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A new milestone was recently achieved in the field of air launching rockets, on the way to the key test in 2019. Stratolaunch, the world’s biggest airplane, hit a new milestone recently, taxiing down the runway at 46 mph. The 500,000-pound huge aircraft with twin fuselages and a wingspan of 385 feet lumbered down the concrete. The goal is to use the airplane as a platform for lifting rockets into the stratosphere before launching them into space. Some see it as a cheaper, more reliable route to low Earth orbit — the sweet spot for many kinds of satellites. Thanks to its massive size, the plane is capable of carrying payloads up to 550,000 pounds.

Utilizing six Boeing 747 engines for a payload capacity of over 500,000 lbs. and an operational range of approximately 2,000 nautical miles, Stratolaunch is capable of delivering payloads to multiple orbits and inclinations in a single mission, according to the company website.

It’s a big improvement over a low-speed test conducted last December, in which the Stratolaunch traveled down a runway at just 28 mph. Previously, the massive aircraft successfully conducted a test of its six turbofan engines at the Mojave Air and Space Port in California. All of these incremental milestones are said to be leading up to the Stratolaunch’s first test flight in 2019, according to

Air launching rockets isn’t a new thing. Orbital ATK’s Pegasus XL rocket is capable of launching from air. And while the Pegasus is a fairly small rocket, capable of launching satellites weighing up to 1,000 pounds, the rocket-maker signed a deal with Stratolaunch last year. NASA and Richard Branson’s Virgin Group have similar projects under development, as does the Defense Advanced Research Projects Agency (DARPA).