by CDR David M. Fox, USN
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.
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.
. The Acoustic Tracking and
Communications System (ATACS), which consists of six hydrophones
spread over the bottom of the lake for tracking and controlling the
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
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
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
CDR Fox is the Officer in Charge of the Acoustic Research Detachment.
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