COLORADO SPRINGS, Colo. – A day after the chairman of the Senate Armed Services Committee accused the US Air Force of misrepresenting the cost to end reliance on Russian rocket engines for military space launch, the chief of Air Force Space Command defended the service's position.

Sen. John McCain, R-Ariz., cited a $3.5 billion discrepancy between two separate cost estimates Air Force Secretary Deborah Lee James provided to Congress for transitioning off the Russian RD-180, which powers United Launch Alliance's Atlas V launch vehicle.

Replacing the Atlas V with a combination of ULA’s Delta IV heavy launch system and SpaceX’s newly-certified Falcon 9 could cost as much as $5 billion, James recently testified before the committee.

By contrast, James told the committee shortly before the hearing that splitting future launches between the two domestic vehicles would cost roughly $1.5 billion, McCain said in a Wednesday letter to the secretary.

But it is impossible to accurately predict the cost of launch vehicles by the end of the decade, the point at which McCain wants the Air Force to stop using the RD-180, Air Force Space Command chief Gen. John Hyten said Thursday during the Space Foundation's annual National Space Symposium.

The Delta IV is much more expensive than the Atlas V — on that point Congress and the Air Force generally agree. But exactly how much more the Delta IV will cost in 2020 is difficult to calculate, Hyten said. It all depends on what assumptions the Air Force makes about the state of the launch industry in the next decade.

"The reason I don't know how expensive that's going to be is because I cant tell you what the industry is going to be in 2020, I can't tell you what ULA's business case is going to be in 2020," Hyten said during a media briefing. "I can make certain assumptions that make the Delta IV very attractive, and I can make certain assumptions that make the Delta IV unbelievably expensive — it's all based on the assumptions that you make of what you think the world is going to be like in 2020."

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The Air Force has consistently said the most cost-effective way to bridge the gap between now and 2019 when a domestic launch vehicle becomes available is to continue to use the RD-180, Hyten said.