Billionaire Paul Allen’s Stratolaunch is the biggest fixed-wing in the world — assuming it can fly. The co-founder of Microsoft’s new plan to launch satellites into space is both slightly unhinged and completely simple at the same time: take two huge planes, fuse them into one even bigger plane, and have them carry rockets into the upper atmosphere. And he says it’s still on track to have its maiden flight in 2017.

“The plane is really coming along,” Allen told GeekWire’s Alan Boyle after a speech at the University of Washington’s Paul G. Allen Center for Computer Science & Engineering. “We’re going to hopefully be flying it later this year.”

The Stratolaunch’s parent company, Vulcan Aerospace, isn’t the only one looking into the airplane-borne upper atmosphere rocket launch method. While Jeff Bezos and Elon Musk are putting their money behind reusable vertical-launch rockets, Richard Branson’s Virgin Orbit also has a system to launch rockets from the underside of a large cargo jet, called LauncherOne. But the Stratolaunch is different, mostly because it’s really, really, improbably huge.

“It is … I can’t even figure out the right adjective. Is it ‘ginormous’? I don’t know,” Allen said. “It’s pretty darn big. The tail is 50 feet high, just the tail. It’s probably the biggest carbon-composite vehicle ever constructed.”

The Stratolaunch has six engines spread out over a 385-foot wingspan and two fuselages, cobbled together from parts cannibalized from two Boeing 747 airliners. It’s 238 feet long and has a tail height of 50 feet.

In other words, it’s freakin’ huge.

Vulcan Aerospace has a contract with Orbital ATK to launch the latter company’s Pegasus XL rockets, which can carry small payloads (a little less than 1,000 pounds) into orbit.

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Allen visited the hangar where Stratolaunch is under construction in December and posted some photos, which give an inkling into how big the plane is but don’t really do it justice.

The Stratolaunch’s plan to get to space is relatively simple. The plane will fly up to the stratosphere (hence its name), the second-highest layer of Earth’s atmosphere, and drop a rocket, which will then ignite and power through the rest of Earth’s gravity well.

Diagram of the Stratolaunch's flight path.
Seems pretty simple. 

Allen didn’t give a specific date for the Stratolaunch’s first test flight, but it stands to reason that we’re still quite a ways off from payload-capable flights. Still, if the giant double-plane ever makes it in the air, it’ll be quite a sight to see.

Photos via Vulcan Aerospace