"Fred [Haise] was still in the lunar module. Jack [Swigert] was back in the command module [CM] in the left-hand seat, and I was half-way in between, in the lower equipment bay, wrestling with TV wires and a camera, watching Fred come on down, when all three of us heard a rather large bang -- just one bang. 

_{View of damaged command module}

Now, before that ... Fred had actuated a valve which normally gives us that same sound. Since he didn't tell us about it, we all rather jumped up and were sort of worried about it; but it was his joke and we all thought it was a lot of fun at the time. So when this bang came, we really didn't get concerned right away ... but then I looked up at Fred ... and Fred had that expression like it wasn't his fault. We suddenly realized that something else had occurred ... but exactly what we didn't know."

Haise said he felt a vibration. Up in the CM Swigert reported,

"... about two seconds elapsed when I had a master alarm and a main Bus 'B' undervolt [loss of power] ... I transmitted to Houston that we had a problem (sound clip, AU 96.6K)."

Lovell continued:

"I guess it's kind of interesting to know what the feelings of the crew are when something like this happens. When you first hear this explosion or bang ... you don't know what it is. We've had similar sounds in the spacecraft before that were for nothing. ... and then I looked out the window and saw this venting ... my concern was increasing all the time. It went from 'I wonder what this is going to do to the landing' to 'I wonder if we can get back home again' ... and when I looked up and saw both oxygen pressures ... one actually at zero and the other one going down ... it dawned on me that we were in serious trouble."

Lovell's assessment was, if anything, conservative. The bang was the explosion of liquid oxygen tank #2 in the service module. This tank provided the vital oxygen used by the fuel cells that were Apollo's primary power source. There was a backup battery-powered electric supply in the Command and Service Module (CSM) with a lifetime of up to ten hours, but Apollo 13, at the time of the explosion, was 87 hours from home.

Emergency in Space

The serious nature of the emergency was starkly evident to the crew and Mission Control. Lovell and his crew mates were more than 200,000 nautical miles out in space with a dead Service Module, including its main propulsion engine. The explosion had wiped out the CSM's main supply of life-sustaining oxygen and power. The CM's 10 hours of operating life had to be reserved for the approach to the earth's atmosphere because, of the three components, it alone had a heat shield which would allow the crew to re-enter the atmosphere and splashdown safely.

The crew's salvation rested with the Lunar Module (LM), the oddly-shaped spacecraft designed to separate from the CSM, land two astronauts gently on the moon, sustain them while there and then carry them back to the mother ship in lunar orbit. But that mother ship was a partial wreck, drifting in space, and the LM became the lifeboat.

What followed was an epic struggle of skilled and highly trained astronauts working in close coordination with the ground-based team at Mission Control against the hostile environment of space. For 86 hours and 57 minutes, more than three days, the struggle continued until the final victory came: Odyssey's trio of orange-striped parachutes dropped the spacecraft into the gently rolling Pacific Ocean 3.5 nautical miles from the prime recovery ship, the carrier Iwo Jima.

Minor Problems

In retrospect, those looking for omens of disaster could have found a few in the early stages of the flight. Before launch, a helium tank showed a higher pressure than expected, and was watched with some concern. A member of the prime crew, Lt. Commander Thomas K. Mattingly, was exposed to German measles a few days before launch. Swigert, the backup crewman, replaced him. A liquid oxygen vent valve refused to close on the first command, and had to be re-cycled several times before it would shut.

In flight, the center engine of the S-11 stage cut off more than two minutes early and, to compensate, the remaining four engines were burned 34 seconds longer than planned. As a further remedy, the engine of the Saturn V's third stage was fired for an extra 9 seconds in the orbital insertion burn. The result was a trivial 1.2 feet per second velocity greater than predicted.

These problems, though, were insignificant. They were typical of the minor anomalies that launch and flight crews expect to encounter, and are trained to remedy.

The flight proceeded with gratifying smoothness. After Apollo 13 entered the lunar corridor, the CSM separated from the third stage and manoeuvred to extract the LM from its housing atop the stage.

Change in Trajectory

_{Earth as viewed from Apollo 13}

The change was routinely made on the mission's second day. No one knew this manoeuvre would later complicate the problem of getting the crew back safely because it would have to be reversed.

Mobilizing for the Emergency

 While the astronauts powered up the LM lifeboat, Mission Control set about mobilizing all the talents available to deal with the crisis. In addition to the contractor representatives normally assisting with the flight, the manufacturers of the major systems and sub-systems in the spacecraft made their top specialists immediately available. A coast-to-coast network of simulators, computers and experts was quickly hooked up. The operation was a tour de force of the breadth and depth of American technological competence.

The problem had two parts:

1. Getting Odyssey and Aquarius (call signs for the CM and the LM, respectively) on the quickest course for home.
2. Conserving consumables (power, oxygen and water) onboard.

"first milestone, because without knowing exactly which attitude the spacecraft is in, there's no way to tell how to burn or how to use the engines of the spacecraft to get the proper trajectory to come home. "

Conserving the Consumables

Cutting back on the use of consumables was the first order of business. The CM was shut down completely after Lovell and Haise powered up the LM and made sure it was functioning properly. Except for the final phase of the flight, the CM was only used as a bedroom. Later all the LM's systems except those relating to life support, communications, and environmental control were turned off, drastically reducing Aquarius' power consumption. The LM was designed to support two men for 49.5 hours, but these actions stretched its resources to provide life support for three men for 84 hours.

Using precious power, the first milestone, aligning the LM's guidance platform, was accomplished.

A Shorter Course for Home

The next step was an engine burn to return the spacecraft to a free-return trajectory. Normally, a course correction of this kind presents no difficulty. For Apollo 13, there were complications. The LM's descent engine, not the SPS, was doing the pushing this time, and it was pushing a spacecraft that had a different mass and centre of gravity. After running the manoeuvre through the computers and simulators on the ground, Mission Control ordered a burn that added 38 feet per second to the spacecraft's velocity.

The second "milestone" as Lovell put it, was cutting the time of the return leg. As Lovell described the situation:

"The nominal flight time ... was 153 hours (from launch) if we had done nothing else. Because consumables were critical, Fred was doing the back-of-the-envelope type of calculation and he figured, if we were lucky, we had about one hour of consumables to spare."

To improve this too-close-for-comfort margin, it was necessary to step up the spacecraft's velocity. Several velocity changes were possible. The one which would save the most time would bring the crew down in the Indian Ocean; a possibility, but recovery would be awkward. A second option would reduce the return time from 153 hours to 143 hours and bring the spacecraft in to the Pacific as planned. However, this option required a velocity increase of 860 fps. Mission Control called the rocket specialists at TRW Systems to determine whether the LM's descent engine could do it.

The descent engine can operate for 17 minutes. Engineers at TRW Systems had test-fired Aquarius' engine and were confident it was good. The tests had also given them a useful reading on its performance. Nevertheless, they went to the simulators and computers to make sure, and the findings confirmed that the engine could do the job with plenty to spare. Thus reassured, Mission Control worked out the procedures and relayed them to the crew.

The engine was fired, the manoeuvre successfully executed, and everyone breathed easier.

More Problems

The accident and its effects kept Mission Control, its teams of experts, and the simulators and computers busy for most of the return leg. Course corrections and new detailed timelines had to be worked out. Manoeuvres had to be recalculated to use a minimum of power and water (for cooling equipment). How would the linked LM and CM behave after the SM had been jettisoned? What would be the effect of discarding the LM one hour before re-entry?

Questions of this kind were put through the various simulator-computer complexes until the ground team was certain that all possibilities had been checked out, and the best answers were in hand. Astronauts Alan Shepard and Ed Mitchell operated one of the LM simulators at the Manned Spacecraft Centre in Houston and Gene Cernan and David Scott worked in the other. At Cape Kennedy, Astronaut Dick Gordon simulated emergency procedures in a third LM. One team of simulator specialists worked around the clock without a break. No procedure, no manoeuvre instruction, no checklist was relayed to the crew that hadn't been thoroughly proved out.

A Cold Ride Home

The ride home was a cold one. With the systems in the CM shut down, there was no internal heat source to maintain cabin temperatures. The inert CM settled to a level of 38 degrees F, so cold that the crew stopped using the couches for their sleep periods. They made makeshift beds in the LM, which was warmer than the CM but still uncomfortable. Worse than the discomfort, the cold prevented them from resting well, and Mission Control was concerned that fatigue might impair their ability to function.

Ingenuity At Work

Many of the difficulties that arose on the return were solved by "jury rigs" that were marvels of ingenuity. The atmosphere in the spacecraft cabins is "washed" of carbon dioxide (produced by the crew's exhalations) by canisters of lithium hydroxide. The LM's canister system overloaded and the carbon dioxide in the cabin atmosphere began a potentially dangerous rise.

_{"Jury Rigged" Canisters to Clean Air of Carbon Dioxide}

After studying the problem, Mission Control instructed Lovell to make an adapter that would attach a hose to the lithium hydroxide canisters in the CM so they could help purify the air. Lovell went the ground one better by splicing together two hoses so the rig would reach through the docking tunnel into the CM. Within an hour carbon dioxide levels dropped sharply.

The new re-entry procedure called for two course corrections, the first to get the spacecraft more toward the centre of the re-entry corridor and the second to refine the angle of entry which had to be between 5.5 degrees and 7.5 degrees. Without the guidance platform powered up, the normal method of determining the attitude of the spacecraft would be by taking star sights. However, ever since the explosion in the SM, the spacecraft had been shrouded in a cloud of debris that glittered in the sun and made sighting on a star impossible. A technique worked out during Apollo 8, using the earth's terminator and the sun, was used. Lovell recounted his reaction at a post-flight press conference:

"When the ground read out the procedure to us, I just couldn't believe it. I thought I'd never have to use something as way-out as this. And here I was on Apollo 13, using this very same procedure. Because it was a manual burn, we had a three-man operation. Jack would take care of the time. He'd tell us when to light off the engine and when to stop it. Fred handled the pitch manoeuvre and I handled the roll manoeuvre and pushed the buttons to start and stop the engine."

Readying the Command Module

Six and a half hours before re-entry, ground-based studies showed that the three CM batteries, two of which had been re-charged from the LM, did not have enough power to maintain all the CM systems throughout re-entry. After intensive simulation on the ground, Mission Control relayed a phased power-up sequence to the crew in which all the needed systems were up only 2.5 hours prior to re-entry, well within the capacity of the storage batteries.

On the final phase of the return leg, as Lovell later put it,

"...things were getting better all the time."

"We were in a different situation now, because, normally when you come home you have only the CSM ... now, though, we had a dead service module ... a command module that had no power ... a lunar module ... a wonderful vehicle ... but that didn't have a heat shield and shortly we'd have to abandon it.

Discarding the SM

"Our procedure ... was to make sure we had a good angle of entry and then at about 4.5 hours before re-entry to manoeuvre to a position to get rid of the service module."

"... the ground had read up a very nice timeline. The only nervous moment we had ... normal procedures require arming the logic busses [the pyrotechnic system that explodes the SM away] and letting Houston look at all the relays. At this time, we didn't have any telemetry with Houston and Fred came up and I said ... 'Fred, I'm all ready to jettison the service module ... just getting ready to arm the pyros.'"

"Fred said, 'I'll get a go from Houston.' I said, 'Fred, we don't have any telemetry with Houston so you're just going to have to put your fingers in your ears and stand by."

" ... so I armed the 'A' system and I could hear the relays ... and nothing happened [an encouraging sign] ... and I armed the 'B' systems and nothing happened ... so I kind of felt we were home free."

"The procedure went well -- we used a push-pull method ... Jim and Fred were in the LM and using the translation controller to give us some velocity. ... when Jim yelled 'Fire!', I jettisoned the service module and it went off in the midst of a lot of debris, which is usual."

Farewell, Aquarius

An hour and a half before re-entry, the LM "lifeboat" that had been their salvation was discarded. Mission Control radioed,

"Farewell, Aquarius, and we thank you."

_{Apollo 13 Lunar Module}

Lovell's benediction was:

"She was a good ship."

Clear of the LM, in Swigert's words, Odyssey "came on in" to the most accurate landing in the history of manned space flight.

A Successful Failure: Conclusion

_{Recovery of Crew After Splashdown}

By a matchless display of tenacity, resourcefulness, ingenuity and courage, a determined group of men at Mission Control working closely with a cool, expert crew averted catastrophe and brought the astronauts through a brush with death.

As an aborted mission, Apollo 13 must officially be classed as a failure, the first in 22 manned flights. But, in another sense, as a brilliant demonstration of the human spirit triumphing under almost unbearable stress, it is the most successful failure in the annals of space flight.