Atlas V 551 launches AEHF-4 for the US Air Force

by William Graham

United Launch Alliance’s workhorse Atlas V rocket launched an advanced communications satellite for the US Air Force Wednesday, following an early-morning liftoff from Florida’s Cape Canaveral Air Force Station. Atlas launched at the opening of a two-hour window at 00:15 local time (04:15 UTC), successfully deloying the spacecraft after a three-and-a-half-hour mission.
Wednesday’s launch deployed the Advanced Extremely High Frequency 4 (AEHF-4) satellite into a geostationary transfer orbit. This is the fourth in a series of secure, high-throughput, communications satellites that are being deployed to modernize one of the primary military communications networks used by the United States and its allies.

AEHF satellites operate alongside the legacy Milstar series of communications satellites that they will eventually replace. Six Milstar satellites launched from 1994 to 2003, with one lost to a launch failure and the other five.

Upgrading this constellation allows the US Air Force to take advantage of new technologies to make its communications more secure and resilient, while increasing capacity. AEHF provides jamming-resistant, encrypted communications to ground, air and sea-based terminals.

AEHF-4’s patch on the Atlas V fairing – via Nathan Barker for NSF/L2

The satellites can cross-link communications, allowing data to be routed across the network without downlinking it to Earth. This capability is backwards compatible with Milstar, allowing the two systems to be interoperated. AEHF is designed to be capable of transmitting and receiving data even through the electromagnetic radiation caused by a nuclear explosion.

Initial plans for the AEHF constellation called for six satellites. In 2004, half of the satellites were canceled, with the Transformational Satellite (TSAT) program expected to supersede AEHF by the mid-2010s. Instead, in 2009, TSAT was canceled in favor of procuring additional AEHF spacecraft, with the size of the planned constellation increased back to six.

AEHF spacecraft are being manufactured by Lockheed Martin, with their communications payloads developed by Northrop Grumman. This is a continuation of the partnership between Lockheed Martin and TRW – which was taken over by Northrop Grumman in 2002 – that developed the Milstar satellite network. Each AEHF satellite is based on Lockheed Martin’s A2100M satellite bus.

AEHF-4 during pre-launch processing at Astrotech

Fully fuelled, AEHF-4 has a mass of 6,168 kilograms (13,600 lb) and is designed to operate for at least fourteen years. Propulsion is provided by an IHI BT-4 apogee motor – which is responsible for orbit-raising operations – and four XR-5 Hall effect thrusters which will be used for station-keeping once the satellite arrives in geostationary orbit.

Each satellite carries multiple communications antennae to support different users. These include a low-gain antenna to transmit and receive across the face of the Earth visible to the satellite, six medium-resolution coverage antennae (MRCA), producing spot beams to cover 24 simultaneous areas, two high-resolution coverage antennae (HRCA) for tactical communications that can cut through attempts to jam the beam, and additional phased array antennae to provide worldwide spot beams where needed.

Each satellite also carries two crosslink antennae, providing bi-directional communications with other AEHF and Milstar satellites.

AEHF uplinks function in the extremely high frequency (EHF) part of the spectrum, at 44 gigahertz, while their downlinks are at 20 gigahertz, in the super high frequency (SHF).

Like Milstar, the system provides Low Data Rate (LDR) and Medium Data Rate (MDR) services – at transmission rates of 75-2,400 bits per second and 4.8-1,544 kilobits per second respectively – while adding a third Extreme Data Rate (XDR) service with speeds of up to 8.192 megabits per second.

Crosslinks between satellites can achieve a bandwidth of up to 60 megabits per second – an increase over the 10 megabits per second that was possible with Milstar.

AEHF-4 was the fourth AEHF satellite to be launched – following the AEHF-1, 2 and 3 satellites – now known as USA-214, USA-235 and USA-346 respectively. These were launched between 2010 and 2013, all by Atlas V rockets, and all remain in service. The first satellite’s entry into service was initially delayed by a problem with its apogee motor, requiring that its Hall thrusters be used for orbit-raising instead. However, the spacecraft eventually reached its station and subsequent satellites have entered service without incident.

Wednesday’s launch was conducted by United Launch Alliance, using an Atlas V rocket in the 551 configuration – tail number AV-073.

Atlas V 551 Stands ready for AEHF-4 launch – via Nathan Barker for NSF/L2

This is a two-stage rocket, with a Common Core Booster (CCB) first stage, a single-engine Centaur upper stage, five Aerojet AJ-60A solid rocket boosters to provide additional thrust in the early stages of flight and a five-meter (16.4-foot) diameter payload fairing that encloses the payload and the upper stage until Atlas has cleared Earth’s atmosphere.

Atlas V can fly in a number of different configurations – varying the number of boosters, number of engines on the upper stage and the size and type of fairing used in order to tailor the rocket’s performance to mission requirements.

While Atlas V vehicles were used for the three previous AEHF launches, these relied on the 531 configuration – which has two fewer solid motors on the first stage. As with the majority of geostationary satellites launching worldwide, AEHF spacecraft are deployed into an intermediate transfer orbit before using their own propulsion to maneuver into their final orbit. The more powerful Atlas V 551 sent AEHF-4 closer to its final orbit, reducing the amount of fuel that the satellite will need to expend before it can enter service.

The launch of AEHF-4 was the seventy-ninth Atlas V mission, and the ninth to use the 551 configuration. Developed by Lockheed Martin for the US Air Force’s Evolved Expendable Launch Vehicle (EELV) program, Atlas V first flew in August 2002 and has since delivered spacecraft to orbit for NASA, the US military and commercial customers.

In seventy-eight launches to date, Atlas V has never lost a payload, although the rocket did experience a partial failure during a June 2006s launch with a pair of National Reconnaissance Office ocean-surveillance satellites which were placed into an off-target orbit. These satellites were able to correct their orbit under their own power.

Atlas V’s 551 configuration made its debut in January 2006, when it deployed the New Horizons spacecraft bound for Pluto. The heaviest-lift version of Atlas V to be flown – although a heavier version of the rocket was planned but later canceled – the 551 has since been used to deploy NASA’s Juno probe to Jupiter, five Mobile User Objective System (MUOS) communications satellites for the US Navy and the Air Force’s AFSPC-11 payload flown earlier this year.

Launches of the Atlas V can take place from the Cape Canaveral Air Force Station in Florida, for missions requiring low-inclination orbits, and California’s Vandenberg Air Force Base which is typically used for launches to polar and retrograde orbits.

Wednesday’s launch was from Space Launch Complex 41 (SLC-41) at Cape Canaveral. This was originally constructed in the 1960s for the Titan IIIC rocket and was later home to Titan IIIE and Titan IV vehicles. Part of the Integrate-Transfer-Launch complex, Complex 41 shared Titan operations with nearby Launch Complex 40. The final Titan launch from SLC-41 took place in April 1999, after which the pad’s fixed and mobile service structures were torn down in preparation for its conversion to the Atlas.

SLC-41 has been used by Atlas V since the type’s maiden flight and is its only launch pad on the East Coast. Rockets are assembled in the nearby Vertical Integration Facility (VIF), atop a mobile launch platform which is then used to transport them, vertically, into position for launch. Ahead of Wednesday’s launch, Atlas was moved to the launch pad on Monday.

With the countdown proceeding smoothly, Wednesday’s launch began with ignition of the first stage RD-180 main engine, 2.7 seconds before T-0. Manufactured by Russia’s NPO Energomash, the RD-180 is a twin-chamber engine derived from the RD-170 series of propulsion systems developed for the Soviet Union’s Zenit and Energia family of rockets. It burns RP-1 propellant – a refined kerosene – and liquid oxygen. About 3.8 seconds after ignition of the main engine, the five AJ-60A boosters also ignited and AV-073 will lift off.

With the five solid rocket motors burning, AV-073 climbed rapidly away from the launch pad. About 3.9 seconds into the flight, Atlas began a series of pitch and yaw maneuvers to establish its easterly trajectory for the ascent into orbit. It took Atlas just 34.6 seconds to reach Mach 1, the speed of sound, with the rocket passing through the area of maximum dynamic pressure 13.2 seconds later.

The AJ-60A boosters burned for the first ninety seconds of flight. After burning out the boosters remained attached to the first stage for a few seconds, allowing more favorable conditions when the boosters do separate so there is no unwanted contact between them and the rest of the vehicle. Separation occurred 110.7 seconds after liftoff, with the first two boosters separating a second or so before the remaining three.

With the solid rocket motors jettisoned, the Common Core Booster’s RD-180 engine continued to thrust Atlas towards orbit. At around the three-minute, 28.6-second mark in the flight, the payload fairing separated from around the second stage and payload. By this time, Atlas had reached space and the fairing was no longer needed to protect the satellite or maintain the rocket’s aerodynamic profile. Discarding the fairing reduces the weight of the rocket, eliminating otherwise dead mass that would have to be carried.

Four minutes and 27.4 seconds after liftoff, Atlas reached the point of Booster Engine Cutoff (BECO). At this stage in the mission, the Common Core Booster exhausted its propellant and shut down. Five seconds later, Centaur separated from the booster and began its prestart sequence. Main Engine Start 1 (MES-1), the ignition of Centaur’s engine for the first of three planned burns, took place ten seconds after stage separation. Centaur is responsible for providing thrust for the rest of the mission, carrying AEHF-4 to its target deployment orbit.

Centaur is powered by an RL10C-1 engine, burning liquid hydrogen and liquid oxygen. Developed in the 1960s for use with modified versions of the Atlas missile, Centaur was the first successful rocket stage to use liquid hydrogen as a propellant and has been upgraded substantially over time to produce the version now used with Atlas V. The engine can be restarted multiple times in flight – a capability that is essential to Wednesday’s complex flight plan.

The first burn of Centaur’s RL10 engine lasted seven minutes and 7.9 seconds, placing it and AEHF-4 into an initial parking orbit. After coasting for ten minutes and 33.9 seconds, the stage fired again for another five minutes and 53.9 seconds. This raised the apogee, or highest point, of the orbit towards geostationary altitude. After the second burn concluded, the mission entered an extended coast before a short final burn.

The coast phase between the second and third burns of the upper stage lasted three hours and 1.3 seconds. The third and final burn of Centaur’s RL10C-1 engine lasted 139.3 seconds, raising the orbit and reducing its inclination to the Equator.

Two minutes and 49 seconds after the end of the third burn, AEHF-4 separated to begin its own mission. At separation, the spacecraft was expected to be in an orbit of 8,914 by 35,299 kilometers (5,539 by 21,934 miles, 4,813 by 19,060 nautical miles), inclined at 12.8 degrees with an argument of perigee of 180 degrees.

After spacecraft separation, Centaur will be passivated, with a blowdown of remaining propellant scheduled for twenty six minutes and twenty seconds after AEHF-4 is deployed. All being well, the mission will be declared complete four hours, 32 minutes and 29.1 seconds after Atlas lifts off from Cape Canaveral.

Wednesday’s launch was the fifth and final Atlas V mission of 2018, and the rocket’s only launch in the second half of the year.

Atlas began 2018 with the successful deployment of the SBIRS GEO-4 missile detection satellite on 20 January, launched the advanced GOES-S (now GOES 17) weather satellite for NOAA in March, the AFSPC-11 mission – with communications and experimental satellites for the US Air Force Research Laboratory – in April and NASA’s InSight mission to Mars in May.

Atlas was scheduled to launch the first test mission of Boeing’s Starliner spacecraft, one of two new vehicles developed for commercial crew missions to the International Space Station, as early as August. However, this launch has now slipped to 2019.

United Launch Alliance has one further launch to make in 2018, with a Delta IV Heavy slated to carry out the NROL-71 mission for the National Reconnaissance Office. This is expected to deploy a large Earth imaging satellite – likely a continuation of or successor to the KH-11 optical reconnaissance series – in early December.

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