Florida ? 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 today’s scheduled early 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.”

Another attempt will not be made until Friday at the earliest.

NASA is up against a tight end-of-month deadline for launching Discovery. If Discovery is not flying by Sunday, the delivery mission will slide into October because of a pair of upcoming launches to the space station from Japan and Russia.