SES-8 had its launch aborted at T+2 seconds. What, exactly, caused the flight computer to decide the engines weren't ready?
The report via Tweet from Elon Musk initially is that a slower than usual ramp up of thrust caused the flight computers to abort.
Elon Musk via Twitter: Launch aborted by autosequence due to slower than expected thrust ramp. Seems ok on closer inspection. Cycling countdown.
There is reported leeway in relaxing some of the sensitivity on this issue for another attempt. What that means was a bit later tweeted by Musk:
Increasing helium spin start pressure. Probably <50% chance of passing all aborts, but worth a try. Countdown resuming ...
Another try was however aborted on T-00:00:48 due to engineers not completing data review of the previous abort and the launch was scrubbed for the day.
When the rocket is in design phase the designers usually design the the graph known as thrust-time graph. Nowadays the flight computers integrated to the rockets measures the thrust build up during lift off (which is important phase) if the thrust build up is not satisfactory the flight computers close the fuel and oxidizer inlet valve and shut downs the turbopump.
According to SPACEFLIGHT101, SpaceX CEO and Chief Designer Elon Musk, some of the Merlin 1D engines were slow to ramp- up thrust. As part of Falcon’s tight red-line limits, the computer aborted when the engine parameters during ignition were not within those limits. The launch team then looked at the data that was collected during the ignition sequence and determined that the engines appeared to the in good condition and were just slow to reach their nominal thrust