<COMMENT>The ABI labs</COMMENT>
<P><B><JC>Understanding Sub-Orbital Spaceflight
<P><JC>University of Chiba City Press February 2004
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<P>A sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not leave the vicinity of Earth, and where the spacecraft does not enter orbit. For example, any object that reaches 100 km above sea level, and then falls back to Earth, is considered a sub-orbital spaceflight. Manned and unmanned sub-orbital flights have been undertaken to test spacecraft and launch vehicles intended for later orbital spaceflight, while other vehicles have been designed exclusively to reach space sub-orbitally: manned vehicles such as the X-15 and SpaceShipOne, and unmanned ones such as ICBMs and sounding rockets.
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<P>The sub-orbital spaceflight should not be confused with a low Earth orbit, or a spaceflight that attains orbit but deorbits after less than one full orbit, known as a partial orbital spaceflight (for example, the Fractional Orbital Bombardment System).
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<P><B>Speed requirements
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<P>Sub-orbital spaceflights are possible with maximum speeds of around 1 km/s, compared to spaceflights in low Earth orbit which need speeds around 8 km/s. However, some sub-orbital spaceflights, such as Intercontinental ballistic missiles, or possibly future commercial spaceflights, speeds of over 7 km/s may be achieved. This difference is caused by the need for the spacecraft to travel a considerable distance horizontally, as opposed to simply flying straight up and coming straight back down.
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<P>So if one's goal is simply to "reach space", for example the teams who were competing for the Ansari X Prize, then there is no need for horizontal motion. Consequently, the astrodynamical measure, delta-v, for a sub-orbital flight can be as low as 1.4 km/s, compared to around 10 km/s required for orbital spaceflights.
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<P>It should be noted that any spaceflight that returns to the surface, including sub-orbital ones, will undergo atmospheric reentry. But because the speeds of sub-orbital spaceflights may be considerably lower than orbital spaceflights, the aerodynamic heating caused will be much less.