Screenshot from ULA Webcast of the AEHF-3 launch. What’s that ticking sound? Can’t you hear it?
Mission Rundown: ULA - Atlas V 531 - AEHF-3
Written: February 27, 2023
None can hear US Air Force squawk
United Launch Alliance conducted its seventy-fifth launch Wednesday morning, with the Atlas V tasked with deploying the AEHF-3 satellite for the US Air Force. Liftoff took place from Space Launch Complex 41, Cape Canaveral Air Force Station.
The launch was delayed from its original T-0 due to range weather, prior to successfully launching at 04:10 local time (08:10 UTC).
United Launch Alliance’s Atlas V rocket was used to launch AEHF-3. The rocket, whose tail number was AV-041, was the 40th Atlas V to fly and the 3rd to use the 531 configuration – which was previously used for the two other AEHF launches.
Notam on debris fields along Atlas 531 flightpath with SRB’s, fairing and core booster crash sites
The AEHF-3 payload
AEHF-3 is the third satellite in the US Air Force’s Advanced Extremely High Frequency program, a series of high-capacity communications satellites which will augment and eventually replace the Milstar spacecraft launched between 1994 and 2003. Previous AEHF satellites were launched by Atlas V rockets in 2010 and 2012.
The AEHF system is expected to consist of six operational satellites. In 2003 two spacecraft were canceled to provide additional funds for the Transformational Satellite Communications System (TSAT), however following TSAT’s cancellation it was proposed that these two satellites be reinstated and late last year Lockheed Martin was awarded a contract worth 1.9 billion dollars to produce them.
Each AEHF satellite has a mass of about 6,170 kilograms (13,600 lb). Built by Lockheed Martin, the spacecraft are based on the A2100M bus, and have design lives of 14 years.
An IHI BT-4 apogee motor is used for initial orbit-raising maneuvers, before Hall Effect thrusters take over to slowly maneuver the satellite into its final orbit. AEHF-3, which will likely be renamed USA-246 in orbit, will provide satellite communications in the Super High Frequency (SHF) and Extremely High Frequency (EHF) bands.
The Atlas V 531 launch
AV-041’s launch began with the ignition of the first stage engine, at T-2.7 seconds. Liftoff occurred at T+1.1 seconds, once the thrust generated by the RD-180 engine and solid rocket motors exceeds the weight of the fully-fuelled rocket and payload.
With three solid motors burning, the rocket rose quickly into the sky, beginning a series of roll, pitch and yaw maneuvers T+5.4 seconds which took it onto the correct trajectory for its ascent to geostationary transfer orbit.
The rocket flew almost due east over the Atlantic, along an azimuth of 90.13 degrees. It took 38.2 seconds for the vehicle to reach a speed of Mach 1, passing through the area of maximum dynamic pressure, or max-Q, eight and a half seconds later. Vibrations from air friction on it and temperature rising from it on the fairings can damage them and the precious cargo inside them.
Shortly before the rocket’s speed reached Mach one, the RD-180 throttled down to reduce external friction induced loads on the vehicle; once max-Q had been passed the engine throttled up to full thrust again.
Approximately 90 seconds into the mission, the solid rocket motors burnt out; however they remained attached to the first stage for about 25 seconds to ensure there is no contact between the solids and first stage. Two SRMs separated at T+114.5 seconds, with the third following suit a second and a half later.
Fairing separation occurred late in first stage flight, about three minutes and 29.1 seconds after liftoff. Five seconds later the Forward Load Reactor, a device which attaches to the top of the Centaur to dampen vibrations within the fairing, was also jettisoned. First stage flight ended with BECO, or Booster Engine Cutoff, at four minutes and 17.3 seconds after launch, when the RD-180 was shut down having expended its propellant.
Stage separation occurred six seconds after BECO, with the Centaur’s RL10 engine beginning its pre-start sequence. Ignition, or Main Engine Start 1 (MES-1), occurred nine seconds later, beginning the first of two planned burns.
The first burn lasted nine minutes and 27.4 seconds, placing the vehicle into a parking orbit which began a seven minute, 57 second coast phase. The second and final burn lasted five minutes and 37.7 seconds, placing AEHF into its transfer orbit.
Spacecraft separation came 23 minutes and 23 seconds after the end of powered flight, at 50 minutes, 58.33 seconds mission elapsed time.
The target orbit for AEHF-2 is a supersynchronous transfer orbit, with an apogee far higher than geostationary orbit to provide additional energy to the spacecraft, reducing the fuel necessary to raise its perigee to geostationary altitude.
The separation orbit is 225 by 50,000 kilometers (140 by 31,000 statute miles, 122 by 27,000 nautical miles) at an inclination of 20.9 degrees, with an argument of perigee of 180 degrees.
The Atlas V 531 rocket
Atlas V 531, AV-041 consists of a Common Core Booster, powered by a Russian-built RD-180 engine burning RP-1 propellant oxidized by liquid oxygen. Three Aerojet Solid Rocket Motors augment its thrust at liftoff, increasing the vehicle’s payload capacity.
The rocket’s second stage, a Single-Engine Centaur (SEC), is encapsulated along with the satellite within the Atlas V’s payload fairing. For the AEHF launch the short five-meter fairing was being flown. This has a length of 20 meters (68 feet) and a diameter of 5.4 meters (17.7 feet).
The second Centaur stage is powered by a single RL10A-4-2 engine, which burns liquid hydrogen and liquid oxygen.
ULA graphic rendering of Atlas 531 with known rocket data inserted from the ULA pdf file
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