'Quiet Death': What to know about the American torpedo that sank Iranian warship, killing 87

The sinking of the Iranian warship IRIS Dena by a single Mk 48 torpedo has put renewed focus on the U.S. Navy’s primary undersea weapon, a heavyweight torpedo that first entered operational service in 1972 and has been steadily upgraded for modern naval warfare.

The strike on the IRIS Dena marked the first time since World War II that a U.S. submarine used a torpedo to sink an enemy ship.

"In the Indian Ocean, an American submarine sunk an Iranian warship that thought it was safe in international waters. Instead, it was sunk by a torpedo. Quiet Death," War Secretary Pete Hegseth said at a press conference on Wednesday.

The Navy says the Mk 48 has long served as its principal undersea weapon, designed to "defeat all threat surface ships and submarines in all ocean environments."

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The Mk 48 is a submarine-launched torpedo that uses information from the launching submarine and its own sensors to find and strike submarines or surface ships.

Physically, the weapon is built for destructive power. According to Navy specifications, the torpedo measures 21 inches in diameter, weighs about 3,744 pounds and carries a 650-pound high-explosive warhead.

According to the Department of the Navy's fiscal year 2025 budget estimates, a single Mk 48 torpedo costs approximately $4.2 million.

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Lockheed Martin, one of the Mk 48 torpedo program’s primary contractors, says it can be guided in real time by wire from the launching submarine, allowing operators to update targeting information and adjust its course after launch. 

If the wire connection is lost, the torpedo can switch to autonomous homing, relying on digital guidance systems and onboard signal processing to continue its pursuit independently.

Over time, the torpedo has evolved through hardware variants known as "Mods," each integrating upgraded sensors, guidance and control systems, and propulsion improvements. 

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The current fleet includes the Mod 7 configuration, developed in partnership with the Royal Australian Navy, while Mod 8 is in development and Mod 9 is being pursued as a rapid prototyping effort, according to the Department of War’s Fiscal Year 2024 Annual Report by the Director of Operational Test and Evaluation.

In addition to hardware upgrades, the Mk 48 undergoes recurring software updates known as Advanced Processor Builds, or APBs, which modify tactics, classification algorithms and operator interfaces to improve performance in increasingly complex undersea environments.