The Air Force is officially looking into a replacement for the Russian-made RD-180 rocket engine.

On Thursday, the service posted a request for information (RFI) on the government contracting website FedBizOps looking for information on "booster propulsion and/or launch system materiel options that could deliver cost-effective, commercially-viable solutions for current and future National Security Space (NSS) launch requirements."

"Air Force Space Command (AFSPC) is considering an acquisition strategy to stimulate the commercial development of booster propulsion systems and/or launch systems for Evolved Expendable Launch Vehicle (EELV)-class spacelift applications," the solicitation said. "The Air Force has relied upon foreign sources for booster propulsion systems in the past."

While the RFI does not specifically say so, it is clear this would be the start of a replacement for the RD-180, used by the United Launch Alliance in its Atlas V launch vehicle.

Congress has expressed interest in developing an American-designed replacement after Russian leadership threatened to cut off the Pentagon earlier this year. on Wednesday, the Alliance took delivery of its first two RD-180s since Russia annexed Crimea.

The RFI shows the early nature of the replacement program, with the service asking for input from industry on a number of options, including whether a new program would be similar to the RD-180 or come in a new configuration and the use of "alternative launch vehicles" for the EELV mission.

Service officials, including Air Force Undersecretary Eric Fanning, have shown enthusiasm for some form of public-private partnership, another area mentioned in the RFI.

Lt. Gen. Ellen Pawlikowski, then head of the Air Force's Space and Missile Systems Center (SMC), said in a June interview that she wants a new engine to create opportunities in the commercial realm, not just for military launch. The RFI reflects that ideas a well, asking for programs ideas that "potentially could result in greater U.S. competitiveness in the commercial space arena."

Overall, the RFI contains 36 questions for industry. Responses to the Air Force are due by September 19. That will be followed by an industry day at Space and Missile Systems headquarters at Los Angeles Air Force Base on September 25 and 26.