From: roberts@cmr.ncsl.nist.gov (John Roberts)
Subject: Re: What kind of computers are in the shuttle?
X-Added: Forwarded by Space Digest
Organization: National Institute of Standards and Technology formerly National Bureau of Standards
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci
Date: Wed, 11 Nov 1992 21:58:49 GMT


-From: choy@skorpio.usask.ca (I am a terminator.)
-Subject: What kind of computers are in the shuttle?
-Date: 11 Nov 92 17:53:36 GMT
-Organization: University of Saskatchewan, Saskatoon, Canada

-The computers used to control the shuttle use very complex programs
-that people don't want to rewrite for newer computers. Has any
-upgrades been done or are the computers still the same old beasts?

Most or all of the orbiters now have the new computers, which are much more
powerful (and have more memory) than the old computers. I believe they're
software-compatible. (I *don't* know whether any new functions were added,
or whether the software has been changed (other than lookup tables, etc.).
The Shuttle control software is claimed to be the most nearly error-free
complex code ever created - millions of dollars went into assuring that.)

I believe one problem with the old computers (and I only saw it once, some
time ago, so don't take it as established fact) was that they did not have
sufficient storage to hold all the abort contingencies. You can imagine the 
displeasure of a Shuttle commander rummaging around for the Once-Around-Abort
tape and trying to get it loaded before the first action is required. :-)

Hm, when in doubt, cheat and look up the original references. Here's a
description of computer and computer-related changes from the STS-49 Press
Kit (the first launch of Endeavour):

.............

>From: yee@trident.arc.nasa.gov (Peter E. Yee)
Subject: STS-49 Press Kit (Forwarded)
Date: Sat, 18 Apr 1992 19:56:05 GMT

			   STS-49 PRESS KIT
			        MAY 1992

...
IMPROVED FEATURES OF SPACE SHUTTLE ENDEAVOUR

	Many systems onboard Endeavour have had design changes or have
been updated from earlier equipment to take advantage of technological
advances and continue improvements to the Space Shuttle.  The upgrades
include several improved or redesigned avionics systems; installation
of a drag chute as part of a series of landing aid additions to the
orbiters; and modifications to pave the way for possibly extending
Shuttle flights to last as long as 3 weeks in the future.

	Some such updated systems already have been installed in the
rest of the shuttle orbiters as well as Endeavour; some will be
installed in all orbiters in the near future; and others will be used
on Endeavour only...

	The advanced general purpose computers (GPCs) are now in the
process of being incorporated into the entire orbiter fleet and will be
installed and used on Endeavour for its first space flight.  The
updated computers have more than twice the memory and three times the
processing speed of their predecessors.  Officially designated the IBM
10-101S, built by IBM, Inc., they are half the size, about half the
weight and require less electricity than the first-generation GPCs.
The central processor unit and input/output processor, previously
installed as two separate boxes, are now a single unit.

	The new GPCs use the existing Shuttle software with only subtle
changes.  However, the increases in memory and processing speed allow
for future innovations in the Shuttle's data processing system.
Although there is no real difference in the way the crew will operate
with the new computers, the upgrade increases the reliability and
efficiency in commanding the Shuttle systems.  The predicted Rmean time
between failuresS (MTBF) for the advanced GPCs is 6,000 hours.  The
flight computers are already exceeding that prediction with an MTBF of
18,500 hours.  The MTBF for the original GPCs is 5,200 hours.

New GPC Specifications
Dimensions: 		19.52S x 7.62S x 10.2S
[I wonder what an "S" is? From context, I'd guess "inches". - JR]
Weight: 		64 lbs.
Memory Capacity: 	262,000 words (32-bits each)
Processing Rate: 	1.2 million instructions per second
Power Requirements:  	550 watts

HAINS Inertial Measurement Units

	The High Accuracy Inertial Navigation System (HAINS) Inertial
Measurement Unit (IMU) will be incorporated into the orbiter fleet on
an attrition basis as replacements for the current KT-70 model IMUs.
The three IMUs on each Shuttle orbiter are four-gimbal, inertially
stabilized, all-attitude platforms that measure changes in the
spacecraft's speed used for navigation and provide spacecraft attitude
information on flight control.
	For Endeavour's first flight, one HAINS IMU will fly with two
accompanying DT-70 IMUs to provide redundancy with proven hardware.
The HAINS IMU for the Space Shuttle is a derivative of IMUs used in the
Air Force's B-1B aircraft.  It includes an improved gyroscope model and
microprocessor and has demonstrated in testing improved abilities to
hold an accurate alignment for longer periods of time.  In addition, it
has proven more reliable than the KT-70 IMUs.  The new IMUs require no
software changes on the orbiter or changes in electrical or cooling
connections.  The HAINS IMU is manufactured by Kearfott, Inc., of
Little Falls, N.J.

Improved Tactical Air Navigation Systems

	A complete set of three improved TACANS will fly on Endeavour's
first flight.  The improved TACAN is a modified off-the-shelf unit
developed by Collins, Inc., of Cedar Rapids, Iowa, for military
aircraft and slightly modified for the Shuttle.  The improved TACAN
operates on 28-volt direct current electricity as compared to the
current TACANs that use 110-volt alternating current for power.  Also,
the new TACANs do not require forced air cooling as do the current
TACANs.
	The TACANs' connections to the Shuttle's guidance, navigation
and control system are identical.  The TACANs provide supplemental
navigational information on slant range and bearing to the orbiter
using radio transmissions from ground stations during the final phases
of entry and landing.

Enhanced Master Events Controller (EMEC)

	The EMEC features improved reliability, lower power usage and
less maintenance than current MECs.  The new design uses 30 percent
less electricity and has more internal backup components.  The MECs,
two aboard each Shuttle, are a relay for onboard flight computers used
to send signals to arm and fire pyrotechnics that separate the solid
rockets and external tank during assent.  The EMEC were built by
Rockwell's Satellite Space Electronics Division, Anaheim, Calif.
Present plans call for Endeavour to be the only orbiter with the
EMECs.

Mass Memory Unit Product Improvement

	Improvements to the current MMUs in the form of modifications
include error correction and detection circuitry to accommodate tape
wear, tape drive motor speed reduction to extend the tape's lifetime.
In addition, modifications were made to the tape drive head to extend
its lifetime.  The improvements have no effect on the current software
or connections of the MMUs.  Two MMUs are on each orbiter and are a
magnetic reel-to-reel tape storage device for the Shuttle's onboard
computer software.  The modification to the MMUs will be done for the
first flight of Endeavour and for the rest of the orbiter fleet during
normal maintenance activities.  The MMUs were built and upgraded by
Odetics of Anaheim, Calif.

Enhanced Multiplexer-Demultiplexer

	The EMDM uses state-of-the-art components to replace obsolete
parts and improve maintenance requirements.  The new components have
simplified the structure of the EMDM by more than 50 parts in some
instances.  The EMDMs are installed on Endeavour, but plans have not
been made to replace the current MDMs in other orbiter.  The MDMs, 19
located throughout each orbiter, act as a relay for the onboard
computer system as it attains data from the Shuttle's equipment and
relays commands to the various controls and systems.  The EMDMs are
manufactured by Honeywell Space Systems Group, Phoenix, Ariz.

Radar Altimeter

	The improved radar altimeter aboard Endeavour already has been
installed and flown on all other Shuttle orbiters since STS-26.  The
altimeter is an off-the-shelf model originally developed for the
military's cruise missile program.  The altimeter has the capability to
automatically adjust its gain control as a function of changes in
altitude.  Along with anti-false lock circuitry, the improvements have
eliminated a problem frequently experienced with the original radar
altimeter caused by interference from the Shuttle's nose landing gear.
The radar altimeter is built by Honeywell, Minneapolis.

Improved Nosewheel Steering

	Improvements to the nosewheel steering mechanisms include a
second command channel, used as a backup in case of a failure in the
primary channel, for controlling the steering through the onboard
computers.  In addition, a valve has been installed in the hydraulic
system to switch in a secondary hydraulic pressure system in case of a
failure in the primary system.  Endeavour will have the modifications
prior to its first flight, and the rest of the orbiter fleet will have
the improvements made during their major modifications periods.  The
improved nosewheel steering was designed by Sterer Engineering and
Manufacturing Components, Los Angeles.

Solid State Star Tracker

	The SSST is a new star tracker design developed for Endeavour
which takes advantage of advances in star tracker technology.  The two
star trackers on each Shuttle orbiter are used to search for, detect
and track selected guide stars to precisely determine the orientation
of the spacecraft.  The precise information is used to periodically
update the orbiter's IMUs during flight.  The SSST uses a solid state
charge coupled device to convert light from stars into an electric
current from which the star's position and intensity are determined.
The solid state design consumes less electricity and provides greater
reliability than the current star trackers.  The SSSTs require no
modification to the orbiter or its software for installation.  Current
plans are for one SSST to be installed on Endeavour and another to be
incorporated into the orbiter fleet on an attrition basis.  The SSST
was developed and built by Ball Aerospace Division, Boulder, Colo.

.............
John Roberts
roberts@cmr.ncsl.nist.gov

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