"6_2_15_5.TXT" (3575 bytes) was created on 11-02-87


         EXTERNAL TANK
              The Space Shuttle external tank has two major roles in the
         Space Shuttle Program -- to contain and deliver propellants to the
         main engines and to serve as the structural backbone of the Space
         Shuttle during launch operations.
              The external tank is composed of two tanks -- a large liquid
         hydrogen tank and a small oxygen tank -- joined together by an
         intertank section to form one large propellant storage container.
              The liquid oxygen tank is the forward portion of the
         external tank.  The forward end of the oxygen tank curves to a
         point to reduce aerodynamic drag.  It also provides lightning
         protection for the entire Shuttle vehicle.  The oxygen tank holds
         approximately 542,000 liters (143,000) gallons) of oxygen.
              The liquid hydrogen tank is located aft of the oxygen tank
         and is about two-and-one-half times larger than the smaller
         oxygen tank.  In this tank is stored liquid hydrogen at a
         temperature of about 251 degrees below zero centigrade (420
         degrees below zero fahrenheit).  The hydrogen tank holds
         approximately 1,458,000 liters (385,000 gallons) of hydrogen.
              The intertank joins the two tanks.  Its primary functions are
         to receive and distribute all thrust loads from the solid rocket
         boosters and transfer loads between tanks for structural
         continuity.
              For launch, the external tank is attached to the solid
         rocket boosters.  The Shuttle orbiter is attached to the external
         tank.  Since thrust is generated by the main engines on the
         orbiter and the two solid rocket boosters, the external tank must
         absorb the thrust loads for the Shuttle vehicle during launch.
              Much of the outer surface of the tank is protected
         thermally.  Spray-on foam insulation is applied over the forward
         portion of the oxygen tank, the intertank, and the sides and
         bottom of the hydrogen tank.  The foam insulation is needed to
         reduce ice or frost formation on the tank during launch and to
         minimize heat leaks into the tank that would cause boiling of the
         liquid propellants.  Ice build-up on the tank would increase
         significantly the Shuttle lift-off weight.
              An ablating material -- a material that chars away -- is
         applied to the external tank bulges and projections to protect
         them from aerodynamic heating during flight through the
         atmosphere.
              The external tank contains systems that are necessary to
         support its operation.  These include a propellant feed system to
         pipe the propellants to the orbiter; a pressurization and vent
         system to regulate the tank pressure; an environmental
         conditioning system to regulate the temperature and humidity in
         the intertank area; and an electrial system to distribute
         electrical power and instrumentation signals and to provide
         lightning protection.
              All fluid control and valves, except for vent valves, for
         operation of the engines are located in the orbiter.  This is
         done to minimize throw-away costs since the external tank is not
         reused.
         
         
         External Tank
              Length      46.9 meters (153.8 feet)
              Diameter     8.4 meters (27.6 feet)