The Ariane 5's Sylda 5 dual payload adapter will be separated next, revealing the Sky Mexico 1 spacecraft for its deployment in a few minutes.
And the upper stage's HM7B engine is now firing to inject the DirecTV 15 and Sky Mexico 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 launch time is set for 2116 GMT (5:16 p.m. EDT). Liftoff actually occurs even seconds later with ignition of the solid rocket boosters.
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 DirecTV 15 telecom satellite occupies the upper position in the rocket’s dual-payload berth. Owned by DirecTV of El Segundo, California, DirecTV 15 will stream 4K Ultra HD television programming to the company's 20 million subscribers in the continental United States, Alaska, Hawaii and Puerto Rico. DirecTV 15 also serves as a backup to DirecTV existing satellites.
The 13,679-pound DirecTV spacecraft, built by in Europe Airbus Defense and Space, will deploy first from the Ariane 5’s payload stack at T+plus 27 minutes, 47 seconds.
A Sylda 5 adapter will be discarded a few minutes later, revealing the 7,014-pound Sky Mexico 1 spacecraft, the mission’s other satellite passenger.
Manufactured by Orbital ATK in Virginia, Sky Mexico 1 hosts Ku-band transponders to broadcast television directly to homes and businesses throughout Mexico, Central America and the Caribbean. It is owned by Sky Mexico, a company jointly held by Mexican media company Televisa and DirecTV.
Separation of the Sky Mexico 1 spacecraft from the Ariane 5 rocket is scheduled at T+plus 37 minutes, 59 seconds.
The payloads have a combined mass of approximately 21,958 pounds, or 9,960 kilograms, including the barrel-shaped Sylda dual-payload adapter. Sign up to follow us on Twitter for the latest launch updates and space news.
Some statistics on today's flight:
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.
The countdown began at 0946 GMT (5:46 a.m. EDT), with clocks programmed for liftoff of the Ariane 5 ECA rocket at 2116 GMT (5:16 p.m. EDT), or 6:16 p.m. local time at the launch site in French Guiana. The launch window extends for 84 minutes.
A check of electrical systems is scheduled to occur around 1346 GMT (9:46 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 flight program for today's launch will be loaded into the rocket's computer.
The launch team will begin the process to fuel the rocket with super-cold liquid hydrogen and liquid oxygen propellants around 1616 GMT (12:16 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 1716 GMT (1:16 p.m. EDT).
Chilldown conditioning of the Vulcain 2 first stage engine will occur at 1816 GMT (2:16 p.m. EDT), and a communications check between the rocket and ground telemetry, tracking and command systems is scheduled for 2006 GMT (3:06 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, 54 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 nearly 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 22,326 miles, a targeted low point of 155 miles and an inclination of 6 degrees.
The release of DirecTV 15 is scheduled for 27 minutes, 47 seconds, after liftoff. The rocket's barrel-shaped Sylda 5 dual-payload adapter will be jettisoned a few minutes later.
Sky Mexico 1 will separate from the lower portion of the payload stack at 37 minutes, 59 seconds.
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Check out photos of the Ariane 5's rollout to the launch pad today.