Small Subs Provide Big Payoffs for Submarine Stealth

by CDR David M. Fox, USN

Intermediate Scale Measuring System, caption follows
Intermediate Scale Measuring System (ISMS). The ISMS consists of a 1,000-foot diameter submerged, horizontal circular hydrophone array, with an associated submerged sound projector array. The data recording and processing equipment 14 miles away is connected to the range by fiber-optic cables.

Have you ever looked at your submarine’s propeller, perhaps during your last dry docking, and wondered, “Why is it shaped like that?” Or maybe you’ve wondered just how someone decided on the shape of the bow, or the sail, or other external parts of the hull.

The answer, of course, is that the configuration of these components was chosen specifically to allow your ship to go fast and employ its sonar effectively while remaining as stealthy as possible. Making submarines quiet, efficient, and effective is our main mission at the Navy’s Acoustic Research Detachment (ARD) at Bayview, Idaho. As an integral part of the Navy’s Research, Development, Test and Evaluation (RDT&E) community – namely, the Carderock Division, Naval Surface Warfare Center under the Naval Sea Systems Command – we execute this mission by operating large-scale submarine models on three ranges in Lake Pend Oreille, Idaho. A fourth range is used to pull submarine towed arrays behind a 60-foot surface vessel to evaluate array self noise using recording equipment on the towing vessel.

Why is the Navy in North Idaho of all places, 350 miles from the nearest ocean? Mostly, to take advantage of the conditions in Lake Pend Oreille. The largest lake in Idaho and the fifth deepest in the United States, Pend Oreille offers a virtually ideal venue for acoustic testing. First, it is deeper than 1,000 feet over an area exceeding 26 square miles, and its flat mud bottom minimizes sound reflection. A low level of particulates in the water results in minimal reverberation and scattering, and its ambient sound level is less than the ocean at Sea State Zero more than one fourth of the time. Moreover, the lake’s water temperature remains at 39.5 degrees Fahrenheit below 300 feet all year, maximizing the repeatability of test results over time. Finally, at eight miles long by three to six miles wide, the testing volume is more than adequate.

Photo of submarine model Dolly Varden. (caption follows)

Group photo atop LSV-2 (caption follows)

The submarine model Dolly Varden is hauled down to the bottom of Lake Pend Oreille in preparation for a buoyancy-propelled return to the surface during which flow-noise measurements will be recorded. Pictured atop LSV-2 are (left to right) Commander, Carderock Division/NSWC; OIC, Acoustic Research Detachment;  the OPNAV Submarine Directorate’s civilian Science and Technology Director; and VADM John Grossenbacher, COMSUBLANT.

While it is clear why the Navy takes advantage of the ideal conditions at Lake Pend Oreille, a more significant question might be why the Navy needs to use large-scale models to test submarine technology at all? The simple answer is cost. We can do model testing here at a fraction of the expense of using full-scale, operational submarines out in the fleet, while the large scale of our models (1/5 size and up) yields performance characteristics in the lake that closely match those of full-scale submarines at sea. Since this quality of data cannot be obtained in small-scale model testing, our large models and large model operating ranges are vital to validating submarine stealth technology. ARD plays a key role in developing submarine stealth by serving as one element of a sequential process in which the RDT&E community validates new technology. This approach – shown in the accompanying sidebar – has been pursued by NAVSEA and the Carderock Division for more than forty years, resulting in the quietest and most capable Submarine Force ever.

Submarine Model Range Facilities at ARD

We have several separate ranges in the lake to test various aspects of submarine sound quieting. The Buoyant Vehicle Test Range (BVTR) measures the noise produced by hydrodynamic flow over the bow and forward section of a submarine, while not masking it with the sound of propulsion or other onboard machinery. By using buoyancy to propel the model upward – like a cork – we avoid having to equip it with a propulsion system.

Operation of the BVTR is very simple. We use a shore-based winch to tow a buoyant submarine model (typically 1/5 the size of an SSN) to the bottom of the lake, stern first. A barge moored above and to the side of the range is used to control test operations, and hydrophones and accelerometers onboard the model are used to measure flow noise and operational data. After the model is hauled to the bottom and its motion settles out, we trip a release, and 15,000 to 25,000 pounds of buoyancy accelerate the model to the surface. As it nears terminal velocity, we have a window of four to six seconds to record the resulting flow noise. Near the end of the run, the stern planes are automatically shifted to dive, forcing the model to pitch over and ascend gently to the surface.

The BVTR has been used to determine the optimal shape, material, coating, mounting scheme, and overall design of the bow dome on every class of nuclear submarine since the USS Sturgeon (SSN-637) class. Modern sonars are much more efficient because of these experiments, since flow noise and its interference as background noise have been significantly reduced.

We use the Intermediate Scale Measuring System (ISMS) to test static (non- mobile) models. The newest of our ranges, ISMS consists of a 1,000-foot diameter submerged, horizontal circular hydrophone array, with an associated submerged sound projector array. We use a shore-based winch to haul the model to the center of the array (at a depth of about 500 feet), where it remains suspended for the duration of the test. The model is attached to a handling platform at the end of the haul-down cable, and operators can position it to present any desired aspect to the projector array. The ISMS can be used to measure the target strength of a submarine hull (that is, how effectively it re-radiates sound from a source not on the model) and how much sound is radiated into the water from a piece of machinery operating onboard. The data recording and processing equipment is on shore in Bayview, and is connected to the range 14 miles away by fiber-optic cables.

Finally, the Large Scale Vehicle (LSV) Range uses large, un-manned, autonomous submarine models to evaluate propeller noise, structural acoustics (overall hull structural vibration), wake production, and maneuvering and powering. In operation since 1987, the range itself consists of three distinct parts:

• The Acoustic Tracking and Communications System (ATACS), which consists of six hydrophones      spread over the bottom of the lake for tracking and controlling the model
• The Radiated Noise Data Acquisition and Analysis System (RNDAAS), which consists of two vertical line hydrophone arrays that listen to the model as it drives by
• The Onboard Data Acquisition System (ODAS), which uses sensors, signal processing, and recording equipment on the model itself to record its self-noise signature and operating parameters

A specially configured Radiated Noise Barge (RNB) contains signal processing, operator control, and data recording equipment. Each time a test is conducted, the self-propelled RNB is driven to the range, where it is moored to a float and electronically connected to the ATACS and RNDAAS arrays. Two sound-isolated diesel generators on the RNB power the onboard instrumentation and the arrays once it is moored at the range. The ODAS system is self-contained on the model. To conserve battery power onboard, the model is towed to the range using a specially configured tender vessel.

Photo of the Kokanee (LSV-1). (caption follows) Photo of the Cutthroat (LSV-2). (caption follows)
Cutthroat (LSV-2), shown here during initial launching, is a 0.294-scale model of the USS Virginia (SSN-774) that will operate as an unmanned, autonomous submarine test vehicle for evaluating new technologies. Kokanee (LSV-1), a self-propelled, quarter-scale model of the USS Seawolf, vents her ballast tanks while cruising on the surface of Lake Pend Oreille, Idaho, during a test at the Acoustic Research Detachment.
Successive steps for submarine technology insertion 
1 Concept development for    potential technology improvements by RDT&E community 
2 Analytical calculations, numerical models, and/or computer simulations
3 Small-scale model testing at the David Taylor Model Basin at the Carderock Division or the Large Cavitation Channel at Memphis, Tenn.
4 Large-scale model testing at the Acoustic Research Detachment
5 Full-scale testing on an operational SSN at the Southeast Alaska Test Facility (SEAFAC), or using USNS Hayes (AG-195) at an Atlantic Fleet open-ocean range.
6 Across-the-board insertion of demonstrated technologies into the Submarine Force

Large Scale Vehicles
As one might expect, the two LSV models operated here are our largest and most complex vehicles. Essentially, they are unmanned, deep-diving submarines that operate under computer control. The LSVs are monitored, but not controlled, by the operators in the RNB and the tender that tows them, except during transit and in emergency situations. The first LSV, Kokanee (LSV-1), is a quarter-scale model of USS Seawolf (SSN-21) and is 90 feet long, 10 feet in diameter, and displaces 155 long tons. Kokanee looks like an SSN on the outside, but inside the forward half of the pressure hull, it contains 1,524 battery cells – about 25 tons worth – to provide power for the electrical propulsion motor (1,440 cells) and instrumentation (84 cells). The after half of the pressure hull contains the instrumentation, including guidance, navigation and control equipment, and the ODAS signal processors and recording equipment. The after compartment also contains a 3,000 horsepower electric propulsion motor, shaft bearings, and the propeller shaft itself. Kokanee’s external stern configuration is similar to that of any SSN. Because they significantly influence the acoustic signature of the model, the pressure hull and external structures simulate a Seawolf-class submarine very closely.

Components inside the pressure hull have less effect on the acoustic signature, so we have substantial freedom there to deviate from the full-scale Seawolf configuration. (Obviously, we don’t need a control room, crew’s mess, or berthing spaces in an unmanned model.) Kokanee’s stern control surfaces operate similarly to those on an SSN, except that they are operated by computer rather than Sailors. Kokanee was used to evaluate propulsor configurations for the Seawolf class, and was a key contributor to achieving the unprecedented stealth of those ships at high speed. Now, the model is also being used to evaluate propulsor and other technologies for the USS Virginia (SSN-774) class.

Our newest model, Cutthroat (LSV-2), is the largest unmanned operational submarine in the world. A 0.294-scale model of the pre-commissioning USS Virginia, it is 111 feet long, 10 feet in diameter, and will displace 205 long tons when delivered. Currently still under the custody of the shipbuilder, a joint team from Newport News Shipbuilding and General Dynamics Electric Boat, Cutthroat will be delivered to the Navy and become operational in the summer of 2001. Construction will be completed at Bayview.

Cutthroat is similar to Kokanee, but more advanced. Enhancements include a larger overall scale – 29 percent, vice 25 percent for Kokanee – which will improve the fidelity of test data to full-scale results. Cutthroat is designed to be more modular than Kokanee, so that major modifications, including radical hull changes, can be made with less impact to other systems onboard the vessel. Another advantage is an increase in ODAS capability. The Cutthroat ODAS will have twice as many data channels recorded as Kokanee at delivery – 512, vice 256 – and this is upgradable to 1,536 recorded channels. The Cutthroat ODAS converts the data from analog to digital form and processes the data digitally. In Cutthroat, data recording can be configured electronically under computer control, whereas Kokanee uses a patch panel. Cutthroat is equipped with a 3,000 horsepower permanent-magnet, radial-gap electric propulsion motor, provided to the Navy under a unique partnership agreement with General Dynamics Electric Boat, the owner of the technology. This motor is easily upgradable to 6,000 horsepower. Other order-of-magnitude improvements were engineered into the guidance, navigation, control, and propulsion systems, including the addition of torque sensors and other sensors of mechanical data for better reconstruction of the scenario.

Payoff for the Navy
The addition of Cutthroat to the ARD model fleet is expected to provide improvements to the Virginia class in the areas of stealth, hydrodynamics, hydroacoustics, and propulsor design, thus supporting technology insertion into current and future SSNs. Two promising areas for future research include submarine maneuverability and electric propulsion development. Cutthroat can be modified extensively – but inexpensively – to determine optimum sail shapes and other parameters for maneuverability, and we can evaluate operating procedures – for example, maximum permissible rudder angles at flank speed – without risking damage to an operational SSN or harm to Sailors. 

We can also use Cutthroat or Kokanee to test SSN electric-drive ideas and components at much less cost than modifying a full-scale SSN. If required, we could completely replace either model’s propulsion system with a completely different version, and evaluate designs before they get into the fleet. The cost to do that to an operational SSN, in dollars and time, would be prohibitive. 
The nation can no longer afford the kind of full-scale submarine prototyping that was pursued in the 1950s and 1960s and which led to the USS Tullibee (SSN-597), USS Jack (SSN-605), and USS Glenard P. Lipscomb (SSN-685). Large-scale model testing provides accurate results at a modest cost. And the ARD represents a low-cost, high-payoff test facility that will help keep our Submarine Force number one in the world for the next 100 years and beyond.

CDR Fox is the Officer in Charge of the Acoustic Research Detachment.