WASHINGTON — A defense authorization bill just drafted in the U.S. Senate would leave United Launch Alliance with fewer Atlas 5 rocket engines than the company says it needs to stay in the competitive national security launch arena until its next-generation rocket becomes available around the end of the decade.

Under the Senate Armed Services Committee’s markup of the National Defense Authorization Act for 2016, ULA would have as few as five of the Russian-made RD-180 engines available for upcoming competitive rounds of the U.S. Air Force’s Evolved Expendable Launch Vehicle program. In a best-case scenario, the Denver-based company could have as many as nine of the engines, which are being phased out due to the downturn in relations with Russia.

ULA and Defense Department officials say the company needs 14 RD-180s to be able to go head-to-head with emerging rival SpaceX until its next-generation rocket, dubbed Vulcan, begins flying. The Vulcan, unveiled in April and featuring a U.S.-made engine, is expected to make its first flight in 2019 and be certified to launch U.S. national security missions by 2021.

In response to Russia’s annexation of Crimea from Ukraine last year, Congress mandated in the National Defense Authorization Act for 2015 that the Air Force stop using the RD-180 by around 2019. The ban does not cover engines ordered to fulfill ULA’s $11 billion Air Force contract for Atlas 5 and Delta 4 rockets and launch services over the next several years.

But ever since the legislation was signed into law in December there have been questions about how many engines ULA would have available for the Atlas 5, its most competitive and versatile rocket, beyond the block buy. The Air Force says the current law leaves only five additional engines available for the upcoming competitions and, along with ULA, has asked Congress for relief in the 2016 defense authorization.

Absent such relief, ULA and the Air Force say, the national security launch market would effectively become a SpaceX monopoly, a reversal of the current state of affairs. Although ULA’s Delta 4 is available, that vehicle is not competitive with SpaceX’s Falcon 9, and ULA says it intends to phase out all but the seldom-used heavy-lift Delta 4 variant by 2018.

The House version of the 2016 defense authorization, which was approved May 15, would grant ULA access to the 14 RD-180s it says it needs to stay in the game until the Vulcan is ready.

That bill must eventually be reconciled with the newly drafted Senate version, which is less generous from ULA’s perspective.

“The nine engines would not make it to a new engine being ready,” Jessica Rye, a ULA spokeswoman, said in a May 14 email to SpaceNews. The new engine she was referring to likely is the BE-1, which would power the first stage of the Vulcan.

John McCain, the chairman of the Senate Armed Services Committee, has been a fierce critic of both ULA’s national security launch monopoly and its reliance — and by extension, the Air Force’s reliance — on the RD-180.

“We cannot and must not be dependent on Russian rocket engines,” McCain said in a May 14 press conference. “The sooner we are away from them, the better we are.”

The Air Force is planning to introduce competition into the EELV program in phases, the first of which features nine missions to be awarded between now and 2017. The Air Force recently said the first of the competitive contracts would be for the launch of a GPS 3 satellite.

The Senate bill stipulates that if ULA wins all nine contracts in the first competitive phase, it would have no RD-180s available for the second phase. If, on the other hand, Hawthorne, California-based SpaceX wins all of the first-phase contracts, ULA would have only five RD-180s available for the second round or for other customers.

The Senate markup was the latest development in the legislative saga surrounding the RD-180 and its replacement strategy. On May 12, the White House said it disagrees with the House bill’s proposed approach for weaning the Defense Department off the RD-180.

Congress appropriated $220 million for new engine development work this year, and is eager to see that program get started in earnest. But the Air Force is wary of investing an estimated $1 billion into an engine nobody wants and instead proposes to spend the money more broadly on launch vehicle development.

The House bill specifies that the money go toward development of a “rocket propulsion system, and the necessary interfaces to the launch vehicle, to replace non-allied space launch engines.”

The White House views this approach as shortsighted.

“While rocket engines are a major component of a launch vehicle, they are only one of many critical components,” the White House Office of Management and Budget said in a statement of administration policy. “These components must be designed and developed together to meet the ultimate cost and performance goals, not only for the launch vehicle but also for the support, operations, and production infrastructure as well. Without a comprehensive strategy that ensures the availability of operational launch systems, the government risks investing hundreds of millions of dollars without any guarantee of ensuring assured access to space.”