A massive shock wave erupts at the moment Atlantis lifts off from launch pad 39B today. Photo: NASA TV/Spaceflight Now

The space shuttle Atlantis, carrying a $164 million airlock module for the international space station, streaked into orbit early today after a flawless countdown, putting on a spectacular sky show as it climbed aloft and rocketed away up the east coast.

Concern about possible showers and thunderstorms evaporated during the final hours of Atlantis' countdown and mission managers cleared commander Steven Lindsey and his four crewmates for an on-time liftoff at 5:04 a.m., the moment Earth's rotation carried the launch pad into the plane of the space station's orbit.

With its three hydrogen-fueled main engines roaring at full power, Atlantis' twin solid-fuel boosters ignited with a ground-shaking roar, instantly turning the pre-dawn darkness into near daylight as the spaceplane vaulted away from its launch stand.

In a first for the shuttle program, one of Atlantis' main engines was equipped with an upgraded high-pressure hydrogen fuel pump built by Pratt & Whitney. The new pump is the final element in a $1 billion main engine upgrade program designed to improve reliability and safety.

The engines are built by Boeing's Rocketdyne Division, which supplied the powerplants' original high-pressure hydrogen and oxygen pumps. As part of a post-Challenger upgrade program, Pratt & Whitney won a contract to develop news pumps with fewer welds and other safety features. Rocketdyne implemented a variety of other improvements.

With the addition of the Pratt & Whitney hydrogen pump, the probability of a catastrophic failure drops from 1-in-438 to 1-in-483. But those numbers include possible booster failures and other catastrophic malfunctions. The odds of a failure due to the engines alone drop from 1-in-999 to 1-in-1,283 with implementation of the block 2 design.

Data from the lone block 2 engine aboard Atlantis are not yet available, but all three engines appeared to work flawlessly and eight-and-a-half minutes after liftoff, the shuttle slipped into its planned preliminary orbit.

At the moment of liftoff, the space station was sailing high above the central Indian Ocean west of Australia. If all goes well, Lindsey will guide Atlantis to a docking with the orbital lab at 10:50 p.m. Friday to wrap up a two-day orbital chase.

The shuttle fliers will be welcomed aboard by Expedition Two commander Yuri Usachev and his two NASA crewmates, James Voss and Susan Helms. Then the real work will begin.

The primary goal of Atlantis' mission is installation of the new airlock module overnight Saturday during the first of three planned spacewalks by Gernhardt and Reilly. The heavy lifting will be done by Helms, operating the station's new Canadian-built robot arm.

The airlock will be attached to the right-side hatch on the Unity node, a multi-hatch gateway module sandwiched between the Russian Zarya propulsion module and the U.S. laboratory, Destiny. the Canadarm2 is required for airlock installation because the shuttle's 50-foot-long arm is too short to reach Unity's starboard hatch.

Canadarm2 was installed during a shuttle mission last April. During tests and checkout in May, the Expedition Two crew ran into subtle problems that eventually forced NASA managers to put Atlantis' launch, originally scheduled for mid June, on hold.

After extensive troubleshooting, engineers traced the primary fault to an intermittent problem with a diagnostic circuit in the robot arm's shoulder pitch joint. A software patch was uplinked to mask out the problem and Atlantis was finally cleared for launch.

After the airlock is installed this weekend, the combined shuttle-station crew will spend two days outfitting and activating the new module before Gernhardt and Reilly stage a second spacewalk overnight Tuesday to attach two high-pressure oxygen and nitrogen tanks.

After additional outfitting, a third spacewalk, this one using the new airlock, is planned for overnight Thursday to install two more oxygen and nitrogen tanks.

The new airlock has two sections: An outer crew lock, the small volume that is actually depressurized and opened to space during a spacewalk, and a larger equipment lock that butts up against Unity's starboard hatch.

"The crew lock is the smaller section, and that's where the two spacewalking astronauts actually go inside, close the hatch and depressurize down to vacuum, and then open up another hatch and do their spacewalk," Lindsey said in a NASA interview. "On the other side of that crew lock is something called the equipment lock and the equipment lock houses the ... meat of the airlock, all of the systems."

Those systems include computers, spacesuit battery chargers, systems to replenish spacesuit cooling water, an atmospheric control system that scrubs carbon dioxide from the air while providing pressurized oxygen and nitrogen for spacesuit tanks.

"One thing that's very unique about the airlock as opposed to how we do spacewalks today is that the airlock actually has a depressurization pump," Lindsey explained. "In the shuttle, when you vent the airlock out so the astronauts can go down to vacuum, you actually vent all that gas out to space and you lose the nitrogen and oxygen, so you have to replenish it from inside."

The new airlock features a system "that will actually pump this atmosphere back into the space station, pump it down, and then we open up the hatches, and, when they come back in, we open the valves and it repressurizes. The advantage of that is it doesn't use very much gas so we don't have to continually replenish gas whenever you [do] a spacewalk."