Screenshot from ULA Webcast of the launch of AEHF-4. It’s midnight for pete’s sake. It’s to bright
Mission Rundown: ULA - Atlas V 551 - AEHF-4
Written: September 22, 2021 - Edit: December 2, 20220
No delays and right on time
With the AEHF-4 mission, we get to experience another spectacular Atlas V launch! Atlas V is built and launched by the United Launch Alliance (ULA). For this mission an Atlas V 551 configuration will launch an Advanced Extreme High Frequency-4 (AEHF-4) satellite to Geostationary Earth Orbit for the United States military. It will aid them in fast and secure communication.
NOTAM hazard areas where Atlas V 551 will drop off five SRB’s, two fairings and 1st stage
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 October 17, 2018 at 00:15 EDT for once right on time, which is a rare occurrence in ULA launches. Okay it happens.
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 three different orbits LEO - GTO - HGTO that AEHF-4 will use during this mission - Centaur 2nd stage will after payload separation be derelict space debris in the last orbit - High GTO
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.
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 SRBs 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.
Graphic of Atlas V 551 split in its major parts. The raceway is the faint brown line on the bottom
The AEHF-4 Payload
The Advanced Extreme High Frequency-4 (AEHF-4) satellite vehicle is built by Lockheed Martin and Northrop Grumman. 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 Atlas 551 delivers it to.
Lockheed Martin is the prime contractor for the AEHF satellites, which are based on the company’s A2100M platform. With a mass at liftoff of 6,168 kilograms (13,600 lb), these large satellites have an expected on-orbit lifespan of at least fourteen years.
Each AEHF carries an IHI Aerospace BT-4 liquid apogee motor for initial orbit-raising, along with four Aerojet Rocketdyne XR-5 Hall thrusters which will complete insertion into geostationary orbit and aid in stationkeeping operations. Monopropellant thrusters are also present on the satellite to assist with control and maneuvering.
The AEHF constellation adds new enhancements over its Milstar predecessors, including an extreme data rate (XDR) capability in addition to the legacy low data rate (LDR) and medium data rate (MDR). XDR became fully operational with the entry into service of the AEHF-4 satellite, now named USA-288.
This satellite completed a ring of AEHF spacecraft in geostationary orbit, allowing signals to be routed around the globe. XDR offers data transfer at speeds of up to 8.192 megabits per second – up from maximums of 2.4 kilobits and 1.544 megabits per second achievable with LDR and MDR respectively. A single AEHF satellite provides greater total capacity than the entire legacy Milstar constellation.
Several different types of communications antennae are carried aboard AEHF-5 to support different applications. A low-gain antenna is used to transmit and receive across the visible disc of the Earth, while six medium-resolution coverage antennas (MRCA) produce 24 spot-beams covering specific areas.
A pair of high-resolution coverage antennae (HRCA) can be used to cut through jamming to ensure tactical communications with units in the field. Phased array antennas are used to provide worldwide spot beams where needed. An AEHF satellite provides enhanced global coverage compared to Milstar, operating up to 68 simultaneous worldwide beams. Two cross-link antennas allow AEHF satellites to conduct bi-directional communications with other spacecraft in the AEHF and Milstar constellations. This allows signals to be routed across the constellation, passing directly between satellites without using a ground station as a relay.
AEHF crosslinks have a bandwidth of 60 megabits per second, compared to the 10 megabits per second possible between Milstar satellites. The enhanced capabilities of AEHF over Milstar give warfighters in the field access to faster and more resilient communications than were previously available to them.
As well as benefiting the United States, AEHF will be used by allied nations, including the United Kingdom, Canada and the Netherlands.
With its focus on secure tactical communications, AEHF is part of a US Military Satcom fleet that also includes the Wideband Global Satcom system geared towards more strategic communications. The US Navy’s Multi-User Objective System (MUOS) and the National Reconnaissance Office’s Satellite Data System (SDS), which relays data from surveillance satellites back for analysis.
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