Less than a decade after its first rocket launch, Elon Musk's SpaceX finds itself in an unfamiliar position.

The upstart venture is the incumbent vying to win the bulk of a $3.5 billion U.S. contract renewal while facing rivals that include Boeing Co., whose spaceflight roots date to the 1950s. At stake: a seven-year agreement to haul supplies and experiments to the International Space Station.

SpaceX is pushing the only made-in-the-USA entry in a four-way derby with Boeing, Orbital ATK Inc. and Sierra Nevada Corp., each of which relies to some extent on rockets with Russian engines. The National Aeronautics and Space Administration will award the work as soon as Thursday as it juggles support for commercial missions while Congress clamors to end U.S. dependence on the imported motors.