Rarely Used Heavyweight Atlas V Delivers Military AFSPC-11 Payload to High Orbit

Pummeling the ground with an estimated 2.5 million pounds (1.1 million kg) of thrust, the largest and most powerful member of United Launch Alliance’s (ULA) Atlas V fleet roared aloft earlier tonight (Saturday, 14 April), to deliver a multi-purpose payload into orbit on behalf of the U.S. Air Force Space Command. Liftoff of the Atlas V 551—equipped with a 17-foot-wide (5-meter) payload fairing, five strap-on solid-fueled rockets and a single-engine Centaur upper stage—occurred from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station, Fla., at 7:13 p.m. EDT, right on the opening of tonight’s two-hour “window”. The AFSPC-11 complement reportedly includes a classified primary customer, together with a geostationary-bound research satellite and a group of other technology demonstration, on-orbit imaging and risk-reduction payloads.

Althought tonight’s flight represented the 77th mission by an Atlas V since its debut back in August 2002, AFSPC-11 is actually only the eighth outing by the rarely-used 551 variant, which carries the potential to deliver up to 41,470 pounds (18,814 kg) to low-Earth orbit and up to 19,620 pounds (8,900 kg) to geostationary altitude, some 22,300 miles (35,900 km) above the planet. This offers an indicator of the size and mass to be delivered to orbit on this mission. First used in January 2006 to loft NASA’s New Horizons spacecraft to Pluto, and described by ULA CEO Tory Bruno as “the bruiser”, the 551 has since seen service to deliver the Juno polar orbiter to Jupiter in August 2011 and five heavyweight Mobile User Objective System (MUOS) narrow-band military communications satellites, between February 2012 and June 2016. Following its AFSPC-11 duty, the power of the 551 will be unleashed again in June 2018 to boost the next Advanced Extremely High Frequency communications satellite (AEHF-5) into geostationary orbit.

Although the precise nature of the AFSPC-11 primary payload remains classified, details have emerged about the suite of secondary passengers heading uphill aboard the Atlas V. Foremost among them is the ESPA Augmented Geostationary Laboratory Experiment (EAGLE), which has been built by Orbital ATK, under contract to the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base, near Dayton, Ohio. According to Spaceflight101, EAGLE comprises an Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter (ESPA), a Moog propulsion module and five “slots” for Space Test Program (STP)-sponsored payloads for technology demonstrations. EAGLE was reportedly ordered from Orbital ATK in 2012 and can support payloads weighing up to 2,400 pounds (1,086 kg) for periods of at least one year.

Riding the STP slots aboard EAGLE are the Hypertemporal Imaging Space Experiment (HTI-SpX) to improve the understanding of hypertemporal imaging, whereby photography acquired of different locations on Earth is “blended” at different spectral wavelengths over time. Other payloads include the ARMOR resilient spacecraft bus development experiment, the Compact Environmental Anomaly Sensor III Risk Reduction (CEASE-III-RR), the Inverse Synthetic Aperture Ladar (ISAL) to conduct high-resolution imagery of other objects orbiting at geostationary altitude and a classified payload, dubbed “MYCROFT”. Details about the latter are sketchy, although it has been reported that the satellite is based on Orbital ATK’s ESPASat bus and is designed for deployment from EAGLE.

The cube-shaped ESPASat is a relatively lightweight spacecraft, with a dry mass of about 154 pounds (70 kg) and a payload mass of about 66 pounds (30 kg). Described by Orbital ATK as “a highly agile spacecraft bus, designed to perform experiments in the Geostationary Earth Orbit disposal orbit”, core missions for ESPASat are listed as technology demonstration, space situational awareness, Guidance, Navigation and Control (GNC) and research in the space sciences. Equipped with hydrazine thrusters and lithium-ion batteries, the satellite can support a total payload power of 30-60 watts and can remain operational for up to three years, with a 12-gigabit data-storage capacity. A previous ESPASat flew in July 2014 aboard a Delta IV booster, part of the $29.5 million Automated Navigation and Guidance Experiment for Local Space (ANGELS) payload, which rode as a secondary passenger on the AFSPC-4 mission.

In readiness for launch, the 206-foot-tall (62.8-meter) Atlas V 551 sailed through a smooth Mission Readiness Review on 26/27 March. “Clean bird,” tweeted ULA CEO Tory Bruno. “Everything about this one is interesting. Too bad I can’t tell you anything about it…” Moving closer to launch day, the Launch Readiness Review (LRR) occurred on 12 April, allowing ULA teams to press ahead with the rollout of the stack on its Mobile Launch Platform (MLP) from the Vertical Integration Facility (VIF). It trundled the approximately quarter-mile (400-meter) distance to the SLC-41 pad surface, where it was carefully centered and propellant umbilicals and electrical and data connections were made. “Rollin’, rollin’, rollin’,” tweeted ULA on Friday. “#AtlasV is rollin’. See that rocket rollin’, let’s go!” Formal countdown operations commenced the following day at T-6 hours and 20 minutes, with the activation of flight control systems. Ground Command Systems testing filled some 3.5 hours, as engineers validated communications links between the booster and the Launch Control Center (LCC).

Weather conditions for the opening launch attempt were expected to be favorable, with an 80-percent likelihood of acceptable conditions. “On launch day, high pressure continues to migrate east,” noted the 45th Weather Squadron at Patrick Air Force Base in its L-1 briefing on Friday, as a cold front advances into the Gulf Coast states and close to the Florida Panhandle. “Moisture is limited early in the count, trending up gradually by the evening hours with warm southerly winds.” With isolated showers predicted, but no thunderstorms, this created the risk of a potential violation of the Cumulus Cloud Rule. However, it was highlighted that in the event of a 24-hour scrub and turnaround to Sunday, 15 April, the weather outlook would take a significant turn for the worse. “In the event of a 24-hour delay, the weather deteriorates significantly with the potential for strong thunderstorms during the count and window,” the 45th stated. “The aforementioned cold front pushes into Central Florida late Sunday evening.” With potential issues surrounding lightning, cumulus clouds and ground winds, a Sunday launch attempt carried with it only a 20 percent chance of acceptable conditions at T-0.

The Atlas V’s Common Core Booster (CCB) is fueled by a mixture of liquid oxygen and a highly refined form of rocket-grade kerosene (known as “RP-1”). Since the former undergoes cryogenic boil-off and must be continuously replenished until close to T-0, it was loaded relatively late in the countdown, whereas the RP-1 experiences no leakage or reduction in capacity during the interval before launch and consequently was pumped aboard the rocket on Friday. “RP-1 tanking complete,” Mr. Bruno tweeted at 6:30 p.m. EDT Friday. “[Probability of Go] holding at 80 percent.” About 2.5 hours before the opening of Saturday’s two-hour window, a built-in hold got underway, prior to the onset of liquid oxygen loading, and propellant storage areas were chilled down, in order to avoid thermally shocking and fracturing the hardware.

At length, the Launch Director authorized the lengthy process to begin. This was followed by the loading of a pair of cryogens—liquid oxygen and hydrogen—aboard the Atlas V’s Centaur upper stage. Liquid oxygen tanking passed smoothly through Slow Fill and Fast Fill modes, before finally transitioning to Topping, in order to continuously replenish the effects of boil-off. This was followed by the onset of liquid hydrogen loading. All told, the 551 was enjoying a trouble-free countdown, as it headed towards at T-0 at 7:13 p.m.

Finally, at 6:54 p.m., the countdown entered its last 15-minute built-in hold, with the clock halted at T-4 minutes. During this time, the ULA Launch Conductor took the opportunity to perform checks on the vehicle, payload and weather, before closing out with a “Go/No-Go” poll of all stations. At 7:09 p.m., with a declaration of “Green” across the board, the clock was released from its hold point at T-4 minutes and resumed counting into the Terminal phase. The Atlas V’s systems were transferred to internal power, the Flight Termination System (FTS) was placed onto internal power and armed and at T-60 seconds the Launch Control System was enabled and on-board computers took control of all critical functions. “Range Green” came the welcome call from the Range Operations Co-ordinator.

The launch sequence got underway at T-2.7 seconds, with the ignition of the Russian-built RD-180 first-stage engine of the CCB, which burned liquid oxygen and hydrogen and punched out 860,000 pounds (390,000 kg) of thrust. The five Aerojet-built solid-fueld boosters, each standing 67 feet (20.4 meters) tall, then ignited with a staccato crackle, producing a total thrust of 1.7 million pounds (790,000 kg) to supplement the initial boost away from SLC-41. Climb-out of the rocket commenced at T+1.1 seconds and the Atlas V executed a “fast-climb” to about 85 feet (26 meters), whereupon the Centaur avionics commanded a pitch, roll and yaw program maneuver to establish the stack onto the proper flight azimuth of 89.9 degrees to deliver AFSPC-11 into orbit.

Fifty-one seconds into the flight, the vehicle passed through maximum aerodynamic turbulence—colloquially known as “Max Q”—and the rapidly ascending stack throttled down its RD-180 engine and began a nominal zero-pitch and zero-yaw angle of attack to minimize these loads. At T+107 seconds, now exhausted, two of the five strap-on boosters were jettisoned, followed by the others about 1.5 seconds later. A little over 3.5 minutes into the mission, with the RD-180 still burning hot and hard, the bulbous payload fairing was discarded, and when the 551 reached a peak load of 4.6 G the engine was throttled back to maintain this level. Shutdown of the RD-180, properly identified as Booster Engine Cutoff (BECO), came at T+273 seconds, after which the 41.5-foot-long (12.6-meter) Centaur and AFSPC-11 separated from the stack.

To deliver the payload stack into orbit, the upper stage supported a series of three burns by its 23,900-pound-thrust (10,840 kg) RL-10C liquid oxygen/hydrogen engine. The first burn lasted for six minutes, after which the Centaur/payload combo coasted for 12 minutes, ahead of the second firing, which ran for a slightly shorter duration of six minutes. With the second burn complete, the stack coasted again for more than five hours, prior to the final burn, lasting a mere three minutes. Following the end of the Centaur’s third burn, an hour-long “separation window” opened for the deployment of the AFSPC-11 primary payload and its secondary passengers.

Completion of today’s mission brings to three the total number of Atlas V flights in 2018, following the latest geostationary-orbiting Space-Based Infrared System (SBIRS GEO-4) in January and last month’s launch of the newest Geostationary Operational Environmental Satellite (GOES-S). Coming up on 5 May is the first Mars-bound mission to be lofted from the West Coast, with NASA’s InSight lander targeted to ride a “barebones” Atlas V 401, with no strap-on boosters. This will be followed by AEHF communications satellites in June and October, as well as the unpiloted and piloted voyages by Boeing’s CST-100 Starliner vehicle in August and November.

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