WASHINGTON — The race to replace the Russian RD-180 rocket engine is steadily moving forward as companies plug away on developing and testing new engines and launch systems.

Help in the form of additional government funding could be on the way. In March, the Pentagon put forward a draft request for proposals seeking out input from companies interested in public-private partnerships for their launch systems.

The Air Force entered into similar partnerships with SpaceX, Orbital ATK, United Launch Alliance and Aerojet Rocketdyne in 2016 for the development of new rocket engines. The winners of this solicitation have the opportunity to attain more money — this time for launch system development — which would be made available as early as fiscal year 2017.

"The focus of this solicitation is to facilitate development of prototypes for up to three launch systems as early as possible, allowing those launch systems to mature prior to a future selection of two [national security space] launch service providers," the draft RFP reads.

Congress has set Dec. 31, 2022 as the end date for use of the RD-180 engine, which is manufactured in Russia. United Launch Alliance — a joint venture of Lockheed Martin and Boeing that was once the Pentagon's sole launch provider — has long relied on the RD-180 to power its Atlas V rocket. But although Congress is eager to do away with the politically unpopular RD-180, which they say fosters a dependence on Russia for access to space, the Air Force wants to retain at least two launch providers to keep costs down.

One of those companies will likely be SpaceX, which has started conducting missions for NASA and will complete its first national security launch in 2018 with a GPS 3 satellite. SpaceX already uses a homegrown rocket engine — the Merlin 1D — for its Falcon 9 and Falcon Heavy rockets, but the company has sought out government funds to help pay for the development of the Raptor, a next-generation methane-fueled rocket engine that could power Falcon 9, Falcon Heavy and the planned Interplanetary Transport System launch system.

Two companies — Amazon founder Jeff Bezos' Blue Origin and legacy defense contractor Aerojet Rocketdyne — have stepped forward to build RD-180 replacements that could be integrated on ULA rockets. Unsurprisingly, executives from both companies lauded their engine as the lowest-cost, lowest risk option for the Air Force during a March 22 event hosted by the Center for Strategic and International Studies.

Blue Origin's BE-4 engine is the furthest along in testing, with a flight certification scheduled for 2017 and a first flight on the Vulcan slated for 2019. The company began developing the engine four years ago with its own funding, before the Air Force began subsidizing the RD-180 replacement effort.

"We were developing that engine first for our own purposes," explained Brett Alexander, the company's director of business development and strategy. "United Launch Alliance came along at just the right time where we were able to shift the sizing of the engine from a 400,000 pound thrust engine to a 550,000 pounds of thrust, which gives them more than enough capability to meet the nine reference orbits that national security community has [required], but to do it at a cheaper price point than they were with the RD-180."

Blue Origin recently shipped the first BE-4 from its manufacturing and design facility in Washington to its test site in Texas, he said.

"We've done over 220 stage combustion tests, we've done a couple hundred power pack tests, we've done all sorts of bearing tests and everything else that goes into the engine itself, and so we're at that point where seeing the first assembly on the test stand is very exciting," he said, adding that the first hot fire test would be "coming up soon."

Aerojet Rocketdyne is manufacturing an engine called the AR1 that will be able to power both the Vulcan and the Atlas V, something the BE-4 will not be able to do. The AR1 has 550,000 pounds of thrust uses the same propellant as the RD-180.

"We're on the path we laid out two years ago on schedule. We'll have a certified production engine coming off a production line, which we have outside of Los Angeles right now, in 2019," John D. Schumacher, Aerojet Rocketdyne's vice president of Washington operations, said at CSIS.

"Depending on the pace of integration with ULA and the pace they're moving on with Vulcan or Atlas, it's ready for integration —the work can start ahead of that — but it's a 2019, 2020 proposition."

The company has built a full scale flight type components and conducted "several hundred" component tests in advance of a critical design review next month, Julie Van Kleeck, vice president of advanced space and launch programs and strategy, told Defense News in an interview.

Jim Simpson, its senior vice president of business development, added that Aerojet Rocketdyne is currently evaluating the launch system draft RFP and is still deciding whether and how to bid.

"I think the key is, is the timing such that we'll be able to accommodate disruptive technology appropriately to enable us to really take full advantage of that in the next generation of EELV," he said. "I think we're also looking at what are the requirements that the government has imposed and how does it all fit together."

Orbital ATK could be the wild card in a future competition, having announced in 2016 that it had begun work on a next-generation launch vehicle. The company has received funding from the Pentagon to create prototypes of Orbital ATK's GEM 63XL strap-on solid rocket motor, the Common Booster Segment solid rocket motor and an Extendable Nozzle for Blue Origin's BE-3U engine. However, the company has not fully committed to building the system, and its development would lag behind other options.

"We expect a go/no-go decision in late 2017 or an early 2018 concerning the next phase of activity to actually build and test this new launch vehicle family," Dave Thompson, it's president and CEO said in a March 8 earnings call.