Aviation Week reports that "plans to begin technology development for a reusable booster system to replace its existing expendable launch vehicles beyond 2025 are being finalized by the U.S. Air Force."
The plan calls for replacing the Atlas V and Delta IV with two versions of the [Reusable Booster System]: a single reusable first stage and expendable cryogenic upper stage for medium-lift missions; and two reusable boosters, cryogenic core stage and upper stage for heavy-lift and growth missions. Initial operational capability is set for 2025, with the EELVs being phased out in 2030 once the Air Force is comfortable relying on the RBS ...
The article concludes that NASA may get involved with the project.
Cancellation of its Ares I may be helping drive up EELV costs, but NASA still plans to develop the heavy-lift Ares V and needs a large hydrocarbon rocket motor, so it has begun talks with the Air Force on joint development.
"NASA is coming into the picture," says [Ken Hampsten, head of spacelift for the Space and Missile Systems Center’s developmental planning division]. "We realize there is only enough money to develop one engine."