Faulty Strut Likely Caused SpaceX Rocket Explosion

A steel strut holding a bottle of helium likely gave way, causing the liquid oxygen tank in the upper-stage of a SpaceX Falcon 9 rocket to over-pressurize, triggering an explosion minutes after launch last month, company chief executive Elon Musk said Monday.

After poring over thousands of bits of engineering data, analyzing video and studying wreckage salvaged after the accident, a materials defect is the leading cause of the June 28 failure. The accident claimed a load of cargo heading to the International Space Station.

GALLERY: How to Fly Rockets Back to Earth

"It's a really odd failure mode," Musk told reporters on a conference call.

Engineers tested thousands of the struts and found a few that failed far below the force they were designed -- and certified -- to withstand.

"We have been able to replicate the failure by taking a huge sample, essentially thousands of these struts, and pulling them. We found a few that failed far below their certificated level. That's what led us to think that there was one just far below its rated capability that happened to be at the wrong place at the wrong time," Musk told Discovery News.

PHOTOS: Dragon's Bounty: SpaceX Mission Complete

Over the course of 18 successful Falcon 9 flights, thousands more struts were flown, apparently without issue.

As a result of the accident, SpaceX will look for a new strut design, most likely from a new vendor, and test each one before they are installed in the rocket's tanks.

Flights are not expected to resume until September at the earliest, Musk added.

The work also will delay the debut flight of Falcon Heavy, a 27-engine version of the Falcon rocket, from late this year until next spring.

NEWS: SpaceX Commercial Crew Will Have Launch Escape Route

The accident investigation remains underway, with the company casting a wide net to consider other issues and contributing factors to the June 28 accident, as well as potential concerns for future missions.

One lesson learned: Future Dragon cargo ships will have new software so that if a launch accident occurs, the capsules' parachutes will deploy. The upgrade already was planned for the passenger version of Dragon, which SpaceX is developing in partnership with NASA to fly astronauts to and from the International Space Station.

"If the software had initiated the parachute deployment then the Dragon spacecraft would have survived," Musk said. "We're now including contingency software that if something were to go wrong with the vehicle, Dragon will always attempt to save itself."

The launch of the Falcon 9 rocket appeared to be progressing as normal, but at 2 minutes 19 seconds into the flight on June 28, a launch anomaly caused the vehicle to break apart.

When Dragon Met Harmony May 26, 2012 -- On Saturday morning at 5:53 a.m. EDT, astronauts aboard the International Space Station unlocked the Dragon hatch and ventured into the first commercial spacecraft in history to be attached to the orbiting outpost. The Dragon, built and operated by the private space company Space Exploration Technologies (or SpaceX), was launched on May 22 and successfully carried out a complex series of orbital maneuvers before berthing with the space station on May 25. Throughout Dragon's rendezvous with the Harmony module of the station, European astronaut and flight engineer André Kuipers -- who is currently on board the space station as a member of Expedition 30/31 -- captured some key moments in high-resolution through the lens of his camera. Here are the highlights.

Dragon Over Namibia During one of the Dragon's "fly-unders," Kuipers snapped the private spacecraft as it passed over the Namibian desert.

10 Meters and Closing The Dragon moves toward the station's Canadarm2 robotic arm.

Dragon Grab On Friday at 9:56 a.m. EDT, NASA astronaut Don Pettit radioed Mission Control in Houston, Texas, to say: "Looks like we've got us a Dragon by the tail." Using the space station's Canadarm2, Pettit grappled the Dragon to begin the gentle process of berthing the unmanned cargo capsule with the station's Nadir port on its Harmony module.

Sci-Fi Spaceship "Like this it looks a bit like a model from a 70's sci-fi film," Kuipers remarks on his Flickr page.

Teamwork in the Cupola NASA flight engineer Don Pettit (front) and André Kuipers work in the space station's Cupola during the Dragon berthing.

Dragon Glow The Dragon capsule and Canadarm2 reflect lights from the space station as sunlight catches the Earth's horizon.

On Target The docking camera shows the Dragon's hatch as it nears berthing.

Berthed! Kupers: "And the Dragon is in its lair! Task accomplished."

"The gate to the Dragon's lair" Kuipers' reflection can be seen the Dragon's hatch window.

"Of course it is from Los Angeles." Once the hatch was opened on Saturday morning, Don Pettit remarked that the Dragon's interior smelled "like a brand new car." Kuipers went even further, saying: "Inside of the Dragon module. Beautiful. Spacious, Modern. Blue LEDs. Feels a bit like a sci-fi filmset. Of course it is from Los Angeles." Now the Dragon is attached to the station, the astroauts will unload the cargo and then reload the capsule with equipment and experiments to be returned to Earth. Undocking is scheduled for May 31 before the capsule will reenter and splash-down off the coast of California.

For more incredible space station photographs and high-resolution versions of the photos shown here, be sure to browse André Kuipers' Flickr photostream.

MORE: Space Station Astronauts Log One Million Photos

MORE ARTICLES BY IAN O'NEILL