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Gravel Haulers: NASA's 747 Shuttle Carriers
Space Shuttle Columbia hitched a ride on a NASA 747 Shuttle Left: The Space Shuttle Columbia hitched a ride on a NASA 747 aircraft.

Unbeknownst to most people, NASA's two massive 747 Shuttle Carrier Aircraft (SCA), in addition to their well-known roles as piggyback-ride providers for the Shuttles, carry a much less glamorous cargo of gravel and iron.

Gravel and iron, one might ask? Sure, just ask Pete Seidl, NASA's 747 SCA maintenance boss at NASA's Dryden Flight Research Center in California.

"You see, the nearly 2 tons (1710 pounds) of pig iron up-front in the former first class section of the aircraft, and the 3.5 tons (7000 pounds) of pea gravel in the cargo hold are for keeping the aircraft’s center of gravity forward when a heavy Shuttle is mounted on top," says Seidl.

All aircraft need to be balanced, and keeping their center of gravity in the right place is key. Without the gravel and iron, the tail-heavy Shuttles, made so by their rocket engines, could cause the 747s to experience flight instability. So, the gravel and iron provide much-needed weight toward the noses of the 747s to balance out the problem.

Pete Seidl and Todd Weston hold pea gravel used as ballast Left: Pete Seidl and Todd Weston hold pea gravel used as ballast.

The pea gravel, so-called due to the size of the pebbles, is contained in cargo containers in the lower forward cargo bay. The pig iron, which is the raw material of steel, is secured to the floor in the first class section forward of the first row of seats. Not the most high-tech ballast around, but affordable and effective.
With small hatches and ladders in various locations leading down three decks, NASA's 747s might remind one of being aboard a big ship.

What other little-known secrets lurk aboard these giant aircraft? Other lesser-known facts about the SCAs include:

  • A crew escape tunnel was installed aboard NASA 747 aircraft number 905 (NASA 905) during the aircraft's modification process for the Shuttle program. The tunnel extended down three decks, from the flight deck to the bottom left side of the fuselage. In a catastrophic emergency, the parachute-clad pilots and flight engineer would activate explosives that would blow a hole through the fuselage for bail-out. Next, they would slide down and out the hole, escaping into the air below the airplane. Activation of the tunnel hole explosives also activated pyrotechnic devices designed to blow out 10 windows above each wing in order to equalize the on-board air pressure allowing the crew to slide down the tunnel. However… this escape system might have led into the inboard engine inlet, obviously not a good thing! The escape tunnel system was removed from the aircraft following completion of the Space Shuttle Approach and Landing Tests (ALT) project.

  • Inside, one might expect to see that the three struts protruding from the top of the SCAs that support the Space Shuttles extend down into the aircraft. Not so. The struts are actually mounted to the 747's skin frames, which are beefed-up for added support to distribute the weight of a mounted Shuttle. Inside the fuselage, extra partial-bulkhead supports were added.

  • Shortly after acceptance by NASA in 1974, NASA 905 was flown in a series of wake vortex research flights at NASA Dryden in a study to seek ways of reducing turbulence produced by large aircraft. Pilots flying as much as several miles behind large aircraft have encountered wake turbulence that has caused control problems. The NASA study helped the Federal Aviation Administration modify flight procedures for commercial aircraft during airport approaches and departures.

  • Along with ferrying the Shuttle Enterprise and the space flight rated orbiters between Edwards Air Force Base, Calif. and NASA's Kennedy Space Center, Fla., NASA 905 also ferried the Enterprise to Europe for display in England and at the Paris Air Show.

  • Everybody rides first class. As all the coach section seats were removed during the modification process, hard-working maintenance crew flying with the 747s have to use the only passenger seats left -- those easy-chairs in first class section. There are a few seats available in the upstairs lounge section, too.

  • NASA 905, the first SCA, was obtained by NASA from American Airlines in 1974. The second SCA aircraft, NASA 911, was a Japan Airlines aircraft acquired by NASA in 1988.

  • NASA 905 was the only SCA used by the space shuttle program until Nov. 1990, when NASA 911 was delivered as an SCA.
Following the wake vortex studies, NASA 905 was modified by Boeing to its present SCA configuration and the aircraft was returned to Dryden for its role in the 1977 Space Shuttle ALT project. This series of eight captive and five free flights with the orbiter prototype Enterprise, in addition to ground taxi tests, validated the aircraft's performance as an SCA. In addition, the glide and landing characteristics of the orbiter configuration were verified, paving the way for orbital flights.

The two SCAs are under the operational control of NASA's Johnson Space Center in Houston, Texas.

For more on NASA's 747 Shuttle Carrier Aircraft, visit the Web at:


Gray Creech
Public Affairs
NASA Dryden Flight Research Center