13 Things That Saved Apollo 13, Part 9: Position of the Tanks

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Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

The saga of the Apollo 13 accident actually began years prior to the launch of the mission. As Jim Lovell wrote in his book, “Lost Moon” the accident was “an accumulation of human errors and technical anomalies that doomed Apollo 13.” But had coincidences been just a little different Apollo 13 could have been an accident from which there was no rescue. NASA engineer Jerry Woodfill believes where Tank Two was positioned in the Service Module led to a successful rescue. “I contend that the crew would have died if the flawed O2 Tank Two had not been on the outer perimeter of the Service Module,” Woodfill said. “The position of that tank had much to do with the extent of the explosion’s damage. Had Tank One been damaged, no rescue would have been possible.”

Graphic showing the Apollo Service module interior. Credit: NASA

The oxygen tanks were specially insulated spherical tanks which held a “slush” of liquid oxygen with a fill line and heater running down the center. Tank Two used for Apollo 13 had originally been installed in Apollo 10, but was removed for modification. In what was considered a minor mishap, O2 Tank Two was accidently dropped and damaged. The two tanks were on a “shelf” in the Service Module and held in place by two bolts. During removal, inadvertently, only one bolt on the shelf was removed, the side that contained Tank Two. When the lifting fixture picked up the shelf, Tank One stayed in place while Tank Two accelerated upward, striking the fuel cell shelf overhead. It only moved about 5 cm (2 inches) but the jolt displaced a loosely fitted fill tube in Tank Two. This tank was replaced with another for Apollo 10, and the exterior was inspected. Since the interior wasn’t inspected, no one knew about the fill tube damage, and the shelf with the damaged Tank Two was installed in the Apollo 13 Service Module (SM-109) November 22, 1968.

Unfortunately there was another problem with the tank, that were it not for the fill tube damage, may not have been an issue. The oxygen tanks had originally been designed to run off the 28 volt DC power of the Command and Service modules. However, in 1965 the tanks were ordered to be refitted to also run off the 65 volt DC ground power at Kennedy Space Center. All components were upgraded to accept 65 volts except the heater thermostatic switches, which were overlooked. These switches were designed to open and turn off the heater when the tank temperature reached 26 degrees C (80 degrees F — Normal temperatures in the tank were -74 C to -174 C (-300 to -100 F.)

During pre-flight testing, Tank Two would not empty correctly, possibly due to the damaged fill line. The heaters in the tanks were normally used for very short periods to heat the interior slightly, increasing the pressure to keep the oxygen flowing. It was decided to use the heater to “boil off” the excess oxygen, requiring 8 hours of 65 volt DC power. This probably damaged the thermostatically controlled switches on the heater, designed for only 28 volts.

Schematic of the oxygen tank. Credit: NASA

The Apollo 13 review board came to the conclusion that the switches welded shut, allowing the temperature within the tank to rise to over 538 degrees C (1000 degrees F). The gauges measuring the temperature inside the tank were designed to measure only to 80 F, so the extreme heating was not noticed. The high temperature emptied the tank, but also resulted in serious damage to the Teflon insulation on the electrical wires to the power fans within the tank.

When the tanks were put into the Apollo 13 spacecraft, the damaged Tank Two was placed in the exterior position.

“Because the spark which ignited the oxygen in Tank Two was located at the top of the tank,” said Woodfill, the tank acted like a cork on a Thermos bottle. Since it was on the outside perimeter, it simply blew out into space along with the 13 foot panel covering the side of the service module. The oxygen tank shelf served to isolate the explosion from the hydrogen tanks below. But had the inboard oxygen Tank One 1 exploded, likely, this would not have been the case.”

Should the flawed tank have been the inner tank, Woodfill said, its explosive force would have taken with it the sister O2 tank amplifying the force of the explosion, just as using two sticks of dynamite instead of one, the destruction would be a magnitude greater.

Image of the damaged Apollo 13 Service Module, taken by the crew. Credit: NASA

“The added explosive force would have fractured the O2 tank shelf involving the fragile hydrogen tanks below,” Woodfill explained. “The volatile hydrogen gas now having a wealth of oxygen from the overhead tanks would surely have destroyed the entire spacecraft assemblage. Of course, the crew would have immediately perished as well. There would have been no clues, no telemetry data trace to explain what had happened.”

“Oxygen Tank One was given the inboard location adjacent to the flawed tank,” Woodfill continued. “Consider the likelihood of that placement. It is one chance in two. The odds for Apollo 13’s survival were fifty percent, a flip of the coin.”

Next: Part 10: Duct Tape

Other articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 8: The Command Module Wasn’t Severed

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Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

When the Apollo 13 crew jettisoned the crippled Service Module as they approached Earth, they saw the extent of the damage from the explosion of an oxygen tank. “There’s one whole side of that spacecraft missing!” Jim Lovell radioed to Mission Control, his voice reflecting his incredulousness at seeing the damage of a 13-ft panel blown off the spacecraft. However, the situation could have been more dire. The heat shield on the Command Module could have been damaged. What’s more, NASA engineer Jerry Woodfill said that instead of the panel blowing out, the explosion could have — and maybe should have –severed the Command Module from the Service Module.

Graphic of the CSM. Credit: NASA

Photos taken by the Apollo 13 crew after the service module was jettisoned in preparation for the command module’s reentry via the heat shield revealed that not only was the panel missing from the side of the spacecraft — blown into the vastness of space by the exploding pressure of the detonating oxygen – there was also damage to the Hi Gain Antenna, at the right of the vehicle drawing above, indicating the panel had catapulted into space, striking the antenna. What the images couldn’t show, and what the Apollo 13 crew couldn’t see was if there was any damage to the Command Module’s heat shield.

“The structural design of the interior of the Service Module is that it has a long open tunnel-like volume in the center of the module, about 30 inches by 13 feet,” said Woodfill. “The tunnel is much like a chimney such that gases, liquids, or particles could readily move through it toward the main engine bell at the right and the heat shield at the left. The tunnel is not sealed so that the explosive force of the burning oxygen from the exploded O2 tank 2 could escape into and around the tunnel in the direction of both the heat shield and main engine.”

Woodfill said concern was voiced in Mission Control that shrapnel from the exploding tank had entered the tunnel, and perhaps ultimately caused damage to both the heat shield and main engine. The main engine wasn’t the biggest issue, as the crew was able to use the lunar lander’s descent engine. (see our previous article , “Using the LM for Propulsion.”) But there was only one heat shield, and it had to work to enable the capsule and the crew to survive the fiery reentry through Earth’s atmosphere.

Thankfully, as it turned out ,the heat shield wasn’t damaged.

The recovery of the Apollo 13 Command Module. Credit: NASA

But almost miraculously, Woodfill said, the command module and service module remained connected following the explosion, while the internal pressure of the explosion rocketed the exterior panel into space.

“The attachment strength of the Service Module panel to the structure required a considerable internal pressure of 24 pounds per square inch for severing it from the service module,” Woodfill said. “A much lower pressure was required to separate the Command Module with its heat shield from the Service Module, only 10 pound per square inch. One can only speculate on why the panel blew and the crew capsule/service module attachment remained intact.”

Since there is no air pressure in space, Woodfill explained, the force which held the vehicles together was the strength of their mechanical attachments.

“Two pressures were at work,” he said. “Each attempted to overcome respective attachment forces: the force which attached the Service Module to the Command capsule and the force which attached the Service Module panel to the Service Module. Because the explosive pressure force of the oxygen was immediately applied in great strength to the panel, this overwhelming force would be expected to blast that panel apart from the vehicle, exceeding the 24 pound per square inch attachment strength. However, venting of residual explosive oxygen into the framework of the Service Module could well be expected to overcome the attachment strength between the two vehicles, separating them.”

Yet, it did not. Why?

Sequence photo from 16mm motion picture film of test at Langley Research Center which seeks to determine mechanism by which Apollo 13 panel was separated from Service Module. Credit: NASA. Click image for more information

“Apparently, the presence of ‘tankage’ and other structure acted to mitigate and dissipate the sudden pressure spike before it reached the interface between the vehicles,” Woodfill said. “However, if a shard from the exploded O2 tank 2 had punctured any of the adjacent tanks, likely a secondary explosion of any of them would have propagated both the explosion and build up of pressure. In that event, certainly, the vehicles would have experienced either a fatal separation or fatal damage to the heat shield.

A piece of shrapnel did fracture the plumbing between the oxygen tanks that allowed the oxygen to leak out of Tank 1, causing the complete loss of power in the Command Module, for without oxygen the fuel cells couldn’t work.

Some may say that having the Service Module attached to the Command Module wasn’t important – it was just dead weight anyway. However, other problems could have developed without the Service Module attached, according the Apollo 13 Failure Report. Having the heat shield exposed to low temperatures for a long period could have damaged it, and internal Command Module thermal problems could arise if the Service Module was jettisoned too early.

Additionally, flight control problems were anticipated if the Command Module wasn’t attached.
The immediate loss of the Service Module would have meant immediate loss of the residual power from the fuel cells while the crew and mission control wrestled to understand the problem. This would have required a much greater power drain on those emergency batteries to the extent that one wonders if the later “trickle-charge” from the lander’s batteries would have been sufficient for reentry.

The crew of Apollo 13, Jim Lovell, Jack Swigert and Fred Haise, during a post-flight debrief. Credit: NASA

Of course, since the Service Module was jettisoned before the crew re-entered (and the SM itself later burned up in the Earth’s atmosphere) no one could do any “forensic analysis” or an engineering “autopsy” on that part of the spacecraft.

“To me, it is amazing that, one, the heat shield wasn’t damaged from the explosion, and two, the connection that could withstand higher pressure ended up blowing, while the weaker connection stayed together,” said Woodfill.

But those were among the many things that saved Apollo 13.

Next: Part 9: Which tank was damaged

Earlier articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 7: The Apollo 1 Fire

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Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

“Far better is it to dare mighty things, to win glorious triumphs, even though checkered by failure than to rank with those poor spirits who neither enjoy much nor suffer much, because they live in a gray twilight that knows not victory nor defeat.” – Theodore Roosevelt

It’s hard to chronicle any of the Apollo flights without mentioning the Apollo 1 fire. And while many believe the Apollo program perhaps wouldn’t have succeeded without that disaster, the sacrifice made by Gus Grissom, Ed White and Roger Chaffee definitely saved the crew of Apollo 13.

“Among the early space missions, I’ve always believed that the greatest courage was needed by their first crews,” said Apollo engineer Jerry Woodfill. “Whether it was Al Shepard, the Apollo 1 crew, or shuttle astronauts John Young or Bob Crippen, the most likely danger would be the first time any new space craft was launched into space. Flaws in design or manufacture could very well be fatal during maiden missions.”

The crew of Apollo 1: Gus Grissom, Ed White and Roger Chaffee. Credit: NASA

On January 27, 1967, during a test on the launch pad with the crew on board, tragedy struck when a flash fire started in the command module. With the pure oxygen environment inside the capsule, the fire quickly proved fatal for the crew before they or workers at the launch pad could get the hatch open. Although the ignition source of the fire was never conclusively identified, the astronauts’ deaths were attributed to a wide range of design and construction flaws in the early Apollo Command Module. The manned phase of the project was delayed for twenty months while these problems were fixed.

“To suggest the dire event of losing three brave astronauts contributing to Apollo 13’s rescue seems almost ludicrous,” said Woodfill, “but the evidence is striking. What Grissom, White and Chaffee contributed to the rescue of Apollo 13 makes them even more heroic than they were when they gave their lives so that men could go to the moon.”

The irony of the whole situation involves the hatch. Following Gus Grissom’s near fatal drowning when his Mercury capsule sank, the Apollo hatch had been redesigned to avoid the kind of unexpected actuation thought to have caused Grissom’s “Liberty Bell 7” to sink.

Gus Grissom and the Liberty Bell 7. Credit: NASA

“Unfortunately, it led to a hatch impossible to open before the Apollo 1 crew expired,” said Woodfill. “Nevertheless, circumstances used Gus, Ed, and Roger’s sacrifice to save other crews in route to the Moon.”

NASA fire-proofed all future Apollo vehicles with non-flammable materials, used a pad atmosphere of a nitrogen/oxygen mix, and coated of all electrical connections to avoid short-circuits.

“Every switch contact and wire was coated with a moisture proofing substance called conformal coating,” said Woodfill. “Were it not for fire-proofing the Apollo command and service modules, Apollo 13, likely, could not have survived reentry. The cold, damp reentry module interior faced extreme condensation of water vapor from the astronauts’ breath. Droplets of water formed behind the display panels.”

Diagram of the Apollo Command Module control panel. Credit: NASA History Office. Click for larger version.

Woodfill said when Apollo 13’s switches were activated for reentry, the interior would surely have burst into flame, were it not for the fireproofing. Condensed water droplets might have short-circuited panel switches, circuit breakers, and connector wiring.

Woodfill said America might never have landed a man on the Moon without Apollo 1. If a fire had occurred on the way to the Moon, it might have ended the will to land men there. “Imagine the horror of the world at such an event,” said Woodfill, “hearing the crew’s painful cries from deep space, ‘We’ve got a fire in the spacecraft.’”

Apollo 1 and the fireproofing of future Apollo spacecraft prevented such an event.

A favorite quote of many managers of the Apollo program, Woodfill said, is from President Theodore Roosevelt, the one posted at the top of this article.

“In a sense, the Apollo One mission was altogether different from Challenger, Columbia, and Apollo 13,” said Woodfill. “No one had dared such a mighty thing as to man the first Apollo spacecraft into orbit. And it, in this case, was fraught with suffering, failure and defeat, rather than a glorious triumph and victory.”

But later, it allowed for great triumph with the success of the Apollo program, and a defying of the odds of the Apollo 13 crew’s survival.

Tomorrow, Part 8: What the Explosion Didn’t Do

Additional articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

Never Before Published Images of Apollo 13 Recovery

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Today marks the 40th anniversary of the successful return and recovery of the Apollo 13 spacecraft and crew, which has been called the the most satisfying splashdown in the history of human spaceflight. The images here of the safe return of the Apollo 13 astronauts have never been published before, and were sent to Universe Today by reporter Robert Gillette.

“Once in a while, we manage to be in the right place at the right time with a camera in hand,” Gillette wrote Universe Today. “I happened to be on the USS Iwo Jima as a young science reporter (for the-then San Francisco Examiner) in April 1970. By the time I made it back to shore to develop the film it no longer had news value. Maybe 40 years later they have historic value, at least for the emotion written in the faces of Lovell, Swigert and Haise. So I dug the old Kodachromes out and had them digitized.”

Regarding the photo above, Gillette said he overheard Apollo 13 Commander Jim Lovell tell the Admiral of the Iwo Jima, “Thank God for Grumman,” referring to the Grumman-built lunar lander that served as the lifeboat for Lovell, Fred Haise and Jack Swigert following the explosion that crippled the Command and Service Module. Gillette has determined the admiral to Lovell’s left is Rear Admiral Donald C. Davis, Commanding Officer of Task Force 130, the Pacific Recovery Force for the Manned Spacecraft Missions.

See more images from Gillette, below.

Rescue helicopter prepares to touch down on deck of USS Iwo Jima with Apollo 13 astronauts aboard, April 17, 1970. Image courtesy Robert Gillette.
Lovell and Swigert emerge from rescue helicopter, April 17, 1970. Image courtesy Robert Gillette.
Jack Swigert and Fred Haise emerge from rescue helicopter,stepping on deck of the Iwo Jima. Image courtesy Robert Gillette.
Haise and Lovell emerge on deck for helicopter ride to American Samoa. Image courtesy Robert Gillette.
Swigert strides on deck moments later for helicopter ride to American Samoa. Image courtesy Robert Gillette.

Our thanks to Robert Gillette for sending us these unique images on this anniversary of the historic return of Apollo 13. For more unique information on Apollo, see our ongoing series, “13 Things That Saved Apollo 13,” our discussion with Apollo engineer Jerry Woodfill which highlights various turning points of the mission.

13 Things That Saved Apollo 13, Part 6: Navigating By Earth’s Terminator

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Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

The rupture and explosion of Apollo 13’s oxygen tank crippled the spacecraft, endangering the lives of the crew and making a Moon landing not an option. But more problems arose as the perilous flight progressed. Keeping the spacecraft on the right trajectory was a huge challenge for Mission Control, and especially for the crew. Normally, the ship’s computers allowed for much of the navigation, but due to the loss of the Service Module as an electrical power source, even backup navigation and targeting functions were unavailable. The Lander’s limited battery power required the shutting down of its guidance computer. The astronauts also needed to use an on-board sextant to confirm their location by sighting-in the stars, similar to how ancient sailors navigated. “There are thirty-seven stars – and one is the sun,” said Apollo engineer Jerry Woodfill, “that provided an accurate way of aligning the spacecraft’s computer platform to allow the astronauts to steer their way through the heavens.”

But the explosion of the tank had enshrouded the Apollo 13 spacecraft with debris. Commander Jim Lovell and his crew couldn’t discern the stars from the particles that glimmered in the sunlight. “The situation was, without the ability to see the stars, you couldn’t navigate,” Woodfill said.

But NASA had a backup navigation plan, thanks to an insightful NASA contractor employee. This novel way of navigating had only been tried once before in space. And coincidentally, the astronaut who used it was Jim Lovell, during his previous flight — Apollo 8 — which orbited the Moon in December of 1968.

An employee of TRW – which was the contractor for many of the navigational systems and procedures for NASA — thought of an unusual backup navigation plan one day. “This fellow is a friend and neighbor of mine,” said Woodfill, “and by his account of the story to me, he said that a thought came to him one day about Apollo astronauts using stars to navigate. What if the stars couldn’t be seen? Now, that was highly unlikely, as there are no clouds, fog, or smoke to conceal stars from viewing by astronauts. But, nevertheless, the thought simply wouldn’t cease. Soon a follow-up idea came to mind. Why not use the Earth’s terminator?”

The nominal flight plan for a mission to the Moon. Credit: Apollo 13 report.

The terminator is the line which delineates between night and day on Earth; where the Sun is shining and where it is dark.

Woodfill’s friend figured out the geometry and wrote a computer program to validate the idea. He submitted the proposal to the navigation board, which approved the technique so that it was entered into the computers in the Mission Control Center.

Through unusual, and what could be called happenstance circumstances, Lovell experimented with the backup plan during Apollo 8.

Lovell served as navigator for the first manned mission to orbit the Moon. He made a star sighting in preparation for the return to Earth, and entered the coordinates into the Apollo spacecraft’s primitive computer using the “DSKY” (display and keyboard). Instead of pressing the ENTR (enter) key, he inadvertently pressed the adjacent CLR (clear) key erasing the entire navigational alignment.

“Lovell consulted with Mission Control whether to repeat the sextant star sighting,” Woodfill said, “and someone realized this would be an opportunity to test the backup ‘seat of the pants’ means of navigating using the Earth’s terminator. And it worked! But then everyone forgot about it, until…guess when?”

Apollo 13's view of the Moon. Credit: NASA

Initially, the Apollo 13 crew was able to use the Sun as a “marker” to help in guiding the spacecraft to confirm they were on the right path, and were able to fire the LM engines for course corrections using the transferred guidance platform from the Command Module.

But as Apollo 13 headed back to Earth, the Reentry (RETRO) and Guidance, Navigation and Control (GNC) officers looking at the trajectory analysis noticed the spacecraft was coming in too “shallow,” that is, Apollo 13 was headed to skip off the atmosphere and out into space forever. Something seemed to be “blowing” the spacecraft off course. Later, it was discovered that cooling vapor from the lander was responsible. Since no lander had been present for previous missions on a return trip from the Moon, such a mysterious “wind” had never been encountered prior to Earth re-entry.

Another burn was needed, but no help from the guidance system would be available, as powering the lander’s guidance system, its gyros, the computer, etc. would use too much electrical power.

Here’s where the backup navigation approach that Lovell experimented with on Apollo 8 came to the rescue.

“If a ‘dead-reckoning’ approach could be used, no electricity would be needed,” said Woodfill. “Simply point the vehicle correctly, start the engine and stop it based on Mission Control’s prescribed time for its operation.” Lovell eyed up the Earth’s terminator line and controlled the “yaw” of the spacecraft, Haise controlled the “pitch” and Swigert timed it with his accurate Omega Speedmaster watch.

Graphics from the Apollo 13 report on using Earth's terminator for navigation.

The Navigation report for Apollo 13 describes it this way:

“The cusps of the Earth terminator were placed on the Y axis of the COAS. The illuminated part of the Earth was placed at the top of the reticle. Pitch attitude was achieved by placing the Sun in the upper portion of the AOT (see below). This procedure aimed the LM +Z axis at the Earth and aligned the LM +X axis retrograde along the local horizontal. An AGS body axis alignment was performed, followed by transitioning the AGS to the automatic attitude hold mode. A maneuver to burn attitude was performed, followed by another body axis alignment.”

Navigation graphics from the Apollo 13 report.

Woodfill said he enjoyed Hollywood’s re-enactment of the procedure in the “Apollo 13” movie. Though the spacecraft gyrations about the heavens are wholly exaggerated, the scene where Tom Hanks, Bill Paxton, and Kevin Bacon set-up and execute the terminator burn is generally accurate.

Suffice to say, the procedure worked for Hollywood dramatics, but more importantly, it worked to save the lives of Lovell, Haise and Swigert.

Tomorrow, Part 6: Fire

Other articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 5: Unexplained Shutdown of the Saturn V Center Engine

Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

While oxygen tank number two on the Apollo 13 spacecraft was an accident waiting to happen, another problem on the Saturn V rocket could have destroyed Apollo 13 before it reached Earth orbit. During the second-stage boost, the center – or inboard — engine shut down two minutes early. The shutdown wasn’t a problem, as the other four engines were able to compensate for the loss by operating for an extra four minutes. But why the engine shut down is a mystery that may have saved the mission.

“A catastrophic failure should have ensued,” said Apollo engineer Jerry Woodfill, “and would have, except for the unexplained behavior of the engine’s shutoff system. In fact, even the NASA Apollo 13 accident report fails to deal with the seriousness of the event.”

When the center engine shut down, it caused a few moments of uneasiness for Mission Control and the crew. Speaking after the flight, Commander Jim Lovell said that when NASA gave them the OK to carry on with the flight, “We all breathed a sigh of relief on the spacecraft. Hey, that was our crisis over with and we thought we’d have a smooth flight from then on.”

Woodfill said that the quick assessment in Mission Control was that a minor electrical signal failed to keep the engine operating so that it shut down prematurely. But that wasn’t the problem.
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What happened was the Saturn V rocket experienced dangerous so-called “pogo” thrust oscillations, a problem NASA knew about. While a fix had been planned for Apollo 14, time did not permit its implementation on Apollo 13’s Saturn V.

“While a clerical error caused Apollo 13’s oxygen tank to explode,” said Woodfill, “because its heater design had not been updated for 65 volt operation, and the tank was a virtual bomb (see Part 1), similarly NASA’s failure to fix a known serious booster flaw should have destroyed Apollo 13.”

The Saturn V rocket had five J-2 engines, each producing 200,000 pounds of thrust, together creating the 1 million pounds of thrust needed for a mission to the Moon.

On previous Saturn flights, these pogo oscillations had occurred during launch. The phenomenon occurred as the fuel lines and structure of the rocket resonated at a common frequency. The resonance tended to amplify in force and potential destruction with each bounce of the “pogo” mechanism. So damaging was the phenomena on the unmanned Apollo 6 mission that an entire outer panel of the Saturn 5 ejected into space.

Launch of Apollo 6. Credit: NASA

“The oscillations are like a jack hammer and it was so dreadful on Apollo 6 that it tore off a panel on the booster, and threatened the mission,” said Woodfill. “Apollo 6’s orbit was supposed to be circular, but because of the pogo effect and failure of second stage engines, the orbit became an elongated orbit of about 60 by 180 miles.”

Woodfill said if Apollo 13 had ended up in that type of orbit, it would have been bad but not fatal. However, Apollo 13 was a much different situation than Apollo 6.

The Apollo 6 mission carried a mock lunar lander of more modest mass than the “full-up” lander which Apollo 13 carried to orbit. With the added mass for Apollo 13, the pogo forces were suddenly a magnitude greater in intensity. A mission report said that the engine experienced 68g vibrations at 16 hertz, flexing the thrust frame by 3 inches (76 mm).

Woodfill said that if the center engine had continued running a few more seconds, the oscillations may have destroyed the vehicle. “That engine was pounding horizontally up and down, a quarter foot, at the rate of 16 times a second,” he said. “The engine had become a two ton sledge hammer, a deadly pogo stick of destruction, putting enormous forces on the supporting structures.”

What shut the engine down?

“It is, to this day, not fully understood, but it had something to do with fooling the engine’s thrust chamber pressure sensor that pressure was too low,” said Woodfill. He has studied the mission report, but says the complete analysis of why the engine shut down isn’t included.

“Though the shutdown command came from a low thrust chamber pressure sensor assessment, actually, the engine was operating correctly,” he said. ” The sensor had nothing to do with the pogo phenomenon. For some inexplicable reason, it was like something sucked the pressure out of the chamber and a sensor turned the engine off. But no one knows exactly why.”

Woodfill said those who later examined the situation said it was altogether lucky that the sensor shut down the engine. “Something intervened, stopping the engine from pounding its way from the mount into the fragile fuel tanks. This would have destroyed the Apollo 13 launch vehicle.”

As it was, the engine shutdown likely saved the Apollo 13 mission.

Tomorrow, Part 6: Navigation

Other articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 4: Using the LM for Propulsion

Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

After Flight Director Gene Kranz and his team in Mission Control had ascertained the true peril the Apollo 13 crew faced following the explosion of an oxygen tank in the Command and Service Module, they next faced a big decision. What was the best way to get the astronauts back to Earth? Do they get them home as fast as possible, or as safely as possible? The final decision they made likely saved Apollo 13.

“Immediately after the explosion, some recommended a faster return using the powerful service propulsion system (SPS), the engine designed for the retro burn into lunar orbit and the subsequent firing to propel the crew homeward to Earth,” said NASA Engineer Jerry Woodfill.

Using these engines to execute a direct abort burn would allow the crew to turn the spacecraft around, come around the front side of the Moon and be back to Earth within a day and a half. This was the quickest option, but it meant using the SPS, which were very near the area that had exploded on the CSM. No one knew if the engine had been damaged, too.

Vital stores of oxygen, water, propellant, and power were lost when the side of the service module blew off. The astronauts quickly moved into the lunar module which had been provided with independent supplies of these space necessities for the landing on the Moon. Years before, Apollo engineers had talked of using the lunar module as a lifeboat. Credit: NASA

The risk of using using the lunar module’s descent engine was an unknown. If it failed or blew, or if the burn wasn’t executed perfectly, the crew could impact the Moon.

The other option was to go completely around the Moon on a so called free-return trajectory, which would take between four to five days to get back to Earth. But would the crew have enough consumables to survive that long?

This flight plan, too, called for an engine burn to set the spacecraft on the correct path back to Earth. But should they use the SPS engine, which was designed for this maneuver but could be damaged, or use the use the descent engine on the Lunar Module, which had never been designed for this type of use?

In his book, “Failure is Not an Option,” Kranz said it was purely a gut feeling that made him choose to take the long way – to go around the Moon and use the descent engine on the lunar lander rather than the CSM.

“Later, Gene Kranz shared he felt a foreboding about using that engine,” said Woodfill. “Nevertheless, even the use of the lander’s descent engine had some risk. The system was not expected to be fired more than once on a lunar mission. It was designed for descent from lunar orbit to landing. To use it for both Apollo 13’s mid-course correction burn (to return to the free-return trajectory) and a subsequent firing to accelerate the journey home amounted to a second firing.”

With the first burn of the LM engines working as hoped, the crew swung around the far side of the Moon (some records indicate Apollo 13 traveled the farthest distance from the far side of the Moon, making them the crew that traveled the farthest away from Earth), Mission Control considered a second burn.

Without the second burn the ship’s trajectory likely would have successfully returned the crew to Earth approximately 153 hours after launch. This provided less than an hour of consumables to spare, a margin too close for comfort.

After a much discussion and calculating, the engineers in the Mission Evaluation Room (MER) and Mission Control determined the LM’s engines could handle the required burn. So, the descent engine was fired sufficiently to boost their speed up another 860 feet per second, cutting the flight time to 143 hours – which provided a better margin for survival.

Damage to the Apollo 13 spacecraft from the oxygen tank explosion. Woodfill noted the missing four Hi-Gain Antenna “horns” severed by the panel and shrapnel from the explosion. Credit: NASA

But what if the SPS engines had been fired? We will never know for sure, but Woodfill said the final photo taken of the damaged command ship after jettison from the reentry capsule appeared to show a slight deformation of the SPS engine nozzle. He believes the SPS panel adjacent to the exploding O2 tank severed the four horns from the mast of the hi-gain communication antenna system. Likely, the shrapnel from the devastating impact with those four dishes ricocheted into the SPS engine bell compromising its use. A hole in the engine’s thrust nozzle would have been catastrophic.

“The fiery bazooka-like blast of the explosion might have cracked the heat shield and damaged critical parts of that engine,” said Woodfill. “The engine’s systems were adjacent to the tunnel-like chimney located in the center of the service module. If the nozzle was deformed, surely, there would have been a potentially fatal consequence of its firing, akin to the loss of the Challenger resulting from the failed solid rocket (SRB) engine.”

Woodfill said that likely, the use of the SPS would have triggered the caution and warning combustion chamber high temperature alarm. “And its use might have made Apollo 13 a fiery meteor-like streak of light never to reach Earth,” he said. “Though a successful firing would have landed the crew days earlier in the Indian Ocean, the peril was too great.”

Tomorrow, Part 5: Unexplained Shutdown of the Saturn V engine

Other articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 3: Charlie Duke’s Measles

[/caption]
Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

Just 72 hours before the scheduled launch of Apollo 13, Ken Mattingly was removed from the mission and replaced by Jack Swigert from the back-up crew as Command Module Pilot. Charlie Duke, also from the back-up crew caught the measles from one of his children, and exposed Mattingly — the only other member of either the prime or back-up crews who were not immune to the disease. If Mattingly were to come down with the measles, he might contract it while alone in the Command Module while Jim Lovell and Fred Haise were walking on the Moon.

“I think Charlie Duke’s measles contributed to the rescue,” said NASA engineer Jerry Woodfill, who has come up with “13 Things That Saved Apollo 13.” “This is one that probably everyone disagrees with me, but it seems like the astronauts on board were perfect to deal with what happened on the Apollo 13 mission.”

Woodfill says his conviction in no way denigrates the abilities of Ken Mattingly. “Ken was a wonderful crew member,” Woodfill said, “and he is a very detailed guy who helped with the rescue of Apollo 13 in a magnificent way. In the movie, Apollo 13, they capture the essence of how he is an ‘engineer’s engineer’.”

Astronaut Charlie Duke. Credit: NASA

Although, ironically Mattingly and Duke flew together later on the Apollo 16 mission, were it not for Charlie Duke’s measles, Woodfill said that Swigert’s special talents for an Apollo 13-type mission would not have been present.

Jack Swigert. Credit: NASA

First of all, his physique was better suited to the harsh conditions he experienced in the inoperable Command Module, where he was positioned for most of the flight. Woodfill said that likely, Swigert’s brawn as a former University of Colorado varsity football player better served him to withstand the cold conditions and endure the small amounts of water that the astronauts had to ration among themselves.

Water was one of the main consumables – even more than oxygen – of which the crew barely had enough.

“Mattingly and Haise had about the same build,” said Woodfill, “which was not as robust a build as Swigert and Lovell. Haise ended up with a urinary tract infection because of not getting enough water.”

But more importantly were Swigert’s familiarity with the Command Module and his “precise” personality.

Screenshot from Apollo footage of Jim Lovell and Jack Swigert. Credit: NASA

“Among the nearly thirty Apollo astronauts, Jack Swigert had the best knowledge of Command Module malfunction procedures,” said Woodfill. “Some have said that Jack had practically written the malfunction procedures for the Command Module. So, he was the most conversant astronaut for any malfunction that occurred in the CSM.”

Swigert had to quickly and accurately write down the procedure to transfer the guidance parameters from the CSM computers to the Lunar module computers. And the procedure for the reentry of the crew to Earth’s atmosphere had to be re-written, with Mission Control calling up to the crew with hundreds of changes to the original plan. “The team on the ground had to recreate a checklist and a procedural ‘cookbook’ that would normally take three months to create, and they had to do it in just days. Jack had to be accurate when he wrote down these procedures. And the communication system wasn’t always the best – it was sometimes garbled or couldn’t be heard very well. While all the astronauts had to have orderly minds, Jack Swigert was a man of extreme order.”

Woodfill said an account from Swigert’s sister bears out that fact. She at one time asked her brother Jack to put away cans of frozen orange juice and lemon juice in her freezer. When she looked in her freezer later, all the lemon juice cans were lined up in orderly fashion, with the orange juice cans neatly lined up in an adjacent row. Later, she asked her brother why he had neatly lined all the lemon cans in a row then a row of orange juice cans, and according to Woodfill, Swigert answered, “Because “L” comes before “O” in the alphabet.”

“The truth is, Swigert was gifted with a respect for extreme order and precision, and he was onboard for just that reason,” said Woodfill. “Every one of the steps in the rescue checklist had to be ‘in the right order’.”

Fred Haise, in 1966. Credit: NASA

And, equally important, said Woodfill, was the talent Haise brought to recording and rewriting operational procedures. “Fred had been a newspaper stringer for a small newspaper in Mississippi in his youth, taking notes and editing them for his local Mississippi paper’s stories. Utmost among reporters is accuracy in quoting sources. Those transmitted words from mission control had to be flawlessly transcribed if the crew was to survive, and Fred and Jack did an amazing job.

Remarkably, said Woodfill, each man’s talents specifically served the unique need. “Each man exhibited exceptional accuracy in adverse surroundings,” he said. “The lander was noisy, the audio sometimes fuzzy, movement unpredictable, temperatures cold, sleep scarce, and fatigue always present.”

Of course, those familiar with the Apollo 13 story know that Ken Mattingly never got the measles. But the role he played in getting the astronauts back home safely can’t be overestimated.

“Call it luck, call it circumstance,” said Woodfill, “but because of Charlie Duke’s measles the men on board Apollo 13 — and back on the ground — were perfect for the situation they encountered.”

Other articles from the “13 Things That Saved Apollo 13” series:

Introduction

Part 1: Timing

Part 2: The Hatch That Wouldn’t Close

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 2: The Hatch That Wouldn’t Close

[/caption]
Note: To celebrate the 40th anniversary of the Apollo 13 mission, for 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.

When the oxygen tank exploded on the Apollo 13 Command Module, the astronauts on board and everyone in Mission Control had no idea what the problem was. In his book, “Lost Moon,” Apollo 13 commander Jim Lovell thought the “bang-whump-shudder” that shook the spacecraft could have been a rogue meteor hit on the lunar module, Aquarius. Quickly, he told Jack Swigert to “button up” or close the hatch between the Command Module Odyssey, and Aquarius, so that both spacecraft wouldn’t depressurize.

But the hatch wouldn’t close.

Apollo engineer Jerry Woodfill believes the balky hatch was one of the things that helped save the Apollo 13 crew. “They were trying to close off the only way they could save their lives,” he said.

In Mission Control and in the nearby Mission Evaluation Room, several engineers, including Woodfill, thought the only explanation for so many systems to go offline at once was an instrumentation problem. “Initially I thought there was something wrong with the alarm system or the instrumentation,” said Woodfill, who helped develop the alarm system for the Apollo spacecraft. “There was no way so many warning lights could illuminate at once. I was sure I would have some explaining to do about the system.”

Screenshot from Apollo 13 footage of Fred Haise floating through the hatch between Odyssey and Aquarius. Credit: NASA

At first, Lovell thought Fred Haise may have been playing a joke on the crew by actuating a relief valve that made a sort of popping noise – something he had done previously during the flight. But with the surprised look on Haise’s face, along with the noise and all the alarms going off, Lovell’s next thought was the hull had been compromised in Aquarius.

Like a submarine crew that closes hatches between compartments after being hit by a torpedo or depth charge, Lovell wanted to close the hatch into the Command Module so all the air didn’t rush out into the vacuum of space.

Swigert quickly tried three times to close the hatch, but couldn’t get it to lock down. Lovell tried twice, and again couldn’t get it to stay closed. But by that time, Lovell thought, if the hull had been compromised, both spacecraft surely would have already depressurized and no such thing was happening. So, the crew set the hatch aside and moved on to looking at the falling gauges on the oxygen tanks.

And shortly after that, Lovell looked out the window and saw a cloud of oxygen venting out into space.

Earlier in the flight, the Apollo 13 crew had opened the hatches between Odyssey and Aquarius, and actually was far ahead on their checklist of preparing to land on the Moon by turning on equipment in the lander.

Woodfill believes this was fortuitous, as was the hatch not closing, because saving time was of the essence in this situation.

“Some people say that doesn’t amount to much time,” Woodfill said, “but I say it did, because if they had closed and latched up the hatch, and then worked to find the real problem of what was wrong, then they would have to delay and quit working the problem to go remove the hatch, stow the hatch and go power up the lander.”

Why was time so important?

The fuel cells that created power for the Command Module were not working without the oxygen from the two tanks. “Tank 2, of course, was gone with the explosion,” said Woodfill,” and the plumbing on Tank 1 was severed, so the oxygen was bleeding off from that tank, as well. Without oxygen you can’t make the fuel cells work, and with both fuel cells gone they know they can’t land on the Moon. And then it became a question of whether they can live.”

But over in Aquarius, all the systems were working perfectly, and it didn’t take long for Mission Control and the crew to realize the lunar module could be used as a lifeboat.

Screenshot from Apollo 13 footage of Jim Lovell and Jack Swigert during the mission. Credit: NASA

However, all the guidance parameters which would help direct the ailing ship back to Earth were in Odyssey’s computers, and needed to be transferred over to Aquarius. Without power from the fuel cells, they needed to keep the Odyssey alive by using the reentry batteries as an emergency measure. These batteries were designed to be used during reentry when the crew returned to Earth, and were good for just a couple of hours during the time the crew would jettison the Service Module and reenter with only the tiny Command Module capsule.

“Those batteries are not ever supposed to be used until they got ready to reenter the Earth’s atmosphere,” said Woodfill. “If those batteries had been depleted, that would have been one of the worst things that could have happened. The crew worked as quickly as they could to transfer the guidance parameters, but any extra time or problem, and we could have been without those batteries. Those batteries were the only way the crew could have survived reentry. This is my take on it, but the time saved by not having to re-open the hatch helped those emergency batteries have just enough power in them so they could recharge them and reenter.”

It’s interesting when the hatch had to work correctly, when the lander was jettisoned for re-enty, it worked perfectly. But at the time of the explosion, it’s malfunctioning kept the pathway to survival into the LM open, saving time. Being able to get into the lunar lander quickly was what helped save the crew’s life.

Tommorow: Part 3: The measles

Additional articles from the “13 Things That Saved Apollo 13”
series:

Introduction

Part 1: Timing

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.

13 Things That Saved Apollo 13, Part 1: Timing

[/caption]

Note: To celebrate the 40th anniversary of the Apollo 13 mission, for the next 13 days, Universe Today will feature “13 Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill. Click here for our preview article.

Oxygen Tank two in the Apollo 13 Service Module exploded at Mission Elapsed Time (MET) 55 hours and 55 minutes, 321,860 kilometers (199,990 miles) away from Earth. If the tank was going to rupture and the crew was going to survive the ordeal, the explosion couldn’t have happened at a better time. “Not everyone agrees with all the things I’ve come up with in my research,” said NASA engineer Jerry Woodfill who has studied the Apollo 13 mission in intricate detail, “but pretty much everyone agrees on this, including Jim Lovell. The timing of when the explosion happened was key. Much earlier or later in the mission would have prevented a successful rescue.”

If the explosion happened earlier (and assuming it would have occurred after Apollo 13 left Earth orbit), the distance and time to get back to Earth would have been so great that there wouldn’t have been sufficient power, water and oxygen for the crew to survive. Had it happened much later, perhaps after astronauts Jim Lovell and Fred Haise had already descended to the lunar surface, there would not have been the opportunity to use the lunar lander as a lifeboat.

But looking at why the explosion happened when it did shows how fortuitous the timing ended up to be.

The control panel of the Apollo 13 capsule. The module is on display at the Kansas Cosmosphere and Space Center in Hutchinson, KS. Photo courtesy Kansas Cosmosphere and Space Center.

The explosion occurred when Jack Swigert flipped a switch to conduct a “stir” of the O2 tank. The Teflon insulation on the wires to the stirrer motor in O2 tank 2 had unknowingly been damaged because the manufacturer failed to update the heater design for 65 volt operation, and the tank overheated during a pre-flight test, melting the insulation. The damaged wires shorted out and the insulation ignited. The resulting fire rapidly increased pressure beyond its nominal 1,000 psi (7 MPa) limit and either the tank or the tank dome failed.

The O2 tanks were stirred in order to get an accurate reading on the gauging systems, as the cryogenic oxygen tends to solidify in the tanks, and stirring allows for a more accurate reading on the quantity of O2 remaining in the tank.

But this was not the first time the crew had been ordered to stir the tank. It was the fifth time during the mission. And most interestingly, the tanks normally were stirred approximately once every 24 hours. So, why was it stirred that often?

In what Woodfill said was a problem unrelated to what caused the explosion, the quantity sensor or gauge was not working correctly on O2 tank 2. The EECOM (Electrical Environmental and Consumables) flight controller in Houston discovered that the quantity sensor was not reading accurately, and because of that Mission Control asked the astronauts to perform additional actuations of the stirrer to try and troubleshoot why the sensor wasn’t working correctly.

So, it took five actuations until the short circuit and the resulting fire and explosion occurred. If the gauge had been working correctly and the normal stirring of the tank had been done, that would have put the time of the fifth stirring after Lovell and Haise had departed for the lunar surface, and the rescue scenario that ultimately was carried out couldn’t have happened.

“Check the arithmetic,” said Woodfill. “Five actuations at 24 hour periods amounts to a MET 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.”

Apollo 13 crew: Jim Lovell, Jack Swigert and Fred Haise. Credit: NASA

Who knows what would have happened to the crew? The fuel cells required the liquid oxygen tanks. This meant no production of electrical power, water and oxygen. The attached lunar lander had to be available. Likely, the two ships couldn’t even have docked back together. And what if the accident had happened behind the Moon without mission control’s help? Alone in the Command module, Swigert would have had difficulty analyzing the problem. Without a fueled lunar lander descent stage attached, lacking its consumables and engines as well as the needed battery power, water and oxygen, the crippled Command Module could not have returned to Earth with live astronaut(s). Not only would Lovell and Haise have perished but Swigert’s fate would have been the same. Even if the damaged Service Module’s engine had worked, no fuel cells meant the ship would die. The situation that the Apollo 13 crew actually faced was dire, but the alternative scenario would certainly have been fatal.

Woodfill contends that the quantity sensor malfunction assured the lunar lander would be present and fully fueled at the time of the disaster. It was an extremely fortuitous event. Had it not occurred, the timing of the explosion would have been far different and the crew would have perished.

Additional Articles from the “13 Things That Saved Apollo 13” series that have now been posted:

Introduction

Part 2: The Hatch That Wouldn’t Close

Part 3: Charlie Duke’s Measles

Part 4: Using the LM for Propulsion

Part 5: Unexplained Shutdown of the Saturn V Center Engine

Part 6: Navigating by Earth’s Terminator

Part 7: The Apollo 1 Fire

Part 8: The Command Module Wasn’t Severed

Part 9: Position of the Tanks

Part 10: Duct Tape

Part 11: A Hollywood Movie

Part 12: Lunar Orbit Rendezvous

Part 13: The Mission Operations Team

Also:

Your Questions about Apollo 13 Answered by Jerry Woodfill (Part 1)

More Reader Questions about Apollo 13 Answered by Jerry Woodfill (part 2)

Final Round of Apollo 13 Questions Answered by Jerry Woodfill (part 3)

Never Before Published Images of Apollo 13’s Recovery

Listen to an interview of Jerry Woodfill on the 365 Days of Astronomy podcast.