The Pentagon is growing concerned that the United Launch Alliance, one of its key partners in launching national security satellites to space, will not be able to meet its needs to counter China and build its arsenal in orbit with a new rocket that ULA has been developing for years.

In a letter sent Friday to the heads of Boeing’s and Lockheed Martin’s space divisions, Air Force Assistant Secretary Frank Calvelli used unusually blunt terms to say he was growing “concerned” with the development of the Vulcan rocket, which the Pentagon intends to use to launch critical national security payloads but which has been delayed for years. ULA, a joint venture of Boeing and Lockheed Martin, was formed nearly 20 years ago to provide the Defense Department with “assured access” to space.

“I am growing concerned with ULA’s ability to scale manufacturing of its Vulcan rocket and scale its launch cadence to meet our needs,” he wrote in the letter, a copy of which was obtained by The Washington Post. “Currently there is military satellite capability sitting on the ground due to Vulcan delays.”

He added: “As the owners of ULA, and given the manufacturing prowess of Boeing and Lockheed Martin corporations, I recommend that you work together over the next 90 days to complete an independent review of ULA’s ability to scale its launch cadence to meet its current” contract requirements.

ULA launched the Vulcan booster for the first time earlier this year and needs to fly it a second time to earn certification from the Pentagon to begin flying national security and intelligence missions. ULA hopes that the second certification launch will occur later this year.

ULA originally won 60 percent of the Pentagon’s national security payloads under the current contract, known as Phase 2. SpaceX won an award for the remaining 40 percent, but it has been flying its reusable Falcon 9 rocket at a much higher rate. ULA launched only three rockets last year, as it transitions to Vulcan; SpaceX launched nearly 100, mostly to put up its Starlink internet satellite constellation. Both are now competing for the next round of Pentagon contracts, a highly competitive procurement worth billions of dollars over several years.

ULA is reportedly up for sale; Blue Origin is said to be one of the suitors.

In addition to its contract with the Pentagon, ULA has committed to 38 launches of Amazon’s Kuiper internet satellite constellation over the next few years, a pace that would require ULA to increase its flight rate well beyond what ULA normally has achieved.

That, Calvelli wrote, raises his concern. To meet its commitment to the Pentagon alone, ULA must launch 25 national security missions by the end of 2027. In all, ULA has said it has sold 70 launches on Vulcan. But over the past five years, Calvelli noted in his letter, the company has had “an average launch cadence of fewer than six launches per year.”

Calvelli did not say in the letter what his specific concerns were with the rocket’s development, and he declined to comment for this report. But in the letter, he cited the Pentagon’s need to move quickly in the space domain as adversaries build their capabilities there.

“The United States continues to face an unprecedented strategic competitor in China, and our space environment continues to become more contested, congested and competitive,” he wrote. “We have seen exponential growth of in-space activity, including counterspace threats, and our adversaries would seek to deny us the advantage we get from space during a potential conflict.”

As the Air Force’s acquisition executive for space, he said he is “focused on driving speed in our acquisitions and delivering programs on cost and schedule to transform our architecture.”

In a statement to The Post, ULA said that its “factory and launch site expansions have been completed or are on track to support our customers’ needs with nearly 30 launch vehicles in flow at the rocket factory in Decatur, Alabama.”

Last year, ULA CEO Tory Bruno said in an interview that the deal with Amazon would allow the company to increase its flight rate to 20 to 25 a year and that to meet that cadence it was hiring “several hundred” more employees. The more often Vulcan flies, he said, the more efficient the company would become.

“Vulcan is much less expensive” than the Atlas V rocket that the ULA currently flies, Bruno said, adding that ULA intends to eventually reuse the engines. “As the flight rate goes up, there’s economies of scale, so it gets cheaper over time. And of course, you’re introducing reusability, so it’s cheaper. It’s just getting more and more competitive.”

In a statement, Lockheed said that “the pace and seriousness of the threats our customers face are not to be underestimated, and with our ULA joint venture partner Boeing, we are committed to providing reliable and swift launch capabilities to meet our customers’ mission demands. We are reviewing Mr. Calvelli’s request and will work together to address it with urgency.”

Boeing said in a statement: “We are getting on more of a wartime footing to stay ahead of the threat, and a quicker and more reliable launch cadence is critical to meeting that need.” It said it would also work to address Calvelli’s concerns.

ULA decided to eventually retire its workhorse Atlas V rocket after concerns within the Pentagon and Congress that it relied on a Russian-made engine, the RD-180. In 2014, the company entered into a partnership with Jeff Bezos’ Blue Origin to provide its BE-4 engines for use on Vulcan. However, the delivery of those engines was delayed for years — one of the reasons Vulcan’s first flight didn’t take place until earlier this year. (Bezos owns The Washington Post.)

In prepared testimony before the House Armed Services Committee earlier this month, Calvelli wrote that the Pentagon needs Blue Origin to dramatically speed up production of the engines to allow Vulcan to meet its national security requirements. Blue Origin also intends this year to start flying its New Glenn rocket, which is powered by seven BE-4 engines. Blue Origin is also seeking to launch satellites for the Pentagon.

“Blue Origin needs to scale its production of BE-4 engines,” he wrote. “We are keeping an eye on whether these two companies can scale to meet our needs.”

Calvelli addressed his letter to Kay Sears, who oversees Boeing’s Space, Intelligence and Weapons Systems division, and Robert Lightfoot, president of Lockheed Martin’s space division. Bruno, ULA’s CEO, was copied.

For years, ULA was the Pentagon’s only launch provider. Then, in 2014, SpaceX, the space venture founded by Elon Musk, sued the Air Force, arguing it should have the right to compete for the launch contracts. The parties settled in 2015. SpaceX has since flown multiple missions for the Pentagon, forcing ULA to compete against a hard-charging and nimble competitor that has upended the industry by launching several times a month.

The U.S. Space Force has said in recent years it wants to harness the capabilities coming from the growing commercial space sector, which is innovating faster than the government. Recently, it released a commercial space strategy that said it would seek to avoid “overreliance on any single provider or solution.”

Competition is key, Pentagon officials have repeatedly stressed, to lowering costs and driving reliability, and the department has maintained that it needs multiple rocket providers to get its assets into orbit.

“Launch is critical to our ability to transform our space architecture,” Calvelli wrote. “We are counting on Boeing, Lockheed Martin, and the ULA team to be successful in getting critical capabilities into space for our warfighters.”