单语范例（未经PONS编辑处理）
More problematic is the fact that ramjets rely on forward speed to compress the incoming air, and only start to operate efficiently above Mach 1.
en.wikipedia.org This effect occurs at speeds approaching Mach 1 but ceases above Mach 1.
en.wikipedia.org Its estimated airspeed at thirty thousand feet was Mach 4.2.
en.wikipedia.org Although its maximum speed was on the order of Mach 2.5, it had a low initial velocity which limited its engagement capability against supersonic targets.
en.wikipedia.org The turbine engine accelerates the vehicle to around Mach 3 before the ramjet takes over and boosts the vehicle up to Mach 6.
en.wikipedia.org