Acoustic stealth of Indian submarines
– Raghu Varier
Stealth aspects of a submarine are very crucial to ensure secrecy of operations. The most important component is the under-water radiated noise. Sound travels faster in water than in air. So reducing the under-water radiated noise is critical. Main sources of submarine noise include:
- Hydrodynamic flow noise
- Propeller noise
- Machinery noise
The hydrodynamic flow noise depends on hull shape and flow velocity. Propeller noise is governed by its shape and speed. Both these noise are in broadband in nature. The machinery noise however can produce discrete frequency peaks. Using a narrow band filter these peaks and its harmonics can be detected. These spikes make SONAR platforms easy to detect and classify the submarine based on its characteristic noise spectrum. Usually SONAR platforms employ large arrays to improve antenna gain and Low Frequency Analysis and Ranging (LOFAR) for narrow band analysis which makes use of processing gain.
World history of acoustic stealth
The first record of developing acoustic stealth features can be found during the cold war era. In 1956 at Nobska, the US Navy brainstormed to develop the first external hull sound-absorbing coatings. This technology was incorporated in the Thresher class SSN commissioned in 1961. However in the late 1970s, the Soviet Navy realized the acoustic advantages on US submarines that were developed 1960s onwards. The Soviet through sustained efforts significantly reduced the machinery noise levels and the high frequency narrow band tonals. The three broad means of reducing machinery generated noise are:
- Quieter equipment
- Resilient isolation mounts
- Anechoic Tiles to attenuate radiated noise
The first two methods are employed over multiple engineering branches. These will be covered in the next article. Here we will focus on hull coatings and its subsequent development.
Use of elastomeric/anechoic tiles
The relatively latest advancement in reducing submarine noise is the use of elastomeric tiles on the submarine hull. These tiles absorb the energy from acoustic signals and therefore attenuate them. The UK’s astute class SSN has about 39000 tiles fixed on the outer hull.
Anechoic Tiles in Indian submarines for acoustic stealth
In India NMRL developed acoustic rubber tiles for the Indian Navy submarines. They are manufactured by Andhra Polymers based in Hyderabad. The tiles are of 20 mm and 51 mm thickness. These rubber tiles are multi-layered and have cavities / void each targeting a specific sound frequency range at different sea depths. They have an outer protective layer and an inner sound absorbing layer.
Following variables govern the acoustic performance of these tiles:
- Distribution of resonance cavities
- Dynamic properties of the rubber matrix
One key feature is that they have tapered cavities of varying dimensions. These tiles are also designed to withstand variable hydrostatic pressure up to 4 MPa.
These tiles are used in the SONAR dome of submarines to isolate the SONAR transducers from structural vibrations and acoustic noise. These tiles are multi-layered with perforated layers in the core. They are attached to each other by a special adhesive. NMRL developed a Vibration Damping Elastomeric Coating (VDC) and is being used in the Indian Navy submarines. Using high performance adhesives, putty and sealants, the acoustic tiles are secured on the hull surface.
The gaps between the metal to metal joints are filled with epoxy-based putty and NMR88C adhesive, thereby levelling it off. This putty has excellent metal to rubber adhesion due to the high performance adhesive NMR51K with a service life exceeding 20 years. Its primary usage is adhering the acoustic rubber tiles onto the underwater surface of marine vessels. Following this the gaps are then sealed using polyurethane sealant to prevent sea water seepage from the underwater surface of vessels.
The speciality of these sealants is its ability to respond to reversibly dynamic conditions at various depths and vessel speeds. This material is used as an encapsulant for transducer assemblies of SONAR. Apart from that, it is also used for acoustic transparent and water barrier coating for composite SONAR domes.
Low Frequency Sound isolation
Acoustic rubber title Mk3 is one such product from NMRL, whose prototype has been fabricated, tested and approved by the Indian Navy for induction. In addition to this NMRL developed damping mastic which is based on nanofiller reinforced epoxy resin. This is used by the Indian Navy for attenuation of structural vibrations. It is applied as a single layer of coating through free layer damping treatment. They provide excellent damping in the low and medium range frequency when applied to machinery and engine room foundations.
High frequency range stealth
NMRL has developed a ferrite base RAM to reduce RCS in the X-band range (8-12 GHz) i.e the high frequency range. It has been incorporated in an epoxy resin and rubbers. This enables it to be made in the form of paints, sheets and structures. They offer more than 10dB attenuation in the X-band. The Indian Navy has used this for snort mast and periscope of their submarines.
Usage of these tiles on Indian Navy submarines
The Kilo class submarines of the Indian Navy were the first ones with fully covered anechoic tiles and it has the reputation of being silent in combat. These tiles are also seen on the Arihant class of submarines. Given the advancements made by NMRL in this field, future Indian Navy submarines might have these tiles given their tremendous benefits.
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