OnBoard Computer (DCU)

The following comes from discussions with Mr. Ed Percasky.

The DCU:
- Teledyne special purpose computer with 16K memory, 24 bit, no parity
- Last few years - 256,000 memory, 16 bit words, 2 cpus

Tasks of the DCU
1. Navigation - where we are, where we want to be next. Takes input from
inertial system. Shoot for a point in space
at a certain velocity.
2. Guidance - determine proper orientation of vehicle, output position in
terms of pitch and roll axes to the control
system. Determine proper times to turn main engines on and off and command
these events.
3. Flight Control System - Servo-control. Determine and issue commands to
gimbal main engines and to fire attitude
control thrusters. (engine conditions are taken each 20 msec and a
newcommand is issued.
4. Sequencing - Determine when discrete events should occur and command them.
5. Propellant utilization - Determine proper positions of PU (propellant
utilization) valves and command them to
proper positions.
6. Determine pressurization or venting needed for propellant tanks and
command actions. 7. Instrumentation and
telemetry - Request telemetry data in the desired sequence, format this
data and output at selected bit rate.

Interrupt priorities
1. power dropout
2. power on
3. DCU telemetry data - generated each time data is placed on the PCM bit
stream 4 Ground support equipment
5. real-time data
Original technology was transistors on boards - 1965 (MIMAs)

System had no direct feedback to the computer about the vehicle. Didn't know:
- tank pressures
- gyro
- PU (propellant utilization) System

They sent up weather balloons on the day of the flight to get mission
constants for the prevailing conditions.

Computer controlled pressure and venting systems (CCVPAPS).
Purpose is to maintain proper LH2 and LO2 tank pressures. Constants are
stored in the computer for upper and
lower bounds of pressure. May cause pressurization or venting of He.

Keeps a dynamic pressure profile as flight through the atmosphere occurs.
windshears and gusts. Once out of
atmosphere, structure people know aerodynamic loads at altitudes.

System has 12 inputs - 6 transducers, 3 for O2 and 3 for H2.
w Pair and space to utilize sensors - Typically compare 2 transducers, eg:
1 and 2. As long as they agree, Ok. If they
disagree, use the third for the rest of the trip.