The launch of Singapore’s final Invincible-class submarine on April 22 marked a new era in the Republic’s submarine journey.

Here is a closer look at the evolution of the Republic of Singapore Navy’s (RSN) submarines, from the Challenger class and Archer class, which are refurbished Swedish vessels, to the new Invincible-class that was designed for Singapore’s needs from the get-go.

Between 1995 and 1997, the RSN bought four Sjoormen-class submarines from the Swedes, refurbished them and renamed them the Challenger class. These 51m-long subs were built for the Royal Swedish Navy in 1968.

The first of the four Challenger-class submarines was launched in 1997 in Sweden. RSN personnel trained with the Royal Swedish Navy. Two of these submarines were retired in 2015 while the other two, RSS Conqueror and RSS Chieftain, remain operational.

These earlier generations of submarines had to be retrofitted to operate in warm tropical waters, which included replacing steel pipes with copper nickel iron ones to reduce corrosion, and installing a system to minimise barnacle growth on the submarine surface.

In 2005, Singapore inked another deal with the Swedes to buy two newer submarines from them. Built by Swedish company Kockums in 1987, the submarines underwent upgrading and refurbishment to prepare them for the warmer, more corrosive waters around Singapore.

In 2013, Singapore acquired two Type-218SG submarines from German manufacturer thyssenkrupp Marine Systems (TKMS). Two more were ordered in 2017. With the launch of all four Invincible-class submarines, the Challenger- and Archer-class submarines will be progressively decommissioned.

Here is how the 70m-long Invincible-class submarines measure up against other things found on land.

The Invincible-class submarines will replace the Challenger- and Archer-class submarines that are now about 60 and 40 years old respectively.