F-117A of The Seas: Lockheed Skunk Work's Sonar-Invisible Submarine

Lockheed Stealth Submarine
Illustration with help from Dr Rachal Pawling (Twitter), Naval Architect and Lecturer in Ship Design, UCL.

F-117A of The Seas: Lockheed Skunk Work's Sonar-Invisible Submarine

Flag While developing the stealth aircraft which we know today as the F-117A Nighthawk (aka Stealth Fighter), Lockheed chanced upon something. It turned out that the faceted sides of the aircraft didn't only deflect radar, they deflected sonar too. They found this out because a polaroid camera couldn't focus on the plane. This was because it used a form of sonar to measure the distance.

Armed with this knowledge, Lockheed's famous Skunk Works set out to design a stealthy submarine using this principle. The idea was to be invisible to the active sonar used by enemy submarines, aircraft and ships. And incoming torpedoes. We think of Cold War sonar technology as primarily about passive sonar. Listening silently to detect the target before they detect you. That is broadly true, but active sonar, where you transmit sound and listen for the rebounds, still played an important role. Especially in the Soviet navy where the passive sonar was, mostly, less capable. The faceted sides didn't offer an advantage in passive sonar, but against active sonar it could be a game changer.
Lockheed Stealth Submarine
Illustration with help from Dr Rachal Pawling (Twitter), Naval Architect and Lecturer in Ship Design, UCL.

This is relevant today because faceted sides are slowly becoming a thing on submarines. Several German, British and possibly other countries' designs take advantage of this, to varying extents.

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Actually, Lockheed were not the first to hit on the idea of adding sloping sides to a submarine to decrease the active sonar cross-section. During World War Two the Nazi German Navy (Kriegsmarine) designed the Type-XXIXH U-boat with angled lines. But the Nazis did not have the computers or experimental knowledge that Lockheed had, so it is debatable whether the XXIXH would have worked. And there is no suggestion that Lockheed knew of the Nazi research or where influenced by it in any way.

Instead, the stealth submarine was born directly from the stealth aircraft project. The design was the brainchild of Ben Rich, then Director of the Skunk Works. It leveraged extensively on the stealth aircraft and actually looked a bit plane-like. The cylindrical pressure hull was encased in a wide wing-like outer-hull which gave it a flattened diamond cross-section. The edges of the facetted sides were softer than the aircraft, more F-22 than F-117A. At the stern it broadened out into two hulls with twin screws. Inward canted upper rudders were on top of these.

Lockheed Stealth Submarine
Lockheed HAVE BLUE demonstrator, which led to the larger F-117A fighter. Note the inward canted tail surfaces, a feature carried over to the submarine.

It may have had lower rudders also (that would make sense). But unfortunately we only have memories to go on currently. My best attempt at a likeness is above. If anyone has more information, I would welcome it!

Tests showed that the facetted design reduced the submarine's sonar signature by 1,000 times. Enthusiastically, Ben Rich took his proposal to the Navy. However it was not well received, partly because it compromised speed for stealth. No order was made. The project was not entirely wasted however. Instead it led to the Sea Shadow stealth boat project.
Lockheed Stealth Submarine
The Sea Shadow stealth boat leveraged the research

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