Date: 11 Nov 83 14:05:07-PST (Fri)
To: space @ Mit-Mc
From: decvax!microsoft!uw-beaver!ssc-vax!eder @ Ucb-Vax
Subject: Re: SRB failure recovery

                                                   11 November 1983

     If one SRB fails to light on the pad or, alternatively, the holddown
bolts on one side are not released, the results are catastrophic.  In both
cases the External Tank is immediately sheared (ripped in half the long way).
The reason is there is a thrust beam inside the tank which crosses between
the SRB's.  It connects the forward ends of the SRB's to each other and
to the skin of the ET.  It is there to distribute any thrust imbalance
between the SRB's equally among the vehicle components.  The nozzles of the
various engines are steerable to correct for this possibility.

     The thrust beam is sized for about a 10% difference in thrust.  If
one engine runs and the other doesn't, or one is freed from the pad and
the other not, the ET will be ruptured.  The ET is .1 inch aluminum, while
the SRB's are more like 1 inch high strength steel.  The tank goes first.

     During flight there is no way to shut down an SRB intentionally or
accidentally short of destroying it.  This is because the solid fuel
contains both fuel and oxidizer.  Once ignited, the combustion is self-
sustaining.  The combustion RATE is dependant on pressure.  The only
way to slow it down is to lower the pressure by blowing the nozzle out
the rear end (which is not designed in), or by splitting the motor case
with an explosive charge (which is).

     Pity the poor astronauts, since they will almost certainly die in
any of these accidents.  If you go to see a launch in person, be aware
that the press and VIP viewing areas (3.5 mi from pad) are within
shrapnel range should the vehicle explode on the pad.  They told us this
AFTER STS-1 (I was in the press area).

                                                Dani Eder
                                                Boeing Aerospace Company
                                                ssc-vax!eder