The Submarine Stability Problem
As the numbers, types and capabilities of modem weapons ~and weapon systems have proliferated, the pace and tempo of modem warfare has increased. As this has occurred, the ability of human beings to manually control their weapon systems has decreased. A major problem exists in controlling the underwater trajectory, or “flight path”, of submarines during high speed maneuvers. This first became known in 1954 shortly after the experimental research submarine ALBACORE (AGSS 569) began operations. Officially described as a hydrodynamic test vehicle, ALBACORE had the hull design of a low drag “body of revolutio”, and a high capacity battery. Her submerged speed was somewhat in excess of thirty knots. With considerable foresight, the designers provided ALBACORE with a one-man control system with modes varying from manual to fully automatic. In concept, she was to be “flown” by the “pilot” like a high speed aircraft.
When operations began, ALBACORE performed splendidly while submerged on a steady course. However, it was discovered that her design permitted a roll/yaw force-coupling to take over when she was put into a high speed tum. In the SUBMARINE REVIEW of January 1988, Henry E. Payne ill discussed submarine instability during high-speed maneuvers. He drew the dramatic picture of a modem high-speed sub pilot in a melee situation. He “tries to tum too sharply at too high a speed” and finds himself “in a snap roll, hanging from his seat belt and with a loss of several hundred feet in depth at a markedly slowed speed.” In support of his article, Mr. Payne discussed the characteristics of water flow about the hull, sail and planes, and the generation of vortices of turbulent water. He stated that vortices result from ship motion through the water, and are the root cause for the inability of modem submarines to maneuver under water with the same sort of stability as airplanes in the atmosphere: The article included pictures of smoke-flow patterns made during wind tunnel tests of a tn5th scale model of SKIPJACK (SSN 585), another submarine with a “body of revolution” hull design. The purpose of the tests was to examine flow patterns about the hull during high speed maneuvers. It is evident from the pictures ‘that significant pressure differentials existed in various locations on the hull. Such pressures cause val}’ing forces to be exerted on the hull at different roll and yaw angles. Applying the basic law of physics that Force = Mass x Acceleration, it is clear that these forces would cause gyrations of the hull about all three axes, and also affect the submarine’s depth and speed. Mr. Payne states, “With a sail height over 60% of the hull diameter, the sail rolling·moment alone at 20 knots can be several MILLION foot pounds.” Forces of that magnitude cannot be neglected if stability is to be maintained.
In a later article (SUBMARINE REVIEW, January 1989), Mr. Payne confirmed the existence of ALBACORE’s instability problem. He stated that rumors had begun to surface about the “submariner’s J. C. maneuver” where “the crew nearly found itself hanging upside down from its seat belts after attempting a high-speed 300 rudder turn.” Not too much was understood at the time as to why the submarine could not be controlled during such turns. In any case, ALBACORE’s control system had difficulty in satisfactorily handling the instability problem as the ship was originally designed.
A number of alterations were made to ALBACORE over the next eighteen years. These included moving the sail mounted hydroplanes to the sides of the hull, substituting stern planes of a “X” configuration, substituting counter·rotating propellers, and adding dive brakes and a dorsal fin rudder. These changes did not completely solve the instability problem before ALBACORE was decommissioned in 1972. In addition, doubts were raised in some quarters as to the advisability of relying on submarine automated control systems.
When nuclear power was introduced for submarine propul· sian, the Navy placed great emphasis on submerged speed. Therefore, the low drag “body of revolution” hull form was applied to the design of attack submarines despite the instability and control problems encountered in ALBACORE. SKIPJACK (SSN 585) with that configuration was laid down in May 1956 and was followed by THRESHER (SSN 593) and STIJRGEON (SSN 637). In 1972, LOS ANGELES (SSN 688), the lead submarine of its class, was also laid down with a “body of revolution” hull form.
Investigations of stability and control problems continued. For example, Ken Hart (SUBMARINE REVIEW July 1988) reported on automatic control system experiments conducted with LOS ANGELES in early 1977. His comments were amplified by Alfred J. Giddings (SUBMARINE REVIEW January 1989). As operational experience with these submarines accumulated, a number of steps were taken to learn even more about the causes of the instability problem, as well as means for correcting it. These included studies, analyses and tests with various hull and control surface configurations. Recommended corrective actions included the addition of a fin keel to balance forces acting on the sail, better fairing of the sail into the hull, attachment of tab controls to the after end of the sail, placing “spoilers” and holes in/on outer hull surfaces to affect water flow, varying the stem plane configuration, and others. Alterations were made in some cases. For example, diving planes have been relocated from the sail to the sides of the hull, and a cruciform tail plane configuration has been used.
It appears that U.S. high speed submarines are not the only ones that have instability problems. In the April1988 issue of the SUBMARINE REVIEW, W. J. Rube described what appear to be steps taken in the design of 1YPHOON to minimize the formation of vortices at rudder, planes, sail and main deck areas. He also commented that in the design of VICfOR m, the “coke bottle” shape was used to improve laminar flow and that polymer stain was applied for changing boundary layer flow conditions.
Based on these and other articles on submarine design, control aberrations and steps taken to find solutions, it is clear that the problem of controlling submarines during high speed maneuvers has not been solved.
The Basic Diving Control Problem
Depth control of the World War II vintage, Fleet type, diesel-electric submarines was purely a manual operation. The diving officer received information required for depth control by viewing the depth gauge, dive/rise angle (bubble) indicators, plane and rudder angle indicators, pitometer log speed, and course changes shown on a gyro compass repeater. Based on this information he issued orders to the bow and stem planesmen, and to the trim and high pressure air manifold operators. At submerged speeds of less than nine knots, (almost all operations were performed at speeds of less than five knots), forces exerted by bow and stem planes and minor adjustments in water ballast were normally adequate for diving, and depth and trim control. Diving officers became fairly competent in maintaining depth control in calm seas after a few months of training. However, diving to two hundred feet or more to avoid air attacks, and depth control at radar and periscope depths in rough seas to track targets and launch torpedoes was another story. As a result, it was not unusual for the diving team to “lose the bubble.” A major cause was sluggish ship response to bow and stem plane forces at low submerged speeds. Control was worsened by the fact that the diving officer had no knowledge of the location and magnitude of forces acting on the outer hull. He knew only that dive angle and depth responded very slowly to orders given the diving team. To aggravate this situation, opportunities to train diving officers were limited during wartime because patrols were conducted largely on the surface. Since it was normal practice for the OOD to take the dive when necessary to submerge, and because none of the diving procedures were automated, each officer tended to conduct a dive differently. As a result, few became truly skilled diving officers, and few became familiar with the degree to which external water forces could cause loss of depth control. For example, when PIKE (SS 173) exceeded a dive angle of greater than SO, pressure on the forecastle deck caused the angle to increase further. The only recourse was to back full and blow bow buoyancy tank.
Depth control became an even more serious problem when ALBACORE and nuclear submarines became operational. High speeds coupled with the “body of revolution” hull design and a large sail area caused extremely great and variable water forces to act suddenly on the hull when large rudder angles were applied. Without knowledge of the magnitude and moments of these forces, diving officers could not know the actions to take to maintain dynamic stability, and the very serious problem descnbed earlier resulted. In order to cope with such forces, a means must be provided for assessing all the force-moments working on the hull.
Control Limitations Imposed By The Human Brain
Without that knowledge, the diving officer of a high speed submarine is worse off than the diving officer of a Fleet type submarine. Even if these forces were to be continually assessed by a suitable sensor system, the human brain lacks the rapid computational capability to continuously compute the resultant 3-dimensional moments of external and internal forces, integrate them into overall moments, select appropriate control devices, direct the application of those devices to counteract the destabilizing forces, and at the same time mentally program course, roll and depth changes. Simply put, the humaQ brain does not operate with the speed of light Consequently, it cannot do all of these jobs in time to maintain a stable attitude during a high speed maneuver.
The Approach To Full Maneuverability
Dynamic instability of vehicles in motion is caused by unbalanced forces. If a submarine is to be “flown by a pilot like a high speed aircraft, ” two things must be done. The inherent design features of the ship which produce upsetting moments must be altered so that their moments are decreased, and a control system must be developed which is able to automatically exert adequate and timely counter moments.
Reduction of upsetting moments is a job for bydrodynamicists and submarine design engineers. Their task is twofold; i.e. modify the bull design to reduce the upsetting moments, and design improved control devices capable of creating greater counter moments. Primary contnoutors to upsetting moments are the sail and various vortices formed in water flow patterns. Reduction of these moments can best be achieved by reducing the size of the sail, improving the fairing of the sail into the hull, and adopting other vortex minimizing features and devices. A compromise must be reached between sail size and requirements for access trunk, antennas, periscopes and piping. Great engineering ingenuity will be required to make a significant reduction of upsetting forces in this area. Development of control devices capable of exerting greater counter moments is a fairly straight-forward engineering task.
Development of a means for continuously measuring the pressure field acting on the external hull is a necessity. It is a task for hydrodynamicists and instrumentation engineers. The concept for sensing external pressures can be illustrated by imagining the external hull divided into approximately six to eight lateral sections. Each of these sections is divided into four subsections to represent top, bottom, port and starboard hull areas. Each subsection is instrumented with pressure sensors exceptthatthe sail is instrumented separately. Sensed pressures are continuously transmitted to the submarine automatic control system.
Finally, computer hardware and software, control system and human engineers must develop a computer system for automatic control. Based on maneuver instructions from the diving officer and data from the external pressure measuring system, the control system must actuate control devices to execute a stabilized maneuver in three dimensional space. In concept, the control system receives maneuver instructions from the diving officer and computes a program of “safe” roll, pitch and yaw angles necessary for making the maneuver. In a continuous process, the system senses external forces acting on the hull, computes their moments and combines them with the internal force moments working on the submarine. The system then computes the counter-forces required to stabilize the submarine as it maneuvers, and selects and actuates control devices to generate those counter-forces.
An automatic control system must perform the following functions simultaneously and continuously to provide this capability:
- Provide an interface with the diving officer to: (1) receive his maneuver instruction inputs, and (2) present him with status information on internal and external forces and moments, the ship’s attitude, and progress of the maneuver in terms of heading, heel and dive angles, depth and speed.
- Compute a program of roll, pitch and yaw angles for carrying out the desired turn, plus depth and speed changes,
- Sense water pressures acting on the hull in a manner to allow external forces and their moments to be calculated,
- Calculate and resolve all internal and external force-moments working on the ship into three orthogonal moments about the e.g., referenced to the true vertical, true north and the sea surface, and
- Actuate control devices to provide dynamic stability while carrying out the ordered maneuver.
One design concept for an automatic closed-loop control system is composed of three major subsystems; an Automatic Attitude Control Subsystem, a Sensor Subsystem, and an Automatic Maneuver Subsystem.
The Automatic Maneuver Subsystem
This subsystem contains a Man/Machine Interface Element to provide the diving officer with a means for defining the desired maneuver. The diving officer enters maneuver instructions, for example, a 500 yard tactical diameter tum at 25 knots at constant depth, or a tum with 25° right rudder and increase in depth to 450 feet. The interface also provides the diving officer with data on submarine attitude and maneuver status.
A Maneuver Programmer Element for generating a maneuver program of time related roll, pitch and yaw angles is also a part of the Automatic Maneuver Subsystem. It transmits this program to the Automatic Attitude Control Subsystem.
The Sensor Subsystem
As previously described, this subsystem senses the sea pressures acting on the external hull and transmits that information to the Automatic Attitude Control Subsystem.
The Automatic Attitude Control Subsystem
The primary function of this subsystem is to automatically operate attitude control devices to maintain dynamic stability while carrying out the desired maneuver. It contains three elements; an Inertial Reference Element, a Computer Element and a Control Actuation Element. The Inertial Reference Element provides an independent orthogonal reference system for measuring roll, pitch and yaw angles and their rates of change. The Computer Element performs all required calculation, data handling, storage, retrieval and display functions for the entire system. It provides inputs to the Control Actuation Element to actuate all attitude control devices including rudder(s) planes, fins, tabs, and spoilers.
Force moments experienced during maneuvers are monitored by the Attitude Control Subsystem to ensure that they do not exceed upsetting force limits previously established during system development testing. Corrective attitude control device actions are automatically applied by this closed-loop control system.
- Future development of attack and ASW submarines will require safety of maneuvers at high speeds. A major effort to solve the dynamic control problem would permit a shift of emphasis from pure high speed to controllability at speed,
- Pressures generated by water Dow along the hull cause sudden and variable high magnitude forces to develop as the flow patterns change during maneuvers. Knowledge of the locations and magnitudes of these forces is essential for the development of a control system that will allow quick tum maneuvers at high speeds.
- Submarine design must evolve further toward the true submersible. The fixed height and area of the sail must be reduced to lessen destabilizing moments. In addition, the distortion of Dow patterns experienced during maneuvers must be minimized. Modification of the “body of revolution” hull form may be made if it eases the control problem by increasing stability.
- The brain does not permit human control of submarines during high speed maneuvers due to the number of complex thought processes involved. Therefore, a fully automatic, highly reliable, attitude and maneuver control system must be developed to program maneuvers ordered by a human operator. It must be able to generate force moments capable of counteracting the upsetting moments created during high speed maneuvers.
- To accomplish this, a Sensor Subsystem must be developed to provide external pressure inputs for calculating external force moments,
- The diving officer must be provided with a control system interface for entering maneuver instructions. The interface must also provide the diving officer with output data on submarine attitude and maneuver status, including visible and audible warnings of the build up of dangerous upsetting forces,
- Stability during high speed maneuvers must be such that the crew has freedom of movement, and that loose materials and equipment are not dislodged from their normal resting and stowage spaces. A fin keel, if added, should not eliminate appropriate banking during high speed turns,
- An automatic control system will permit the standardization of submerged maneuver tactics, thereby reducing the time required to train skilled diving officers,
- There is no alternative to an automatic control system despite a reluctance to rely upon one. A very high degree of reliability can be built into automatic systems by such means as use of high reliability components, functional redundancy, incorporation of computer error detection, extensive development testing and thorough quality control processing during system development and production.
THE SUBMARINE REVIEW
THE SUBMARINE REVIEW is a quarterly publication of the Naval Submarine League. It is a forum for discussion of submarine matters. Not only are the ideas of its members to be reflected in the REVIEW, but those of others as well, who are interested in submarines and submarining.
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