The once-dominant rocket maker trying to catch up to Musk’s SpaceX

These days, the military has a different number on speed dial: Elon Musk’s.

United Launch Alliance, the Colorado-based company that long had a virtual monopoly on national-security missions, has been usurped over the past decade by Musk’s SpaceX. The billionaire-led company has grown to become the world’s busiest rocket launcher and, over the past couple of years, the chief partner to the U.S. military, flying many of its most sensitive space missions.

ULA, a joint venture of Boeing and Lockheed Martin, is striving to reclaim its position by moving past problems that have hamstrung its new Vulcan Centaur rocket, leaving the vehicle years behind schedule. While it is pushing to speed production, the company’s struggles are drawing scrutiny from Congress and Pentagon officials, who want several companies capable of blasting off defense and spy satellites, as military powers jockey in orbit.

“Vulcan delays are now impacting national-security launches, leaving military satellite capability on the ground," said a spokeswoman for the Air Force, the parent organization for the military’s Space Force.

ULA has been preparing its Decatur, Ala., rocket factory for higher Vulcan production rates, including by clearing space dedicated to older vehicles and deploying engineers to keep manufacturing lines moving, Chief Executive Tory Bruno said.

“We know that a lot of people are depending on us," Bruno said. “We’re really pushing hard to ramp up."

ULA’s future is in question as its owners have discussed selling the company to potential buyers. The company, which has nearly 3,000 employees working across its facilities and launch sites, doesn’t report financial details, but filings from Lockheed Martin indicate earnings have fallen in recent years.

ULA was formed in 2006 when Boeing and Lockheed fused their rival launch businesses. The Pentagon supported the merger, wanting to ensure rockets Boeing and Lockheed long operated separately would continue to be available for national-security missions.

The company built a sterling record handling Pentagon launches. Between 2007 and 2017, ULA blasted off more than 70 national-security missions, according to data collected by the astrophysicist Jonathan McDowell.

SpaceX, founded by Musk in 2002 and for years an upstart, battled in Washington and in court to compete for military flights. Meanwhile, it developed a fleet of partially reusable rockets that proved to be cheap, trustworthy and capable of rapid launches.

In 2016, it won its first launch contract with the Air Force—breaking ULA’s monopoly—and handled its initial flight for the National Reconnaissance Office, a spy agency, the following year.

Betting on Vulcan Centaur

ULA bet big on a new rocket, Vulcan Centaur, which has a powerful main booster and a smaller craft, Centaur, to propel satellites and other vehicles to orbital destinations. Vulcan is designed to reach targets in deeper space as well as near-Earth orbits, the destination for such launches as Amazon.com’s satellite network.

The company began developing Vulcan about a decade ago after Congress required ULA to phase out use of a Russian-built engine on its older Atlas V rockets following the Kremlin’s 2014 invasion of Ukraine’s Crimea.

ULA later hired Blue Origin, the space company founded by Jeff Bezos, to supply Vulcan with new engines. Bruno said six years ago, when the engine deal was announced, that he hoped to fly Vulcan for the first time in 2020.

Blue Origin missed early goals for engine production and struggled after opening a Huntsville, Ala., factory four years ago as it developed a production system and brought on scores of new employees, people familiar with the facility said.

Bezos’ space company has been pushing to churn out engines for ULA and its own use. The company hired a manufacturing leader from SpaceX, Ian Richardson, to take on a high-level operations job, work that has included improving the engine factory.

ULA’s Bruno said in April that Blue Origin has been closing in on building one engine a week, versus one a month. Each Vulcan booster needs two of the engines.

In January, ULA launched Vulcan Centaur for the first time. The company plans three more Vulcan launches this year: a mission in September designed to show the U.S. military the rocket can operate as designed and then two operational flights for the Pentagon.

A Blue Origin spokesman said the company recently delivered its eighth engine to ULA, giving ULA enough for Vulcan’s next three flights. “We have dozens more in our manufacturing flow and continue to scale our factory and supply chain to ensure we meet all future engine demand," he said.

Lost business

ULA now has 23 Vulcans in various stages of production, but the company’s struggles have cost it business and drawn attention from the Pentagon.

A Space Force command recently reassigned three missions from ULA to SpaceX because Vulcan is behind schedule.

Frank Calvelli, the top acquisition official for space at the Air Force, said in a May letter to Boeing and Lockheed that he was concerned about ULA’s ability to scale up Vulcan production and conduct more flights.

Calvelli recommended an outside review. Former military and aerospace officials, including Susan Mashiko, who once served as deputy director at the National Reconnaissance Office, are working on the effort. Mashiko declined to comment.

In May, Calvelli also visited ULA’s Alabama rocket factory, Blue Origin’s nearby engine plant and another Vulcan supplier. The Washington Post reported earlier on his letter. Boeing and Lockheed said they were committed to ensuring ULA reaches a faster launch cadence.

Some federal lawmakers have pushed the Pentagon to work with a larger number of rocket launch providers, in addition to ULA and SpaceX, which now has a de facto monopoly on U.S. launches.

“If you have a monopoly situation and inadequate competition it will be more difficult to get the best price for the technologies you need," Rep. Adam Smith (D., Wash.), ranking member of the House Armed Services Committee, said in an interview earlier this year.

Emily Glazer contributed to this article.

Write to Micah Maidenberg at micah.maidenberg@wsj.com