Abort Request Command

From a draft NASA requirements document: Both the crew and the CVCC [Commercial Vehicle Control Center] shall be capable of initiating the pad and the ascent abort sequence.

From NASA Generic Flight Rules Volume A, Space Shuttle:



I need to do some homework on Mercury, Gemini, and Apollo abort initiation.  Gemini crew members could actuate their ejection seats on their own, but whether both the crew and the ground could initiate an abort is something I don’t really know.  Famously, the Soyuz crew members cannot initiate an abort.  Both the “April Anomaly” of 1975 and Soyuz T-10-1 pad abort of 1983 (http://www.youtube.com/watch?v=UyFF4cpMVag) crews were unable to initiate abort action and had to rely on the ground control, as is still the case today even with the newest model Soyuz.

The Shuttle on the other hand . . . well, here is a mostly true Flight Director story.

In the “old” mission control center at JSC the Flight Director console is preserved much as it was during Apollo and also as it was when I was first training to be a Shuttle Ascent Flight Director in the late 1980’s.  Among the old fashioned push buttons and lights (and a rotary phone!) is a formidable piece of hardware always referred to in capital letters:  the Abort Switch.  The face of the Flight Director console has the (black&white) computer monitor screens, the comm panels with all the flashing lights, and the DDD (dedicated display driver) lights which illuminated for various events to keep the Flight Director situationally aware.  The surface of the console is a flat desk covered with Plexiglas (all the better to keep coffee spills off the  reference papers below. Between these two surfaces, one horizontal, one vertical, is a short, inclined surface with various controls.  One of these controls is the Abort Switch.  A large handle, maybe 3 inches long, fits into its base snugly by virtue of triangular metal fittings and strong springs.  The Abort Switch is a “lever locking” switch, which means it cannot be accidentally bumped.  The protocol, after loudly announcing on the Flight Director comm loop to the CAPCOM:  “Abort  RTLS (or TAL or ATO)” which the CAPCOM would immediately repeat to the crew over all three air to ground circuits, was for the Flight Director to pull the Abort Switch Handle out from the console, moving it up (to send the “A” command), then releasing the handle to let the switch pop back to center neutral (locked position), pull the Abort Switch handle out a second time from the console and move it down (to send the “B” command), and finally let the switch freely pop back to the center neutral/locked position.

Why all the rigmarole? 

The hardware Abort Switch had a nasty habit of sticking on and flooding the command buffer with “Abort A” or “Abort B” commands unless the switch was allowed to pop freely back to the neutral position.  If the Abort Switch got stuck in this continuous command mode, no other commands could be sent to the shuttle and the Ground Control officer’s back room had to do computer terminal magic work to make it go away – remember, those were the days of main frame computers and ordinary mortals – or even the Flight Director – were not allowed to touch a computer terminal. 

Funny thing, all this effort was merely to illuminate a light on the Shuttle Commander’s dashboard.  The entire sequence sent a series of “Abort Request Commands” which really did very little.  But we paid a lot of attention to the sequence.  When properly initiated the Abort Switch first sent three times the single stage “Abort Request A Command” which, once received onboard, was routed from the radio equipment to the General Purpose Computers to a Multiplexer/Demultiplexer to one of the Annunciator Control Assembly electronics to light one bulb of the Commander’s abort light.  The second actuation sent the “B” command out three times through the same radio equipment to the GPCs then to a redundant MDM and ACA to light the redundant bulb in the Commander’s abort light.  The Shuttle Commander, after having the confirming cue as described in the flight rule above, then moves the rotary abort switch from its “off” position to the desired abort mode (RTLS, TAL, ATO, or AOA) and punches the Abort push button indicator (which the abort commands have illuminated), and then the computer software modes to the desired abort mode and away you go.  Whew.  Go back and read that slowly.  All of that, just to avoid making a mistake.  Does it sound overly complex to you?

When we moved to the “new” control center in the mid 90’s, the builders wanted to eliminate the abort switch and just have the Flight Director send the abort commands by mouse click on the computer screen in front of him.  The Flight Director office rebelled against this affront to tradition and demanded a dedicated PBI on the console to send the abort command.  Probably the only PBI in the “new” control center is the Flight Director’s Abort button.  Which doesn’t do anything but light a light in front of the spacecraft commander.  But don’t worry; it has complex and convoluted software to control it!

During multitudinous simulations in both the old and new control centers, I have had the opportunity to send the abort command to the crew in the shuttle mission simulator probably a thousand times.  The only Flight Director to have a reason to send the abort request command in real flight was Orion Flight, Flight Director #21, Cleon Lacefield, on STS-51-F in 1985.  The Center SSME was erroneously shut down early by bad instrumentation resulting in an Abort To Orbit (which is principally a dump of fuel from the Orbital Maneuvering System).  STS-51-F made it to orbit, and the mission was accomplished completely successfully albeit at a lower than planned orbital altitude.  There is an apocryphal story that Cleon was so busy with the ascent that he neglected to send the Abort Request Command.  I don’t know if that is true, I need to ask Cleon.  In any event, Orion Flight is the only American Flight Director to declare an ascent abort in history of American manned spaceflight.

All of that is background to my real story for the day. 

Every countdown, the Abort Command System is checked about 10 hours pre-launch.  One of the non-ascent Flight Directors is on console in Houston with a team to babysit the vehicle (really under the control of the Launch Director), the network, and the Mission Control Center until the Ascent team shows up about four hours prior to launch.  Now, the orbit certified Flight Directors do not have the training that the Ascent Flight Directors have, and there is considerable uncertainty about this test. The FD is directed to actually send the Abort Request commands (both A and B) to the vehicle and one of the Caped Crusaders sitting in the Commander’s seat verifies the light comes on.

So when Alpha Flight (FD #23, you have to look it up) was doing prelaunch for the very first time, the NTD called from KSC and said:  “Houston Flight, step 16-xxx, send Abort Command” he wouldn’t do it!  As he later said, “I thought it might blow up the vehicle, and I didn’t want to be responsible for that!” 

I remember this incident well, because as the rookie trainee Flight Director I got the assignment to write up the Handbook procedure on the pre-launch abort light test. 

But if you hadn’t read this, wouldn’t you agree with Alpha Flight?  Who would believe that the Shuttle Flight Director sending the abort command merely lights a light in the cockpit for the Commander to see?!

Oh, next you will want to know how to turn the light off . . . .

About waynehale

Wayne Hale is retired from NASA after 32 years. In his career he was the Space Shuttle Program Manager or Deputy for 5 years, a Space Shuttle Flight Director for 40 missions, and is currently a consultant and full time grandpa. He is available for speaking engagements through Special Aerospace Services.
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30 Responses to Abort Request Command

  1. Jess Lucero says:

    Very interesting…thanks. Is it true that sts-51 was almost lost due to another SSME almost shutting down?

  2. Baylink says:

    You *are* working on the book, right Wayne?

    • waynehale says:

      Of course there is the “Wings In Orbit” book on the history of the space shuttle coming out from the Government Printing Office Bookstore in the spring. Check out NASAWatch. Keith was kind enough to feature it today!

  3. Dxbear says:

    Awesome insight to the LCC environment.. 28 years working on and around the orbitier, OPF, VAB and Pad.. I always assumed that an abort switch would send a self destruct signal.. very cool to know after all these years..

    In the early days whe the astronauts were around the hardware alot, and right after Challenger 51L, I was in an engineering meeting for the investigation team.. we were working on the escape slide and Thermal tile changes when the topic of RTLS (return to launch site) abort came up.. it was said to be an unsurvivalable water landing, thus the crew HAD to stabilize the vehicle, and then use the escape slide to rocket down the pole and chute into the water..

    It would neat to one day hear you discuss these facts.. alot of my pilot friends have always asked me these type of questions..

    Great story Wayne.. keep up the interesting blog..

    • waynehale says:

      Of course the intent of RTLS to to arrive safely back at the Shuttle Landing Facility and avoid that nasty ditching scenario. About that ditching . . . . it is not considered survivable since the shuttle stall speed is somewhere above 150 kts (at light weight). So rather than ditch in the ocean, the crews are trained to blow the side hatch, extend the pole (so that they clear the wing) and bail out in a line rather like some WWII movie about paratroopers. The rocket idea was not implemented.

  4. David Fuller says:

    In the early days of Shuttle I worked the RTC (Real Time Command) console. We enabled commanding on consoles such as INCO, COMM SUPPORT, and Flight Director.

    On launch I had enable Flight for his Abort Command, and he came back and said he couldn’t get his console configured. Instead of trying to talk him through it, I unplugged and ran over to the front room (we were both on the third floor). Back then, they posted armed guards at the doors, and people were given mission badges that showed access to various rooms, including the front room.

    I ran past the guard and scooted up to the Flight console. I heard the guard’s metal folding chair clang on the floor, and then I heard GC (Ground Controller) say something like, “Its all right, he’s one of us!”

    I never looked back to see if he had drawn his revolver. I just quickly configured Flight’s console for the Abort command, and went back to my RTC console.

    • waynehale says:

      We always believed that the security guards (like Barney Fife) were not allowed to put bullets in their guns. Anyway, that was long ago and security around the MCC doors has been turned over to little computer chips in your badge and security guards are rarely seen in the MCC these days.

  5. Mike Fair says:

    Beyond fascinating. For a design engineer like me, this is no-joke riveting, as well as an important example. The lesson would be all the more complete to have insight into how the system came to be this way. What insight or demands did the FD office give when this sequence, and the equipment and software, were being designed. By what process involving requirements, legacy, group consensus, or an individual decisionmaker was the evolution of this system conducted?

    • waynehale says:

      That would be a fair question for a space archeologist. I think a lot of the people involved with the system pre shuttle (or in Mercury, Gemini, and Apollo) have passed away. NASA is terrible about keeping this kind of detailed engineering history. Other than the work on the “new” control center in the mid 90’s I would be of no help in establishing how this complex system came into being. But you can imagine it all started with a requirement to have two ways to alert the crew . . . and got more and more complex over time.

  6. Bill Foster says:

    Great write up from Turquoise Flight. Testing the Abort PBI is still a major sequence in the pre-launch count, now occurring three times, about a day before launch by all three positions that can send the command (Flight Director, Flight Dynamics Officer and Mission Flight Control Officer (MFCO) (who also has a much more devastating button to push if necessary)), about 10 hours before launch by Flight Director, and about 6 hours before launch by the MFCO. With only 2-3 missions left to go, fervently hoping that no one needs to push it during powered flight again. It can also be used during orbit operations as a means of trying to signal the crew during certain comm loss situations, but this is rarely simmed and has never been necessary during a real mission (to date).

  7. Craig Hutchinson says:

    Slightly off-topic, but what was the ejection envelope when the shuttles had ejection seats?

    • waynehale says:

      That was a long time ago but my recollection is that those ejection seats (active for the first 4 shuttle flights where the crew was only 2) were intended to be used late in entry if the shuttle could not make the runway. Basic parameters I remember are subsonic and less than 50,000 ft. But that was a long time ago and you shouldn’t trust my memory on that one.

  8. charlie barber says:

    Oh the memories you brought back for me as a Command Controller (on of the Ground Controllers bathroom staff). I was one of those who in a pinch would have to find the Abort A/B Off commands stored on an 8 inch floppy and send it upon your or another Flight Directors request.On time by your request during a sim I had to send the Abort A and B commands, and the Off commands when Sim Sup had a failure put in both FD and FIDO toggle switches.

    In regards to Mercury, Gemini and Apollo Abort lights, I do believe all 3 spacecraft had them available. I suppose Chris Craft or Gene Kranz would have the insight into their operations. I have the manuals at home, but unfortunately am not there. I did see those indicators in the Gemini 4 Reentry Module the other day at the NASM here in DC.

  9. P. Savio says:

    A Big Button that does nothing much other than light up a light bulb…..interesting

    There is a photo on wikipedia showing the Abort Switch from 51-F in the ATO position

    Off topic – good to see Spacex get up and back from orbit. Maybe NASA can “loan” them the Orion Launch Escape System – and we can get back into US crewed launches not long after the Shuttle is retired.

    • waynehale says:

      The picture you speak of is of the shuttle cockpit and I would expect that to be accurate. The question that I don’t know the answer to is whether or not Orion Flight sent the commands from MCC or not.

      The Orion LES is sized for a much heavier capsule than Dragon. During the Pad Abort 1 test, the Orion capsule could have hit 16 G’s I believe. A lighter capsule would hit higher G loads which would not be good for the crew, and might not be within the structural margin of the Dragon. A Launch Escape System has to be tailored to its capsule. You can’t move it around like putting a old Chevy V-8 in your Prius.

      • P. Savio says:

        Maybe load the LES with less propellant? I vaguely remember reading somewhere the LES thrust could be tailored to a particular version (weight) of the Orion capsule. I’m no engineer – just keen to see US back in crewed flight post Shuttle.

  10. Steve Pemberton says:

    This article was perfect timing for me and icing on the cake. A relative of mine works in the Shuttle Avionics Integration Lab (SAIL), which I got to visit for the first time a couple of weeks ago. I got to sit in the cockpit during a couple of sims, both of which had aborts take place. One of the runs had an early engine failure resulting in RTLS which was really fascinating. The other run had a later engine failure which started out as ATO, but that was followed by another engine failure resulting in changing to AOA. One thing I noticed was that there was no AOA setting on the commander’s rotary abort switch. Unlike the photo referenced in a previous reply, the abort switch in OV-95 (and thus I assume current shuttles) only has positions for RTLS, ATO and TAL. It looked to me like the decision to go AOA was more of a procedure decision as all they did was get to orbit then go through what seemed to be the normal preparations for deorbit burn and landing at KSC.

    Touring SAIL, watching them go through simulated flight (and seeing all of that wiring!) made a deep impression on me and gave me an even greater appreciation for the complexity of what you and others had to do to launch these incredible machines.

    • waynehale says:

      The other thing to note is that the positions on the switch are changed from the early days. RTLS is on one side of “Off” and the other abort modes are on the other side. Selecting RTLS is a one way trip – you can’t change it. I remember the sim where the sim sup had failed the PBI contacts to “ON” and the instant that the crew moved the switch to the first position they were off on an RTLS when they really wanted to go ATO. The next day we started the process to change the switch to avoid that trap. AOA (or AOA-shallow) is currently selected via OMS-1 and -2 targets rather than the abort rotary switch.

  11. Becca says:

    Even though it only turns on a little light, when I was a newbie FDO, pressing one of the only buttons in the MCC was the highlight of my first pre-launch planning shift. 🙂

  12. Iron Flight says:

    Great story Turquoise, and completely true. Having done more prelaunch planning shifts than most, I still get sweaty palms every time I have to do the Abort Light checks – even though I know the commands don’t do anything but light the lightbulbs. The real worry is that there are so many people watching who will see you mess up (if you do). I don’t get nervous talking in front of a large crowd, but I would HATE to be the cause of an IPR that has to be talked about and justified to the Launch Director (and the MMT) and that everyone at all the Centers is aware of. It’s something that, as you say, we nver train for.

    It’s really great to have a good GC who keeps the building safed so that when a book gets left on the keyboard, the thousand (or so) abort commands that result get trapped in Houston.

  13. James Pendergrass says:

    Wayne, thank you so much for creating this blog and for posting such fascinating writeup such as this. I have been a huge fan of the space program since I was a little kid, not that I am that old (36), but write-ups and stories like this make me feel like a little kid again.

    Like a few others have posted, I always thought the abort button on the flight directors console actually did more than turn on a light, but without stories like this I would have never known. Thanks to all of you that have worked on the program for sharing your personal stories of working on such an awesome machine.

  14. David Seidel says:


    This isn’t a comment on this blog entry but since “Wings in Orbit” is due out in February I think there is a companion to it that you may want to share with your blog fans. It is Jeff Hoffman’s and Aaron Cohen’s MIT “Aircraft Systems Engineering” course on the Space Shuttle from the fall of 2005. It is available through iTunes as well as the MIT Open Courseware site. The MIT site includes the related materials but iTunes does not. I’ve only gotten a few lectures in (and haven’t gotten to yours yet) but it is an unprecedented collection of presenters and unedited set of presentations. I communicated with Jeff briefly and he recognizes the once-in-a-lifetime opportunity he and Aaron created. I’m sharing these thoughts with Steve Garber in the NASA History Office and Roger Launius at NASM.

    The MIT direct URL is: http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-885j-aircraft-systems-engineering-fall-2005/index.htm

    Also, in May I’ll be conducting a weekend educator conference here at JPL to celebrate the end of the Shuttle program. I’ll be getting some Rockwell old-timers from the Aerospace Legacy Foundation to participate. I plan to promote “Wings,” the MIT course and your blog as indispensable resources for fans of the Space Shuttle.

    With respect bordering on awe,


    David Seidel
    Manager, Elementary and Secondary Education
    Jet Propulsion Laboratory

    • John Mosier says:

      I just found your website and blog.

      I was involved with the Gemini and Gemini B (MOL) programs in 1965 and 1966. I worked in the Propulsion Department at McDonnell Aircraft in St. Louis. The job in which I had the most involvement was with a computer program that simulated the “popgun” effect that would occur when the retro-rockets were fired in a closed space. The “popgun” effect caused a sudden pressure to build up in the conical adapter shell that had been instantaneously severed at the firing of the retro-rockets. The “popgun” effect would only last for in the order of one tenth of a second, but this simulation was to predict the initial conditions for the aerodynamics calculation of a complete trajectory. I had proposed several tests for validation of the computer model, which was loaded with empirical guesses for some of the most important gas dynamics effects which would be present during the “popgun” process. As I was writing the specifications for these multi-million dollar tests, the Air Force began to cancel them one-by-one. As the tests were cancelled, I began to feel enormous pressure as my computer simulation really needed the tests in order to validate the computer model for the process. I went to my boss, Henry Overall, and asked him “how could management be so stupid to allow the tests to be cancelled when men’s lives were at stake.” He told me that “in a case like this, you can bet that management knows something we don’t know.”

      A few years later, before the Gemini B and the MOL ever flew, the entire program was cancelled. This has been one of the most significant life lessons that I learned.

  15. Charlie Barber says:

    A short summary of initiating Mercury Aborts…

    Mercury Abort Initiation; Abort shall be initiated by application of a 28-volt signal to the “abort junction” in the escape system electrical network. Upon receipt of the 28-volt signal, the 28-volt source shall be instantly “locked in” at this junction and shall provide the necessary power source to initiate the abort sequence, consistent with the flight mode in which the abort maneuver is necessary.
    Mission aborts may be initiated under any of the following conditions:
    a. Prior to capsule umbilical separation, an of-the-pad abort may be initiated from the blockhouse via the blockhouse hardline which energizes the capsule “Mayday relays”. NOTE: Energizing the Mayday Relays initiates an abort sequence in this abort case, and as well the following cases (b. through e.). Additionally, the capsule instrument panel mounted red-colored ABORT light is illuminated when the Mayday relays are energized in order to inform the astronaut that an abort sequence has been initiated.
    b. After capsule umbilical sep, and prior to missile umbilical liftoff signal (2 inches altitude), an abort can be initiated by radio cmd (aka ground cmd abort RF signal), by hard-line which bypasses the missile lockout relay via missile umbilical, or by the astronaut (astronaut utilizes the on-board “Abort Handle” aka ‘Chicken Switch’).
    c. After missile liftoff, but prior to missile umbilical sep, an abort can be initiated by radio cmd, by hard-line via missile umbilical, by the missile Abort Sensing and Implementation System (ASIS), or by the astronaut.
    d. After missile umbilical separation and prior to booster, and prior to booster (BECO) and/or sustainer (SECO) cutoff, an abort can be initiated by radio cmd, ASIS, or by the astronaut.
    e. After booster shutdown and escape tower jettison, but prior SECO, an abort can be initiated by radio cmd, ASIS, or by the astronaut.
    f. After SECO, an abort can be initiated by radio cmd, or by the astronaut.

    After receipt of an Abort signal, the following actions would occur (flight-phase dependent, but never the less, the following illustrates the complexity in the abort execution and sequencing that would occur shortly after an atlas missile liftoff with the booster and sustainer engines running).

    a. Shut-down booster and sustainer engines, fire (pyro)the capsule/Atlas adapter bolts.
    b. Fire escape rocket.
    c. Sense capsule to booster adapter separation, jettison retro rocket package, and jettison retro rocket umbilicals.
    d. Maximum altitude sensor “runs out”, fires tower separation bolts (jettison ring).
    e. Sense tower jettison ring separation, fire tower jettison rocket.
    f. Sense tower jettison through electrical disconnect command, command capsule attitude rate damping, after 3 second time-delay eject antenna fairing and deploy main parachute.
    g. Attitude rate damping stops at main parachute deployment.

    The above is a highly condensed version of what may be found in regards to in the ”Mercury Capsule No.16 Configuration Specification (Mercury-Atlas No.8”, which ultimately became the spacecraft known as Sigma 7 which was piloted by Walter Schirra. This document is available to the public on the Internet at the NASA Technical Reports Server website.

    Next -Gemini Abort…

  16. David Buchner says:

    I truly enjoy these stories. And the comments.

    So I hope you don’t get bored with sharing them, anytime soon.

    (I suppose this relegates me to Very Geeky, but that’s not exactly news)

  17. Charlie Barber says:

    Gemini Aborts… caution – it’s a long one..

    Gemini Abort Initiation:

    In all cases, the Abort has to be initiated by the crew after an Abort Command has been received. An abort indication consists of illumination of the red ABORT indicator lights which are located on the Command Pilot and Pilot control panels. The ABORT indicators are illuminated by three different methods:
    a. During pre-launch prior to launch vehicle umbilical plug disconnect, the ABORT indicator may be illuminated from the blockhouse via hardline via the launch vehicle tail plug.
    b. After umbilical release, both ABORT indicators may be illuminated by ground command (RF) to the spacecraft via a channel of the Digital Command System .
    c. Ground command (RF) to the launch vehicle to shut down the booster or second stage.

    The abort system is comprised of the abort indicators, controls, relays and pyrotechnics. The part of the abort system which the crew uses is determined by the Abort Mode in effect at the time when the Abort Command is received, or the decision to abort is made. The Abort Mode to be used is determined by calculations made on the ground and depends on the altitude and velocity attained by the spacecraft.
    Critical abort altitudes are 15,000, 75,000, and 522,000 ft. Below 15,000 ft., seat ejection (Mode 1) is used. Between 15,000 and 70,000 ft., seat ejection or modified retro rocket abort are available (Mode 1-2); where either Abort Modes 1 or 2 are used at the option of the Command Pilot. Between 75,000 and 522,000 ft., retro rocket abort (Mode 2) is used. Above 522,000 ft., normal reentry (Mode 3) is used.

    Abort Mode 1:
    If an abort becomes necessary during pre-launch, it is accomplished by using Abort Mode 1. The Abort Command is executed from the blockhouse via hardline through the launch vehicle tail plug umbilical. This command illuminates both red-colored ABORT indicators on the Command Pilot and Pilot’s instrument panels. When the pilots see the ABORT indicators illuminate, they immediately pull the ejection seat “D” rings which are mounted on the seat. When either of the “D” rings are pulled, both ejection systems are energized. One half second later, the hatches are pyrotechnically opened, and one half second after that, both seats are ejected from the spacecraft. One quarter second after the seats are ejected, a sustainer rocket under each seat is fired, which extends the distance between the pilots and the launch vehicle by approximately 275 ft in max altitude and 700 ft distance to landing.). The seats are then pyrotechnically separated from the pilots, the main chutes open, and the pilots are lowered safely to the ground.
    After normal liftoff, and before the Gemini-Titan reaches an altitude of 15,000 ft., an abort condition could develop. The crew monitor their booster health and status indicators throughout powered flight (Two ENGINE 1 under pressure indicators, an ATT RATE indicator, and the ABORT indicator must remained extinguished for nominal powered flight to proceed; ENGINE 2 indicator remains extinguished (illuminates AMBER if an engine thrust chamber under pressure condition exists); STAGE 1 FUEL and OXIDIZER vertical gauges must indicate pressures within operating limits, and the LONGITUDINAL ACCELEROMETER gauge must indicate an increasing acceleration within limits vs. the flight time as indicated by the Event Timer. Booster operation is telemetered to the ground for analysis and interpretation. The range safety officer (RSO), the booster systems engineer (BSE), the flight director (FD), or the flight dynamics officer (FIDO) individually can decide that danger is imminent and an abort is mandatory.
    A channel of the Digital Command System is used to send the Abort Command to the spacecraft and ground command tones are separately sent to the launch vehicle to shut down the booster engines. When the engine shutdown command tones are received, the destruct switches of the launch vehicle are also armed. The two ENGINE 1 under pressure indicators and the ABORT indicators illuminate red. The pilots evaluate these displays and pull the “D” rings to initiate ejection from the spacecraft.

    Abort Mode 1-2:
    Abort Mode 1-2 is the modified retro abort mode. It is effective at altitudes between 15,000 and 75,000 ft., which is approximately in the 50 to 100 second time frame after liftoff. Abort Mode 1-2 is used when a Mode 1 Abort is inadvisable and when a delay to permit entry into Mode 2 conditions is impractical. However, the crew has the option to eject or to ride-it-out depending on the assessment of the abort condition(s). Therefore the “D” rings are not stowed during the Abort Mode 1-2 time frame.
    Abort Mode 1-2 begins during stage 1 boost approximately 50 seconds after liftoff. If an abort condition develops, and the crew decides to ‘ride-it-out’, the Command Pilot moves the Abort Control Handle from NORMAL to SHUTDOWN. He waits approximately 5 seconds for booster thrust to decay, then moves the handle from SHUTDOWN to ABORT.
    Through a series of electrical relay and pyro squib actions, the Gemini Equipment Adapter separates from the Retro Adapter. Simultaneously, the four retro rockets are salvo-fired and the spacecraft thrusts away from the launch vehicle. If the abort altitude is between 15,000 and 25,000 ft., the Retro Adapter is jettisoned 7 seconds after the retro rocket salvo fire is initiated. If the abort altitude is between 25,000 and 75,000 ft., the Retro Adapter is jettisoned 45 seconds after the salvo fire.
    After Retro Adapter separation, the spacecraft is maneuvered to re-entry attitude by the nose mounted reaction control system thrusters. If the abort altitude is above 40,000 ft., the drogue chute is deployed at 40,000 ft., and the main parachute is deployed at 10,600 ft. If the drogue chute fails, or has not been deployed before the spacecraft descents to 10,600 ft., the emergency sequence is used to deploy the main chute.
    NOTE: If one of the two first stage engines should fail and the launch vehicle is above 40,000 ft., the pilots may elect to remain with the spacecraft until the operating engine has boosted them to 75,000 ft. At this altitude, Abort Mode 2 would become effective.

    Abort Mode 2:
    Abort Mode 2 becomes effective above 75,000 ft. Approximately 100 seconds after liftoff on a normal mission, the launch vehicle has boosted the spacecraft to an altitude of 75,000 ft. Note that the ground station based computers calculate the time of changeover from Abort Mode 1-2 to Mode 2. The crew is notified via UHF voice of the Abort Mode change. The crew then stows the “D” rings. Note that ejection at the speed the launch vehicle/spacecraft are travelling would most likely be fatal at this point.
    Abort Mode 2 begins during stage 1 boost and ends at second stage cutoff (SSECO). The crew continues to monitor the stage(s) as they would in the previous Abort Modes. Any decision to abort may be theirs or can be requested by ground ops. If the ground sends the command to abort, both ABORT indicators illuminate red. In Abort Mode 2, the Command Pilot must act; he moves the Abort handle to SHUTDOWN. The operating stage engine(s) are cutoff. He will then immediately move to Abort Control Handle to ABORT. The spacecraft is separated from the launch vehicle as it is in Abort Mode 1-2. Since orbital velocity is not reached below 522,000 ft., the Reentry Module immediately begins a reentry trajectory. The spacecraft is maneuvered to retro (blunt end fwd) attitude, the Retro Adapter is jettisoned, and nominal landing procedures are initiated.

    Abort Mode 3:
    At approximately 310 seconds after liftoff, the second stage reaches the altitude of 522,000 ft., and a velocity of 21,000 feet/sec. The ground station will call the crew via UHF to inform them of the Abort Mode change from Mode 2 to Mode 3.
    If an abort after this time becomes necessary, via ground command the ABORT indicators would illuminate red. The Command Pilot responds and moves the Abort Control Handle to the SHUTDOWN position. The second stage engine shuts down as a result of this action. The Abort Control Handle remains in the SHUTDOWN position. The Command Pilot then depresses the SEP SPCFT (Separate Spacecraft) telelight/switch on the Sequence Panel. This switch fires shaped charges and separates the complete Gemini spacecraft assembly from the second stage. Two 85 lb. OAMS (Orbital Attitude Maneuvering System) thrusters maneuver the spacecraft away from the second stage. The Adapter Section is separated, followed by the nominal pre-reentry retrorocket firing sequence (each of the 4 retro rockets are fired separately in the nominal sequence) followed by reentry and nominal landing procedure execution.

    The above was referenced from the Gemini Familiarization Manual (SEDR 300 Vol 1), Section 4 (Sequence System).

  18. Charlie Barber says:

    Apollo Abort Systems Summary:
    A. Emergency Detection System

    The Emergency Detection System (EDS) is designed to detect and display emergency conditions of the launch vehicle (Saturn -1b or Saturn-V) to the astronauts. The EDS also provides automatic abort initiation, under certain conditions, between liftoff and T+90 seconds from launch. The display circuitry and automatic abort capabilities are enabled at liftoff. A lockout system provided to prevent enabling the automatic abort circuitry prior to liftoff. The Commander may initiate an abort manually at any time with the Commanders translation control device.

    B. Automatic Abort

    The EDS will initiate an automatic abort signal after launch by sensing excessive rates or engine out conditions. The abort signal will cause booster cutoff, event timer reset (provides a time reference for manual descent operations), and launch escape system (LES) activation.

    C. Manual Abort

    A manual abort can be initiated prior to, or during launch by manual counter clockwise rotation of the Commanders translation control. The launch escape system can be utilized until approximately normal tower jettison. During a normal mission, the LES tower is jettisoned shortly after second stage ignition (approx 280,000 ft. for Saturn-1b, or 320,000 ft. for Saturn-V), and any abort thereafter is accomplished by utilizing the Service Propulsion System (SPS) in the Service Module (SM). An SPS abort must be manually initiated. Upon abort initiation, the booster automatically separates from the spacecraft, the SM reaction control system (RCS)thrusters fire to accomplish the SPS ullage (propellant settling)maneuver, and the SPS engine ignites to thrust the spacecraft away from the booster.

    D. Abort Request Indicator

    The ABORT request indicator light is illuminated by ground control (the decision to initiate ABORT request command may be made by the Launch Operations Manager, the spacecraft communicator (an astronaut) in the Launch Control Center, the Spacecraft Test Conductor, and the Houston Flight Director), or the Range Safety Officer (RSO) after liftoff, using Ground Support Equipment (GSE) or a radio command via the command uplink. When illuminated, the ABORT indicator indicates an Abort request and serves to alert the crew of an emergency situation.


    Abort Mode Definitions/Summaries:
    1. Pad Abort:

    T-51 minutes; Both of the high speed elevators within the mobile launch tower are parked at the 320 foot level. The Command Module (CM) access arm, Swing Arm 9, which is also located at the 320 foot level, is moved back to the retract position. This is a 12-degree standby position. From this position, it can be quickly returned to the spacecraft, if needed, and remains at this parked position until T-5 minutes to launch. At T-minus 5 minutes, it swings back to the full retract position. The astronaut crew aboard the spacecraft, in an emergency situation, could use their Launch Escape Tower to clear themselves well away from the spacecraft. The CM and the launch escape system (LES) separate from the Service Module, with the LES propelling itself and the CM beneath it upward and eastward to the sea using a small solid-fueled engine (the pitch control motor) at the top of the tower on the launch escape system. The launch escape tower would then be jettisoned in anticipation of the parachute deployment and the CM would splash down. In addition, hey also have the option where the swing arm, at the 12-degree position, is called back where they could quickly exit the spacecraft, then go across the swing arm, again having an option of either taking an elevator to safety at the bottom of the pad, or a slide wire which has a cab attached to it which would carry them to the pad perimeter. These would be decisions depending on the type of emergency.
    T-5 minutes; Five minutes before launch, Swing Arm 9 retracts and swings clear of the rocket. If an anomalous condition occurs within the last five minutes before launch, the CM and the launch escape system (LES) separate from the Service Module with the LES propelling itself and the CM beneath it upward and eastward to the sea..
    2. Aborts after Launch; Throughout the powered ascent of the launch vehicle, there are various modes of aborting the mission, each of which are appropriate to the current height and speed.

    a. Mode IA (One Alpha):
    The initial 42 seconds, to an altitude of about 10,000 ft. are flown in Abort Mode IA (one alpha). If a dangerous situation occurs within this period, the CM would separate from the SM, and the LET (Launch Escape Tower,) would carry the CM up from the wayward launch vehicle while a small ‘pitch control’ motor at the top of the LET steers the assembly east out over the ocean and away from a possibly exploding booster below. The main rocket motor of the LES, if used, would burn for eight seconds, generating 147,000 pounds of thrust through four nozzles which were angled to direct the exhaust away from the CM. The tower would be jettisoned only 14 seconds after the initiation of the abort. While this was going on, the highly dangerous hypergolic propellants of the Command Module’s RCS would quickly and automatically be dumped overboard as they would be harmful to the recovery forces. The CM would then descend on parachutes to a normal splashdown.

    b. Mode IB (One Bravo)
    Abort Mode IB extends from 42 seconds into the flight to an altitude of 16.5 nautical miles- as defined by the abort checklist. With the vehicle being further downrange and tilted over, the pitch control motor would not be required in the event of a Mode IB abort. However, it had been discovered during hypersonic testing, that the CM/LET stack could be aerodynamically stable in a tower-first as well as a base-first attitude so a pair of canards were added which would be deployed automatically to force the combination into an attitude where the base of the CM is facing the direction of travel, ready for the safe deployment of the drogue and main parachutes. While the canards have little effect in a low altitude abort, they become increasingly important as the Saturn V gains speed through the IB mode.
    For the first two of these Modes, IA and IB, the Flight Plan defined the safe range of vehicle motion rates as not exceeding ±4° per second in pitch and yaw, ±20° per second in roll. Motion rates exceeding these limits would have entailed an abort.

    c. Mode IC (One Charlie)
    Mode IC was used for aborts occurring between 16.5 nautical miles and the jettison of the tower. As the air is now very thin, the airflow around the pair of canards at the top of the tower would have little aerodynamic effect during an abort, so the Command Module’s RCS would be used to control the orientation of the spacecraft until they become effective. The safe range of vehicle motion rates are now defined as not exceeding ±9° per second in pitch and yaw, ±20° per second in roll. During One-Charlie, the first staging occurs, that is the jettisoning of the spent S-1C (first stage) and ignition of the S-II (second stage). One-Charlie ceases about 30 seconds after the staging when the LES is jettisoned. As the S-IC nears the end of its burn, the crew inhibits the EDS with a switch directly below the computer keypad. The EDS is only needed for flight through the thickest part of the atmosphere where high aerodynamic forces and the structural load they impart to the vehicle could cause loss of control to turn catastrophic too quickly for the crew to react in time. With EDS switched off, any required aborts must be initiated by the crew, giving them more control.

    d. Mode II – tower jettison T+3 min 20 sec
    The first action that occurs with the start of Mode 2 is LES jettison. A single, small, solid-propellant motor near the top of the tower fires for one second, jettisoning the entire LES and the checklist the crew uses moves to abort Mode II. As with most Apollo systems, the crew could manually command the LES jettison if the automatic system failed. Abort Mode II lasts from the jettisoning of the tower to the decision to stage from the S-II to the S-IVB. In a Mode II abort, the Command and the Service Modules will separate from the launch vehicle and the SM main engine or its RCS engines will be used to get the spacecraft away from the launch vehicle. Then the CM and SM will separate before the CM completes a normal splashdown on the ocean.

    During this time, the crew prepares the Service Propulsion System (SPS) engine for use in case it is needed during an abort. The SPS would only be needed in the event of a Mode III or Mode IV abort after T+05:52. In a Mode III abort, the SPS would be used to correct the spacecraft trajectory to achieve the desired landing site in the Atlantic. In a Mode IV abort, the SPS would be used to take the spacecraft to orbit. In each case the SM Reaction Control System (RCS) would first be used to separate the CSM from a wayward S-IVB. There is no possibility to use a fire-in-the-hole maneuver given the proximity of the SPS to the fully loaded LM propellant tanks.]
    When the SPS is firing, the pitch and yaw attitude of the spacecraft must be controlled by positioning the gimbals that control the direction that the massive SPS engine is pointed in relative to the Service Module – the thrust vector. This can either be done automatically by the Command Module Computer (CMC), or manually by the Command Module Pilot (CMP) using the Rotational Controller. This latter is termed Manual Thrust Vector Control (MTVC), and this mode would be used in an abort since it provides more direct control to the pilot. However, the thrust vector must point through the spacecraft’s center of gravity or the spacecraft would start to tumble during any SPS burn. Under Rate command, any tendency to tumble is automatically corrected using signals from the spacecraft gyros. Under the alternative, termed Acceleration Command, the pilot would make the necessary corrections himself. Hence, Rate Command would make it easier to fly the spacecraft in what would be a very challenging situation.

    e. Mode III -aka COI (approx T+ 6 min)
    COI stands for Contingency Orbit Insertion. This is another way of saying “Abort Mode III”. The S-IVB would now have the capability to take the spacecraft to a point where the Service Module’s large SPS engine can ignite and place the CSM into Earth orbit. However, in the event of such an abort, and without the S-IVB, the spacecraft would not be able to depart for the Moon, instead embarking on a planned for, but hopefully unrequired Earth orbit mission.

    f. Mode IV (approx T +9 min 15 sec; S-II cutoff, SIV-b ignition)
    Mode IV was the abort mode where the crew were given a Go decision to continue to orbit using the S-IVB, and should that stage deviate from its allowed limits, the CSM would separate from the Saturn and use the SPS (Service Propulsion System) to continue into Earth orbit.

    The above was referenced and edited from the Apollo 16 Flight Journal and Apollo CSM Sys Handbook

    I hope this all wasn’t too much, but it illustrates the complexity and variations that launch aborts entail… Barely touches the surface….


    • Vaughn says:

      Thank you so very much for the information on abort modes. I’ve been looking for this kind of detailed information for years.

      I was a true space junkie growing up in the 60s and 70s.

      Again, I appreciate your filling in a lot of the details.

  19. Jack Knight says:

    Comments on Mike Fair’s question regarding design considerations in the MCC.

    The MCC evolved over time, as all such things do. I was told that during the Gemini-Agena period, an MCC command buffer, due to some sort of software glitch, dumped the entire Titan command set to the Cape during a ground test. Fortunately, the Cape’s system had the link to the Titan disabled so the commands did nothing. However, it scared folks. This led to re-evaluating the command systems design to minimize any chance of inadvertent commands being issued from the MCC, yet ensure that it worked when you needed it.

    For example, except for the Abort Command function which was unique, software code and a button was added to require enabling a command panel before its command buttons would issue commands. Code was added to isolate certain commands during certain mission phases unless specifically requested to be enabled. Over time, other software protections were built in, especially for Shuttle.

    The reason for the Abort Command A and B was reliability; if one didn’t work, the other one would. The lever-lock feature, which also existed in the cockpits for several critical switches, was to prevent an unintended bump to cause a command to be issued. The Capcom call plus the Abort light in the cockpit was to insure that an Abort was really called for (not just a garbled voice or some sort of sabotage effort).

    The MCC was responsible after tower clear on Apollo and Gemini and after SRB ignition for Shuttle, so its links to the spacecraft had to have an RF element in it. Getting a signal from the MCC may go through several paths including hardline, microwave, fiber-optics, etc., but at the end, it had to go via RF to get to the flying vehicle. Testing during the pre-launch period insured the link(s) were working. Reliability was paramount and redundant paths or other mechanisms were employed.

    Except possibly for Mercury, the only people who had vehicle destruct capability were the Range Safety group, who were independent of either the LCC or the MCC. They had their own criteria for destruct (coordinated with the MCC team for each flight) and the RF links to execute it. Needless to say, the codes were highly classified and protected from potential spoofing.

    Jack Knight

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