Space Shuttle Enterprise (Orbiter Vehicle Designation: OV-101) was the first orbiter of the Space Shuttle system. Rolled out on September 17, 1976, it was built for NASA as part of the Space Shuttle program to perform atmospheric test flights after being launched from a modified Boeing 747. It was constructed without engines or a functional heat shield. As a result, it was not capable of spaceflight. Originally, Enterprise had been intended to be refitted for orbital flight to become the second space-rated orbiter in service. However, during the construction of , details of the final design changed, making it simpler and less costly to build around a body frame that had been built as a test article. Similarly, Enterprise was considered for refit to replace Challenger after the latter was destroyed, but was built from structural spares instead. Enterprise was restored and placed on display in 2003 at the Smithsonian's new Steven F. Udvar-Hazy Center in Virginia. Following the retirement of the Space Shuttle fleet, replaced Enterprise at the Udvar-Hazy Center, and Enterprise was transferred to the Intrepid Sea, Air & Space Museum in New York City, where it has been on display since July 2012. Differences between Enterprise and future shuttles The design of Enterprise was not the same as that planned for , the first flight model; the aft fuselage was constructed differently, and it did not have the interfaces to mount OMS pods. A large number of subsystems—ranging from main engines to radar equipment—were not installed on Enterprise, but the capacity to add them in the future was retained, as NASA originally intended to refit the orbiter for spaceflight at the conclusion of its testing. Instead of a thermal protection system, its surface was primarily covered with simulated tiles made from polyurethane foam. Fiberglass was used for the leading edge panels in place of the reinforced carbon–carbon ones of spaceflight-worthy orbiters. Only a few sample thermal tiles and some Nomex blankets were real. Enterprise used fuel cells to generate its electrical power, but these were not sufficient to power the orbiter for spaceflight. Enterprise also lacked reaction control system thrusters and hydraulic mechanisms for the landing gear; the landing gear doors were simply opened through the use of explosive bolts and the gear dropped down solely by gravity. As it was only used for atmospheric testing, Enterprise featured a large nose probe mounted on its nose cap,