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Shake, Rattle and Roll

Keith Cowing has a report on today's telecon to discuss the Ares 1 vibration issue. Apparently they've settled on a solution before they really understand the problem.

[Late afternoon update]

Bobby Block and Todd Halvorson have blog posts up as well. But I think that Halvorson's reporting is a little garbled here:

Gravitation forces on the astronauts will be reduced to 0.25 Gs from around 5 to 6 Gs, the latter of which is about double the force exerted on shuttle crews.

I think that he's confusing the steady-state acceleration resulting from thrust with the vibration acceleration ostensibly being mitigated by the springs and dampers. Also, it's not a "gravitation force." I'm assuming that NASA meant that they can reduce the oscillations on the crew couch from high gees to a quarter of a gee, but that's independent of the gees imposed by thrust. If they're only accelerating at a quarter of a gee, that would result in horrific gravity losses during ascent.


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lmg wrote:

If they only have 0.25g pressing down on them, either they made it to Mars already, or they're falling.

bbbeard wrote:

The 0.25/5-6 number is the oscillating g component, on top of roughly 3-4 g's average longitudinal acceleration. Yes, that means that the astronauts would be seeing negative axial g's twelve times a second.

Halvorson does seem to be trying to equate the 6 g's oscillating with double the 3 g's average for the shuttle. So, yeah, that's a confusing way to put it and certainly doesn't help put the problem in context.

As for whether it's "gravitation", I'm not sure what your hangup is exactly, here, Rand. It's experienced as an inertial force, like centrifugal acceleration or the sudden stop associated with hitting a brick wall. It's common enough to call these "g forces" [sic], with the "g" short for "gravity". And the last time I worked out the Christoffel symbols, all inertial accelerations show up in the Einstein tensor on the same footing.

Rand Simberg wrote:

Well, my "hangup" is what I said. It is not gravitation. And as written, it's also easily confused with the acceleration due to linear acceleration of the vehicle resulting from thrust, the way it's written (the maximum of this for the Shuttle is three gees). It should have made it more clear that these are oscillatory accelerations, not linear ones. In neither case are they "g" forces, though that's the unit of measurement. "Gravitational" forces are forces caused by, you know...gravity.

bbbeard wrote:

"Gravitational" forces are forces caused by, you know...gravity.

Well, actually, I do know gravity -- I've got a doctorate in theoretical physics. I'm guessing you would like to confine the term "gravity" to the inverse-square-law spooky-action-at-a-distance force that one mass exerts on another. It turns out physics has gotten more complicated since Newton, and it really doesn't make much sense to distinguish between "gravity" and forces like centrifugal force in the context of what we know now. Pretty much any force that can be zeroed out at a point by the appropriate coordinate transformation is "fictitious" in the sense that we used to be taught that centrifugal force is. From the standpoint of general relativity, you can't even ascribe definite meaning to the Copernican doctrine "the Earth revolves around the Sun and not vice versa" -- because you can always pick a coordinate system where the Earth is stationary (it's just got these peculiar fictitious forces in it!)....

Well, whatever... I'm just being pedantic, aren't I?.... The point is that a physicist would not necessarily say it's incorrect to call the oscillatory environment "gravitational".

Rand Simberg wrote:

Well, whatever... I'm just being pedantic, aren't I?

Yes, you are. ;-)

While I'm familiar with the equivalence principle, most people aren't, and when you're trying to convey information for the masses, most people are going to think (rightly, IMO) that "gravitational" means that "spooky inverse R-squared action at a distance" thingie. To use that term to apply to an acoustic vibration is quite misleading, if not outright wrong, and doesn't aid in their comprehension of what's going on. It only confuses.

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This page contains a single entry by Rand Simberg published on August 19, 2008 9:39 AM.

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