The most powerful member of United Launch Alliance’s (ULA) Atlas V fleet—the mighty “551” variant, previously used on eight occasions, from lofting NASA’s New Horizons to Pluto in January 2006 to this year’s AFSPC-11 mission for the Air Force—saw service again earlier today (Wednesday, 17 October), when it boosted the fourth Advanced Extremely High Frequency (AEHF-4) communications satellite into supersynchronous transfer orbit. It was ULA’s 50th mission for the U.S. Air Force and the 250th flight of the liquid-fueled Centaur booster, whose heritage extends back over five decades. Liftoff the Atlas V 551 took place just after midnight at 12:15 a.m. EDT, rising into the darkness on a pillar of flame from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station, Fla. Tonight’s flight marked ULA’s fifth Atlas V mission of the year and the Centennial, Colo.-headquartered organization’s eighth overall launch in 2018. Next up in December will be the classified NROL-71 payload for the National Reconnaissance Office, riding this year’s second Delta IV Heavy.
AEHF-4 also marks ULA’s 50th mission for the Air Force, coming on the heels of a raft of launches which have delivered the geosynchronous-orbiting elements of the Space-Based Infrared System (SBIRS GEO), the Wideband Global Satcom (WGS), the interim Global Positioning System (GPS) IIF constellation and four flights of the X-37B Orbital Test Vehicle (OTV) mini-shuttle. And with three previous AEHF launches before today—in August 2010, May 2012 and September 2013—we can expect to see AEHF-5 and AEHF-6 lofted before the end of 2019 to close out a six-spacecraft network. Yet AEHF-4 is somewhat distinct from its predecessors, in that it utilizes a far more powerful variant of the Atlas V. The three previous satellites flew aboard 531 rockets, with three strap-on boosters, as opposed to five, a peculiarity explained by the specific design of its journey to Geostationary Transfer Orbit (GTO). “Interesting trajectory tonight,” tweeted ULA CEO Tory Bruno on Tuesday. “All our orbits are customized. This is a GTO, but we will coast for several hours out to apogee and then do a major lift of the perigee. This will leave the spacecraft with only a small amount of remaining energy to add in order to circularize.”
Previously nicknamed “the bruiser” by Mr. Bruno, the 551 has been used eight times since January 2006, lofting NASA’s New Horizons and Juno missions to Pluto and Jupiter, respectively, together with five Mobile User Objective System (MUOS) narrow-band military communications satellites and this year’s AFSPC-11. Standing 206 feet (62.8 meters) tall, the booster can haul up to 19,620 pounds (8,900 kg) of payload to geostationary altitude, some 22,300 miles (35,900 km) above the planet. Atop its Mobile Launch Platform (MLP), the booster departed the Vertical Integration Facility (VIF) at SLC-41 early Monday, 15 October, and traversed the quarter-mile (400-meter) distance to the pad surface. There it was carefully centered and propellant umbilicals and electrical and data connections were established. Formal countdown operations commenced shortly after 5 p.m. EDT Tuesday.
When operational, the 13,600-pound (6,170 kg) AEHF-4 will take its place as the fourth member of a “constellation” of high-powered satellites, providing fast and secure communications to link civilian leadership with military assets, anywhere in the world. Built by Lockheed Martin in Sunnyvale, Calif., and with a “protected communications payload” developed by Northrop Grumman Corp., the AEHF system replaces the outdated Milstar network and—as its name implies—operates at extreme high frequencies (44 GHz uplink) and super-high frequencies (20 GHz downlink) and can relay communications directly, without the need to pass through ground stations. Their phased-array antennas help to eliminate potential sources of radio jamming and each AEHF satellite can support data rates as high as 8.192 Mbits/sec. The system features encryption, low probability of intercept and detection and the ability to penetrate the electro-magnetic interference caused by nuclear weapons to route communications, real-time video, maps and targeting data to users on land, at sea or in the air. By the end of 2019, when the final two members of the six-satellite network are hopefully operational, AEHF will provide full-surface coverage between 65 degrees North and 65 degrees South latitude.
Northrop Grumman delivered electronics units to Lockheed Martin for AEHF-4 back in June 2013 and the “protected communications payload” itself was shipped in December 2014, reportedly some 4.5 months ahead of schedule. The complete AEHF-4 spacecraft was transported from Sunnyvale, Calif., to Cape Canaveral Air Force Station, Fla., on 27 July, arriving on the East Coast in the cavernous cargo bay of a Galaxy C-5 heavylift aircraft. It was encapsulated within the Atlas V’s 17-foot-diameter (5-meter) Short Payload Fairing (SPF) on 26 September, clearing another milestone on the road to launch. Although Interim Operational Capability (IOC) for the AEHF network was achieved in mid-2015, following the first three satellites, the arrival of AEHF-4 heralds an unprecedented quantum leap in operational capability, providing global Extended Data Rate (XDR) connectivity to warfighters and international partners and an estimated ten times greater communications throughput than was achievable by Milstar.
“Four AEHF satellites in orbit means protected global connectivity for those who need it most, from the President to deployed soldiers,” explained Michael Cacheiro, Lockheed Martin’s vice president of protected communications. “We offer powerful end-to-end systems, so that more operational users can have assured connectivity in contested environments. Delivering this fourth satellite in orbit will be critical to the Air Force, as it will connect all four satellites in orbit, forming a geostationary ring to provide uninterrupted global communications.”
Launch of AEHF-4 was targeted for a two-hour “window”, extending from 12:15 a.m. through 2:15 a.m. EDT Wednesday, 17 October. The weather outlook was described as 80-percent favorable, with a chance of violating the Cumulus Cloud Rule, a situation expected to deteriorate to 70-percent favorable in the event of a 24-hour scrub to Thursday. “The Bermuda High pressure ridge remains to the north, causing southeasterly flow over the Space Coast,” noted the 45th Space Wing at Patrick Air Force Base in their launch day weather briefing. “Today and through the countdown this evening, a front will move into the Southeast U.S., but the high-pressure ridge will remain just north of the Space Coast, causing southeast flow and isolated coastal showers.” As the countdown progressed through Tuesday evening, the weather outlook improved to 90-percent favorable.
Ignition of the Russian-built RD-180 engine of the Atlas V’s Common Core Booster (CCB) occurred at T-2.7 seconds, quickly ramping up to 860,000 pounds (390,000 kg) of thrust. Next, the five strap-on boosters—each generating a propulsive kick of 348,500 pounds (158,000 kg)—were commanded to fire and the 551 began a fast climb away from SLC-41 under a combined impulse of 2.6 million pounds (1.1 million kg). Shortly after clearing the tower, the giant rocket executed a combined roll, pitch and yaw maneuver to establish itself onto the proper trajectory to deliver AEHF-4 into orbit. Forty-eight seconds into the climb, it passed through the period of maximum aerodynamic pressure on its airframe and at 120 seconds the five strap-on boosters, exhausted of their solid fuel, were jettisoned.
Meanwhile, the stack continued under the impulse of the RD-180. The bulbous payload fairing was discarded 3.5 minutes into the flight and the engine shut down on time about 60 seconds later. Over the course of the next quarter-hour, the Centaur upper stage and attached AEHF-4 payload separated. With the CCB and strap-on boosters gone, the payload stack weighed a mere 7 percent of what it did at liftoff. The Centaur’s RL-10C-1 engine, producing a thrust of 22,900 pounds (10,390 kg), burned for seven minutes, then shut down on time, after which the stack coasted, prior to the second of three firings. AEHF-4 was released from the Centaur at 3.5 hours after launch, at an initial orbit with an apogee of 21,933 miles (35,299 km) and a perigee of 5,538 miles (8,913 kg), inclined 12.8 degrees to the equator. Tonight’s flight was the 250th flight of a Centaur. First flown in May 1962, the liquid-fueled booster’s plaudits included Surveyor-1 to the Moon, NASA’s first successful landing on another celestial body, as well as the twin Viking missions to Mars and the twin Voyagers to the outer Solar System and beyond.
Looking ahead to next year’s launch of AEHF-5, Northrop Grumman announced the completion of its protected communications payload in March 2017 and delivered it to Lockheed Martin for integration and testing. In May 2018, AEHF-5 was put through extensive thermal-vacuum and acoustic tests, ahead of systems-level testing, with delivery to the Air Force slated for 2019. Meanwhile, it was previously reported that the final member of the network, AEHF-6, will boast the first 3D-printed part—a remote interface unit, holding avionics circuitry—ever flown aboard a Lockheed Martin-built military satellite.
With only one more launch scheduled for 2018—December’s long-delayed flight of the mammoth Delta IV Heavy, carrying the classified NROL-71 payload for the National Reconnaissance Office—this year is expected to close with nine missions for ULA, its lowest number since 2010. And as ULA gradually retires its Delta fleet, with the Delta II having completed its swansong flight last month, 2019 will also see the “single-stick” Delta IV Medium wrap up almost 17 years of operational service with its last mission. Elsewhere, ULA’s fleet of Atlas V boosters will see a full plate of work in 2019, including the maiden voyage of the Dual-Engine Centaur (DEC) and the first Atlas to carry humans since May 1963.