Captain Jim Patton is a retired submarine officer who is an active.consultant in submarine matters to government and industry. He commanded USS PARGO (SSN 650).
Background
In a distant galaxy a long, long time ago, a 637-class submarine was given an unusual one-time tasking. It was to spend what turned out to be about three weeks operating throughout about one square mile of ocean, at 10 or so feet above the bottom, in about 600 feet of water. A diurnal tidal shift of more than a knot existed in the area and the tasking would require frequent periods of zero speed with respect to the bottom or even slight stern way. Precise control of the pitch and yaw axes and positioning of the ship to within a few feet of areas of interest were also necessary. Fortunately, acoustic quietness was not an overriding consideration.
The tactical solution to this unusual problem was to rig out the Secondary Propulsion Motor (SPM), a single speed high RPM electric motor, at 000 degrees relative, and run it continuously. Safety procedures required that the SPM only be rigged out or retracted while above 200 feet, but once out and locked, the ship could operate down to test depth. The same procedures, however, required that it be trained in azimuth only when de-energized. Since frequent adjustment of ship’s heading was anticipated, this was an unacceptable alternative to control the yaw axis.
With the SPM running, a backing bell was put on the Main Engines. When about 30 turns astern were applied, their thrust just counteracted the SPM, and the ship was essentially Dead in the Water (DIW). At 20-25 turns astern, the ship acquired slight headway, and at 35-40 turns, stern way. At all times, however, there existed significant wash over the stem planes and rudder which permitted a quite nice control of both pitch and yaw. Although pitch control combined with some head or stern way provided a degree of vernier depth control, the greater burden in this dimension fell upon the Diving Officer of the Watch. Great skill was required in keeping on top of fore.aft and overall trim, particularly during periods of watch change or blowing of sanitaries and steam generators.
In any case, all concerned were impressed with the fact that there are some things that a large submarine might be asked to do that would require an unusually precise degree of slow-speed controllability, an almost NR-1 level of finesse in the x, y and z planes (and their first derivatives).
Discussion
Slow-speed controllability in the vertical plane is not a new phenomenon for large U.S. nuclear submarines. The entire feasibility of the submerged launch of a ballistic missile, essentially an airframe which was unforgiving of shear forces across the deck from any headway at all, required that the SSBN be DIW. When this evolution was further complicated by the rapid and violent changes in displacement as many missiles left and launch tubes back-flooded, incredible design innovation had to be exercised in the hovering and ballasting systems, now taken so much for granted, to nicely control the z axis. It’s interesting to note that the control system for the hovering system employed not only depth error, the difference between ordered and actual depth, but also its first derivative, vertical velocity, the second derivative, vertical acceleration, and even the third derivative, a parameter called quickening, the time-rate of change of acceleration. There was something decidedly non-intuitive about having the boat below ordered depth and sinking, yet having the system (correctly as it turned out) actually flooding water into the hovering tanks. Precise control of the x and y-axes and heading, however, were of little concern during the launch evolution. After all, if you ‘re not going anywhere, it doesn’t matter in which direction you’re pointed.
From about 1999 to 2001, the Defense Advanced Research Projects Agency (DARPA) ran a Submarine Payload and Sensors program in which consortia of commercial and government agencies were challenged to envision what U.S. nuclear attack submarines would evolve into within a decade or two. During this period, the two competitive consortia were selected and both converged on amazingly similar visions. Not only would submarines increase their payload by an order of magnitude or more, and have far greater access to the e11viro11ment but modularity would also permit more rapidly converting to an increasingly broad range of missions. Many of these missions would require the same degree of near-bottom slow-speed maneuverability discussed above, but not necessarily with the luxury of not having to be concerned with acoustic covertness. In fact, the ability to conduct covert in-theater replenishment/change of payload or stores was not ruled out as a value-added characteristic.
It is one thing to envision the great things to come, but quite another to deal with the things that are, and the transitions that must occur as today’s reality becomes that of the future. There are now two SEA WOLFS, JIMMY CARTER with its greater access to the environment is well along in construction, the first of four SSBN to SSGN conversions (with an order of magnitude greater payload) will show up during the last half of this decade, and VlRGINIAs will begin trickling into the inventory in a few years. However, as a point in fact, the Submarine Force will consist primarily of 688 class SSNs until officers just now reporting to their first submarine become Commanding Officers. If there is a frequent need for good slow-speed controllability between now and then, how will it be provided?
Perhaps the SPM gimmick described above also works on a 688, but the author is unaware of its having been tried. Besides, even in third world littoral waters, the noise levels associated with a 688 SPM might easily be unacceptable. TRIDENT-class SSBNs have two (to achieve enough thrust to propel the much larger hull) non-trainable SPMs, but it is not as yet clear as to whether those four to be converted to SSGNs will have any alterations that would better support slow-speed controllability in shallow littoral waters. The SEA WOLF’s have an altogether different secondary propulsion device -the Secondary Propulsion Unit (SPU). The SPU is still single speed, but this shrouded, rim-driven induction motor propulsor incorporporates a much higher number of electrical poles, and therefore offers intrinsically quieter performance at lower RPMs. Also, the SPU does not penetrate the pressure hull mechanically, as the SPM does, but only electrically. (Although not unsafe per se, it was somewhat unsettling while operating near the bottom at slow speeds in deep water to dwell too much on that I 0 or so inch cylinder that penetrated the pressure hull to support the SPM). SPUs designed for JIMMY CARTER are not only even quieter than those on SEA WOLF and CONNECTICUT through improved propulsor design, but are also capable of variable speed. VIRGINIAs will receive SPUs that are variable speed, but are not the quieter JIMMY CARTER variant.
Employment
If indeed the bulk of the U.S. Submarine Force had quiet, slow-speed controllability as a general characteristic, how would this enable new missions or make existing missions safer or more effective? As starters, those who would benefit greatly are the SEALs and other Special Forces who are more routinely operating on and off these ships. It is rumored that the 688-class, as it exists today, has a terrible reputation among SEALs for being very difficult to get on and off, due to very poor submerged controllability at speeds much below 4-5 knots. Since anything over one knot of relative velocity represents a challenge to even as accomplished a swimmer as a SEAL, any improvements in this regard would pay immediate dividends. Launching and landing such as the Swimmer Delivery Vehicle (SDV) and the Advanced SEAL Delivery System (ASDS) would also be significantly easier and safer.
It has become apparent, not only through studies and the DARPA Payloads and Sensors program, but also through Fleet experiments such as last year’s GIANT SHADOW exercise where a large Autonomous Underwater Vehicle (AUV), Penn State University’s Sea Horse was launched from an SSBN’s missile tube, that great benefits can be gained in the near term through exploitation of AUVs and their cousins, tethered Remotely Operated Vehicles (ROVs). The Navy will soon deploy the Long term Mine Reconnaissance System (LMRS) -a torpedo-sized vehicle which will conduct an independent search for moored and bottom sea mines and return to its launching ship for recovery. It is patently intuitive that the launch, recovery, and even mission profile of such vehicles are significantly enhanced, if not basically enabled, through quiet slow-speed controllability. In addition, much is read about submarines aspiring towards the ability to implant or retrieve devices such as temporary bottom-mounted sensor arrays, evolutions that imply a fine degree of positional control at very slow speeds.
Conclusions
In general, the Cold War saw very few requirements for the vast majority of U.S. attack submarines to possess or exercise precise 3-axis slow speed controllability. Many indications imply that may no longer be the case. As the entering sea story implies, there may have been cases where ad hoc solutions had to be invented in response to emergent problems, but these solutions probably do not satisfy the existing and emergent requirements in all respects as regards precision and/or acoustic stealth. A serious shortcoming in the near and mid term is that the mainstay of the Submarine Force for many years, the LOS ANGELES class SSN, has a reputation of having especially poor maneuvering characteristics at slow speeds. It would appear almost a certainty that, sooner or later, some sort of backfit to this class will be operationally desirable to provide it with the near-zero speed controllability that VIRGINIAs are likely to have. Similarly, with as much hype that the forthcoming SSGNs are rightfully receiving as regards a quantum jump in littoral submarine operations, attention to their ultra-slow or zero speed capabilities is warranted.
