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Pegasus II was an air-launched orbital rocket under development in 2012–2015 by Orbital Sciences Corporation for use by Stratolaunch Systems.〔 〕 The vehicle was to be carried by the Stratolaunch aircraft, developed by Scaled Composites, which is the world's largest aircraft by wingspan.〔 Originally designed with solid lower stages and a cryogenic upper stage, the Pegasus II design was subsequently modified in 2014 to exclusively use solid rocket motors. However, the failure of the design to meet the economic cost and revenue targets led to the design being "shelved" by Stratolaunch in May 2015. Stratolaunch "has reopened the design plan and is evaluating over 70 different launch vehicle variants" for use on the Scaled Composites Stratolaunch.〔 〕 ==History== Stratolaunch Systems announced plans for its air-launched orbital rocket system in December 2011.〔 〕 Funding would be provided by Vulcan, an investment company created and chaired by Microsoft co-founder Paul G. Allen. The launch system would consist of three primary components: a carrier aircraft, contracted to Scaled Composites; a launch vehicle, originally contracted to SpaceX as a derivative of the Falcon 9—later contracted to Orbital Sciences as the Pegasus II; and a mating and integration system to enable the aircraft to safely interface with, carry, and release the booster, contracted to Dynetics. The Falcon 9 Air was to be powered by four Merlin 1D engines in its first stage,〔 〕 and would deliver up to into low Earth orbit (LEO) or to geosynchronous transfer orbit (GTO).〔(【引用サイトリンク】url=http://www.space.com/18747-stratolaunch-orbital-sciences-replaces-spacex.html )〕 As development progressed, the scope of alterations to the Falcon 9, including significant structural modifications to the first stage for aerodynamic surfaces,〔 and the disruption to manufacturing processes these modifications would have required,〔 led Stratolaunch and SpaceX to part ways and cease development of the Falcon 9 Air. Stratolaunch then engaged Orbital Sciences Corporation (Orbital) to develop the Pegasus II.〔 〕 Orbital received a study contract in November 2012, and a full vehicle development contract in 2013. The rocket consists of two solid stages produced by Alliant Techsystems (ATK) and, as originally designed, a cryogenic third stage. In 2014, the cryogenic stage was replaced by two additional solid rocket stages.〔 In February 2015, Orbital Sciences Corporation merged with Alliant Techsystems to create Orbital ATK Inc. In April 2015, Stratolaunch announced that they are considering options for multiple launch vehicle options to be used with Stratolaunch over a range of satellite sizes, and that some development work on the Orbital launch vehicle has been slowed down.〔 〕 However, by May 2015, since the evaluation of the Orbital solid-rocket launch vehicle economic case was not achieving revenue targets, Stratolaunch had "shelved" the Pegasus II option and "reopened the design plan and is evaluating over 70 different launch vehicle variants."〔 〕 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「Pegasus II (rocket)」の詳細全文を読む スポンサード リンク
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