More recently,NASA abandoned hope for the X-33,a pro posed replacement for the space shuttle that was supposed to fly from the ground to orbit in a single stage,rather than using a booster rock et,because the technology never quite panned out.

• 最近，国家航空航天局放弃了研制X-33的希望，因为它的技术一直没有得到充分验证。X-33是被提议用来代替航天飞机的产品，原本预计它可以无需火箭推动器而仅靠单级火箭就能直接从地面飞抵轨道。