ULA releases OA-6 launch anomaly report, clears Atlas V for next flight
When United Launch Alliance’s (ULA) Atlas V 401 rocket sent the OA-6 Cygnus cargo vessel, the S.S. Rick Husband, to the International Space Station (ISS) on March 22, 2016, mission controllers detected an anomaly as the rocket headed to low-Earth orbit with its several tons of important supplies and equipment for the astronauts aboard the station.
The aerospace company has now released a report on what occurred with that particular Atlas V launch. ULA also ensured the rocket is now ready for its next space launch on June 24. That booster will launch the U.S. Navy’s fifth Mobile User Objective System (MUOS-5) satellite, which was delayed more than a month due to the anomaly.
As the first stage of the Atlas V underwent its planned burn during the OA-6 Cygnus mission, an oxidizer-rich blend of fuel flowed into the RD-180 main engine. That caused the first stage to run out of liquid oxygen and shut down about six seconds prematurely.
“The Atlas V’s robust system design, flight software, vehicle margins and propellant reserves enabled the successful outcome for this mission,” ULA stated in a June 15 press release. “The Centaur upper stage identified the first stage performance shortfall and compensated with an extended burn to deliver Cygnus to the precise orbit, well within the required accuracy.”
The ULA engineering team, their engine supplier, and several government customers formed a team to review the flight and operational data to determine direct and root causes of the incident and implement corrective actions for future flights.
The RD-180 engine propelling the Atlas V rocket off the launch pad to the ISS was supposed to burn for four minutes and 16 seconds. According to Spaceflight Now, the rocket telemetry revealed, at 3 minutes and 42 seconds into the flight, an unexpected shift in fuel pressure differential across the RD-180 Mixture Ratio Control Valve (MRCV). This valve caused a reduction in fuel flow to the combustion chamber, which in turn led to the inappropriate propellant mixture and the premature rocket firing cutoff.
“Thanks to the robust vehicle system design of Atlas V, the OA-6 mission was delivered successfully to its intended orbit and Cygnus completed its mission to the ISS,” said Gary Wentz, ULA’s vice president of Human and Commercial Systems. “ULA applied a rigorous anomaly review process to identify and implement all necessary corrective actions to ensure we continue to reliably deliver critical capabilities for our customers with 100 percent mission success.”
Since the March 22 launch, a “minor” change to the MRCV assembly has been implemented. Details about the valve change are proprietary.
“We would like to thank our customers and supplier partners for their outstanding collaboration in the detailed review of this anomaly. We are honored to be entrusted with delivering America’s most critical space assets to orbit and appreciate the outstanding team effort in delivering 100 percent mission success—one launch at a time,” said Laura Maginnis, ULA’s vice president of Custom Services. “We remain on plan to launch all of our manifested 2016 missions within the year.”
When MUOS-5 is launched next week, it will be delivered to geostationary orbit from Cape Canaveral Air Force Station’s Space Launch Complex 41. The tasks of the military satellite will be to provide clear and secure communications for mobile military forces around the globe.
Other space vehicles set to be lofted into the heavens atop Atlas V rockets in 2016 include the OSIRIS-REx planetoid sample return mission on Sept. 8 and the GOES-R meteorology satellite on Nov. 4.
Larry Klaes is an author and freelance journalist specializing in news and educational work on the sciences. Klae's past endeavors include editor of SETIQuest magazine and President of the Boston chapter of the National Space Society (NSS). Klaes joined SpaceFlight Insider in 2016.