A comment following Gessler's fine April 25, 2026 article is:
As well as Kilo square sail characteristics influencing the Arihants, Russia's Delta SSBNs (with similar square sails side on) and hump back would influenced India's Arihant designs.
What INS Arihant (and Arighaat) may look like. Note square sail, planes on sail and hump back (Artwork courtesy H I Sutton at Covert Shores).
---
I theorise that Russia has suffered so much financially from Ukraine war sanctions and higher tri-service defence spending that Russia may have traded accelerated SSBN and SSN design secrets to India. Unlike China (1969) India has no record of fighting Russia. So little risk for Russia that its defence tech exports to India would be used against Russia.
In return guaranteed Indian purchases of Russian oil and gas and joint energy projects have been considerable.
Its possible the now 30 year old Russian Borei/Borey SSBN design features will influence India's future full size S5 SSBNs. Also design features of the French and now German SSKs sold to India will influence India's S5 SSBNs.
Possible Borei/Borey SSBN design (Artwork courtesy Mike1979 Russia - Own work at Wikipedia (right sidebar).
---
---
Overt and covert intelligence gathering (proud professions) about current UK, US, Chinese and French SSBN programs, would also influence India's S5 designs.
See previous Submarine Matters articles on the S5s HERE.