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An orbital spaceflight (or orbital flight) is a spaceflight in which a spacecraft is placed on a trajectory where it could remain in space for at least one orbit. To do this around the Earth, it must be on a free trajectory which has an altitude at perigee (altitude at closest approach) above (this is, by at least one convention, the boundary of space). To remain in orbit at this altitude requires an orbital speed of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires higher delta-v. The expression "orbital spaceflight" is mostly used to distinguish from sub-orbital spaceflights, which are flights where apogee of a spacecraft reaches space but perigee is too low. ==Orbital launch== Orbital spaceflight from Earth has only been achieved by launch vehicles that use rocket engines for propulsion. To reach orbit, the rocket must impart to the payload a delta-v of about 9.3–10 km/s. This figure is mainly (~7.8 km/s) for horizontal acceleration needed to reach orbital speed, but allows for atmospheric drag (approximately 300 m/s with the ballistic coefficient of a 20 m long dense fueled vehicle), gravity losses (depending on burn time and details of the trajectory and launch vehicle), and gaining altitude. The main proven technique involves launching nearly vertically for a few kilometers while performing a gravity turn, and then progressively flattening the trajectory out at an altitude of 170+ km and accelerating on a horizontal trajectory (with the rocket angled upwards to fight gravity and maintain altitude) for a 5-8 minute burn until orbital velocity is achieved. Currently, 2-4 stages are needed to achieve the required delta-v. Most launches are by expendable launch systems. The Pegasus rocket for small satellites instead launches from an aircraft at an altitude of 12 km. There have been many proposed methods for achieving orbital spaceflight that have the potential of being much more affordable than rockets. Some of these ideas such as the space elevator, and rotovator, require new materials much stronger than any currently known. Other proposed ideas include ground accelerators such as launch loops, rocket assisted aircraft/spaceplanes such as Reaction Engines Skylon, scramjet powered spaceplanes, and RBCC powered spaceplanes. Gun launch has been proposed for cargo. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「orbital spaceflight」の詳細全文を読む スポンサード リンク
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