Engineers working on a propulsion system to enable hypersonic air travel say they have passed a development milestone.

U.K.-based Reaction Engines is developing technology for Synergetic Air-Breathing Rocket Engines (SABRE), which could one day allow aircraft to fly up to five times faster than the speed of sound — that’s Mach 5 or 3,836 miles per hour.

At that speed, hypersonic flights between London and Australia could be over in just four-and-a-half hours.

While some commercial plane makers are racing to bring back supersonic jets, Boeing has gone further than many of its competitors by unveiling a design for a hypersonic aircraft that could potentially travel five times quicker.

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