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Watch our live coverage of the Flight Readiness Firing for United Launch Alliance's first Vulcan Centaur rocket at Cape Canaveral Space Force Station. Follow us on Twitter.

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United Launch Alliance's first Vulcan Centaur rocket is back on its launch pad at Cape Canaveral for a critical test-firing of its Blue Origin-built BE-4 main engines as soon as Wednesday, a test that was delayed from last month by an issue with the booster engine ignition system.

ULA plans to load methane, liquid hydrogen, and liquid oxygen propellants into the Vulcan first stage and its Centaur upper stage Wednesday afternoon. If the tanking procedures go well and the rocket's ignitors pass a readiness check, ULA's launch team is expected to proceed with the test-firing of the BE-4 engines some time after 6 p.m. EDT (2200 UTC).

The engine test window extends for several hours Wednesday evening, but ULA did not provide a schedule of events. A ULA spokesperson said the company would not release a timeline because the engine hold-down firing is a test, and the company declined a request for an interview to discuss plans and objectives for the test-firing.

Live video the engine test, called a Flight Readiness Firing, will be available on this page.

ULA technicians rolled the Vulcan Centaur rocket from its vertical hangar to pad 41 Tuesday at Cape Canaveral Space Force Station in preparation for the test-firing.

The Flight Readiness Firing will demonstrate day of launch timelines and procedures, propellant loading operations, and the countdown sequence for the Vulcan Centaur rocket through ignition of the booster's twin BE-4 engines, according to ULA. The test is meant to check the performance of the launch vehicle, engines, ground systems, and software.

After coming out of a final built-in hold, the automated countdown will tick the final few minutes until ignition of the BE-4 engines. The Vulcan Centaur rocket will switch to internal power and the propellant tanks will rise to flight pressure before valves open to allow methane and liquid oxygen to flow into the BE-4 engine thrust chambers.

The twin engines will flash to life, throttle up, and fire for about six seconds, sending a plume of exhaust out of the east-facing flame trench at pad 41. Hold-down restraints will keep the Vulcan rocket attached to its mobile launch platform throughout the test-firing.

Then the rocket's flight computer will command the engines to shut down at the conclusion of the test.

"The engines will execute a nominal terminal count, then ignite and throttle, holding trust before the engines begin throttle down to to simulate in-flight throttling prior to booster engine cutoff, then execute a flight-like shutdown," said RJ Sansom, ULA's Vulcan chief engineer, in a blog post on the company's website.

The Flight Readiness Firing will be the culmination of a series of tests and countdown rehearsals at Cape Canaveral to prepare for the first Vulcan test flight. Most recently, ULA's launch team loaded methane, liquid hydrogen, and liquid oxygen propellants into the Vulcan booster and its Centaur upper stage during a May 12 tanking test. The countdown for the Flight Readiness Firing will follow a similar timeline as the May 12 tanking test, including propellant loading operations, built-in holds, and readiness polls of the launch team.

ULA moved the Vulcan Centaur rocket back to the Vertical Integration Facility after the May 12 tanking test to make "adjustments" to the vehicle. The changes included adjusting a setting with ground hydraulic pressure, changing the topping rate for liquid oxygen, and changing the flow of purge and chill gas to the BE-4 engine igniters, according to Tory Bruno, ULA's chief executive officer.

With those changes complete, ground teams planned to perform the Flight Readiness Firing on May 25, but ULA postponed the test-firing after discovering a problem with the BE-4 engine ignition system. That prompted a return of the rocket to the hangar for troubleshooting before ULA rolled the Vulcan launcher back to pad 41 Tuesday.

"FRF is really about confirming the operational readiness of the integrated system: launch vehicle, ground systems, facilities and the associated software. In addition, we will demonstrate the ability to successfully execute the engine start sequence and validate our hot-fire abort response procedures," said Dillon Rice, ULA's Vulcan launch conductor, in a post on the company's website.

ULA says it installed additional instrumentation on the rocket to monitor the performance of the engines during the Flight Readiness Firing.

In parallel with the preparations for the Flight Readiness Firing, ULA engineers continue investigating a hydrogen explosion in March that cut short a structural test of the Vulcan rocket's Centaur upper stage at NASA's Marshall Space Flight Center in Huntsville, Alabama. The blast damaged the test stand and a Centaur upper stage test article. The Vulcan rocket will use a larger, upgraded model of the Centaur upper stage currently flying on ULA's Atlas 5 rocket.

If engineers determine they don't need to make any changes to the Centaur upper stage on the first Vulcan rocket, the test flight could take off this summer. In remarks last month, Bruno said the mission could delay until later this year if corrective actions are required on the Centaur.

ULA is a 50-50 joint venture between Lockheed Martin and Boeing, which merged their Atlas and Delta rocket programs in 2006. The Vulcan rocket will fly in several configurations, with varying numbers of strap-on solid rocket boosters and different payload fairing sizes available on each flight, depending on mission requirements.

The Vulcan rocket for the program's first test flight sports a colorful paint job with a bright red flame emblazoned on the side of the 17.7-foot-side (5.4-meter) first stage. For the tanking tests and the Flight Readiness Firing, the Vulcan rocket is not fitted with any solid rocket boosters or a payload fairing. In that configuration, the vehicle stands about 166 feet (50.7 meters) tall.

Once the test-firing is complete, ULA will drain the rocket's propellant tanks and return the Vulcan Centaur to its hangar for inspections. Technicians will install two of the Northrop Grumman-built solid-fueled boosters and the payload shroud supplied by Beyond Gravity, formerly known as Ruag Space.

ULA's ground teams will also inspect the Vulcan rocket and its engines after returning the vehicle to the hangar, and technicians may need to adjust or replace thermal blankets around the engines that could be singed by the test-firing. ULA will also swap out single-use igniters on the BE-4 engines before moving forward with final launch preparations.

The Vulcan rocket's inaugural flight will be the first first launch to use new methane-fueled BE-4 engines from Blue Origin, founded by billionaire Jeff Bezos. At full throttle, each BE-4 engine can generate about 550,000 pounds of thrust. Two of them will power each Vulcan core stage, with zero, two, four, or six solid rocket boosters to add thrust in the first couple of minutes of flight.

The Vulcan rocket's Centaur upper stage, called the Centaur 5, is an upgrade to the upper stages currently flying on ULA's Atlas 5 rocket. The Centaur 5 has a wider diameter to accommodate larger cryogenic hydrogen and oxygen propellant tanks, along with two Aerojet Rocketdyne RL10 engines. The Centaur flying on the Atlas 5 rocket typically flies with a single engine.

Once all of the Vulcan rocket configurations are operational, the new rocket will fully replace and grow the lift capability currently offered by all of ULA's rockets. The largest Vulcan rocket variant, with a single core stage, will have more payload lift capability than ULA's Delta 4-Heavy, which has three liquid-fueled first stage boosters connected together.

Eventually, ULA plans to recover the reuse BE-4 engines from Vulcan launches, but not the entire first stage.

ULA unveiled the Vulcan rocket in 2015, then targeting a first launch of the new vehicle in 2019. The company selected Blue Origin's BE-4 engine for the first stage propulsion system in 2018. At that time, ULA aimed to launch the first Vulcan test flight in 2020.

But delays, primarily caused by issues discovered in BE-4 engine production and testing, forced the first Vulcan test flight to slip several years. Bruno said earlier this month that Blue Origin and ULA completed final qualification testing of the BE-4 engine ahead of the first Vulcan launch, clearing a hurdle that was still threatening to delay Vulcan's debut earlier this year.

On its first flight, the Vulcan rocket will launch a commercial moon lander developed by Astrobotic, which will attempt to deliver a batch of NASA experiments and tech demo payloads to the lunar surface. The Astrobotic lander, named Peregrine, is part of NASA's Commercial Lunar Payload Services Program, which buys rides to the moon for agency payloads on commercially-owned spacecraft.

Two prototype satellites for Amazon's Kuiper broadband network will also be aboard the first Vulcan launch.

ULA's Vulcan rocket has been selected by the U.S. Space Force to launch the majority of the military's large national security satellites for the next five years. The military requires to "certification flights" of the Vulcan rocket before it is approved for national security launch missions.

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Follow Stephen Clark on Twitter: @StephenClark1.