How about 5x the speed of sound?

Boeing, one of Hitemco's best indirect customers, just released a new development: A new ship that can travel at speeds of up to Mach 5. That's 5 times the speed of sound.

“It’s a really hard problem to develop an aircraft that takes off and accelerates through Mach 1 all the way to Mach 5 and beyond,” said Kevin Bowcutt, Boeing’s chief scientist for hypersonics, speaking to Aerospace Daily & Defense Report. “The specific impulse of an air breathing engine goes down with increasing velocity, so you have to make the engine bigger to get to Mach 5. But doing that means a bigger inlet and a bigger nozzle, and trying to get that through Mach 1 is harder.” See more here.