Launch timeline for Atlas 5’s AEHF 4 mission

A United Launch Atlas 5 rocket is set to launch the U.S. Air Force’s fourth Advanced Extremely High Frequency communications satellite from Cape Canaveral. This timeline shows the major mission events planned over a three-and-a-half-hour flight to an optimized geostationary transfer orbit.

The 197-foot-tall (60-meter) rocket, propelled by an RD-180 main engine and five solid rocket boosters, is set for liftoff during a two-hour launch window Wednesday that opens at 12:15 a.m. EDT (0415 GMT).

The AEHF 4 mission will be the 79th flight of an Atlas 5 rocket, and the fifth Atlas 5 launch of 2018.

Built by Lockheed Martin, the AEHF 4 satellite joins three previous satellites in the AEHF constellation launched by Atlas 5 rockets in 2010, 2012 and 2013. With four AEHF satellites in orbit, the Air Force’s new generation of secure, nuclear-hardened voice, video and data relay spacecraft will provide global coverage.

An overview of the Atlas 5/AEHF 4 launch sequence and a ground track map illustrating the rocket’s path after liftoff are are posted below.

Credit: United Launch Alliance

T+0:00:01.1: Liftoff

After igniting its RD-180 main engine at T-minus 2.7 seconds, the Atlas 5 rocket fires its five solid rocket boosters and rises away from Complex 41 at Cape Canaveral Air Force Station, Florida, with approximately 2.6 million pounds of thrust.

T+0:00:34.6: Mach 1

The Atlas 5 rocket exceeds the speed of sound, flying east from Cape Canaveral Air Force Station.

T+0:00:48.4: Max-Q

The Atlas 5 rocket passes through the region of maximum dynamic pressure during ascent through the lower atmosphere.

T+0:01:50.7: Jettison SRBs

Having burned out of propellant approximately 20 seconds earlier, the five spent Aerojet Rocketdyne-built solid rocket boosters are jettisoned once dynamic pressure conditions are satisfied.

T+0:03:28.6: Payload Fairing Jettison

The Atlas 5 rocket’s payload fairing, made in Switzerland by Ruag Space, is jettisoned in a clamshell-like fashion once external heating levels drop below predetermined limits after climbing through the dense lower atmosphere. The Forward Load Reactor deck that connected the payload fairing’s structure to the Centaur upper stage is released five seconds after the shroud’s jettison.

T+0:04:27.4: Main Engine Cutoff

The RD-180 main engine completes its firing after consuming its kerosene and liquid oxygen fuel supply in the Atlas first stage.

T+0:04:33.4: Stage Separation

The Common Core Booster first stage of the Atlas 5 rocket separates from the Centaur upper stage. Over the next few seconds, the Centaur engine liquid hydrogen and liquid oxygen systems are readied for ignition.

T+0:04:43.4: Centaur Ignition 1

The Centaur RL10C-1 engine ignites for the first of three upper stage firings. This burn will inject the Centaur stage and the AEHF 4 satellite into an initial parking orbit.

T+0:11:51.3: Centaur Cutoff 1

The Centaur engine shuts down after arriving in a planned low-Earth parking orbit. The vehicle enters a 10-minute coast period before arriving at the required location in space for the second burn.

T+0:22:25.2: Centaur Ignition 2

Producing 22,900 pounds of thrust, the Centaur re-ignites to accelerate the payload into a highly elliptical transfer orbit from the parking altitude achieved earlier in the launch sequence. This burn lasts nearly six minutes.

T+0:28:19.1: Centaur Cutoff 2

The second Centaur firing places the AEHF 4 satellite into an elliptical transfer orbit stretching more than 20,000 miles above Earth, beginning a three-hour coast period for the mission’s final orbital adjustment maneuver.

T+3:28:20.4: Centaur Ignition 3

After a three-hour coast, the Centaur’s RL10 engine reignites for a roughly one-minute, 40-second firing to place the AEHF 4 satellite in the proper orbit for spacecraft separation.

T+3:30:00.1: Centaur Cutoff 3

The powered phase of flight is concluded as the Centaur reaches the planned elliptical geostationary transfer orbit with an apogee, or high point, of 21,933 miles (35,299 kilometers), a perigee, or low point, of 5,539 miles (8,914 kilometers), and an inclination of 12.8 degrees.

T+3:32:49.1: AEHF 4 Separation

The AEHF 4 spacecraft deploys from the Centaur upper stage.

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