(Bloomberg) -- Boeing Co. and Lockheed Martin Corp.’s rocket venture will fly a dummy payload instead of Sierra Space’s tardy Dream Chaser plane during its key Vulcan rocket certification flight in September.

Sierra Space “have told us that they will step aside in order to support our critical national security space missions,” Tory Bruno, Chief Executive Officer of Boeing-Lockheed’s United Launch Alliance told reporters on Wednesday.

United Launch Alliance is racing to certify its new Vulcan rocket for a flurry of launches carrying the nation’s most-sensitive satellites for the US Defense Department. After certification, ULA plans to launch two national security missions on Vulcan before the end of the year. The rocket is also under contract for future trips to put Amazon.com Inc.’s internet satellites into orbit. 

Vulcan is meant to be ULA’s primary rocket moving forward and is critical for the company’s ability to compete against Elon Musk’s SpaceX.

ULA now will fly a mass simulator — essentially a hunk of metal designed to mimic a satellite’s weight, shape and instrumentation — during its long-awaited September certification flight. The US Space Force requires Vulcan to fly two successful missions to prove it can handle national security cargo. In January, Vulcan deployed a robotic lunar lander during its debut launch.

ULA also plans to fly experiments during the September mission and conduct specific maneuvers to put Vulcan through its paces. 

Asked during the briefing why Amazon satellites couldn’t replace Dream Chaser, Bruno said the online shopping giant had always planned to fly its first satellites on an Atlas V, ULA’s workhorse rocket that is being retired. Flying satellites on the Atlas V requires a different configuration and different spacecraft dispenser than Vulcan uses. 

“They were never, probably, really going to be ... set up to jump from an Atlas to a Vulcan in an unplanned way,” Bruno said. 

In May, Frank Calvelli, the Air Force’s assistant secretary for space acquisition, told ULA that it needed to pick up the pace of its Vulcan launches in order to meet the “growing demand” for national security missions. The Air Force also imposed financial penalties on ULA for delays with Vulcan.

Bruno said he is confident in ULA’s ability to increase the flight cadence and that he is working with an independent review team to help with the issue. 

“It’s absolutely appropriate for everyone, including my customer, to focus on ramping up the rocket,” Bruno said. “For us, that is the primary thing we’re working on right now.”

(Updates with details on the mission from tenth paragraph.)

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