Three weeks ago, on its seventh scheduled launch to resupply the International Space Station, SpaceX's Falcon 9 rocket inexplicably burst into flames less than three minutes into its flight. Today, Elon Musk finally announced what led to the explosion—sort of.

Emphasizing that their findings were only an “initial assessment,” Musk said that they believed that a broken strut—a two-foot-long piece of steel, an inch wide at its thickest point—was responsible for the rocket’s explosion, which had previously only been attributed to a "decompression event" in its liquid oxygen tanks. No one was injured, but the 4,000 pounds of cargo in the attached Dragon space capsule got blasted into smithereens.

By design, the faulty strut should have been able to take 10,000 pounds of force—well above the 3,500 pounds that the struts actually experience during a launch. But after screening “some enormous number” of them, SpaceX found that some could only withstand 2,000 pounds. The struts were made by an outside supplier that Musk wouldn’t name.

The original point of launching the rocket, in addition to resupplying the ISS, was to try to recover the Falcon 9 on a floating barge in the ocean so that SpaceX could reuse it. Commercial spaceflight companies like SpaceX want to find a way to reuse their rockets, dramatically decreasing the cost of space delivery and travel. (A NASA-funded study reported that using commercial spaceflight could make a trip to the moon up to ten times cheaper.)

SpaceX has never succeeded in recovering Falcon 9, this being the third time in the last eight months that the rocket has crashed or burned. But it's the only time that the company has failed to even deliver the ISS supplies (and the third time this year that a resupply mission has failed).

Falcon 9 is a two-stage rocket, which means that as it shepherds the the Dragon capsule into space, it ejects its weight in two steps. The suspected broken strut, which held helium bottles in place in its liquid oxygen tank, was part of Falcon 9’s second stage. A rocket uses liquid oxygen to burn its fuel1, and as the oxygen gets used up, something—in this case, helium—has to replace the consumed oxygen to maintain proper pressure in the oxygen tank. During launch, the strut snapped, making too much helium leak into the oxygen tank, which led to too much pressure in the oxygen tank, which led to...boom.

Musk says in addition to losing biomedical experiments and high school science projects, SpaceX probably lost hundreds of millions of dollars in revenue in the explosion. Plus, a US and European climate satellite called Jason-3, originally scheduled for launch on August 8, had to be postponed.

So how is SpaceX planning to recover from this one?

First, it will go after the obvious—Musk says the rocket will use a different strut, which the company's engineers will screen individually instead of trusting the guarantee on the label. In the event of another explosion, SpaceX is also rushing the installation of software on the capsule, Dragon, that will let it parachute back to Earth before the rocket explosion gets to the valuable contents in the capsule.

But Musk says that this explosion doesn’t affect the company's overall commercial timeline—SpaceX doesn’t have any planned commercial launches until after September. Keeping a game face, he used the catastrophe as a reminder that going to space, after all, is hard. “The fundamental nature of rocketry is that it is a case where the passing grade is 100 percent, every time,” he says. “From the moment of lift-off, it's 100 percent or nothing.”

Musk also blames overconfidence for the accident. The size of SpaceX increased from 500 employees to about 4,000 since their last failed launch seven years ago. “When you’ve only ever seen success, you don’t fear failure quite as much,” he says. Leave it to Elon to slip in some humble bragging while talking about one of his smoky failures lying on the ocean floor.

1UPDATE 7/30/15 6:50 PM This story originally referred to liquid oxygen as fuel. Oxygen is not the fuel; oxygen is needed to burn the fuel.