WASHINGTON – Orbital ATK will be the sole provider of solid-rocket boosters for United Launch Alliance’s Atlas 5 and next-generation Vulcan rockets starting around 2019, the companies announced Sept. 22.

The new boosters, which Orbital ATK will begin designing, are expected to be available about the same time ULA has said the Vulcan would make its first flight.

“We have relied for decades on Orbital ATK’s industry leading rocket motor technology, which is ideally suited to support our future rocket launch plans,” Tory Bruno, ULA’s president and chief executive, said in a press release.

Orbital ATK of Dulles, Virginia, has long provided solid-rocket boosters for ULA’s Delta 4 rocket, while Aerojet Rocketdyne has supplied similar motors for ULA’s Atlas 5. All but the heaviest version of the Delta 4 — this variant does not use solid-rocket motors — are being retired in the coming years.

Under the deal, Orbital ATK is developing two similar rocket motors: one for the Vulcan and one for Atlas 5, which ULA has said will fly operate concurrently with the Vulcan for several years.  The new motors also will be commercially available for other customers, the release said.

Specifics, including costs, of what the companies described as a  “long-term strategic partnership,” were not immediately available.

“ULA and Orbital ATK will offer customers better value and reliable access to space,” David Thompson, president and chief executive of Orbital ATK, said in the release. “The capabilities and technology of the newly-merged Orbital ATK enabled us to expand the partnership with ULA to help lower costs and maintain the highest standards of mission assurance.”

In addition to the solid rocket boosters for Delta 4, Orbital ATK already provides composite structures, nozzles, propellant tanks and booster separation motors for ULA’s vehicles.

When Vulcan was officially unveiled in April, Bruno said the rocket could be augmented by up to six solid-rocket boosters, giving it greater lift capability than the largest version of the Atlas 5 but not as much as the Delta 4 Heavy, which features three core stages in a side-by-side configuration.

ULA’s selection of Orbital ATK would appear to be a loss for Aerojet Rocketdyne of Sacramento, California, which as the only other U.S. maker of large solid rocket boosters likely bid for the work.

Aerojet Rocketdyne makes liquid-fueled upper stage engines for the Atlas 5 and Delta 4 as well as the main engine for the Delta 4, but its relationship with Denver-based ULA has become strained over the latter’s plan to use Blue Origin’s planned BE-4 as Vulcan’s first-stage engine. Aerojet Rocketdyne has proposed its AR1, which could be plugged in to the first stage of the Atlas 5, whose current Russian-built main engine is subject to a congressional ban for ULA’s core national security launch market.

ULA continues to fund work on the AR1 as a backup option and says it will make a final decision next year. Meanwhile, Aerojet Rocketdyne has inquired about obtaining the rights to the Atlas 5 design and more recently offered to buy ULA from its parent companies, Boeing and Lockheed Martin, for $2 billion.

Boeing executives say the company is not interested in the offer, while Lockheed Martin has remained silent on the matter.

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.