Your Questions about Apollo 13 Answered by Jerry Woodfill

Article written: 6 May , 2010
Updated: 24 Dec , 2015

Now that our series on “13 Things That Saved Apollo 13” is complete, NASA engineer Jerry Woodfill has graciously agreed to answer questions from our readers. We have a lot of questions, so we will post some of Jerry’s answers today and more over the next few days.

Question from Daniel Roy: Did we ever find out why Apollo 13’s trajectory was too shallow on the way back in spite of TCMs? I have trouble believing that the low impulse/ slow venting/ random pointing from ruptured tanks could explain the delta V.

Jerry Woodfill: The shallowing trajectory resulted from the lunar lander’s cooling system discharging vapor during the coast back to Earth. It was not a result of residual release of remnant gases from service module damage. No Apollo mission returned to Earth with a LM attached except for Apollo 13. For that reason the slight but, nevertheless, noticed contribution to the shallowing entry angle had to be dealt with by the Apollo 13 retro. To this day, I find it remarkable that, though the retro did not know the source of the shallowing, he was certain it would cease after the last corrective compensating burn. And, of course it did, after the LEM was jettisoned.

Question from wjwbudro about how much residual power was provided by the fuel cells after the explosion

Jerry Woodfill: Your question about how much residual power the fuel cells contributed prior to employing the emergency (or some call them reenty batteries) launched me into some research about the chemistry of fuel cell operation. I’ve always shared that the reaction of hydrogen and oxygen produce electricity with two by-products extremely useful to human space exploration, breathable oxygen and water. Both oxygen and hydrogen must be present for the reaction to continue.

For Apollo 13, the sequence of the loss of the ability of the fuel cells to produce power relates to the loss of O2 and H2 entering them. Sy Liebergot has a wonderful CDROM where he deals with “how the data read.” Sy had to contend with analyzing what was going on (IN REAL TIME) with regard to the timing of loss of the O2 cryo-tanks, the fuel cells, etc. Google Sy on the Internet, and you’ll find a wealth of information discussing the issue. My admiration of how Sy dealt with such an overwhelming failure so masterfully continues 40 years after the event. But the bottom line is…no O2 into the cells no water, oxygen, or electrical power out. That was the reason for employing the emergency batteries. The fuel cells weren’t much help after because the rupture of the plumbing caused O2 tank One’s O2 to vent into space after O2 tank 2 exploded (I always say “exploded” though some disagree contending it to be a rapid heating of cryogenic O2 being vented into space, sort of like heating air in an empty sealed container until the vessel ruptures.)

Question from science teacher Christopher Becke from Warhill High School: What were the specs of the onboard computers, both in the LM and the Command Module? What was the clock speed and how much (and what type of) memory did they have? I’m trying to impress upon my students that their graphing calculators are more powerful than the computers that brought astronauts to the moon.

Jerry Woodfill: About a year ago, I felt like comparing Apollo 13’s computer to today’s state of the art. Besides the computers (CSM and LM), the only integrated circuit contained among the millions of spacecraft parts was an octal counter in my lunar lander’s caution and warning system’s brain known as the Caution and Warning Electronic Assembly or C&WEA for short. There was an excellent article I discovered at this link from the Download Squad.

Additionally, a wealth of information is given in the Apollo Experience Report which can be accessed at this link.

These documents are a national treasure for recreating the technical history of Apollo. I authored the warning system portion of the Apollo Experience Report on the lunar lander’s Caution and Warning System.

I recall that the strength of the Apollo computer, though it was a “lightweight” in RAM and Hard-Memory, was its “multi-tasking” ability. (Better than an iPhone, since Apple chose not to include that capability presently in mine.) However, when my warning system began to ring “Program Alarms,” (warnings, five of them to be exact) this multitasking capability proved altogether helpful in making Armstrong the first man on the Moon.

One of the Apollo Computer’s “subtasks” was akin to a kind of low level housekeeping info thing which generated an alarm. But the priority executive routine of providing landing control continued undisturbed. Ignoring the program alarms by Flight Controllers Steve Bales and John Garman was a huge reason Neil Armstrong was first on the Moon, that President Kennedy’s prediction and challenge was fulfilled in that decade, and, most importantly, for me…that I didn’t go down in engineering/aerospace infamy whose warning system sounded a “false-alarm” making Pete Conrad and Allan Bean the first men on the Moon on Apollo 12. Thanks Steve and John!

Question from Greg: Should NASA be spending more time reviewing the Apollo 13 mission and other mishaps in order to better anticipate and respond more effectively to new and unexpected mishaps in future missions?

Jerry Woodfill: The neat thing about every one of these questions is they launch potential investigations which can only help future space travelers. Whether it was Apollo One, Apollo 13, Challenger or Columbia, each tragedy resulted in fixing a later situation which might have been fatal if corrective steps had not been taken to learn from failure. This question is one that I’ve addressed extensively in unpublished books I’ve authored.

Now, regarding failure to fix potentially fatal items; yes, over the course of my 45 year career, it is easy to reflect and study failures after the fact and cite instances where people, groups, circumstances resulted in disaster and tragedy. I’m one of those guilty people. I should have done a better job with regard to the Apollo One warning system. Collectively, and, perhaps, individually, we share the burden of not having done a better job for Gus, Roger, and Ed.

Specifically, I remember the final review at North American of Spacecraft 012 where Ed, Gus, and Roger sat at the front of the conference room. They were included with a NASA review panel determining how to disposition “open items” or “squawks” needing fixing before or after shipment of their Apollo One spacecraft to the Cape.

My warning system was a problem for me because it became sort of the “wolf crying boy” who is always the one to aggravate those who want to ignore a root problem blaming it on the messenger. During the initial factory tests of this, the first of the litter of subsequent Apollo Command modules, there were dozens of times the alarm system sounded Master Alarms.

In summary, virtually none were the fault of the alarm system. But, nevertheless, it was blamed until I could find the actual culprit. Some said, “The electronics are simply too sensitive ringing alarms when all that has happened is a momentary switch actuation causing a brief electrical transient which triggers that Master Alarm.”

After dealing with all the culprits, I had only one unexplained alarm remaining. This was the one I was called to present to the board which included Ed, Gus and Roger. “Next item, O2 FLOW unexplained Caution and Warning Alarm.” It was July of 1966. My wife Betty and I had been married less than a month, and here I was dealing with a life-threatening situation.

To digress here, I think the movie APOLLO 13 would have been better served with this event as the opening scene because all the players in the Apollo program were involved. I remember Apollo 7 crewman Walt Cunningham, one of the Apollo One back-up astronauts along with Wally Schirra and Donn Eisele, rooting around in the Spacecraft 012 mockup. Walt emerged with some kind of handle he had accidentally severed from the ship’s interior. Amazed and disgusted, Walt held it up for all to see. Perhaps, that was a precursor for what was to follow?

My explanation was that the O2 Hi alarm was another of those momentary transient things. I shared that nonthreatening events like a routine turning on of the cyclic accumulator demanded added O2 flow into the cabin actuating the alarm. In fact, in route to the Moon, even a urine-dump would cause the O2 flow to increase ringing the alarm. (Later, that was one of my jobs, to indicate in Apollo 11’s check-list that an O2 Hi master alarm could be expected for that reason.) If it was a problem, it would surface once more during Cape testing and be dealt with then. My assessment was accepted by the board.

On January 27th, 1967, Ed, Gus, and Roger were hours into what was called a “plugs-out” test simulating a voyage to the Moon. Suddenly came the call, “We’ve got a fire in here!” In seconds three men perished. When Deke Slayton arrived later and surveyed the interior of Spacecraft 012, he looked up at the alarm panel. The O2 flow hi light was still on. Likely, the ECS (Environmental Control System) should have called for the high flow of Oxygen feeding the fire, but I will never know if it came on before the fire to warn the astronauts to take action. So that is why I cannot “white-wash” this question because it is simply these kinds of events that result in the failures we have experienced over the course of human space flight. Whenever one happens, it is because of people like me who should have done a better job.

Question from Dirk Alan: My question is about the free return trajectory. After rounding the moon, could a spacecraft head back to earth – travel round the earth and head back to the moon? Could it round the moon and head back to earth again and again ? I’m asking if a space station would be feasible in a circumlunar orbit re-supplied now and again with fuel for course corrections to shuttle between the earth and moon?

Jerry Woodfill: The short answer is yes to all of the above. For Apollo 13, the free return trajectory has been much discussed. I’ve often reflected about it, as well. In fact, the first consideration in the rescue was to return to the free return trajectory after the explosion. (BTW, I think I erred in my No. 12 submittal of the “13 Things..” in suggesting that a lander-less-Apollo 13 would have resulted in cremating the crew days later if the explosion had occurred in the circumstance at 55 hours 54 minutes 54 seconds. They were not in the free return mode at that time having departed from it by an earlier burn.)

In actuality, the crew, shortly after the explosion, used the lander’s descent engine to return to free-return. Recently, in conjunction with Apollo 13’s 40th anniversary, added study has been done. The investigation sought to determine how close Apollo 13 would have come to Earth based on its free-return orbit. Here is the link to a YouTube video summarizing the effort. It’s really neat!

Hey, I just listened once more and watched this again. Apparently, I was right predicting the crew without the lander would have been cremated after all, five weeks later in May of 1970. Don’t ascribe this to any talent I have. It’s just lucky. But watching the video will do much to answer every question you have above about space stations, etc. You might Google other terms like Hohmann Transfer Orbit, Aldrin Cycler Orbit, Libration Points, and Sling-Shot orbits. These are strategies in orbital mechanics considered when planning planetary exploration, manned and unmanned.

Questions from Gadi Eidelheit, Quasy and Tom Nicolaides about the Hatch That Would Not Close

Jerry Woodfill: I’ve shared the account of “the hatch that would not close” virtually every time I’ve shared the Apollo 13 story. ( This is approaching a 1000 talks. Do the math. Simply telling the story once a month for nearly 40 years adds up to nearly 500 times.) One man believed the inability to make the hatch close resulted from differential pressure between the vehicles. I tend to discount that because the hatch had been open for some time stabilizing the interior atmospheric pressure throughout the assemblage.

Others who have considered the problem, think that Jack Swigert and Jim Lovell’s belief that a meteor had punctured the LM caused Jack and Jim’s hasty efforts to be flawed and inexact. The misalignment in the hurried closing was responsible. This was addressed in one of the crew debriefs I reviewed several years ago.

Now, I just had the thought, “The Apollo 13 capsule is available at the Kansas Cosmosphere.” To my knowledge, no one since the rescue has actually tried to reproduce the hatch closing problem. But, again, I simply don’t know if that has been the case. (As we press on, I’m going to be honest about what I know and don’t know. This is one of those things I really can’t answer satisfactorily.)

From Hans-Peter Dollhopf: Question about Why an Apollo 13 Movie and not an Apollo 11 Movie:

Jerry Woodfill: Another question I wanted to address among those left at the close of each of the “13 Things…” articles concerns why a movie was made about Apollo 13 and not about Apollo 11. My thought is because of the circumstance of how the movie came into production. I have a close friend named Jerry Bostick. Jerry was the lead FIDO for Apollo 13. We knew one another through the local Methodist Church, too. Jerry’s son Mike was in one of the Sunday school class sessions I taught.

Well, Mike went on to work for Ron Howard as a producer for Universal Studios. Being familiar with the Apollo 13 rescue because his dad, Jerry Bostick, had played a key role, Mike suggested to Ron Howard that Universal buy the rights to Jim Lovell’s book LOST MOON, for a movie. Incidentally, Jerry Bostick is the source of the quote, “Failure is not an option.”

Google Jerry Bostick’s name, and you’ll be able to read the story. Now had Neil Armstrong’s child worked for Ron Howard, and, if Neil had written a book focused on Apollo 11, it might have competed for an academy award like Apollo 13. Incidentally, there are moments in Apollo 11’s mission just as perilous and potentially fatal as the Apollo 11 mission. Perhaps, Nancy will let me address them in another Universe Today series! I can count a half dozen so it won’t be “11 Things That Saved Apollo 11.”

Question: Didn’t the Soviets Plan also use LOR?

Jerry Woodfill: About the Soviet Direct Ascent approach. Prior to the dismantling of the “iron curtain” and the cooling of the “Cold War”, information about Soviet Manned Space endeavors was sketchy. I found, in 1977, that a Soviet rocket scientist had proposed a lunar orbit rendezvous technique in the early days of rocketry, even before Sputnik. Unfortunately, or fortunately, with regard to America’s efforts, his approach was not accepted initially. Earliest Soviet approaches, like America’s, tended toward the Direct Ascent scheme. Probably the same debate ongoing with American lunar planners existed in the Soviet Union.

The simplicity of a single vehicle based on a NOVA class booster led at the onset. Ultimately, perhaps, as Soviets studied America’s choice of LOR, and its LEM offspring, an approach similar to America’s was pursued. Nevertheless, the ultimate Soviet booster N-1 was much more powerful than the Saturn V. (10,000,000 pounds of first stage thrust versus approximately, 7,500,000.)

I was altogether astounded to discover the evolution of the Soviet approach when sketches, and even videos, were released with the collapse of the Soviet Union and its posture of manned space secrecy. But, I still contend, that the early focused efforts by NASA championed by Dr. Houbolt on the LOR lunar architecture won out over, I believe, tardy acceptance by the same in the Soviet Union. One of the finest compliments one receives is the adoption of a competitor’s approach. Simply comparing BURAN to the Space Shuttle tends to make this case as well.

Check back tomorrow for more answers from NASA engineer Jerry Woodfill.

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12 Responses

  1. Greg says

    Many Thanks for your response. Once again you provided more insight and interesting information than I could have hoped for. I certainly can see how difficult it can be to see problems in advance of them cascading into mishaps and also how easy it is to spot in retrospect.
    What I hope to see is something akin to what some big corporations do to test their internet security: hire expert hackers to try to beat the system. In other words, I was thinking I would like to see NASA form a separate think-tank of engineers (call them the “devil’s advocates”) who’s specific purpose is to find flaws and project how these might cause mishaps so that corrective and preventitive action can be taken to preempt mishaps.
    For example just the fact that the shuttle was strapped to the side of and not the top of it’s fuel tank should have prompted the creation of an early ejection and recovery system for the crew cabin during the design stage.
    The other thing that I thought of in brainstorming is the concept of having redundant materials on board so that failed or failing systems can be repaired on site. For the Apollo 13 mission it could have meant having spare batteries and C02 scrubbers on board so that the lunar module would not be needed if either of these failed.

  2. wjwbudro says

    Thank you Mr. Woodfill for taking your valuable time to answer our questions. You have given me enough information and links to reference to keep me busy for quite awhile. The link to “how powerful was the Apollo 11 computer” brought back a few memories for me. I joined Control Data after my hitch in the navy in the early sixties and I remember well the the programming tricks I had to incorporate in order to overcome those limitations.

  3. wjwbudro says

    Forgot to thank you as well Nancy for coordinating all this. I have been with U.T. since Fraser was a pup (lol) and thoroughly enjoy the reviews and articles you all provide for us.

  4. ka9q says

    I don’t think Jerry understood wjwbudro’s question. Although O2 tank 2 was lost immediately in the explosion, it took several hours for O2 tank 1 to lose pressure and it kept fuel cell 2 going during that time. (Fuel cells 1 and 3 were lost almost immediately because, unknown to anyone at the time, the shock of the explosion had closed their oxygen reactant valves.)

    It wasn’t enough to support the entire CSM load, which is why one of the entry batteries was partially drained, but the battery situation would have been MUCH worse had both O2 tanks and all three fuel cells been instantly lost in the explosion. In fact, EECOM even managed to partly recharge one of the entry batteries before O2 tank 1 pressure fell too low to keep the last fuel cell going.

    I’d list this as one of the lucky things that kept Apollo 13 from being a tragedy.

  5. wjwbudro says

    Thank you ka9q,
    I followed the Apollo drama intently back then and although my memory is fading quicker these days, that stuck in the back of my mind. Where did you find this info? I Googled for hours and although I may have missed it, I couldn’t find anything in any of the archives. And your right, it extended their life support and should be listed as one of the “lucky” things that helped save the crew.

  6. ka9q says

    This is documented in great detail in the Cortwright Commission report, available here among other places:

    Also, Sy Liebergot’s autobiography Apollo EECOM: Journey of a Lifetime discusses the immediate aftermath of the explosion in great detail. His book includes a CDROM with audio recordings the controller voice loops during the Apollo 13 and 15 missions. The Apollo 13 recordings, which are absolutely fascinating, start a few minutes before the explosion and continue for several hours. During the first two hours they talk more or less continuously about the remaining pressure in O2 tank 1 and the load on fuel cell 2, the only one still operating after the explosion because the shock closed the reactant valves to fuel cells 1 and 3. They didn’t know this at the time because of the way the indicator circuits were wired.

    I don’t know the numbers offhand, but it shouldn’t be too hard to compute the number of amp-hours produced by FC2 after the explosion until the pressure in tank 1 decayed too low for it to continue operating. Had tank 1 emptied immediately along with tank 2 in the explosion, that many more amp-hours would have had to come from one of the entry batteries.

    The crew and MOCR were so good at conserving the batteries in the LM that they had enough of a reserve to recharge the CM entry batteries by feeding power back from the LM to the CSM over the umbilical that normally carried power from the CSM to the LM during translunar cruise. Had all three fuel cells died immediately in the explosion, I don’t know if it would have been possible to top off the entry batteries from the LM before entry; this would be interesting to figure out.

  7. ka9q says

    By the way, one of the many changes made after Apollo 13 was the addition of a 400 amp-hour silver-zinc battery to the service module. The lunar module had four of these batteries in its descent stage (five starting with Apollo 15).

    Even if this spare battery had been available during the Apollo 13 emergency it wouldn’t have been nearly enough to get the CM back to earth without the fuel cells. But it would have provided a “cushion” to keep the CM alive for quite a few hours while the problem was being worked on. Firing up the LM wouldn’t have been quite so time-critical, and it wouldn’t have been necessary to discharge the CM entry batteries and then figure out how to recharge them afterwards.

    As an aside, the Apollo CSMs used to ferry crews to the Skylab space station carried several more of these aux batteries. The fuel cells shut down after a couple of weeks docked at the station because the service module couldn’t store enough hydrogen and oxygen to last through each stay, so the CSM was entirely dependent on these batteries for the return flight from Skylab.

  8. wjwbudro says

    I thank you for taking your time to provide this information.
    Apparently I didn’t enter the right search phrases. I am now looking through transcripts.
    From your posts, it sounds like you were more than just and interested party, yes? lol and thanks once again.

  9. wjwbudro says

    I thank you for taking your time to provide this information.
    Apparently I didn’t enter the right search phrases. I am now looking through com transcripts. What a ride they had.
    From your posts, it sounds like you were more than just and interested party, yes? lol and thanks once again.

  10. wjwbudro says

    that was supposed to read; an interested party. What chair did you occupy? lol

  11. wjwbudro says

    While reading the mission comm transcripts, a thought crossed my mind; odd but, maybe interesting for anyone that might still be watching this thread.
    Would it not be probable that at least some of the dumped waste product moving along the spacecraft trajectory would be gravitational attracted by the moon and end up on its surface? Not good for future bio search missions if so.

  12. wjwbudro says

    I forgot to add that would be the closest dump of course.

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