THE STIR THAT SAVED THE LIVES OF APOLLO 13's CREW

by Jerry Woodfill

Former Apollo 13 Warning System Engineer


Though the Apollo 13 rescue was an event more than three decades past, frequent revelations of God's care and keeping of her crew surfaces. For example: A recent improved internet search program called "Google" ferreted out thousands of references to Apollo 13. Simply instructing the program to find all web pages with the name "Apollo 13" yielded a count of 110,000. That the two I had authored, one for NASA, the other for a Christian testimony called WHAT REALLY HAPPENED TO APOLLO 13 numbered as the 28th and 31st of more than a 110,000 was a miracle in itself. After examining nearly a thousand of those pages beyond the 28th and 31st, I tired of the exercise though a number of unique testimonies of God's hand in the rescue resulted.

Because the WHAT REALLY HAPPENED page was among the first listed, I chose to enter my name in the Google search field. The hope was to find those sites who had commented on the testimony. I was pleased to find 110 pages whose content contained the name "Jerry Woodfill." Among these was an obscure usenet debate about my views about the Apollo 13 rescue. Apparently, one of the debaters had cited me and the WHAT REALLY HAPPENED TO APOLLO 13 page as evidence for his case. He spoke of the warning system I managed as "the first indication of the Apollo 13 explosion." He referenced the account I gave about the optimum time of the explosion as evidence for his case (whatever his case was). A terse retort from his opposition was in summary, "Though Jerry Woodfill was among the first to know of the problem, he was certainly no expert concerning when the disaster happened. There were hundreds who knew more about the specific systems involved than he did." Then was added the comment, "the explosion happened when it happened because the liquid oxygen simply stratified at that time requiring powering of the flawed O2 Tank heater which ignited the oxygen."

These comments encouraged deeper examination of "Why Apollo 13 exploded so fortuitously?" My contention has always been that Apollo 13 might have exploded on the launch pad. But a worse time would certainly have been when the Lunar Lander had departed for the Moon's surface. Without the lunar lander lifeboat option, no rescue would have been possible. In the former case, not only Apollo 13, but the entire launch pad, including the Saturn 5 booster, would have been incinerated.

I would agree that hundreds (in fact tens of thousands) of those who worked on Apollo had more intimate knowledge of various systems than I had or have. However, none (to my knowledge) have spent more than three decades reconstructing and examining the workings of the engineering, designs, and circumstances that dealt with the rescue of Jim Lovell. My efforts have specifically focused on the spiritual perspective, i.e., God's hand of providence and His answers to world-wide prayer. Additionally, as the systems' manager for the alarm system, my expertise focused on how various components worked together to avoid just the kind of disaster which nearly killed the Apollo 13 crew. In this sense, my research has been unique and qualifies me as an "expert."

So that, there were, perhaps, only a handful of such experts, and as the responsible Caution and Warning Project Engineer, I was, in all humility, an expert concerning such malfunctions aboard the command module and its lunar lander. My "credentials" included the preparation of the actual check list of alarms Neil Armstrong, Buzz Aldrin, Jim Lovell and others could expect as they journeyed to the Moon. To that end, I authored the official NASA Apollo Experience Report concerning the design of the Lander's Caution and Warning System.

But enough of such "bragging." My purpose is a defense of God's providence as having caused the explosion to occur at the optimum moment. There is a scripture in the Old Testament which promises just this type of divine intervention for those in peril on land or sea, in air or space. Isaiah 65:24 affirms that "before they call, I will answer; and while they are yet speaking, I will hear." Before that famous call of "Houston, We've had a problem," before anyone was praying for the rescue, God was answering through a mechanism which assured the tank would explode about 200,000 miles from Earth. The strategy God used is so very logical and premeditated that it is certain evidence of His hand in the ultimate rescue of Apollo 13.

I am not alone in the contention that the detonation time of O2 Tank 2 at approximately 55:55 g.e.t. (ground elapsed time since launch of Apollo 13) was the most important reason for the successful rescue. Apollo 13's commander Jim Lovell voiced the same belief when questioned: "Aside from the explosion, what was the most important moment during the flight of Apollo 13?" Summed up Lovell's response was: "When it happened...much earlier or later in the mission would have prevented a successful rescue." Examination of why it happened when it happened demonstrates conclusively that God scripted the moment far more artfully than Director Ron Howard's screen writers who were nominated for APOLLO 13's academy award.

As an engineer, the study of cause and effect relationships has always fascinated me. Apollo 13's flawed O2 tank 2 is such a case study. Because the tank had been dropped, and because its heater design had not been updated for 65 volt operation, the tank was a virtual bomb. The tank's vent tube was jarred out of alignment by being dropped five years before the flight of Apollo 13. When Apollo 13 had completed a launch pad test weeks before launch using full 02 tanks, 02 tank 2 would not vent through that flawed vent pipe. The normal approach was to use gaseous oxygen to push the liquid 02 out of the tank through the vent pipe.

In each 02 tank were heaters and a paddle wheel fan. The heater and fan (stirrer) device encouraged a portion of the cold liquid 02 to turn into a higher pressure 02 gas and flow into the fuel cells. A fan also known as the cryo-stirrer was powered each time the heater was powered. The fan served to stir the liquid 02 to assure it was uniformly consistent in density, i.e., not stratified.

Because the tank could not vent the liquid 02 from its vent pipe, it was decided that the heater/fan would be used to boil off the liquid 02 over a period of 8-10 hours. In this way, the clogged vent plumbing would not be needed. Venting would be via heating the liquid 02 so that it would become higher pressure gaseous oxygen and vent through the fill pipe. To protect the heater from being overly hot, a switch like device called a relay was in the tank to turn off heater power anytime the temperature exceeded 80 degrees F. Also, there was a temperature gage on the tank which technicians on the ground could monitor if temperature exceeded 80 degree F. Unfortunately, five years prior to the Apollo 13 manned Countdown Demonstration Test (CDDT), NASA had instructed the 02 tank manufacturer to redesign the tanks to be compatible with 65 volt DC power supplied by the power supplies at the Cape.

The tank's maker failed to comply and left the design of the heater safety switch for 28 volt operation. When the heater was powered up to vent the tank, the higher voltage "fused" the relay contacts so that the switch could not turn off power when the temperature of the tank exceeded 80 degrees F. As a result, the heater and the wires which powered it reached estimated temperatures of around 1000 degrees F., hot enough to melt the Teflon insulation on the heater wires and leave portions of them bare. Bare wires meant the potential for a short-circuit and an explosion since these wires were immersed in the liquid oxygen. Anytime power was applied to those heaters to stir the tank's liquid oxygen, an explosion was possible. How the potential short circuit occurred is an example of divine intervention.

First of all, the exposed heater wires immersed in the highly flammable liquid Oxygen of Tank 2, would, of course, only "short-circuit" with the application of heater power known as "stirring the cryos." Indeed, the first instance of this was ON THE LAUNCH PAD when the very failure causing the heater wire insulation to burn off happened. This created a potential "short circuit" within the tank. It occurred weeks before launch. Following this powering of those bared wires, no additional "power-up" was planned. However, it is not uncommon for pre-launch re-verification to be preformed. One such re-check might easily have been these heater circuits since they had been used in a non-standard way to empty the oxygen from the cryo tanks after the Countdown Demonstration Test (CDDT) weeks earlier. Such a routine re-test involving cryo stirring would have jeopardized the launch vehicle, support persons, or astronaut crew. Such re-do's often occur for myriad reasons. For Apollo 13, despite the compromised system, none occurred until the craft was safely on its way to the Moon.

Research indicated that "standard operating procedure" (SOP) had Mission Control request a stirring of the cryos approximately every twenty-fours. For the Apollo 13 mission, the first stir came about 24 hours into the mission (23:20:23). Ordinarily, the next cryo stir would not be called for until 24 HOURS later. The period between stirs could be shortened or lengthened considerably to allow for crew sleep periods. The heater-cryo stir procedure was done to assure accuracy of the quantity gauge and proper operation of the system through the elimination of O2 stratification. The sensor read more accurately because the stir mades the liquid Oxygen more uniform and less stratified. After the first stir, 87 % remaining O2 quantity was indicated, a bit ahead of expectations. The next stir came about a day later according to SOP at 46:40 GET

At the time of the second heater-cryo-stir, 02 Tank 2's quantity sensor failed. Post mission analysis indicated the failure was not related to the bare heater wires. The loss of ability to monitor 02 tank 2's quantity caused mission control to issue comment to the crew: "(Because the quantity sensor failed,) we're going to be requesting you stir the cryos every six hours to help gage how much 02 is in tank 2." However, Mission Control chose to perform some analysis of the situation in Tank 2 by calling for another stir, not at 53 hours g.e.t. but at 47:54:50 g.e.t. and still another at 51:07:41 . Because the other 02 Tank, Tank No. 1, indicated a low pressure, both tanks were stirred at 55:53. Count the number of stirs since launch: 1 - at 23:20:23, 2 - at 46:40, 3 - at 47:54:50, 4 - at 51:07:44 and 5 - at 55:53. There were five applications of current to those bare heater wires. The last three over a period of only 8 hours rather than 72 hours. Had it not been for the non-threatening failure of Tank 2's quantity probe and the low pressure in O2 Tank 1, this would not have been the case.

The failure of that probe is, perhaps, the most important reason Apollo 13's crew lived. Anyone who has analyzed hardware failures understands that the more frequent and shorter the period between operation of a flawed component hastens ultimate failure. NASA performed stress testing on hundreds of electrical systems using this approach. More frequent power-ups at shorter intervals encourages flawed systems to fail sooner.

Based on the actual occurrence of the short circuit in O2 Tank 2 after the fifth heater-cryo-stir resulted in the explosion of Apollo 13's 02 Tank 2. Had the normal sequence of stirs been performed at 24 hour intervals, and the failure come after the fifth stirring, the explosion would have occurred after the lunar module, the life boat, was no longer available. Check the arithmetic: 5 heater actuations at 24 hours periods amounts to a g.e.t. of 120 hours.. The lunar lander would have departed for the Moon at 103.5 hours into the mission. At 120 hours into the mission, the crew of Lovell and Haise would have been awakened from their sleep period, having completed their first moon walk eight hours before. They would receive an urgent call from Jack Swigert and/or Mission Control that something was amiss with the Mother ship orbiting the Moon.

Analysis of Swigert's ship's problems would be clouded by the absence of his two comrades on the lunar surface. Added problems for Mission Control would have been the interruption of communications each time the command ship went behind the Moon, interrupting the telemetry so crucial to analyzing the failure. When it became evident, the cryogenic system would no longer produce oxygen, water, and electrical power, those command module emergency batteries would have been activated. Likely, Mission Control would have ordered an abort of the lunar lander earlier, but, of course, that would have been futile. Had the tiny lander's ascent stage rendezvoused and docked with the depleted CM, all the life supporting descent stage consumables would remain on the Moon. The nightmare would have the Apollo 13 crew saying their last farewells to their families and friends making their peace with God. One can only speculate how the end might have come: ingesting scores of the capsules on board for colds or slowly bleeding the remaining oxygen into the vacuum of space?

In summary : Why was the frequency of the application of O2 cryo tank 2 heater power increased for Apollo 13. Why was the time between stirs so much reduced? It was not only because of stratification of liquid O2. The EECOM (the responsible flight controller) discovered that the quantity sensor in the O2 tank was not reading accurately, a problem unrelated to the exposed heater wires. I contend that the quantity sensor malfunction was ordained by God to assure the lander would be present and fully fueled at the time of the disaster. For those who believe in this type of "preventive Providence" as an overseeing act of the mercy of God, I would further my case in this way. Scripture speaks of God "answering before they call.. " (Isaiah 65:24). Before the call, "Houston, we've had a problem," the failure of that quantity sensor in O2 tank 2 confirmed that age old promise.

This discussion has demonstrated that failure of the sensor at 55:55 g.e.t. meant enough residual power, water, and electricity would be available in the lunar lander to reach Earth, but what if the explosion had occurred at the time of the first heater-cryo stir 23 hours 20 minutes 20 seconds from launch? Based on the condition of those bared wires, some might argue that this could have been the case. Despite what has been demonstrated concerning the fifth actuation as the determiner, consider the early case for the disaster. Could the crew have survived? All would agree, no explosion could have been possible until electrical current was applied to those bared wires. The first application came about a day after launch.

From actual mission events, the LM was jettisoned at 141 hours 30 minutes after launch of Apollo 13. Again, some simple arithmetic demonstrates the importance of the explosion happening when it happened as the major reason for the successful rescue. At the time of jettison, after nearly 86 hours of extreme conservation of the LM's water, electricity and oxygen since the 55:55 g.e.t. explosion, only 4.5 hours of electricity and 5.5 hours of water for cooling remained. There was yet an ample oxygen supply of over a hundred hours duration. Though sufficient oxygen remained, the loss of all electricity and water would prove fatal to the crew.

To make the LM last, Mission Control considered the quickest route home to Earth. Several choices were considered. The large powerful command ship's engine was thought to be compromised (and evidence from photos indicate it was). Likely, its use would have proven fatal because the engine's nozzle appeared to have been slightly deformed by the O2 tank blast. The only alternative was to accelerate the journey home with the LM's descent engine. This meant the earliest splash down (in the Atlantic) would be 133 hours from launch. If the Indian Ocean were chosen, the earliest return would be 152 hours from launch. The path selected was a splashdown in the Pacific which split the difference in time of return at 142 hours 40 minutes (ground elapsed time from launch). Choosing the best case return of 133 hours meant that the residuals in the LM would be about 14.5 hours of electricity and 15.5 hours of water. Using these margins as an established limit of life support is further evidence that 55 hours 55 minutes for the explosion was altogether fortuitous..

Again, do the arithmetic: If the explosion occurs at the first possible opportunity of 23:40:23 g.e.t., and the best possible return is 133 hours, the time from the explosion the LM must function becomes (133 hours minus 23 hours 40 minutes 23 seconds) or about 108 hours 20 minutes. The best the LM could have done would have been the time it lasted plus its residuals. In actuality, tt lasted 86 hours with margins of 4.5 hours for electricity and 5.5 hours for water. This amounts to a life of 90.5 hours of electricity and 91.5 hours for water. If the explosion had occurred at the earliest opportunity, the first time heater power was applied at 23 hours 20 minutes 23 seconds, 108 hours 20 minutes would be needed to return to Earth. The rescue would fall about 18 hours short. It has been shown previously that the crew would have perished during the routine fifth activation of the heaters on the 16th of April, 1970, a Thursday afternoon around 2:00 PM Houston time. In this second case of an early explosion, the crew would have expired early Thursday morning around 2:00 AM Houston time, the 16th of April 1970.

Throughout Scripture, five is often associated with God's grace. David had five stones as he faced Goliath. There were five porches at the Pool of Bethesda for healing. The good steward was given five talents. The alarm which rang on board the Eagle prior to Neil Armstrong's landing sounded five times. Cryo stir number five triggered the explosion of Apollo 13 at a time when the crew could be saved. The simultaneous alarm which sounded as a result of the malfunction was reset at 55 hours 55 minutes, (a string of four fives) g.e.t. God's grace was, indeed, at work in the rescue of Jim Lovell and his crew.


The following question was asked on 21st September 1999 in Chicago Illinois. The interviewer was Robert Godwin. Special thanks to Jim Lovell for his time and cooperation.**

Question: Aside from the explosion, what was the most important moment during the flight of Apollo 13?

*From digital video clip of Jim Lovell on accompanying CDROM to APOLLO 13 THE NASA MISSION REPORTS - Editor Robert Godwin, Copyright by Apogee Books 2000.

Copyright 2002 JRWIV INTERESTS

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