With lots of talk of dire budget choices, members of a Senate committee found out Thursday that the military may have to shell out hundreds of billions of dollars more for the Pentagon's Joint Strike Fighter project.

At a hearing, one senior Pentagon official acknowledged the increase in costs of the F-35 was "unbelievable," as Ashton Carter told Senators that the planned purchase price for over 2,400 of the Joint Strike Fighters was soaring over original estimates.

"Right now it's not an affordable program and the sustainment costs are not an affordable program," said Sen. John McCain (R-AZ), who led a series of tough questions for military officials on the futuristic plane, slated to be the centerpiece of U.S. air defenses.

One red flag during the hearing was over delays in developing the software to run the plane - that's a familiar trouble spot for U.S. fighter jets, as the F-22 has also encountered software issues that have made the plane difficult to fly.

One story is that when a group of F-22's were flying West from Hawaii across the International Date Line, their software avionics went haywire.

That plane - which like the F-35 is being developed, built and produced by Lockheed Martin - has never flown a single combat mission since the first F-22's were delivered six years ago.

"The software is behind schedule," one official told McCain, who looked for more answers, but received no detail on how long it might take to finish the source code for the F-35, which was described as being two to three times the size of the source code in the F-22.

Senate Armed Services Chairman Carl Levin (D-MI) went so far as to ask Carter to put together back up plans for a future that does not involve the F-35, or fewer numbers of the planes.

That prompted Senators from a series of states with Lockheed Martin faciltiies to lob Carter and others repeated softball questions extolling the virtues of the plane, and why it is needed in the U.S. military arsenal of the future.

The Joint Strike Fighter - which would be used across the military and by U.S. allies - is the largest current acquisition program in the Pentagon.

And with tight budgets, it may get a lot of scrutiny as lawmakers look to save money in the future.

Earlier this week, Defense Secretary Robert Gates was asked about the F-35, and whether some of it should be trimmed back to save money. His basic answer was - not really - saying the U.S. needs a "fifth-generation fighter."

"So how do you pay for that in the context that we're talking about? Those are the kinds of hard choices that I want to surface and have people address," who argued that just cutting the military by "X percent...I think that would be the most dangerous approach of all."

Just as the F-22 has faced budget pressure in the past, so too, may the F-35.

This isn't just about a second engine for the plane. This is about how many of them will you build.