For every airplane, there's a region of the flight envelope into which it dare not fly.
- By Peter Garrison
- Air & Space magazine, March 2001
(Page 5 of 5)
Resonance is at the root of the phenomenon of flutter in aircraft. Just as a crystal goblet, set to vibrating by just the right "forcing frequency," may shatter, a structure on an airplane, set in motion by another structure whose natural frequency is very nearly the same, may vibrate so violently that it breaks.
The Saturn V rocket experienced a novel and unexpected kind of structural resonance. The five first-stage engines were ignited at 0.3-second intervals, the center engine first, followed by the others in symmetrical pairs. Each ignition sent a jolt through the rocket and built up tension in its hold-down mechanism.
On launch the hold-downs flipped back to release the rocket--and the 360-foot-tall, six-million pound monster might have crumbled on its pad if aeroelasticity analysts at Boeing had not discovered in advance that the rocket resonated in tune with the rhythmically timed additions of thrust. The "twang" of the sudden release would then make the fuel and oxidant, which accounted for 90 percent of the Saturn's total weight, settle downward in the gigantic tanks, stretching their thin aluminum skins. The tanks would recoil like rubber bands, pumping the liquids back upward. The rhythmic bouncing of the entire fluid mass, which engineers nicknamed "Ka-Doing-a-Doing-a-Doing mode," resembled the motion of a shaken water balloon. It produced structural loads well beyond the rocket's flimsy safety margins.
Various solutions to the problem were investigated, including baffles in the tanks--discarded as too heavy--and releasing the hold-downs before full thrust had been attained--too dangerous for the human occupants, in case the engines failed to attain full thrust. The eventual solution included altering the ignition timing and adding what came to be nicknamed the shoe-in-the-mud: a simple mechanical damper that slowed the rocket's initial acceleration after release, just as deep mud slows the extraction of a foot.
Early Saturn flight tests revealed that the bouncing liquids in the tanks was also excited by the random vibrations of the engines. It launched a vicious cycle: pressures in fuel and oxidant lines began to fluctuate, throttling the engines up and down in time with the bouncing liquids. This "pogo effect" was cured by placing accumulators in the fuel and oxidant lines to damp out the pressure fluctuations.