WASHINGTON — A congressional mandate to stop using the Russian-made RD-180 engine for military space launch by 2019 may not be realistic, the Air Force's top official warned Wednesday.

The comments from Air Force Secretary Deborah Lee James came during a budget hearing in front of Senate appropriators, in the first of several hearings the service will take part in over the next two months as it attempts to defend its 2016 budget request.

The RD-180 is a key component on the Atlas V launch vehicle, produced by the United Launch Alliance (ULA). The Russian-made engine has been used successfully for years, but received negative attention from Congress following Russia's invasion of Ukrainian territory last year.

As a result, congressional leaders inserted language into the fiscal 2015 National Defense Authorization Act directing the service to end use of the RD-180 by 2019. That timetable to develop, test, certify and produce a replacement engine has been viewed as tight by analysts, something James alluded to in response to a question from Sen. Richard Shelby, R-Ala.

"All of the technical experts with whom I have consulted have told me this is not a one- or two- or three-year deal," James said. "You're looking at six years, maybe seven years to develop an engine, and another year or two beyond that to integrate. This truly is rocket science. These are hard technical problems and so to have that 2019 date there is pretty aggressive, and I'm not sure we can make it."

James added that she would like to bring technical experts in to talk to Congress about this issue, because "I'm not sure 2019 is doable."

Two competitors for a replacement have already popped up. ULA is working with Seattle-based Blue Origin to develop a new engine known as BE4, while Aerojet Rocketdyne is also developing a solution, known as AR1, which could be used as a backup if BE4 has challenges.

Asked on Twitter to respond to James' comments, Tory Bruno, ULA CEO, said that if all goes well, the BE4 is planned to make its first flight in 2019, with certification coming by 2022-2023.

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@AaronMehta Blue on track for 2019 flight. Just enough RD180s to make it. Backup with AJR about a year behind — Tory Bruno (@torybruno) February 25, 2015

@AaronMehta Rocket development is always hard. Engines are the hardest part. That's why we have a back up. Happier if we had more RD180s... — Tory Bruno (@torybruno) February 25, 2015

.@AaronMehta We need AT LEAST 29 engines to make it to the BE4 American Engine, if nothing goes wrong. More to make it to AR1. — Tory Bruno (@torybruno) February 26, 2015

.@AaronMehta Limit of less than 29 leaves both a gap in NSS capability and a period with only one provider (not ULA). Hence the Sec's resp. — Tory Bruno (@torybruno) February 26, 2015

If the new engines are not certified by 2019, that would leave a potential gap where the RD-180 is no longer allowed to be used.

Another alternative does exist. SpaceX's Falcon 9 launch vehicle is expected to be certified by midyear; once that occurs, SpaceX will begin competing for military space launch contracts.

If the Air Force cannot certify a replacement for the RD-180 in time for the congressional deadline, SpaceX could find itself only competing with ULA's heavier, and more expensive, Delta IV vehicle, at least until the Atlas V could re-enter the competition.

James' comments came a day after Russia announced a plan to end its participation in the International Space Station program in 2024, after which it will launch a Russian-only space station.