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And the Centaur liquid oxygen tank reached the 95 percent level. The topping off process is starting now.
The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds 48,750 gallons of cryogenic oxidizer for the RD-180 main engine.
The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10C engine along with liquid hydrogen to be pumped into the stage a little later in the countdown. The Centaur will perform three firings to propel the MUOS satellite into its intended orbit today.
Clocks have one more built-in hold planned at T-minus 4 minutes. During that pause, the final "go" for launch will be given. All remains targeted for liftoff at 7:43 p.m. EST from Cape Canaveral's Complex 41.
In the next couple of minutes, chilldown thermal conditioning of the mobile launch platform upon which the rocket stands will begin. This is meant to ease the shock on equipment when supercold cryogenic propellants start flowing into the rocket.
Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.
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This initial pause was designed to give the team some margin in the countdown timeline to deal with technical issues or any work that could fall behind schedule before fueling starts. But all is going smoothly today, with officials not reporting any troubles in the count.
The final hold will occur at T-minus 4 minutes.
Weather conditions remain "GO" at the present time.
The outlook for the launch window now predicts scattered clouds at 3,000 and 12,000 feet and broken clouds at 22,000 feet, good visibility, southerly winds of 8 gusting to 12 knots, a temperature of 61 degrees F.
If the launch should slip to Wednesday for some reason, there is an 80 percent chance of acceptable wether.
Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff at 7:43 p.m. EST (0043 GMT).
Soon the launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the C-band system used to track the rocket as it flies downrange, plus a test of the S-band telemetry relay system.
A planned half-hour hold begins when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled to verify all is in readiness to start fueling the rocket for launch.
Supercold liquid oxygen begins flowing into the Centaur upper stage, followed by the first stage filling. Liquid hydrogen fuel loading for Centaur will be completed a short time later.
A final hold is scheduled at the T-minus 4 minute mark. That pause will give everyone a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.
The launch window extends from 7:43 to 8:27 p.m. EST (0043-0127 GMT).
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The United Launch Alliance rocket was wheeled to Cape Canaveral's Complex 41 today aboard a mobile platform, emerging from the assembly building where the rocket's two stages, five strap-on boosters and the payload were integrated over the past two months.
This slow half-hour drive from the 30-story Vertical Integration Facility to the launch pad used a pair of specially-made "trackmobiles" to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.
The 206-foot-tall satellite booster was moved to Complex 41 for launch of MUOS No. 3, the latest satellite to join the Mobile User Objective System in geosynchronous orbit.
"It is an exciting time for the Navy, for the program and for the Department with the upcoming third launch of five MUOS satellites," said Navy Capt. Joe Kan, MUOS program manager.
"To deliver a satellite like MUOS is no small task and the team worked around the clock and through every holiday. We are honored to do so, because we know that our warfighters never stop in their own mission to keep us safe," said Iris Bombelyn, vice president of Narrowband Communications at Lockheed Martin.
The rocket is flying the 551 vehicle configuration for the fifth time in 52 flights. The version features two stages, five solid rocket boosters and a five-meter-diameter nose cone. It is powered off the launch pad by an RD AMROSS RD-180 main engine and the Centaur upper stage is equipped with an Aerojet Rocketdyne RL10C.
Countdown clocks begin ticking Tuesday at 12:33 p.m. EST, leading to activation of the rocket, final testing and system preps. Fueling operations start at 5:33 p.m.
The day's available launch opportunity begins at 7:43 p.m. and closes at 8:27 p.m. EST to send the rocket on an easterly heading into an equatorial orbit.
There is an 80 percent chance of acceptable weather, a 20 percent improvement over earlier forecasts.
Watch this page for live updates throughout the countdown and flight, plus live streaming video.
See our earlier MUOS 3 coverage.
And see our Atlas archive for further information.
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Preparations for the launch continue to move forward. Rollout of the rocket from its assembly building to the launch pad is scheduled for Monday.
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