WASHINGTON — U.S. government launch services provider United Launch Alliance is evaluating a proposal to purchase as many as 30 more RD-180 engines for its Atlas 5 rocket despite a congressionally imposed ban on using the Russian-built hardware in its primary national security market.

The new engines would be used for future commercial Atlas 5 missions, ULA spokeswoman Jessica Rye said. The Atlas 5 today is used primarily for U.S. government missions, but the vehicle is being shopped more aggressively on the commercial market and is expected to begin launching astronaut-carrying capsules to the International Space Station on a commercial basis as early as 2017.

Word of the negotiations follows the passage of legislation barring the use of the RD-180 on future military missions. Specifically, the National Defense Authorization Act of 2015, which was signed into law in December, directs the U.S. Air Force to wean itself from the Russian-built propulsion system by 2019.

The ban was drafted after a swift deterioration of U.S.-Russian relations, prompted by Russia’s incursions into Ukraine last year, raised concerns about the U.S. Defense Department’s reliance on the RD-180 to launch some of its most critical national security satellites. A separate piece of defense legislation provided $220 million to the Air Force this year to begin development of a U.S. alternative to the engine.

The RD-180 is built by NPO Energomash, which is located near Moscow, and imported by RD-Amross, a joint venture between Energomash and United Technologies Corp. of Hartford, Connecticut. Energomash’s director general, Vladimir Solntsev, disclosed the discussions of a new sale in a Russian television interview, and the story was picked up by the Russian Sputnik news agency Jan. 16.

In a Jan. 20 email to SpaceNews, Matthew Bates, a spokesman for Pratt & Whitney, a division of United Technologies, confirmed that RD-Amross “submitted a proposal to ULA with an option to deliver up to 30 Energomash-produced RD-180s.”

Rye said ULA is “assessing the business case for additional engines in support of anticipated commercial launch services.”

ULA, a joint venture of Boeing and Lockheed Martin, builds and operates two main rockets, the Atlas 5 and Delta 4. The latter vehicle is used almost exclusively to launch military satellites, while the Atlas 5 frequently handles NASA missions as well as the occasional commercial satellite.

Commercial Atlas 5 missions are marketed by Lockheed Martin, which in the past two years has been pushing hard to make sales in that market. In a recent interview, ULA Chief Executive Tory Bruno said he expects the company to be able to bring its rocket prices down in the future.

Perhaps more significantly, Boeing will use the Atlas 5 to launch its CST-100 commercial crew capsule to the space station starting as early as 2017. Boeing Space Exploration of Houston has a NASA contract for two CST-100 missions, with options for an additional four.

Moreover, NASA has adopted the Atlas 5 as its workhorse for launching science satellites and these missions do not fall under the congressionally imposed ban. Whether there is sufficient commercial and civil demand for the Atlas 5 to justify a 30-engine order is unclear, however.

ULA is working with the secretive Blue Origin rocket company to develop an RD-180 replacement dubbed BE-4 that the companies say could be ready to fly as early as 2018. But because the BE-4 would use a different fuel than the RD-180 — liquified natural gas as opposed to denser kerosene — it would require modifications to the Atlas 5 first stage.

Engine-maker Aerojet Rocketdyne of Sacramento, California, is working on a kerosene-fueled RD-180 replacement that it says could be ready in four years.

However, many in the industry are skeptical that any new U.S. engine will be ready by 2019, and even Gen. John Hyten, the commander of Air Force Space Command, has conceded that the congressional timetable is aggressive.

In a Jan. 20 email, Dee Valleras, a spokeswoman for Lockheed Martin’s Denver-based Commercial Space division, declined to be specific about projected commercial demand for the Atlas 5. “While we consider market forecast figures proprietary, we believe the assured, affordable launch services powered by Atlas will continue to be of value for commercial customers worldwide.”