The Pentagon and the United Launch Alliance agree on the need for rockets that are made in the United States. In testimony before the Senate in April, the secretary of the Air Force, Deborah Lee James, said the invasion of Crimea “made it abundantly clear to all of us that we have to stop relying on Russian engines.”

But she also said that halting any new purchases made after the invasion of Crimea, as the law now requires, could leave the Air Force without a viable engine for the Atlas after 2018.

Ending the reliance on the RD-180 “is not as simple as it appears,” said David A. Deptula, a retired Air Force general who runs the Mitchell Institute for Aerospace Studies in Arlington, Va. He said “underinvestment, lack of a clear vision and stifling bureaucracy” had slowed innovation of alternatives for years.

“We must always remember that this nation went to the moon in less than a decade,” he wrote in an email.

The debate over the Russian engines has become entangled in an emerging rivalry among the companies vying for the lucrative business of space launches, amounting to $70 billion in contracts for military and intelligence missions alone between now and 2030, according to an estimate cited by the Government Accountability Office.

The United Launch Alliance, formed in 2006, has a monopoly on military and intelligence contracts, but it faces competition from Space Exploration Technologies Corporation, or SpaceX, founded by Elon Musk, the entrepreneur behind PayPal and Tesla. SpaceX has developed its own rocket, called the Falcon 9, which has carried out 18 successful missions for NASA and private companies, including the first private resupply of the International Space Station. After months of review, negotiations and a lawsuit that was settled last year, the Air Force last month certified the Falcon 9 for use in national security missions, ending United Launch Alliance’s monopoly.

According to Ms. James, SpaceX can now compete for two launches scheduled this year and seven more planned for the next two years. The Air Force announced this week that it had opened bidding on the first of those, with applications due by June 23.