The world's biggest plane is a step closer to its first flight, after passing another major milestone.Named Stratolaunch, the colossal aircraft travelled down the runway under its own power for the first time, firing all six of its Pratt and Whitney turbofan engines - each weighing 8,940lbs (4,000kg).During the aircraft's first low-speed taxi test, Stratolaunch successfully travelled down the runway at 25 knots (28mph/45km/h).Once testing has been completed, the aircraft - which has a wingspan longer than a football field - will be used to launch satellites and other objects into space.This could include a Dream Chaser spaceship, which could act as a mini-shuttle to reach low Earth orbit destinations and return astronauts or payloads to a runway within 24 hours.It is expected to take the skies for the first time in 2019.The main purpose of the test was to put the aircraft’s ability to steer and stop through its paces.A ground team from Stratolaunch Systems Corp monitored a number of systems, including steering, braking, anti-skid and telemetry.All objectives of this test were achieved.In a written statement George Bugg, aircraft program manager at Stratolaunch, said: 'This was another exciting milestone for our team and the program.'Our crew was able to demonstrate ground directional control with nose gear steering, and our brake systems were exercised successfully on the runway.'Our first low speed taxi test is a very important step toward first flight.We are all proud and excited.' Since the first engine runs in September, the Stratolaunch team has performed a series of engine tests from a newly established Stratolaunch Mission Control Center (MCC) located at its facility at the Mojave Air and Space Port in California.The MCC serves as the Seattle-based firm's hub for testing communications and will eventually be the centre of its aircraft and launch operations.Once low-speed taxi tests have been safely completed, the company will begin the next phase of taxi testing, which will include increased speeds.The plane is the vision of Microsoft co-founder Paul Allen who wants it to act as a giant air pad in the sky, allowing payloads to reach space faster and at a lower cost than existing technologies.The aircraft is so huge if it sat in the centre of a football field, it would be wide enough for its wings to reach 12.5feet (3.8metres) beyond each goalpost.Test flights were expected for 2016 and 2017, but project delays have pushed back the date to sometime in 2019.The Stratolaunch team completed fuel testing of all six fuel tanks on September 19, to ensure their proper operations, at the Mojave Air and Space Port.Each of the six tanks were filled independently to check their fuel mechanisms were working correctly and to that they were properly sealed.In addition to fuel testing, engineers began testing the flight control system.So far, they have have exercised the full limits of motion and rate of deflection of the wings control surfaces and stabil

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