> new cold-war weapon intended to give the upper hand in a full-scale invasion of the U.S.

This is not true. For one, the Soviet Union never had any plans for the invasion of the U.S.

One thing they did better in the cold war was understanding the political results of potential military actions.

Because of this, they never had any misconceptions of a major conventional war directly between superpowers that lasts more than two weeks before either being resolved or going nuclear. This informed everything about how they built up their military, from the tiniest piece of equipment to overall doctrine. Once you understand this, a lot of their equipment and thinking is a lot more sensible. (For example, the way their logistical organization is set up. A tank division, and the tanks in it, are supposed to give maximum possible effort for about two weeks after which it doesn't matter what shape they are in.)

Any concept of a Soviet invasion of the US is fanciful to the point of being stupid. Just because if the Soviets somehow attain the necessary conventional superiority to attempt it, and then push it, why exactly wouldn't the Americans just nuke them?

The US military maintains plans for invading Canada, defending Puerto Rico against alien attack, and flooding the Netherlands to deny them to the militant Tibetan Buddhist cult which has discovered an effective indoctrination-hypnosis video and is conquering Europe. Plans are just plans. Lots of them are stupid. Planning officers often make strategic plans as simple exercises in considering all factors. You can be assured that Soviet invasion of the US mainland was hardly one of the more farfetched ones, as such things go, though the conditions never came close to presenting themselves.

Nuclear weapons make most military plans "fanciful to the point of being stupid". We've been in a state of unavoidable mutually assured destruction since the late 1960's. This hasn't stopped weapons development. Being able to move large amounts of soldiers and weaponry around at high speed is a generally useful thing.

But you do not develop a new weapons system just to run a speculative planning exercise. If this was not just a military-industrial complex boondoggle - which I suspect it was - I imagine there were some somewhat realistic plans around what it might actually be useful for.

I'll give you one right off the bat where something like this might be useful, in antiship or transport roles: https://en.wikipedia.org/wiki/Turkish_Straits_crisis

Russia has a lot of inland lakes, and has always defined its empire with non-negotiable geopolitical requirements for blue-water ports on all sides. The countries that pose a threat to those ports generally have a lot of vulnerable coastlines. Fly an ekranoplane empty to a staging area, fill it with a regiment, fly it in boost mode over any land obstacles, and you can rapidly drop significant troop numbers at any point on the coast without control over the body of water.

Larger ekranoplanes that can operate higher off the water are more useful as transports, and things plausibly get much bigger than the KM.

Incompetent bureaucracies do exactly this - "you do not develop a new weapons system just to run a speculative planning exercise". One of significant reasons why USSR failed as a state.

There is no way this would’ve be useful the cross the Atlantic or pacific oceans to invade the continental US.

The only area where this might be useful is in the Great Lakes, at that point the USSR already had to invade Canada at the least and establish a sufficient presence to support these things, and even then it’s not clear if it would be more useful than traditional transport.

These things are cool from an engineering point of view but are quite useless in real world applications.

Probably not across the ocean, but how about having a carrier (like an aircraft-carrier) to carry them across said ocean and then launching these for the last few dozen kms? (This is maybe a dumb plan, because the slow carrier can be easily detected, and the sea might still be bad a few km close to the coast...).

I wonder if cruising over the Arctic ice would've worked...

I don't think an aircraft carrier can launch the second biggest plane in history, which is also a boat.

Alaska is only 55 miles (88km) from Russia.

"THE BERING SEA, near the chain of the Aleutian Islands, is one of the most intense patches of ocean on Earth. Strong winds, freezing temperatures, and icy water are normal conditions. The combination makes for some of the most ferocious waves on the planet, where the water can rise and fall 30 feet on a normal day."

https://www.nationalgeographic.com/photography/proof/2017/03...

Definitely not the place for these.

> full-scale invasion of the U.S.

I don't think that's realistic nor has it been the goal of Soviet military doctrine, more likely targets for the Ekranoplan would have been Western Europe and Scandinavia (via the Baltic Sea), Turkey (Black Sea) and Iran (Caspian Sea).

I don't believe you are correct about a full scale invasion of the U.S.

There are two reasons.

The Caspian Sea is a sea and not an ocean. The waves are a fraction of the size of those in the Atlantic. If 'waves were roads' then you would need a four wheel drive equivalent of the 'sea monster' to make it an 'ocean monster'. Ground effect isn't ground effect in quite the same way if you have to fly 50m above the water instead of 5m.

The other reason is that the Soviet Union did not have any realistic ambitions to invade the United States. Their thinking was much more defensive than Western Cold War thinkers were allowing their paranoia to believe.

Ekranoplanes function better the larger they get. The Caspian Sea Monster was not the endpoint of ekranoplane engineering. It deals well with ~1m wave height. For the Black Sea / Mediterranean sea or the Baltic, a larger version would be required, and for the Atlantic or Pacific, larger still. You could plausibly even fly it over Arctic sea ice fields without bergs.

Boeing was throwing around proposals for a ground effect aircraft several times this large in the early 2000's, with the largest bumping up against the weight range of naval destroyers at ~5000 tonnes.

https://en.wikipedia.org/wiki/Boeing_Pelican