Now,NASA has earmarked$4.8billion for a Space Launch Initiative to develop a replacement for the shuttle that will be an other two-stage reusable launch vehicle but cost much less to maintain and operate.

• 国家航空航天局现在已经拨出48亿美元用于空间发射计划，研制航天飞机的替代品，这是另一种两级可再利用发射器，但维护和运行费用要少得多。