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And the upper stage's HM7B engine is now firing to inject the Intelsat 30/DLA-1 and Arsat 1 satellites into orbit.
The residual hydrogen burn flares will fire beneath the Vulcain engine at Minus-6 seconds to burn away any free hydrogen gas. At Minus-3 seconds, onboard systems take over and the two inertial guidance systems go to flight mode. Vulcain main engine ignition occurs at Minus-0 seconds with checkout between Plus+4 and 7 seconds. If there are no problems found, the solid rocket boosters are ignited at Plus+7.0 seconds for liftoff at Plus+7.3 seconds.
The synchronized sequence has started. Computers are now in control of this automated final phase of the launch countdown to prepare the rocket and ground systems for liftoff. There are three computers running the countdown - one aboard the Ariane 5 and two redundant computers at the launch complex.
The new launch time is set for 2143:45 GMT (5:43:45 p.m. EDT), according to Arianespace. Liftoff actually occurs even seconds later with ignition of the solid rocket boosters.
The only item listed red on the status board at the Guiana Space Center is the Intelsat 30/DLA-1 spacecraft, one of the mission's two passengers. All other parameters are green.
The reason for this hold is unfavorable weather at the launch base.
At Minus-4 seconds, the rocket's onboard computer will take over control of main engine start, health checks of the powerplant and solid rocket booster ignition commanding for liftoff.
The satellites will use their on-board engines to raise their orbits and position themselves over the equator.
The 13,933-pound Intelsat 30/DLA-1 satellite, built by Space Systems/Loral, will begin a 15-year operational mission beaming direct-to-home television services to Latin America.
Based on the Loral 1300-series satellite bus, the satellite carries 72 Ku-band transponders and 10 C-band transponders and will be positioned at 95 degrees wast longitude.
The 6,576-pound Arsat 1 spacecraft, manufactured by INVAP of Argentina, is owned by the Argentine national telecom operator Arsat.
Positioned at 71.8 degrees west, it will support television broadcasts, Internet, telephone and data transmission services to Argentina, Chile, Uruaguay and Paraguay with a payload of Ku-band transponders.
The rocket will take nearly 34 minutes to inject the satellites into orbit and release them. The payloads have a combined mass of approximately 22,229 pounds, or 10,083 kilograms, including the barrel-shaped Sylda dual-payload adapter.
Get an overview of the launch sequence.
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Here are some statistics on today's launch:
The 17.7-foot-diameter first stage's Vulcain 2 engine burns 149.5 metric tons, or about 329,000 pounds, of liquid oxygen and 25 metric tons, or about 55,000 pounds, of liquid hydrogen. The cryogenic upper stage's HM7B engine consumes about 14.7 metric tons, or more than 32,000 pounds, of oxygen and hydrogen.
The fluids are stored at super-cold temperatures and naturally boil off in the warm tropical atmosphere in French Guiana. More propellant is slowly pumped into the rocket for most of the countdown to replenish the cryogenic fuel.
The topping sequence ends in the final few minutes of the countdown as the fuel tanks are pressurized and the fueling system is secured.
Built by a consortium of European contractors led by Safran in Vernon, France, the Vulcain 2 engine generates up to 300,000 pounds of thrust during its 9-minute firing. It burns about 320 kilograms, or 705 pounds, of liquid oxygen and liquid hydrogen propellant per second.
The engine's nozzle has an exit diameter of 2.1 meters, or about 6.9 feet. It weighs more than 4,600 pounds and its liquid oxygen and liquid hydrogen turbopumps spin at 12,300 rpm and 35,800 rpm, respectively.
The Vulcain 2 replaced the Vulcain engine used on the initial version of the Ariane 5. The newer engine produces 20 percent more thrust.
The Ariane 5's upper stage is powered by an HM7B engine, a modified version of the HM7 engine used on the upper stage of the Ariane 4 rocket. The 364-pound HM7B engine is manufactured by Airbus Defence and Space in Ottobrunn, Germany.
The HM7B engine produces more than 14,500 pounds of thrust in vacuum.
The Ariane 5 configuration with a Vulcain 2 engine and HM7B-powered cryogenic upper stage is known as the Ariane 5 ECA.
The Ariane 5's twin solid rocket boosters are packed with propellant near the launch site in French Guiana before they are assembled and positioned on each side of the cryogenic core stage.
With the rocket now fully fueled for launch, the vehicle weighs 1.7 million pounds. At liftoff, the rocket produces 2.9 million pounds of thrust.
The cryogenic propellant will be gradually pumped inside the rocket to maintain proper levels as the fuel evaporates over the rest of the countdown.
Chilldown of liquid oxygen and liquid hydrogen propellant lines at the ELA-3 launch pad has begun. The chilldown procedure helps condition the ground plumbing before the cryogenic propellants are pumped inside the Ariane 5's first and second stages.
The countdown began as scheduled at 0930 GMT (5:50 a.m. EDT), followed by power-up of the rocket's computer and avionics systems for an electrical check beginning at 1330 GMT (9:30 a.m. EDT).
Workers finished their hands-on tasks on the launch pad, including the closure of doors, removal of safety barriers and configuring fluid lines for fueling. The ground team then evacuated the ELA-3 launch pad before the start of fueling.
The launch window extends for 62 minutes.
A check of electrical systems was scheduled to occur around 1330 GMT (9:30 a.m. EDT).
Workers will also put finishing touches on the launch pad, including the closure of doors, removal of safety barriers and configuring fluid lines for fueling.
The launch team will begin the process to fuel the rocket with super-cold liquid hydrogen and liquid oxygen propellants after 1600 GMT (12 p.m. EDT). First, ground reservoirs will be pressurized, then the fuel lines will be chilled down to condition the plumbing for the flow of super-cold liquid hydrogen and liquid oxygen, which are stored at approximately minus 423 degrees Fahrenheit and minus 298 degrees Fahrenheit, respectively.
It will take approximately two hours to fill the Ariane 5 core stage tanks.
A similar procedure for the Ariane 5's cryogenic upper stage will commence at 1700 GMT (1 p.m. EDT).
Chilldown conditioning of the Vulcain 2 first stage engine will occur at 1800 GMT (2 p.m. EDT), and a communications check between the rocket and ground telemetry, tracking and command systems is scheduled for 1950 GMT (3:50 p.m. EDT).
The computer-controlled synchronized countdown sequence will begin seven minutes before launch to pressurize propellant tanks, switch to on-board power and take the rocket's guidance system to flight mode.
The Vulcain 2 engine will ignite as the countdown clock reaches zero, followed by a health check and ignition of the Ariane 5's solid rocket boosters seven seconds later to send the 1.7 million-pound launcher skyward.
Five seconds after blastoff, the rocket will begin pitching east from the ELA-3 launch pad, surpassing the speed of sound less than a minute into the mission. The Ariane 5's twin solid rocket boosters will jettison 2 minutes, 23 seconds after liftoff.
Once above the dense atmosphere, the launcher's payload fairing will fall away at an altitude of about 68 miles. The Ariane 5's first stage will shut down 8 minutes, 50 seconds after liftoff, followed moments later by stage separation and ignition of the hydrogen-fueled cryogenic HM7B upper stage engine.
The rocket's upper stage will fire for approximately 16 minutes, accelerating to a velocity of 21,000 mph, or more than 9.3 kilometers per second, to reach an orbit with a planned high point of 22,236 miles, a targeted low point of 155 miles and an inclination of 6 degrees.
The release of Intelsat 30/DLA-1 is scheduled for 27 minutes, 50 seconds. The rocket's barrel-shaped Sylda 5 dual-payload adapter will be jettisoned a few minutes later.
Arsat 1 will separate from the lower portion of the payload stack at 33 minutes, 42 seconds.
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The rocket and launch platform were towed by a Titan tug powered by a 540-horsepower engine with dual transmission modes to control its movements with millimeter precision while running at full power.
The one-hour transfer of the launcher to the ELA-3 launch zone was to be followed by the careful positioning of the rocket's mobile launch platform over the flame trench, then the connection of the rocket with the launch pad's electrical, telemetry and propellant loading systems.
Ground crews planned to fill the Ariane 5 first stage's helium pressurization system later Wednesday.
The Ariane 5 rocket is set for its fifth flight of the year, with liftoff scheduled for 2100 GMT (5 p.m. EDT; 6 p.m. French Guiana time). The launch will mark the 76th Ariane 5 mission since its debut in 1996.
The liftoff from the European-run Guiana Space Center on the northern coastline of South America is set for 2100 GMT (5 p.m. EDT; 6 p.m. French Guiana time) Thursday, the opening of a 51-minute launch window.
It will take nearly 34 minutes for the Ariane 5's powerful solid rocket boosters and two-stage hydrogen-fueled core to release the Intelsat 30 and Arsat 1 communications satellites into a geostationary transfer orbit with a high point of 22,236 miles (35,786 kilometers), a low point of 155 miles (250 kilometers), and an inclination of 6 degrees.
Thursday's flight will mark the ninth launch of the year for Arianespace, which oversees Ariane 5, Soyuz and Vega rocket operations from the tropical spaceport at the edge of the Amazon jungle.
Officials in charge of the launcher, satellites and ground systems convened for a launch readiness review Tuesday. The customary meeting concluded with approval to proceed with final launch preparations, including rollout of the Ariane 5 rocket from its final assembly building to the ELA-3 launch zone Wednesday.
The 1.7-mile rollout is should take about an hour to complete under the power of a 540-horsepower Titan tug with dual transmission modes to control its movements with millimeter precision while running at full capacity.
Standing more than 179 feet tall, the Ariane 5 rocket will make the journey on top of a mobile launch platform. Once the launcher arrives at the pad, workers will carefully position the rocket's mobile platform over the flame trench, then the connect the rocket with the launch pad's electrical, telemetry and propellant loading systems.
Later Wednesday, workers will fill the rocket's first stage liquid helium sphere, which contains pressurant for the cryogenic propellant tanks to be loaded with liquid oxygen and liquid hydrogen during the final countdown Thursday.
Thursday's Ariane 5 flight, called VA220 in Arianespace's launch naming scheme, will mark the heavy-lift launcher's fifth mission this year.
Intelsat 30 will ride in the Ariane 5 rocket's upper berth, heading for a 15-year mission to provide Ku-band direct television services for DirecTV Latin America and C-band services for Intelsat's own business in Latin America.
The smaller Arsat 1 spacecraft is the first geostationary communications satellite built in Argentina. It will support television broadcasts, data transmission and Internet access for customers in Argentina, Chile, Uruguay and Paraguay.
Intelsat 30, also known as Intelsat DLA-1, will raise its orbit to geostationary altitude 22,300 miles over the equator. Its final operating position will be at 95 degrees west longitude.
Arsat 1 is destined to be positioned in geostationary orbit at 71.8 degrees west longitude.
The 6.9-ton Intelsat 30 satellite was manufactured by Space Systems/Loral of Palo Alto, Calif., and carries 72 Ku-band transponders and 10 C-band transponders.
DirecTV Latin America is leasing the spacecraft's Ku-band capacity -- along with 72 more Ku-band transponders on sister satellite Intelsat 31 set for launch next year -- to grow the company's television broadcasting portfolio in Central and South America and the Caribbean.
Intelsat 30 will be co-located with the Galaxy 3C spacecraft, which is also used by DirecTV Latin America.
Built by INVAP, an Argentine high-tech contractor, the 6,580-pound Arsat 1 spacecraft is designed for a 15-year mission. The satellite's operator is Arsat, a national telecom company backed by the government of Argentina.
The Arsat 1 project reportedly cost about $250 million.