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From: karn@faline.bellcore.com (Phil R. Karn)
Newsgroups: sci.space.shuttle
Subject: Re: Throttling the Challenger
Message-ID: <1482@faline.bellcore.com>
Date: Fri, 16-Oct-87 21:50:30 EDT
Article-I.D.: faline.1482
Posted: Fri Oct 16 21:50:30 1987
Date-Received: Sun, 18-Oct-87 05:44:47 EDT
References: <340@ablnc.ATT.COM>
Organization: Bell Communications Research, Inc
Lines: 21
Keywords: Challenger Throttle
Summary: throttle control is normal and automatic

During a normal launch the crew does little more than watch the displays
and twiddle their thumbs; all engine control (gimbaling, throttling,
etc) is completely under onboard computer control.  Control of a launch
vehicle is something that humans do very badly but computers do
extremely well, so there's no practical alternative. The crew has the
ability to take manual control, but that is little more than a
last-ditch backup. Even aborts are done under computer control, at least
as long as engines are firing.

The "callouts" you hear during a launch are simply confirmations of
events that happen automatically; no one is pushing any buttons. The
reduction of engine throttle during the first minute of flight is
programmed in to reduce the stresses on the launcher as it hits "Max-Q",
the region of maximum aerodynamic pressure. Once altitude has increased
sufficiently the throttle may again be increased because the less dense
atmosphere no longer generates enough drag to worry about.  The SRBs
are also "throttled back" during this time, but this is not under control
of the onboard computers; their thrust-vs-time curves are established by
the way propellant is loaded into them.

Phil