Three types of External Tanks or ET’s were manufactured for the Space Shuttle program: Original ET’s used during the first six missions were standard weight tanks (SWTs); improved tanks used throughout the 1990’s were lightweight tanks (LWTs); and super lightweight tanks or SWLTs. All three models were designed prior to the Space Shuttle Columbia accident, and all had the same appearance on the outside. ET-94 is a LWT and differs in internal construction and materials from the other two types.
The Last of the Flight External Tanks
- ET-94 was one of three LWT tanks NASA ordered specifically to support science missions for the Space Shuttle Columbia.
- Referred to as a “deferred-build” tank since production of SLWT’s had already begun.
- ET-94 was delivered to NASA In January 2001 but stored at the Michoud Assembly Facility.
- The first of the “deferred-build” tanks, ET-93, was involved in the Space Shuttle Columbia accident.
- The Columbia Accident Investigation Board (CAIB) and NASA were interested in finding out if there was anything unique to the deferred-build tanks that contributed to the accident. Investigators spent a lot of time examining ET-94.
- The investigation team dissected foam from various parts of the tank and generally inspected every aspect of the tank looking for answers.
- This explains why there are significant pieces of foam missing from ET-94 and the need for cosmetic restoration before it is assembled with the Orbiter.
- After Columbia was lost, NASA did not fly any dedicated low-earth orbit science missions, eliminating the need for the remaining deferred-build tanks.
Since the external tank was discarded on every mission, ET-94 is the only flight external tank remaining.
- Three of the original test tanks also remain – one in a museum in Florida, one in Alabama (Huntsville), and one disassembled at Michoud Assembly Facility.
- The last of the deferred-build tanks, ET-95, was never assembled.