Russia's only aircraft carrier, the Admiral Kuznetsov, is one of the five worst carriers in the world, according to The National Interest.

In October 2016, the Kuznetsov was sailing to Syria through the English Channel on a combat deployment when it was spotted belching thick clouds of black smoke.

“The main problem with the ship is that is has a very problematic propulsion system,” Dmitry Gorenburg, a senior research scientist at the Center for Naval Analyses, previously told Business Insider. “It's just unreliable.”

Before it was sent back to the yards for repairs, the Kuznetsov was always accompanied by a special tugboat in case it broke down, and the plumbing was said to be so bad that most of its toilets didn't work.

It's being refitted at Russia's 35th Ship Repairing Yard in Murmansk and won't be ready to return until 2021.

Inside the Admiral Kuznetsov

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Russian aircraft carrier Admiral Kuznetsov in 2012Russian Ministry of Defense/Wikimedia Commons

The Kuznetsov was laid down in 1983 but not commissioned until 1990.

Kuznetsov-class carriers are about 930 feet long, 235 feet wide, and 210 feet high. They also have a maximum displacement of about 59,000 tons.

Kuznetsov-class carriers are powered by eight turbo-pressurized boilers, four steam turbines, and six diesel generators, bringing the carrier to a maximum speed of about 33 mph.

A defensive beast

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Sukhoi Su-33 launching from the Russian aircraft carrier Admiral KuznetsovRussian Ministry of Defense

Kuznetsov-class carriers have a crew of 1,600 sailors. But unlike U.S. carriers, the Kuznetsov was built as a defense carrier.

It's armed with 12 P-700 Granit anti-ship cruise missiles (housed in silos underneath its flight deck), a Kinzhal air defense missile system armed with about 200 anti-air missiles, one Udav-1 anti-torpedo multiple launch rocket system, and eight Kashtan air defense gun/missile systems.

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The naval missile and anti-aircraft artillery weapon system Pantsir-ME developed by the Tula KBP aboard the Russian aircraft carrier Admiral KuznetsovRostec

The Kuznetsov will also be fitted with a Pantsir-M/EM air defense system with eight 57E6-E surface-to-air missile launchers — as well as two GSh-6-30K/AO-18KD 30mm six-barrel rotary cannons — upon its return from the shipyard.

It can also carry up to 41 aircraft, including Su-27s, Su-33s, MiG-29s, as well as Ka-27, Ka-31, and Ka-52K helicopters.

It's getting old

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Russian aircraft carrier Admiral Kuznetsov undergoing repairs on June 27, 2015Jared Keller

According to The National Interest, the end of the Soviet Union “also meant the collapse of funding for the carrier project,” leaving the Kuznetsov to languish without substantial upgrades. The flight deck has a built-in ski jump, angled at 12 degrees, to help with takeoffs. The aircraft are brought up from the hangar bay using an elevator.

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The elevator aboard Russian aircraft carrier Admiral KuznetsovRussian Ministry of Defense

The Kuznetsov is set to return to service in 2021, but it may take longer. The Diplomat reported in 2016: “All Soviet carriers were constructed in Ukraine and Russia has lost valuable expertise and technology — particularly surface ship propulsion technology — due to the ongoing crisis in Ukraine.”

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