WASHINGTON — The Pentagon’s congressional overseers appear divided over the timetable for weaning the U.S. Air Force from a controversial Russian rocket engine, a matter that sources say likely will not be settled until House and Senate lawmakers meet later this year to finalize the 2016 defense authorization bill.

At issue is the RD-180, the main engine on United Launch Alliance’s Atlas 5 rocket, which launches the majority of U.S. national security satellites. Denver-based ULA operates two main rockets, the other being the Delta 4, which, though powered by an American-made engine, is 25 percent more expensive and used less frequently than the Atlas 5.

In response to Russia’s annexation of Crimea from Ukraine last year, Congress mandated that the Air Force stop using the RD-180 and begin work on a U.S.-built replacement. That language was finalized late last year in conference negotiations on the 2015 defense authorization bill between a group of lawmakers informally known as the Big Four — the chairmen and ranking members of the House and Senate armed services committees.

But in recent weeks, the Defense Department and ULA have complained that the law is too restrictive and would hamstring the company in upcoming launch competitions against emerging rival SpaceX. Under the most conservative reading of the law, ULA would have only five RD-180 engines available for the competitions, which are slated to begin this year and could cover nine launches over the next three years.

Moreover, Air Force Secretary Deborah Lee James has warned Congress that the law’s 2019 deadline for fielding a new American-made rocket engine is unrealistic, meaning the presumptive beneficiary of the program, ULA, which has announced plans to phase out the Delta 4, would effectively be relegated to the sidelines.

ULA and the Air Force are pushing for a relaxation of the ban. Hawthorne, California-based SpaceX argues that there is no need to change the language, while members of Congress appear to be split.

In a hearing March 17, Reps. Mike Rogers (R-Ala.), chairman of the House Armed Services strategic forces subcommittee, said a change in the law might be necessary to make clear that Congress’ intent was for ULA to have enough RD-180s to compete through 2018 and avoid giving SpaceX, whose Falcon 9 rocket is awaiting Air Force certification, a monopoly on intermediate-class missions. Under that scenario, the Defense Department would benefit from the lower prices resulting from competition.

A day earlier, however, Rep. Mac Thornberry (R-Texas), chairman of the full House Armed Services Committee, was more guarded, saying lawmakers “would want to understand much more clearly why they don’t think that we can have an American engine” by 2019 before they would be inclined to tweak the language. He added, “I would say the anxieties about being dependent upon a Russian engine have only grown” since last year.

Key members of the Senate appear even less sympathetic to the Pentagon-ULA arguments. Sen. John McCain, chairman of the Senate Armed Services Committee, left little ambiguity about his position during a hearing March 18, accusing the Air Force of dragging its feet on a new engine, for which Congress appropriated $220 million this year.

“Instead of giving this effort the level of attention needed, the Air Force has wasted a year doing very little to end our reliance on Russian rocket engines,” McCain said. “If the Air Force is unwilling to do what’s necessary to meet the 2019 deadline, they are going to have to figure out how to meet our space launch needs without the RD-180.”

Sens. Bill Nelson (D-Fla.) and James Inhofe (R-Okla.), meanwhile, wrote Defense Secretary Ashton Carter March 10 to express concern over what they see as Air Force inertia. “Given the urgency of the situation and clear guidance from Congress, I am concerned at the lack of action exhibited by DOD and the Air Force,” they said.

Air Force officials argue that under the law as currently written, the Defense Department could wind up merely trading ULA’s current monopoly for a SpaceX monopoly in the intermediate-class market.

“Without relief from this language, coupled with ULA’s recent decision to retire the non-price competitive Delta medium-class launch vehicle, we will no longer meet our long standing assured access to space policy, where we attempt, to the maximum extent practicable, to have two paths to space for each of our satellites,” said Bill LaPlante, assistant secretary of the Air Force for acquisition.

ULA Chief Executive Tory Bruno has said the company intends to phase out the intermediate class, or single-core, version of the Delta 4, which is similar in capability to the Atlas 5, as early as 2018. Because there is no heavy-lift variant of the Atlas 5, the company will continue flying the triple-core Delta 4 Heavy as long as the Air Force wants, he said.

Bruno has said that the RD-180 ban should be delayed until an American-alternative is ready.

For its part, SpaceX argues that as long as the Delta 4 exists there is no need to relax the bill language. “Not a single additional RD-180 is necessary to ensure American access to space,” SpaceX President Gwynne Shotwell said during the March 17 hearing. “Two American-made launch vehicle families, the ULA Delta 4 series of rockets, and the SpaceX Falcon rockets, have the capability to fulfill 100 percent of the nation’s launch requirements.”

Meanwhile, the Air Force said March 17 that it plans to issue a draft solicitation for new engine concepts in early April. LaPlante said Air Force is also planning to pay for the new engine via a public-private partnership that is “dependent on the level of maturity of the prospective rocket engines.”

ULA is working with Blue Origin on the BE-4, a liquid natural gas-fueled engine that Bruno has said will not be ready to fly Air Force missions before 2021. Aerojet Rocketdyne of Sacramento, California, also is working on an RD-180 alternative dubbed the AR-1 and says it can meet the 2019 deadline, although ULA and Air Force officials remain skeptical.

LaPlante said the Air Force also is planning a broad area announcement for developing software tools for modeling combustion stability, advances in heat-resistant coatings, and fuel injection components. That technology would be made available to industry for the new engine program, he said.