Path: utzoo!utgpu!water!watmath!clyde!bellcore!tness7!killer!pollux!ti-csl!DMeyer@mips.csc.ti.com
From: DMeyer@mips.csc.ti.com (Dane Meyer)
Newsgroups: sci.space.shuttle
Subject: Re: Orbiter/SRB separation
Message-ID: <50665@ti-csl.CSNET>
Date: 3 Jun 88 14:15:49 GMT
Sender: root@ti-csl.CSNET
Lines: 69
Posted: Fri Jun  3 09:15:49 1988


  While it may be true that SRB/ET - OV separation during liftoff
realisticly will never be developed due to one or more of; technical
complexity, expense, or politics, I find this discussion quite
informative and interesting.  I've been forwarding your messages to
Ken Scofield who really asked the original question and sent me this
note regarding Roger's comments.
                                                                              
Dane Meyer  (Texas Instruments, Dallas)
                                                                 
ARPA/CSnet: dmeyer@csc.ti.com
UUCP:       {convex!smu im4u texsun pollux ihnp4!infoswx rice}!ti-csl!dmeyer
 
----------------------------------------------------------------------------
In reply to Roger Noe:

>From: rjnoe@uniq.UUCP (Roger J. Noe)
>Subject: Re: Orbiter/SRB separation
>
> ... (much text on thrusts and weights deleted; I'll assume it's correct)
>
>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.  

Why?  The SSME's *are* throttlable, and there is some on-board fuel which 
normally is used for final insertion into orbit, as well as the subsequent
de-orbit burn.  So, the SSME's could be throttled down to some suitable
level, and be allowed to burn while the disconnect/separation is accomplished.

>		   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.  

So What?  What you've described is the derivative of acceleration (known
as "jerk") which, in this context, I don't think particularly matters.  As I
understand the Challenger situation, the thing that wiped it out were the
aerodynamic forces which were imposed on it by the sudden change in flight
path (i.e., it was thrown askew by the exploding ET).  As long as the various
components separate cleanly, and don't make any extremely radical changes in 
relation to flight path, I see no reason why the OV and ET/SRB (as separate
units) couldn't stay intact.  Besides, if the SSME's were still burning, at
least at partial throttle as I suggested above, the "jerk" would be greatly
reduced (if it matters at all), and the OV could quickly accelerate away 
from the impending ET/SRB fireball (or whatever).  All the thrust-to-weight
ratios I've seen discussed indicate that the OV could easily "out-run" the
ET/SRB.  And by the way, although the current connecting struts can't be 
released while engines are burning, I can't imagine that it would be very
difficult to redesign them so that they could be released while under load.


             *
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