Atlas 5 launch timeline for the AEHF 6 mission

atlas-5-launch-timeline-for-the-aehf-6-mission

A United Launch Atlas 5 rocket is set to launch the U.S. Air Force’s sixth and final Advanced Extremely High Frequency communications satellite from Cape Canaveral. This timeline shows the major mission events planned over a nearly six-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 Thursday, March 26, that opens at 2:57 p.m. EDT (1857 GMT).

The AEHF 5 mission will be the 83rd flight of an Atlas 5 rocket, and the second Atlas 5 launch of 2020.

Built by Lockheed Martin, the AEHF 6 satellite joins five previous satellites in the AEHF constellation launched by Atlas 5 rockets in 2010, 2012, 2013, 2018 and 2019. The first four AEHF satellites in orbit allowed the Air Force’s new generation of secure, nuclear-hardened voice, video and data relay spacecraft to span the globe, and the addition of two more AEHF geostationary relay satellite will grow the network’s capacity and resiliency.

The Atlas 5 launch sequence will last 5 hours, 40 minutes, from liftoff until deployment of the AEHF 6 spacecraft. On this mission, ULA added extra hardware and maneuvering fuel to the Centaur upper stage, enabling the launcher to deliver the AEHF 6 satellite closer to its final orbit more than 22,000 miles (nearly 36,000 kilometers) above the equator.

The GSO kit was added to the Atlas 5 rocket on the fifth AEHF launch last August, after the first four AEHF missions did not employ the additional capability.

The extra performance provided by the GSO kit will allow the Centaur upper stage to coast an additional two hours before its third firing. Spacecraft separation is scheduled for T+plus 5 hours, 40 minutes.

Thanks to the mission profile change, the AEHF 6 satellite will separate in an orbit with a perigee, or low point, several thousand miles higher than would be possible without the GSO kit. That means AEHF 6 — like AEHF 5 last year — will need to consume less of its own finite propellant supply to circularize its orbit, leading to a longer operating lifetime for the mission.

According to an Atlas 5 user’s guide published by ULA, the GSO kit includes additional battery power, a full load of hydrazine to control the upper stage’s orientation in space, and additional shielding over sensitive components, including the Centaur’ hydrogen and oxygen tanks.

An overview of the Atlas 5/AEHF 6 launch sequence and a ground track map illustrating the rocket’s path after liftoff are are posted below.

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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+0:00:34.4: Mach 1

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

T+0:00:46.2: Max-Q

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

T+0:01:46.7: Jettison SRBs

Having burned out of propellant approximately 15 seconds earlier, the five spent Aerojet Rocketdyne-built solid rocket boosters are jettisoned once dynamic pressure conditions are satisfied. The first pair of boosters will jettison, followed a second-and-a-half later by the remaining three.

T+0:03:24.9: 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+0:04:26.3: 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+0:04:32.3: 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+0:04:42.3: 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 6 satellite into an initial parking orbit.

T+0:11:46.9: Centaur Cutoff 1

The Centaur engine shuts down after arriving in a planned low-Earth parking orbit. The vehicle enters an 11-minute coast period before arriving at the required location in space for the second burn.

T+0:22:44.3: Centaur Ignition 2

Producing 22,900 pounds of thrust, the Centaur re-ignites to accelerate the AEHF 6 payload into a highly elliptical transfer orbit from the parking altitude achieved earlier in the launch sequence. This burn lasts more than six minutes.

T+0:28:52.7: Centaur Cutoff 2

The second Centaur firing places the AEHF 6 satellite into an elliptical transfer orbit stretching more than 20,000 miles above Earth, beginning a five-hour coast period for the mission’s final orbital adjustment maneuver. A secondary U.S. Air Force smallsat payload named TDO 2 will separate from the Centaur upper stage’s aft bulkhead around 30 seconds after the RL10 engine shuts down. The TDO 2 mission support space domain awareness through optical calibration and satellite laser ranging.

T+5:36:39.1: Centaur Ignition 3

After a five-hour coast, the Centaur’s RL10 engine reignites for a roughly one-minute, 28-second firing to place the AEHF 6 satellite in the proper orbit for spacecraft separation.

T+5:38:07.8: Centaur Cutoff 3

The powered phase of flight is concluded as the Centaur reaches the planned elliptical geostationary transfer orbit with an expected perigee, or low point, of 6,758 miles (10,876 kilometers), an apogee, or high point, of 21,933 miles (35,298 kilometers), and an inclination of 13.9 degrees. The final RL10 engine burn will fire until depletion of the Centaur upper stage’s propellant, giving the AEHF 6 satellite the most favorable orbit possible before spacecraft separation.

T+5:40:56.9: AEHF 6 Separation

The AEHF 6 spacecraft deploys from the Centaur upper stage.

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