Stratolaunch, the hybrid aircraft made up of two new fuselages and wings, plus six engines from old Boeing 747's, is conducting taxi tests prior to its first flight later this year. It's enormous, designed to hang a satellite launch rocket from beneath its center wing, and lift it to 30-40,000 feet before dropping it, to get it through the thickest layers of atmosphere and give it a "head start" to orbit.
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.
Last weekend’s tests built on an initial round of low-speed runway tests in December, and were aimed at evaluating updates made to the plane’s steering and primary braking systems, Vulcan spokeswoman Alex Moji said in an email.
“We are excited to report all objectives of this test were achieved – the aircraft reached a runway speed of 40 knots (46 mph),” she said. “The data collected will be used to evaluate and update our flight simulator for crew training.”
There's more at the link.
Here's what the behemoth looks like during taxi tests. Compare it to the size of the pickup trucks nearby.