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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.

LoveForChrist
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    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. – SF. Jun 05 '20 at 12:28
  • @SF. SpaceShipOne and Two were far from that (and One did surpass the FAI-set definition) but they launched from the air, not from ground. My question regarding the minimum speed includes air-launched spaceplanes. – LoveForChrist Jun 05 '20 at 12:31
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    @SF OP is asking about suborbital flight. – Russell Borogove Jun 05 '20 at 12:50
  • By minimum speed, do you mean if the spaceplane is launched at that particular velocity instantaneously then it crosses the Karman line? Because generally any rocket/spaceplane accelerates throughout the way till the end of their powered flight and then it either descends after gliding in zero G (parabolic arc) or reach the parking orbit for which they must acquire the required orbital speed by that time. – OrangeDurito Jun 05 '20 at 14:06
  • @OrangeDurito No, I mean a pattern like that of the X-15, SS1 and 2 spaceplanes: the spaceplane is brought as high as possible by another plane, there it jacks out and fires the engine. The question is what minimum speed it would need in order to reach space, not when reaching space. – LoveForChrist Jun 05 '20 at 14:28
  • A minimum velocity could be given by simple ballistics. A more in-depth look at the aerodynamic effects could be derived by methods similar to: https://space.stackexchange.com/a/3692/31429 – Anton Hengst Jun 05 '20 at 14:50
  • some related information can be found in https://space.stackexchange.com/questions/tagged/x-15?tab=Votes – uhoh Jun 05 '20 at 14:51
  • A lower bound is 1.4 km/s to reach 100 km by ballistic motion only. – Anton Hengst Jun 05 '20 at 15:00
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    @AntonHengst The SpaceShipOne's achieved speed was 961 m/s and it reached 100.1 km (flight 15P). – LoveForChrist Jun 05 '20 at 15:08
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    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. – Russell Borogove Jun 05 '20 at 16:13
  • @RussellBorogove At some point of your ascent you would be unable to counteract gravity at too low speeds anymore, therefore even with enough fuel there's a lower speed limit. You can't reach space at 1000 ft/s (300 m/s), can you? – LoveForChrist Jun 05 '20 at 16:34
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    @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. – Russell Borogove Jun 05 '20 at 16:39
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    @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.

     – Anton Hengst Jun 05 '20 at 18:11
  • @LoveForChrist I think "outer space" is the space out of the Solar System. The space around the Earth is called simply "space" or "LEO". – peterh Jun 05 '20 at 20:30
  • Btw, LEO $\approx$ 25 mach and at least $\approx$ 200 km height. – peterh Jun 05 '20 at 21:03
  • @peterh-ReinstateMonica "Outer space" is the (near-perfect) vacuum outside a celestial body's atmosphere (if it has any). From Earth, outer space starts at either 50 or 62.14 miles (most common definitions). On the Moon you're in outer space when standing on its surface already because it has no atmosphere (only an exosphere). – LoveForChrist Jun 06 '20 at 04:34
  • @AntonHengst I'm asking for what is practically possible (realistic point of view) and the minimum speed, so as written in my comments the spaceplane may be taken as high as possible by another plane (how high would that be?) and there it jacks out, fires the engine until it reaches a fast enough speed, and what would a minimum speed be to reach 55 mi (88.5 km) altitude? – LoveForChrist Jun 06 '20 at 04:38

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