The U.S. Air Force has delayed the launch date of its next Boeing Wideband Global Satcom (WGS) satellite owing to additional work needed to mitigate possible risk discovered in the last Delta IV rocket launch, during which the RL10B-2 upper stage engine malfunctioned.

The WGS launch was slated for February. It is set to take place no earlier than March 28, says U.S. Air Force Lt. Connie Dillon, a spokeswoman for the service’s Space Command.

At issue is how to avoid a repeat of the anomaly that occurred during the Oct. 8 launch of the third Boeing GPS IIF when the Pratt & Whitney Rocketdyne RL10B-2 upper stage engine experienced reduced thrust. The problem is linked to a fuel leak, though investigators have not yet found a root cause or, as a result, a way to ensure the problem will not happen again.

Air Force Gen. William Shelton, Space Command chief, took the unusual step of convening a formal accident investigation board on Oct. 12. Though there was no materiel loss – the satellite made it to orbit – the RL10 is the upper stage used for both the Atlas V and Delta IV fleet. And, he was worried that a problem with the engine would be a single point of failure, potentially grounding a host of satellites awaiting launch on the United Launch Alliance Evolved Expendable Launch Vehicle (EELV) family. “Our goal is to return the entire EELV fleet to operations, but we still have work to do to complete the analysis and determine the root cause of the anomaly,” Dillon says.

Thus far, investigators have compiled a “list of credible scenarios” and developed a “series of tests required to confirm findings and the credibility of the proposed scenarios,” Dillon says. Additional information on these scenarios and tests was not provided because the investigation is ongoing.

Despite the investigation, not all EELV operations have come to a stop. The Air Force is allowing operators to evaluate the risk associated with each launch and move forward if they accept that risk.

This was the case for the Dec. 12 Atlas V launch of the Air Force’s X-37B Orbital Transfer Vehicle. NASA is planning to go ahead with an Atlas V launch of the Tracking and Data Relay Satellite as soon as Jan. 29.

However, it seems that the willingness of operators to continue Atlas V launches points to a reduced risk of using the RL10A, the engine variant on the launch vehicle.

The two upper stage engines share much of the same hardware. But the RL10B-2, used for the Delta IV, has unique performance requirements. The RL10A provides lower thrust because of the improved performance of the Atlas V rocket itself. The RL10B-2, by contrast, provides about 10% higher thrust and employs a larger composite nozzle extension.

Dillon says that “applicable mitigations will be evaluated for both RL10 variants.” And, that “potential mitigation actions for future launches continue to evolve with the investigation.”