This photo depicts the installation of an External Tank (ET) into the Marshall Space Flight Center Dynamic Test Stand, building 4550. It is being mated to the Solid Rocket Boosters (SRB's) for a Mated Vertical Ground Vibration Test (MVGVT). At 154-feet long and more than 27-feet in diameter, the ET is the largest component of the Space Shuttle, the structural backbone of the entire Shuttle system, and is the only part of the vehicle that is not reusable.