This article was originally published April 20, 2016, at 1:14 p.m. EST. It has been updated to include comments from Sen. John McCain.

WASHINGTON — Pentagon acquisition chief Frank Kendall, in congressional testimony Wednesday, defended the controversial path to end the US reliance on Russian rocket engines for military space launch—and Senate appropriators signaled their continued support.

That path, for now, involves using more of the engines while the industrial base ramps up to offer competing engines. DoD could replace the RD-180 engines by 2021 at the soonest, but the politicization of the issue is slowing down needed Congressional action, Kendall, the undersecretary of defense for acquisition, technology and logistics, said at a Senate Appropriations Defense Subcommittee hearing Wednesday.

"It's a complicated issue, but the [Defense] Department's goals have never changed, to develop two engines, so if one of them has a failure, and we have a big gap in capability, we still have access to space," Kendall said. "We need competition to keep the expense down."

Ultimately, DoD will seek public-private partnerships for reasonably priced launch services, versus buying a lone replacement engine and subsidizing a single company, Kendall said. Replacing the Atlas V with a combination of ULA's Delta IV heavy launch system would cost, Kendall said, as much as $50 million per launch.

"It would be over $1 billion out of our defense budget to get us off RD-180s, and I don't think that's a good tradeoff," Kendall said.

The US Air Force contracts for launch services with United Launch Alliance, a joint venture of Boeing and Lockheed Martin, which uses the RD-180 rocket engine to power its Atlas V launch vehicles. SpaceX is ULA's main competition for Pentagon business, as the company's Falcon 9 rocket won certification last year to compete for military space launches.

For their support of DoD's plan, two key appropriators, the SAC-D's ranking member, Sen. Richard Durbin, D-Ill., and subpanel member Sen. Richard Shelby, R-Ala., have attracted the ire of the Senate's lead authorizer, Senate Armed Services Committee Chairman John McCain, R-Ariz. McCain sought a ban on Russian rocket engines in the 2016 defense policy bill that was trumped by the 2016 appropriations bill.

Should McCain make another attempt in the coming 2017 National Defense Authorization Act, Durbin and Shelby signaled their positions had not softened in exchanges with Kendall Wednesday.

McCain last year accused both lawmakers "pork barrel parochialism" as a rocket factory in Alabama may benefit from their advocacy, as would Boeing, headquartered in Illinois.

"We're planning on doing plenty, and that's a practice of corruption that they're engaged in," McCain said Wednesday.

Air Force officials recently had to defend against McCain over a $3.5 billion discrepancy between two separate cost estimates Air Force Secretary Deborah Lee James provided to Congress for transitioning off the RD-180.

In an exchange with Shelby, Kendall acknowledged discrepancies in estimates, attributing them to differences in the number of engines and how soon they are adopted.

Durbin said there is "war of words" in the Senate over the issue, and that he rejected the idea of stopping the use of the RD-180 cold, which he said DoD officials would leave the US hostage to a sole supplier or force the department to use an ill-suited engine. By contrast, using the RD-180s during the projected five-year transition period could foster competition between the various players.

"I don't want to subscribe to these Russian engines any longer than we have to but I think it is short-sighted, as some on the authorizing committee have said we're cutting them off cold turkey, we're finished with them," Durbin said.

To illustrate the potential impact of abandoning the RD-180, Durbin displayed a chart equating the projected $1 billion to $2 billion cost to: replacing every Humvee in the Marine Corps; a 2.5 percent raise for troops for five years; doubling funds for National Guard equipment; and 10 to 20 space launches through an open competition.

Otherwise in the hearing, Assistant Secretary Of Defense For Research And Engineering Stephen Welby and Defense Advanced Research Projects Agency Director Arati Prabhakar testified alongside Kendall about the Pentagon's budget request for science and technology.

DoD's budget request for science and technology is $12.5 billion, 1.9 percent above the previous year's request and 2.4 percent of DoD's $524 billion top-line, according to defense officials who testified Wednesday. Of that, $2.1 billion is for basic research.

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