The Republic of Singapore Navy (RSN) operates a technologically advanced naval force of two Challenger-class (Sjöormen-class) submarines and two Archer-class submarines (Västergotland-class).  
Capabilities at a Glance
Total Submarines in Fleet: 4
- Ballistic Missile Submarines (SSBNs): 0
- Nuclear-Powered attack submarines (SSNs): 0
- Diesel-electric attack submarines (SSKs): 4
- Air-independent propulsion (AIP) enabled: 2/4
Between 1995 and 1997, the RSN acquired four second-hand Sjöormen-class submarines from Sweden: the HMS Sjöbjörnen, HMS Sjölejonet, HMS Sjöormen, and HMS Sjöhunden. In the early 2000s, RNS re-commissioned these vessels respectively as the RSS Challenger, RSS Conqueror, RSS Centurion, and RSS Chieftain.  In November 2005, the RSN signed a contract with Sweden’s Kockums Naval Solutions to purchase two retired Västergotland-class submarines: HMS Hälsingland and HMS Västergötland. The RSN re-commissioned these vessels in 2011 and 2013 as the RSS Archer and RSS Swordsman. 
Modernization and Current Capabilities
In December 2013, Singapore purchased two Type 218SG submarines from the German group Thyssen-Krupp for 1.36 billion USD. The long-range vessels will be equipped with the Air Independent Propulsion system (AIP).  In February 2019, the first vessel, the RSS Invincible, was launched in Kiel. It will be delivered to Singapore in 2022.  In 2017, Singapore’s Ministry of Defence announced a decision to procure two additional Type 218SG submarines; the new boats were laid down in January 2018 and expected for delivery in 2024. 
Singapore possesses two Challenger-class diesel-electric attack submarines. These submarines are 51 meters long with a 6.1-meter-wide beam and can travel up to 16 knots when submerged. With its teardrop-shaped hull design and X-rudder configuration, the Challenger-class has improved underwater maneuverability, albeit with a limited diving depth of only 150 meters (compared to 300 meters for Archer-class submarines). Their weapons systems are capable of firing modern wire-guided torpedoes. 
Singapore possesses two Archer-class diesel-electric attack submarines. These submarines are 60.5 meters long with a 6.1-meter-wide beam and can travel up to 15 knots when submerged. They feature Stirling Mk 3 AIP technology to enhance stealth and endurance capabilities.  The vessels were originally designed to operate in the cold waters of the Atlantic; however, they have since undergone the process of ‘tropicalization’ which included the installation of corrosion-resistant piping and valves, marine growth protection systems, and additional cooling.  The two Archer-class submarines have an advanced sonar system, the DCNS SUBTICS combat data system, and a pressurized diver’s lockout to facilitate special forces operations. Additionally, these have a new flank array sonar that was developed by Singapore’s Defence Science and Technology Agency and ST Electronics. Their weapons systems are capable of firing WASS Black Shark heavyweight torpedoes. 
Import and Export Behavior
Singapore operates submarines imported from Sweden. It has also placed orders for submarines from Germany.
Singapore is not an exporter of submarines.
 "Archer Class Submarines, Singapore," Naval Technology, accessed 8 August 2019, www.naval-technology.com.
 Kate Tringham, "RSN takes delivery of second Archer-class submarine," Jane's Navy International, 3 January 2013, www.janes.com.
 Singapore Ministry of Defence, "Keynote Address by Dr. Tony Tan Keng Yam, Deputy Prime Minister and Minister for Defence, for the Launch of RSS Challenger in Sweden on 26 Sep 97," News and Events, 27 September 1997, www.mindef.gov.sg; Keith Jacobs, "Naval Procurement Programs in Southeast Asia," Naval Forces, Vol. 23, Issue 2, April 2002.
 Ridzwan Rahmat, “TKMS begins work on Singapore’s third and fourth Type 218SG submarines,” IHS Jane’s Navy International, 16 January 2018; "Archer Class Submarines, Singapore," Naval Technology, accessed 8 August 2019, www.naval-technology.com.
 Tom Kaeckenhoff and Maria Sheahan, “ThyssenKrupp wins submarine order from Singapore,” Reuters, 2 December 2013, www.reuters.com; Wendell Minnick, “Singapore Contracts for 2 New German Subs,” Defense News, 2 December 2013, www.defensenews.com.
 “First German Invincible-class submarine to be delivered to Singapore Navy in 2022,” Navy Recognition, 6 July 2020, www.navyrecognition.com.
 Ridzwan Rahmat, “TKMS begins work on Singapore’s third and fourth Type 218SG submarines,” IHS Jane’s Navy International, 16 January 2018.
 "The Republic of Singapore Navy –Mission, Organizational structure and Naval Bases," Naval Forces, Special Issue, 2007; "Challenger class," Jane's Underwater Warfare Systems online edition, 28 September 2009, http://juws.janes.com.
 "Singapore," Jane's World Navies, 15 February 2013.
 "Factsheet – Submarine Tropicalisation Programme," Singapore Ministry of Defence, May 2001, www.mindef.gov.sg.
 "Singapore Submarine Forces," Jane's Underwater Warfare Systems, 25 June 2009.
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- Ship, Submersible, Ballistic, Nuclear: A hull classification for a submarine capable of launching a ballistic missile. The "N", or nuclear, refers to the ship's propulsion system. SSBN's are generally reserved for strategic vessels, as most submarine launched ballistic missiles carry nuclear payloads. A non-strategic vessel carries the designation SSN, or attack submarine.
- Diesel-electric submarine
- Diesel-electric submarine: A submarine with a diesel-electric transmission. Diesel-electric transmissions require access to oxygen for the diesel generator to charge the submarine’s batteries or drive the motor. This type of submarine is thus louder and must surface more frequently than a nuclear-powered submarine. A diesel-electric submarine can fire conventional cruise missiles against land targets, and in theory, can also carry nuclear-tipped cruise missiles. Diesel-electric submarines are significantly cheaper to build and purchase than nuclear-powered vessels, which makes them the vessel of choice for smaller navies.
- Air Independent Propulsion Technology (AIP)
- Air Independent Propulsion Technology (AIP): A propulsion system that uses liquid (or compressed) oxygen or hydrogen fuel cells, thereby allowing submarines to stay submerged for longer periods without the need for external sources of oxygen. This increased endurance also increases a submarine’s survivability.