There was one abort where the engines were starting to throttle up. The computer aborted when thrust wasn't developed fast enough. The problem was determined to be with the hypergolics (stored outside the rocket) used to light the engines. Any work done on the engines would probably be for cleaning, since the engines weren't at fault.
Topped with a television broadcasting satellite, SpaceX’s Falcon 9 rocket fired its engines and was moments away from liftoff from Cape Canaveral on Thursday, but the commercial booster aborted the launch after computers detected the engines were too slow building up thrust.
The countdown marched smoothly toward liftoff Thursday, with the rocket clearing key hurdles that hamstrung Monday’s launch attempt. The Falcon 9 pressurized its propellant tanks, switched to internal power and ignited its nine Merlin 1D first stage engines a few seconds before the appointed launch time.
But the Falcon 9’s computer-controlled countdown sequencer recognized a problem and called off the launch, shutting down the engines after they flashed to life and sent a wave of sound across the Florida rocket base.
The launch was “aborted by autosequence due to slower than expected thrust ramp,” Musk posted on Twitter.
As engineers continued to study the problem, SpaceX elected to restart the countdown to preserve a chance to launch Thursday.
Ultimately, however, SpaceX said they could not get comfortable with the issue in time and ordered another hold with less than a minute left in the day’s second countdown.
“We called manual abort,” Musk tweeted. “Better to be paranoid and wrong. Bringing rocket down to borescope (inspect) engines.”