WASHINGTON – The U.S. Air Force contends United Launch Alliance needs as many as 22 RD-180 rocket engines to compete against SpaceX for dozens of national security launches that start going out for bid later this year, according to a U.S. senator.

ULA has ordered 29 RD-180 engines from Russia for its Atlas 5 rocket. Fifteen of those engines are for Air Force launches already under contract. The remaining 14 are what ULA has said it needs to import in order to compete for military launches until its next generation rocket, known as Vulcan and powered by a U.S.-made engine, is ready around 2020.

The Air Force plans to begin soliciting bids later this year for an initial batch of nine missions, all of which Air Force officials say Atlas 5 is suited to launch. A further 28 missions will be put out for bid starting in 2018, with 25 of those suited to the Atlas 5.

Air Force Secretary Deborah Lee James told the Senate Armed Services strategic forces subcommittee in April that ULA needs to be allowed to buy 18 RD-180 engines for missions not already under contract in order to ensure the government receives competing bids when it buys launch services over the next several years.

But the Air Force now appears to be revising that estimate upward, complicating its quest for relief from a 2015 law barring the RD-180’s use for future military launches.

Sen. Richard Shelby (R-Ala.), a senior member of the Senate Appropriations defense subcommittee, told James in a July 15 letter that Air Force leaders, including Gen. Mark Welsh, the service’s chief of staff, told him ULA needed to be allowed to buy 18 to 22 engines to maintain uninterrupted access to space and ensure competition.

Responding on James’ behalf the next day, Bill LaPlante, the assistant secretary of the Air Force for acquisition, told Shelby that 18 engines “is a reasonable starting point” for enabling competition while ensuring national security payloads do not get stranded on the ground by a single launch failure.

“As the competitive environment develops and evolves, we will re-assess the number of engines required to ensure we maintain assured access to space,” LaPlante wrote in his July 16 letter.

The Air Force is required by law to phase out its use of the RD-180 under legislation Congress enacted last year following Russia’s incursion into Ukraine.

Specifically, the 2015 National Defense Authorization Act bars the Defense Department from entering into new contracts or modifying existing contracts for launch services with companies that rely on Russian suppliers.

Because ULA has only partially paid for the 14 engines it ordered from the RD-180’s Florida-based importer RD-Amross in anticipation of winning new Air Force business, Pentagon lawyers have said a strict interpretation of the law could limit ULA to using just five of those engines for the upcoming launch competitions.

That could eventually create a new national security launch monopoly in SpaceX, the Air Force has said.

Both ULA and the Air Force have asked Congress for legislative relief to prevent Atlas 5 from being disqualified from all but a small handful of upcoming launch awards.

While lawmakers appear inclined to grant it, the House and Senate are at odds over how much relief to give.

The House in May approved a defense authorization bill that would permit ULA to use all 14 on-order engines for the competitions. The Senate version of the same bill, approved in June, allows only nine. The difference is a major sticking point between leaders of the two chambers, although Capitol Hill sources expect the Senate position to prevail when the bill emerges from conference.

Neither bill, however, grants ULA the leeway to ride an RD-180-powered Atlas 5 as far into the competition as ULA and the Air Force would like.

That is OK with Sen. John McCain (R-Ariz.), the chairman of the Senate Armed Services Committee, who has been keeping the pressure on the Air Force to end its dependence on the RD-180.

ULA says it is committed to phasing out the RD-180 and switching to Vulcan, a next-generation rocket featuring a main engine being developed by Kent, Washington-based Blue Origin.

But Vulcan is not expected to make its debut before 2019 and win certification to launch military payloads until 2022. That schedule, ULA has said, assumes no technical setbacks and that ULA does not find itself a few years from now sitting on the sidelines with no Atlas 5 and a Delta 4 it admits is too expensive to give SpaceX’s Falcon 9 a run for the Air Force’s money.

If Congress does not give the Air Force the relief it thinks it needs to keep Atlas 5 in the game, the service could ask the U.S. secretary of defense for a waiver.

The 2015 law stipulates that a waiver is to be used for “the national security interests of the United States” and when space launch services could not otherwise be obtained at a fair and reasonable price.

Air Force officials have been studying how and when to potentially use the waiver.

But for now, the Air Force is focusing on getting Congress to change the law.

“The Department is working closely with Congress to request some relief from the FY15 [National Defense Authorization Act] language in order to guarantee assured access to space while maintaining a competitive environment, while also transitioning expediently off Russian-made engines,” Capt. Annmarie Annicelli, an Air Force spokeswoman, said in a July 22 email to SpaceNews.