Why SpaceX scrubbed its third launch attempt in five days

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A Falcon 9 rocket lit its engines Sunday night but failed to get off the pad as SpaceX scrubbed its third attempt in five days to launch a commercial communications satellite from Cape Canaveral Air Force Station.

No new target date was immediately set, but the Eastern Range said a fourth attempt to launch the SES-9 satellite would not occur before Tuesday.

CEO Elon Musk said on Twitter that a low thrust alarm led computers to cut off the rocket’s nine Merlin 1D main engines about a second before it would have lifted off from Launch Complex 4

That was caused in part, Musk said on Twitter, by the rocket sitting on the pad an extra 35 minutes longer than expected, after a boat wandered into a safety zone below the rocket’s flight path.

“Hold, hold, hold!” an Eastern Range safety officer called shortly before a planned 6:46 p.m. liftoff.

No description of the boat was given, but Musk said helicopters were scrambled to clear it from the hazard area.

The launch time was pushed back to 7:21 p.m.

The wait caused liquid oxygen loaded into the rocket’s propellant tanks to heat up, and a “helium bubble triggered (the) alarm,” Musk said.

It was not clear how quickly the rocket would be ready for a fourth launch attempt.

“The engineering team has a chance to determine what changes if any then need to make before we proceed into another launch countdown,” SpaceX engineer John Insprucker said during the company’s launch Webcast.

The mission’s first two launch attempts, on Wednesday and Thursday, scrubbed after SpaceX said it was having trouble keeping the rocket’s liquid oxygen supply chilled to near its freezing point. It’s a more dense form of the propellant used by an upgraded version of the Falcon 9 now trying to launch for the second time.

The liquid oxygen appeared to be behaving Sunday until after the boat entered the picture and delayed the liftoff.

The nearly 12,000-pound SES-9 satellite, built by Boeing, is the heaviest yet that a Falcon 9 will try to boost to an orbit more than 22,000 miles over the equator.

After the launch, SpaceX has said it will try to land the first-stage booster on a ship stationed 400 miles down range, but that success is unlikely.

(credit: James Dean – Florida Today, Feb 29,2016)
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