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Tuesday, January 28, 2014

USS Scorpion (SSN-589) - Updated

Scorpion was a Skipjack-Class submarine.  The Skipjack class was the nuclear-powered attack submarine class previous to the Thresher/Permit class.  As such, they were not quite as deep-diving, although they did have a very sleek hull, which allowed for a high submerged speed.  Skipjack-class submarines held (classified) speed records while using somewhat noisy 5-bladed screws, but lost this high speed when the screws were eventually replaced with quieter 7-bladed screws.

Skipjack class boats had several improvements over previous classes of nuclear submarine.  The forward planes were moved to the sail to reduce flow noise near the sonar equiment, which shared space with the torpedo tubes in the bow.  Skipjacks were the first teardrop-shaped hulls on a nuclear submarine, which finally broke with designs that were based on diesel boats.

This design made the Skipjack class the first nuclear submarine with a hull optimized for submerged operations, rather than a surface vessel that could dive for brief periods of time.  The original nuclear submarine, the USS Nautilus, was basically a Tang-Class diesel submarine hull, with nuclear propulsion replacing the diesel engines.

The Skipjack class were very handsome ships, with a short hull and quite a large sail for their size.

Below, from Wikipedia, Skipjack class layout.

1.Sonar arrays
2.Torpedo room
3.Operations compartment
4.Reactor compartment
5.Auxiliary machinery space
6.Engine room

Scorpion was the third ship of six built in the Skipjack class.   Her keel was laid in August 1958, and she was launched in December 1959.  From 1960 to 1967 she went on several patrols in the Atlantic, out of Norfolk, Virginia.

Below, a photo of USS Scorpion and some of her crew, shortly before she was lost.

In February 1967, Scorpion entered shipyard for what should have been an extended overhaul, for repairs and also for installation of SUBSAFE systems.  The SUBSAFE program was put in place following loss of the USS Thresher.  The demand for nuclear submarines' surveillance capabilities was very high during this time period, and the number of nuclear subs available was small.  

Scorpion's extended overhaul was shortened to 6 months, and the very desirable SUBSAFE systems would not be installed.  She was released from the shipyard in June 1967.  She went on patrol in Feb 1968 to observe Soviet naval activities near the Azores.

On May 16, 1968, Scorpion departed from the patrol area to return home to Norfolk, Virginia.  For an unusually long period of time, beginning shortly before midnight on 20 May and ending after midnight 21 May, Scorpion attempted to send radio traffic to Naval Station Rota in Spain but was only able to reach a Navy communications station in Nea Makri, Greece, which forwarded Scorpion's messages. Six days later, the media reported she was overdue at Norfolk.  The Navy suspected possible failure and launched a public search. Scorpion and her crew were declared "presumed lost" on 5 June.

Unlike the sinking of USS Thresher, it wasn't clear where the USS Scorpion had gone down, nor was there any idea of what had gone wrong.  After a long and difficult search, the wreckage of Scorpion was found at a depth of 9800ft. 

A court of inquiry was convened to determine the cause of the loss of Scorpion.  Unfortunately, unlike Thresher, there was no vessel nearby in communication with her when she was lost.  The court of inquiry had to make conclusions based on a handful of undersea photographs, testimony from engineers, and a handful of sonar recordings from the SOSUS network.

The court of inquiry was not able to conclusively determine what had caused the sinking, but the following possibilities were evaluated:

Accidental Torpedo Activation:  MK-37 torpedo was accidentally fired.  The accoustically activated torpedo might then swim back to the only nearby source of sound (Scorpion), and destroy it.  This was considered unlikely for a couple of reasons.  Scorpion could have fired a second torpedo to destroy the first one, and all torpedoes have a safety mechanism that disarms them in the event they make a 180 degree turn.

Hot Run:

Torpedo Hot Run in the tube:  Torpedo begins running in a dry torpedo tube.  The danger here is twofold - the torpedo motor can overheat without seawater to cool it, and explode.  Secondly, after the motor runs a certain length of time, the warhead activates and begins listening for noise, then detonates.  For a Hot Run in the torpedo tube, procedure is to quicky open the outer door and eject the torpedo.  The caveats about a torpedo 180 degree turn apply to this scenario as well.

Torpedo Hot Run in the torpedo room:  In this scenario, the torpedo is accidentally activated while inside the ship, and cannot be ejected overboard before it will detonate.  In this case, the entire ship is placed into a tight 180 degree turn to deactivate the warhead before it arms.  Support for this theory is that Scorpion is on the ocean floor facing the opposite direction it was traveling, so perhaps the captain ordered a 180 degree turn, hoping to shut down an onboard hot run.

Torpedo malfunction/fire/detonation: This theory proposes that a torpedo battery (which activates the motor), leaked acid or internally shorted, creating enough heat to ignite torpedo fuel or other combustibles inside the weapon, causing a fire which eventually detonated the warhead. Plausible if it took place in the torpedo tube. Implausible (due to crew training) if it took place in the torpedo room itself.

Below, a MK-37 Torpedo.  Phased out in 1972.

Soviet Attack:  In this scenario, the Scorpion was sunk by a Soviet Submarine, possibly in retaliation for the unexplained loss of K-129, a Soviet Golf-class submarine lost in the mid-Pacific on March 8, 1968.  A couple of books have been written about this scenario, invoking the Walker Spy Ring as giving away the position of Scorpion to the Soviets.  There would need to be a major US/Soviet coverup for this one to be likely, and it's difficult to believe someone would not have talked by now.

The most siginficant evidence against all of the above theories is that the SOSUS network did not record a primary explosion, followed by the sound of the hull being crushed.  The only sound was the implosion of Scorpion's hull.   During the more recent sinking of the Kursk, SOSUS detected two distinct explosions (one small and one much larger), which both now are generally understood as a single torpedo warhead detonation, followed later by the entire torpedo room armament exploding.

For this reason we are left with non-explosive explanations for the sinking, and that brings us back to what sunk the Thresher: Flooding.  Scorpion had been passed over for SUBSAFE retrofit.  That means that the remote-operated hydraulic seawater shutoff systems were never installed.  Any flooding would have had to be attacked in person and isolated using manual valves.

Torpedo tubes are 21" holes in the ship.  Trash disposal units are one foot holes in the ship.  Seawater cooling systems are numerous and come in various pipe diameters.  The largest seawater penetrations in a ship are for the main condensers in the propulsion plant.  Because Scorpion was not part of the SUBSAFE program, none of these critical seawater systems could be considered robust, due to lack of a solid quality control program. 

Unfortunately we will never know exactly what sunk Scorpion, and that is a shame.  The families of the crew and those of us in the larger family would probably feel better knowing what happened to Scorpion in the depths of the Atlantic.

Scorpion's name was struck from the Naval Vessel Register on June 30, 1968.   She and her 99 crew members have been on Eternal Patrol since that date.

Thanks to an anonymous commenter, I revisited the Wikipedia article about the USS Scorpion, here, and decided to add another possible scenario for her loss.  That scenario is a hydrogen explosion.
There has always been a rumor floating around the fleet that perhaps the loss of Scorpion had to do with her main storage battery.

U.S. nuclear submarines, although independent of the disadvantages of a diesel-electric drive train propulsion system, are still equipped with a single main storage battery.  On a nuclear ship, the battery is installed to provide the ship with a temporary source of electrical power to operate vital equipment in the event of a reactor scram, or the loss of both steam turbine generators.  This battery power can be supplemented and extended by a small emergency diesel generator that is started up once the ship reaches shallow (periscope) depth.

For some background on the possibility that Scorpion might have been lost due to a battery mishap, let me explain how submarines would charge the batteries back in the diesel-electric era.  The diesel sub would need to come to periscope depth, and then extend the snorkel mast.  The snorkel mast is a large diameter pipe with a valve at the top which will go shut if a wave briefly submerges the snorkel intake.  Large diesel-generators thus perform double-duty: They charge the ship's batteries while also massively ventilating the ship with fresh air.

So... the diesel-generators would be started, and a battery charge would commence.  Diesel-electric submarines had 3-4 massive batteries, and so several batteries might need to be charged at once, or perhaps each one would be charged at a different time of day, after a period of deeper operations.  Thus it was common for diesel electric boats to be at very shallow depth ("periscope depth") for long periods of time, vulnerable both to detection, and to collision, because a snorkel mast isn't all that visible to surface vessels.

Here is the interesting part of this theory:  Diesel boat commanders would normally set "Condition Baker" (the closure of all watertight doors) when operating at periscope depth, so that if a collision with a surface vessel occurred, only one compartment *might* flood, and the sub should survive.

Meanwhile, down in the battery compartment(s):  When a lead-acid battery is being charged, hydrogen gas is released due to electrolysis, which is catalyzed by the sulfuric acid present in the battery's electrolyte.

The battery receives two types of charge:  Normal and Equalizing.  A normal charge is just that - a topping off that is done fairly frequently.  An equalizing charge is done less often and much greater voltage is applied to the battery, at just below the threshold that will create damaging internal heating in the battery cells.  As a result of this extra voltage, an equalizing charge generates a great deal more hydrogen than a normal charge.

Due to the production of hydrogen during a battery charge, on a nuclear submarine, there is a very specific ventilation line-up that must be aligned beforehand.  This line-up is to ensure the battery compartment has enough air circulation that dangerous amounts of hydrogen do not accumulate.  Just 4% hydrogen can burn in air, and 8% hydrogen is explosive.

Diesel submarines require massive amounts of fresh air, drawn in through the snorkel mast, to operate the diesel-generators, and so hydrogen accumulation during a battery charge is not much of an issue.  Nuclear subs don't require any air to recharge the main storage battery, and so maintaining the ventilation line-up is much more critical.  The only mechanism for removing hydrogen on a nuclear ship are air scrubbers, which work very slowly, and only process a small slipstream of recirculating air.

The Battery Theory conjectures about the possibility of a clash of old diesel submarine habits and new nuclear submarine technology:  That when Scorpion ship came up to periscope depth (perhaps during an equalizing battery charge), that "Condition Baker" was set, just like they did in the diesel boat days. This closure of watertight hatches would, without any doubt, interfere with the battery charging ventilation line-up.  An explosive concentration of hydrogen may have then developed and ignited from static or a stray spark.

I am pretty confident that a hydrogen explosion would not be powerful enough to rupture the hull.  Rupturing the hull would take far more energy than a pocket of hydrogen gas could generate.   Submarine hulls are *tough*.  Perhaps a hydrogen blast could cause a fire that got out of hand, or rupture an internal seawater fitting that could not be isolated.

As with the hot run scenarios and the hostile fire, the caveats about SOSUS only recording the sound of the hull failing (rather than being preceded by an explosion) might apply.  On the other hand, a smallish hydrogen explosion (as compared to Kursk's nitrate-laden torpedo warheads detonating) might have gone un-noticed by SOSUS.

Thanks for the tip, anon commenter!


Anonymous said...

Worth updating with the relatively new (and yet old) theory that Scorpion sank due to a hydrogen explosion.

See Wikipedia for related details.

Mark said...

Yes, thanks for commenting. I had intended to mention the theory that perhaps the main storage battery had an issue, but forgot to put that in the original post. I will update it ASAP!