Stratolaunch Systems has completed the first phase of engine testing on its Roc aircraft at the Mojave Air & Space Port in the Antelope Valley, according to the company.

Stratolaunch Chief Executive Jean Floyd wrote in a posting Tuesday on the company’s website that all six of the aircraft’s turbofan engines were started and allowed to idle.

“In these initial tests, each of the six engines operated as expected,” Floyd wrote in the posting. “Over the next few months, we will continue to test the aircraft’s engines at higher power levels and varying configurations, culminating to the start of taxi tests.”

Stratolaunch is building the first-of-its-kind airplane to launch satellite-carrying rockets into orbit. Northrop Grumman Corp.’s subsidiary Scaled Composites is building the craft. When completed, Roc will be the largest aircraft in the world with a wingspan of 385 feet – longer than a football field.

The Stratolaunch flight system uses the Roc to take a rocket to high altitude and release it so the rocket can fly into space. Stratolaunch was founded in 2011 by billionaire Paul Allen to launch satellite-carrying rockets into orbit as a lower cost alternative to ground-launched rockets.

The Roc’s first launch demonstration could come as early as 2019.