NASA hoping this shuttle 'window' stays clear

December 4, 1998
by Paul Hoversten

NASA  will try again today to launch the space shuttle Endeavour and its astronaut-hardhats to the International Space Station after a last-minute alarm foiled Thursday's attempt.

Endeavour has just a 10-minute "window" -- from 3:31 a.m. to 3:41 a.m. ET today -- to be launched.  NASA  prefers a 3:36 a.m. liftoff.

Any time within the 10-minute window will put Endeavour on the proper trajectory with the right amount of fuel to rendezvous with the station's first piece, a Russian-built propulsion module that is orbiting 240 miles above Earth.

Endeavour is carrying the station's second piece, a U.S. connecting tunnel.

If the shuttle can be launched on time today, the crew will try to join the two pieces in space Sunday night.

Thursday the clock ran out on the attempt. A cockpit alarm that sounded 41/2 minutes before a planned 3:38 a.m. liftoff forced managers to stop the countdown while they studied the problem.

	?What prevented the Thursday launch of the space shuttle Endeavour? Do you think that NASA officials were too cautious or not cautious enough at launch time? ?Why is there such a narrow window of time (ten minutes) during which Endeavour can be launched? What must scientists take into account to ensure a Unity-Zarya link? ? Explain why the International Space Station may be important for future space research. What role might space have in people's everyday lives 100 years ahead?

By the time they determined the source and concluded it would be safe to proceed, the 10-minute window had passed. The alarm sounded after a momentary drop in pressure in one of the shuttle's three hydraulic power units. The units are used to steer the shuttle's main engines during launch and control its wing flaps, rudder, rakes and nose wheel during landing.

NASA missed the window "by between one and two seconds," launch director Ralph Roe said.

Usually, short windows themselves aren't a problem.  NASA  launched nine shuttles to the Russian space station Mir on time -- each with just a 5-minute window.

But, coupled with problems, tight windows can sink a launch attempt, says George Diller, a Kennedy Space Center spokesman.

"If it's mechanical, you don't have time to discuss it and still make the window. If it's weather, you don't have time to wait it out," he says. "Under the normal 21/2-hour launch window, we'd have plenty of time to discuss whatever's wrong. If it's safe to launch, we can work through it."

Thursday's scrub of a fully fueled shuttle launch was the first for a mechanical reason since October 1995. That was when the shuttle Columbia had six delays. It finally got off the ground on the seventh attempt, three weeks after the original target date.

Of the 92 shuttle missions that NASA has flown since 1981, most made it off the pad on time.

In any countdown, there are built-in holds for managers to check the systems before proceeding. Two-hour holds come at six hours and three hours before liftoff, and a 10-minute hold comes at 20 minutes before launch. The final hold, lasting 40 minutes, comes just nine minutes before liftoff. The clock restarts from where it left off after each hold.

Problems can cause a scrub at any time. Shuttle launches have been delayed even up to three seconds before liftoff.