NASA selects 'tuned' mass dampers to solve Ares I oscillation problem

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NASA's Constellation programme leadership has selected actively 'tuned' mass dampers to solve the Ares I crew launch vehicle's (CLV) first-stage oscillation problem.

The damper mass is a spring or springs that will be located either inside or outside the first-stage's aft skirt and can be actively "tuned" to respond to the oscillation frequency of the Ares I first-stage solid rocket motor (SRM).

The Ares I uses five steel casing segments that contain solid polybutadiene acrylonitrile (PBAN) fuel for its first-stage SRM that separates at 194,000ft (59,100m).

The problem to be solved was a potential coupling between thrust oscillations in the SRM and vibration modes in the CLV.

Identified in October 2007 NASA came to understand that the frequency of pressure pulses in the five-segment SRM was close to the natural frequency of the second longitudinal vibration mode of the complete launch vehicle.

NASA's leadership was concerned that there was a risk of a "pogo stick" resonant vibration.

The active damper solution was presented to the Constellation programme's managemernt on 8 August along with other options that were rejected as not as effective.

"[The tuned mass damper] is looking to be very effective. The  dampers have been flown before on the Shuttle. We are just adding the active element to allow the tune up," says Constellation programme manager, Jeffrey Hanley during a teleconference on 11 August. He also described the active dampers as "electromagnetic mass absorbers".

Hanley added that he did not know what the mass impact on Ares I the damper would have but that it was "not out of the realm of unreasonableness". He expected such details would be available at a teleconference with the Ares project office management and Marshall Spaceflight Center senior engineers in a "couple of weeks".

During the 11 August teleconference Hanley and NASA's exploration systems mission directorate's deputy associate administrator Doug Cooke announced that the agency's internal planning date for the first crewed flight of the Orion crew exploration vehicle, Orion-3, was being put back 12-months from September 2013 to September 2014.

This was due to a better understanding of the known funding, including the requested fiscal year 2009 budget, and likely progress after all the Ares I and Orion contracts had been placed. If the FY2009 budget is not approved by its start date of 1 October this year Hanley and Cooke did not know what the impact of such a funding delay would have on the 2014 date.

The space agency has a "committed" first crewed flight date of March 2015, for which it claims a 65% confidence level. According to Cooke and Hanley the 12-month delay for the internal planning date will not affect the March 2015 target.

Read more about NASA's Constellation vehicle development at Rob Coppinger's Hyperbola blog