Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!umd5!uflorida!novavax!proxftl!greg
From: greg@proxftl.UUCP (Gregory N. Hullender)
Newsgroups: sci.space.shuttle
Subject: Re: Orbiter/SRB separation
Summary: change in acceleration not stressful
Message-ID: <308@proxftl.UUCP>
Date: 13 Jun 88 13:40:50 GMT
References: <1869@bigtex.uucp> <4706@hplabsb.UUCP> <1934@ssc-vax.UUCP> <478@uniq.UUCP>
Organization: Proximity Technology, Ft. Lauderdale
Lines: 43

In article <478@uniq.UUCP>, rjnoe@uniq.UUCP (Roger J. Noe) writes:

> The fact that the OV is exerting a net force on the ET is not the problem.
> Consider what would happen if you tried separating the orbiter vehicle
> from the rest of the system while everything's running.  First you have to 
> shut down the SSME's and disconnect the OV from the ET propellant lines.
> In that time, all the thrust is from the SRB's and both the ET and the OV
> are dead weight.  If about one fourth the ET fuel is gone, the ET weighs a
> total of about 1.24e6 lbs.  The OV still weighs about 200000 lbs. and the SRBs
> may be down to about 460000 lbs. each.  So the total weight of the stack is
> down to about 2.36e6 lbs. with a thrust of around 6.2e6 lbs. for a thrust
> to weight ratio of about 2.63.  Cutting off the SSMEs loses around 1.2e6 lbs.
> thrust for a ratio of 2.12, a loss of about 0.5g.  If this cutoff takes 0.1
> second, that's a change of 5g/sec.  Now what happens when you detach the OV
> from the ET?  The OV suddenly loses another 2.12g of acceleration!  If this
> separation takes 0.1 second, that's 21g/sec!  The ET+SRBs combination would
> gain about 0.19g in this same time from the dropped mass.  I think we've
> already seen what happens when you subject the STS to such rates of change
> in acceleration while it's flying:  the ET disintegrates, the SRBs fly off,
> and the OV becomes something you stuff down an abandoned missile silo.

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 destroyed the Challenger wasn't change in acceleration; first one of
the O-rings burned through (about 120 degrees of arc).  Then strong
high-altitude winds buffeted the vehicle and allowed a plume of hot gasses
to leak out where the O-Ring had been. This plume touched off the ET, which
detonated with the energy of a small nuclear weapon.  The orbiter was then
exposed to AERODYNAMIC forces of more than 20 g's, and it was those forces
(not the explosion) that broke it into pieces.

That problem with separating the orbiter during launch is still there,
though; even if you used something like explosive bolts to accomplish the
separation, how do you make the orbiter survive the aerodynamic forces?
-- 
------
My opinions are not necessarily those of my employer.

	Greg Hullender  allegra!novavax!proxftl!greg