The culprit was a misbehaving valve on the rocket's Centaur upper stage, which has two RL10 engines fed by super-cold liquid hydrogen and liquid oxygen propellants.
The flight rules had been laid out for this flight ahead of time. With the crew at the launch pad, the proper action was to scrub.
NASA announced overnight that officials decided to skip a launch opportunity Tuesday night to allow engineers more time to study the valve problem and decide whether they need to replace it.
It is the culmination of nearly a decade-and-a-half of development by Boeing, which has a $4.2 billion contract with NASA to ready Starliner for crew missions, then carry out six long-duration crew ferry flights to and from the International Space Station.
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