http://www.aviationweek.com/aw/generic/story.jsp?id=news/aw012808p2.xml&headline=Thrust%20Oscillation%20Issue%20Threatens%20Ares%20I%20Design&channel=space The link above has more info, but in a nutshell, the Ares I's resonant frequency is 12hz, due to the upper stage's rocket. This is causing some serious concern as to astronaut's safety, and for good reason... they'll be hitting about 4g's while oscillating at that frequency. Can we say "shake the **** out of you"? :o