Senate Armed Services Committee members questioned top Pentagon officials Thursday on how they plan to rein in spending on the nation’s next-generation fighter, whose life-cycle costs could eventually reach $1 trillion.

Defense Undersecretary for Acquisition Ashton Carter described current projected costs for the F-35 as “unacceptable” and said the plane would be “unaffordable at that rate.” To remedy the situation, he said, the Pentagon is doing a new “should cost” analysis, “scrutinizing every aspect of the cost of the airplane . . . and seeing how they can be driven [down] over time of the program.”

The current plan is to buy nearly 2,500 F-35s over the next 25 years, by which time it would make up 95 percent of the nation’s fighter aircraft force. Despite the more than 90 percent rise in costs (from $69 million apiece to $133 million) and falling years behind in delivery, Carter said, “We looked at alternatives and we didn’t have any good alternatives. We want the airplane.”

He added: “Our objective is to make sure that those [cost] estimates don’t come true and that we do have an affordable program.”

Sen. Rob Portman (R-Ohio) reopened the issue of General Electric’s alternative engine for the F-35, which Defense Secretary Robert M. Gates has ordered halted. Ending the competition between GE and Pratt & Whitney over the engine was supposed to save $2.9 billion, but Portman questioned what would happen if GE spends its own money to continue work over the next two years in the hope that the competition is reopened. Committee Chairman Carl Levin (D-Mich.) asked Carter to supply that information to the panel.

Among the major outstanding issues is the computer software used to run the sophisticated electronics of the aircraft. Pilots are to wear helmets designed to display some of the data now shown in front of them in the cockpit. Early tests found that some of the data was out of eye focus.

The F-35 is to have a stealth capacity to avoid radar and electronic attack measures that can disable enemy air defenses. It will also have onboard sensors that will “allow it to acquire . . . airborne targets, ground targets and so forth, much better than predecessors,” David Van Buren, deputy assistant Air Force secretary for acquisition, told the panel.

J. Michael Gilmore, Defense Department director of operational testing, said the biggest challenge “is integrating and testing the mission system software . . . on this aircraft.” He described how information from multiple sensors had to be fused to provide pilots with “threat warnings, modes of attack” and other capabilities “that we can’t discuss in open session.”

Gilmore said the test programs have “a reasonable chance of being executed,” but he would not guarantee they would meet expectations and warned that the final software package is not due to be integrated into the aircraft until 2015, under current schedules.

Despite the pitfalls that remain because of the aircraft’s complexity, Carter told the members, “The last place I want to go is to dumb down the airplane. I don’t think that’s necessary.”