Apr 7

Mine Over Matter

Thursday, April 7, 2016 12:01 AM

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On the evening of 14 April 1988, Samuel B. Roberts (FFG-58) was steaming though the Persian Gulf when her forward lookout spotted several ominous spherical canisters with floating on the surface: sea mines! Though the crew managed to stop the ship in time to avoid the ones they saw — it was one that was not spotted lurking just below the surface with which guided-missile frigate collided. 253 pounds of TNT detonated, blowing a huge hole in Roberts’ hull, flooding several compartments, and sending several sailors to the hospital. The mines were identified as being put down by the previous September by the Iranian minelayer Iran Ajr. Four days later, the United States retaliated against Iran in Operation Praying Mantis.

A Polish-made M-08/39 Naval mine identical to the type which damaged the Samuel B. Roberts (FFG-58) on 14 Apri 1988.

A Polish-made M-08/39 Naval mine identical to the type which damaged Samuel B. Roberts (FFG-58) on 14 April 1988.

Consisting of a buoyant metal sphere filled with explosives tethered to a heavy cylinder that anchors it to the sea bottom, the M-08/39 contact mine that disabled Roberts had not changed much at all in the eighty years since it had been first introduced in the Imperial Russian Navy under Tsar Nicholas II in 1908. Though contact mines like it had been long since joined by magnetic, acoustic, and other types of influence mines, the 19th-century technology at the heart of the contact mine was — and is — still an effective means of offensive naval weaponry.

Popular conceptions of naval mines such as in Gilligan’s Island and the 2007 film Hot Fuzz generally depict a ball bristling with spikes that when pressed or broken off, activate a ticking timer — and then kaboom. “Except for the tendency to overpopulate with horns,” notes one World War II publication on mines, “this picture is correct — for contact mines. A ship must come into physical contact with this type of mine in order to fire it, hence the name.”

A typical depiction of a sea mine, shown here bristling with spikes.

But the reality of contact mines is much more interesting. Many of the most common types owe their operation to a fancy bit of electrochemistry.

Diagram from a 1944 US Navy publication showing the operation of a contact mine. Naval Institute Photo Archive.

Diagram from a 1944 US Navy publication showing the operation of a contact mine. Naval Institute Photo Archive. Click to enlarge.

The “spikes” seen on actual naval mines are not spikes so much as protuberances commonly called Hertz horns after their inventor Dr. Albert Hertz (and not, as is commonly believed, German physicist Heinrich Hertz, for whom the unit of frequency is named), a native of Königsberg and a naval veteran of the Franco-Prussian War. His first designs for the “horn” date to about 1868, though interest in the design didn’t really take off until the 1870s after that war had concluded.

Contact mines on the deck of a British destroyer some time during the First World War. The S-class destroyer HMS Saladin can be seen in the background. Naval Institute Photo Archive.

Contact mines on the deck of a U. K. Royal Navy destroyer some time during the First World War. S-class destroyer HMS Saladin can be seen in the background. Naval Institute Photo Archive.

The horns consist of a metal horn generally made of a soft, bendable metal like lead encasing a glass ampule. The ampule within each horn contain an electrolytic mixture of chromic acid (H2CrO4) and sulfuric acid (H2SO4). When the horn is bent, the ampule breaks, spilling the electrolyte onto carbon and zinc electrodes that form the plates of a battery cell. The acid mixture reacts with the plates by the following reaction:

3 Zn → 3 Zn2+ + 6 e

Cr2O72— + 6 e+ 14 H+ → 2 Cr3+ + 7H2O

The chromic acid serves to react with the hydrogen gas as a depolarizer, so the actual reaction taking place in the cell may be regarded as:

Zn + H2SO4 → ZnSO4 + H2

The now-activated chromic acid cell generates about 2 volts of electrical potential, enough to set off an electrical detonator and then the whole rest of the mine with deadly results.

A World War II-vintage diagram showing the construction of a contact mine. Naval Institute Photo Archive.

A World War II-era diagram shows the construction of a contact mine. Naval Institute Photo Archive. Click to enlarge.

Because the chromic acid cells are not used up until activated (and even then they only have to be used once), naval mines employing the Hertz horn can remain deadly for many years.

Not many 19th-century technologies can be employed in modern warfighting, but the contact mine and its deadly, elegant simplicity still pack a potent threat to ships to this day.

An Explosive Ordnance Disposal (EOD) technician examines an Iraqi contact mine that washed ashore in a Kuwaiti harbor during the Gulf War in 1991. Naval Institute Photo Archive.

An Explosive Ordnance Disposal (EOD) technician examines an Iraqi contact mine that washed ashore in the Kuwaiti port of Ash Shuaybah during the Gulf War in 1991. Note the horns. Naval Institute Photo Archive.