SpaceX Traces Weak Strut to Falcon 9 Failure
[Via Satellite 07-21-2015] After reviewing data consisting of more than 3,000 telemetry channels from the June 28 Falcon 9 rocket failure, SpaceX has identified the probable cause as a defective strut in the second stage. Going forward, SpaceX intends to replace the strut entirely, though thousands of them have flown without issue on previous missions.
On a July 20 conference call, SpaceX CEO and Lead Designer Elon Musk told reporters that the investigation is still ongoing, but after vetting other possibilities the company’s preliminary conclusion is that a strut used to hold a helium bottle in place broke, releasing the bottle into the second stage and rapidly compromising its integrity, leading to the overpressure event that ultimately upset the rocket. The strut was supposed to withstand up to 10,000 pounds of force, but failed around 2,000 pounds, despite undergoing testing while on the ground.
Musk said the helium bottles in the liquid oxygen tank on both stages of the Falcon 9 are meant to withstand high pressure during launch, and are typically at about 5,500 pounds per square inch (psi). As the rocket burns through its fuel, helium is used to replace its volume and structurally stabilize the stage. SpaceX believes that when the strut failed, the unrestrained helium bottle inside the second stage shot to the top of the tank due to the buoyancy of 3.2 times Earth’s gravity it experienced during that segment of the flight. In less than a second, this led to an overpressure event that obliterated the vehicle. The Dragon capsule initially survived the incident, but Musk said the spacecraft remained in an inert mode during ascent and did not deploy parachutes that could have saved it. Once Dragon hit the surface of the ocean, it, too, was destroyed. SpaceX is adding software to Dragon V1 to deploy parachutes should they be needed in the ascent phase again. Dragon V2 will have the parachute software as well as built-in thrusters.
Musk emphasized that this conclusion is preliminary, and highlighted more research is ongoing to confirm the cause with confidence. SpaceX has deployed a submersible vehicle to search the bottom of the ocean for any debris that can contribute to the understanding of the anomaly. Immediately following the mishap, the vehicles positioned to monitor an attempted landing of the first stage were repurposed to locate debris as well, from which SpaceX retrieved some fragments, but the majority of the wreckage remains underwater. Still, the company was able to reproduce the strut failure through its own research, adding weight to this conclusion.
“We have been able to replicate the failure by taking a huge sample of these — potentially thousands of these struts — and pulling them and we found a few that pulled far below their specificated level, and that’s what led us to think that there essentially was one that was just far below its rated capability,” said Musk.
SpaceX is known for building the majority of its rockets in-house, but the strut that failed came from a separate provider. Each Falcon 9 uses hundreds of these struts. Musk said SpaceX would not name the manufacturer to circumvent their vilification, but confirmed that SpaceX intends to find a new supplier of this component. Furthermore, the launch provider will personally test each strut on its own rather than put its faith in material certification, as it did before.
“Addressing the strut is pretty straightforward,” said Musk. “We are switching to something which has an even higher margin of safety than what we had, which already was very high.”
SpaceX evaluates the struts by firing the rocket stages on the ground prior to launch, but Musk said the environment on the ground compared to in atmospheric ascent could have masked any issue. In the company’s post-failure analysis, Musk said the lowest point at which a strut failed was at first, 6,000 pounds, but after testing an “enormous number” they found one that was overwhelmed at a mere 2,000 pounds. The increased testing will result in some cost increase for the Falcon 9, but Musk said the price for customers should stay the same. SpaceX does not plan to build the struts internally as a result of the sullied object.
“While SpaceX does all of the major components in house, there are hundreds of suppliers that supply minor components. It would be very difficult for us to in-source all of those,” Musk explained.
NASA and the U.S. Air Force participated in the SpaceX-led investigation, for which the Federal Aviation Administration (FAA) provided oversight. The Falcon 9’s second stage failed 138 seconds into the Commercial Resupply Services (CRS) 7 mission, and SpaceX reported it had 0.893 seconds between the first indication of an issue to loss of all telemetry. Musk said SpaceX briefed the U.S. government and, within the limits of International Traffic and Arms Regulations (ITAR), commercial customers last week.
One peculiarity found in the telemetry data was a drop in helium pressure followed by a rise back to starting pressure. Musk described this event as confusing, and that a plausible conclusion is that, as the helium bottle broke free it may have pinched off the line to the helium manifold. This could have restored pressure, but released enough helium to cause the tank to fail.
Going forward, Musk said the Falcon 9 rocket is tentatively expected to return to flight no sooner than September, and that the Falcon Heavy — originally anticipated to make its debut launch this year — will have to wait until 2016. He added that it is not clear which customer will be first to fly with the Falcon 9 rocket when it returns. In a written statement, SpaceX said the company still anticipates flying all the customers originally intended to fly in 2015 by the end of year.
Musk said SpaceX may have gotten too comfortable with the cycle of launching rockets as the company grew, and that it has learned a valuable lesson from the failure. The last major rocket failure the company experienced was with the Falcon 1 about seven years ago. At the time, Musk said the number of total SpaceX employees came to about 500. Today it is approximately 4,000.
“The vast majority of the people at the company today had only ever seen success. And when you’ve only ever seen success, obviously you don’t fear failure quite as much,” he commented.
Musk said he sends a company-wide email out ahead of every flight urging anyone who has reason to believe they should hold off on launching to contact him directly. By the 20th time, he said, it simply did not resonate with the same force.
“I think now everyone at the company appreciates just now difficult it is to get rockets to orbit successfully and I think we’ll be stronger for it,” he said.