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By William P. Gruner and Henry E. Payne III
Submarine warfare is no longer limited to launching weapons against enemy ships, submarines, and land targets and evading enemy antisubmarine (ASW) attacks. It includes collecting intelligence information on activities near foreign shores and at sea and tracking, intercepting, and escaping from foreign submarines. To meet these challenges, U.S. submarines must be able to maneuver rapidly while engaged in __ _ their missions.
The public generally is not aware of the close encounters our ' \ submarines experience.
Table 1 shows a few examples.
The need for great maneuverability in these deadly games of ob- k „ ^ serve, tag, and evade is evident. To make an effective and safe maneuver in the proximity of other submarines, ships, and mines, a diving officer needs two things—precise control of his own submarine and accurate knowledge of the positions and movements of other vessels and objects. Our existing attack and ASW submarines are hampered by three major shortcomings.
> Although capable of speeds in excess of 30 knots, they are unable to make small-radius turns at high speeds safely, because they become unstable.
► The normally used four-man, manually operated control system lacks the rapid-response capability for directional control of an unstable submarine.
► The captain of a submerged submarine has little knowledge of the precise locations of other submarines and ships in his vicinity. Consequently, he does not know how best to maneuver.
The Stability Problem. A submarine at rest has static stability. When it starts to pitch or roll, moments are generated by buoyancy and weight forces to restore it to the rest position. When propelled, it has dynamic stability if it can be made to follow a predictable path. However, pitch-yaw hydrodynamic coupling (the dreaded snap roll) causes severe stability problems when maneuvering at high speeds. Dynamic stability is a prerequisite for directional control, and directional control is a prerequisite for maneuverability. High maneuverability cannot now be achieved because directional control cannot be maintained when large rudder angles are applied at speeds in excess of 15-20 knots. Once control is lost, the submarine may be lost if the diving officer cannot regain control.
Recognition of this instability problem did not come until very powerful propulsion plants and better streamlining made high speeds possible. The problem was highlighted in 1954 when the experimental high-speed, battery-powered submarine USS Albacore (AGSS-569), designed with a sleek body-of-revolution hull, became operational. With a top speed greater than 30 knots, she was equipped with specially designed control surfaces and a fully automated control system that could be operated with from one to four men, with or without selected automation. Instability soon became evident. One commentator noted, “If in a melee situation, a modern highspeed sub pilot tries to turn t o o
sharply at too high a speed, he might find himself in a snap roll, hanging from his seat belt and with a
| Table 1: Some U.S. Submarine Close Encounters' | |
Submarine/Area | Date | Remarks |
Baton Rouge (SSN-689) Off Kola Inlet | Feb. 1992 | Submerged collision with Soviet nuclear submarine. |
Augusta (SSN-710) North Atlantic | Fall 1986 | Collided with a very quiet Soviet sub while testing a new sonar system. |
Baltimore (SSN-704) North Atlantic | 1985 | Engaged in close observation of unusual Soviet submarine activity. |
Tautog (SSN-639) Not available | June 1970 | Rammed by a Soviet sub while shadowing her. Both subs were damaged. |
Lapon (SSN-661) North Atlantic | 1969 | Trailed a Soviet SSBN for 40 days. |
Barbel (SS-580) Oulf of Tonkin | 1966 | Accidently rammed and sank North Vietnam’s largest freighter. |
Swordfish (SSN-579) North Pacific | 1963 | Tried to evade Soviet ASW forces while observing Soviet naval exercise. She bent her periscope on a Soviet ship and was depth charged. |
Grenadier (SS-525) Off Iceland | May 1959 | Detected, tracked, and held down a Soviet sub until she was forced to surface. |
Gudgeon (SS-567) Off Vladivostok | Aug. 1957 | Tried to evade several Soviet destroyers for 30 hours during which time she was depth charged. |
'°ss of several hundred feet in depth at a markedly slowed speed.”2
An early lack of concern about the stability problem Probably was because, until the end of World War II, very evv °f the world’s submarines had submerged speeds in excess of ten knots. Manual control of bow and stern P*anes and helm by separate operators directed by a div- lng officer was considered adequate at normal speeds of fw° to six knots. If “the bubble was lost” while increas- lng depth, an alert diving officer could soon regain con- r°l by backing and blowing tanks.
. Later, when nuclear-powered submarines of the Skip- (SSN-585), Sturgeon (SSN-637), and Los Angeles SN-688) classes—all characterized by body-of-revolu- '°n hull designs and large sails—became operational, they, ■°0’ had stability problems similar to the Albacore's. Their '^stability is caused by external variable forces acting on e'r hulls—a result of pressures exerted by the swirling Jesses of water that are continuously generated by the u 1 as it turns and drives its way through the ocean. These asses of water, termed vortices, rotate inward toward the ^nter of the hull. The higher the submarine’s speed, the °re energy is imparted to them, and the greater the pres- r®s acting on the hull.
h'gure 1 is a computer simulation of vortices generated by a submarine during the mid-stage of an evasive maneuver at 24 knots with a 30” rudder angle. Note the displacements of the twin vortices as the ship progresses through the water and the effect of the sail on their location. Under the influence of the forces generated, the stem will squat down and the bow pitch up. Meanwhile, the forces will cause the submarine to sink and slow, and it will go into a snap roll unless speed is reduced by backing and the rudder put amidships.
Sample patterns of wind tunnel airflow tests, using a scale model of the Skipjack hull, are shown in Figure 2. The first pattern shows the airflow to be quite smooth when the submarine is on a straight course. However, when the model is placed at significant roll and yaw angles as in the second pattern, the strong influence of the sail pulls the upper vortex out of its place along the hull and into the wake of the sail. Comparisons of such patterns at different roll angles, yaw angles, and speeds reveal dramatic variation in the oscillating flow and pressure patterns.
Full-scale submarines experience similar flow conditions when large rudder angles are applied at high speeds. Resulting unbalanced pressures on the sail, for example, can produce forces of millions of pounds, with corresponding moments great enough to rotate a submarine
about its roll axis. This results from a submarine’s relatively small righting moment (because of its small metacentric height) about the roll axis.
The Directional Control Problem. When a roll begins under these conditions, a coupling of motions about the roll, pitch, and yaw axes occurs to cause a submarine to veer suddenly and radically in unexpected directions and to change depth and speed. Manual control under such conditions becomes highly difficult, if not impossible.
In discussing directional control of the Albacore, an early commanding officer stated, “It was evident that the Albacore performed significantly better with automated
controls programmed by a single operator than she did with a standard four-man team of diving officer, helmsman, bow planesman, and stem planesman.” He likened the submarine control problem to that of a high-speed aircraft in which “designers turn to higher performance control machines to offset human limitations in sensing and reacting, a lack of uniformity of performance, and limited adaptability to performing multiple requirements simultaneously.” He believed future submarines would be “flown” with automatic controls with pilot override.3
Limited human capabilities—the human brain and muscular control systems operate too slowly and have other limitations—caused airplane designers to turn to higher- performance-control machines. Pilots are unable to keep pace with rapidly changing situations while faced with multiple tasks during aerial combat, low-level flying, and landing. Further, human systems, if under stress, are likely to err, function inconsistently, and freeze. The repeatability and reliability of modern computer-aided systems have made them the only choice for high-speed airplane maneuvers. For similar reasons, they are the only choice for controlling high-speed submarine maneuvers.
Despite the Albacore’s favorable experience with the automatic one-man control system in the mid-1950s, the system was looked upon with suspicion. Although other work was performed to improve the controllability of submarines at high speeds, the basic four-man diving team used in low-speed submarines was applied to nuclear-powered submarines, with limited provision for a one-man control system. To minimize the risk of loss of control, maneuverability is now limited by operational procedures that constrain rudder angles to a very few degrees at speeds above about 15 knots. As another safety measure, the rudder control system prevents the application of large rudder angles at high speeds.
The Approach to Greater Submarine Control
For our submarines to achieve greater maneuverability, they must be modified through the application of current technical knowledge in control systems, hydrodynamics, human engineering, and physics. Much of this knowledge can be borrowed from aircraft and missile engineers, who were faced with stability and control problems as soon as Orville and Wilbur Wright demonstrated a capability for powered flight in 1903.
The well-known author and aeronautical engineer Neville Shute described the state of airplane control during World War I, when most airplanes flew at speeds of 60-130 knots. “We knew that a clumsily executed turn might have the effect of putting an aeroplane into a spinning nose-dive (a Parke’s Dive, some of us called it, be* cause Lieutenant Parke was one of the very few people who had come out of it alive). In general, a spin, once started, continued to the ground, the machine hitting very violently. And that literally, was all we knew about it.”4 ;
As airplane speeds increased, it became obvious tha* manual flight control was no longer acceptable. It was recognized that, “human pilots were incapable of adequately compensating for the rapid changes encountered during
tactical engagements, adverse weather conditions, ground- skimming flight, and other maneuvers. To make flight feasible, pilots were provided with assistance in the form of automatic flight-control systems. Such systems now are incorporated in all modem high-performance aircraft, including C-141 and C-5 transports, B-l and B-2 bombers, and F-4, A-6, F-14, F-15, and F-16 fighters and fighter- bombers.
“Individual systems vary from aircraft to aircraft, but ln general provide pitch, roll, and yaw control augmentation, autopilot modes of operation, and altitude hold. The pilot normally enters maneuvering orders by means °f stick or control column and rudder pedals. Major elements of the flight-control system include control-force transducers, pressure and temperature sensors, directional gyros, accelerometers, a central air-data computer, and a flight-control computer. The latter interprets transducer mput from pilot actions and other data sources, processes data, and sends commands to the various servo-actuators to assure that proper control surface and device responses are made to pilot orders within safe flight limits. Control system reliability is achieved by rigid parts selection; inspection and test at component, subassembly, and system levels; and redundancy in electronic, hydraulic, and power supply elements.”5
While scientists and engineers were developing systems t° control supersonic aircraft, others were developing control systems for intercontinental ballistic missiles, to enable them to deliver payloads accurately and reliably to targets thousands of miles distant. At that time, subma- nne designers seemed content to modify two-dimensional control systems for submarines operating in a three-dimensional ocean. Since these systems do not provide the oegree of maneuverability required, a new control capa- ’bty must be developed—one similar to aircraft and mis- Sl*e control-system technology.
All bodies in motion—including submarines and airplanes—are subject to the same inviolable laws of Physics, These laws govern relationships between mass, 0rce, torque, inertia, and acceleration. One important law states that rotational acceleration about an axis is proportional to the moment applied and inversely proportional to the inertia characteristic (moment of inertia) of the body about that axis. The high energy vortices noted previously and the large sail are the main causes of the upsetting moments that cause U.S. submarines to lose control in high-speed turns. Therefore, special attention must be paid to reducing sail size and its resultant interaction with the hull vortices.
Concept for an Automatic Submarine-Control System
Because of the complexity of submarine dynamic motions, the rapidity with which upsetting moments are generated, the speed with which control forces must be applied, and the inability of humans to exercise manual control, the new control system must employ computer technology. Figure 3 presents a concept for an automatic submarine-control system composed of three major subsystems: an automatic maneuver subsystem, a pressuresensing subsystem, and an automatic attitude-control subsystem.
The Automatic Maneuver Subsystem. This subsystem performs two major functions. First, it provides the man/machine interface by which the diving officer enters maneuver instructions and receives information. To initiate a maneuver, the diving officer specifies the required maneuver within the framework of an earth-oriented, north-referenced, three-dimensional orthogonal coordinate system. Maneuver instructions may call for a simple turn and a change in depth or for more complex maneuvers to avoid a collision or to reach a distant weapon launch position or a position offset from a moving-target track.
Next, this subsystem generates a time-oriented program to comply safely with the maneuver instructions in terms of roll (bank), pitch and yaw (course) angles, depth, and speed and transmits it to the automatic attitude-control subsystem. As the maneuver progresses, the interface provides the diving officer with displays of own-ship time- related track and track status in terms of course, depth, speed, submarine bank, and pitch attitudes. Other displays show force amplitudes and moments, plus visual and au-
dible warnings of the buildup of unsafe forces. When other ships and submarines are present in the maneuver area, their tracks are displayed to the extent data are available.
The Pressure-Sensing Subsystem. Of the many forces affecting the motions of a submerged submarine, the most significant destabilizing forces are caused by water flow about the hull. The pressure-sensing subsystem measures the locations and magnitudes of these external forces. To do this, pressure sensors are distributed on external hull areas determined through wind- and water-tunnel tests to be areas of concern. For example, the hull surface may be divided into sections instrumented to continuously monitor pressures. These pressure measurements are transmitted to the automatic attitude-control subsystem.
The Automatic Attitude-Control Subsystem. Currently, a diving officer has little knowledge of the forces acting on the ship. His knowledge is limited to the pressure on his feet, the inclination of his body, his own balance sensors, and instrument readings at the diving station showing dive angle, depth, bank, course, and speed. In essence, the diving officer observes the effects of forces, but not their magnitude, location, or direction. That information is inadequate and acquired too late to permit control at high speeds.
The automatic attitude-control subsystem, on the other hand, converts the pressure data it receives into forces acting on the various sections of the hull and measures all other forces acting on the submarine—including buoyancy, weight, propulsion thrust, and centrifugal forces resulting from rotational motions. It then converts these forces into moments about the center of mass moving through the water and resolves them into three orthogonal moments referenced to the submarine’s pitch, roll, and yaw axes as aligned in inertial space.
Finally, it activates control devices to cause the submarine to comply with the maneuver program. To perform these functions, the subsystem includes an inertial reference element (independent of the ship’s navigational system), a computer element, and a control actuation element. The inertial reference element provides the basis for determining the alignment of the ship’s axes and all forces and moments in terms of an inertial space coordinate reference system. The computer element receives, stores, and retrieves data and instructions, formulates displays for user interface, and performs all control-system calculations. Using these data, it calculates control forces and actions required to bring existing bank, pitch, and course angles into agreement with their corresponding programmed angles and delivers those instructions to the control actuation element. This element (which includes control signal generators, actuators for all control surfaces, tabs, fins and other control devices, and the control surfaces and devices themselves) implements the control action orders.
Improved Underwater Vision
Vastly improved underwater “vision” is required for our submarines to engage safely and successfully in operations such as those listed in Table 1. Toward that end, a strong effort must be made to develop sensor systems that can provide better information on the positions and movements of ships, submarines, and other objects near our submarines. Vigorous research and development must be conducted in fields other than sonar, including electrical, electromagnetic, and magnetic energy propagation through seawater, over wide frequency spectra. “Frequency windows” must be defined that promise longer range detection and better object definition. In addition, equipment promising more effective energy collection, signal amplification, processing, and display must be developed and applied to the underwater vision problem at an accelerated rate.
Conclusions
With existing technology, our attack and ASW submarines can be modified to allow them to change course and depth safely at high speeds with full rudder angles. In conjunction with the development of the automatic submarine-control system, the sail and all equipment enclosed within it must be redesigned to reduce destabilizing moments created by the height and area of the sail and its vortex distorting characteristics. At the same time, control surfaces should be redesigned and relocated to minimize the interaction of plane and hull flow patterns. Depending upon the outcome of these redesign efforts, it may be necessary to resize control surfaces and add fins, tabs, and other devices. It also appears advisable to provide independent movement for each control surface to obtain sufficient control forces and moments for safe maneuvering at all speeds with maximum rudder angles.
With the control problem solved, improved underwater vision—to the degree attainable—will determine our submarines’ ability to maneuver around submarines, ships, mines, harbor nets, and underwater obstacles and to take offensive and defensive actions more effectively.
POLARIS,” June 1991. Publication of Submarine Veterans of World War II, and Pacific Fleet Memorial Association letter of 3 July 1991.
Henry E. Payne III, “Submarine Maneuvering Instability,” The Submarine tic view, January 1988.
VAdm. Jon L. Boyes (USN [Ret]), “Flying the Albacore,” The Submarine He- view, April 1987.
‘^evil,e Shute. “So Disdained,” Cassel and Co., Ltd., 1928.
ichael K. Ryan, Chief Engineering Test Pilot, Aeronautics Division, Lockheed Missiles and Space Co., Inc.
Mr. Gruner, a 1935 graduate of the U.S. Naval Academy, served as e*' ecutive officer on board the Pike (SS-173), Sunfish (SS-281), and ApogO* (SS-308). During World War II, he commanded the Skate (SS-305); l»s ship was awarded the Navy Unit Commendation, and he was awarded the Navy Cross. After leaving active duty in 1947, he worked for Lock heed Missile and Space Company in a variety of programs, including the Polaris and Tomahawk missiles and the P-3 and S-3 ASW aircraft'
Dr. Payne is an aeronautic engineer who has worked on submarine flu^ dynamics, beginning at the Princeton-Forrestal Research Center in 1958-
Proceedings / July