RENTON, Wash. — Aerojet Rocketdyne and the U.S. Air Force have revised an existing agreement supporting development of a new large rocket engine to include work on an updated version of an upper stage engine.

The company announced June 25 that it signed a modification of its Rocket Propulsion System other transaction authority agreement with the Air Force to incorporate work on the RL10C-X engine. The original agreement, signed in February 2016, covered work on the AR1 booster engine.

The RL10C-X is an updated version of the RL10 currently used on the Atlas 5 and Delta 4 launch vehicles. The updated version makes used of advanced technologies, such as additive manufacturing, to lower production costs while maintaining performance and reliability.

“Aerojet Rocketdyne has extensive experience building rocket engines for most of the nation’s preeminent launch vehicles and we will continue that legacy with the RL10C-X engine,” Eileen Drake, president and chief executive of Aerojet Rocketdyne, said in a statement.

The modified agreement comes after two companies announced plans to use the RL10 in the upper stages of next-generation launch vehicles they are developing. In April, Orbital ATK (now Northrop Grumman Innovation Systems) announced it selected the RL10 for the upper stage of its OmegA rocket. In May, United Launch Alliance announced it would use the RL10 in the upper stage of its Vulcan rocket, including working with Aerojet on the development of the RL10C-X.

The modified agreement, Aerojet said, continues to support for work on the AR1 engine. Drake said in the statement that development of the “first complete AR1” is in progress, with hotfire tests scheduled for 2019.

The AR1 is one of two engines under consideration by ULA to power the first stage of the Vulcan rocket. The other, Blue Origin’s BE-4, started hotfire tests last October. In a June 19 speech at the Amazon Web Services Public Sector Summit in Washington, Rob Meyerson, senior vice president of Blue Origin, said testing of the BE-4 was continuing with “full qualification” of the engine expected in 2019.

ULA executives have said for months that a decision on the engine it will use for Vulcan will come “soon,” but have not offered more specific timelines or other updates on the engine selection process.

ULA is one of several companies seeking awards from the Air Force called Launch Service Agreements to support continued development of their new vehicles. Other competitors include Blue Origin, Northrop Grumman and SpaceX. The Air Force is expected to award up to three such agreements later this summer.

The Aerojet announcement didn’t disclose financial terms of the revised agreement, but a June 22 contract announcement by the Defense Department listed the value of the award modification at $69.8 million. That announcement did not disclose how much funding was supporting the new RL10C-X work versus continued AR1 development.

The announcement also did not disclose if any other terms of the agreement changed. In February, the Air Force confirmed that Aerojet approached the Air Force about reducing the share of overall development costs paid for by the company from one third to one sixth.

“The Air Force has gained the necessary approvals to do so, if a mutually beneficial arrangement can be reached with Aerojet Rocketdyne,” the Air Force Space and Missile Systems Center (SMC) said in a Feb. 14 statement about a potential modification of the the cost-share portion of the agreement. “The Air Force and Aerojet Rocketdyne are still in discussions, but are working very hard to find closure on a restructured agreement.”

Aerojet Rocketdyne spokesperson Mary Engola referred questions on contract details, including the split of funding between the AR1 and RL10 projects and any changes in the cost-sharing arrangements, to SMC. An SMC spokesperson was not able to immediately answer questions on those topics late June 25.