NATIONAL HARBOR, Md. — Top executives from United Launch Alliance’s parent companies reaffirmed their support for a next-generation Vulcan rocket, but said uncertainty about the best way to end reliance on a Russian rocket engine has caused them to invest cautiously.

In a pair of press briefings March 8 at the Satellite 2016 trade show, Rick Ambrose, executive vice president of Lockheed Martin Space Systems, and Craig Cooning, president of Boeing Space and Network Systems, offered the clearest view to date for how ULA’s parent companies would fund development of Vulcan. Ambrose is the chair of ULA’s board of directors this year. Cooning is vice chair.

The Air Force has so far agreed to invest as much as $201 million in Vulcan under a private-public partnership announced Feb. 29 covering integration of Blue Origin’s BE-4 engine with the Vulcan design. ULA has pledged $134 million toward that specific effort, which covers only a fraction of Vulcan’s total projected development cost.

ULA has yet to put a firm price tag on Vulcan’s development, but in unveiling its proposed next-generation rocket last April at the Space Symposium in Colorado Springs, Tory Bruno, the joint venture’s CEO, said new rockets typically cost $2 billion, including $1 billion for the main engine.

“We see a way to get there based on what the government’s telling us,” Cooning said of Vulcan’s development cost. “If you look at the [internal research and development] that comes with the missions previously booked on United Launch Alliance, together with the government making this investment, there’s enough money.”

For more than a year, Boeing and Lockheed Martin have been investing in the rocket on a quarter-by-quarter basis and the ULA board leaders said this week that the practice would continue.

“We have to be prudent, disciplined stewards of any kind of investment,” Ambrose said. “Vulcan would be like any other investment decision.”

In September 2015, ULA’s leaders said a ban by Congress on the Russian RD-180 rocket engine, which powers ULA’s Atlas 5 rocket, was a leading driver behind the measured investment in Vulcan. But that issue was temporarily resolved in December, when Sen. Richard Shelby (R-Ala.) used a must-pass spending bill to eliminate the engine restrictions that had become law just weeks earlier.

Now, Ambrose pointed to “uncertainties” with launch policy, while Cooning said disagreements between lawmakers and the Air Force on the best approach for ending RD-180 dependence have given them pause, further justifying a “cautious and conservative approach.”

Cooning called for what he described as greater “alignment” between the Air Force and Congress. The Air Force and ULA want the government to invest at least $1.2 billion over the next five years to develop a new U.S. rocket.

Some influential lawmakers, however, want to limit the government’s investment to development of a new main engine.

That view is supported by Aerojet Rocketdyne, which will get $115 million from the Air Force this year — as much as $536 million over five years — to complete AR1, a liquid oxygen- and kerosene-fueled engine that its designers say could be used as a direct replacement for Atlas 5’s RD-180 or as a substitute for the BE-4 engine ULA favors for Vulcan.

ULA and its corporate parents don’t want Congress to limit the Air Force’s investment to propulsion work.

“An engine alone does not make a rocket,” Cooning said.