The Shuttle Carrier Aircraft or SCA as they were called were heavily modified B-747-100 aircraft that were converted to provide ferry service for the space shuttle orbiters. Weight was cut by stripping the cabin and the airframe was strengthened to handle the weight of the orbiter. Mounting struts were added and additional vertical stabilizers were added to the tail for stability. When flying with an orbiter fuel and altitude were severely affected. The range dropped from 5,500 nautical miles to only 1,000 nautical miles before refueling was required. Only two of the SCAs were ever built.