WASHINGTON — The U.S. Air Force is changing the way it buys Atlas 5 and Delta 4 rockets from United Launch Alliance (ULA) in a bid to cut costs and increase throughput at its launch ranges, which have suffered from bottlenecks in recent years, according to a service official.

Col. Gary Henry, commander of the Air Force Launch Range Systems Wing, said the service plans to begin buying Atlas 5 and Delta 4 rockets in blocks, assigning satellite payloads to individual vehicles at a later date. Under current practice, the Air Force buys the rockets one at a time for specific payloads roughly three years ahead of the target launch date.

The Air Force also is considering changing how it pays Denver-based ULA, a Boeing-Lockheed Martin joint venture, to maintain two launch vehicle production lines when demand in recent years has only been sufficient to support one at healthy production rates, Henry said in a Sept. 15 interview. The Air Force currently has separate Evolved Expendable Launch Vehicle (EELV) Launch Capability contracts with Boeing and Lockheed Martin, which developed the Delta 4 and Atlas 5 rockets, respectively, separately prior to the 2006 creation of ULA.

The EELV Launch Capability contracts do not cover individual launches; rather, they pay for infrastructure, work force and overhead costs associated with maintaining both the Atlas 5 and Delta 4 production lines. “We are working with ULA to consolidate those into a single contract, and that will help generate some efficiencies on the business side,” Henry said.

Individual launches aboard Atlas 5 and Delta 4 are procured under EELV Launch Services contracts. Buying those rockets in bulk would give the Air Force better economies of scale and decoupling them from specific payloads at the time of purchase — such vehicles are referred to in industry parlance as white tails — would provide more flexibility in terms of scheduling launches, Henry said.

The current scheme has created bottlenecks at Cape Canaveral Air Force Station, Fla. — by far the busier of the Air Force’s two main launch ranges — due in part to satellites not being delivered on time. Having a stable of rockets available to be assigned as payloads roll off the production line would help alleviate the problem, Henry said.

Henry said demand for Atlas 5 and Delta 4 rockets from both the government and commercial sector is at an all-time high. The Delta 4 and Atlas 5 have been largely relegated to the sidelines of the commercial launch market in part because of their relatively high cost but also because their manifests tend to be booked with government satellites that in many cases wind up not making their target launch windows. But commercial satellite operators say they would like to see these rockets more easily accessible in the wake of the bankruptcy filing by Sea Launch, which in recent years has been one of only three main providers of launches for geostationary orbiting telecommunications satellites.

“We believe that we need to make the actual slots here on Atlas or Delta kind of generic,” Henry said. “So you define gates where you narrow the field of [satellites] that will be competing for a specific slot. For Atlas, as an example, on the eastern range a slot exists every two months. To manage that you have to have kind of a white tail fleet, basically a set of boosters agnostic to what payload we want to fly on them.”

ULA believes the new practice will help create a more stable business environment for the entire EELV production chain. “ULA appreciates the Launch Range Systems Wing leadership in developing more efficient launch acquisition processes to support all U.S. government customers that will have benefits to the nation’s space industrial base by providing a more stable business forecast,” ULA spokesman Mike Rein said in a written response to questions.