Screenshot from ULA Webcast of the launch of AEHF-6. LOX line to Centaur is being rechilled
Mission Rundown: ULA - Atlas V 551 - AEHF-6
Date: September 10, 2021 - Edit: November 28, 2022
Another spoonful of Alphabet Soup
With the AEHF-6 mission, we get to experience a spectacular Atlas V launch! Atlas V is built and launched by United Launch Alliance (ULA). For this mission an Atlas V 551 configuration will launch an Advanced Extreme High Frequency-6 (AEHF-6) satellite to Geostationary Earth Orbit for the United States military.
AEHF-6 was launched by United Launch Alliance, riding aboard an Atlas V rocket. For Thursday’s launch, Atlas flew in its 551 configuration, the most powerful version of this workhorse rocket, with this specific vehicle having tail number AV-086.
Atlas V 551 burned less fuel to reach orbit, while also leaving the rocket’s upper Centaur stage in a higher disposal orbit where it is less likely to collide with other objects. AEHF-6 gets deployed in a higher orbit so reaching its geostationary orbit is easier.
The attachment of five side-mounted solid rocket boosters (SRBs) to the Atlas first stage will generate three-quarters of the energy necessary at liftoff to power the vehicle on a complex, six-hour flight. The core Atlas booster will do the remaining 25%.
ULA’s Atlas V 551 launched from SLC-41, CCAFS Thursday March 26, 2020 at 16:18 EDT.
NOTAM hazard areas where Atlas V 551 will drop off SRB boosters, fairings and 1st stage
The number 5 obviously means a 5 meter fairing. The second number determines the number of strap on solid rocket boosters (SRBs). It can range from 0 to 5, and in this case, there are five on various sides of the center common core. The third and final number refers to the number of engine bells on the Centaur Upper Stage, which can be either one or two. In this case there will be one engine.
The AEHF-6 Payload
The Advanced Extreme High Frequency-6, AEHF-6 satellites themselves were built by Lockheed Martin and Northrop Grumman. They are based around the A2100M platform and each has a mass of about 6,168 kilograms (13,598 pounds). The spacecraft is designed to operate for at least fourteen years.
Its main goal is to provide a fast, highly reliable and secure connection for United States soldiers in all levels of conflict. The backbone of the Department of Defense communication are these satellites. Once they are in orbit they will be integrated into the Milstar (Military Strategic & Tactical Relay) constellation of military satellites.
For power, the A2100M satellite has two expandable five segment solar arrays that will use solar energy and convert it into electrical energy for the satellite to use. That conversion happens in the “bus” or the main power unit. There are batteries for nighttime operations, and fuel tanks feeding the Aerojet Rocketdyne XR-5 Hall Thrusters for station keeping purposes and a BT-4 liquid apogee motor to reach geostationary orbit from the transfer orbit which Atlas 551 delivers AEHF-6 to.
The BT-4 liquid apogee motor built by Japan’s IHI Corporation, which is used for initial orbit-raising operations. Smaller monopropellant thrusters – also developed by Aerojet Rocketdyne – will be used where additional attitude control is needed – on top of the reaction wheels that will provide day-to-day control.
AEHF’s communications payloads were developed by Northrop Grumman. To ensure backward compatibility with users of the legacy Milstar satellites, AEHF spacecraft support the same 2.4 kilobit-per-second low data rate (LDR) and 1.5 megabit-per-second medium data rate (MDR) signals as their predecessors. A new extreme data rate (XDR) signal provides speeds of up to 8.192 megabits per second.
The AEHF satellites carry multiple antennas to provide these services to users with varying requirements and use cases. A low-gain antenna provides coverage of the whole disc of the Earth visible to the satellite, six medium resolution coverage antennas (MRCAs) produce 24 spot beams for focused coverage of specific areas, while two high-resolution coverage antennas (HRCAs) support jam-resistant tactical communications. Phased array antennas generate further spot beams that can be targeted around the world as required.
A pair of crosslink antennas allow direct satellite-to-satellite communications at rates of up to 60 megabits per second, allowing signals to be relayed between Milstar and AEHF spacecraft without passing through ground stations – enhancing its survivability.
The AEHF spacecraft are numbered under the “USA” series, used to designate most US military satellites. After reaching orbit, AEHF-1 was named USA-214, while the next four satellites became USA-235, USA-246, USA-288 and USA-292 respectively. Following Thursday’s launch, AEHF-6 is expected to be redesignated USA-298.
The Atlas V 551 rocket
Atlas V is a two-stage rocket consisting of a Common Core Booster (CCB) first stage with a Centaur upper stage. Depending on the size and mass of its payload, the target orbit and other mission requirements, Atlas can fly in several different configurations.
These use varying numbers of solid rocket boosters to augment the first stage, single and dual-engine versions of the Centaur and four or five meter (13.1 or 16.4 foot) diameter payload fairings to accommodate different satellites.
Thursday’s launch used the 551 configuration, with a five-meter fairing, five solid rocket boosters and a single-engine Centaur. This is the most powerful version of Atlas V to have been developed. The rocket has tail number AV-086.
Atlas V with production number AV-086 launched from Space Launch Complex 41 (SLC-41) of the Cape Canaveral Air Force Station on Florida’s Space Coast.
AV-086 was moved to the launch pad on Tuesday ahead of the AEHF-5 launch. With Atlas in position, on Wednesday RP-1 propellant, a form of rocket-grade kerosene was loaded into its first stage tanks.
The Atlas V first stage will hold 94.6 cubic meters or 25 000 gallons of RP-1 kerosene fuel and 185.5 cubic meters or 49 000 gallons of liquid oxygen to feed the RD-180 main engine during the initial four-and-a-half minutes of the rocket's ascent.
Varius tweets from ULA launch stating the following numbers: 48800 gallon lox in 1st stage - 66000 gallon lox and H2 in both 1st and 2nd stage - 4150 gallon lox in 2nd stage - 12300 gallon H2 in 2nd stage.
It’s a numbers game until confirmed by other sources.
The fully-assembled Atlas V 551 rocket stands 197-foot-tall. Weighs 1.3 million pounds. Or more exactly 1278919 pounds equal to 580107.9 kg ~ 580.1 tons.
The three different orbits LEO - GTO - HGTO that AEHF-6 will use during this mission - Centaur 2nd stage will after payload separation be derelict space debris in the last High GTO one
The first stage burns RP-1, oxidized by liquid oxygen, while Centaur uses liquid hydrogen and liquid oxygen. Because of their extremely low boiling points, these cryogenic liquids were not loaded onto the rocket until Thursday’s countdown was well underway.
Thursday’s launch began with ignition of the RD-180 main engine at the base of Atlas’ Common Core Booster, which roared to life 2.7 seconds before the countdown reached zero. Atlas lifted off at T+1.1 seconds, with the five Aerojet AJ-60A solid rocket motors clustered around the first stage igniting.
The AEHF-6 mission, this rocket has a long five-meter fairing, five solid rocket boosters, and one engine on the Centaur Upper Stage.
It’s unique to the Atlas rocket to have their solid rocket boosters (SRBs) positioned in this way. When, like in this case, 5 SRBs are used, they are positioned with two on one side and three directly opposite of them.
If you notice carefully in the image of Atlas core boosters, there are long and somewhat flat pipes “running” down the side of the first core stage. These are raceways and carry fuel from the tanks down to the engines and some carry gasses back up to the tanks to pressurize them so the fuel stays flowing out the pipes.
Graphic of Atlas V 551 split in its major parts. Four fairing parts are put together when Centaur and AEHF-6 are stacked on top of each other in the High Integration Facility HIF.
When Atlas was designed, these two raceways were placed in their positions without the thought of SRB placement. So when more SRBs were needed, they were placed in the most convenient spot. Two SRB between the raceways and three opposite them.
The offset of the thrust won’t make it fly in the wrong direction. The engines on the core stage can gimbal, they counteract that offset of thrust by vectoring their thrust which is known as thrust vector control, or TVC. The SRBs, and most of them for that matter, do not have TVC abilities, but their nozzles can be angled and turned slightly sideways. That will counteract some of that offset SRB thrust.
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