Next Atlas V Mission Delayed, As ULA Investigates Anomalous RL-10C Behavior

Although successful, last month’s SBIRS GEO-5 launch atop an Atlas V experienced anomalous behavior with the RL-10C engine of its Centaur upper stage. United Launch Alliance (ULA) seeks a deeper understanding of the issue before committing STP-3 to flight. Photo Credit: Jeff Seibert/AmericaSpace

For the second time this year, United Launch Alliance (ULA) has announced a delay to the joint U.S. Space Force/Space and Missile Systems Center (SMC) Space Test Program (STP)-3 mixed-manifest research and technology mission, scheduled to fly atop the most powerful variant of its Atlas V booster out of Space Launch Complex (SLC)-41 at Cape Canaveral Space Force Station, Fla. In a blog update posted Friday afternoon, the Centennial, Colo.-headquartered organization explained that the cause of the delay was “to evaluate launch vehicle readiness” following anomalous observed behavior of the Centaur upper stage’s RL-10C engine during last month’s SBIRS GEO-5 launch.

A new target date for the mission—previously set to fly on 23 June—has yet to be revealed, with ULA noting only that it is “working with our customer” to determine a new launch opportunity. However, tweeted comments from ULA CEO Tory Bruno seemed to preclude the risk of an “indefinite” delay and the targeted 30 July launch of another Atlas V with the second Orbital Flight Test (OFT-2) of Boeing’s CST-100 Starliner to the International Space Station (ISS) appears unaffected by the anomaly.

The STPSat-6 payload arrived at Astrotech Space Operations in Titusville, Fla., in early May. Photo Credit: Staff Sgt. Luke Kitterman/U.S. Space Force/Space and Missile Systems Center (SMC)

The mission was originally targeted for late February, which would have made it ULA’s first flight of 2021. Its presence atop an Atlas V in its “551” configuration—the most powerful variant of the rocket, equipped with a 17-foot-diameter (5-meter) payload fairing, five strap-on boosters and a single-engine Centaur upper stage—offered an indicator of the size, mass and complex orbital requirements of this mixed-manifest payload.

Capable of lifting up to 41,000 pounds (18,800 kg) to low-Earth orbit and up to 19,600 pounds (8,900 kg) to Geostationary Transfer Orbit (GTO), the 551 has been used 11 times between January 2006 and March 2020, including the launches of NASA’s New Horizons to explore the Pluto/Charon system and Juno to Jupiter.

The Atlas V hardware for the STP-3 mission arrived in Florida back in February. Photo Credit: ULA

Contracts worth $191 million to launch STP-3 were awarded to ULA back in June 2017, under the Air Force’s Phase 1A procurement strategy, with initial expectations that the mission would fly within the June-August 2019 timeframe, although it met with significant delay. The payload’s primary space vehicle is STPSat-6, the design and integration contracts for which were awarded to Orbital Sciences Corp. (now Northrop Grumman Corp.) in February 2017.

Based upon the A500 satellite “bus”, the geostationary-bound STPSat-6 will house a total of nine payloads for the Department of Defense, the National Nuclear Security Administration (NNSA) and NASA to “operationally demonstrate advanced communication capabilities, collect space weather data and support nuclear detonation detection in the Earth’s atmosphere or in near space”.

STP-3 mission artwork. Image Credit: ULA

Leading this suite of payloads is the NNSA’s third Space and Atmospheric Burst Reporting System (SABRS)-3, built by Los Alamos National Laboratory (LANL) in Los Alamos, N.M., which is part of an ongoing effort to replace neutron, gamma-ray and particle detectors flown aboard Defense Support Program (DSP) satellites from the 1970s through the 1990s. It augments the optical, radio frequency, X-ray and particle sensors of the Global Burst Detector (GBD) payload aboard Global Positioning System (GPS) satellites.

Also aboard STPSat-6 is the Laser Communication Relay Demonstration (LCRD), built by NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Md., which provides an end-to-end optical relay, capable of sending and receiving data from orbiting spacecraft to ground control stations.

The powerful Atlas V 551 previously saw service to lift the sixth Advanced Extremely High Frequency (AEHF-6) military communications satellite to orbit in March 2020. Photo Credit: Jeff Seibert/AmericaSpace

“This evolution to more internet-like communications will reduce the amount of processing required before data can be sent to science and mission operations centers,” NASA previously reported. “LCRD will demonstrate the robust capabilities of optical communications.” Specific benefits from optical communications systems include lower sizes, masses and power requirements and bandwidths 10-100 times greater than traditional radio frequency systems.

Seven additional experiments from the Department of Defense’s Space Experiments Review Board are also assigned to fly STPSat-6. One of them is thought to be the NNSA’s Space and Endo-Atmospheric NuDet Surveillance Experiment (SENSER), which seeks to reduce the developmental risks for future Nuclear Detection sensors by testing and evaluating critical technologies in the space environment, ahead of production and integration into next-generation systems.

An Atlas V 551 lifted NASA’s Juno mission to Jupiter in August 2011. Photo Credit: Alan Walters (

However, delays with STPSat-6 prompted the first postponement of the mission. In March, ULA revealed that a delay had been effected “to enable the customer to evaluate the launch readiness of the spacecraft”.

Finally, early last month, STPSat-6 was delivered from Northrop Grumman’s Dulles, Va., facility to Astrotech Space Operations in Titusville, Fla., for pre-launch processing. By this time, the Atlas V hardware for the mission—the Common Core Booster (CCB) and the Centaur—had also arrived on the Space Coast and were being put through their own checks ahead of stacking. Northrop Grumman announced on 12 May that its five GEM-63 solid-fueled boosters, each measuring 66 feet (20 meters) in length and 5.5 feet (1.6 meters) in diameter, were also in transit to the launch site.

Northrop Grumman’s five GEM-63 motors were delivered to the Cape last month. Photo Credit: Northrop Grumman Corp.

The second element of the STP-3 payload is the Space Force’s Long Duration Propulsion ESPA (LDPE)-1, also built by Northrop Grumman. The LDPE-1 payload—reportedly now redesignated Rapid On-Orbit Space Technology and Evaluation Ring (ROOSTER)-1—is the first of a series of experimental spacecraft designed to accommodate small experiments and facilitate the deployment of small satellites.

Its design is based on Northrop Grumman’s ESPAStar platform, which utilizes a modified Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter (ESPA) “ring” and supports a modular capability for hosting technology development and operational payloads.

Video Credit: AmericaSpace

The ESPAStar concept provides power, pointing, telemetry and command-and-control functionality. The LDPE-1/ROOSER-1 payload was delivered from Northrop Grumman’s Gilbert, Ariz., facility to Cape Canaveral Space Force Station in December 2020 and reportedly marks a step closer “to advancing rideshare capabilities”.

Friday’s announcement of yet another delay to the STP-3 mission is apparently tied to anomalous behavior seen in the Centaur’s RL-10C engine during last month’s mission to deliver the latest geostationary Space-Based Infrared System (SBIRS GEO-5) aloft. “Those watching the live feed may have noticed some “ringing” of RL-10’s new carbon nozzle extension,” tweeted ULA CEO Tory Bruno.

The SBIRS GEO-5 launch in May. Video Credit: AmericaSpace

“While it did its job, boosting RL-10’s eye-watering performance even a bit higher, we want to make sure we fully understand that behavior before flying this configuration again.”

In terms of how this might impact the launch dates for successive Atlas V/Centaur missions—including the long-delayed second Orbital Flight Test (OFT-2) of Boeing’s CST-100 Starliner to the International Space Station (ISS), currently targeted for 30 July—Mr. Bruno offered a more positive update. “OFT-2 does not have the SBIRS-5 configuration of RL-10,” he tweeted in response to a question from Ars Technica‘s Eric Berger. “STP-3 is not indefinitely delayed. We have a structured process for manifest changes. Underway. New schedule shortly.”

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