That first stage is composed of five reusable solid rocket boosters derived from the type that NASA uses to launch the shuttle.
The shaking problem, which is common to solid rocket boosters, involves pulses of added acceleration caused by gas vortices in the rocket—similar to the wake that develops behind a fast-moving boat, said Arenas, who has researched vibration and space-launch issues.
Those vortices happen to match the natural vibrating frequencies of the motor's combustion chamber, and that combination causes the shaking.
Senior managers were told of the findings last fall, but NASA did not talk about them publicly until the AP filed a Freedom of Information Act request earlier this month and the watchdog Web site Nasawatch.com submitted detailed engineering-oriented questions.
The response to those questions, given to both Nasawatch and AP, were shared with outside experts, who judged it a serious problem.
NASA engineers characterized the shaking as being in what the agency considers the "red zone" of risk, ranking a five on a one-to-five scale of severity.
"It's highly likely to happen, and if it does, it's a disaster," said Fischbeck, an expert in engineering risks.
The first launch of astronauts aboard Ares I and Orion is set for March 2015.
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