CAPE CANAVERAL – NASA called off the launch of space shuttle Discovery for a second time Tuesday after a critical fuel valve failed to work properly.

Launch officials halted the countdown midway through the fueling process. The seven astronauts had not yet boarded the shuttle for the scheduled early Wednesday morning flight to the international space station.

“Drats!” said astronaut Jose Hernandez in a Twitter update. Fellow astronaut Christer Fuglesang, a Swede, chimed in with this tweet: “Bad luck again.”

A new launch date was not immediately set. It was also unclear whether NASA would be able to meet the end-of-month deadline for sending Discovery on its way. Over the weekend, managers said if Discovery was not flying by Aug. 30 or so, the delivery mission almost certainly would slide into October because of a pair of upcoming launches to the space station from Japan and Russia.

The problem cropped up while launch controllers were trying to shut the fill-and-drain valve in Discovery’s engine compartment. There was no indication the valve closed, and it appeared to be broken, said NASA spokesman Allard Beutel. He stressed that the exact condition of the valve was not known; it could be simply sticky.

Workers will need to get into the engine compartment in order to check and possibly replace the valve, a potentially time-consuming operation.

The valve, part of the main propulsion system, is used for the flow of liquid hydrogen from the external fuel tank to the main engines. It needs to be closed for launch, and open in order to drain the tank following a launch delay.

“You don’t want it to get stuck in the position where it’s closed because you cannot drain the tank that way,” Beutel said. Rules prevent engineers from cycling the valve back and forth because “you don’t take chances” with it, he said.

“Teams here are looking at the next steps,” he added.

The valve worked fine during the first launch attempt, but thunderstorms prevented Discovery from blasting off early Tuesday morning.

The first launch attempt was scuttled in the wee hours of Tuesday. Sixteen hours later, try two ended with a surprising scrub.

Discovery is loaded with thousands of pounds of space station supplies and equipment, including a new treadmill named after Comedy Central’s Stephen Colbert.