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Boy, Does This Need A Follow Up
One bit of explanation is required, I think. When Keith refers to a "five-by-five" matrix, he's talking about the standard risk assessment tool that NASA (and ARES Corporation, for whom I casually consult, and others) use to track program risk.
Here's an example from the Mil Standard, but it's a five by four (five levels of probability, four levels of consequence). Anything that is in one corner (low likelihood, low consequence) can be ignored, and anything that is in the opposite corner (high for both) should be receiving the bulk of the program resources. Things that are in between are tracked, and measures are taken to move them down to the 1,1 corner of the matrix. Though I can't find an example of one at my fingertips, the five by five is a little more fine grained in consequence level.
It can be used either for safety issues (in which case, "catastrophic" corresponds to loss of mission or crew), or for programmatic issues, in which case "catastrophic" would probably be complete program failure. It's a little harder to evaluate in this case, though, because that depends on how "program failure" is defined. Does it mean that the program is cancelled? Or does it mean that the program is restructured beyond recognition? Ares 1 seems to me to be vulnerable to either one.
What exactly is the issue? The problem is that any structure has a resonant frequency at which it naturally vibrates. If you excite the structure at that frequency, you can develop a positive-feedback system that will literally shake it apart (the Tacoma Narrows Bridge is the classic example).
Solid rocket motors don't run particularly smoothly (compared to well-designed or even poorly designed liquids) and large solid motors provide a very rough ride. Everyone who has ever ridden the Shuttle to orbit has commented on how much smoother the ride gets after staging the SRBs.
Now, one way to mitigate this is to damp it out with a large mass. The Shuttle does this by its nature, because even though it has two of the things, they are not directly attached to the orbiter--they are attached to a large external tank with one and a half million pounds of liquid propellants in it, and it can absorb a lot of the vibration. Moreover, the large mass has a frequency that doesn't resonate with the vibration.
As I understand it (and I could be wrong, and I'm not working Ares, but this is based on discussions, many off the record and all on background with insiders on the program), there is a very real concern that the upper stage on top of the SRB in "the Stick" will be excited at a resonant frequency, but that even if not, the stage will be too small to damp the vibrations of the huge SRB below.
If this is the case, there is no simple solution. You can't arbitrarily change the mass of the upper stage--that is determined by the mission requirement. Any solution is going to involve damping systems independent of the basic structure that are sure to add weight to a launch vehicle that is already, according to most reports, underperforming. Or it will involve beefing up the structure of the upper stage and the Orion itself so that they can sustain the acoustic vibration loads. In the case of the latter, it is already overweight, with low margins.
So this constitutes a major program risk, that could result in either cancellation, or a complete redesign (that no longer represents the original concept, because the problem is fundamentally intrinsic to it).
Now, let's take apart the response a little:
Thrust oscillation is...a risk. It is being reviewed, and a mitigation plan is being developed. NASA is committed to resolve this issue prior to the Ares I Project's preliminary design review, currently scheduled for late 2008.
The problem is that NASA can "commit" to resolve it until the cows come home, but if it's not resolvable, it's not resolvable. They can't rescind the laws of physics, and we're approaching a couple of anniversaries of times when they attempted to do that, with tragic results.
Now this next part is (to put it mildly) annoying:
NASA has given careful consideration to many different launch concepts (shuttle-derived, evolved expendable launch vehicle, etc.) over several years. This activity culminated with release of the Exploration Systems Architecture Study in 2005. Since then, the baseline architecture has been improved to decrease life cycle costs significantly.
The problem is that NASA didn't give "careful consideration" to the previous analyses after Mike Griffin came in. As far as can be determined, all of the analysis performed under Admiral Steidle's multiple CE&R contracts, performed by major contractors, was ignored, and put on the shelf to collect dust while NASA decided to build what the new administrator, along with Scott Horowitz and Doug Stanley, were predisposed to build. I have never seen "NASA's analysis" that supports this statement. Steve Cook made a valiant attempt to justify it at the Space Access Meeting last March, and was given kudos, at least by me, for having the guts to come in and defend it to a hostile audience, but no one was convinced, or even saw convincing data. He simply stated the conclusions, but didn't show the numbers.
But the most troubling thing to me is the end:
Thrust oscillation is a new engineering challenge to the developers of Ares - but a challenge very similar to many NASA encountered during the Apollo Program and development of the space shuttle. Every time NASA faces an engineering challenge - and it faces many - agency engineers examine all the options for addressing the issue. NASA has an excellent track record of resolving technical challenges. NASA is confident it will solve this one as well.
The problem is that, in reality, despite its confidence (or at least its stated confidence) NASA's record on this score is, at best, mixed. For instance, think about (as just two examples) the X-33. Or the OMV (I did a Google on it, and couldn't come up with any good histories of it--one needs to be written). Or many of the original space station concepts, which required complete redesigns. Sometimes engineering challenges are just too great to overcome, and a new approach is required to overcome a flawed concept. I don't know whether that's the case with Ares 1 or not, but this response doesn't instill in me any confidence that it's not.Posted by Rand Simberg at January 17, 2008 04:02 PM