NASA managers have concluded they don't need an active damping system in the aft skirt of the Ares I first stage to prevent dangerous vibrations from rattling the crew of the Orion crew exploration vehicle on top, and will use a passive system instead.

Steve Cook, the Ares project manager at Marshall Space Flight Center, said Dec. 17 that a follow-up review held 90 days after preliminary design review in September concluded a passive series of weights and springs, coupled with the spring/ring design originally baselined as an isolator between the first and second stages, should keep thrust oscillation in the system at acceptable levels.

The Ares I program has kept thrust oscillation generated by the solid-fuel first stage shortly before it burns out as an open item for a "delta-PDR" early next year, and has been working options to mitigate it. Early calculations suggested the vibrations, when transmitted through the stack to the Orion seats, could endanger crew health or make it difficult for them to read instruments and perform tasks.

Most of those calculations were based on ground-test data from space shuttle solid-fuel boosters, which will form the basis for the Ares I first stage. Since the problem surfaced, the Ares I project has worked with the space shuttle program to instrument shuttle boosters for actual flight data, and that has suggested the early calculations may have overstated the problem.

"We were able to verify that we are definitely getting less thrust oscillation in flight than we see in our ground-test motors," Cook said. "That's good, because where we care about this issue is in flight and not on the ground."

In developing systems to detune the Ares I stack so the thrust oscillation that occurs in all solid-fuel rockets isn't amplified to dangerous levels, Ares I engineers had considered a set of 36 active dampers using electromechanical actuators in the aft skirt to counter vibrations. The passive system eliminates those actuators and still will damp out the vibrations, Cook said.

The spring/ring design for the vibration isolator between the stages was one of three studied, and proved to be the lowest-risk approach, Cook said.

Artist's concept of Ares I: NASA