WASHINGTON — With growing doubts it will be selected by United Launch Alliance for its Vulcan rocket, Aerojet Rocketdyne is looking to smaller launch vehicles as potential customers for its AR1 engine.

In a Sept. 24 statement to SpaceNews, company spokesman Steve Warren said that Aerojet Rocketdyne believed that the AR1, which it had been developing for Vulcan, could be used instead on unspecified medium-class launch vehicles.

“Citing threats to U.S. space capabilities, senior U.S. defense officials have emphasized a need to transition to smaller spacecraft that can be developed and launched more quickly,” Warren said. “A medium-class launch vehicle powered by a single AR1 is ideally suited to become a new workhorse rocket for the nation.”

There is, for now, little work on launch vehicles in that category, with the focus instead on both much smaller vehicles for dedicated smallsat launches, for which the AR1 would likely be oversized, as well as larger EELV-class vehicles like Vulcan. Warren didn’t elaborate on any specific concepts for such vehicles that could make use of the AR1.

Those comments come as evidence grows that the AR1 will not be ready for the Vulcan, which ULA plans to have ready for a first launch in 2020. In a filing with the U.S. Securities and Exchange Committee, first reported by Ars Technica, the company said that it had completed renegotiations of an agreement with the U.S. Air Force that funds development of the AR1. Both Aerojet and the Air Force confirmed in February that they were renegotiating that agreement, formally known as an other transaction authority (OTA).

The original agreement had a total value of $804 million, of which the Air Force contributed two-thirds of the cost, and called for completing development of the AR1 by the end of 2019. The renegotiated agreement, finalized in June, reduces its overall value to $353.8 million, with the Air Force contributing five-sixths of the cost. A total of $290.1 million has been spent to date on AR1 work, and with the revision, Aerojet said in the filing that its “cash contributions to this OTA are now complete.”

That revision also changes the scope of work, with Aerojet now required to only “design, build, and assemble” a single prototype engine by the end of 2019. That suggests the AR1 will not be in full production in time for ULA to use it for Vulcan launches starting in 2020. Warren said that while AR1 “will be ready for testing in 2019,” the company “cannot comment on ULA’s integration schedule.”

Speaking on a panel at the World Satellite Business Week conference in Paris Sept. 11, ULA President and Chief Executive Tory Bruno confirmed that the company still planned to perform a first launch of Vulcan in 2020. The company has not disclosed when it will select either the AR1 or Blue Origin’s BE-4 engine for Vulcan, although Bruno said at that conference that a decision should come “very soon.”

The BE-4 has long been the frontrunner to power the first stage of Vulcan, and its development has been well ahead of the AR1. While the AR1 has yet to begin hotfire tests, Blue Origin started such testing of the BE-4 nearly a year ago both for potential use on ULA’s Vulcan as well as its own New Glenn launch vehicle.

“It’s performing quite well,” Blue Origin Chief Executive Bob Smith said of BE-4 on the same panel as Bruno. “We’ve gone through several hundred seconds of firing, including an over 200-second firing of that engine, so we’re feeling very good about its progress and what we’re going to be able to deliver to the market, as well as for our own consumption.”

At an investment conference Sept. 13, Aerojet Rocketdyne President and Chief Executive Eileen Drake also acknowledged that the AR1 was “secondary” to the BE-4 in the Vulcan competition.

“United Launch Alliance continues to say that this is a competition,” she said. “They have also said that Blue Origin is probably primary. We might be secondary and that’s based on a couple of things when it comes to funding, but we feel strong about the AR1.”

Drake, at that meeting, said that other vehicles could use AR1 if it’s not selected for Vulcan, but was not specific about the opportunities. Warren, in the statement to SpaceNews, argued that the engine’s use of kerosene and liquid oxygen propellants, the same as those used by a number of other vehicles, makes it compatible with existing launch infrastructure.

“A single AR1 engine generates 500,000 pounds of thrust, making it ideally suited to power the core stage of medium-lift launch vehicles,” he said. “AR1 is the ideal engine for many possible solutions; it brings the right thrust level, size and performance to a wide variety of launch vehicles.”