During a system definition review (SDR) which finished up in October, NASA and contractor engineers found a potentially serious vibration/oscillation problem with the five-segment solid-fuel "stick" Ares I booster design intended to loft the Orion CEV.
A NASA spokeperson said, in response to a question from NASAWatch:
"Thrust oscillation or resonant burning is a characteristic of all solid rocket motors. It is caused by vortex shedding inside the solid rocket motor, similar to the wake that follows a fast-moving boat. When the vortex shedding coincides with the acoustic modes of the motor combustion chamber, pressure oscillations generate longitudinal forces that may affect the loads experienced by the Ares I during the last phase of first-stage flight. NASA is assessing the analyses in more detail, looking for any potential impacts to the integrated stack and ways to mitigate those impacts. Results are due in spring 2008. It is a normal part of the development process to identify, mitigate and track challenges such as this."
NASA confirmed this meant a delay of six months in the Critical Design Review (CDR) but denies a rumor that this means a slip of over a year in the first human flight, currently scheduled for March 2015. According to NASA, there's enough flexibility in the development schedule to solve the new problem without long-term impact.
COMMENT: All large rockets, being elongated structures through which tremendous and complex stresses are acting, do have challenges of this type at some point in the design phase. Going all the way back to 1958, the spinning upperstage package on the Jupiter-C/Juno I had to vary its rotation rate to avoid a "coupling" of vibrations as the first stage burned fuel and thus changed its own characteristics.
That said, no large rocket has ever been this elongated: the Ares/Orion stack will have a ratio of height to diameter of 18:1.
NASA made an early decision for a Shuttle SRB-dervived booster for Orion over alternatives like man-rating a Delta or Atlas EELV to simplify the whole Constellation program, keeping the cost down and getting the program moving sooner. To me (insert my usual "I'm not an engineer" disclaimer here), the idea that this was a "safe, simple, soon" solution, as manufacturer ATK described it, is increasingly hard to argue for. The current Ares design is no more a "simple" modified SRB than the Vanguard booster was the "modified Viking sounding rocket" the Navy used to sell its program in 1955. (Insert usual "If that's not familiar, read my book" comment here.)
I understood the initial basis for proposing the Ares family, but that logic may not be valid any more. Some human spaceflight experts have always been leery of a solid-fuel booster, which cannot be turned off, despite the addition of an Apollo-type escape tower. If I were the NASA Administrator, I'd continue with the Ares program but invite the EELV makers to provide a new round of proposals for a backup booster option.
(Insert one more comment, emphasizing even more than usual that this is a personal opinion and not related to any company or organization I'm affiliated with.)