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50 Years Ago: “Houston, We’ve Had a Problem”

During their third day in space, the crew of Apollo 13, Commander James A. Lovell, Command Module Pilot John L. “Jack” Swigert and Lunar Module Pilot Fred W. Haise, traveling some 205,000 miles from Earth, had just concluded a television broadcast that included views of their Lunar Module (LM) Aquarius and Command Module (CM) Odyssey. Controllers in Mission Control at the Manned Spacecraft Center (MSC), now the Johnson Space Center in Houston, enjoyed the broadcast from their consoles, while Lovell’s wife Marilyn, accompanied by Dr. Charles A. Berry, MSC’s Director of Medical Research and Operations, watched the broadcast from the Visitors Gallery. The astronauts were looking forward to entering lunar orbit in less than day, followed by Lovell and Haise making a landing in the Moon’s Fra Mauro highlands the day after. During their 33-hour stay on the Moon, Lovell and Haise planned to complete two spacewalks, conducting geologic surveys and placing scientific instruments that would return data to scientists on the ground long after their departure. Swigert had his own science to conduct from lunar orbit.

apollo_13_mcc_tv_transmission_just_before_explosion_w_kranz apollo_13_marilyn-and-berry_in_mcc_minutes_before_accident
Left: Flight Director Kranz (in foreground) watches the Flight Day 3 TV broadcast
from Apollo 13 just minutes before the accident. Right: Marilyn Lovell (at left) chats
with Dr. Berry in the MCC Visitors Gallery just minutes before the accident.

As the astronauts completed the routine housekeeping chores of stirring the cryogenic liquid oxygen and liquid hydrogen in their tanks in the Service Module (SM), they heard a dull bang throughout the docked spacecraft, with Haise in the LM and Lovell and Swigert in the CM. At that precise moment, 55 hours 55 minutes and 4 seconds into the flight, Mission Control recorded a sudden shift in the spacecraft’s High Gain Antenna (HGA) communications accompanied by a strange crackle in the otherwise quiet radio stream. Within a few seconds, controllers at several consoles monitoring spacecraft systems began reporting to Flight Director Eugene F. “Gene” Kranz that they were seeing strange data from Apollo 13, including a restart of the main computer, and immediately began troubleshooting the new and unknown problem. Aboard the spacecraft, about 1 to 2 seconds after the bang, the Master Alarm light and the Main Direct Current (DC) B Bus Undervolt light went on, followed by a Fuel Cell light. That prompted Swigert to call Mission Control, “Okay, Houston, we’ve had a problem here.” Capsule communicator (Capcom) astronaut Jack R. Lousma asked, “This is Houston. Say again, please.” Lovell replied, providing some more detail on their condition, “Ah, Houston, we’ve had a problem here. We’ve had a Main B Bus Undervolt.” Lovell reported that sensors picked up a drop in voltage across one of the two main DC electrical buses supplying power to the Command and Service Module (CSM), triggering the warning lights to alert the crew to the problem.

apollo_13_fd3_kranz_post_accident_apr_13_1970 apollo_13_fd3_capcoms_post_accident_apr_13_1970
Left: Flight Director Kranz shortly after the accident. Right: Capcom Lousma
(seated) assisted by (left to right) Slayton, Mattingly, Stafford and Young,
shortly after the accident.

At this point, neither the crew nor the team in Mission Control knew the cause of the problem. Because the voltage on Bus B had come back to normal, everyone’s first inclination was to blame instrumentation or some unknown transient event. But Haise, having left Aquarius and now back in Odyssey, radioed down that despite the now normal voltages, there was “a pretty large bang associated with the Caution and Warning there.” The crew first suspected that something had happened to the LM, perhaps a micro-meteoroid strike, and Lovell and Swigert rushed to close the hatch to isolate the two spacecraft. Haise noted that the quantity sensor for oxygen tank 2, which had failed earlier in the mission by showing off-scale high and located in the same tank that had experienced anomalies during the Countdown Demonstration Test back in March, was now oscillating between 20% and 60%, perhaps rocked back to life by the jolt. The crew suddenly faced a cascade of malfunctions. First Lovell noted that sensors in the helium tanks used to pressurize the SM’s Reaction Control System (RCS) thrusters were showing alarms as were some of the propellant systems. The DC Main B Bus that showed normal just a few minutes before now showed zero volts, the Alternating Current (AC) bus 2 read zero and the DC Main A Bus showed an undervolt, meaning instead of the normal 27 volts it was reading 25.5 volts. Two of the three fuel cells (FCs) that combined oxygen with hydrogen to supply power to the DC busses appeared to be offline, explaining why Main Bus B showed zero volts as it received power from FC 3 and why Main Bus A’s voltage was dropping because it drew power from FC 1 and 2 and FC 1 was offline. Haise attempted to reconnect the fuel cells to get the busses back up but neither FC 1 nor FC 3 appeared to be operational. In Mission Control, the team was trying to ascertain if they were dealing with an instrumentation issue or if there was a real problem. Five minutes had elapsed since the crew heard the bang.

apollo_13_sm_sector_4_schematic_colorized apollo_13_sm_electrical_schematic
Left: Schematic of the SM tanks and fuel cells. Right: Schematic of the SM Electrical
Power System at the time of the accident.

About 14 minutes after the incident began, Lovell called down to say that the quantity in oxygen tank number 2 now read zero, and even more worrisome, “it looks to me, looking out the hatch, that we are venting something. We are venting something out into space. It’s a gas of some sort.” The venting couldn’t be just instrumentation. Kranz got on the internal Mission Control loop to talk to his controllers with some of the most memorable words spoken in human spaceflight:

“Okay now, let’s everybody keep cool, we got the LM still attached, the LM spacecraft’s good so if we need … to get back home we’ve got a LM to do a good portion of it with. Okay, let’s make sure that we don’t do anything that’s going to blow our CSM electrical power with the batteries or that will cause us to lose the main or the fuel cell number 2. Okay, we want to keep the O2 and that kind of stuff working. We’d like to have RCS, but we got the Command Module system, so we’re in good shape if we need to get home. Let’s solve the problem but let’s not make it any worse by guessing.”

Fifteen minutes had elapsed since the crew heard the bang. 

apollo_13_mcc_fd3_kranz_shortly_after_the_accident apollo_13_fd3_kranz_surverys_mocr_after_the_accident
Two views of Flight Director Kranz shortly after the accident.

To Mission Control and the crew onboard Apollo 13 several things were becoming clear even as the underlying cause remained unknown. The CSM was losing oxygen required for the crew to breathe and to generate power in the fuel cells. Mission Control estimated that at the rate the quantity was decreasing in oxygen tank 1 they had less than two hours of power generation remaining. The LM appeared unaffected and the idea of using it as a lifeboat started to be discussed. The continued venting caused the SM’s RCS thrusters to fire to try to maintain the spacecraft’s attitude, important to maintain good communications. With FC 1 and FC 3 off line, the crew wanted to preserve FC 2 as long as possible. Haise called down to Mission Control that it might be a good idea to think about the consumables available in the LM, even though it could support two crewmembers for two days and now it would have to support three crewmembers for four days. Critical consumables included oxygen for the crew, power to run the spacecraft’s systems including the navigation and life support equipment, water both for drinking and to cool the electronic equipment, and carbon dioxide removal.

Of immediate urgency the crew needed to preserve enough consumables such as electricity and oxygen in the CM to allow for a successful reentry and splashdown at the end of the mission. The two sources of oxygen in the CM were a surge tank and a repress package, the latter a set of three tanks used to repressurize the cabin in case it had to be vented – the crew needed to isolate the surge tank to ensure it retained sufficient oxygen for the crew to breathe during reentry. Thinking the leak might be coming from FC 3, Mission Control asked the crew to shut it down – the action ruling out a Moon landing since flight rules stated the spacecraft cannot enter lunar orbit with only two working fuel cells. Shutting down FC 3 had no effect on stopping the leak because the problem was not with any of the fuel cells but with the tanks supplying oxygen to all three of them – tank 2 was empty and tank 1 getting there. 

Less than an hour after the crew heard the bang, Flight Director Kranz instructed his team that the new mission plan involved abandoning the Moon landing, looping around the Moon and getting the crew home safely as quickly as possible. The first order of business was getting back onto a free-return trajectory. A course correction on the mission’s second day specifically maneuvered Apollo 13 from such a path onto a hybrid trajectory to enable the landing at Fra Mauro. With the landing off the table, they needed to make another course correction to maneuver back to the free return path. Kranz knew that the SM’s Service Propulsion System (SPS) engine could not be used so he ordered his team to prepare plans to use the LM’s Descent Propulsion System (DPS) engine, normally used to land on the Moon, to perform the maneuver. Fortunately, engineers had previously drawn up contingency plans to use the DPS to maneuver the docked spacecraft in case of an SPS failure. 

apollo_13_managers_during_the_crisis apollo_13_fd3_lunney_post_accident_apr_13_1970
Left: Flight Directors Lunney and Griffin (at left) confer with managers McDivitt
and Slayton (at right). Right: Lunney in a thoughtful moment shortly after the accident.

His shift over, Kranz handed Mission Control over to Flight Director Glynn S. Lunney and his Black Team of controllers with Lousma staying on as Capcom. Normally the team going off-shift would go home for much needed rest, but this night they stayed and pored over all the available data to understand what happened aboard Apollo 13. Lunney began directing his team to come up with procedures to activate the LM Aquarius as quickly as possible to ensure that its communications, navigation and life support systems were all functioning before the CM Odyssey had to be shut down. Lousma called up to the crew that as the quantity in tank 1 “was slowly going down to zero, … we’re starting to think about the LM lifeboat.” Ninety minutes had elapsed since the crisis began.

apollo_13_fd3_lunney_post_accident_apr_13_1970 apollo_13_fd4_capcom_lousma_with_slayton_mattingly_brand_and_young_apr_14_1970
Left: Flight Director Lunney (seated) shortly after the accident. Right: Capcom Lousma
(seated in white shirt) assisted by (left to right) Slayton, Mattingly, Brand, and Young.

It was now a race to activate the LM and get all three crewmembers inside AquariusOdyssey had just 15 minutes of power left. Haise already in Aquarius radioed to Lousma with the good news, “Jack, I got LM power on.” He performed an abbreviated activation to get oxygen and water flowing, and the electrical, navigation and communications systems enabled. One unexpected communications issue arose – the LM used the same frequency as the spent S-IVB stage, on its way toward an impact with the Moon. Under normal circumstances, this interference would not have occurred since the stage would have crashed before the crew activated the LM. Meanwhile, back in Odyssey Swigert busied himself with deactivating that spacecraft, completing the task less than three hours after the crisis began.

apollo_13_fd3_press_conference_kraft_mcdivitt_sjoberg_apr_14_1970 apollo_13_fd3_mcc_engineers_working_the_problem_apr_13_1970
Left: NASA managers (left to right) Kraft, McDivitt and Sjoberg discuss Apollo 13’s
condition a few hours after the accident. Right: Controllers in Mission Control
work to save the Apollo 13 crew.

With the immediate crisis of finding the crew a safe haven over, Lunney addressed his team concerning the major issues they all faced – the maneuvers needed to get them home as quickly and safely as possible; reducing power usage in the LM to ensure its batteries lasted as long as possible; and assessing the onboard consumables especially carbon dioxide removal. During a midnight press conference, Deputy Director of the Manned Spacecraft Center, now the Johnson Space Center in Houston, Christopher C. Kraft, James A. McDivitt, Apollo Spacecraft Program Office Manager, and Sigurd A. “Sig” Sjoberg, Director of Flight Operations, provided reporters with a status of the problems with Apollo 13, with Kraft describing it “as serious situation as we’ve ever had in manned spaceflight.” The managers expressed confidence that the crew would return safely to Earth, although McDivitt added, in comparing the current crisis to the Gemini 8 accident in 1966, that that crew was able to return to Earth in 20 minutes while Apollo 13 had many hours to go.

apollo_13_trajectory_corrections_to_get_home apollo_13_fd5_darkened_cm_during_transearth_coast
Left: Apollo 13’s trajectory before and after the accident. Right: Darkened CM
after its shutdown, the only light coming from the Sun shining through the windows.

At 61 hours 30 minutes into the mission, at a distance of 216,626 miles and about 5 and a half hours after the accident, the astronauts conducted a 34-second firing of the LM’s DPS engine to put the spacecraft back onto a free-return trajectory. The maneuver raised the altitude of their closest pass around the back side of the Moon from 69 miles to 156 miles. If the crew did nothing else, on this trajectory they would splash down in the Indian Ocean at about 151 hours and 45 minutes. This was not an optimal scenario since the prime recovery ship, the USS Iwo Jima (LPH-2), was stationed in the Pacific Ocean and Mission Control wanted them home sooner to ensure adequate consumables remained. A second maneuver after they rounded the Moon, called PC+2 for two hours after pericynthion or closest point to the Moon, would speed up their return and move the landing zone into the Pacific Ocean.

apollo_13_post_accident_capcom_brand_w_young apollo_13_mcc_fd4_kranz_and_griffin_apr_14_1970
Left: Capcom Brand (left) confers with fellow astronaut Young (right).
Right: Flight Directors Kranz (left), Griffin (right) and Lunney (just visible at far left).

In Mission Control, Joseph P. Kerwin took over the Capcom position from Lousma and Flight Director Gerald D. “Gerry” Griffin’s Gold Team took their seats at their consoles. For a short time, backup Lunar Module Pilot Charles M. Duke, recovered from his bout of German measles, took over the Capcom position, followed by Vance D. Brand. For the next few hours as the astronauts continued to approach the Moon, they continued conserving power in Aquarius and making preparations for the PC+2 burn. Kerwin advised them to move the lithium hydroxide canisters, used to scrub the cabin’s atmosphere of carbon dioxide (CO2), from Odyssey to Aquarius. A buildup of condensate in the cold CM cabin could render the canisters unusable. The crew entered a rest period although sleep was understandably hard to come by. Shortly before Apollo 13 entered the Moon’s shadow, Kranz’s White Team of controllers arrived back in MCC to take their consoles, just 18 hours after the accident.

apollo_13_as-13-60-8636_keeler apollo_13_as13-60-8637_tsiolkovsky apollo_13_as-13-60-8648_mare_muscoviense
Three images from Apollo 13 as they round the farside of the Moon.
Left: Keeler Crater. Middle: Tsiolkovsky Crater. Right: Mare Muscoviense.

At 76 hours 42 minutes, Apollo 13 entered the Moon’s shadow and the astronauts could now see stars that had been obscured by the cloud of debris following the spacecraft. Precisely on schedule, 26 minutes later and continuing to accelerate as the Moon’s gravity pulled them on, Apollo 13 disappeared behind the Moon and communications stopped. While behind the Moon, the crew of Apollo 13 set a record that stands to this day as the most distant human space travelers. Because the Moon was further in its elliptical orbit around the Earth than during other Apollo missions and because Apollo 13’s closest approach was farther from the Moon than other missions, Lovell, Swigert and Haise hold the distance record of 248,655 miles. For Lovell, this was his second time seeing the Moon up close, but for Swigert and Haise this was their single opportunity to peer down at Earth’s satellite and like tourists anywhere they took a number of spectacular photographs especially of features on the farside.

apollo_13_as13-60-8683_maria_crisium_tranquilitatis_serenitatis_fecunditatis apollo_13_s-ivb_impact_seismic_recording apollo_13_mattingly_around_the_pc2_burn
Left: Photo of the Moon as Apollo 13 began its homeward journey. Right: Apollo 12’s seismometer recording the impact of Apollo 13’s S-IVB stage. Right: Mattingly in MCC during the PC+2 maneuver.

Precisely 25 minutes after disappearing behind the Moon, Apollo 13 reappeared on the other side, with Lovell greeting Mission Control, “Good morning, Houston.  How do you read?” As they departed the Moon, the astronauts snapped a few more photographs including the area of the Apollo 11 landing site, the Sea of Tranquility. Thirty minutes later, Brand informed the crew that the S-IVB stage had impacted the Moon and Apollo 12’s seismometer recorded the event from 85 miles away, prompting Lovell to wryly respond, “Well, at least something worked on this flight.” Haise added, “I’m sure glad we didn’t have a LM impact too,” a reference to the planned impact of the LM’s ascent stage after he and Lovell had returned from the Moon. An hour and a half later, with the Moon now more than 6,200 miles behind them, the astronauts fired the LM’s DPS engine for 4 minutes and 24 seconds to accelerate the spacecraft and return them to Earth 10 hours earlier and in the Pacific Ocean. Mission Control filled with many astronauts and managers, including NASA Administrator Thomas O. Paine, to observe the PC+2 maneuver. After completing the burn, the astronauts powered down Aquarius to conserve the spacecraft’s batteries. One unpleasant side effect of the LM power down along with the total shutdown on the CM was cabin temperatures dropping to uncomfortable levels without the electronic equipment generating heat, making sleep more difficult.

apollo_13_co2_system_test_in_vacuum_chamber apollo_13_mcc_slayton_showing_lioh_adaptor apollo_13_fd4_lioh_setup_swigert_and_lovell_hands
Left: Setup to use CM LiOH canisters in the LM during an altitude chamber test.
Middle: Slayton demonstrating CM LiOH canister setup to managers in Mission Control.
Right: Swigert (right) and Lovell assembling the CM LiOH setup in the LM

The next order of business involved rigging up a system to use the LiOH canisters from Odyssey that are square in shape with the system in Aquarius that uses round canisters. This provided the crew with adequate CO2 removal for the remainder of the flight. Brand informed the astronauts that a team was working on a procedure to solve this “square peg in a round hole” problem. Indeed, a team led by Robert E. “Ed” Smylie, Chief of the Crew Systems Division, his Deputy James V. Correale and Test Director James C. “Jim” LeBlanc did exactly that, using only replicas of items the crew had onboard. The setup involved using a spacesuit’s liquid cooled garment and inlet hose, plastic bags, a cue card from the flight plan and of course duct tape to attach the CM canister to the LM’s outlet hose to pass air through to scrub the CO2. The engineers tested the setup in an altitude chamber and Capcom Kerwin called the procedure up to the crew. Swigert and Lovell built two of the makeshift contraptions and within minutes after the astronauts installed the first one in Aquarius, CO2 levels dropped significantly. 

apollo_13_lovell_swigert_eating_in_lm apollo_13_fd5_haise_in_lm
Left: Lovell (left) and Swigert eating in the LM during the Earthward coast.
Right: Haise in the LM during the Earthward coast.

Brand called up to the astronauts about the status of their consumables and all looked adequate to get them home safely. To reassure them further, Chief of Flight Crew Operations Donald K. “Deke” Slayton got on the Capcom loop, and knowing they hadn’t had much sleep since well before the accident, advised them, “We think you guys are in great shape, all the way around. Why don’t you quit worrying and go to sleep.” Lovell responded, “Well, I think we just might do that – or part of us will,” a reference to the fact that one astronaut stayed awake on watch duty at all times since the accident. He and Swigert slept for about five hours in Odyssey with Haise keeping watch in Aquarius. After that exchange, Lousma relieved Brand on the Capcom console while Flight Director Windler’s Maroon Team of controllers took their respective consoles. 

apollo_13_fd5_lunney_99_hour_presser apollo_13_fd5_mcc_griffin_windler
Left: Flight Director Lunney at a press conference during the Earthward coast.
Right: Flight Directors Griffin (left) and Windler during the Earthward coast.

The astronauts put the spacecraft in a Passive Thermal Control (PTC) or barbecue mode, rotating about once every 11 minutes along the longitudinal axis to evenly distribute temperatures. The spacecraft experienced a slight wobble, likely due to venting from one of the SM’s hydrogen tanks as they heated up during the sunlit part of the PTC rotation. Lunney’s Black Team of controllers came on to relieve Windler’s engineers and Kerwin came back in to replace Lousma at the Capcom console. At 90 hours and 25 minutes, at a distance of 216, 277 miles, Apollo 13 passed from the Moon’s gravitational field back into the Earth’s and slowly began to accelerate toward its home planet.

apollo_13_mcc_kerwin_around_get_90 apollo_13_fd5_darkened_cm_including_hatch_and_docking_probe
Left: Capcom Kerwin during the Earthward journey.
Right: Darkened CM during the Earthward journey.

Kerwin and Swigert spent some time to properly configure all 417 switches and circuit breakers in Odyssey in preparation for a brief partial activation to check on its systems and also for the eventual reactivation for reentry. Swigert returned to the dark CM and verified that both main busses operated properly, a good sign since they were needed for reentry. Shortly after Brand replaced Kerwin as Capcom at about 97 hours, Haise reported hearing a thump in the LM’s descent stage, followed by a “shower of snowflakes.” Unbeknownst to the crew and Mission Control, battery number 2 (of four) in the descent stage experienced a short circuit, with a potentially dangerous release of oxygen and hydrogen gas. Two hours later, the crewmembers reported a warning alarm regarding battery 2 and Mission Control, with Griffin’s Gold Team having relieved Lunney’s Black Team, advised them to turn it off only to reverse the decision a few hours later, not fully understanding the problem. Swigert returned to Odyssey and powered it up briefly so Mission Control could receive enough telemetry to check on the status of its systems. They all appeared to be working well, especially given the cold temperatures, a positive sign for when they powered the capsule up for entry. For the next few hours, the astronauts prepared themselves for the next maneuver, again to be conducted using the LM’s DPS engine but without using the LM’s computer to conserve power and cooling water, they would have to maintain the spacecraft’s attitude using visual sightings of the Earth and Sun out the windows. Lousma had returned to the Capcom console to help the crew with the maneuver. At 105 hours and 18 minutes into the flight, and still 175,000 miles from Earth, the astronauts ignited the LM’s DPS engine for 14.8 seconds. The burn had the desired effect of fine tuning their reentry path into the Earth’s atmosphere. Windler and his Maroon Team resumed their positions in MCC, replacing Griffin’s Gold Team. With about 38 hours remaining in the perilous journey, Lovell took over the watch aboard Apollo 13 while Swigert and Haise got a few hours of much needed sleep.

To be continued…

John Uri
NASA Johnson Space Center