Senator John McCain is once again trying to stop the US military from using Russian rocket engines. McCain and House Majority Leader Kevin McCarthy will introduce legislation on Wednesday to repeal a provision that was added to the $1.1 trillion spending bill shortly before it passed in late December. The provision allowed commercial space companies to buy and use Russian rocket engines, which had previously been banned. McCain argues that it was added "in secret, with no debate."

McCain issued a statement on his website about the legislation, calling it "vital to ensuring the United States does not depend on Vladimir Putin’s regime for assured access to space," and saying that it's "morally outrageous and strategically foolish to ask American taxpayers to subsidize Russia’s military industrial base."

One particular space company is primarily at the center of this issue: the United Launch Alliance, a joint venture between Boeing and Lockheed Martin, and SpaceX's main commercial competitor. ULA uses Russian-made RD-180 engines on the Atlas V, the company's flagship rocket. The US Air Force pays ULA to launch satellites on the Atlas V because it's one of the more relatively cheap options on the market, has never blown up, and — until recently — SpaceX wasn't cleared to launch military payloads.

But that all became a problem when Russia invaded Ukraine. Not long after, Congress passed the National Defense Authorization Act of 2015, which was filled language that forbade the Air Force from dealing with "Russian suppliers of rocket engines" for its launches. Specifically, the US military (and the companies it contracts with) would be immediately banned from buying Russian engines, and banned from using them after 2019. The ban was an effort to distance the US military from reliance on Russian technology, and was also meant to stop funding the Russian government. (NPO Energomash, which makes the RD-180, is owned in part by the Russian government.)

ULA fought this ban, even though it already had a stockpile of RD-180 engines big enough to last through nearly four more years of launches. ULA claimed that, once that stockpile is used up, the company will be left with no other option to power its rocket. (That claim has been disputed, both by space industry manufacturing outfit Aerojet Rocketdyne — which has tried to sell its own rocket engines to ULA, and even attempted to buy the company outright last fall — and by McCain, who says ULA "manufactured a crisis.")

Development of both the engine and the rocket will still take years, though. The new rocket won't be ready to fly commercial missions until the early 2020s, so ULA was worried it would have to stop performing military launches for a number of years and leave all the business to SpaceX.

The provision that was added to the spending bill essentially lifted the ban on the RD-180 on December 18th, and so ULA bought 20 more of the Russian engines the following week. If Senator McCain is able to repeal the provision, it's possible that ULA would once again face losing national security space missions to SpaceX.