The U.S. military grounded all of its new F-35 Joint Strike Fighters after an incident on June 23, when one of the high-tech warplanes caught fire on the runway of a Florida air base. The no-fly order began on the evening of July 3 and continued through July 11.

All those F-35s sitting idle could be a preview of a future in which potentially thousands of the Pentagon's warplanes can't reliably fly.

To be fair, the Pentagon routinely grounds warplanes on a temporary basis after accidents and malfunctions to buy investigators time to identify problems and to give engineers time to fix them.

But there's real reason to worry. The June incident might reflect serious design flaws that could render the F-35 unsuitable for combat.

For starters, the Lockheed Martin-built F-35 — which can avoid sensor detection thanks to its special shape and coating — simply doesn't work very well. The Pentagon has had to temporarily ground F-35s no fewer than 13 times since 2007, mostly due to problems with the plane's Pratt & Whitney-made F135 engine — in particular, with the engines' turbine blades. The stand-downs lasted at most a few weeks.

Pratt & Whitney has totally redesigned the F135 in an attempt to end its history of frequent failures. But there's only so much engineers can do. In a controversial move during the early stages of the F-35's development, the Pentagon decided to fit the plane with one engine instead of two. Sticking with one motor can help keep down the price of a new plane. But in the F-35's case, the decision proved self-defeating.

That's because the F-35 is complex — the result of the Air Force, Marines and Navy all adding features to the basic design. In airplane design, such complexity equals weight. The F-35 is extraordinarily heavy for a single-engine plane, weighing as much as 35 tons with a full load of fuel.

By comparison, the older F-15 fighter weighs 40 tons. But it has two engines. To remain reasonably fast and maneuverable, the F-35's sole engine must generate no less than 20 tons of thrust — making it history's most powerful fighter motor.

All that thrust results in extreme levels of stress on engine components. It's no surprise, then, that the F-35 frequently suffers engine malfunctions.

In 2008, two analysts at the RAND Corp. programmed a computer simulation to test out the F-35's fighting ability in a hypothetical air war with China. The results were startling.

“The F-35 is double-inferior,” John Stillion and Harold Scott Perdue concluded in their written summary of the war game, later leaked to the press. The new plane “can't turn, can't climb, can't run,” they warned.

Yet the F-35 is on track to become by far the military's most numerous warplane. It was designed to replace almost all current fighters in the Air Force and Marine Corps and complement the Navy's existing F/A-18 jets.

The Pentagon plans to acquire roughly 2,400 of the radar-evading F-35s in coming decades, at a cost of more than $400 billion.

Minor fixes might get America's future warplane flying again soon — for a while. But fundamental design flaws could vex the F-35 for decades to come, forcing the Pentagon to suspend flying far too often for the majority of its fighter fleet, potentially jeopardizing U.S. national security.

David Axe is the national security editor at Medium.com.

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