After successfully completing low-speed taxi testing earlier this year, the giant Stratolaunch aircraft successfully hit a new milestone recently, reaching 90 mph (145 km/h) during its latest stage of runway testing. The announcement comes days after billionaire co-founder of the company Paul Allen passed away, just months before his long-dreamed creation finally hits the sky.

In development since 2011, the massive dual-fuselage Stratolaunch is undoubtedly an immensely ambitious engineering challenge. Designed to be the first commercial air-launch vehicle capable of sending relatively small payloads into a variety of Earth orbits, the world's largest plane has passed a series of key milestones over recent years.

After emerging from its hangar for the first time in mid-2017, it has undergone a series of engine tests, and earlier this year it successfully stormed down a runway in Mojave, California. These initial runway taxi tests took it up to a modest speed of 46 mph (74 km/h) but now it has almost doubled that with a newly revealed round of testing.

The latest video to be released by the company shows the plane reaching 80 mph (128 km/h) during its new phase of medium-speed taxi testing, but the description on the video suggests the plane ultimately hit a top speed of 90 mph (145 km/h) during the test. After further testing over the coming months, it's hoped the aircraft's maiden flight will take place within the next 12 to 18 months.

The timing of the new Stratolaunch milestone is bittersweet for all involved, with the successful taxi test taking place just days before the passing of founder Paul G Allen. Best known for co-founding Microsoft with Bill Gates, Allen's aerospace aspirations started in childhood and the Stratolaunch project was the focal point of those dreams.

CEO of Stratolaunch Jean Floyd tweeted his condolences following Allen's passing, suggesting everyone working on the project will be dedicated to fulfilling Allen's legacy. "Incredibly sad news," Floyd tweeted. "We deeply respect and admire Mr. Allen's vision. His legacy will be honored."

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