Vibration Analysis Delays Ares I-X Stacking

Crews at Kennedy Space Center will wait to start stacking the Ares I-X test vehicle so engineers will have more time to analyze three vibration-loads issues that could threaten range safety during its suborbital test flight, which probably will slip into October.

Stacking is expected to begin the week of July 6. Constellation Program Manager Jeff Hanley, who oversees development of the Ares I crew launch vehicle, said July 2 that the extra analysis may give test managers more confidence that they won't have to destack the vehicle later to correct one of three potential problems. If it doesn't, more work may be necessary before stacking can begin.

One of the issues involves concerns by some engineers that the shaking of the four-segment space shuttle solid-rocket booster that forms the active portion of the Ares I-X stack will overwhelm the hydraulic mechanism that drives the thrust vector control system that helps guide the vehicle during powered flight.

Engineers also want more calculations on whether the vibrations would disable the avionics box linking the flight termination system on the vehicle with the range safety officers who would destroy it if it veers off course. And they want extra analysis on whether any of the secondary structures inside the steel boilerplate simulating the Ares I upper stage - ladders, railings and the like - could shake loose during launch and damage instrumentation or other hardware.

Shaking from the Ares I-X first stage isn't the same as that which is driving the design of the final Ares I vehicle. In both cases, thrust oscillation seen as the solid-fuel motor nears burnout sets up the vibration, but on the Ares I the frequency is lower and the harmonics into the crew compartment at the top of the stack is the main concern.

On the Ares I-X the expected frequency is about 15 hertz, versus 12 Hz on the five-segment Ares I solid motor, and the concern is with command receiver decoders that ride in an avionics box installed in the forward skirt between the solid motor and the upper stage simulator.

If the box is knocked out, the range safety officer would not be able to ignite a pyrotechnic charge designed to break open the motor's casing longitudinally and halt the flight if it strays off course.

The Orlando Sentinel reported last month that the Air Force 45th Space Wing, which handles range safety at Kennedy, has raised the issue with NASA.

Hanley said if the avionics box is found to be threatened by the shaking, NASA may argue for clearance anyway on the grounds that the thrust oscillation only appears in the last 10 to 20 seconds of the motor burn, when the vehicle would be too far downrange to pose a safety threat to occupied areas.

Thrust oscillation also is a concern with the thrust vector control system, which rides in the aft skirt below the motor. Vibroacoustic loads are the concern in the upper stage simulator, and strengthening secondary structures in one or both areas may be necessary to resolve the problems, Hanley said.

The issue surfaced during a "mate review" held in advance of the start of stacking on a mobile launch platform in High Bay 3 of Kennedy's Vehicle Assembly Building (VAB). Subsequent reviews will be held at each step of the stacking, which could further slow the process, Hanley said.

Ultimately, though, the biggest hurdle in getting Ares I-X off the ground probably will be the integrated systems test, an electronics launch simulation to be held in the VAB. While NASA is still officially holding Aug. 30 as the earliest launch date for Ares I-X, Hanley said the most optimistic timeline for the integrated systems test - three weeks - wouldn't allow a launch until "late September."

"When you look realistically at that kind of testing, our experience is that it's going to be more like four, five, six weeks, so that could push us into October," Hanley said.

Artist's concept of Ares I-X launch: NASA