Key Excerpts From NASA’s Investigation into Antares Failure

The bottom of the Antares explodes right after liftoff.
The bottom of the Antares explodes right after liftoff.

NASA Independent Review Team
Orb–3 Accident Investigation Report

Executive Summary

Key Excerpts

The IRT performed detailed analysis and review of Antares telemetry collected prior to and during the launch, as well as photographic and video media capturing the launch and failure. Based on this analysis, the IRT determined that the proximate cause of the Antares launch vehicle failure was an explosion within the AJ262 rocket engine installed in the Main Engine 1 position. Specifically, there was an explosion in the E15 Liquid Oxygen (LO2) turbopump, which then damaged the AJ26 rocket engine designated E16 installed in the Main Engine 2 position. The explosion caused the engines to lose thrust, and the launch vehicle fell back to Earth and impacted the ground, resulting in total destruction of the vehicle and its cargo. Figure 3 shows a single AJ26 engine stored on its transportation and processing skid. Figure 4 shows the aft end of a typical Antares launch vehicle with both AJ26 engines installed.

The IRT also developed a detailed system-level fault tree, timeline of events, and failure scenarios, and performed analysis and forensic investigation of the hardware recovered from the accident. The IRT concluded that the cause of the explosion on launch was loss of rotor radial positioning resulting in contact and frictional rubbing between rotating and stationary components within the Engine LO2 turbopump Hydraulic Balance Assembly (HBA) seal package. This frictional rubbing led to ignition and fire involving LO2 within the turbopump HBA. This conclusion is consistent with the proximate cause determination made by the Orbital ATK AIB investigation findings. Figure 3 highlights the general location of the HBA within the turbopump, but further detail about the turbopump and HBA design is not provided due to proprietary restrictions.

Technical Root Causes

The IRT was not able to isolate a single technical root cause for the E15 fire and explosion. The IRT identified three credible technical root causes (TRCs), any one or a combination of which could have resulted in the E15 failure:

  • TRC-1: Inadequate design robustness of the AJ26 LO2 HBA and turbine-end bearing for Antares. After performing extensive technical design evaluation and a number of sensitivity analyses of the LO2 turbopump, it became apparent to the IRT that the HBA and thrust bearing designs have several intricacies and sensitivities that make it difficult to reliably manage bearing loads. As a result, this area of the turbopump is vulnerable to oxygen fire and failures. The AJ26 engines were not subjected to a thorough delta-qualification program to demonstrate their operational capability and margin for use on Antares. Performing a thorough delta-qualification program for Antares would likely have revealed these issues. Furthermore, the Acceptance Test Program (ATP) established for the AJ26 engines was not sufficient to test and screen the engines for these design sensitivities and potential workmanship issues that could exacerbate those sensitivities.
  • TRC-2: Foreign Object Debris (FOD) introduction to the E15 LO2 turbopump. Forensic investigation identified the presence of both titanium and silica FOD with in E15 prior to its impact on the beach. However, no firm conclusions can be drawn with respect to the quantity of FOD introduced to or already present within the engine prior to or at the time of the explosion. The lack of significant particle impact damage to the recovered impeller and other components indicates that there were not gross-levels of FOD present within the system. In addition, there is no clear forensic evidence that FOD directly or indirectly led to the E15 failure.
  • TRC-3: Manufacturing or other workmanship defect in the E15 LO2 turbopump. Forensic investigation performed by Orbital ATK and NASA discovered the presence of a defect on the turbine housing bearing bore that was not consistent with baseline design requirements.3 The investigation determined that the defect was introduced during machining of the bearing bore housing and was therefore present prior to the engine ATP and Antares launch for Orb-3. Forensic investigation of Engine E17, which failed during ATP in May 2014, discovered the presence of a similar non-conforming defect in the housing bearing bore. A limited number of other engine turbine housings (i.e., Engine E16 and the 1998 test engine) previously and successfully subjected to extended ground tests and ATP, as well as an untested spare turbine housing, were inspected. Neither E16 nor the spare housing showed any evidence of a similar manufacturing defect. However, the 1998 test engine that had been subjected to extensive ground testing exhibited a similar defect to that observed in Engines E15 and E17, but it was not possible to conclude whether the defect was introduced during manufacturing or was the result of wear from extended operation of the engine. Sufficient information is not available without further engine inspections and tests to conclude that the presence of this manufacturing defect would always result in failure of the engine during operation.

The IRT determined that all three of these technical root causes would need to be addressed as part of any return to flight efforts for Antares. The IRT concluded that the failure could have been the result of any one of the identified root causes above; however, the presence of the machining defect and FOD could have increased the possibility of failure associated with the engine design and Antares operating conditions.

Footnotes:

2 The AJ26 engine used for Antares is based on a core Russian NK-33 rocket engine designed and manufactured in the early 1970s in support of the Russian N-1 moon program. Aerojet-Rocketdyne modifies the NK-33 configuration for use on U.S. launch vehicles. For Antares, the AJ26 also includes several operational variations from the NK-33 operations originally intended for the N-1 program, such as but not limited to operation at a higher power level and engine gimballing.

3 Further information about the turbine bearing bore housing design and the location and configuration of the defect is not provided due to proprietary restrictions.