Screenshot from twitter handle Wolf Pakcs

India secretly launched the S4 nuclear submarine (SSBN) last month. It is India's 3rd Arihant class SSBN and has 8 launch tubes for K4/K5 SLBMs and a more powerful reactor than INS Arihant according to twitter handle Wolf Pakcs.

Looking back 5 years after she was commissioned in 2016, the Indian Navy’s INS Arihant remains something of an enigma. Her existence is no secret, in fact it is a proud achievement of Indian industry. But photographs are very few. And nearly all those you will find on the internet are many years old. It is a very secretive submarine program writes submarine expert H I Sutton in NavalNews.

The Arihant is a unique design which can be characterized as a ‘pocket boomer’. It is much smaller than other ballistic missile submarines. Its hull is shorter and thinner than its contemporaries and it only carries four missile silos. But this does not take away from the industrial achievement of an indigenous nuclear-powered submarine. And in many respects its modest size seems pragmatic. Other countries now taking the nuclear submarine path,  are also going for smaller types he adds further.

The Arihant-class submarines are nuclear powered ballistic missile submarines built under the Advanced Technology Vessel (ATV) project. They will be the first nuclear submarines designed and built by India. The submarines are 111 m (364 ft) long with a beam of 11 m (36 ft), a draught of 15 m (49 ft), displacement of 6,000 tons (5,900 long tons; 6,600 short tons). The complement is about 95, including officers and sailors. The boats are powered by a single seven blade propeller powered by an 83 MW pressurised water reactor and can achieve a maximum speed of 12–15 knots (22–28 km/h) when surfaced and 24 knots (44 km/h) when submerged.

The submarines are powered by a pressurised water reactor with highly enriched uranium fuel. The miniaturised version of the reactor was designed and built by the Bhabha Atomic Research Centre (BARC) in Kalpakkam, Tamil Nadu. It included a 42-metre (138 ft) section of the submarine's pressure hull containing the shielding tank with water and the reactor, a control room, as well as an auxiliary control room for monitoring safety parameters

Four boats of this class are planned. The first boat of the class, INS Arihant, was commissioned in August 2016. The first four vessels are expected to be commissioned by 2025.

While the original plan was to have four Arihant class submarines, it was changed by the UPA government, sources in the know said. Now, the two Arihant class submarines will have a displacement of 6,000 tonnes while two other SSBNs will be of a larger size (7,000 tons displacement).

A key differentiating factor will be that the two larger vessels under construction — S4 and S4* at the Ship Building Centre in Visakhapatnam — will have eight missile tubes instead of four.