Bent Out of Shape

The Space Shuttle Is Still a Death Machine, Warns Engineer Ali AbuTaha

All previous spacecraft were designed with perfect symmetry in mind. The Saturn V, which carried the Apollo astronauts to the moon, and its smaller cousins were basically enormous, hollow, fuel-filled cylinders with the payload perched on the top.

The shuttle broke decades of design precedent by mounting the airplanelike orbiter on the side of the solid-fuel boosters and fuel tank - a highly asymmetrical arrangement that puts enormous stress on the whole assembly at liftoff. When the orbiter's main engines fire, they do so about 30 feet from the vehicle's geometric center of the attachment of booster to pad. This off-center thrust produces torque - a tendency to bend or rotate - so powerful that it would wrestle the shuttle to the ground if the vehicle were not securely fastened to the pad.

The 185-foot-tall shuttle is designed to bend forward several feet under the brunt of the launch force before snapping back. It vibrates for several seconds after liftoff to dissipate the energies, a motion called "twang," because it's essentially what happens to a guitar string when plucked.

Shuttle hardware was designed to match the stress and strain of precise launch procedures. The procedures laid out in the late Seventies as the first shuttle was being designed and constructed called for the restraining bolts to be blown at 3.8 seconds after ignition of the orbital thrusters. Just prior to this moment, the load on the base of the shuttle (also called the "bending moment") is 350 million inch-pounds, well within the hardware's capacity to absorb. (An inch-pound is the torque you would exert on a bolt if you were to apply one pound of force with a one-inch wrench. A torque of twenty inch-pounds is enough to unscrew the top from any jar.)

But before the maiden voyage of the shuttle, NASA engineers grew worried. Their engineering studies showed that if it were released 3.8 seconds after ignition, the vehicle would snap back far enough to scrape the launch-pad rigging. This could damage the orbiter, and the force of snap-back alone could harm delicate payloads.

NASA considered several launch options to evade this difficulty. One option was to ignite only two of the orbiter's three engines at liftoff; the other was to offset the bending by tilting the vehicle in the opposite direction on the launch pad. NASA found these cures worse than the disease. Instead, the agency settled on an apparently simple solution: delay launching to 6.5 seconds after ignition of the orbiter's main engines. At this point the bending moment is reduced to about 190 million inch-pounds - once again well within hardware limits. The shuttle snaps back a smaller distance and clears the launch pad with ease.

But AbuTaha says it's not that simple. According to his calculations, the bending moment doesn't decrease steadily between four and seven seconds after the orbiter's main-engine ignition. On the contrary: it reaches a peak of nearly 590 million inch-pounds at five seconds after ignition, then declines. Although he worked out the curve independently, AbuTaha's figures agree with the results of a February 1981 ground test of the shuttle Columbia engine, as cited in aeronautics expert R.E. Gatto's article, "Effects of System Interactions on Space Shuttle Loads and Dynamics," which was presented to the International Council of Aeronautical Sciences Congress in 1982.

Why should the maximum stress on the shuttle occur five seconds after ignition? AbuTaha makes a "twang" analogy with a bathroom scale. If you weigh 150 pounds and step onto the scale gingerly, the needle will register 150 pounds with a minimum of fluctuation. But if you pounce on the scale from atop the nearby sink, the needle will oscillate wildly, perhaps ranging between 100 and 200 pounds in a series of increasingly smaller arcs, before settling on 150. Revving the orbiter's main engines to 100-percent capacity in one second delivers a shock to the assembly similar to jumping on the scale. The five-second point, says AbuTaha, is equivalent to the moment when the needle on the bathroom scale records its highest weight.

The Rogers Commission was not oblivious to shuttle "twang." But it rejected the idea that twang had anything to do with the Challenger disaster. Page 54 of the first volume of the commission's report states, "The resultant total bending moment experienced by [the Challenger] was 291 x 106 inch-pounds, which is within the design's allowable limit of 347 x 106 inch-pounds." However, on page 1351 of Volume Five of the report, the commission cites the same figure, written as "291,000,000," as the bending moment for the right solid booster only. The effect on the entire assembly, argues AbuTaha, should be the combined bending moments of both boosters. Multiply by two, and you arrive at the maximum force that AbuTaha calculated.

This figure is 70 percent greater than the design's allowable limit, as cited in the Rogers report. And every shuttle mission up to the Challenger explosion (and possibly afterward) has experienced this force. "This is the kind of error that catches up with you," warns AbuTaha.

Not only does this miscalculation explain the shuttle disaster that killed seven astronauts and set the U.S. space program back nearly three years, as AbuTaha suggests, it also reveals the source of the mysterious malfunctions that have plagued the shuttle program since its first launch in 1981, from tiles knocked off and booster segments warped to satellites that inexplicably failed to work.

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