On September 27, 1995, the
orbiter Discovery,
atop a special NASA 747,
stopped in Salt Lake City,
Utah, on its way back to
Edwards Air Force Base for reconditioning after
STS-70 where it launched a TDRS satellite.
The flight to Salt Lake City had been delayed from the previous day by
bad weather and by a faulty
bolt which attaches the orbiter to the 747. The shuttle carrier
aircraft, a specially modified 747, appeared about 5 o'clock PM over
Utah County from the southwest, flying low. It flew along the bench
over the Salt Lake Valley north to Ogden, then returned to land
at the Salt Lake International Airport.
Tens of thousands of Wasatch Front area residents watched as the
carrier aircraft flew overhead. It stayed overnight on display at the
Utah Air National Guard Base, and resumed its trip the next day.
The last time residents were treated to such a close visit was 16 years ago when a 747 carrying the orbiter Enterprise landed at Hill Air Force Base, near Ogden. The Enterprise was the first orbiter in the fleet, but was never meant to go into orbit. It was used for fit checking, mechanical and electrical compatibility checks and flight worthiness tests [2]. The plane that carried the Discovery today also carried the Enterprise for the flight worthiness ("drop") tests [3], according to Frank Marlow, one of the carrier aircraft pilots.
Tens of thousands of people came to see the Discovery and its carrier. It sat on the tarmac outside the Utah Air National Guard installation just east of the Salt Lake City International Airport. Volunteers from the Utah Air National Guard helped with crowd and traffic control so that the general public could be allowed within about 30 meters of the aircraft, according to Staff Sgt. Leigh Mullin. She also explained that members of the guard were eager to help.
VIP's and members of the press were allowed inside the 747. We talked with Frank Marlow, one of the carrier aircraft pilots. Mr. Marlow, a former US Navy pilot, joined NASA in 1969. In total, six pilots take turns flying the 747 when transporting an orbiter from one place to another.
The Discovery weighs 150,000 pounds (68 000 kg) while being
transported. Other orbiters weigh as much as 230,000 pounds
(100 000 kg).
Having the orbiter on top changes the flight
characteristics of the 747.
It adds a lot of lift, but also
a lot of drag. Mr. Marlow reports that the 747/orbiter combination is
no harder to fly than a regular 747: it is nice and docile, but there
is more vibration. In fact, vibration limits the airspeed to 250
knots (290 mph or 460 kph). If they flew faster, the vibrations
would cause excessive stress on the carrier aircraft and orbiter aging
their airframes greatly.
The orbiter's additional lift makes the 747 tend to raise its nose or
"pitch up."
The only real flight problem occurs during take off and landing. To
add stability, the carrier 747 has
outboard stabilizers: two extra vertical tails on the ends of the
elevators.
The stabilizers extend below the rear
part of the carrier's fuselage.
This limits the carrier's maximum pitch up to about 5 degrees. If
the aircraft pitched up any higher, the stabilizers would scrape on the
runway.
Because of the increased drag, the carrier aircraft with the orbiter burns much more fuel than a conventional 747. One gallon of fuel propels it only one length of the plane. In flight, the 747 burns 40,000 pounds (18 000 kg) of fuel an hour. The carrier can't carry enough fuel to fly nonstop across the country, so it must land two and sometimes three times per flight. Normally, it stops at Air Force bases, but the office of Senator Robert F. Bennett (R-Utah) helped arrange for it to stop in Salt Lake City.
A pathfinder plane flies in front of the carrier aircraft to check
for turbulence and rain or clouds. The tiles on the orbiter are
so fragile that 20 seconds of flying though rain causes millions of dollars
of damage. The pathfinder plane flies back and forth about 100 miles
ahead of the 747 scouting for a clear path. NASA uses C-135's,
Gulfstreams, and other airplanes as pathfinders.
The body of the 747 is mostly empty
(see interior picture below)
since it is devoted to carrying the orbiter.
There is some conventional seating for additional pilots or
passengers forward of the front door on the main deck and on the upper
deck of the 747. The forward part of the main deck also has two
tables for conferences or planning.
The orbiter is bolted to support struts on the 747. Workers must go
inside the shuttle to attach the bolts. A single 3 inch (76 mm) bolt
in each of the rear struts secures the orbiter. In the front there
are 12 bolts. One of the reasons Discovery was delayed
getting to Salt Lake City was a problem in the right, rear bolt. We
asked Mr. Marlow, one of the carrier pilots, if there was any way to
release the orbiter in flight in case of emergency. He reported there
is no way to detach the orbiter.
It takes 5 days to prepare the orbiter to be carried after landing. NASA crews must remove all experiments and stored articles, drain volatile gases and fuels, and examine the craft. [4] It takes a full day to mate the orbiter to the 747.
Most of the load of the orbiter is carried in the skin of the 747.
The internal supports are actually secondary. The skin is almost 1
inch thick.
Temperature and pressure restrictions on the orbiter limit the 747 to
an altitude of 16,000 feet. The orbiter is partially
powered during flight by an electrical unit in the body of
the 747. (The electrical unit is the white instrument rack in the
center of the picture.) The power heats the orbiter during flight and
helps control trim settings during flight.