A lot of energy has been expended over the last three weeks about the issue of vibration of Ares I, the new expendable launcher being developed by NASA as part of the Constellation hardware to get us back into the manned space business after the Shuttle is retired. This revelation by NASA of a possible Ares I issue morphed into calls for re-thinking whether the Ares development was a good idea when we already have an expendable launcher, the Atlas V, that could with a good amount of modification do the job. For those hoping to change Ares, the fun has passed–according to a New York Times article, it’s beginning to look like NASA Administrator Mike Griffin’s opinion that the vibration issue was a “mountain out of a mole hill” was spot-on.
It was discovered a few weeks ago that the Ares I “might” vibrate at a frequency of 15 Hertz, the same frequency as the resonance frequency of the Orion spacecraft. I wrote might because Ares is still under development, so we can’t put an Ares I launcher on a rocket test stand, light the (roman) candle (which is essentially what a solid rocket is), and collect vibration data that would show if Ares does in fact vibrate at 15 Hertz, because the Ares I doesn’t exist yet as a launcher.
Since Ares I is still in the development phase, and because NASA does not want a problem like this vibration issue cropping up after the design is writ in stone, NASA has used old launch data from the space shuttle and from software models to guesstimate the performance of the Ares I rocket. That old launch data from the Shuttle showed that the solid rocket motors (SRB’s in NASA-speak) could vibrate at 15 Hertz. This has not been an issue in the past for the Shuttle since its resonant frequency is not 15 Hertz.
But it turns out that the Orion spacecraft does have a natural, or resonant, frequency of 15 Hertz. I didn’t know this–this comes from the NYTimes article mentioned above. So putting Orion together with a launcher producing the same frequency as its resonance frequency is like putting gasoline on a fire.
A resonance frequency is that frequency at which your structure will naturally vibrate. Give that frequency a kick by…say, another device producing the same frequency, and those vibrations combine, raise the frequency amplitudes (height) can get pretty bad…bottom line: a structure can actually break up if it hits resonant frequency too long. Think of Tocoma Narrows bridge.
So what can NASA Marshall Space Flight Center engineers do to attenuate, or deal, with the possible Ares I vibration issues? A lot, apparently. One possible solution is to put a set of shock absorbers, which could be as simple as some rubber bumpers, between the solid part of Ares I and the rest of the rocket. Another thing would be to change how the fuel burns inside the rocket, which could change the exhaust velocity, and thus the frequency. And, as some rocket engineers have stated, the problem might not even exist, that issue has been a virtual mountain built of nothing.
For now, Marshall engineers are stating for the record that they can deal with this issue. They are the experts who are also there, on the ground, so as an once-upon-a-time engineer, I have to trust their opinions over anyone else’s.
I personally think that NASA Administrator Mike Griffin has forgotten more about rockets than even those of us with advanced aerospace engineering degrees will ever know, so I’m going to trust Griffin. Besides, space activists need to focus on getting NASA sufficient funding and making sure our next President doesn’t cancel out all of this work to pay for a pet program, as one has speculated about doing.