Saturn V - Apollo   Launch Aborts

Banner abort switches

Credit to Houston Space Center
Banner abort switches

Credit to The Other Real
Figure 1

Manual abort
An abort could be iniated by the Apollo crew by turning a lever (marked in red in the figure above).
There were basically two types of cues for aborts:

  1. the cues which were provided by the displays reflecting the flight behavior and the technical status of the launch vehicle and which were pointing to an imminent danger;
  2. an abort request from Mission Control.
Abort request by Mission Control
There were three flight control officers who could request such an abort by using a dedicated switch on their control panel: the Booster engineer (Booster), the Flight Dynamics Officer (FDO) and the Flight Director (Flight). A valid request required the abort switch to be moved to the "A" side, then to the "B" side and finally back to the neutral position. This action would have resulted in a bright red abort light to be lut up at the spacecraft's control panel. But it was still up to the crew to grant the abort request from Mission Control and turn the abort lever.

Automatic abort by EDS
However during the two three minutes of the ascent an abort would have been triggered automatically by the Emergency Detection System (EDS):

  1. in case of a structural failure between the command module and the instrument unit on top of the S-IVb stage;
  2. if two or more S-IC engines drop below 90% of rated thrust;
  3. and/or if pitch, yaw or roll redline rates would have exceeded.
This EDS has been conceived for rapidly developing life-threatening situations, which would have been too rapid for humans to react adequately. The EDS auto mode was manually disabled by the crew about 10 seconds after the beginning of abort mode IC phase and automatically disabled when the Launch Escape System (LES) was jettisoned (beginning of the mode II abort phase). In this launch phase the crew and Mission Control had more time to react. And human intervention became necessary to be able to select between the various abort options which had been conceived for this launch phase.

In the picture above, top right, two abort request switches, side by side, are shown. It is a photo of a part of the recently restored console of the Flight Director at the Johnson Space Center. However, in practice one switch was for requesting an abort and the other for cancelling the abort request. The explanation of the switch at the right should contain the text "A/B OFF" twice. In this photo the distinction between the switches has probably accidentally not been made.

The NASA Manned Spacecraft Center's Mission Planning and Analysis Division had designed emergency and contingency procedures for various phases in a moon mission.
These phases have been organized into two categories:
  1. The powered flight phases
    1. Launch or ascent phase
    2. Translunar Injection (TLI)
    3. Lunar Orbit Insertion (LOI)
    4. Lunar Descent
    5. Lunar Ascent
    6. Trans Earth Injection (TEI)
  2. The coast phases
    1. Earth Parking Orbit (EPO)
    2. Translunar Coast (TLC)
    3. Lunar Parking Orbit (LPO)
    4. Trans Earth Coast (TEC)
In this page we will focus on the launch phase of the mission.
In figure 3 is shown that three flight regimes in the launch phase were distinguished: atmospheric, transitional and space. Four abort procedures have been designed taking into account the three flight regimes.

Further two kinds of emergencies were distinguished which could prompt an abort:

  1. Slowly developing emergencies
    Mission Control was monitoring the flight trajectory and was assessing the deviations of altitude, downrange distance, flight path angle and velocity.
  2. Rapidly developing emergencies (too rapid to allow for human intervention)
    Emergency detection by an onboard system (EDS: Emergency Detection System) leading to an automatic abort of the flight.

In this page are discussed:
  1. The launch abort modes I, II, III and IV and two additional abort modes;
  2. Reentry trajectory control using the aerodynamic properties of the Apollo command module;
  3. The various launch abort recovery areas;
  4. The plot charts for selecting the most suitable abort mode;
  5. An explanation of the plot charts as used by the flight controllers.

Banner MOCR
1 2 3 4 5 6

Credit to NASA
Figure 2
The group displays in de the Mission Operations Control Room (MOCR) during a Saturn V - Apollo launch.

During one of the most critical phases of an Apollo mission, the launch, the flight was monitored meticulously. The three dark charts in the middle (3, 4 and 5) are plotboards. The actual flight trajectory was plotted in realtime on a template which contained predrawn expected nominal trajectories, limit lines and abort mode boundaries.
These displays enabled flight controllers to spot trajectory deviations, to make assessments and decisions on flight abort options.

The primary charts for the flight controllers
The three dark charts in the middle might look somewhat complicated at first glance. But these charts were designed to display the plots at three different scales for a compact and a cohesive presentation of graphical information. The smaller the scale the more details were displayed. (see figure 9)
The trajectory from S-II / S-IVB staging until orbit insertion was displayed on the smallest scale for a better view on the various abort options which were available near the orbit insertion.

The left dark chart (no.3) in the photo is an Altitude - Range chart.
The middle one (no.4) is an Inertial Flightpath Angle - Inertial Velocity chart
The right one (no.5) is also an Inertial Flightpath Angle - Inertial Velocity chart on three somewhat different scales. This chart was focussed on the S-IVB powerd flight phase up to orbit insertion. The three predrawn somewhat triangular shaped curves were representing the boundary lines of a particular abort option (mode IV) for different altitudes which involved the use of the Service Propulsion System (SPS) of the Apollo spacecraft to achieve a contingency orbit.

The various abort options near the end of the launch phase
The smallest scale in the three charts was meant to monitor the last phase of the launch, which was the S-IVB powered flight up to orbit insertion, more closely. In this phase, in which the launch vehicle would have been approaching the African coast rapidly, various abort options became available in rapid succession for which a quick decison making process was required. All these abort options were much depending on rapid changing flight parameters and were devised to avoid a high risk landing on African soil.

    All these options would have led to the folowing three situations :
  • landing short: in the Atlantic Ocean a couple of hundreds kilometers in front off the African coast;
  • landing long: landing across the African continent in the Indian Ocean;
  • insertion into a contingency orbit.
For all those three options a proper preparation was required beginning with an upload of flight data provided by Mission Control via ground based stations as input for the onboard Apollo Guidance Computer (AGC). The AGC was needed for a controlled retrograde burn using the SPS, in case of a reentry, or a controlled posigrade burn in case of a contingency orbit insertion. In case of an reentry the AGC was also used to control the reentry trajectory to land in a selected area.

If for any reason the required preparations for a controlled reentry was inadequate a high risk landing on the African continent would have been the last option. Because of the expected (very) long arrival time of recovery forces, astronauts were trained to survive in harsh conditions.


Picture illustrating flight regimes and abort modes

Diagram based on figure 5 in ref.1 "Mission Design for Flight Safety"
Figure 3
Atlantic abort modes

In the ascent or boost phase three flight regimes have been distinguished: atmospheric, transitional or sub-orbital and space. For each regime an abort procedure has been designed.
In the picture shown above four abort modes have been depicted. However mode IV is not an abort procedure in the strict sense, because this mode will not result in a quick reentry. The objective of abort mode IV is to achieve a contingency orbit.

Abort mode I
Abort mode I is explained in figure 4.

In the abort modes II and III the reentry trajectory could be controlled by controlling the roll angle of the command module during descent

Abort mode II
In mode II the Service Module RCS engines or the SM main engine propel(s) the CSM away from the launch vehicle. When the CSM is at a safe distance, the CM is separated from the SM and manoeuvred into a reentry attitude.
In the mode II abort procedure, the CM's roll angle would have been set to obtain a full lift up attitude. The blue dotted curve represents the last possible mode II trajectory which began at the socalled mode switch-over point at which so much velocity would have been gained that it would have become necessary to switch to mode III.

Abort mode III
In mode III the SPS has to be used, not only to separate from the booster as in mode II, but also for a retrograde burn in order to be able to land some 1000 km off the African coast, some 6000 km downrange. This specific landing area is called the Atlantic Discrete Recovery Area (ADRA).
Besides the retroburn, the descent trajectory needs to be actively controlled to land in the ADRA. In order to have a controlled burn for the required Δv and a controlled descent, flight data need to be uploaded from ground stations into the Apollo guidance computer as input for its guidance software.

    It is a rather complicated and hectic sequence:
  • data transmission from Mission Control to ground station to spacecraft's guidance computer;
  • program initiation for the SPS retroburn;
  • retroburn;
  • SM-CM separation;
  • program initiation for reentry control.
Abort mode IV
In mode IV the Service Module main engine (SPS) is used to separate the CSM from the launch vehicle and insert the CSM into a contingency orbit.

Contingency orbit insertion using the S-IVB stage
The S-IVB stage could also be used to bring the spacecraft into a contingency orbit. This option was designed for cases in which the S-II stage would fail in midflight and was endangering the crew. The option became available when threshold values for altitude and velocity were met (see figure 7).

The "Fixed-Δv" abort mode: a beyond-Atlantic abort mode
The window of opportunity for mode III was closing when the booster (S-IVB stage) was about to cut-off and orbit insertion was almost achieved. When nevertheless an immediate return of the crew would suddenly have become necessary, a socalled "fixed-Δv" abort mode could be used which would result in a landing in the Indian Ocean across the African continent. The selected landing area was called the Indian Ocean Recovery Area (IORA) (see figure 6).

For this abort mode to execute, it had to be determined first, based on the predicted initial velocity at abort, whether a retrograde burn (to loose velocity) or a posigrade burn (to gain velocity) would be required to land at IORA.

IORA had the advantage that this landing area could be used for all launch azimuths. The choice for the geographical location of IORA was deliberate. The geographical central angle between the launch pad and IORA is 180°, IORA is therefore the location where all groundtracks, having different launch azimuths, intersect.

Personal note:
This "Fixed-Δv" abort mode is discussed in ref. 2 and 5. But it has never been mentioned in voice callouts between Mission Control and the crew during the launch phase. And so far I have not been able to find any reference to this mode in flight manuals. I might wonder whether astronauts have been trained for such an unlikely occurence.

From ref.2 figure 2a

From ref.2 figure 2b

From ref.2 figure 2c
Figure 4
Abort mode I

In abort mode I the motor (L/E motor) of the launch escape tower (LET) would have been used to propel the CM away from the launch vehicle. A pitch control motor (P/C motor) at the top of the tower would have been used to provide the proper pitch angle for the evasive maneuver.

The hardware of the Earth Landing System (ELS) consisted of two drogue chutes, three main chutes with their three pilot chutes and five chute mortars to have the parachutes deployed. The ELS had to be armed to enable the parachute deployment sequence.

Abort mode I had been divided into the submodes IA, IB and IC mainly because of the rapid decreasing atmospheric pressure during the ascent.
The only difference between mode IA and IB however was that in mode IA a propellant dump of the Reaction & Control System (RCS) would have been executed to prevent a possible fire source at landing. This procedure was ommitted in mode IB
In mode IA and IB a canard system in the top of the LET would have been used to induce a pitch tumble to put the CM in a proper attitude, blunt end forward.

Mode IC would have been used for altitudes above 37 km where the atmosphere is too thin for the canard system to be effective. The crew would also have to operate the small engines of the CM's RCS manually to induce the required pitch tumble.


CM aerodynamics
Figure 5
Reentry trajectory control using the aerodynamic properties of the Apollo command module.

Controlling the lift vector by controlling the roll angle
The reentry trajectory control is left to the Apollo Guidance Computer (AGC) by controlling the roll angle of the spacecraft during its reentry. The Apollo command module has been deliberately designed to have a center of gravity which doesn't coïncide with its geometric center. This will result in an unbalance of the aerodynamc forces acting on the heat shield causing it to tilt slighty with respect to the direction of flight during the descent. This tilt will result in a lifting force perpendicular to the drag. By controlling the roll angle the lifting vector could be pointed in all directions in a plane perpendicular to the direction of flight. So by controlling the roll angle an upward, downward and sideward lift can be created.

This mechanism enables the software by employing the Reaction Control System (RCS) to control the descent trajectory to have a controlled landing in a pre-selected landing area.


Recovery areas

Redrawn from ref.2 figure 9
Figure 6
Groundtracks from the launch pad to the Indian Ocean Recovery Area (IORA) for trajectories having 72°, 90° and 110° launch azimuths with their recovery areas.
    Three types of recovery areas were distinguished:
  1. ACRA: the Atlantic Continuous Recovery Area, stretching from the launch pad to 6000 km downrange;
  2. ADRA: the Atlantic Discrete Recovery Area, a specific area 6000 km downrange;
  3. IORA: the Indian Ocean Recovery Area, that is an area, some 19 000 km downrange, which the trajactories of various launch azimuths have in common.
    The socalled central angle between the launch pad and IORA is 180°
Mode I and II
ACRA was applicable for the abort modes I and II.

Mode III
Mode III came into effect when the landing area in case of a mode II abort had approached ADRA. In order to land at ADRA a retro maneuver with the SPS became necessary to reduce the velocity. And after SM-CM separation, the descent trajectory of the CM had to be controlled to create safe reentry conditions and to land at or near the ADRA.

The "Fixed-Δv" abort mode
If however at the moment of abort, the velocity would have become too high to make it to the ADRA, and an immediate return of the crew was required, then a retrograde burn maneuver or a posigrade burn maneuver needed to be planned to aim for a landing in IORA across Africa. These procedures were designed to avoid a high risk landing on the African continent.

In order to execute the mode III or the "Fixed-Δv" abort mode procedure, flight data needed to be uploaded to the spacecraft's guidance computer for a precise burn and to enable the guidance computer to control the reentry trajectory by controlling the spacecraft's roll angle during the descent.

Mode IV
The mode IV abort, a Contingency Orbit Insertion (COI) maneuver using the SPS, was highly preferred above the very complicated and risky mode III procedure. In figure 8 is illustrated which trajectory conditions had to be met to achieve an orbit when using the mode IV abort procedure. Otherwise mode III was left as the only option for a safe return of the crew.

Personal note:
From the map shown above it can be concluded that the decisions about the abort mode options III and IV were time critical for trajectories with launch azimuths between 72° and 92°. For launch azimuths between 92° and 110° however, there would have been plenty of time. Mission rules might have allowed for a prolongation of mode II and effectively extending ACRA. Mission rules might also have allowed to do the reentry in the technical less demanding "full lift" mode after the retrograde burn in abort mode III and effectively extending ADRA.


Diagram or plot 1?

Redrawn from ref.4 figure 2
Figure 7
Abort modes in the entire launch phase

In this chart all the available abort modes are depicted.
The ranges of the abort mode I, II and the early S-IVB to orbit capability were relatively easy to manage.
But near orbit insertion a far more closer look was necessary to be able to make adequate abort decisions in case of emergencies. This closer look is shown in figure 8.

Diagram or plot 1?

Redrawn from ref.4 figure 3
Figure 8
Near orbit insertion abort modes

This chart illustrates the complexity of the abort decision making process near the moment of orbit insertion. The objective to avoid a high risk landing on African soil as much as possible had a large impact on the abort mode management. In the last three minutes of the launch phase the various abort options became available in rapid succession. Using mode III or "Fixed-Δv" mode were considered highly unlikely. The boost flight was considered as the most risky part of the launch. But so close to orbit insertion and with the mode IV (SPS to orbit) capability available, only a fatal spacecraft system failure which could have endangered the crew for which an immediate return became necessary through a mode III or a "Fixed-Δv" abort. However such an event was considered as highly unlikely.

The mode IV abort partly overlaps mode II but entirley overlaps mode III. Mode IV was preferred because it was a far less hectic procedure than mode III and could provide some options for an alternative mission in Earth orbit.

Shifting retroburn times in mode III
There was a range in which a retrograde SPS burn was not needed to land in ADRA. By controlling the descent trajectory the intial velocity might vary within a certain range as indicated with the blue area in the chart.
But beyond that range (into the light red area) a controlled retroburn was needed to obtain the proper -Δv. The required -Δv is depending on the initial velocity (velocity at abort). The velocity was measured constantly by the onboard inertial platforms in the spacecraft and the Instrument Unit on top of the S-IVB stage and a nearby tracking station (Canary Island Tracking Station). These data were sent to Mission Control via the Manned Space Flight Network (MSFN) and enabled Mission Control to determine the required retroburn time to obtain the required -Δv.
In the case that a mode III abort had to be initiated, the calculated required retroburn time and other flight data had to be uploaded into the Apollo Guidance Computer for a controlled retroburn and for a controlled descent.

Shifting posigrade burn times in mode IV
The required +Δv to achieve an orbit was depending on altitude, pitch angle and initial velocity. The Flight Dynamics Officer and his support group had a collection of graphs at their disposal to oversee the required +Δv to be achieved with the posigrade burn. Like in mode III, the required flight data as input for the onboard AGC could always, almost instanteneously, be provided by a computing and data processing system, the Real Time Computer Complex (RTCC) of the Mission Control Center (MCC).
(See pages on this site for more information about the MCC and its RTCC)


Flight Controller's Plotboard

Redrawn from a photo of an original plot chart

Flight Controller's Plotboard with commentary overlay

Figure 9a and 9b
Flight Controller's Plotboard (without and with a commentary overlay).

This chart has been redrawn from a photo of an actual chart to be used during the launch phase of an Apollo mission. These kind of charts were used by the Flight Dynamics Support Group in one of their plotters. This group was located in a room adjacent to the Mission Operations Control Room at the Johnson Space Center in Houston.
(For more information about these mission support groups.)

The graphical information is displayed at three different scales. That is because the monitoring requires an increasing rate of meticulousness. The last phase of the mission is depicted in red. In this phase of the launch between S-IVB stage ignition and orbit insertion various abort options become avaialable in rapid succession with the aim to avoid a riskful landing on African soil.

The chart contains curves of predrawn nominal trajctories, limit lines and abort mode boundaries. The actual trajectory was plotted in real-time on this chart to spot deviations from the expected nominal trajectories. The chart is presenting the successive available abort modes along the ascent trajectory which enables the flight controllers to make adequate decsions with regard to the abort options in case of emergencies.

In figure 9b comments have been added to the chart.
To retain some of the authenticity of the original chart, the same US customary measurement units have been used in the redrawn version.

Markings for mode I, II and III have been left out
In the chart only two abort modes are represented: the S-IVB to contingency orbit insertion and the mode IV abort (SPS to contigency orbit insertion). Mode I, II and III have been left out.

S-IVB to COI capability
COI stands for Contingency Orbit Insertion. This option became available when the Saturn V stack had obtained a certain velocity and altitude. In case of a mid-flight the S-II stage failure, the S-IVB stage could be used to take over the powered flight to bring the spacecraft into orbit. In such an occurence the capability for a TransLunar injection burn is however lost.

Personal note:
Maybe there was a maximum time (one minute?) by which an S-IVB ignition could be moved up for a COI burn before the amount of propellant had dropped below a threshold value at which the TLI burn capability would have been lost.

Mode IV: SPS to COI capability
The abort mode IV involved a COI by using the SPS. For inertial flighpath angles above zero this mode IV could be performed in two ways:

  1. An SPS posigrade burn as soon as the mode IV option becomes available;
  2. Suspend the SPS posigrade burn until the apogee was achieved. The burn at the apogee was called an apogee kick. This procedure was meant to optimize the efficiency of the burn to achieve an orbit with a perigee of 139 km (75 nautical miles).
AOS stands for Acquisition Of Signal. The range at which Grand Canary Island Tracking Station (CYI) is able to track the spacecraft was limited by the curvature of the Earth. To be able to assess the apogee kick an abort should happen within a period of time after the S-IVB ignition. Otherwise the apogee would be beyond the tracking horizon and this assessment capability would be lost.

Chart no.4 in figure 2
That chart shown in the group displays of the Mission Operations Control Room, was a projection version of more or less the same chart.
But a perceptive eye might discover that on that version there are a predrawn mode II limit line, a predrawn short line marking the moment of LET jettison and a predrawn exit heating limit line (the small wiggling curve left, near the chart boundary).
The long limit line is an element of the part that is drawn on the smallest scale which focuses on the near orbit insertion trajectory and is marking the upper limit of mode II. The short line belongs to the medium scale part of the chart and is marking the moment of LET jettison, the ending of mode IC and the beginning of mode II.
The small wiggling curve belongs to the medium scale part too.
The three limits have been drawn in figure 9b (dotted black lines) for comparison.

Also in this projection version of the chart the markings for abort mode III have been left out. For mode III however boundary markings would be helpful, if not essential, to make the proper decisions. However mode III is overlapped by mode IV, so there is no gap in the abort mode coverage. Mode III would have been used if an immediate return of the crew, that close to orbit insertion, became necessary. It would have required a failure of the Service Module or the Command Module for that to happen. The likelihood of such an occurence was estimated to be very small.

"Fixed-Δv" abort mode has also been left out
Like in abort mode III the markings for a "Fixed-Δv" abort mode, which would result in a landing in the Indian Ocean, has been left out. But it can be assumed that in case of emergencies the flight controllers could quickly switch over to suitable plotboards to oversee these two rather complex abort modes.

ACRA Atlantic Continuous Recovery Area
ADRA Atlantic Discrete Recovery Area
AGC Apollo Guidance Computer
AOS Acquisition Of Signal
COI Contingency Orbit Insertion
CSM Command & Service Module
CYI Canary Island Tracking Station
EDS Emergency Detection System
ELS Earth Landing System
EPO Earth Parking Orbit
IORA Indian Ocean Recovery Area
LET Launch Escape Tower
LOI Lunar Orbit Insertion
LPO Lunar Parking Orbit
MCC Mission Control Center
MOCR Mission Operations Control Room
MSFN Manned Space Flight Network
RCS Reaction Control System
RTCC Real-Time Computer Complex
SPS Service Propulsion System
TEC Trans Earth Coast
TEI Trans Earth Injection
TLC Translunar Coast
TLI Translunar Injection

  1. Mission design for flight safety
    MSC Internal note No. 67-FM-175, November 17, 1967
    by Carl R. Huss, Claiborne R. Hicks Jr., Charlie C. Allen

  2. Preliminary Launch Aborts Analysis For Manned Apollo S-V Missions
    Manned Spacecraft Center, Houston
    Flight Analysis Branch
    MSC Internal Note No. 67-FM-7, January 18, 1967
    by Bobbie D. Weber

  3. Apollo Experience Report - Abort Planning
    NASA TN D-6847, June 1972
    by Charles T. Hyle, Charles E. Foggatt, Bobbie D. Weber

  4. Operational abort plan for the Apollo 9 mission
    Volume I - Launch Phase
    MSC Internal note No. 69-FM-15, January 27, 1969
    Manned Spacecraft Center, Houston
    NASA, Mission Planning and Analysis Division, Flight Analysis Branch
    by Edward M. Henderson

  5. Spacecraft Operational Abort Plan For Apollo 7 (mission C)
    Volume II - CSM Launch Aborts
    MSC Internal note No. 68-FM-170, July 19, 1968
    Manned Spacecraft Center, Houston
    NASA, Mission Planning and Analysis Division, Flight Analysis Branch
    by Edward M. Henderson
    TRW System Group, Mission Operational Section
    by J.V. Butler

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