United Launch Alliance’s Atlas V rocket carried out its first mission for the United States Space Force Thursday, deploying the final Advanced Extremely High Frequency (AEHF) satellite to provide secure communications for the US military and its allies. Liftoff from the Cape Canaveral Air Force Station ahead of a long mission to deployment.
The AEHF-6 satellite carried into space by Thursday’s launch will complete a constellation of satellites at the core of the US military communications. Originally a US Air Force program, deployment of Advanced Extremely High Frequency (AEHF) satellites began in August 2010.
United Launch Alliance (ULA) has launched all six satellites using Atlas V rockets. AEHF-6 will be the first satellite to be launched for the Space Force, which was formed in December when the former Air Force Space Command became a separate branch of the US armed forces.
Designed to augment and eventually replace the older Military Strategic and Tactical Relay (Milstar) satellite network, AEHF was conceived as a six-satellite constellation providing secure, survivable and jam-resistant protected communications to warfighters, military units and facilities around the globe. AEHF was later scaled back to three satellites before being expanded back to a six-satellite system with the cancellation of the successor Transformational Satellite (TSAT) program in 2010. Thursday’s launch will bring the constellation up to its full complement of satellites.
The AEHF satellites themselves were built by Lockheed Martin. 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. Power comes from two deployable solar arrays, each consisting of five panels.
The propulsion systems aboard each satellite include a BT-4 liquid apogee motor built by Japan’s IHI Corporation, which is used for initial orbit-raising operations. Aerojet Rocketdyne XR-5 Hall Thrusters will be used for station-keeping one the satellite reaches geostationary orbit, while 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 antennae 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 resolutions coverage antennae (MRCAs) produce 24 spot beams for focused coverage of specific areas, while two high-resolution coverage antennae (HRCAs) support jam-resistant tactical communications. Phased array antennae generate further spot beams that can be targeted around the world as required.
A pair of crosslink antennae 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 the system’s survivability.
AEHF-6 via Lockheed Martin
AEHF is one of several satellite communications networks maintained by the US military. It is designed for secure tactical communications while the Wideband Global Satcom (WGS) system enables strategic communications. The US Navy operates its own MUOS satellites to connect mobile terminals, while the National Reconnaissance Office has its own Satellite Data System (SDS) to relay data from its reconnaissance satellites back to analysts. The United States has welcomed its allies into the AEHF project, with participants including the United Kingdom, Canada, Australia and the Netherlands.
AEHF’s predecessor, Milstar, consisted of six satellites launched by Titan IV rockets between February 1994 and April 2003. Despite being designed for ten years of operation, most Milstar satellites are still in service – aside from the third satellite which was lost in an April 1999 launch failure. Milstar was brought under the control of the AEHF program office in August 2010 ahead of the AEHF-1 launch.
The deployment of AEHF satellites has taken a similar amount of time, with Thursday’s final launch coming nine and a half years after the first satellite rode to orbit. After a successful launch aboard an Atlas V 531, AEHF-1 encountered problems with its apogee motor and was forced to rely on its station keeping and attitude control thrusters to reach its operational orbit. Although this delayed the satellite’s arrival on station by nine months, AEHF-1 was able to enter service and has operated successfully ever since. Subsequent AEHF satellites were launched in May 2012, September 2013, October 2018 and August 2019.
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.
Like the five satellites before it, 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. Although earlier AEHF satellites were launched using the less powerful 531 configuration, the switch to the 551 from the fourth launch onwards has allowed the satellites to be deployed into a transfer orbit with a higher perigee. This means they need to burn less fuel to reach geostationary orbit, while also leaving the rocket’s upper stage in a higher disposal orbit where it is less likely to collide with other objects.
ULA photo of the Atlas V at the pad
Atlas V was developed by Lockheed Martin under the Evolved Expendable Launch Vehicle (EELV) – now National Security Space Launch (NSSL) – program and made its maiden flight in August 2002 with a communications satellite for French company Eutelsat. Atlas competed with Boeing’s Delta IV rocket for military launch contracts until December 2006, when Lockheed Martin and Boeing formed United Launch Alliance (ULA) to consolidate their satellite launch operations and settle a lawsuit surrounding Boeing’s illegal acquisition of proprietary information from Lockheed Martin during an earlier stage of the EELV program. ULA took over operation of Atlas V and Delta IV, as well as the older Delta II rocket which retired in 2018.
A two-stage rocket, Atlas V consists of a Common Core Booster (CCB) with a Centaur upper stage. Up to five AJ-60A solid rocket motors can be attached to the first stage to provide additional thrust during the initial stages of flight.
Atlas can be launched in several different configurations, allowing it to carry out missions with different size and weight of payload and different target orbits in an efficient and cost-effective manner. Each configuration is assigned a three-digit code, with the first digit indicating the diameter of the rocket’s payload fairing, the second giving the number of solid rocket motors and the third digit giving the number of engines on the rocket’s Centaur upper stage. The smallest configuration – the Atlas V 401 – has a four-meter (13.1-foot) fairing, no solid rocket motors and a single-engine Centaur upper stage.
The 551 configuration that was used for the AEHF-6 launch has a five meter (16.4-foot) fairing, five solid rocket motors and a single-engine Centaur. This heaviest-lift version of the Atlas V had flown ten times prior to Thursday’s launch. It was first used in January 2006 to boost the New Horizons probe on its way to Pluto, and next in August 2011 with the Juno mission bound for Jupiter. Subsequent launches have carried five MUOS communications satellites for the US Navy and two previous AEHF satellites for the Air Force, and the AFSPC-11 mission for the Air Force which included the CBAS communications satellite and a collection of technology demonstrators.
Thursday’s launch took place from Space Launch Complex 41 (SLC-41) at the Cape Canaveral Air Force Station. This former Titan III and Titan IV launch pad was constructed in the 1960s and has been used for Atlas V launches since the type was introduced in 2002. Atlas rockets are assembled atop a mobile launch platform in the nearby Vertical Integration Facility (VIF), before being rolled out to the launch pad for liftoff. In preparation for Thursday’s mission, Atlas left the VIF at 11:04 local time (15:04 UTC) on Wednesday, taking about 50 minutes to reach its launch position. The launch platform was “hard down” at the pad – lowered onto its supports – at 12:38 local (16:38 UTC).
Atlas V at SLC-41 – via ULA
With rollout completed, technicians worked to install umbilical and electrical connections between the launch platform and pad infrastructure. Loading of RP-1 kerosene propellant into the first stage tanks was also completed on Wednesday. The launch countdown will begin from T-6 hours, 20 minutes at 07:57 Eastern Time on Thursday (11:57 UTC). The count includes 40 minutes of planned built-in holds, leading to an initial T-0 of 16:57 Eastern (20:57 UTC).
Loading of liquid hydrogen propellant into the second stage and the liquid oxygen used as oxidizer by both stages of the rocket was conducted in the last two hours of the countdown due to the extremely cold temperatures of the fluids. The tanks continued to be topped off as they boiled off as the countdown progressed.
About 2.7 seconds before launch, the RD-180 first stage engine ignited. At T+1.1 seconds the five AJ-60A solid rocket motors lit and AV-086 will lifted off on the first leg of its journey to deploy AEHF-6. After clearing the launch pad, Atlas began a series of pitch, roll and yaw maneuvers to establish an easterly trajectory out over the Atlantic Ocean.
It took about 34 seconds for Atlas to reach Mach 1, the speed of sound. About 12 seconds later the rocket encountered the moment of maximum dynamic pressure – Max-Q – where the combination of increasing velocity and decreasing atmospheric density put the greatest stress on the vehicle.
About 90 seconds into the mission, the solid rocket motors burned out. These briefly remained attached to the first stage until flight conditions are more favorable to ensure they can separate cleanly without impacting the rocket as their spent casings are jettisoned. This separation event occurred at the one minute and 46.7-second mark in the flight.
Three minutes and 24.9 seconds after launch, having climbed above most of Earth’s atmosphere and reached space, Atlas shed its payload fairing. While the fairing provides protection for AEHF-6 during its ascent through the atmosphere, once the rocket reaches space this protection is no longer needed and jettisoning the fairing reduces the weight of the vehicle. The five-meter fairing also encapsulates the Centaur stage, so must separate so that the mission can continue after the first stage burns out.
Booster Engine Cutoff (BECO) – the end of first stage flight – occurred four minutes and 26.3 seconds into the launch. At this point in flight, the RD-180 concluded its burn and shut down. Six seconds after BECO stage separation occurred, with Centaur released from the Common Core Booster to continue the mission. Centaur’s RL10C-1 engine began its prestart sequence after separation, igniting ten seconds later for the first of three planned burns.
This first burn lasted seven minutes and 4.6 seconds, setting up an initial parking orbit. After a ten-minute, 57.4-second coast Centaur performed its second burn, firing for another six minutes and 8.4 seconds to reach an intermediate geostationary transfer orbit.
Thirty-one seconds after the end of the second Centaur burn, a small secondary payload was deployed from the stage’s Aft Bulkhead Carrier (ABC). The TDO-2 satellite was built for the Air Force Research Laboratory by the Georgia Institute of Technology, and will serve as a target for laser ranging and calibration tests.
With TDO-2 deployed, AV-086’s mission entered an extended coast phase prior to the third Centaur burn and deployment of AEHF-6. From the end of the second burn to ignition for the third burn, this coast lasted five hours, seven minutes and 36.4 seconds. When Centaur restarts, its RL10 engine fired for one minute, 38.7 seconds to raise the perigee of its orbit. Spacecraft separation occurred two minutes and 49.1 seconds after the burn ends, with AEHF-6 separating to begin its mission.
The target orbit for spacecraft separation is 10,876.3 by 35,298.7 kilometres (6,758.2 by 21,933.5 miles, 5,872.7 by 19,059.7 nautical miles), inclined at 13.9 degrees to the equator. After separation, AEHF-6 will perform a series of burns with its liquid apogee motor to circularise this orbit and reduce its inclination, bound for a final geostationary orbit.
With its primary payload safely deployed, Centaur will conduct a blowdown about 26 minutes and 20 seconds after separation, venting any remaining propellant to reduce the risk of an explosion on orbit. The official end of mission is expected to be at six hours, thirty-eight minutes and 56.9 seconds elapsed time.
Thursday’s launch was the second of the year for United Launch Alliance and its Atlas V rocket, following on from the successful launch of the Solar Orbiter mission in February. The next Atlas launch is currently scheduled for May with the Space Force’s X-37B spaceplane to begin its sixth mission in low Earth orbit.