A year after China unveiled the world’s first operational air-breathing hypersonic missiles, the United States is launching a programme to double down on developing similar weapons.The US Defence Advanced Research Projects Agency (Darpa) will bring contractors together later this month to lay out what it wants from the Next Generation Hypersonic Cruise Missile, with air-breathing propulsion at the heart of the project, according to the agency’s website.Unlike rocket-powered missiles, air-breathing weapons draw oxygen from the atmosphere instead of carrying all they need on board, freeing up weight for fuel or payload.China put that technology on public display at a massive…