WASHINGTON — United Launch Alliance carried out a static-fire test of its Vulcan Centaur rocket June 7, one of the final milestones before the vehicle’s first launch.

A Vulcan rocket fired its two BE-4 engines in a static-fire test called the Flight Readiness Firing (FRF) at 9:05 p.m. Eastern from Cape Canaveral’s Space Launch Complex 41. The engine start sequence started at T-4.88 seconds, ULA said in a statement an hour after the test, with the engines throttling up to their target level for two seconds before shutting down, concluding the six-second test.

The test appeared to go as planned. “Nominal run,” Tory Bruno, president and chief executive of ULA, tweeted moments after the test.

“This is a huge milestone. This is as close as you can come to launching a rocket without actually launching the rocket,” Mark Peller, vice president of Vulcan development at ULA, said on a company webcast shortly after the test.

The test exercised all the vehicle and ground systems up through ignition of the engines, stopping just before releasing the rocket. “It’s our last major milestone on the path to launch,” he said. “So, a big accomplishment.”

ULA planned to carry out the FRF May 25. However, the company called off the test several hours in advance after detecting a “delayed response” in the ignition system for the booster’s engines. ULA rolled the vehicle back to the Vertical Integration Facility to correct the problem, although the company did not disclose further details.

In February, Bruno said the FRF would be the final major test milestone before the launch of the rocket on its inaugural flight, called Cert-1. After the test, the rocket will be rolled back to the VIF for final preparations, including integration of its payload, before returning to the pad for launch.

However, there was an incident in late March during testing of a Centaur upper stage at NASA’s Marshall Space Flight Center. Hydrogen leaked from the structural test article and ignited, creating a fireball.

Bruno said in a May 16 interview that the company was still investigating the source of the leak and what corrective actions, if any, were needed to fix it. If ULA determines no changes are needed to the upper stage, the Cert-1 launch could take place later in the summer. That would slip to later in the year if ULA decides it needs to make modifications to the Centaur.

An additional factor is that the primary payload for the Cert-1 launch, the Peregrine lunar lander by Astrobotic, had launch windows open for about four to five days a month. ULA will also have to work around other Atlas launches at the pad, although one potential conflict, the first crewed flight of Boeing’s CST-100 Starliner, has slipped from late July because of spacecraft issues.

Other payloads on Cert-1 are the first two demonstration satellites for Amazon’s Project Kuiper broadband constellation, as well as a payload for space memorial company Celestis.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...