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From: stu@splut.UUCP (Stu Cobb)
Newsgroups: sci.space.shuttle
Subject: Re: Throttling the Challenger
Message-ID: <199@splut.UUCP>
Date: Sat, 24-Oct-87 00:55:37 EST
Article-I.D.: splut.199
Posted: Sat Oct 24 00:55:37 1987
Date-Received: Mon, 26-Oct-87 05:48:25 EST
References: <340@ablnc.ATT.COM>
Organization: Confederate Microsystems, League City, TX
Lines: 60
Keywords: Challenger Throttle
Summary: "Go at Throttle-Up"

In article <340@ablnc.ATT.COM>, rcpilz@ablnc.ATT.COM (Robert C. Pilz) writes:
> 
> One thing about the crash has been haunting me. When I saw
> the replays of the explosion, I heard the commands to the
> crew to be at 105% throttle. They acknowledged that. Then they
> were told to go to 80% (or so). They acknowledged that. Then
> they were told (very quickly) to go back to 105%. At this point,
> the explosion happened.

OK.  There is sort of a relationship here, but you've got it backwards.
There's no cover-up.

Aerodynamic pressure on a booster is a function of speed and atmospheric
density.  As you launch, speed increases and density decreases.  Obviously,
there is a point at which aerodynamic pressure ("q-bar") is a maximum.  This
point is called "Max Q."  For the Shuttle, this happens between 40 and 70
seconds after launch.

During Max Q, the aero stresses on the vehicle are at a maximum.  This is
one of the design points for the vehicle.  Lots of turbulence, vibration,
etc.

Because of the great aero stresses, the Shuttle flight planners do what they
can to remove additional stresses.  Among other things, they throttle the
engines back during that period.  This happens automatically; it's built
into the flight software.  In fact, the SRB's also "throttle back."  That's
done by proper design of the grain when the booster is poured.  All this is
thought out well in advance.

Now then.  The last call we heard to Challenger was "Go at Throttle-up."
This meant, among other things, that Houston had seen all three engines
throttle back up to 104% (I believe), after passing through the "thrust
bucket" at Max Q.

Nothing sinister about that.  So why did I say they're related?

It is thought by some that the O-ring, after partially opening at ignition,
sealed itself again.  During Max Q, Challenger passed through a particularly
vicious wind shear.  It is thought (again, by some) that some combination
of this wind shear and the already intense vibration of Max Q opened the O-ring
back up, causing the fatal leak.

It took some ten to twenty seconds for the leak to destroy the vehicle, long
enough for the shuttle to leave Max Q behind and bring the engines back up
to full power.  Thus, you heard the throttle-up call right before the
explosion.  They're both related to Max Q; they're not related to each
other.

> ... areas like this need to be addressed beyond fixing
> the boosters. There need to be better plans to abort the mission.

We're working on it.  Unfortunately, you simply can't abort until you drop
the solids.  This is obviously not good, but there's no way to fix it
without rebuilding the entire fleet.  This would cost several billion
dollars.  Congress has decided (perhaps implicitly) that the lives of a few
astronauts are not worth that kind of money.  They may well be right -- I
know the thing's not particularly safe (that much explosive never will be),
but I'd ride it anyway...

Stu