Defense Secretary Ash Carter has said the Pentagon must continue its purchases of Russian RD-180 rocket engines until the U.S. can find an alternative to the United Launch Alliance‘s Atlas V, Breaking Defense reported Wednesday.

Colin Clark writes that Carter told the Senate Appropriation defense subcommittee the military needs to buy at least 18 RD-180 engines or fly its payloads on Delta IV launch vehicles, which he claimed would require additional funds.

U.S. Air Force officials and some senior Pentagon officials have promoted the use of 18 RD-180 rocket engines until ULA, SpaceX and Blue Origin can offer new rockets that can carry military and other national security satellites into space, according to Clark’s report.

Moscow, Russia-based rocket engine company NPO Energomash designed and manufactures the RD-180 engines used on the Atlas V rocket.