A mysterious, strange-looking experimental airplane has appeared at an airfield in Southern California.

The sleek, egg-shaped passenger plane with the rear-mounted propeller engine burning diesel fuel is the Celera 500L. Its secretive inventor, the physicist William Otto, claims that the Celera 500L could be nearly an order of magnitude more fuel-efficient than current equivalent aircraft. And that it could transform commercial aviation.

Experts, however, are skeptical. After 100 years of aviation development, it’s getting harder and harder to squeeze a few extra miles per gallon out of any airplane, however exotic it might be.

The Celera 500L first appeared in public in 2017 at the Southern California Logistics Airport, a former U.S. Air Force base in Victorville, California. Two years later the bulbous plane, which is roughly the size of a six-passenger business jet, reappeared with a few minor modifications.

In May a passing pilot observed the Celera 500L taxiing on the ground, perhaps in preparation for flight tests some time this year. Sightings of the Celera 500L caused a minor buzz among aviation nerds on Twitter.

A few news organizations questioned the plane’s purpose and compared it to the similar-shaped Bell X-1, the rocket-propelled plane in which Chuck Yeager broke the sound barrier for the first time, in 1947.

Otto, the president of California-based Otto Laboratories, developed the Celera 500L under the auspices of his company Otto Aviation. And he apparently isn’t keen to talk about it. Otto didn’t respond to an email seeking comment for this story.

But patent documents and the Celera 500L’s federal registration help to reveal Otto’s thinking. He wants to fundamentally change the way everyday Americans fly. “The present invention was borne out of frustration with the cost and inefficiency of the airlines’ hub-and-spoke transportation model,” Otto wrote in one patent.

In the hub-and-spoke system, passengers fly on smaller planes along “spoke” routes to one of a fairly small number of large “hub” airports, where they transfer to larger planes for onward travel to a hub airport near their destination.

Generally speaking, hub-and-spoke operations connect small airports and their small airplanes to large airports with large airplanes. The system “requires dense concentrations of passengers both at the relatively few hub facilities and in ever larger aircraft flying to fewer and fewer destinations,” Otto wrote.

“The inefficiencies for the traveler arise out of the time wasted traveling long distances from their true origin to the large hub or major airport, enduring the lengthy lines at check-in and security checkpoints, and the ever-longer boarding process on the ever larger aircraft,” he continued. “In addition, the traveler must often fly to cities that are well out of the way to his final destination, and transfer with additional wasted connection times.”

In theory, a very efficient, small airplane could connect practically any continental U.S. airport, however tiny, to any other continental airport, making big hub airports unnecessary. Otto wants the Celera 500L to be the first plane that can skip all the hubs.

But to fly directly from, say, the tiny regional airport in Columbia, South Carolina to, for example, Los Angeles, the Celera 500L would need to cover more than 2,000 miles on a single tank of gas. One of today’s most efficient small passenger planes, the single-propeller PC-12, can haul nine passengers a maximum distance of 1,700 miles.

The PC-12 burns conventional aviation gas, as opposed to diesel, and gets around five miles to a gallon, according to calculations that reporters Joseph Trevithick and Tyler Rogoway made at the website The War Zone. Otto wants the diesel-powered Celera 500L to get between 30 and 42 miles per gallon, a near order-of-magnitude improvement over the PC-12.

Juan Alonso, a professor in the Department of Aeronautics at Stanford University, has his doubts. A 30 percent improvement in fuel-efficiency is possible in an airplane with a new, more aerodynamic wing design, an ultra-light airframe made from high-tech composite materials and a super-efficient engine.

But an 800 percent improvement? “Unlikely,” Alonso told The Daily Beast. The Celera 500L’s rear-mounted propeller is a good choice for a more fuel-efficient plane, Alonso said. But the egg-like fuselage probably is less fuel-efficient than the narrower fuselages on planes such as the PC-12, he pointed out.

Mark Drela, an MIT aeronautical engineer, is equally skeptical. “The 500L looks fairly well-designed,” Drela told The Daily Beast, “but I cannot say whether it’s close to optimum, or whether the diesel engine makes sense.”

To say for sure, Drela said he’d need to know how much the Celera 500L weighs and how efficient its engine is. Since Otto isn’t talking, Drela can only guess. And he’s guessing that the Celera 500L isn’t nearly as efficient as Otto hopes it will be.

“Celera’s claim that it will have 10 percent of the gallons per mile compared to other aircraft with the same payload does seem difficult to square,” Drela said.

If and when the Celera 500L starts flying and Otto reveals hard numbers, we might finally know whether the physicist stands any chance of shattering America’s hub-and-spoke air-travel system.

“Clearly, there are compelling reasons for wanting an air-transportation system that is economically superior to our current air-transportation system in acquisition, operation and maintenance costs,” Otto wrote.

“Such a transportation system requires a unique aircraft,” he added. It’s not clear yet whether the Celera 500L is that aircraft.