No civilian travelers have flown aboard supersonic jets since the Concorde fleet was grounded in 2003, but a handful of entrepreneurs are hoping to change that, with a flurry of Mach-plus business-jet designs competing to get to market. The technical challenges of achieving supersonic speeds are difficult, and the cost of certifying civilian aircraft for such speeds are enormous—and then even if those hurdles are cleared, many countries, including the United States, forbid supersonic flight over land because o…