The promise of hypersonic flight sending us halfway around the world in a matter of hours is being bandied about again, this time by a British company that declares, with all due humility, that it has made “the biggest breakthrough in aerospace propulsion technology since the invention of the jet engine.” Reaction Engines Limited says its hypersonic engine will send us streaking across the sky at speeds well over Mach 5, allowing us to have bagels for breakfast in New York and sushi for lunch in Tokyo.