Origin
The initial plans for the International Space Station envisaged a small, low-cost 'Assured Crew Return Vehicle' (ACRV) which would provide emergency evacuation capability; the X-38 was the prototype of this. Following cancellation of the ACRV in 2002, the program led to the more capable Orbital Space Plane concept.
The first variant of the Orbital Space Plane was designed to serve as a crew rescue vehicle for the ISS; this replaced the previous plans for a dedicated station Crew Return Vehicle, which had been sidelined by budget cuts.
This early version of the plane had been expected to enter service by 2010.
Read more about this topic: Orbital Space Plane Program
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