Ares I engineers are scheduled to present their final recommendation this week on fixing a potentially dangerous thrust oscillation on the new crew launch vehicle.

“Conservative” analysis last year predicted potentially dangerous vibrations in the Orion cabin triggered by harmonic vibrations across the stack in the final few seconds that the solid-fuel first stage is burning.

After considering mounting the first-stage recovery parachutes on springs to “detune” the stack, the engineering team assigned to tackle the problem has settled on an approach designed to minimize changes to the Orion vehicle by handling almost all of the vibration in the first stage.

NASA still is gathering flight- and ground-test data on the phenomenon. But for a worst-case scenario, the Ares I project and its first-stage contractor – ATK – would develop an active tune mass absorber that would detect the frequency and amplitude of the thrust oscillation with accelerometers, and use battery-powered motors to move weights up and down to damp it out. The concept calls for mounting 16-20 of the devices on the aft skirt of the Ares I first stage (Aerospace DAILY, June 17).

Reducing loads

Garry Lyles, an experienced launch vehicle engineer at Marshall Space Flight Center who heads up the effort to fix the thrust oscillation problem, said the approach will be able to reduce vibration loads on the crew in Orion to 0.25 gs, which is considered low enough for astronauts to be able to read displays and react to changing conditions effectively.

Steve Cook, exploration launch vehicles project manager at Marshall, said upcoming testing and analysis, including centrifuge tests at Ames Research Center that may update the Project Gemini-vintage human-loads guidelines, could eliminate the need for an active-damping concept.

Lyles’ team has also studied passive damping in the aft skirt, which may be sufficient for handling the actual loads. A passive “compliance structure” – essentially a spring-loaded ring that would detune the stack by softening the interface between the first and upper stages – also is included in the design concept.

The active aft-skirt tune mass absorber would weigh about 6,500 pounds and the compliance structure would add another 6,000 pounds. Both would drop away with the first stage, Cook said, meaning the maximum hit to overall vehicle performance in terms of payload to orbit would be 1,200-1,400 pounds. That is within performance margin maintained at this point in vehicle development, Cook said, and could drop if new data reveal the fixes are more than is needed.

“The first thing we’re going to look to see if we can leave off is the compliance ring,” Cook said. “Then we’d like to be able to go from an active system down to a passive system, and then the last thing we would do is to take the whole thing off as we go forward.”

The Ares I is in the final stages of preliminary design review (PDR), with a final PDR board tentatively set for Sept. 10. But the thrust oscillation issue will be handled in a “delta PDR” next spring, and developing the mitigation designs should fit within the overall schedule.

“The solution we’ve got works,” Cook said. “It pounds it flat, and it gives us a lot of flexibility, and we can handle it from a performance perspective.”