May 2002 Excerpt
Magnetic Silencingthe Legacy Continues
By Leslie Spaulding
month, Carderock Division will dedicate a Magnetic Fields Laboratory at its
West Bethesda site. This facility replaces the Navys previous magnetic
silencing labs, which were closed by Base Realignment and Closure (BRAC) actions
at Annapolis and White Oak.
The scientists and engineers who work in this facility have been co-located for some time. The 31 people from White Oak were reassigned and consolidated in West Bethesda in January 1997. The 15 Annapolitans transitioned to this site in September 2000. Six additional people have been hired since. However, the working relationship between the two former labs is an old one, with personnel collaborating on many tests throughout the years.
Magnetic silencing first became important during World War II, when it was discovered that the Germans posed an increased mine threat. The earths magnetic field corresponds roughly to that of a very large magnet near its center, orientated north-south. Its magnitude at the earths surface is roughly half a gauss or 0.4 ampere turns per centimeter. Any mass of iron stressed in the earths magnetic field becomes a magnet. A ship, stressed by riveting, etc., during construction attains a sizable magnetization which then changes gradually, depending on its heading and location, when the ship is under way. Before WWII the Germans had developed sensors capable of detecting a ships field and detonating a charge close enough to the hull to cause major damage. This development prompted the United Kingdom and the United States to develop countermeasures for mines triggered by magnetic fields. And a new field of technology was opened.
The post-WWII years brought dramatic changes to the Engineering Experiment Station (EES) in Annapolis as a period of modernization was underway. It was during this time that the Ralph K. James Magnetic Fields Laboratory was opened. The facilitys principal mission was to aid in the research and development of magnetic countermeasure devices and low magnetic field shipboard equipment, as well as to provide a means of rapid testing and analyzing of prototype equipment under actual running conditions and prior to permanent installation in the Fleet.
This facility used high-tech instrumentation for its day. It was designed to measure the vertical component of magnetic field intensity below the items which were being studied. The output of the system was digitized and recorded on punched paper tape in the same format used by the digital computer which was present at the EES. Extensive use was made of the computer to perform different analyses on the measured field data for each experiment.
Over the years, the Magnetic Fields Laboratory at Annapolis changed with the changing needs of the Navy. When it was transitioned by the 1995 BRAC actions, the lab consisted of two main buildings: Control and Acquisition and Test and Measurement. Additionally, there were the Sensor Development and Electrolytic Tank/Staging buildings. Branch personnel and light laboratories were located in portions of two other buildings. The Test and Measurement Building was four stories tall and built into the side of a hill, providing ground level entrances on both the top and bottom floors. It was designed to test full-sized ship equipment, such as motors and generators, while they operated under load.
To meet the stringent permeability requirements, the building was designed around the use of a heavy concrete base support reinforced with aluminum rods and large 40-foot laminated wooden beams located on each floor. Increased flexibility, as well as conformance to the low permeability requirements, were provided through the use of lightweight aluminum gratings on each of the probe levels. The walls were constructed of cinder block reinforced with aluminum mesh. The roof was made of large cuts of composition material laid across wooden beams, which ran the full width of the building. The same permeability restriction was placed upon any materials used within the 288-foot nonmagnetic radius. To insure that this area was magnetically clean, a series of borings were taken, and a magnetic survey of the area was made with a conventional land mine locator.
Located in this building was the Large Scale Model Facility (LSMF), which was used to measure scaled physical models of submarines and ships. In the LSMF, submarine and surface ship models were suspended from a trolley on a 40-foot monorail. The models were moved along a north-south line, passing over a bar of triaxial sensors and through rings of triaxial magnetic sensors. This allowed for the simulation of full-scale measurement ranges, as well as a more complete spatial coverage of the source than was available at other full-scale measurement ranges. An Earths field reference sensor, located about 300 feet from the building, was used to track and compensate for changes in the background fields. The control and data acquisition system was located in a second building. These buildings were connected by an underground walk-through tunnel used to run all the cables.
The Magnetic Fields Laboratory at Annapolis witnessed many advancements in magnetic silencing over the years. Most notably were the early investigations into the permanent magnetic signature of engines, and later the qualification of each piece of large DC equipment for minesweepers oceangoing (MSO), minehunter coastal (MHC), and mine countermeasure (MCM) ships. Other notable work involved the investigations of designs for Trident, Seawolf, and Virginia Classes.
Mines and countermine research and development were the initial products of the White Oak Laboratory with the technical spin-offs into torpedoes and products in the special warfare areas and underwater detection and fire control. It also fueled the initial research in magnetic and other materials, batteries, and degaussing. To test the products, innovative equipments, facilities, and techniques were designed and implemented. One of the first buildings constructed at the White Oak site was the magnetic test building that was used to develop a counter measure to the German magnetic influence mine that was used in World War II.
When the facilities were in the planning stages, the White Oak site was carefully surveyed by magnetometers to assure its suitability for magnetic research and to locate the area where the earths field was most stable. For the magnetic silencing buildings, the construction contracts required that no magnetic materials would be included.
In its prime, the Magnetic Silencing Complex in White Oak consisted of the Magnetic Ship Model Lab, the Magnetic Structures Test Facility and the Long Coil Test Facility, which were all contained within a designated magnetic quiet zone. The Magnetic Ship Models Laboratory had fully automated data acquisition and control and was used to test physical magnetic models up to 12 long. This lab was capable of producing uniform magnetic fields and generating magnetic fields to replicate the magnetic environment for anywhere on Earth and space using a magnetic field feedback stabilization system.
The Long Coil Facility was capable of handling items 32 long and 2 in diameter. It could produce a very uniform field magnetic center in its coil system and had a magnetic signature ship simulator. The Magnetic Structures Test Facility also had an automated data acquisition and controls system. It could handle items 9.5 wide, 13 long, and 10 high and produce very uniform magnetic fields. The facility was used to generate magnetic fields to replicate the magnetic environment for anywhere on Earth and space and had a multiple sensor array installed to map field signatures of items under test. It was also capable of performing eddy current measurements.
White Oak, too, contributed greatly to advancement in magnetic silencing. Its greatest accomplishments include:
- Development of the
closed loop degaussing concept for minesweepers.
- Specification, development,
procurement, and installation of the Electromagnetic Roll Facility at NAVSTA
- Specification, procurement,
and in-service support for fixed magnetic ranges and for the Forward Area
and Acoustic Range.
- Development of the
magnetic and electric sensors for mines and range uses.
- Development, test, and evaluation of a low frequency electromagnetic signature reduction countermeasure.
Now, the combined talents of these two former labs will continue the legacy. NSWCCD scientists and engineers have been providing the skills, knowledge, technical leadership and specialized labs and facilities to the Navy in this area for more than 40 years.
The Magnetic Fields Laboratory, which will be dedicated next month, is a unique facility which can simulate ambient magnetic conditions that a ship will encounter at any location on Earth. It is the only U.S. facility that can provide accurate measurements of magnetic field signature beneath full-size operating electrical machinery and magnetic scaled physical models. The capabilities include electric scaled physical models, electrolytic measurements tanks, and simulation models. The facility will be used to define the signature mechanisms, as well as to develop countermeasures for the electric fields emanating from surface combatants.
The lot on which the new facility stands was first checked with magnetometers to ensure that the ground was magnetically clean. Geology, traffic, heavy industry can all impact magnetism. The site had to be quiet with no background noise. To ensure a magnetically clean site, a 300-foot magnetic radius was defined. The new facility was constructed entirely of non-magnetic material. Division personnel sampled everything that went into the building, including the concrete and the fixtures. Magnetic searches were conducted to ensure the building was clean.
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