuclear capable but is no longer carried. The Tomahawk cruise missile can be launched from torpedo tubes or on the submarines so equipped with the VLS. With terrain following and GPS navigation systems, a range of over 1,200nm, and a warhead of over 500lb of high explosive, it is an extremely formidable stand-off weapon. Below that is a portrayal of the Harpoon antiship missile as it breaks the surface and heads off to its assigned target. This view is often the first indication an enemy has that there is a US fast attack submarine in the area.