Three weeks after the catastrophic loss of a Falcon 9 v1.1 booster, which exploded just 139 seconds after leaving Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla.—dooming the seventh dedicated Dragon cargo ship, bound for the International Space Station (ISS)—SpaceX CEO Elon Musk described the status of his company’s ongoing investigation on Monday, 20 July, and revealed that a failed helium tank strut appeared to be the root cause. Over the past weeks, engineering teams have spent thousands of hours matching up data across Falcon 9 v1.1 systems down to the millisecond, in order to properly understand the event which befell their vehicle, within a period of just 0.893 seconds, prior to loss of telemetry. In summarizing the developments, Mr. Musk noted that the accident occurred rapidly, that software modifications might have saved its precious Dragon payload, and that the failure is not expected to impair SpaceX’s goal of launching its first Commercial Crew missions from 2017 onward. At the same time, he added that the Hawthorne, Calif.-based launch services company does not expect to fly another Falcon 9 v1.1 until September 2015, at the soonest.
As described in a previous AmericaSpace article, the loss of the CRS-7 mission on 28 June shattered an impressive track record of 18 successful launches by SpaceX since June 2010, which have delivered a wide range of payloads into low-Earth orbit, Geostationary Transfer Orbit (GTO), and further afield, to the L2 Lagrange Point. Thirteen successful missions have been flown by the upgraded Falcon 9 v1.1 since September 2013. This variant of the two-stage booster benefits from the enhanced Merlin 1D powerplant—whose nine first-stage engines are capable of 1.3 million pounds (590,000 kg) of propulsive yield and whose restartable Merlin 1D Vacuum second-stage engine generates 180,000 pounds (81,600 kg) of thrust—and can deliver up to 28,990 pounds (13,150 kg) into low-Earth orbit and up to 10,690 pounds (4,850 kg) into GTO. Key payloads launched successfully to date by the v1.1 have included four ISS-bound Dragons, the Deep Space Climate Observatory (DSCOVR) to the L2 Lagrange Point, and six telecommunications satellites into Geostationary Transfer Orbits at an approximate altitude of 22,300 miles (35,600 km).
However, the heart was torn from SpaceX on 28 June, when the 14th Falcon 9 v1.1 vanished into a fireball, just 139 seconds after liftoff, dooming not only the CRS-7 Dragon cargo ship—the seventh dedicated mission under the terms of a $1.6 billion Commercial Resupply Services contract with NASA, signed back in December 2008—but also the first of two critical International Docking Adapters (IDA-1). These adapters, the second of which remains earmarked to fly aboard the CRS-9 Dragon mission, were intended to be installed onto the forward-facing and space-facing (or “zenith”) ports of the space station’s Harmony node, thereby providing primary and backup docking interfaces for Boeing’s CST-100 and SpaceX’s own Dragon V-2 piloted vehicles from mid-2017 onward. It is understood that IDA-2 will now be repurposed for the former IDA-1 role, whilst a set of structural spares will be used as the basis for the construction of IDA-3 to fulfil the original IDA-2 role. Although it is expected that IDA-3 will ride a Falcon 9 v1.1 uphill, it remains to be seen when that launch will now take place.
Speaking earlier today, in a media teleconference, SpaceX CEO Elon Musk explained that a support strut—measuring about 2 feet (60 cm) in length and about an inch (2.5 cm) in thickness at its widest point—appeared to be the root cause of the failure. Developed by a supplier which Mr. Musk refused to name, the strut’s role was to support one of the composite over-wrapped helium tanks in the Falcon 9 v1.1’s second stage. Designed to handle up to 10,000 pounds (4,530 kg) of force, the strut actually failed at just 2,000 pounds (900 kg), barely a fifth of its rated strength, catastrophically releasing helium into the second stage Liquid Oxygen (LOX) tank at about 3.2 G. “The pressurization system itself was performing nominally, but with the failure of this strut, the helium system integrity was breached,” SpaceX explained in a news release Monday evening. “This caused a high-pressure event inside the second stage, within less than one second, and the stage was no longer able to maintain its structural integrity.” It is understood that the struts will no longer be used on future missions and that SpaceX plans to “implement additional hardware quality audits throughout the vehicle” to ensure that “all parts received perform as expected per their certification documentation.”
Subsequent inspection of pre-flight images of the strut revealed no apparent trace of damage. “No evidence of assembly errors in the strut in hi-res closeout photos taken before launch,” Mr. Musk stressed, adding that SpaceX engineers will now move toward individual testing of the struts. He noted that the accident unfolded rapidly, with the first indication of a problem through to the structural breakup spanning just 0.893 seconds, and described the circumstances of the CRS-7 accident as a “really odd failure mode.”
These findings are in keeping with early speculation on 28 June, which highlighted that first-stage performance was entirely nominal and that the CRS-7 Dragon spacecraft continued to communicate, until it passed over the horizon after the failure. Mr. Musk pointed out that, in spite of no other issues with the launch, SpaceX was looking at other issues, admitting that the company may have become complacent in recent years. “Rockets are a fundamentally difficult thing,” he explained, adding that no further Falcon 9 v1.1 launches are anticipated before September 2015 at the earliest, with the maiden flight of the mammoth Falcon Heavy—which, when operational, carries the potential to deliver up to 117,000 pounds (53,000 kg) into low-Earth orbit and up to 46,700 pounds (21,200 kg) into Geostationary Transfer Orbit—pushed back to no sooner than March 2016.
This is an intense pity, for SpaceX accomplished five flawless Falcon 9 v1.1 missions in the first four months of 2015, together with the highly successful Pad Abort Test (PAT) of its crewed Dragon vehicle in May. The company has delivered a pair of Dragons—CRS-5 in January and CRS-6 in April—to restock the ISS, despatched the DSCOVR spacecraft to the L2 Lagrange Point in February, and lofted the Eutelsat 115 West B, ABS-3A and TurkmenÄlem52E/MonacoSat communications satellites into Geostationary Transfer Orbit in March and April. Had the loss of CRS-7 not occurred, as many as eight more missions were timetabled for the second half of 2015, including a further two Dragons to deliver the Bigelow Expandable Activity Module (BEAM) and the IDA-2 docking adapter to the space station, together with the Jason-3 ocean surface altimetry mission, several commercial payloads, and the maiden voyage of the Falcon Heavy.
In spite of the disappointment, Mr. Musk does not expect the CRS-7 loss to significantly impact the Commercial Crew timeline, describing it as being “not on the critical path,” and kindling some hope that SpaceX may still be in a position to launch its first uncrewed Dragon as soon as the fall of 2016 and a piloted mission several months later. He added that had the “right software” been aboard CRS-7, the Dragon and its payload might have been salvaged. “The Dragon spacecraft not only survived the second-stage event,” a subsequent SpaceX news release stated, “but also continued to communicate until the vehicle dropped below the horizon and out of range.”
Software to permit the deployment of parachutes in the event of a launch failure will be included aboard the CRS-8 Dragon mission, which was originally scheduled for early September, but whose target date presently remains in flux. Although Mr. Musk hinted at a No Earlier Than (NET) target of September for the next Falcon 9 v1.1 mission—with no information yet on whether it will carry Jason-3, CRS-8 or another payload—he was adamant that precise return-to-flight dates would only be issued after all data had been fully analyzed and properly understood. What is clear, though, is that lost revenues from this failure are expected to be “meaningful,” stretching into the hundreds of millions of dollars. “Our investigation is ongoing until we exonerate all other aspects of the vehicle,” SpaceX stressed, “but at this time, we expect to return to flight this fall and fly all the customers we intended to fly in 2015 by end of year.”
The author would like to express thanks to AmericaSpace’s Michael Galindo, for his support in the preparation of this article.