From: roberts@cmr.ncsl.nist.gov (John Roberts)
Subject: Re: Acceleration
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: Tue, 29 Dec 1992 03:34:57 GMT


-From: prb@access.digex.com (Pat)
-Subject: Re: Acceleration
-Date: 27 Dec 92 18:04:22 GMT
-Organization: UDSI

-In article <1992Dec22.220405.26976@wuecl.wustl.edu> gene@wucs1.wustl.edu (_Floor_) writes:
->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
->
->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.  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.

-Certainly electronics can be built to be flexible,  it's just what degree
-of flexibilty you desire.  Flex is a stress/strain relationship.
-Steel is flexible, rubber is rigid.  you just need to define these terms
-first.  besides, if you build with amorphous materials,  you can get
-quite a flex out of silicons.

Are the solar panels of HST made of amorphous silicon? They were rolled up
very tightly before deployment.

Something that didn't get adequately covered in the previous posts: in both
mechanical and electrical systems, there's a significant difference between
*being able to survive* a period of high acceleration or intense vibration,
and *continuing to operate* during that period. The hard disk drives in the
laptop computers used in the Shuttle generally survive the vibrations of the
launch, but they're not expected to run during that time. If an electrical
system uses spring-loaded contacts, an intense shock may cause the contacts
to open momentarily. I just got some literature from a company at the
Technology 2002 conference that makes metal-on-silicone contacts that are
rated for 50 G - there may be other kinds of contacts for more demanding
situations. Come to think of it, the components I'd worry about the most are
the oscillators - anybody know if it's possible to make quartz crystal
oscillators sturdy enough to continue working under an acceleration of
thousands of gravities?

-and i believe the designers understnad the material characteristics
-of their probes quite well.

I'm inclined to agree - the Galileo atmospheric probe's designers must have
considered acceleration and vibration in their design.

->] 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

-Actually,  i think the term is Impulse.

You inspired me to look it up in a reference book:
 # impulse [MECH] the integral of a force over an interval of time.
 # jerk [MECH] the rate of change of acceleration; it is the third derivative
     of position with respect to time.

 (from the McGraw-Hill Dictionary of Scientific and Technical Terms, 1989)

->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. 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 :-)

-Hopefully this kid will take a physics class.

-I think he is mistaking  Work with Force and energy.

I think he's introducing the concept of cumulative damage where it might
not be applicable. If the magnitude of the forces and the margins of the
design are such that there is some cumulative damage (i.e. fatiguing of
components that flex slightly), the system could still be designed so that
it's likely to survive days or months of such conditions. That's another
design parameter that can be tested prior to launch.

-Work is force through a distance,  Energy is work*time, Force is mass*Accel
-( boy i hope i got these right :-) )

You got work and force right, but energy is power * time. Energy is the
generic term, while work, heat, etc. are alternate ways of expressing it.
They can all be expressed in the same derivative SI units (joules, for 
instance). Power is energy (work, heat flow, etc.) divided by time.
Joules are kg * m^2 / s^2. Watts (power) are kg * m^2 / s^3.

It should be noted that in some calculations, "per unit time" or "per unit
area" or "per unit mass" is taken for granted, and is implicit in the 
expression of the problem. That's just a shorthand notation, to avoid having
to carry the units through all the calculations.

-It takes energy to achieve a momentum change.

-A probe has high momentum hitting atmosphere.  it gets  a high acceleration,
-on a small mass.  not a lot of force,  exerted through several miles of
-atmosphere,  for a few minutes.

-I think the kid is missing the fact that while the accelerations of dropping 
-a watch and hitting it with a sledge are the same, the work products are significantly  different.  

If you smash the watch between two objects, then college-freshman-type
mechanics is no longer a good way to describe the system. All sorts of 
complex materials properties start to come into play.

-Try this.   drop a timex.  work out the acceleration.

You have to make a guess at the degree of deformation, which is very
nontrivial. If you assume no deformation, then the peak acceleration
comes out infinite.

-Now,  hang the timex from a string.   Let a pendular mass strike it, at low spe
-ed.  work out the acceleration.  keep increasing the mass and speed.
-continue until the timex dies.   I suspect you will be surprised at how
-high you can go.  

Same problem. You might be able to work out a lower limit for peak 
acceleration, accepting that the peak is probably *much* higher than
this lower limit.

John Roberts
roberts@cmr.ncsl.nist.gov