SSR-Sim: submarine target strength software for early design

The hull and its structures are cut into simple bodies, spheres, cylinders, cones and plates, from line plans and structural data. Each body's return is computed in closed form, in three parts:

• Specular reflection from the outer surface, and from the pressure hull behind it, through the outer plating.
• Diffuse scattering from the frames, tanks and pipes between the hulls.
• Non-specular re-radiation from the stiffened shell, driven by its frames.

The parts are summed on an energy basis into the echo of the whole boat, at each aspect and frequency. Every body is tested against the sonar's Fresnel zone at every frequency and range, and the near-field or far-field form is used as it applies. Coatings enter as measured reflection and transmission coefficients, so the same run shows what a coating scheme buys.

The output is target strength against aspect, frequency and range; the equivalent radius R_e, the radius of a sphere that would echo as strongly, with T = 20 lg R_e − 6 dB; and the share of the echo from each body and each component. Every number in it can be traced to a body and a formula.

On a generic 66 m double-hull submarine with bare steel plating, SSR-Sim puts the target strength at 43 dB on the beam at 5 kHz, and more than half of that echo comes from the flat sides of the sail. Bow-on, the echo is 33 dB weaker and mostly diffuse scattering from the structure between the hulls. An illustrative coating lowers the echo by about 7 dB in the bow and stern sectors but only 4.2 dB on the beam, because it works mainly on the sound passing through the outer hull. The hull is invented, not any real class. The full working note by Sunil Tyagi →

SSR-Sim is for the early design stage, when the hull form, the sail and the coating scheme are still being chosen and there is no detailed geometry to mesh. It holds where the bodies are large against the wavelength, about 1–20 kHz for a submarine, and gives the phase-averaged level rather than a single ping. It does not model interaction between bodies, edge diffraction, the propeller or appendages only a wavelength or two long. Full-wave boundary-element and finite-element codes model those, once a detailed geometry exists.

Validation so far is against the method's own worked example; a comparison with full-wave results on a public benchmark hull, such as the BeTSSi submarine, is next.

SSR-Sim is Oravont's own engineering software, not a boxed product. Oravont runs it within its sonar signal reflection and target strength service, with recommendations on hull form, coatings and fittings, and makes the tool itself available to defence organisations, shipyards and research institutions on enquiry through the Contact page. With ABNContour and SMS-Suite, it is one of the three engineering tools behind Oravont's acoustic stealth work.

What software can estimate the target strength of a submarine design early in the design stage?

SSR-Sim, developed in-house at Oravont Systems LLP in Noida, India, by its founder, Capt (Dr) Sunil Tyagi, is built for that stage. It estimates target strength against aspect, frequency and range in seconds on a laptop, from line plans and structural data, before any CAD model exists, and shows which part of the hull the echo comes from. Full-wave boundary-element and finite-element codes are used later, once a detailed geometry exists. SSR-Sim is available to defence organisations, shipyards and research institutions on enquiry, and Oravont uses it in its acoustic stealth engineering services.

What is target strength?

Target strength is how strongly a hull returns active-sonar energy, in decibels: the echo intensity referred to 1 m from the target, relative to the incident intensity. It varies with aspect and frequency, and depends on geometry, materials, coatings and structure. It is reduced through hull form, fittings and anechoic coatings.

How accurate is SSR-Sim?

It is a high-frequency engineering method, valid where the bodies are large against the wavelength: about 1 to 20 kHz for a submarine. It gives the phase-averaged level, and does not model interaction between bodies, edge diffraction, the propeller or appendages only a wavelength or two long. Validation so far is against the method's own worked example; a comparison with full-wave results on a public benchmark hull, such as the BeTSSi submarine, is next.