The cost of maintaining the U.S. Air Force’s aging T-38 fast jet trainer increases “steeply” in the outyears, but Air Force Gen. Edward Rice, who leads the training command, says the fleet is safe.

The aging T-38 will remain in the fleet for an uncertain amount of time due to repeated delays in launching a replacement program called T-X. Rice says it is not yet known if the T-X effort will get support from the Air Force in its fiscal 2015 budget, which is being refined this fall. Delays have pushed the initial operational capability to around 2023. “Timing is the issue, and funding is the issue with timing,” Rice told reporters during a Sept. 16 press conference at the annual Air Force Association conference here. “I can’t go to the leadership of the Air Force and say if we don’t buy a new airplane something catastrophic is going to happen.”

Once the program does garner funding, Rice says he will have to revalidate a five-year-old analysis of alternatives on how to proceed with a procurement. The service plans to buy as many as 350 aircraft to replace the T-38. In the meantime, the Air Force is structurally enhancing 125 of its T-38s that have seen the hardest use.

Teams competing for work include BAE/Northrop Grumman, General Dynamics/Alenia Aermacchi and Lockheed Martin/Korea Aerospace Industries. Boeing officials say they will continue work on a brand new aircraft to address the forthcoming requirement. The company is said to be in talks with Saab on a potential partnership, though neither company is confirming the talks.

The safety of the T-38 fleet at a reasonable cost should hold out “for longer than I think it is going to take us to make this decision” to start T-X, Rice says.

Meanwhile, the industry teams hoping to compete for T-X are in a holding pattern at a time when defense spending is being cut back and some companies are taking a conservative approach toward spending internal research and development dollars.

BAE announced its exclusive partnership with Rolls-Royce for the Adour Mk951 engine, which already powers the U.K.’s new Hawk T2 jet trainers. As BAE’s team awaits a decision on T-X, the companies are looking to “tighten up the supply chain [and] maybe focus on more U.S. content,” says Thomas Hartmann, senior vice president of customer business in North America.

The Adour Mk951 would be final-assembled in the U.S.

Northrop Grumman has not yet decided where to place its final assembly plant for the Hawk jets, says Mark Lindsley, director of the company’s T-X effort.

BAE is now producing 22 Hawks for Saudi Arabia and another eight for Oman. Poland is expected by year’s end to announce the winner of its plans to buy 30 fast-jet trainers.