The line of potential contractors to send U.S. Department of Defense satellites into space got shorter this week when United Launch Alliance (ULA), a joint venture of Boeing Co. (NYSE: BA) and Lockheed Martin Corp. (NYSE: LMT), pulled out of the bidding for the contract. The sole remaining bidder is Space Exploration Technologies, better known as SpaceX, the company founded and led by Tesla Motors chief Elon Musk.

ULA has been battling the Pentagon over a congressional ban on the use of Russian-built rocket engines to launch military satellites. In last year’s National Defense Authorization Act, Congress forbade the use of the Russian engines after 2019. ULA’s plea for an exception was denied in October.

According to a report at Defense News, the ban affects nine of 29 Russian engines ULA ordered before Russia’s annexation of the Crimea in early 2014. A ULA spokeswoman said in a statement:

Under the restrictions imposed by the 2015 National Defense Authorization Act (NDAA), ULA does not currently have any Atlas engines available to bid and therefore is unable to submit a timely proposal. … ULA wants nothing more than to compete, but unfortunately we are unable to submit a compliant bid for GPS III-X launch services. The [request for proposals] requires ULA to certify that funds from other government contracts will not benefit the GPS III launch mission. ULA does not have the accounting systems in place to make that certification, and therefore cannot submit a compliant proposal.

The GPS III (or GPS 3) satellite is one of the new generation of GPS navigation, positioning and timing satellites being built by Lockheed Martin. The Air Force typically maintains a constellation of more than 30 such satellites in medium Earth orbit, replacing older ones at a rate of a few launches each year, according to SpaceNews.

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Under the NDAA of 2016, Congress has authorized ULA to use a total of nine more Russian-built rocket engines to compete for military and spy satellite launch contracts using its Atlas V rockets.

Earlier this year SpaceX, was certified to compete for launches of some military and spy satellites, ending a ULA monopoly that began in 2006 when former competitors Boeing and Lockheed created ULA.

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