An RL-10B-2 upper stage engine. Photo: Pratt & Whitney

Boeing won't send its first Delta 4 rocket into space Saturday as planned after a last-minute engine issue was raised and meteorologists predicted virtually no hope of favorable weather conditions.

Blastoff from pad 37B at Cape Canaveral Air Force Station in Florida carrying the Eutelsat's W5 telecommunications satellite had been planned for sunset.

But officials called a scrub Friday morning to give engineers additional time to sort through a problem with internal cracks found after an RL-10 engine test at manufacturer Pratt & Whitney. The Delta 4's upper stage features a single RL-10B-2 engine.

"Last night, as we were closing out our final actions for the Launch Readiness Review we did come upon some new information from Pratt & Whitney that they provided us," Delta program manager Dan Collins told reporters Friday afternoon.

"A rotor that had been recently fired was undergoing extremely detailed inspection. During that inspection, one very small and another small linear indication, or crack, were discovered. And our teams, Pratt & Whitney and Boeing, worked very hard together through the night to understand the ramifications of those.

"As we approached our LRR this morning, it had not been enough time for our team, especially the Boeing team, to have been able to digest the information, fully understand it and be able to give me, from my propulsion team, a 'yes, we are ready to go forward.'

"At that point in time, I felt it was prudent to allow my engineers the time they needed to make sure everything was good and that we would end up passing the chance on Saturday. I do have to admit the weather on Saturday is not looking that good. But the reason for waiting was to make sure my engineers had completed what they needed to do and felt comfortable in order to go towards launch.

"So we are off working that now...I am confident over the weekend we will be able to reach the full understanding, exonerate the latest data."

Boeing has been reviewing concerns with the RL-10 for a couple of weeks and had delayed moving the W5 spacecraft from its payload processing facility to the launch. But engineers had determined the problem was isolated to engine parts made this year. The Delta 4 engine on the launch pad was built in 1998. However, the additional test results became apparent Thursday night.

"It is part of the same investigation. As we have gone further, we have been looking extremely hard," Collins said. "We had one action item, which was to do this very detailed inspection...In doing it we found some more information and we are just taking the time to make sure we understand what the information means to us."

Boeing says, despite these late concerns, the liquid hydrogen/liquid oxygen RL-10B-2 engine on the first Delta 4 rocket is defect-free.

"This is an absolutely beautiful RL-10. Its test program was clean. The data from that test program is well within family and well to the good side of the family at all the critical areas," Collins said.

Officials will convene the Launch Readiness Review on Monday to review the weekend's analysis of the RL-10 to decide whether to press ahead with the countdown and liftoff on Tuesday. The launch window on Tuesday extends from 5:39 to 6:49 p.m. EST (2239-2349 GMT).

Besides the RL-10 constraint, the weather likely would have kept Delta 4 on the ground Saturday anyway. Launch Weather Officer Joel Tumbiolo said there was just a 10 percent chance of acceptable conditions.

"Tomorrow is not a good beach day," Tumbiolo joked. "Tuesday will be a good Chamber of Commerce day."

In a mirror image of Saturday's forecast, forecasters are calling for near-ideal weather Tuesday and Wednesday, with a 90 percent chance of good conditions.

"As bad as tomorrow is going to be, Tuesday and Wednesday are as about as good as it gets," Tumbiolo said.

Sunday and Monday weren't available launch dates due to the Leonids meteor shower.

If the launch doesn't happen on Tuesday, Boeing has a backup opportunity available on Wednesday. The Eastern Range would then have to spend Thursday reconfiguring its systems to support Friday night's rescheduled launch of space shuttle Endeavour from nearby Kennedy Space Center.