Absalon F341 ASW in 5 minutes

10/17/21

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ASW (anti submarine warfare) is the discipline where, a surface ship, want to find enemy submarines at as great a distance as possible, and preferably before the submarine can fire torpedoes at your own ships. If the submarine's torpedo has an effective range of e.g. 20 nmil, then you must be able to detect the submarine at a greater distance. This can only be done if you use ( Active Towed Array Sonar ) . An effective active sonar is lowered into the water, where it emits some very powerful sound pulses, which are dispersed in the water. If the sound wave meets a submarine, some of these waves are reflected back. It is in exactly the same way that a bat catches its prey. In order to be able to receive the weak sound waves that are reflected from the submarine, a long series of hydrophones are used which are lowered into the water, preferably 400 to 1000 meters after the ship, to reduce the propeller noise that the ship itself generates. (TAS) Towed Array Sonar. A TAS typically consists of 60 to 120 hydrophones, which hang like beads on a string. By calculate all the signals from all hydrophones, however, it is easier to distinguish between the reflected signal from the submarine and the background noise. (it's again the same way the bat does, it only has two ears)

Of course, the submarine designers try to make the submarine as invisible as possible. The entire Submarine is therefore typically coated with a thick layer of Polyurethane, to absorb the sonar sound instead of reflecting the sound. How good the submarine is at absorbing the sound / reflecting is called (TS = target strength) The lower the TS, the better the submarine is to sneak past unhindered, in the same way as a "stealth plane" is very difficult to detect with radar.

In the F341 model:

The model F341 has a Hull mounted sonar and a TAS Towed Array Sonar system.

Design of the hull mounted sonar

The Sonar is a 344kHz piezoelectric (PZT) disc element with a diameter of 27mm (equal to 2m if 1:1)

The Hull mounted Sonar is placed below the keel, for optimal visibility in all directions.
The sonar is designed with an acoustic front “lens” and a backing material made of a combination of tungsten powder and epoxy. The backing layer closed to the PZT is heavy density, to match the PZT acoustic impedance, and by gradually reducing the amount of tungsten power, changing the density. The result is a well damped transducer with a good transient response. (It may sound easy to make at home on the desk, it is not and it sucks, and a lot of epoxy and power is wasted because I only produced a single piece)

The Sonar can be powered by a 50W amplifier and controlled by the 1GHz raspberry pi. The main processor.

 

The TAS Towed Array Sonar and ATAS Active Towed Array Sonar design

The TAS is design to reach a depth of a ship length (200cm) at a speed of 2 knots.

A real life TAS: A hydrophone array is made up of a large number of hydrophones placed in known locations and towed in a horizontal line behind a boat or ship. Sound arriving at the array from a distant source, such as a submarine, will reach each hydrophone at slightly different times, depending on the direction from which the sound is coming. Using this information from all the hydrophones in the array, the direction from which the sound is coming can be pinpointed. If the background noise is uncorrelated, then noise can be reduced by the square root of the number of hydrophones. An array with 100 hydrophones can therefore listen / find noise sources that are (√100) 10 times below the individual hydrophone noise floor.

In the model, TAS is implemented with a sinking class 10 fly-line. If you use a "sink tip" line, it places itself flat in the water at a nice depth a few meters behind the ship, it is also so rigid that it can be wound in and out completely automatically without any problems.

The TAS winch 3D drawings

The TAS winch is designed with a modified servo and a multi-turn potentiometer. The modified servo is mounted with a wheel with teeth to be able to count how many meters the array is pulled out. A 2nd motor ensure that the winch coil rolls the array back onto the coil. This motor is only active during retrieve.

The picture is showing the actual implementation, everything is controlled via a single PWM signal.

At the bottom of the pictures you can see the shock absorber which holds the whole "winch" in place, as well as the motor which is used to pull the TAS in

The ATAS is designed with a 100mm linear servo motor that can open the rear gate, and at the same time move the ATAS out into the outer position. ATAS itself is designed as a two ring transmitter with a resonant tube in the middle.

It does not work but if I had to design it, I would probably design it this way :)

The transmitter can be designed in many ways, but it is important to be able to send as much sound out into the horizontal plane in the water as possible. Sound sent towards the surface or towards the bottom must be reduced.

 

Towed Array Sonar in action (link to video)

 

The model is prepared for defense against torpedoes with a "hard kill" ATT Linkl to the real ATT "Sea Spider" by Atlas in Germany

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This site was last updated 10/17/21