NASA engineers are adding instrumentation to the first full-scale flight version of the Ares I crew launch vehicle to gather real data about vibrations from its solid-fuel first stage that initially were predicted to be seriously out-of-spec.

Those predictions, which could mean expensive modifications to the Ares I and the Orion crew exploration vehicle that will ride atop it, are based largely on ground-test data. Managers hope flight-test results from the Ares I-X flight will give them a much better idea of just how bad the problem is, and what it will take to solve it.

In the works since August 2006, the Ares I-X vehicle is scheduled to lift off from Launch Complex 39B at Kennedy Space Center on April 15, 2009. Its flight will mark the beginning of the transition from the space shuttle to the next U.S. human launcher.

Since the vibration issue started raising concerns, the Ares I-X flight-test team has devised an approach to measuring the phenomenon that involved both the forcing function imparted by the solid-fuel first stage as it burns and the way the vehicle responds to it. Early "conservative" calculations based on ground-test data suggest the forcing function will set up a harmonic response with the rest of the vehicle that could damage delicate hardware and even injure the crew. But the data's usefulness is limited.

"That's what's missing from a ground test," says Robert Ess, Ares I-X mission manager. "You get the forcing function. You don't get the response."

To gain the maximum advantage from human-rated heritage hardware, NASA has built the Ares I around a five-segment version of the four-segment reusable solid rocket motor (RSRM) that lifts the space shuttle off the launch pad. The Ares I-X test will use a four-segment RSRM to fly a simulated Ares I upper stage and Orion crew launch vehicle, drawing data from some 750 instruments to learn as much as possible about the flight dynamics of the stack as possible.

The Ares I project already is negotiating with the shuttle program office to install sensors in an RSRM on a shuttle mission early next year to gather data on thrust oscillation - a common phenomenon in solid-fuel rockets - and how it transfers into the shuttle. "What we'd like to do is put some instrumentation on the motor and on the shuttle itself to, one, understand the forcing function in flight, and two, understand what kind of response is it transmitting into the shuttle," says Steve Cook, Ares projects office manager at Marshall Space Flight Center.

The test flight will be the first to use modified space shuttle ground infrastructure - itself modified from its original Saturn V support design - to launch a version of the shuttle-follow-on vehicle. Plans call for the Ares I-X project to begin building up its vehicle on a mobile launch platform as soon as the shuttle program turns it over - probably late this year.