(Reuters) – The rocket launch company run by Boeing Co and Lockheed Martin Corp on Monday said it had signed contracts with multiple U.S. firms to work on a next-generation rocket engine as an alternative to a Russian engine now used for key launches.

United Launch Alliance (ULA), the joint venture run by the two largest U.S. weapons makers, said it would choose one of the companies to develop a new engine by the fourth quarter, which would enable initial launches by 2019.

The U.S. Defense Department, spurred by growing concerns over Russia’s actions in Ukraine, has said it wants to end its dependency on Russian engines to power rockets that launch national security satellites into space.

Controversy about the Russian rocket engine has also put a spotlight on Space Exploration Technologies, a privately held company, that is seeking U.S. Air Force certification so it can compete for some of the U.S. government rocket launches.

SpaceX last month sued the Air Force for excluding it from a multibillion-dollar 36-launch contract awarded to ULA.

ULA said it had signed commercial contracts with multiple firms to investigate next-generation liquid oxygen/hydrocarbon first stage propulsion concepts. It provided no details on the names of the companies involved or the cost.

Aerojet Rocketdyne, a unit of GenCorp, has said it is potentially interested in working on a new engine. It was not immediately clear which other companies might be involved.

ULA said each firm would carry out a technical feasibility analysis, develop plans, identify schedule, cost and technical risks, and cost estimates.

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