Managers of NASA's Ares launch vehicle project are in early discussions with their space shuttle counterparts over using a shuttle launch early next year to gather flight-test data on a potentially dangerous vibration that ground-test data suggest might occur in the Ares I first stage.

That stage is to be a five-segment version of the four-segment reusable solid rocket motor (RSRM) that is fired in pairs to boost the shuttle stack off the launch pad. "Conservative" calculations of the potential frequency and amplitude of a thrust oscillation that could occur in the Ares I first stage as it nears burnout suggest the vibration could damage critical components and harm the crew.

Those calculations are based on static ground tests of four-segment boosters conducted over the history of the shuttle program, and some very limited information extracted from data collected during shuttle flights for other purposes, according to Steve Cook, Ares Projects Office manager.

To get better data, Ares project engineers want to mount pressure sensors inside an operational shuttle RSRM, probably for a flight early in 2009, to measure the changes in pressure inside the motor as it burns. Cook said Jan. 22 the planning is in the early stages.

"What we'd like to do is put some instrumentation on the motor and on the shuttle itself to, one, understanding the forcing function in flight, and two, what kind of response is it transmitting into the shuttle," Cook said. "This is not something we've tried to go off and measure before."

In-house project calculations presented in October 2007 during the Ares I system definition review warned that the thrust oscillation inside a five-segment RSRM motor - a normal phenomenon in solid-fuel rocket motors - could set up a harmful resonance with the structure above it.

Ares project engineers were concerned that the forcing function from the oscillation "will couple with vehicle axial modes, localized components modes or other fluid/acoustic modes which could cause adverse effects on humans, lead to structural failures, and/or cause unacceptable controllability issues," according to their Oct. 30, 2007, presentation papers.

In response, NASA has set up a "focus team" headed by Garry Lyles, an experienced launch vehicle engineer at Marshall Space Flight Center, to give the problem a thorough analysis, improve the estimates of the magnitude of the problem, and recommend fixes. The team - which has pulled experts from across NASA and elsewhere - has an interim report due at a preliminary design review "checkpoint" scheduled in March, and has not finished its work.

Nominally the oscillation frequency of a five-segment booster is 12 Hz (compared to 15 Hz for the four-segment version). But after that it gets complicated translating the RSRM ground-test data into accurate forcing function figures - the amount of force that would be exerted on the whole Ares I stack by the first-stage oscillation - and the stack's response to it.

Lyles said Jan. 16 early results suggest the original calculations are conservative and that the oscillation may not be as well organized - and destructive - as feared. If they wind up approaching worst-case levels, potential fixes include isolating specific components such as avionics that may be damaged by the vibration; changing the geometry inside the motor that sets up the thrust oscillation in the first place, and even unlocking the seat shock absorbers designed to protect the crew of an Orion vehicle atop the stack on landing so that they are protected during ascent as well.