April 29, 2013
At Marshall Space Flight Center, a relatively simple technology developed to smooth potentially dangerous vibrations in NASA's defunct Ares I crew launch vehicle is finding its way into the wider world as a way to steady buildings, aircraft, ships and other structures reacting to winds, waves and even earthquakes. The passive approach uses the weight of a liquid coupled to a structure to dampen shaking, swaying, fluttering and other oscillations.
NASA has spent about $5 million refining the technique it calls fluid structure coupling (FSC), but has been reluctant to reveal details because of the military potential growing out of the launch-vehicle application that spawned it originally. Now engineers here have expanded their early analytical and experimental work on the Ares I thrust-oscillation problem to encompass a host of potential applications, including stabilizing nuclear power plants and tall buildings in earthquakes and violent storms, ships and drilling platforms in rough seas, and fuel-filled aircraft wings in turbulent flight conditions.
“Once you [understand] the concept, it has allowed us to ask a lot more questions in a lot more places,” says Rob Berry, chief technologist and manager of the FSC project at Marshall. “We're saying anywhere fluid and structures coexist, you can control the coupling. The question is, 'can you control enough fluid, enough coupling, to make it worthwhile?'”
The Ares I application used an FSC device immersed in the upper stage liquid oxygen (LOX) tank to calm vibrations set up in the vehicle stack as its solid-fuel first stage neared propellant burnout. The thrust oscillation posed a danger to astronauts in the Orion crew capsule at the top of the stack (AW&ST July 6, 2009, p. 42). Ultimately, opponents of a government-owned orbital crew vehicle seized on the thrust-oscillation issue as ammunition in their successful efforts to kill the project. But work on the FSC technology continued at a low level, using surplus hardware scrounged from the boneyards of this Apollo-vintage propulsion center and funds from NASA's Office of the Chief Technologist, the heavy-lift Space Launch System (SLS) program, and other sources.
The basic idea is what Jeff Lindner, one of the engineers who invented the FSC launch-vehicle application, calls “a compressible degree of freedom.” In the Ares I, Lindner and his colleagues used the weight of the LOX in the upper stage to dampen thrust oscillation by building a system that gave the relatively heavy cryogenic liquid another place to go instead of transmitting vibrations upward from the solid-fuel first stage.
“The bottom of the tank moves up or down, and that fluid goes along for the ride,” says Lindner. “If you put a compressible degree of freedom, a bubble—think of a balloon—in the tank, when it compresses the fluid moves toward it. When it expands, the fluid moves away from it . . . Now we have a very large percentage of the fluid which we control the dynamics of, all by controlling the dynamics of that compressible degree of freedom.”
The FSC project is using the 40-story vehicle dynamics test facility originally built for the Saturn Moon rocket, and later modified to handle the Ares I, to demonstrate just how little fluid is needed to stabilize a tall building. The team has mounted oscillating weights near the top of the structure that are massive enough to set the whole building moving with an easily perceptible sway.
In the photo, Lindner handles part of the off-the-shelf green plastic pipe holding 13,000 lb. of water that has an FSC device inside. As long as the system is engaged at the top of the 4.5-million-lb. structure, the oscillating weights barely move the building. But when the valve in Lindner's right hand is closed, isolating the device, water in a nearby transparent tank begins sloshing dramatically as the building sways perceptibly.