WASHINGTON — A bipartisan group of 20 House members has asked the Defense Department not to alter the U.S. Air Force’s plans to fund development of new launch systems.

In the letter to Secretary of Defense James Mattis, dated April 10, the members said the Air Force should continue efforts to develop “complete, robust launch systems” rather than focus on specific components, such as an engine to replace the Russian-built RD-180. That approach, they argued, is the best way to end reliance on the RD-180 while providing assured access to space at reduced cost.

“Investing in the entire launch system through government and industry cost-share partnerships — rather than a specific component — is the fastest, safest, and most affordable way for the taxpayer to achieve these objectives,” they wrote. “Restricting funding only for a domestic engine will result in higher costs for the taxpayer and risks delays in ending use of the RD-180 engine.”

The Air Force made several Rocket Propulsion System awards in early 2016 to support development of both engines and full-scale vehicles, with the winning companies contributing one third of the cost of each award. In March, the Air Force issued a draft request for proposals (RFP) for the next phase of the program, called the Launch Service Agreement, with the full RFP expected this summer.

Under that program, the Air Force is expected to make up to three awards in early 2018 to fund continued development of those vehicles, including certification test flights. Among the companies expected to compete for those awards are Orbital ATK, which is developing a vehicle through its Next Generation Launch program; SpaceX, which received funding to support work on its Raptor methane engine last year; and United Launch Alliance, for its Vulcan vehicle.

Aerojet Rocketdyne also received a Rocket Propulsion System award from the Air Force last year to support work on the company’s AR1 engine, which the company is billing as a replacement for the RD-180 used on ULA’s Atlas 5. ULA has yet to make a formal decision on the engine that will power the first stage of Vulcan, but has indicated as recently as April 5 that Blue Origin’s BE-4 remains the frontrunner over the AR1.

The members who signed the letter include Rep. Adam Smith (D-Wash.), ranking member of the House Armed Services Committee. Twelve Republicans signed the letter, led by Rep. Will Hurd of Texas. Five of the members who signed the letter are from Colorado, where ULA is headquartered, while four are from Washington state, where Blue Origin is based. Three members are from Texas, where both Blue Origin and SpaceX have test facilities.

Absent from the list of members who signed the bill are Reps. Mac Thornberry (R-Texas) and Mike Rogers (R-Ala.), the chairmen of the full House Armed Services Committee and its Strategic Forces Subcommittee, respectively. In February, the two sent a letter to Acting Secretary of the Air Force Lisa Disbrow and James MacStravic, performing the duties of the under secretary of defense for acquisition, calling on the government to have “full access to, oversight of, and approval rights over decision-making about any engine down-select for Vulcan (assuming they will be requesting government funding).”

In the letter, they argued that since ULA is accepting government funding to support the development of Vulcan, the government should also have insight into that process, “especially where one of the technologies is unproven at the required size and power.” That was a reference to Blue Origin’s BE-4, which will be the largest rocket engine developed to date using methane as a fuel, rather than the kerosene used by the RD-180 and AR1 engines.

Thornberry has since backtracked on the comments in that letter, telling reporters last month it was not his intent to micromanage subcontracting decisions.

Rogers, in a recent SpaceNews interview, said he was not satisfied with the pace of development of an RD-180 replacement, but also praised the capabilities of commercial launch companies. “My subcommittee, our full committee, this Congress, is committed to not stop until we have an American-made engine that can get our national security space assets launched,” he said.