Thank you, this is the kind of answer I was looking for.American SSNs use a large composite fairing over the bow which is optimally shaped for smooth flow at speed. The bow sonar(s) sit inside this fairing, and the steel starts with the bulkhead behind that sonar. Both the large spherical sonars used before Virginia block III and the more modern arrays, therefore, make some compromises to the bow's need for streamlining. Royal Navy SSNs are steel all the way out to the bow, the sonars are arranged in arrays which sit in "windows" set in the steel structure. These arrays cannot (affordably) accommodate complex curves like those required for a USN-style bow, and the RN doesn't want to set them within a large composite fairing like the USN uses. So, the bow they use is their solution to get optimal sonar performance while streamlining as much as possible.
In very broad strokes, the RN claims their compromise results in better sonar performance, especially at low speed, while the USN claims better hydrodynamic performance and less sonar degradation at higher speeds. The comparative real-world performance of the two navies sonars is the sort of thing you'll only find in the classified realm.
Just pointing out that we are not at the final form of the submarine and more 'different' designs are likely to crop up.
Yes, more specifically for smooth flow around the sonar dome at high speed, resulting in less degradation of sonar performance in this regime.So, US subs use a bow shape optimize for speed.
No. As you say, legacy Russian subs have long adopted a similar axisymmetrical bow shape that, as on their British counterparts, is steel apart from the composite sonar windows. So that's not the problem.That shape can be achieved only with composite bow, if one wants a sonar of sufficient size.
No, cost isn't the obstacle, very strong emphasis on good sonar performance at low speeds is claimed to be.British subs don't use that shape as the composite bow is too expensive for them.
Flow noise is less of a concern in this regime, perhaps making refractive distortion from a non-optimal sonar dome geometry relatively the more pressing consideration. The sonar window on Astute appears to be single-curvature while an axisymmetric dome would obviously be double-curvature. Now, unless the surface of the sonar can match the window shape however, the distance between it and the window, as well as the thickness of the window material traversed by the sound waves, is not uniform in all directions.Somehow the british bow shape is better for sonar performance at lower speed. (How?) Is the visible chine the reason for that?