From a realistic point of view there must be a minimum speed in order to reach space in a suborbital spacecraft, let's say in a spaceplane. Like there's a first cosmic velocity (for orbit) and a 2nd cosmic velocity (escape velocity) the speed required to reach outer space is actually something like a cosmic velocity 0. Surely, a plane aiming to reach space (let's say an altitude of 55 mi / 88.5 km) needs to go supersonic and probably more than twice the speed of sound.
The SpaceShipOne reached Mach 2.9 and an altitude of 100.1 km in its first spaceflight, and Mach 2.92 and Mach 3.09 in its subsequent spaceflights. The SpaceShipTwo (VSS Unity) reached Mach 2.9 and 82.72 km in its first spaceflight and Mach 3 and 89.9 km in its 2nd one. It seems to me that in order to reach 55 mi altitude you need to go at more than Mach 2 or am I wrong?
I'm talking of rocket-powered planes of course, since it is unlikely to reach space by other means.
Sorry if the question has been asked already.