From: roberts@cmr.ncsl.nist.gov (John Roberts)
Subject: Acceleration, Galileo, X-rays
X-Added: Forwarded by Space Digest
Organization: National Institute of Standards and Technology formerly National Bureau of Standards
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci
Date: Wed, 23 Dec 1992 05:21:49 GMT


-From: gene@wucs1.wustl.edu (_Floor_)
-Subject: Re: Acceleration
-Date: 22 Dec 92 22:04:05 GMT

-In article <BznC82.74x.1@cs.cmu.edu> roberts@cmr.ncsl.nist.gov (John Roberts) writes:
-] That applies to things that are somewhat resiliant (like humans with their
-] limbs not locked), because if deformation continues throughout the period
-] of acceleration, then the entire body is not really subjected to the full
-] acceleration. (For another example, putting rubber feet or a springy internal
-] suspension in a piece of equipment can greatly reduce the maximum shock if
-] you drop it.)

-Hmmm...you think maybe rigidity has something to do with this?
-If something is rigid, it is much more likely to break than something
-flimsy, which will bend.  

It's not that simple - you can also get reinforced oscillations, shock waves
that propagate through the structure (generating traveling wavefronts of
internal stress, which can perhaps reinforce, or be concentrated at certain
points), etc. Other things being equal, it's probably safe to say that you'd
prefer a structure that dissipates the energy of internal shock waves, and 
an external structure that minimizes the transmission of such.

-Electronics certainly canot be built in
-a manner that will bend. Any flexing of the probe would have to
-be somehow accounted for in the design.

Electronic devices are not utterly inflexible - they do have some degree of
resiliance.

-] But other than that, and factors such as prolonged stress on human hydraulic
-] systems, the greater problem can be with rapid changes in acceleration, which
-] are of course associated with short bursts of acceleration. (I believe the
-] usual term for the time derivative of acceleration is "jerk".) These rapid
-] changes can cause very high internal stresses that are not found with slow,
-] steady increases in acceleration.
-] 
-] Just as an example, compare your body lying in a bed with a downward 
-] acceleration of one gravity, or being clamped in a device that repeatedly
-] shakes your body back and forth, with a maximum acceleration of half a gravity.
-] Which do you think would be likely to place more stress on your body, and
-] which would be quicker to cause discomfort? :-)
-] 
-] John Roberts

-You're joking me if you think the Galileo probe will experience constant
-deceleration. There's going to be buffeting worse than we could imagine,
-I imagine (:-). Especially at speeds many times that of sound (which I'm
-sure will be different for the Jovian atmosphere)! So you're point is
-very applicable. 

If you've ever seen video from inside the Shuttle during launch, you've
probably noticed that the astronauts are shaken pretty hard (at least
while the SRBs are thrusting). Payloads for launch are routinely tested
for resistance to vibration. I'd certainly *hope* that that's been factored
into the design of the Galileo atmospheric probe.

-Experiencing this jolting for milliseconds (as per
-a dropping watch) may not cause any damage. But if you dangled the watch
-from the ceiling and proceeded to place a jackhammer at its face,
-slamming into its face for a couple of minutes, liklihood is that
-the watch will no longer function! Ditto for an atmospheric probe.
-That thing is going to get one whale of a beating. You've helped me
-emphasize my point even more! Thanks :-)

- Gene Van Buren, Kzoo Crew(Floor), Washington U. in St. Lou - #1 in Volleyball

There are at least two mechanisms for damage from acceleration/vibration:
instantaneous peak load (which has nothing to do with continuation of
the forces much beyond the natural resonant period of the object), and
cumulative damage which gradually builds up. In either case, it's fun to
speculate on the stresses the probe will be subjected to, but idle to
attempt any authoritative claims unless accompanied by a large amount of
very nasty math, and detailed information on the structure of the probe.
I'm inclined to believe the Galileo people probably did their homework on it.

(I just read an article on testing portable personal computers for resistance
to damage. Among the tests were dropping them from desk height onto a 
carpeted or uncarpeted concrete floor, putting them in a paint can shaker,
and running them through a (slightly padded) clothes dryer. Surprisingly,
most of the PCs survived at least some of the tests, and some of them survived
all of the tests! I also found a short article on portable PCs on the Shuttle -
I'll try to post some details next time I find the article. :-)

One factor I'm interested in - the generation of X-rays. Black body radiation
at 28000 degrees Fahrenheit ought to have a considerable amount of emission
at X-ray wavelengths. Unfortunately, the one book in the library that might
give the actual levels is checked out. Can anybody give an idea of the
magnitudes involved?

John Roberts
roberts@cmr.ncsl.nist.gov