□□1998～1999年，美国航宇局（NASA）牵头进行了航天运输体系研究（STAS）工作，并在此基础上提出了一项新计划——综合航天运输计划（ISTP）。ISTP规划了未来四五十年内美国全部的航天运输计划，主要包括航天飞机安全改进计划、第2代可重复使用运载器（RLV）计划和第3代及其 □ From 1998 to 1999, NASA took the lead in the Space Transport System Study (STAS) and proposed a new plan, the Integrated Space Transportation Program (ISTP). The ISTP plans all of the U.S. space programs in the next four or five decades, including the Space Shuttle Security Improvement Program, the Generation 2 Reusable Launcher (RLV) Program, and Generation 3 and its