One year after Orbital ATK’s Antares rocket suffered a launch failure that destroyed their third commercial resupply mission to the International Space Station, NASA’s Independent Review Team has published their official accident investigation report. Over the last year, NASA has worked alongside Orbital ATK and the Federal Aviation Administration (FAA) to determine the cause of the accident and give recommendations for avoiding future mishaps. Due to the proprietary nature of the engineering data used in the investigation and International Traffic in Arms Regulations (ITAR), the full report is not available to the public. However, NASA was able to publish an executive summary of the report, which outlines the conclusions of their investigation.
Immediately after the accident, NASA set up the Independent Review Team with approval from the FAA in order to carry out an independent analysis of the accident and compare results with Orbital ATK’s internal Accident Investigation Board. Orbital ATK gave NASA access to all their telemetry, launch imagery, and physical evidence so they could determine the root cause of the accident.
The most visible cause of the launch failure was the explosion of one of the two Aerojet AJ26 engines that powers the first stage of Antares. The explosion originated in the liquid oxygen turbopump of engine E15, damaging the entire main engine assembly and causing Antares to fall back to the ground. Further evidence pointed to a mechanical failure in the turbopump’s hydraulic balance assembly, which created friction and started a liquid oxygen fire. The exact cause of this failure could not be determined, but NASA narrowed it down to three possibilities which will all be addressed.
One possible root cause is that the bearings in the AJ26 turbopump were not capable of withstanding the operational loads experienced in flight on Antares. Due to sensitivities in the AJ26’s turbopump design, the hydraulic balance assembly is especially prone to liquid oxygen fires. Because of a lack of thorough qualification testing, the engine’s performance margins for Antares were not fully understood. The Orb-3 mission was carrying a heavier payload than previous Antares flights, so its engines were throttled up to 108 percent rated thrust to accommodate the additional mass. Without fully understanding the turbopump’s design limitations, Orbital ATK could have simply pushed the engine too far.
A defect in the liquid oxygen turbopump could have also caused the failure. Both Orbital ATK and NASA discovered a manufacturing defect that was introduced before the AJ-26 engines underwent acceptance testing. This defect was present in an engine that failed similarly in a hot-fire test at NASA’s Stennis Space Center in May 2014, but it was never determined whether the defect contributed to that failure. Aerojet’s original test engine from 1998 showed signs of a similar defect, which never affected ground tests and could have been caused by wear from extended operation. While it is unclear whether such a defect contributed to the explosion, it is certainly a probable cause.
The evidence that sparked the most controversy in the investigation was the detection of foreign object debris in the recovered components of engine E15. Particles of titanium and silica were found to be present in the engine sometime before the rocket’s impact. Aerojet used this evidence to claim that the accident was the result of negligent vehicle processing by Orbital ATK. However, NASA did not find particle impact damage in the recovered components, suggesting that there likely wasn’t a dangerous amount of debris within the engine. Aerojet has since dropped their allegations against Orbital ATK. In September, Aerojet agreed to pay a $50 million settlement to Orbital ATK over the accident. While it appears unlikely that foreign object debris caused the accident, NASA believes it still could have been a contributing factor.
Both Orbital ATK and NASA agree that the best technical solution for returning Antares to service in a timely manner is to simply replace the AJ-26 with a more reliable engine. Antares has been modified to use the DR-181 engine from NPO Energomash, which is a single-chamber variant of the RD-180 engine that powers the United Launch Alliance Atlas V. In the future, Orbital ATK must also install better sensors in the engines to improve the quality of telemetry during acceptance testing and Antares flights. They will also take extra measures to prevent foreign debris and moisture from infiltrating engines during processing.
In addition to investigating the technical causes of the accident, NASA also addressed organizational issues that impacted risk management in the CRS program and the accident investigation. Because so much information about the launch vehicle and engines is proprietary, communication barriers were present leading up to the accident and during the investigation. Both Aerojet and Orbital ATK had insufficient knowledge of the design and operational history of the Russian NK-33 that the AJ-26 began its life as. This resulted in overly optimistic risk assessments which gave NASA a false sense of security. NASA criticized Orbital ATK for accepting greater launch vehicle risk over time and put much of the blame on the lack of partnership between Orbital ATK and Aerojet.
Orbital ATK will learn on Nov. 5 whether Cygnus will continue delivering cargo to the ISS for years to come. NASA will be announcing the CRS-2 contract winners, which will deliver cargo to the ISS through 2024. Orbital ATK is competing with SpaceX, Sierra Nevada, Boeing, and Lockheed Martin for two available contracts.
Meanwhile, the enhanced Cygnus is being readied in Cape Canaveral, Fla., for the cargo craft’s return to flight. The OA-4 mission is on schedule to launch on a ULA Atlas-V 401 rocket on Dec. 3 to the International Space Station for NASA and Orbital ATK, while back on Wallops Island repairs are now complete at the Antares launch pad. The first pair of RD-181 engines have already been integrated with an Antares core stage, which is on track for a hot-fire test at Wallops in January.
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