Space track

ULA – NROL-71 Mission patch.

Dec. 9, 2018

Image above: The mobile service tower retracts into position for launch during a countdown Dec. 8. Image Credit: United Launch Alliance.

A dramatic automatic abort 7.5 seconds before the planned liftoff of a United Launch Alliance Delta 4-Heavy rocket Saturday night kept the towering launcher on the pad at Vandenberg Air Force Base, California, with a top secret spy payload for the National Reconnaissance Office.

The 233-foot-tall (71-meter) rocket was counting down to launch at 8:15 p.m. PST Saturday (11:15 p.m. PST; 0415 GMT Sunday), but an automated sequencer detected a technical issue and triggered an abort.

“Hold hold hold,” a member of the ULA launch team declared on the countdown net.

A burst of flame appeared at the base of the rocket, a normal occurrence in the final seconds of a Delta 4 countdown as sparklers activate near the engines to burn off excess hydrogen gas before ignition, a measure aimed at eliminating the risk of a fireball or explosion.

It was not immediately clear whether any of the rocket’s three Aerojet Rocketdyne RS-68A main engines started their ignition sequences, but a statement later released by ULA said the computer-controlled countdown sequencer ordered an abort at T-minus 7.5 seconds.

In the statement, ULA said the abort was “due to an unexpected condition during terminal count at approximately 7.5 seconds before liftoff.

Image above: The mobile service tower retracts into position for launch during a countdown Dec. 8. Image Credit: United Launch Alliance.

“The team is currently reviewing all data and will determine the path forward. A new launch date will be provided when available,” ULA said.

The Delta 4-Heavy is made up of three Delta 4 first stage boosters bolted together, each with an RS-68A engine burning liquid hydrogen and liquid oxygen propellants. ULA commands the three RS-68A engines to start in a staggered sequence, beginning with the starboard engine at T-minus 7 seconds, followed two seconds later by ignition of the center and port engines.

The timing of the abort at T-minus 7.5 seconds suggests the countdown stopped around a half-second before the first of the Delta 4-Heavy’s three main engines was supposed to ignite.

ULA’s launch team quickly “safed” the rocket, disarmed ordnance, and drained the Delta 4-Heavy of its supply of cryogenic propellants. The launch team did not set a new target launch date, but officials were instructed to plan for an extended turnaround after Saturday night’s scrub, and the Delta 4-Heavy flight was expected to be delayed at least a few days.

Image above: The mobile service tower retracts into position for launch during a countdown Dec. 7. Image Credit: United Launch Alliance.

A similar cutoff in the final seconds of a Delta 4 countdown in 2010 resulted in a three-day slip to resolve the problem responsible for the abort — and replace the hydrogen burn-off sparklers on the pad — before the rocket successfully launched from Cape Canaveral with a GPS navigation satellite.

The upcoming mission from Vandenberg, located around 140 miles (225 kilometers) northwest of Los Angeles, is codenamed NROL-71 by the National Reconnaissance Office, which owns the U.S. government’s classified intelligence-gathering satellites. The NRO has not released any information about the spacecraft aboard the Delta 4-Heavy, but independent observers of NRO space launches believe the payload is heading for an unusual, high-inclination orbit, and is likely a new Keyhole-type high-resolution optical imaging satellite, with an Earth-pointing telescope capable of capturing extremely detailed imagery of sites around the world for review by government intelligence analysts.

The Delta 4-Heavy is ULA’s biggest rocket, and can loft up to 51,950 pounds (23,560 kilograms) of payload mass to a 120-mile-high (200-kilometer) low Earth orbit inclined 90 degrees to the equator.

The heavy-lift variant of the Delta 4 rocket has launched 10 times to date. The NROL-71 mission will be the 11th flight of a Delta 4-Heavy, and the 38th mission overall for the Delta 4 family since November 2002. It will also be ULA’s ninth and final launch of the year, following five Atlas 5 launches, a pair of Delta 4s, and the final liftoff of the company’s now-retired Delta 2 rocket.

A launch attempt for the NROL-71 mission Friday night was scrubbed after the Delta 4 team encountered a problem with a communications link between the control center and the rocket associated with the holdfire system.

For more information about United Launch Alliance (ULA): https://www.ulalaunch.com/

Images (mentioned), Video, Text, Credits: ULA/Spaceflight Now.com/Stephen Clark.

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