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 almost 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.”...

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 almost 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.