United Launch Alliance is preparing for its second rocket flight for the Air Force in just eight days, a Thursday evening blastoff from Cape Canaveral that returns the Delta 4 rocket to action carrying a military communications satellite to serve the Americas.

A Delta 4 rocket with four boosters will launch WGS 5. Credit: Justin Ray/Spaceflight Now

The Delta 4 fleet has been idle since last October when the launch of the GPS 2F-3 navigation satellite suffered a low-thrust condition on its upper stage RL10B-2 engine on the way to successfully reaching the proper orbit.

Although the seven-month inquiry into that close-call has not yet been closed, the Delta 4 rocket has been cleared for flight this week.

"Given the comprehensive investigation that included extensive analyses and engine testing, along with the mitigating actions that have been implemented, we have concluded that the risks have been mitigated and that it is safe to proceed with the WGS 5 launch," said Jim Sponnick, ULA vice president of mission operations.

The Air Force concurred with ULA on the decision to grant flight clearance for the engine to fly Thursday aboard the rocket to place the $342 million Wideband Global SATCOM 5 satellite into space for communications to military forces and civilian leaders.

"ULA completed a flight clearance assessment recently for the WGS 5 mission and our Air Force customer also assessed and approved flight clearance for this Delta 4 mission," Sponnick said.

During the Oct. 4 launch, a small fuel leak began at the moment the engine was lit, robbing the rocket of its expected top-level thrust settings and forcing the vehicle to improvise to overcome the anomaly during the flight.

A leak began when the upper stage was ignited for first time. Credit: United Launch Alliance

Launch officials were helpless to intervene as they monitored telemetry readings and wondered if the rocket would have enough fuel to reach the correct orbit during the tense, three-and-a-half-hour mission.

The first stage and its strap-on boosters had done their jobs during the morning blastoff, separating to leave the cryogenic upper stage to perform three firings to lob the 3,400-pound bird into an orbit 11,000 nautical miles up.

But as the RL10B-2 engine was ignited for the initial time and reached its peak chamber pressure, a leak started above the narrow throat portion of the thrust chamber, officials revealed in December.

The situation reduced the engine thrust output below the expected 25,000 pounds, causing the powerplant to burn longer to compensate and still achieve the proper orbit targets on its circuitous route into the GPS constellation.

It could have doomed some launches, but the coupling of the relatively light-weight GPS and the generous fuel margins on the Delta 4 allowed the flight to persevere.

In the wake of the anomaly, rocket-maker United Launch Alliance engine supplier Pratt & Whitney Rocketdyne convened a team to investigate what created the problem and prevent its reoccurrence.

Although that inquiry remains open, officials have instituted measures aimed at proving the particular engine flying Thursday will successfully deliver the Boeing-built WGS 5 satellite into the intended orbit.

"Based on the progress of the investigation, additional process and operational mitigations are in place for the WGS 5 mission. We are ready to launch," said Col. Ron Fortson, chief of the EELV generation operations division within the launch systems directorate at the Air Force's Space and Missile Systems Center.

Thursday's launch is scheduled minutes after sunset. Credit: Pat Corkery/United Launch Alliance

Extra inspections, officials said, looked for any signs of existing damage or foreign objects within the engine that could impact the mission.

"We've done additional inspections to make sure that the hardware is sound," Fortson said.

In addition, this and future Delta 4 launches will include in-flight helium purges to critical areas of the engine system and change how the engine is thermally conditioned during ascent to prepare for its initial ignition after first stage separation.

"We have done enough analysis and testing through the investigation and put mitigations in place that we're confident we are ready to fly WGS 5. But the investigation will continue to further understand the causes and further mitigations we can put in place to burn down the risk," Fortson added.

Engine testing conducted over the past few months replicated fuel leaks like those observed during the October launch, ULA said.

The Delta 4's RL10B-2 engine, fueled with liquid hydrogen and liquid oxygen, will be required to fire twice Thursday night -- to reach a preliminary parking orbit and then boost the satellite into a supersynchronous transfer orbit with a high point of 36,000 nautical miles during the 41-minute flight.

A WGS satellite in the factory. Credit: Boeing

The rocket was rolled horizontally to the launch pad and erected in mid-March for the start of final preparations. A countdown dress rehearsal was conducted in April.

"WGS 5 factory testing was successfully completed in the first quarter of 2012 and WGS 5 was put into storage in March of 2012," said Boeing WGS Program Director Mark Spiwak.

"The satellite came out of storage earlier this year, was finalized and shipped to the launch site on March 8. The spacecraft completed fueling on March 31 and was fully encapsulated in the launch vehicle fairing on April 25."

The payload was moved to Complex 37 and mounted atop the rocket on May 7.

Thursday's liftoff is scheduled just after sunset, during a window extending from 8:27 to 8:57 p.m. EDT.

It comes only a week after a ULA Atlas 5 rocket launched from the Cape's Complex 41 to successfully place the GPS 2F-4 spacecraft into orbit.

This Delta 4 will be ULA's fifth launch of the year and the 71st since its creation 77 months ago.