Path: utzoo!attcan!uunet!husc6!think!craig
From: craig@think.COM (Craig Stanfill)
Newsgroups: sci.space.shuttle
Subject: Re: Orbiter/SRB separation
Message-ID: <22274@think.UUCP>
Date: 20 Jun 88 18:20:39 GMT
References: <1869@bigtex.uucp> <4706@hplabsb.UUCP> <1934@ssc-vax.UUCP> <478@uniq.UUCP> <308@proxftl.UUCP>
Sender: usenet@think.UUCP
Reply-To: craig@mneme.think.com.UUCP (Craig Stanfill)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 41

>This article was great up to this point.  Unfortunately, the comments about
>rate of change of acceleration are wrong; even if the system INSTANTLY
>stopped accelerating ALTOGETHER (experiencing an infinite rate of change in
>acceleration) that wouldn't stress the system.  INCREASING acceleration
>can damage things, but it doesn't matter how fast or how slowly the
>increase happens, although how long it lasts could be important.

What you are saying here makes sense for static analysis, but you are
completely ignoring system dynamics.  The first thing to do is to straighten
out the terminology in this discussion.  The engines in a spacecraft do
not apply an acceleration, they apply a force.  When engines are turned on or
off, the result is not an instantaneousl change in the acceleration of the
structure, but an instantaneous change in the force applied to
point in that structure.

While the static equilibrium of the structure depends only on the
magnitude of the force, the dynamics of the system very much depend on
how quickly the force is applied.  The problem is that the structure
has massive components connected by elastic elements.  All such
systems are oscilators.  When a force is applied, the entire system
starts oscilating, with the suddenness of force application determining
how energetic these oscilations are.  If the force is applied suddenly
enough, these oscilations may cause structural failure.

A simple thought experiment should suffice to convince you of this.
Suppose you have two bricks, each having a mass of 1 KG, and a spring
that can withstand a tension of just slightly over 1 Newton.  If we
gradually apply a force of 2N to the front brick, the entire assembly
will accelerate at a rate of 1 m/s/s.  If, however, we suddenly apply
a force of 2N to the front brick, then the first brick will accelerate
at a rate of 2 m/s/s, while the rear brick is stationary.  Until the
spring can stretch to the point where it is under a tension of 1N, the
first brick will be accelerating faster than the second.  During this
time, the first brick will acquire a considerably higher velocity than
the second and, because it has momentum, will continue moving faster
for some time.  This will cause the spring to lengthen and, as a
result, will increase the tension on the spring.  The spring will then
break.  If you are uncomfortable with words, work out the differential
equations; they are not terribly complex.

						- Craig Stanfill