The Crew Return Vehicle (CRV), sometimes referred to as the Assured Crew Return Vehicle (ACRV), was the proposed lifeboat or escape module for the International Space Station (ISS). A number of different vehicles and designs have been considered over the past two decades with several flying as developmental test prototypes but no one single design has been built as the dedicated CRV. In the original space station design, emergencies were intended to be dealt with by having a "safe area" on the station that the crew could evacuate to, pending a rescue from a U.S. Space Shuttle. However, the 1986 Space Shuttle Challenger disaster and the subsequent grounding of the shuttle fleet caused station planners to rethink this concept.