And the RD-180 has its supporters. "There's nothing out there that's better in terms of weight-to-power ratio than the RD-180," said Marco Caceres, an analyst with the Virginia-based Teal Group. "I don't know if you can come up with an engine as powerful as the RD-180 in a short time from scratch. "It's really more about developing the least expensive engine that will make the Atlas V much cheaper commercially."

Two companies vying with SpaceX for a NASA commercial crew program – Boeing(BA:US) and Sparks, Nevada-based Sierra Nevada Corp. – - may also find themselves relying on Russian engines, because of their plans to use Atlas V rockets, said Marcia Smith, a former director of the space studies board at the National Research Council and now editor of Arlington, Virginia-based spacepolicyonline.com. Even so, mutual dependencies in space may make this one area that’s immune to disagreements on the ground. “The Russians are making money off these sales,” said Marco Caceres of the Fairfax, Virginia-based Teal Group. “It would make sense from a political standpoint to snub us, but from a financial standpoint, it’s not so good.”

If Russia did cut off supplies of the RD-180, it might be more symbolic than actually painful for the US, according to Marco Caceres, director of space studies with the Teal Group. “The Atlas V isn’t launching that much, so short-term impact would be minimal,” Caceres said. “The long-term impact would be that Atlas V would have to find another engine and that wouldn’t be easy.” The biggest impact might come not to US military launch, but to the corporate firms that provide it. Right now, military launch is provided either by ULA’s Atlas V or Delta IV. For years, the US has wanted to maintain two families of launch vehicles in case one failed. However, SpaceX looks poised for certification this year. The combination of having a third launch option, along with the lack of RD-180 parts, might lead to the end of the Atlas V. That may be all hypothetical, however, as Caceres doubts Russia would block sales of RD-180s, primarily because of the financial impact. “It’s not to the benefit of the Russians to do this. These are engines that bring in hard currency to Russia, the same way Russian oil and gas does,” he said. “Russia doesn’t really export much else of any consequence. ”