ULA conducts NROL-107 launch, last Atlas NRO mission

by Ian Atkinson

United Launch Alliance (ULA) launched its Atlas V with the classified NROL-107 Silent Barker mission set on Sunday, following several days, including a rollback due to a hurricane threat.

For this demanding mission, sending the payload to a near-geosynchronous orbit, Atlas V flew in its most powerful active configuration: Atlas V 551.

NROL-107 Silent Barker launch

Silent Barker is a joint project of the National Reconnaissance Office (NRO) and the United States Space Force that has been in development for approximately three years. According to published budget reviews, “SILENTBARKER will provide the capability to search, detect, and track objects from a space-based sensor for timely custody and event detection.

This essentially means that Silent Barker will form a “watchdog” constellation of satellites used for tracking other countries’ activities in geostationary orbit. This departs from the current method of geostationary tracking using ground-based assets, which can track objects down to the size of a basketball — depending on the weather on the ground. Silent Barker can track smaller objects and continuously observe their position and movement.

This launch, expected to be one of two in total for the initial constellation, will have multiple satellites onboard — although the exact number is classified. NRO will operate the constellation, which should be operational by 2026.

From the publicized launch requirements, the Silent Barker payload will be launched into a 41,849 by 42,479-kilometer orbit with zero inclination, positioned at the 105-degree East slot. This places the satellites around 7,000 kilometers above a proper geosynchronous orbit. It is unknown where the satellites will maneuver themselves following deployment.

But before any on-orbit activities take place, Silent Barker must first launch.

The payload’s Atlas V launcher began stacking in early August. The first stage booster was first installed on the mobile launcher, followed by the five GEM-63 solid rocket motors (SRMs) and the Centaur upper stage — situated inside the base of the five-meter payload fairing.

On Aug. 11, the complete stack — minus the payload — was rolled out to the launch pad at SLC-41. There, the vehicle performed a wet dress rehearsal (WDR). As the name suggests, this event is a complete rehearsal of launch day activities, from propellant loading until just before engine ignition. This helps work out any potential issues in the integrated launch vehicle and ground support systems prior to launch day.

Many ULA launches do not include a pre-flight WDR. However, customers can request them for more important or expensive payloads. Missions such as Boeing Starliner flights, NASA science missions, and NRO payloads have commonly utilized such tests.

An exploded view of the Atlas V 551 configuration – the most powerful flown version of Atlas V. This rocket will lift NROL-107 to orbit. (Credit: United Launch Alliance)

After completing the WDR, Atlas V was rolled back into the vertical integration facility to have the payload installed. Although the cryogenic liquid oxygen and liquid hydrogen propellants were drained from the vehicle before the rollback, the first-stage RP-1 fuel was not. This is because RP-1, a refined form of kerosene, is stable at room temperatures and can remain inside the vehicle long-term compared to cryogenic propellants.

NROL-107, situated inside its five-meter-wide payload fairing, was lifted onto the rest of the launch vehicle before the now-complete rocket returned to the launch pad.

On launch day, Atlas V was loaded with its remaining, cryogenic propellants — liquid oxygen and liquid hydrogen. The first stage RD-180 engine burns RP-1 and liquid oxygen, while the Centaur second stage’s RL-10C-1-1 engine burns liquid hydrogen and liquid oxygen.

Both the RD-180 engine and Centaur upper stage can trace their roots back to previous Atlas vehicles. Centaur has been flying as a second stage for Atlas rockets since May 1962 on Atlas-Centaur, the rocket that launched the Surveyor lunar probes, multiple Mariner missions to Mars, and Pioneer 10 & 11 to Jupiter and Saturn. The RD-180, however, is much more modern, beginning its service life on the short-lived Atlas III in 2000. While the RD-180 will be retired alongside Atlas V later this decade, Centaur has been further developed for ULA’s upcoming Vulcan rocket.

The countdown entered a planned hold at T-4 minutes, during which Atlas mission control performs a final go/no-go poll of all flight controllers to ensure the vehicle is safe and ready for launch. Should all controllers respond with “go,” the hold is released shortly afterward.

During the final four minutes of the countdown, the propellants were topped off and the tanks pressurized for flight. At T-2.7 seconds, the RD-180 engine ignited, and the flight computers autonomously check all engine and vehicle systems.

The computers command the simultaneous ignition of the five SRMs and the release of the launch clamps, allowing the vehicle to lift off from the launch pad at T+1 second.

Thanks to the high thrust of the five GEM-63 SRMs, Atlas V goes supersonic just over 35 seconds after liftoff. Only 11 seconds later, the vehicle reaches max-Q, which is the period of maximum dynamic pressure on the vehicle. After this point, aerodynamic loading on the vehicle decreases until it reaches near-zero as the rocket enters the vacuum of space.

Onboard view from the Centaur second stage on a previous Atlas V launch, showing the booster stage falling away. (Credit: United Launch Alliance)

At one minute and 44 seconds after launch, the SRMs burn out and are jettisoned. They separate in one group of two and one group of three, with a gap of two seconds in between.

The next event is payload fairing jettison at T+3 minutes and five seconds, which exposes Centaur and the Silent Barker payload to space. This eliminates some dead mass; the protective fairing is no longer needed as the vehicle will be in the vacuum of space. The two Centaur forward load reactors jettison a few seconds later. These pieces of hardware redirect some structural load from the payload adapter to the sides of the fairing, lowering the overall stresses on the thin tank walls of Centaur during the initial high-acceleration portions of the flight.

Given the classified nature of this mission, the published launch timeline does not detail any further mission events. However, based on previous Atlas V 551 flights, it can be inferred that the Atlas booster — or first stage — should burn out and separate around four and a half minutes into the flight. Centaur will then begin its first of likely three or four burns, bringing the payload into a low-Earth parking orbit. The RL-10 engine will then shut down and the stage will coast.

After coasting for some period of time, the RL-10 will ignite for a second time, sending the stack into a geostationary transfer orbit. Once again, the stage will enter a coast phase, this time for approximately five to six hours.

Near apogee, the highest point in the geostationary transfer orbit, Centaur will ignite its RL-10 engine for the third time, boosting Silent Barker into a near-geosynchronous orbit. The satellites will then deploy, and Centaur will either vent the remaining propellants or ignite its engine for a final time, entering a graveyard orbit for disposal.

This mission was the 98th for Atlas V, and the second this year for United Launch Alliance. It also closes a chapter in Atlas V’s history. NROL-107 will be the last NRO payload to launch on the Atlas V, as all further ULA-launched reconnaissance missions will fly on their upcoming Vulcan rocket.

(Lead image: Atlas V rolling out to the launch pad at SLC-41 ahead of the AEHF-6 mission in March 2020. Credit: Sawyer Rosenstien)

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