For decades, aviation enthusiasts and conspiracy theorists alike have whispered about Aurora, a top-secret American aircraft said to have reached speeds beyond Mach 6. Officially, no such project has ever existed. Unofficially, the evidence paints a far more intriguing picture. In the late 1980s and early 1990s, mysterious “donuts-on-a-rope” contrails and thunder-like sonic booms were repeatedly reported…