Launch Vehicle Digital Computer (in the Saturn V instrument ring)
The astronaut Commander monitors several sources of information during ascent. He is trained to abort on two separate but related abort cues.
- He may use his own physiological senses.
- He may be instructed by verbal commands from Mission Control.
ABORT light is a cue that can be turned on by certain officers in Launch Control or Mission Control.
LIFTOFF light turns on when the umbilical to the instrumentation ring is disconnected. The same signal arms all of the Saturn's pyrotechnic systems (that is, pyrotechnics are disabled while the umbilical is connected).
- The Flight Director Attitude Indicator displays attitude, rate of attitude change, and attitude errors.
LV GUID light turns on if the attitude exceeds a certain range, or if there is a discrepancy between attitude sensors.
LV RATE light turns on if attitude change rates exceed certain thresholds; there is a switch that can make this signal cause an automatic abort.
- The launch vehicle engine lights show engine problems, detected by dedicated (non-computer) circuits in the instrument ring. (The Apollo 13 movie shows one of these warning lights turning on.)
- The angle of attack meter is "a pitch and yaw vector summed angle-of-attack/dynamic pressure product. It is expressed in percentage of total pressure for predicted launch vehicle breakup (abort limit equals 100%). It is effective as an abort parameter only during the high q flight region from
+50 seconds to + 1 minute 40 seconds."
- An accelerometer and altimeter.
- Stage separation lights.
- Many of the above lights also turn on the
MASTER ALARM light and tone.
A few of these conditions can cause an automatic abort, if the
EDS switch is set to
Should the Commander determine that an abort is necessary, he turns the T-shaped handle of translational controller counter-clockwise. This causes two redundant, dedicated (i.e. non-computer) sequencers to cause an abort. If either of these sequencers fail, the
NO AUTO ABORT light turns on, and the crew can perform the sequence by manually pushing buttons:
LES MOTOR FIRE,
CSM/LM FINAL SEP (to ditch the docking ring),
APEX COVER JETT,
MAIN DEPLOY, and
Thus, there are multiple ways to safely abort a launch if the LVDC fails.
Apollo Guidance Computer (in the Command Module)
The AGC is backed up by the Stabilization and Control Subsytem. The AGC is digital; the SCS is analog. Each system had its own set of sensors to determine spacecraft attitude. When the T-shaped translational controller is turn clockwise, the SCS is in control; turning it back to the center position returns control to the AGC. (Turning it counter-clockwise causes a launch abort, as per the previous section.)
The SCS has several modes:
- A "direct" mode which provides manual control of the RCS thrusters. The translational controller causes translational thrust, and the rotational controller causes spacecraft rotation. The amount of thrust is proportional to how far the controller is moved. As these are accelerations, they require pilot skill.
- The "attitude set" mode has a set of thumbwheels where the astronauts can set a desired attitude. The SCS will automatically turn the spacecraft in that desired direction, using minimal impulses. It also useful for holding attitude.
- The "attitude rate" mode causes the spacecraft to rotate along any one of the three axes, at a rate set by the astronauts. It was used for the "barbecue roll".
- Should the SCS electronics fail, pushing the translational or rotational controls all the way in any direction directly fires the associated RCS engines full blast (instead of proportionally).
DIRECT ULLAGE button can fire all of the aft-facing RCS thrusters.
In addition, the service module main engine can be fired by pressing the
SPS DIRECT THRUST button.
Thus, everything can be manually done without the AGC, albeit with much less precision.
Lunar Guidance Computer and Abort Guidance System (in the Lunar Module)
Normally, control of the LM is done by the LGC. The AGS is a backup, but is only designed to put the LM into orbit so the CSM can pick it up.
Should both computers fail, there is a manual mode which gives a limited amount of control:
- The rotational controller can be used to cause rotational acceleration proportional to the movement of the controller, using the RCS thrusters. When the controller is released, it returns to center, which maintains an attitude hold.
- The translational controller causes linear acceleration by the RCS thrusters, proportional to the controller's displacement.
- The descent engine can be throttled manually.
- The ascent engine can be manually turned on and off with the stabilization and control electronics.
Thus, there is at least some minimal manual control in the LM. Again, it is not as precise as computer control.
The set of computers which are absolutely essential are actually those in Mission Control. Many aspects of the mission depended on accurate attitude, thrust, and timing calculations. These were too complicated to be performed on the spacecraft computers; rather, the calculations were done back in Houston. Even the "manual" burns that occurred during Apollo 13 depended on calculations back at Mission Control. Without those computers, rendezvous and safe landings are not possible.