X-Main

The Flight of Apollo 13

by W. David Compton

Apollo 13, the third manned lunar landing and exploration mission, had been
tentatively scheduled in July 1969 for launch in March 1970, but by the end of
the year the launch date had been shifted to April. In August 1969 crew
assignments for Apollo 13 were announced: James A. Lovell commanded the prime
crew, which included Thomas K. Mattingly II as command module pilot and Fred
W. Haise as lunar module pilot. Their backups were John Young, John Swigert,
and Charles Duke. The target for the mission was the Fra Mauro Formation, a
site of major interest to scientists, specifically a spot just north of the
crater Fra Mauro, some 550 kilometers (340 miles) west-southwest of the center
of the moon's near side. On March 24, 1970, during the countdown demonstration
test for Apollo 13, KSC test engineers encountered a problem with an oxygen
tank in the service module. The spacecraft carried two such tanks, each
holding 320 pounds (145 kilograms) of supercritical oxygen. They provided the
oxygen for the command module atmosphere and (along with two tanks of
hydrogen) three fuel cells, which were the spacecraft's primary source of
electrical power. Besides power, the chemical reaction in the cells produced
water, which not only supplied the crew's drinking water but was circulated
through cooling plates to remove heat from certain critical electronic
components. The tanks were designed to operate at pressures of 865 to 935
pounds per square inch (psi) (6,000 to 6,450 kilopascals) and temperatures
between -340F and +80F (-207C to +27C). Inside each spherical tank were a
quantity gauge, a thermostatically controlled heating element, and two
stirring fans driven by electric motors. The fans were occasionally operated
to homogenize the fluid in the tank; it tended to stratify, leading to
erroneous quantity readings. All wiring inside the tank was insulated with
Teflon, a fluorocarbon plastic that is ordinarily noncombustible. Each tank
was fitted with a relief valve designed to open when the pressure rose above
1,000 psi (6,900 kilopascals); the tanks themselves would rupture at pressures
above 2,200 psi (15,169 kilopascals) . Both tanks were mounted on a shelf in
the service module between the fuel cells and the hydrogen tanks. The
countdown demonstration test called for the tanks to be filled, tested, and
then partially emptied by applying pressure to the vent line, thus forcing
oxygen out through the fill line. Number one tank behaved normally in this
test, but number two released only 8 percent of its contents, not 50 percent
as required. Test engineers decided to proceed with the rest of the test and
investigate the problem later. The next day, after KSC engineers had discussed
the problem with colleagues at MSC, North American Rockwell (builders of the
service module), and Beech Aircraft (manufacturers of the oxygen tanks), they
tried emptying the tank again, with no success. Further talks led to the
conclusion that the tank probably contained a loose-fitting fill tube, which
could allow pressure to escape without emptying the tank. When normal
procedures again failed to empty the tank, engineers decided to use its
internal heaters to boil off the contents and applied direct-current power at
65 volts to the heaters. This was successful but slow, requiring eight hours
of heating. It was then decided that if the tank could then be filled normally
it would not cause a problem in flight. A third test gave the same result as
the second, requiring heating to empty the tank. In view of the difficulty of
replacing the oxygen shelfa job that would take at least 45 hoursand the
possibility that other components might be damaged in the process and the
launch delayed for a month, NASA and contractor officials decided not to
replace the tanks. The spacecraft was launched on April 11,1970, and the
mission was quite routine for the first two days. At 30 hours and 40 minutes
after launch (30:40 ground elapsed time, or g.e.t.), the crew ignited their
main engine to put the spacecraft on a hybrid trajectory, a flight path that
saved fuel in reaching the desired lunar landing point. At 46:40 the crew
routinely switched on the fans in the oxygen tanks briefly. A few seconds
later the quantity indicator for tank number two went off the high end of the
scale, where it stayed. The tanks were stirred twice more during the next few
hours; and at 55:53, after a master alarm had indicated low pressure in a
hydrogen tank, the Mission Control Center (MCC) directed the crew to switch on
all tank stirrers and heaters. Shortly thereafter the crew heard a loud bang
and felt unusual vibrations in the spacecraft. Mission controllers noticed
that all telemetry readings from the spacecraft dropped out for 1.8 seconds.
In the CM, the caution and warning system alerted the crew to low voltage on
d.c. main bus B, one of two power distribution systems in the spacecraft. At
this point command module pilot Jack Swigert told Houston, Hey, we've had a
problem here. Because of the interruption of telemetry that had just occurred,
flight controllers in the MCC had difficultly for the next few minutes
determining whether they were getting true readings from the spacecraft
sensors or whether the sensors had somehow lost power. Before long, however,
both MCC and the crew realized that oxygen tank number two had lost all of its
contents, oxygen tank number one was slowly losing its contents, and the CM
would soon be out of oxygen and without electrical power. Among the first
actions taken were shutting down one fuel cell and switching off nonessential
systems in the CM to minimize power consumption; shortly after, the second
fuel cell was shut down as well. When the remaining oxygen ran out, the CM
would be dead; its only other power source was three reentry batteries
providing 120 ampere-hours, and these had to be reserved for the critical
reentry period. An hour and a half after the bang, MCC notified the crew that
we're starting to think about the lifeboatusing the lunar module (LM) and its
limited supplies to sustain the crew for the rest of the mission. Plans for
such a contingency had been studied for several years, although none had
anticipated a situation as grave as that of Apollo 13. Many of these studies
were retrieved and their results were adapted to the situation as it
developed. Shortly after the accident, mission commander James Lovell reported
seeing a swarm of particles surrounding the spacecraft, which meant trouble.
Particles could easily be confused with stars, and the sole means of
determining the spacecraft's attitude was by locating certain key stars in the
onboard sextant. Navigational sightings from the LM were difficult in any case
as long as it was attached to the command module, and this would only
complicate matters. Flight controllers decided to align the lunar module's
guidance system with that in the command module while the CM still had power.
That done, the last fuel cell and all systems in the command module were shut
down, and the crew moved into the lunar module. Their survival depended on
this craft's oxygen and water supplies, guidance system, and descent
propulsion engine (DPS). Normally all course corrections were made using the
service propulsion system (SPS) on the service module, but flight controllers
ruled out using it, partly because it required more electrical power than was
available and partly because no one knew whether the service module had been
structurally weakened by the explosion. If it had, an SPS burn might be
dangerous. The OPS would have to serve in its place. When word got out that
Apollo 13 was in trouble, off-duty flight controllers and spacecraft systems
experts began to gather at MSC, to be available if needed. Others stood by at
NASA centers and contractor plants around the country, in touch with Houston
by telephone. Flight directors Eugene Kranz, Glynn Lunney, and Gerald Griffin
soon had a large pool of talent to help them solve problems as they arose,
provide information that might not be at their fingertips, and work on
solutions to problems they could anticipate farther along in the mission.
Astronauts manned the CM and LM training simulators at Houston and at Kennedy
Space Center, testing new procedures as they were devised and modifying them
as necessary. MSC director Robert R. Gilruth, Dale D. Myers, director of
manned space flight, and NASA administrator Thomas O. Paine were all on hand
at Mission Control to provide high-level authority for changes. Soon after the
explosion, the assessment of life-support systems determined that although
oxygen supplies were adequate, the system for removing carbon dioxide (CO2) in
the lunar module was not. The system used canisters filled with lithium
hydroxide to absorb CO2 as did the system in the command module. Unfortunately
the canisters were not interchangeable between the two systems, so the
astronauts were faced with plenty of capacity for removing CO2 but no way of
using it. A team in Houston immediately set about improvising a way to use the
CM canisters, using materials available in the spacecraft. Flight controllers,
meanwhile, were addressing operational problems. Their first critical decision
was to put the crippled spacecraft back on a free-return trajectory, which was
accomplished by firing the LM descent engine at 61:30. Mission Control then
had some 18 hours to consider the remaining problems; the next was a possible
adjustment to change the spacecrafts landing point on earth. If this was to be
done, it was scheduled for PC + 2 two hours after pericynthion (closest
approach to the moon), after the spacecraft emerged from behind the moon. In
the interval, Houston worked out a new flight plan that would minimize the
consumption of oxygen, water, and electricity while keeping vital systems
operating. The alternatives for the PC +2 maneuver were worked out by about 64
hours g.e.t. A major consideration was the total time to splashdown. Left on
its free-return course the command module would return at about 155 hours
g.e.t. to a landing in the Indian Ocean. Three options would bring it back in
the mid-Pacific and could reduce the total mission time to as little as 118
hours. The fifth possibility returned the spacecraft in 133 hours, but to the
South Atlantic. For one reason or another, all but one of these choices were
discarded. The free-return (no course correction) choice was abandoned, since
there was no known reason not to use the LM descent propulsion system.
Recovery in either the Atlantic or the Indian Ocean was far from ideal; the
main recovery force was deployed in the mid-Pacific and there was not enough
time to move it or to make adequate arrangements elsewhere. Two options giving
the shortest return time (118 hours) had other drawbacks. Both would require
using virtually all of the available propellant, and it was not prudent to
assume that no additional course corrections would be required. One of them
involved jettisoning the service module, which would expose the CM heat shield
to the cold of space for 40 hours and raise questions about its integrity on
reentry. After five and a half hours of weighing the choices and their
consequences, flight directors met with NASA and contractor officials and
presented their findings and recommendations. The decision, made some ten
hours before the scheduled engine burn, was to go for mid-Pacific recovery at
143 hours. During all of these deliberations the atmosphere in the lunar
module was gradually accumulating carbon dioxide as the absorbers in the
environmental control system became saturated. Members of MSC's Crew Systems
Division devised a makeshift air purifier by taping a plastic bag around one
end of a CM lithium hydroxide cartridge and attaching a hose from the portable
life-support system, allowing air from the cabin to be circulated through it.
After verifying that this jury rig would function, they prepared detailed
instructions for building it from materials available in the spacecraft and
read them up to the crew. For the rest of the mission the improvised system
kept the CO2 content of the atmosphere well below hazardous levels. The
decision to recover in the Pacific fixed the time line for the remainder of
the mission and imposed some rigid constraints on preparations for reentry.
The final course correction had to be made with the LM engine; command module
systems had to be turned on and the guidance system aligned; the service
module had to be discarded; and when all preparations had been made, the lunar
module would be cut loose. In all these preparations the power available from
the CM's reentry batteries was a limiting factor. From the PC + 2 burn until
about 35 hours before reentry the sequence of activation of CM systems was
worked out, checked in the simulators, and modified. Fifteen hours before
beginning reentry the revised sequence of activities was read to the crew, to
give them time to review and practice it. The husbanding of expendable
resources, particularly electrical power, paid off on the morning of landing,
when it was discovered that power reserves in the LM were adequate to allow
use of it in the CM. Some of the early CM activities could then be done at a
less hurried pace. The Apollo 13 command module splashed down within a mile of
the recovery carrier with about 20 percent of its battery power remaining.
Three weary, chilled astronauts came aboard the U.S.S. Iwo Jima on April 17
and were flown to Hawaii for an emotional reunion with their families. Mission
Control teams and their hundreds of helpers were no less drained. The usual
cigars were lighted up after recovery, but the splashdown parties that evening
were subdued: most of those who went quit early and went home to bed. Their
efforts were recognized the next day when President Richard M. Nixon, on his
way to Hawaii, stopped in Houston to present the Presidential Medal of
Freedom, the nation's highest civilian award, to the entire team. NASA
immediately convened an investigation board to determine the cause of the
accident and postponed Apollo 14 until its results were in. Lacking the
spacecraft itselfthe service module had been jettisoned before reentry, and
the crew had been able to take only a few rather poor photographs of itthe
board initially had only the data from inflight telemetry to work with. When
it became clear that the fault lay in oxygen tank number two, the board
carefully reviewed its entire history, from fabrication to launch, as recorded
in the detailed documentation that followed every piece of equipment from
plant to launch pad. Under the board's direction, MSC and other NASA centers
conducted tests under simulated mission conditions to verify its findings. The
investigation, which concluded in a few weeks, turned up a highly improbable
sequence of human error and oversight that led inexorably to the failure in
flight. Board Chairman Edgar M. Cortright, director of Langley Research
Center, explained the board's findings to congressional committees in June.
The accident, he reported, was not a random malfunction but resulted from an
unusual combination of mistakes as well as a somewhat deficient and
unforgiving design. As the board's report reconstructed the events leading up
to the accident, the tank left Beech Aircraft's plant on May 3, 1967, after
passing all acceptance tests. It was installed as part of a shelf assembly in
service module no. 106 on June 4, 1963, having passed all tests conducted at
North American Rockwell during assembly. Design changes in the service module,
however, necessitated removing the entire shelf from SM 106 for modification.
During removal, which was accomplished by use of a special fixture that fit
under the shelf to lift it upward, workmen overlooked one bolt that held down
the back of the shelf, with the result that the removal fixture broke,
dropping the shelf two inches. The board concluded that this incident might
have jarred loose a poorly fitting fill tube. Subsequent tests did not detect
any flaws, and after modification the shelf was shipped to Kennedy Space
Center for installation in SM 109, the Apollo 13 spacecraft. What was not
known was that this oxygen tank was fitted with obsolete thermostatic switches
protecting its heating elements. Original specifications for the switches
called for operation on 28 volts d.c.; in 1965 this was changed to 65 volts
d.c. to match the test and checkout equipment at the Cape. Later tanks
conformed to the new specifications, but this one, which should have been
modified, was not, and the discrepancy was overlooked at all stages
thereafter. In a normal checkout of a normal tank, this would not have
mattered, because the switches would not have opened during normal operation.
But the improvised procedure used when this tank failed to empty (the result
of a loose fitting, as noted above) raised the temperature in the tank above
80F (27C), at which point the switches opened. Tests conducted during the
investigation showed that the higher current produced by the 65-volt power
source caused an arc between the contact points as they separated, welding
them together and preventing their opening when the temperature dropped. This
went undetected during the detanking procedure at the Cape; it could have been
noticed if anyone had monitored the heater current, which would have shown
that the heaters were operating when they should not have been. But all
attention was on the specific malfunction, and no one was aware that the
heaters were on continuously for eight hours on two separate occasions. The
result, as tests showed, was that the heater tube reached 1,000F (538C) in
spots, damaging the Teflon insulation on the adjacent fan-motor wiring and
exposing bare wire. From that point on, the board concluded, the tank was
hazardous when filled with oxygen and electrically powered. Teflon can be
ignited at a high enough temperature in the presence of pure oxygen, and the
tank contained small amounts of other combustibles as well. Unfortunately for
Apollo 13, the tank functioned normally for the first 56 hours of the mission,
when the heaters and the fans were energized during routine operations. At
that point an arc from a short circuit probably ignited the Teflon, and the
rapid pressure rise that followed either ruptured the tank or damaged the
conduit carrying wiring into the tank, expelling high-pressure oxygen. The
board could not determine exactly how the tank failed or whether additional
combustion occurred outside the tank, but the pressure increase blew off the
panel covering that sector of the service module and damaged the directional
antenna, causing the interruption of telemetry observed in Houston. It also
evidently damaged the oxygen distribution system, or the other oxygen tank, as
well, leading to the loss of all oxygen supplies and aborting the mission. The
board pointed out that although the circumstances of the tank failure were
highly unusual and that the system had worked flawlessly on six successful
missions, Apollo 13 was a failure whose causes had to be eliminated as
completely as possible. It recommended that the oxygen tanks be modified to
remove all combustible material from contact with oxygen and that all test
procedures be thoroughly reviewed for adequacy. Compared to the AS-204 fire in
1967, Apollo 13 was only a frightening near-miss, and because its cause was
localized and comparatively easy to discover, it had fewer adverse effects on
the program. Only the skill and dedication of hundreds of members of the
often-celebrated manned space flight team saved it, however, and the accident
served to remind NASA and the public that manned flight in space, no matter
how commonplace it seemed to the casual observer, was not yet a routine
operation. The same lesson had to be learned once more sixteen years later,
when on January 28,1986, the space shuttle Challenger and all seven of its
crew were lost a minute after launch. An unforgiving design and the failure of
human judgment under pressure combined again to bring a program to a halt
while corrective measures were taken. 

[Excerpted from W. David Compton, Where No Man Has Gone Before: A History of
Apollo Lunar Exploration Missions
(Washington, D.C.: NASA SP-4214, 1989), pp. 386-93.]