WASHINGTON NASA’s managers have settled on a fix they say will protect astronauts from potentially dangerous levels of vibrations that could otherwise reach the planned Orion Crew Exploration Vehicle during its climb to orbit atop the Ares 1 rocket, according to information posted on a NASA Web site.

NASA Constellation program officials decided Dec. 17 to update the Ares 1 vehicle design to include upper-plane C-spring isolators and an upper-stage liquid oxygen (LOX) damper intended to keep vibrations originating in the Ares 1 main stage from reaching Orion and its crew.

The Constellation program is a 5-year-old effort to replace the space shuttle with new rockets and spacecraft optimized for the Moon, including Orion and the Ares 1 booster. Since 2007, NASA engineers have been looking at options for “detuning” Ares 1 to prevent oscillations originating in its solid-rocket main stage from synching up with the natural resonance of the rest of the vehicle, potentially exposing astronauts to unsafe levels of shaking by the time the vibrations reach the Orion capsule.

Thrust oscillation is common to solid-rocket motors, the casings of which tend to resonate like organ pipes as their propellant burns from the inside out. Rocket designers commonly stiffen joints or make the vehicle heavier to prevent these up-and-down vibrations — or oscillations — from growing stronger as they travel up the length of the rocket.

Although Ares 1 passed its preliminary design review in September 2008, engineers at NASA’s Marshall Space Flight Center in Huntsville, Ala., said at the time that they had not yet settled on a hardware fix for the problem.

In September 2009, Constellation program officials said data gleaned from an Ares 1 ground test conducted Sept. 10 suggested the thrust oscillation problem was not as serious as initially thought. A suborbital flight test of an Ares 1 prototype conducted Oct. 28 provided additional reassurance, though program managers said incorporating the upper-plane C-spring isolators and LOX damper at this phase of the rocket’s design makes sense, according to information posted Dec. 21 on the Constellation program’s blog.

“When we discover an engineering risk, like thrust oscillation, we tackle it with full rigor,” Jeff Hanley, Constellation program manager, states in the blog post. “That’s what this team has done with thrust oscillation. We assumed the worst when the problem was first discovered. The good news is there is no empirical evidence of problematic oscillations from our ground test of the first stage development motor or during the Ares 1-X first test flight.”

Hanley says the Ares 1 C-spring isolators are intended to work like shock absorbers to detune the Ares launcher while the LOX damper is expected to counter the vehicle’s acoustic response by absorbing and disrupting the oscillation.

“Together these options will give us added confidence in the tuning of the vehicle as we mature the Ares and Orion designs,” Hanley says.

The blog post states, “The NASA team, along with the prime contractors, has worked this issue carefully, understanding and minimizing any effects of the integrated vehicle response by introducing new thrust oscillation hardware into the design. The team will … prepare the upper stage design to accommodate the addition of this mitigation hardware at a later time, if desirable.”

“The options approved today puts us on a robust foundation as we move forward,” Hanley says in the blog. “Finalizing the thrust oscillation design now allows us to keep to our schedule and provides contractors specific requirements about what we need them to build.”

Hanley makes no mention of when the Constellation program expects to fly Ares 1 for the first time, but  he and other officials maintain that the program remains on track to conduct the first crewed Ares launch in early 2015. A blue-ribbon panel convened by the White House this summer, however, concluded that 2017 is the earliest NASA could expect to field Ares and Orion, and only if the agency is given a major budget boost.



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