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Delta 4 rocket overcomes engine issue during launch

BY JUSTIN RAY

SPACEFLIGHT NOW

Posted: October 6, 2012



Generating less than its normal 25,000 pounds of thrust because of a still unknown problem, the upper stage engine on the Delta 4 rocket had to fire for longer periods of time Thursday morning before ultimately delivering the GPS payload into the right orbit, overcoming the adversity to achieve success.

Credit: Pat Corkery/United Launch Alliance

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Delta 4 rocket-maker United Launch Alliance and RL10B-2 engine provider Pratt & Whitney Rocketdyne have convened a review panel to look into the telemetry recorded during the ascent and determine what caused the low-thrust condition. Officials say it is too soon to know what, if any, impact the situation will have on plans to launch an Atlas 5 rocket with the Pentagon's X-37B miniature space shuttle using a Centaur upper stage equipped with a similar-yet-different RL10 powerplant Oct. 25. "Though the GPS 2F-3 mission was a complete success, ULA fully understands the challenges of launch and will thoroughly investigate and implement appropriate actions to reliably deliver our customer's critical capabilities to the orbital positions required," Jim Sponnick, ULA's vice president of missions operations, said in a press release Friday night. Thursday's voyage of the 20-story rocket set sail at 8:10 a.m. EDT (1210 GMT) from pad 37B at Cape Canaveral Air Force Station topped with the Global Positioning System 2F-3 navigation satellite. A pair of strap-on solid-fuel boosters provided extra power for the first 95 seconds of flight, assisting the cryogenic first stage in climbing away from Earth. The RS-68 main engine finished its burn about four minutes after liftoff and separated, leaving the upper stage with the RL10B-2 to deploy its extendible carbon-carbon nozzle and ignite. The mission sequence planned three burns of the upper stage, initial reaching a low-altitude parking orbit, then a highly elliptical transfer orbit and eventually achieving a circular orbit in line with the GPS constellation 11,000 nautical miles up. That first burn was supposed to last nearly 8 minutes. However, it ran around a half-minute longer than expected as a consequence of the lower-than-planned thrust output, to reach the parking orbit of roughly 213 by 88 nautical miles, tilted 41.6 degrees to the equator. The rocket coasted over the central Atlantic for about 9 minutes before restarting the engine to run for a scheduled three minutes -- but went about a minute beyond the anticipated duration -- to inject itself into an orbit with a high point of 11,001 nautical miles, a low point of 129 nautical miles and inclined 43.3 degrees. Precise numbers on the actual burn durations and exactly how long the firings went overtime were not immediately available to the press Saturday. There was no official indication that the launch was in trouble as it unfolded live. Behind the scenes, however, there were worries about the engine's performance.

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



A three-hour quiet coast period then began as the rocket motor and attached payload traveled away from Earth toward the high-mark of the orbit for one final firing. The third burn raised the orbit's low point and increased the inclination to ascend into the GPS network, then Delta released the satellite cargo into an approximate 11,047-nautical-mile perch tilted 55 degrees to the equator. Officials indicate the final burn produced the necessary boost to finish shaping the orbit for GPS 2F-3. Whether this firing also suffered the low-thrust condition or not has not been confirmed. But the satellite arrived in space exactly where it was intended to fly despite the engine situation that, remarkably, was not a detriment to the launch's end result. "The Delta 4's robust system design, flight software, vehicle margins and propellant reserves enabled the successful outcome for this mission," ULA said in a statement to reporters Friday night. "The unexpected signature was seen during second stage performance as evidenced by a reduced thrust level of the RL10 engine. The onboard inertial guidance and flight control systems compensated for the lower thrust conditions and the Delta second stage delivered the satellite to the proper orbit." The investigation team assembled by ULA and PWR will have oversight from major customers, the press release said. The panel will work to determine what caused the low-thrust and identify what actions should be taken to prevent a reoccurrence in the future. Fed with supercold liquid hydrogen and liquid oxygen, the 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 has been fired in space 23 times to date. Its specs include a nominal thrust of 24,750 pounds, mass of 664 pounds, an overall length of 13.6 feet, including 7 feet just for the nozzle extension and a specific impulse of 465.5 seconds.



