Differences between Enterprise and future Shuttles
The design of Enterprise was not the same as that planned for Columbia, 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 RCS thrusters (which were useless in atmospheric flight) 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, common on test aircraft because the location provides the most accurate readings for the test instruments, being mounted out in front of the disturbed airflow.
Enterprise was equipped with Lockheed-manufactured zero-zero ejection seats like those its sister Columbia carried on its first four missions.