水平起/降可重复使用的航天器是今后的发展方向,但由于单级入轨方案的关键——组合发动机研制面临着许多难题,短期内无法上马,直接影响到航天计划的兑现。且所冒的技术风险又大,开发投资巨额,故现在有更多国家主张优先开发由运载级与轨道级组成的二级水平起/降的航天器运载系统。目前,苏、德、英、美等国均有各自的方案。 Horizontal / vertical reusable spacecraft are the future direction of development, but due to the single-stage orbit program key - combined engine development faces many problems, the short term can not be launched, a direct impact on the fulfillment of the space program. And because of the huge technical risks and the huge investment in development, more countries now advocate giving priority to the development of a spacecraft delivery system that is composed of two levels of carrier-grade and orbital levels. At present, each country such as the Soviet Union, Germany, Britain and the United States has its own plan.