The next launch of a Global Positioning System satellite is back on track after engineers gained fresh insight into the circumstances surrounding a previous GPS flight and its low-thrust condition on the upper stage.

The Delta 4 rocket is aiming for a Feb. 20 launch of the GPS 2F-5 navigation satellite. The evening launch window at Cape Canaveral extends from 8:40 to 8:59 p.m. EST.

Animation of the upper stage firing. Credit: United Launch Alliance

Engineers have been studying the situation since the October 2012 flight, even continuing to investigate after putting in place vehicle modifications that allowed three high-profile launches to go forward in 2013.

Rocket-operator United Launch Alliance and the Air Force were preparing to launch GPS 2F-5 in October 2013 when work was halted.

"The cause for delaying the GPS 2F-5 launch was not a new observation, but rather a potential new understanding the dynamic signatures that were measured during the October 2012 launch, along with the need to assess whether this could result in any change in previous flight clearance assessment," officials said.

"The investigation had previously determined that a fuel leak occurred within the engine system and that this fuel leak caused the low thrust. The ongoing Phase II investigation included very detailed characterization and reconstructions of the instrumentation signatures obtained from the October 2012 launch. These efforts resulted in some updated preliminary conclusions last October that perhaps low-frequency dynamic responses occurred on the engine system during the first engine start event."

By taking the flight telemetry from 2012, teams were able to replicate the charateristics in ground testing over the past few months.

"It was new ground testing of the launch vehicle instrumentation system that led to the preliminary conclusion that there may have been low-frequency dynamic responses during the first engine start," officials said.

During the 2012 flight, a fuel leak started as the engine was lit for the first time. That dropped the thrust level down five percent.

Two subsequent burns of the upper stage also experienced five percent drops as well, but the flight did achieve the planned orbit successfully.

"Over the past few months, the investigation team has conducted additional high-fidelity dynamic testing and analysis related to the hardware system structural and telemetry system characteristics," officials said.

"[I]t has been reconfirmed that the mitigations/system improvements previous implemented were appropriate. The flight constraint that was imposed last October has been removed."

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

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

During the Oct. 4, 2012 launch, a small fuel leak began at the moment the engine was ignited, robbing the rocket of its expected top-level thrust settings and forcing the vehicle to improvise to overcome the anomaly during the flight.

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 2012.

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.

Fed with supercold liquid hydrogen and liquid oxygen, the Aerojet Rocketdyne RL10B-2 is the latest in a long line of upper stage engines dating back a half-century. The original version of the RL10 debuted successfully on an Atlas rocket in 1963 and has been part of Centaur for more than 200 space missions.

The RL10 has dispatched robotic expeditions to every planet in our solar system, plus multiple missions to the moon and countless military spacecraft and commercial communications satellites in orbits around Earth.

This latest RL10 variant was introduced in 1998 as part of Boeing's Delta 3 program, which served as a stepping-stone to the Delta 4 rocket and development of its cryogenic upper stage.

The engine design has been fired in space 26 times to date.