Ares Woes

Aviation Week explains the latest series of problems with the design of the Shuttle replacement vehicle, Ares.

Engineers at the Ares I project office here are working with experts from across the country to better understand the thrust oscillation issue in the first stage, a five-segment version of the four-segment reusable solid rocket motor (RSRM) that is fired in pairs to power the space shuttle stack off the launch pad.Conservative calculations of the potential frequency and amplitude of a thrust oscillation that could occur in the first stage as it nears burnout, and of the way that vibration links to the rest of the vehicle, suggest that it could set up a resonance that would damage critical components and harm the crew (AW&ST Dec. 10, 2007, p. 60).

Serious? Yes. Solvable? Perhaps.

A thrust-oscillation focus team, convened in November 2007, has since calculated that the problem may not be as severe as it appeared earlier in the fall. But the work continues under a looming March deadline, set so designers on both the launch vehicle and Orion can start work in earnest on mitigating the effect, if necessary, before preliminary design review (PDR) at the end of the summer.

It’s a lot of vibration we’re talking about here.

Of particular concern was the effect of the oscillation on the crew. Based on static ground tests of four-segment boosters conducted throughout the history of the shuttle program, and some very limited information extracted from data collected during shuttle flights for other purposes, the tiger teams best estimate for the vibrations average amplitude at the Orion command module would be 4.3g. But the Orion specifications set a crew health vibration limit of less than 0.6g rms (root mean square) in any axis over 1 min. during ascent.

In other words, the crew portion of the shuttle replacement could experience, in theory, up to eight times the vibrational acceleration designers would like it to have.

Rand Simberg points out:

If it “may not be,” it also “may be.” This goes beyond risk (which is quantifiable), into uncertainty, which by definition is not, and that’s an unhappy place for an engineer to be.

…and then after quoting the NASA Administrator (“I hope this is the worst we’ve got to deal with,” says NASA Administrator Michael Griffin.), points out the politics.

Well, apparently, they’re not allowed to see it as a show stopper. People get fired for pointing out that the emperor is naked.

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One Comment on “Ares Woes”


  1. […] I wrote here about the problems that NASA engineers had concerns about with excessive vibrations during launch […]


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