The submarine was named after the Nanggala, a powerful, divine short spear wielded by Prabhu Baladewa, a Hindu god mentioned in the Mahabharata and a character in wayang puppet theatre. Legend states that the spear is capable of melting mountains and splitting oceans.
The vessel was also known as Nanggala II in order to differentiate it from RI Nanggala (S-02), an older Whiskey-class submarine sharing the same name.
During the 1960s, Indonesia was known as one of the largest Asian naval powers, with 12 Soviet-made Whiskey-class submarines in its fleet. However, by 1981, during the Indonesian New Order, when Cakra and Nanggala arrived in Indonesia to reinforce the country's naval defenses, only one of the 12 Whiskey-class submarines had still retained the ability to dive. The Indonesian government had planned to purchase a Type 206A submarine from Germany in the late 1990s, but was unable to do so due to funding issues.
During the beginning of the Reform Era, an embargo on military equipment imposed by the U.S., as well as continuing financial problems experienced as a result of the Asian financial crisis, meant that the Indonesian Navy was unable to procure any additional submarines until 2017. As a result, Cakra and Nanggala were the only active submarines in the Indonesian Navy between the decommissioning of KRI Pasopati in 1994 and the commissioning of KRI Nagapasa in 2017.
By 2020, Indonesia had made plans to own and operate eight submarines by 2024.
The submarine conducted a number of intelligence gathering operations in the waters around Indonesia, including one in the Indian Ocean from April to May 1992, and another around East Timor from August to October 1999, in which the boat tracked the movements of the International Force East Timor as it landed in the region. During May 2005, the submarine was tasked with scouting, infiltrating, and hunting down strategic targets around Ambalat, after Indonesian KRI Tedong Naga [id] and Malaysian KD Rencong were involved in a minor collision near the area.
Nanggala underwent a refit at Howaldtswerke that was completed in 1989. Roughly two decades later, the boat underwent a full refit for two years in South Korea by Daewoo Shipbuilding & Marine Engineering (DSME) that was completed in January 2012.[a] The refit cost US$ 63.7 million, replaced much of the submarine's upper structure, and upgraded its weaponry, sonar, radar, combat control and propulsion systems. After the refit, Nanggala became capable of firing four torpedoes simultaneously at four different targets and launching anti-ship missiles such as Exocet or Harpoon. Its safe diving depth was increased to 257 metres (843 ft), and its top speed was increased from 21.5 knots (39.8 km/h) to 25 knots (46 km/h). Roughly five years later in November 2016, the submarine was equipped with an ASELSAN KULAÇ echosounder system.
The navy announced in a written statement that Nanggala had requested permission to dive to fire a SUT torpedo at 03:00 WIB (20:00 UTC, 20 April). About an hour after being given clearance, the boat lost contact with surface personnel.
According to the navy, at around 04:00, Nanggala should have been flooding its torpedo tubes in preparation for the firing of the torpedo. Indonesian military spokesperson Major GeneralAchmad Riad [id] reported that the last communication with Nanggala was at 04:25, when the commanding officer of the training task force would have authorized the firing of torpedo number 8.Chief of Staff of the Indonesian NavyYudo Margono reported that Nanggala had fired a live torpedo and a practice torpedo before contact was lost.
The navy subsequently sent a distress call to the International Submarine Escape and Rescue Liaison Office at around 09:37 to report the boat as missing and presumably sunk. The navy stated that it was possible that Nanggala experienced a power outage before falling to a depth of 600–700 m (2,000–2,300 ft). Widjojono stated that Nanggala was able to dive to a depth of 500 m (1,600 ft). The deepest areas of the Bali Sea are over 1,500 m (4,900 ft) below sea level.
At the time it went missing, Nanggala had 53 people on board, including 49 crew members, 1 commander, and 3 weapons specialists. The highest-ranking naval officer in the submarine was ColonelHarry Setyawan, the commander of the submarine unit of the 2nd Fleet Command. Subordinates with him were Lieutenant Colonel Heri Oktavian, the commander of the submarine, and Lieutenant Colonel Irfan Suri, an officer from the Weapons Materials and Electronics Service.
At noon on 22 April, Yudo Margono stated that the oxygen reserves on Nanggala would be sufficient for the entire crew and passengers for three days after it had submerged, noting that the oxygen would run out on Saturday, 24 April, at 03:00 (20:00 UTC, 23 April). Submarine experts stated that submarines have backup systems that may provide sufficient oxygen for some time depending on the state of the equipment. Sources in the Indonesian Navy reported that the underwater telephone (UWT) of the submarine was defective during the drill, hampering communications between the boat and rescue vessels in the area.
Around 07:00 on 21 April, an aerial search revealed traces of an oil spill on the surface of the water near the location where the submarine was believed to have dived.
Achmad Riad later reported that an oil slick had been observed at multiple locations. He added that Raden Eddy Martadinata had detected movement underwater at a speed of 2.5 knots (4.6 km/h), but was unable to obtain enough information to identify the contact before it disappeared. Yudo Margono also reported on Thursday that an Indonesian naval vessel had detected an object that was magnetic at a depth of 50 to 100 metres (160 to 330 feet).[c]
The location where Nanggala was found
On 24 April 2021, the Indonesian Navy announced the finding of debris, including a part associated with torpedo tubes, a coolant pipe insulator, a bottle of periscope grease, and prayer rugs. Because the debris was found within 10 nmi (19 km; 12 mi) of the point of last contact and no other vessels were believed to be in the area, the debris was believed to have come from the submarine, and Nanggala was declared sunk.[d]
Yudo Margono stated that a sonar scan had shown the submarine at a depth of 850 m (2,800 ft), and its crush depth was presumed to be 500 m (1,600 ft).
He stated that on 25 April 2021 at 01:00, KRI Rigel pada conducted multibeam echosounder and confirmed the final position of KRI Nanggala-402 at 838m on the sea floor.
But due to the maximum operational depth limitation of 800m of Rigel's two underwater ROVs, the Rigel was unable to launch them and was assisted by the submarine rescue ship MV Swift Rescue of the Republic of Singapore Navy, which launched its drone and made visual contact with Nanggala at 09:04.
On 25 April 2021, the Indonesian Navy confirmed that all 53 hands on board were lost.
Politician Zeng Wei Jian of the Gerindra political party, whose founder is defence minister Prabowo Subiyanto, stated that the Chief of Navy should explained the reason behind the Nanggala being allowed to train in a sea area that is considered hazardous and dangerous.
“How could the submarine KRI Nanggala, carried 53 people and trained in a dangerous zone. These should be clarified,” said Zeng Wei Jian.
The Nanggala was designed for 34 people.
Marine specialist I Ketut Sudiarta of the Bali's Marine and Fisheries Agency said that waters north of Bali Sea is categorized as "sea trenches" with average depth of 700-1300 meter, getting deeper eastwards, reaching depth of 1,000m or more and is characterized by steep elevation with strong and turbulent eddy current, and that under the sea, there is a very active upward fault line, extending farther eastward to the north of Flores Island.
Sudiarta said that the usual naval training areas are the waters off Situbondo, Banyuwangi, East Java and northwards to north Singaraja, Bali.
Overloaded Crew Capacity
Zeng Wei Jian also questioned why 53 people were allowed onboard the Nanggala when it sunk, when it was designed for 34 crew.
The possibility of over-capacity was also raised by a member of the Parliamentary Committee I on Defence Maj-Gen. (Ret.) TB Hasanuddin
"That means an overloading of 15 persons. Why was an excess load of 15 people allowed?" he asked. 
Death of three crew in a torpedo launch failure.
TB Hasanuddin also said that three crew members of the Nanggala died in a failed torpedo launch exercise in 2012. The submarine was then sent to South Korea for repair.
Lack of oxygenation gel.
Further Hasanuddin stated that oxygen, used in the Nanggala to release additional oxygen gas, was not carried onboard the submarine when it sailed out to sea for the execise.
"I was informed that during diving, KRI Nanggala 402 was said not to carrying oxygen gel, but was still ordered to sail."
1,000m Depth Rating of Rigel's Two Underwater ROVs
The Nanggala was found at a depth of 838m, which was within the 1,000m depth rating of the underwater drones of the Rigel.
The Rigel operated two underwater drones:
a) Hugin-1000 Autonomous Underwater Vehicle (AUV) with a depth rating of 1,000m manufactured by Kongsberg Gruppen Norway, and
b) H-800 Hytec ROV, also rated to 1,000m deep, manufactured by French ECA Group.
Yudo Margono claimed that as the Nanggala was detected by Rigel sensors beyond 800m depth, the Rigel could not sent its ROVs which he said has a depth rating of maximum 800m.
However that claim seemed to contradict the manufacturer specification of both the Hugin-1000 and the H-800, as both explicitly stated that their ROV model delivered to the Rigel and her sister ship the KRI Spica, have depth rating of 1,000m, well within the Nanggala's depth of 838m
Kongsberg had also conducted a 3-weeks practical training for the Indonesian Navy to operate the Hugin-1000 AUV down to 1,000m deep offshore the island of Sumatra, Indonesia in 2015.
MV Swift Rescue - submarine search and rescue ship
After the Rigel claimed to have detected the Nanggala at 838m and was unable to send the ROVs, the location was passed over to the Singapore Navy's MV Swift Rescue.
MV Swift Rescue sent down its ROV Super Spartan AT 07:37am and successfully located the Nanggala at 09.04am and transmitted video recordings of the debris.
The launch of the ROV by MV Swift Rescue, at 07:37am on 24 April 2021, almost 8 hours after the Rigel had claimed to have located the Nanggala, also discounted the possibility of a confirmed finding of the Nanggala by the Rigel, since the dwindling oxygen supply inside the missing submarine was calculated to be exhausted by 03:00am on Saturday, 24 April 2021.
If the Nanggala had indeed been located, the ROV would had been launched immediately from the MV Swift Rescue to the sea floor to attached oxygen hose before 03:00am.
"Hopefully before they can be found, the oxygen will be enough," Adm. Yudo Margono, the Indonesian Navy chief of staff, told a news conference.
Paradoxically, while the Indonesian military claimed that the Rigel's two ROVs were unable to reach the Nanggala as they were depth-rated at maximum 800m instead of 1,000m as stated by their manufacturers, the Singapore navy however claimed that MV Swift Rescue's ROV Super Spartan can search waters up to only "500m deep" when the Nanggala was located 838m deep, way beyond the Singapore's military claim maximum depth-rating of 500m for the Super Spartan, when the ROV took the close-up video recordings of the submarine on the sea floor.
Faulty Underwater Telephone
Defence magazine Janes reported that two sources within the Indoneisan military that Nanggala’s multifrequency underwater telephone (UWT) system was defective throughout this operation, and that the boat has been relying on naval frequencies in the VHF and UHF bands for its communications needs while surfaced.
KRI Cakra-401, sister ship of KRI Nanggala-402
PT PAL, an Indonesian state-owned shipyard that services submarines including the Nanggala.
“The lack of this UWT is preventing the boat from communicating with assets on the surface, and this is hampering our attempt to assess if an early intervention can be done to assist the distressed crew prior to a rescue attempt”, said one of the sources who spoke on the condition of anonymity. He also requested that further details of these systems be omitted so as not to compromise the confidentiality of equipment onboard Nanggala’s sister boat KRI Cakra (401), which had been under service since 2018 at the state-owned shipyard PT PAL, which also serviced the Nanggala.
Commander's Complaints Of Poor Workmanships by PT PAL
Nanggala's commander, Let. Col Heri Oktavian, who died in the incident, had voiced his frustration about the maintenance status of the Nanggala to Edna C Pattisna, a close friend, whose is also a reporter with local news media Kompas, which she published under an article titled "Message from KRI Nanggala-402 Commander" in kompas.id website.
He stated that the workmanshop quality and maintenance services performed by state-owned shipyard PT PAL Indonesia were unsatisfactory and encountered frequent delays.
The Nanggala was last serviced by PT PAL last year in 2020.
He lamented that an officer encountered criticism from his superiors for reporting the poor workmanships by PT PAL on the KRI Alugoro (405), a Changbogo-class submarine assembled by PT PAL's Surabaya yard and launched on 11 April 2019
"This submarine (the Alugoro) by PT PAL, there's nothing good about it," said Edna, quoting Heri Oktavian.
Edna said that Heri Oktavian hoped that the "decision makers carefully consider the TNI and its soldiers and not just "make the boss happy" ("Asal Bapak Senang") to attain promotions, positions or material gains."
On 27 April 2021 at a press conference at the naval headquarters at Cilangkap, East Jakarta, he explained the experience of total black-out while submerged. In 10 seconds, the submarine sank 90m, tail first at 45 degree, and the crew were ordered to moved forward to balance the ship. But since the submarine was at a steep angle, he had to crawl on his knees, said Iwan as he paused, emotionally-overcome by the incident.
On 3 May, two Chinese Navy ships, ocean tugNantuo 195 and ocean salvage and rescue ship Yongxingdao, arrived to assist with the recovery of the wreck. Scientific research vessel Explorer 2 was scheduled to arrive the next day.[e] There have also been discussions between the Indonesian Navy and state-owned oil regulator SKK Migas to raise the submarine.
By 18 May, the team had successfully recovered two liferafts that weighed approximately 700 kg (1,500 lb) each. However, they had yet to locate the submarine's pressure hull, and gave up on lifting the bridge after a sling was broken during a failed attempt, as the bridge likely weighed over 20 tonnes (20,000 kg; 44,000 lb).
The Navy said Nanggala might have experienced a power outage. After the finding of debris from Nanggala, Yudo Margono said the submarine might have cracked instead of exploded, as an explosion would have been detected by sonar.
Indonesian legislator and retired Armymajor generalTubagus Hasanuddin suspected the refit, performed by the South Korean firm DSME in 2012, may not have been performed properly. He said that after the refit, the submarine had failed a torpedo firing test, which resulted in three deaths. Hasanuddin also said Nanggala had exceeded its design capacity of 38 with 53 people on board when it sank. Yudo Margono said the vessel was combat ready, had received a letter of acceptance, and had a history of successful firing exercises.
No further refitting of Nanggala was performed at DSME after the 2012 refit despite the need for submarines to undergo maintenance at least once every six years.
Parliamentarian Utut Adianto stated that Indonesia's defences required modernization, while military analyst Connie Rahakundini Bakrie [id] shared similar concerns.Frans Wuwung, former head of the engine room of Nanggala, stated that despite the submarine's age, its equipment was still in good condition due to proper maintenance, and did not consider such a modernization necessary.
After the Indonesian Navy declared Nanggala lost with all hands, the People's Consultative Assembly recommended a posthumous promotion for all personnel on board. Hadi Tjahjanto stated that he would propose the promotions to Indonesian President Joko Widodo. A day later, on 26 April, Joko Widodo announced that the government would award a posthumous promotion and confer posthumously the Bintang Jalasena 'Navy Meritorious Service Star' to everyone on board Nanggala. The ceremony conferring the awards and promotions was held on 29 April, attended by Joko Widodo, Minister of Defense Prabowo Subianto, Hadi Tjahjanto, and Yudo Margono.[f]
Tubagus Hasanuddin recommended that the Indonesian Navy's remaining Cakra-class submarine be taken out of service.
Two days after the sub had been declared sunk, Rahmat Eko Rahardjo, the commander of the 2nd Fleet Naval Combat Squad who had given permission for Nanggala to dive, and ING Sudihartawan, the commander of the 2nd Fleet, were relieved of their commands by Hadi Tjahjanto. Hadi appointed Iwan Isnurwanto, a former Nanggala crew member and chief of staff, to replace the latter.
During the search, use of the hashtag #PrayForKRINanggala402 and #KRINanggala402 became popular on Twitter. After Nanggala had been declared sunk, the phrases "On Eternal Patrol" and "Rest In Peace", and the motto Wira Ananta Rudira 'Steadfast to the End', used by the submarine unit to which Nanggala belonged, saw increased usage.