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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.

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    $\begingroup$ If you mean specifically a spaceplane, using fixed wing for lift, that speed will be the orbital speed (7.84838 km/s), by definition - Kármán line, the internationally recognized 'edge of space' altitude was calculated the altitude where an airplane, to sustain aerodynamic lift, would need to exceed orbital speed. $\endgroup$ – SF. Jun 5 '20 at 12:28
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    $\begingroup$ @SF OP is asking about suborbital flight. $\endgroup$ – Russell Borogove Jun 5 '20 at 12:50
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    $\begingroup$ If you have an unlimited amount of fuel to expend you can do it at an arbitrarily low speed. In practice there are a lot of engineering/aerodynamics variables in play, so I don't think it's possible to give a really firm answer. $\endgroup$ – Russell Borogove Jun 5 '20 at 16:13
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    $\begingroup$ @LoveForChrist No. Speed is not acceleration. If you are going 1 m/s upward, and have enough upward thrust to exactly balance Earth's gravity + air drag, you will continue to go up at 1 m/s until you pass the Kármán line. In practice you can't do it because you need an insane amount of fuel. $\endgroup$ – Russell Borogove Jun 5 '20 at 16:39
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    $\begingroup$ @LoveForChrist That is for purely ballistic motion with no aerodynamic drag. If you had instant impulse from the ground then you would need 1.4 km/s. SpaceShipOne did not start from the ground, and it was not instant impulse. Because you are being a bit unclear about what is theoretically possible and what has been practically achieved, your question may be hard to answer. After all, if I lift my spaceplane to 100 km & then let it go, it needs go to at a speed of 0 m/s to get to space. $\endgroup$ – Anton Hengst Jun 5 '20 at 18:11