For almost a half-century there’s been a clear speed limit on most commercial air travel: 660 miles per hour, the rate at which a typical-size plane traveling at 30,000 feet breaks the sound barrier and creates a 30-mile-wide, continuous sonic boom. The ground-level disturbances that result—shattered windows, cracked plaster, maddened farm animals—have kept supersonic travel mostly off-limits since 1973, when the Federal Aviation Administration banned its use over U.S. soil.

That may be changing. In August, NASA says, it will begin taking bids for construction of a demo model of a plane able to reduce the sonic boom to something like the hum you’d hear inside a Mercedes-Benz on the interstate. The agency’s researchers say their design, a smaller-scale model of which was successfully tested in a wind tunnel at the end of June, should cut the six-hour flight time from New York to Los Angeles in half. NASA proposes spending $390 million over five years to build the demo plane and test it over populated areas. The first year of funding is included in President Trump’s 2018 budget proposal.