"6_2_3_20.TXT" (2982 bytes) was created on 12-30-88

SHUTTLE CARRIER AIRCRAFT

The NASA SCA is a Boeing 747 (100 series) purchased from American
Airlines on June 17, 1974.  It was modified to ferry orbiters to and
from various shuttle facilities and to transport and release OV-101
for the ALT program.

On Feb.  29, 1988, NASA announced plans to acquire a Boeing 747-100
to serve as a second SCA for the space transportation system.  A
letter contract was signed with Boeing Military Airplane Company, a
division of the Boeing Company, Seattle, Wash., to reserve the
aircraft for NASA use.  The additional SCA will increase ferrying
capability and eliminates a potential single-point failure in the
space transportation system.  This 747-100 will be modified to carry
space shuttle orbiter's from landing sites to the processing
facilities at the Kennedy Space Center in Florida.  The modifications
will be made by Boeing at its manufacturing facilities in Wichita,
Kan.  This 747-100 is nearly identical to the original SCA and was
selected to minimize costs associated with modifications and
operation.

Modifications to the basic 747 aircraft included removal of interior
equipment (passenger seats, galleys, etc.); changes to air
conditioning ducts, electrical wiring and plumbing; installation of
higher thrust engines (JT9D-7AHW) and the 747-200 series rudder
ratio-charger; and alteration of the longitudinal trim system to
permit 2 degrees more nose-down trim.  Other changes included
relocation and installation of antennas, addition of bulkheads and
doublers in the fuselage main deck, addition of structural doublers
and tip fins to the horizontal stabilizers to improve directional
stability with the orbiter on top of the aircraft, and addition of
one forward and two aft support assemblies for attachment of the
orbiter.  The modifications increased the basic weight of the
aircraft by approximately 2,800 pounds.

The orbiter's mated location on the 747 was based on consideration of
static stability and control, structural modifications, weight and
performance.  Center-of-gravity limits for the 747 with the orbiter
mated were 15 percent of the 747's mean aerodynamic chord for the
forward limit and 33 percent MAC for the aft limit.  Longitudinal
stability is similar to that of the basic 747; ballast must be added
so that the center-of-gravity limits are not exceeded.  The ballast
is carried in standard 747 cargo containers in the forward cargo
compartment.  The mated configuration allows the 747's center of
gravity to shift approximately 10 feet upward.

For the ferry flight configuration, the tail cone fairing is
installed on the orbiter to decrease aerodynamic drag and buffet, and
aerosurface control locks are added to the orbiter's elevons.  The
orbiter is unmanned and the orbiter systems inert.  A bailout system
also is installed in the 747.

Some modifications to the 747 SCA are removable.  These include
support struts for the orbiter, horizontal tip fans and associated
cabling and umbilicals.