WASHINGTON — Although United Launch Alliance has already selected Blue Origin to develop a new main rocket engine, it will continue to fund a smaller-scale effort by longtime supplier Aerojet Rocketdyne to examine propulsion concepts that include a kerosene-fueled main engine dubbed AR-1.

Aerojet Rocketdyne of Sacramento, California, has been shopping the AR-1 as a potential replacement for the Russian-made RD-180 engine that powers the main stage of ULA’s Atlas 5 rocket. But on Sept. 16, ULA said it had selected Blue Origin’s BE-4 to be the main engine for a future rocket that would combine elements of the Atlas 5 and ULA’s other workhorse vehicle, the Delta 4.

In a Sept. 29 press release, Aerojet Rocketdyne said that under the new agreement with ULA, the companies would jointly fund feasibility studies, cost estimates and risk management for the kerosene-fueled AR-1 engine.

Aerojet Rocketdyne currently builds the hydrogen-fueled RS-68 main engine for the Delta 4 as well as the RL-10, different variants of which power the upper stage for both the Atlas 5 and Delta 4, as well as solid-rocket motors. The new agreement with ULA, financial terms of which were not disclosed, also includes work on those propulsion systems, the press release said.

The U.S. Defense Department is mulling various options to wean itself from the kerosene-fueled RD-180, whose future availability is uncertain amid the recent deterioration in relations between Washington and Moscow. Aerojet Rocketdyne says the AR-1 fits the bill and could be fully developed in four years for less than $1 billion.

In a separate press release Sept. 30, Aerojet Rocketdyne announced it had formally responded to an Air Force request for information on RD-180 replacement options. In the release, the company described the switch from the Russian engine to the AR-1 as a “low-risk transition” to multiple launch vehicles.

The release also said the AR-1 “would require minimal changes” to the Atlas 5 and its ground system and launch infrastructure. Blue Origin’s BE-4, by contrast, is fueled by liquefied natural gas and thus could not be readily plugged into the Atlas 5 vehicle and pad as they are currently designed.

“This is the time to realize that we need a response to a national need at minimum cost and risk to the American taxpayer, that at the same time preserves our industrial base,” Paul Meyer, Aerojet Rocketdyne’s senior vice president for advanced programs and business development, said in the Sept. 30 release.

In June, ULA announced it had asked multiple companies to study hydrocarbon-fueled engine concepts and report back with options accompanied by likely schedules, costs and technical risks. ULA on Sept. 16 said the BE-4 engine had emerged as the winning concept for reasons including the fact that it has already been in development for three years.

Prior to the BE-4 announcement, Congress was preparing defense spending bills that included substantial funding in 2015 to develop an RD-180 replacement. Should the government still opt to back an engine development program outside ULA’s investment in the BE-4, the AR-1 could be a viable alternative given Aerojet Rocketdyne’s familiarity with ULA’s existing vehicles and requirements.

“We clearly understand the requirements of our customer, ULA, and are focused on continuing our long-term partnership as the U.S. propulsion supplier of choice,” Meyer said in a Sept. 29 press release. “We believe we have an unparalleled level of engineering talent, tools, technology, program management, advanced manufacturing capabilities and demonstrated performance to continue to deliver the most reliable propulsion products available today and into the future.”

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.