Stratolaunch Systems, the space launch venture backed by Microsoft co-founder Paul Allen, says it put its super-duper-sized carrier plane through a fresh round of revved-up taxi tests last weekend at California’s Mojave Air and Space Port.

The six-engine, 385-foot-wide aircraft, nicknamed Roc, is the world’s largest airplane as measured by wingspan. It’s designed to carry up rockets for high-altitude launches in midflight.

Stratolaunch has said orbital launches could begin in the 2019-2020 time frame if the test program goes well.