Thunderstorms pounded Florida's Space Coast late Thursday, delaying work to ready the shuttle Endeavour for blastoff Friday afternoon. But the storms passed, a protective gantry was retracted and engineers began pumping rocket fuel into the ship's external tank on schedule to set the stage for the orbiter's 25th and final flight.

Credit: NASA/Spaceflight Now

"Launch teams are not working any issues right now that would prevent us from launching on time at 3:47 p.m. Eastern today," NASA spokesman Allard Beutel said just before fueling began. "Currently, forecasters are calling for a 70 percent of good weather for this afternoon's launch."

The protective rotating service structure was pulled away from Endeavour just before midnight, nearly five hours later than planned because of thunderstorms from an approaching cold front. Working through the early morning hours, engineers made up lost time and after assessing the latest forecast, NASA's Mission Management Team gave the team permission to begin pumping a half-million gallons of supercold liquid oxygen and hydrogen rocket fuel into Endeavour's external tank on time at 6:22 a.m. EDT (GMT-4).

Forecasters expect low clouds to dissipate as the day wears on, clearing out in time for Endeavour's planned 3:47:55 p.m. launch time. But brisk crosswinds are expected at the shuttle's emergency runway, setting up what could be a down-to-the-wire countdown. The forecast calls for scattered clouds at 4,000 feet, winds out of the north-northeast at 12 knots gusting to 18.

Endeavour's crew -- commander Mark Kelly, pilot Gregory H. Johnson, Michael Fincke, Gregory Chamitoff, Andrew Feustel and European Space Agency astronaut Roberto Vittori -- got up at 6 a.m. After breakfast and final medical checks, Kelly and Johnson will get a weather update before donning their bright orange pressure suits and heading to launch pad 39A around noon to begin strapping in for launch.

Kelly's twin brother Scott, who just got back from a long-duration stay aboard the International Space Station, joked Thursday night in a Twitter feed that he was "ready if replacement is required." Kelly's wife, Arizona Rep. Gabrielle Giffords, flew to Florida Wednesday, her first major trip since an assassination attempt in January. President Barack Obama and his family are scheduled to fly in later today, landing at the nearby Cape Canaveral Air Force Station.

The three-hour fueling procedure began when ultra-cold liquid hydrogen and oxygen began flowing through transfer lines to inlets on either side of the shuttle's aft engine compartment. Before entering the tank, the propellants circulate through the ship's three main engines to acclimate the propulsion system plumbing to cyrogenic temperatures.

If all goes well, the tank will be topped off and the system will transition to "stable replenish" mode around 9:22 a.m.

Engineers will be paying close attention to a seven-inch gaseous hydrogen vent line attached to the side of the external tank as the hydrogen section is filled. NASA has had problems with leaks in the vent line but managers are confident the system will work normally this time around.

Here is a timeilne for the remainder of today's countdown (in EDT; best viewed with fixed-width font):

HH...MM...SS...EDT...........EVENT

Fri  04/29/11

09...20...00...06:22 AM......Resume countdown (T-minus 6 hours)

09...20...00...06:22 AM......LO2, LH2 transfer line chilldown
09...10...00...06:32 AM......Main propulsion system chill down
09...10...00...06:32 AM......LH2 slow fill
08...40...00...07:02 AM......LO2 slow fill
08...35...00...07:07 AM......Hydrogen ECO sensors go 'wet'
08...30...00...07:12 AM......LO2 fast fill
08...20...00...07:22 AM......LH2 fast fill
06...25...00...09:17 AM......LH2 topping
06...20...00...09:22 AM......LH2 replenish
06...20...00...09:22 AM......LO2 replenish

06...20...00...09:22 AM......Begin 2-hour 30-minute built-in hold (T-minus 3 hours)
06...20...00...09:22 AM......Closeout crew to white room
06...20...00...09:22 AM......External tank in stable replenish mode
06...17...00...09:25 AM......Ascent flight control team on console
06...05...00...09:37 AM......Astronaut support personnel comm checks
05...35...00...10:07 AM......Pre-ingress switch reconfig
05...12...00...10:30 AM......NASA TV launch coverage begins
04...25...00...11:17 AM......Final crew weather briefing
04...15...00...11:27 AM......Crew suit up begins
03...50...00...11:52 AM......Resume countdown (T-minus 3 hours)

03...45...00...11:57 AM......Crew departs Operations and Checkout building
03...15...00...12:27 PM......Crew ingress
02...25...00...01:17 PM......Astronaut comm checks
02...00...00...01:42 PM......Hatch closure
01...30...00...02:12 PM......White room closeout

01...10...00...02:32 PM......Begin 10-minute built-in hold (T-minus 20m)
01...00...00...02:42 PM......NASA test director countdown briefing
01...00...00...02:42 PM......Resume countdown (T-minus 20m)

00...59...00...02:43 PM......Backup flight computer to OPS 1
00...55...00...02:47 PM......KSC area clear to launch

00...49...00...02:53 PM......Begin final built-in hold (T-minus 9m)
00...24...00...03:18 PM......NTD launch status verification
00...09...00...03:38:55 PM...Resume countdown (T-minus 9m)

00...07...30...03:40:25 PM...Orbiter access arm retraction
00...05...00...03:42:55 PM...Launch window opens
00...05...00...03:42:55 PM...Hydraulic power system (APU) start
00...04...55...03:43:00 PM...Terminate LO2 replenish
00...04...00...03:43:55 PM...Purge sequence 4 hydraulic test
00...04...00...03:43:55 PM...IMUs to inertial
00...03...55...03:44:00 PM...Aerosurface profile
00...03...30...03:44:25 PM...Main engine steering test
00...02...55...03:45:00 PM...LO2 tank pressurization
00...02...35...03:45:20 PM...Fuel cells to internal reactants
00...02...30...03:45:25 PM...Clear caution-and-warning memory
00...02...00...03:45:55 PM...Crew closes visors
00...01...57...03:45:58 PM...LH2 tank pressurization
00...00...50...03:47:05 PM...SRB joint heater deactivation
00...00...31...03:47:24 PM...Shuttle flight computers take control of countdown
00...00...21...03:47:34 PM...Booster steering test
00...00...07...03:47:48 PM...Main engine start (T-6.6 seconds)
00...00...00...03:47:55 PM...Booster ignition (LAUNCH)

Additional coverage for subscribers:
VIDEO: COUNTDOWN UPDATE AND WEATHER BRIEFING PLAY
VIDEO: THE PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: ASTRONAUTS ARRIVE AT LAUNCH SITE PLAY | HI-DEF
VIDEO: COUNTDOWN PREVIEW BRIEFING PLAY | HI-DEF

VIDEO: GUIDED TOUR OF SPACE STATION PLAY | HI-DEF
VIDEO: ENDEAVOUR MISSION PREVIEW MOVIE PLAY | HI-DEF
VIDEO: GET TO KNOW ENDEAVOUR'S ASTRONAUTS PLAY | HI-DEF

VIDEO: PRE-FLIGHT INTERVIEW WITH MARK KELLY PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH GREG JOHNSON PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH MIKE FINCKE PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH ROBERTO VITTORI PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH DREW FEUSTEL PLAY
VIDEO: PRE-FLIGHT INTERVIEW WITH GREG CHAMITOFF PLAY

VIDEO: ENDEAVOUR CLEARED FOR APRIL 29 LAUNCH PLAY

VIDEO: EVACUATION DRESS REHEARSAL PLAY | HI-DEF
VIDEO: ASTRONAUTS BOARD ENDEAVOUR PLAY | HI-DEF
VIDEO: SUITING UP FOR PRACTICE COUNT PLAY | HI-DEF
VIDEO: SHUTTLE INSPECTED FOR STORM DAMAGE PLAY | HI-DEF
VIDEO: CREW TEST-DRIVES PAD ESCAPE VEHICLE PLAY | HI-DEF
VIDEO: ASTRONAUT CHAT AT THE PRESS SITE PLAY | HI-DEF
VIDEO: CREW ARRIVES FOR PRACTICE COUNT PLAY | HI-DEF

VIDEO: PAYLOADS ARRIVE AT LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: CANISTER ROTATED UPRIGHT FOR PAD PLAY | HI-DEF
VIDEO: PALLET OF SPARE PARTS INTO CANISTER PLAY | HI-DEF
VIDEO: AMS PAYLOAD PUT INTO TRANSPORTER PLAY | HI-DEF

VIDEO: SHUTTLE AND STATION PROGRAM BRIEFING PLAY
VIDEO: THE STS-134 MISSION OVERVIEW PRESENTATIONS PLAY
VIDEO: IN-DEPTH WITH ALPHA MAGNETIC SPECTROMETER PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALKS PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: SUN RISES OVER THE LAUNCH PAD PLAY | HI-DEF
VIDEO: ENDEAVOUR ARRIVES ATOP PAD 39A PLAY | HI-DEF
VIDEO: ROLLOUT FROM VEHICLE ASSEMBLY BUILDING PLAY | HI-DEF

VIDEO: HOISTING ENDEAVOUR TO TANK AND SRBS PLAY | HI-DEF
VIDEO: ORBITER GOES VERTICAL PLAY | HI-DEF
VIDEO: ENDEAVOUR LEAVES PROCESSING HANGAR PLAY | HI-DEF

VIDEO: PAYLOAD MEDIA DAY PLAY | HI-DEF
VIDEO: THE ALPHA MAGNETIC SPECTROMETER PLAY | HI-DEF
VIDEO: PAYLOAD REMOVED FROM C-5 PLAY | HI-DEF
VIDEO: AMS ARRIVES FROM EUROPE PLAY | HI-DEF

VIDEO: EXPRESS LOGISTICS CARRIER NO. 3 PLAY | HI-DEF
VIDEO: HIGH-PRESSURE OXYGEN GAS TANK PLAY | HI-DEF
VIDEO: PUTTING STATION ANTENNA ON CARRIER PLAY | HI-DEF

VIDEO: TANK UNLOADED TO ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: THE EXTERNAL FUEL TANK ARRIVES PLAY | HI-DEF
VIDEO: BOOSTER SEGMENT MOVED TO VAB PLAY | HI-DEF
VIDEO: KENNEDY SPACE CENTER SRB CEREMONY PLAY | HI-DEF
VIDEO: ENDEAVOUR FITTED WITH MAIN ENGINES PLAY | HI-DEF
VIDEO: ORBITER TOWED OFF RUNWAY PLAY | HI-DEF

MORE: STS-134 VIDEO ARCHIVE
HDTV: HIGH-DEFINITION COVERAGE
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