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A two-stage-to-orbit (TSTO or ''DSTO'' - Double/Dual-Stage-To-Orbit) launch vehicle is a spacecraft in which two distinct stages provide propulsion consecutively in order to achieve orbital velocity. It is intermediate between a three-stage-to-orbit launcher and a hypothetical single-stage-to-orbit (SSTO) launcher. At liftoff the first stage is responsible for accelerating the vehicle. At some point the second stage detaches from the first stage and continues to orbit under its own power. An advantage of such a system over single-stage-to-orbit is that the entire mass of the spacecraft is not carried into orbit. This reduces the difficulty involved in reaching orbital velocity. An advantage over three or more stages is reduction in complexity and fewer separation events, each of which reduces cost and risk of failure.〔(【引用サイトリンク】url=http://www.spacex.com/falcon1.php#engine_reliability )〕 ==Examples of two stage to orbit systems== * Angara 1.2 * Atlas V (401 and 501) * Blue Origin orbital launch vehicle * Cosmos-3M * Delta IV (medium) * Falcon 1 * Falcon 9 * Saturn IB * Saturn V (Skylab launch) * Tsyklon-2 * Vulcan * Zenit-2 * Blackstar (spacecraft) conjectural system * (proposed by Ukraine) It is not always clear when a vehicle is a DSTO. Many designs which use a very small boost at the beginning of their flight are referred to as single-stage-to-orbit. Some have also coined the expression 1.5STO for 'one-and-a-half-stage-to-orbit', e.g., the Atlas. Also, many launch vehicles have side-mounted booster rockets which are jettisoned early which are called "stage-0". 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「Two-stage-to-orbit」の詳細全文を読む スポンサード リンク
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