But that will take time. “That’s because [the Flyboard] has limited autonomy, it is hardly inconspicuous, and it is difficult to fly,” says Marion Laguës, spokeswoman for the French Defense Innovation Agency. “It is the development prospects that are interesting.”

The French Army, which has funded Mr. Zapata to the tune of $1.5 million, say the hoverboard could one day be a battlefield trump card. French special forces are working closely with Mr. Zapata, a former jet-ski world champion, to fine-tune his machine. “Innovation is not a gimmick,” said Defense Minister Florence Parly of the hoverboard, which she suggested could function as “a flying logistics platform, or an assault platform,” conjuring an image of squadrons of individually airborne infantry descending on an enemy.

On Sunday, standing atop a small platform fitted with five jet engines, inventor Franky Zapata took 20 minutes to cross the English Channel from Sangatte, France, to Kent, England. But while his device, the Flyboard Air, may appear the stuff of science fiction, it is the French military that hopes he will make it a viable reality.

When Franky Zapata flew a jet-propelled hoverboard across the English Channel on Sunday – the first person to do so – he impressed more than sci-fi gadget geeks.

Also watching with interest was the French military high command.

Fantasy film aficionados would recognize Mr. Zapata’s invention. In “Spider-Man,” the Green Goblin sows terror from something very like the Flyboard Air that Mr. Zapata piloted across the channel. Marty McFly uses a similar vehicle in “Back to the Future 2.”

The French Army, though, which has funded Mr. Zapata to the tune of $1.5 million, sees a serious real-life purpose. One day, military planners say, the hoverboard could be a battlefield trump card, and French special forces are working closely with the French inventor, a former jet-ski world champion, to fine-tune his machine.

On Sunday, Mr. Zapata took 20 minutes to fly the 18 miles that separate Sangatte, near Calais in France, to St. Margaret’s Bay in Kent, England. Standing atop a small platform fitted with five jet engines, setting his direction by leaning and his speed with a remote control, he traveled at up to 80 miles an hour about 50 feet above the water.

He stopped once to refuel on a boat positioned halfway across the channel, donning a new backpack full of the kerosene that powered the engines.

On July 14, at the Bastille Day military parade in Paris, Mr. Zapata stole the show as he hovered and swooped above the crowd, brandishing a rifle. “Proud of our army, modern and innovative,” tweeted President Emmanuel Macron alongside video he posted of the Flyboard Air in action.

Michel Spingler/AP Franky Zapata sets out from Sangatte, France, on Aug. 4. Mr. Zapata’s first attempt to traverse the English Channel on a flying board 10 days earlier failed when he crashed into a refueling boat 12 miles into the trip.

Defense Minister Florence Parly was equally taken by the machine. “Innovation is not a gimmick.” The hoverboard could serve as “a flying logistics platform, or an assault platform,” she suggested, conjuring an image of squadrons of individually airborne infantry descending on an enemy.

But not tomorrow. “As it stands, the Flyboard Air has no operational use,” says Marion Laguës, spokeswoman for the French Defense Innovation Agency, a branch of the General Directorate for Weaponry, which is helping to finance the development of new, quieter jet engines.

“That’s because it has limited autonomy, it is hardly inconspicuous, and it is difficult to fly,” she says. “But it prefigures new uses,” Ms. Lagues adds. “It is the development prospects that are interesting, and the Ministry of Defense is studying the kinds of mission that this sort of aircraft might carry out.”

Whether the Flyboard Air is indeed an aircraft or not is a question that nearly wrecked the whole project. French civil aviation bureaucrats grounded Mr. Zapata in 2017 because he had no pilot’s license and his experimental machine had not been certified.

It was then that the inventor approached French special forces (though American military equipment manufacturers are reported to have tried to tempt him to work in the United States). The ties he developed with the French military persuaded the civil aviation authority to give him a special dispensation for the Flyboard Air.

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But that leeway has not been extended to the Ezfly, the more manageable, commercial version of Mr. Zapata’s hoverboard, which resembles an airborne Segway. Even as a rich man’s toy, the Ezfly is not available for sale while Mr. Zapata and the authorities argue about how exactly to categorize what it is that he has invented.

Sometimes being a murderous supervillain has its advantages; the Green Goblin never had to put up with that kind of bureaucratic obstruction.