For the past two years, senior U.S. lawmakers have haggled over how many more RD-180 rocket engines United Launch Alliance should be permitted to import from Russia before the Atlas 5 is sidelined by Fly American restrictions.

The matter appeared settled when President Obama signed the National Defense Authorization Act of 2016 into law in November allowing ULA just nine more RD-180s instead of the 18 the Air Force says are needed to keep America’s national security payloads launching until a replacement is ready. A few weeks later, Obama signed another bill — the 2016 Consolidated Appropriations Act — that lifts the RD-180 limits authorizers spent two years debating. Sen. John McCain (R-Ariz.) and House Majority Leader Kevin McCarthy (R-Calif.) are vowing to reinstate the nine-engine limit.

The RD-180 rumpus is far from over.

And now a new rocket fight is starting to emerge.

Lawmakers are divided over the steps the Air Force should take to end its dependence on the RD-180, which has reliably powered 62 Atlas 5 flights since the rocket’s 2002 debut. While the Air Force has now made clear it wants a new rocket, some lawmakers are still insisting a new engine is all it needs.

Congress has given the Air Force $444 million over the past two years to end RD-180 reliance. A provision in the 2015 NDAA says the money must be spent on a new U.S. liquid propulsion system, not a new launch vehicle.

Several days before the Pentagon unveiled its 2017 budget seeking $1.2 billion over five years to help industry field a new rocket, DoD officials were on the Hill making the case for relief from the engineonly approach.

“If we were to continue down the path of funding rocket engines alone we believe this effort would benefit … only one launch service provider, which we don’t really believe is anyone’s intent,” Air Force Secretary Deborah Lee James told the Senate Armed Services Committee during a Jan. 27 military space launch hearing.

Lest anyone forget, James and Frank Kendall, the Pentagon’s acquisition czar, made a point of noting — several times, in fact — that the engine-only path was set by Congress.

Sens. Jeff Sessions (R-Ala.) and Mazie Hirono (D-Hawaii) took the bait, asking the Pentagon to draft bill language expanding use of the engine money. Kendall said the DoD would be happy to comply.

Rep. Mike Rogers (R-Ala.), the chairman of the House Armed Services strategic forces subcommittee, opposes using the engine money for anything but a new main-stage rocket engine.

“There should be no confusion, we are not using government funds to build a new launch vehicle and new infrastructure,” he told SpaceNews in November. “We have an engine problem — reliance on Russian engines.”

McCain, the Senate Armed Services Committee’s chairman, echoed that sentiment Jan. 27. “It is rocket engines that we’re buying from the Russians, not anything else. … What we want to do is get out of the Russian rocket engine business.”

Launch has been a sticking point in authorization bills in the past. Expect the trend to continue this year.