Air Force officials on Wednesday asked lawmakers to change the law so the service can use Russian-made rocket engines ordered before the 2014 invasion of Crimea and thus keep up with scheduled national security launches.

prior to the Crimean invasion in 2014 while it works to certify new companies for military launches

Congress, in the 2015 defense bill, blocked the military from using engines paid for after the invasion of Crimea. But Air Force Secretary Deborah Lee James asked a Senate Armed Services subcommittee to change the law to let the military use engines it had ordered, but not paid for.

All military space launch is handled by the United Launch Alliance, a joint venture between Boeing and Lockheed Martin. The majority of those launches occur on the company's Atlas V launch vehicle, which relies on the RD-180, a Russian-made first stage engine.

Additional Russian-made engines are necessary until the Air Force can certify new companies for military launches and get away from reliance on Russia , service officials say.

Changing the law " will allow us the flexibility to keep the cost-competitive Atlas in play until we have a domestic alternative," James said in testimony to the committee's strategic forces subcommittee .

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The Pentagon's lawyers have interpreted the law's restriction as allowing ULA to use only five engines that had been fully procured, and prohibiting the use of engines that had been partly paid for but not delivered; James' proposal would allow ULA access to all 18 engines the company had begun to procure before Russia's invasion.

The use of the RD-180 has been highly contentious on Capitol Hill, especially with Senate Armed Services Committee chairman Sen. John McCain, R-Ariz., who says the use means millions of dollars for "cronies" of Russian President Vladimir Putin.

"Given Russian behavior, do you think it's in our best interest to subsidize the Russian military industrial base?" McCain asked.

"No," James replied, continuing that "it's regrettable, but the assured access to space is even more important."

McCain said the committee will look to legislation to strengthen the cut away from Russian engines, because the continued reliance on the Russian rocket makers is not "acceptable to the American taxpayer."

James said the service has a four-step program to transition to domestic engines:

Mature technology to reduce risk for new engines. The service has already obligated $50 million to research and development on this, with another $45 to $50 million expected in the next six months.

Award contracts to work on existing investments in rocket propulsion systems .

Create new contracts for new launch systems .

Award contracts for rockets with American-made systems , for launches in 2018 and beyond.

"With this approach we are confident that we can partner with American industry to develop a domestic propulsion system and integrate it into a launch system," James said in testimony.

The Air Force is working to quickly certify a new entrant, SpaceX, to use its Falcon 9 to carry national security payloads. James said she is confident SpaceX's Falcon 9 vehicle will be certified by June and can compete for contracts for two launches this year, along with seven more in 2016 and 2017.

SpaceX has already carried multiple payloads for NASA, including a Deep Space Climate Observatory satellite earlier this year. SpaceX would not be certified to carry all payloads, at least until its Falcon 9 Heavy rocket is certified at a later date.