A key lawmaker’s call to halt all buys of military equipment from Russia does not include the Russian-made RD-180 rocket engine used in United States national security space launches, sister publication Defense Daily has learned.

Senate Armed Services Airland subcommittee Chairman Richard Blumenthal (D-Conn.) said Wednesday during an Army modernization hearing he was preparing a letter to Secretary of State John Kerry to end buys from Russia. Blumenthal spokeswoman Kamara Jones said Friday the letter, due to be released Monday, won’t mention the RD-180. The Army is managing a program to provide Mi-17 Russian helicopters for the Afghan national security forces, which caught Blumenthal’s attention. Mi-17s are developed by the Russian state company Rosoboronexport.

Though relying on a Russian engine for a critical national security capability like space launches has always been discussed, the RD-180 has become a hot topic on Capitol Hill since the recent crisis between Ukraine and Russia developed. DoD wants to make sure it has a backup plan in case Russia severs supplying the RD-180.

The Air Force uses the RD-180 in its Evolved Expendable Launch Vehicle (EELV) program, designed to assure the federal government access to space. Incumbent launch provider United Launch Alliance (ULA) uses the RD-180 in its Atlas V launch vehicle. ULA also uses the Delta IV, which is made domestically, but also costs more.

DoD is performing a “business case analysis” of what it would cost to produce the RD-180 in the U.S. Air Force Chief of Staff Gen. Mark Welsh said recently it would likely take around $1 billion and five years to develop the RD-180 domestically. A former high ranking Air Force Space Command (AFSPC) officer and a former NASA administrator are participating in this study.

The RD-180 is developed by NPO Energomash and is distributed in the U.S. by RD AMROSS, a joint venture of NPO Energomash and Pratt & Whitney. Pratt & Whitney is a division of United Technologies Corp. [UTX] ULA is a joint venture of Lockheed Martin [LMT] and Boeing [BA].