Given the shorter time period involved, the direct abort was initially favoured. However, Kranz was concerned about damage that might have occurred with the SPS. Would it perform? Would it explode? Would it simply drain the remaining fuel cell completely? Given the LM’s descent propulsion system (DPS) was undamaged and had sufficient fuel, he opted to go with the free return option. It might take longer, but with adjustments, the LM’s systems could support the three men for a four-day flight.
Even so, there were additional challenges. Firstly, Apollo 13 wasn’t actually on a free-return trajectory; it was on what was called a hybrid trajectory, intended to allow it to reach its intended landing site at Fra Mauro. To get back onto a free return to Earth, the crew would have to complete a major engine burn as they passed around the Moon using the less powerful DPS. Such burns also required the use of reference stars to back up what the instruments were reporting – but the cloud of debris around Apollo 13 made it almost impossible to accurately identify the correct stars.
A more deadly issue lay with the fact that while the LM had the necessary fuel for the manoeuvres and a large enough air supply to support three men, the latter required “scrubbing” to remove excess carbon dioxide created by the astronaut’s exhalations, and the filters designed for this were not intended for such extended use.
However, at least with the LM operating as a lifeboat, the crew and mission control would have time to address the other issues. So Haise started powering-up the LM, while Swigert and Lovell started transferring equipment and supplies from the CM and powering it down in an orderly fashion, with Lovell taking the responsibility for manually copying the CM’s guidance orientation data and carrying out hand calculations to transfer it to the LM’s guidance system.
Immediately after doing so, and with the Moon getting closer, the issue of the engine burn became the focus of attention. With its lower power, the DPS would place Apollo 13 on a slow return to Earth that would see it splash down in the Indian Ocean, where the US Navy had few resources to perform recovery operations. The return flight needed to be speeded up. One way to do this would be to jettison the SM prior to reaching the Moon. This, coupled with a single, long burn of the DPS would allow the light CM / LM to reduce the return flight time by 36 hours and bring the CM down over the Pacific Ocean, where recovery vessels could more easily be on hand. But this would also mean the CM’s heat shield would be exposed to the cold of space – something it was not designed to endure, and the fear was it might fail during atmospheric entry as a result.
Instead, it was decided to keep the SM attached to the CM and go for a burn that would reduce the return flight time by just 12 hours, whilst still brining the command module back down over the Pacific and additionally leaving the LM with reserves of fuel for additional course corrections, should they been needed.
This engine burn took place 22.5 hours after the initial explosion and 2 hours after pericynthion, the closest approach to the Moon. It required the skills of all three astronauts working in unison to complete, and at the end of the 4 minute 23 second burn and despite relying on line-of-slight positioning using the Moon and the Sun, Apollo 13 was just 0.3m outside its optimal return flight path. In completing the burn, the crew set the record for the highest absolute altitude attained by a crewed spacecraft to date: 400,171 km.
Following the burn, the real hardship set in. To conserve power, the LM was almost completely powered-down, and the temperature rapidly dropped to 3 °C (38 °F). Some of this could be reduced by passive thermal control – using the LM’s reaction control system (RCS) thrusters to set it and the CM rotating slowly around their central axis at about 3 revolutions an hour. This helped even out the temperature differential between one side of the craft always being in sunlight and the other in shadow, but it still left the cabins terribly cold overall. Because the LM’s fuel cells did not produce water as a by-product (as was the case with those on the CM), water was limited to the tanked supply in the LM and whatever Swigert had been able to transfer over in bags from the LM, with each man rationed to just 0.2 litres (1/2 pint) of water a day.
Swigert, who didn’t have lunar overshoes as he would never set foot on the Moon, suffered particularly from the cold, while Haise would develop a urinary tract infection as a result of dehydration. All three had to face the discomfort of leaking urine bags, as mission control advised against discharging urine into space in case it caused a reaction that gradually pushed the craft off-course, which combined with exhaled breath to create condensation that covered every surface, making the LM a cold, dank environment.
Meanwhile on Earth, and using items known to be aboard the CM and LM, engineers started cobbling together what came to be known as “the mailbox”, an improvised means of using carbon dioxide filters from the CM (and which were otherwise incompatible with the LM’s system) to support the latter, thus extending the LM’s supply of usable air. Once built and confirmed, CAPCOM Joe Kerwin relayed the instructions step-by-step to Swigert and Haise over the course of an hour, enabling them to build it and confirm it was working – and not a moment too soon; CO2 levels in the LM were getting alarmingly high.
Despite the accuracy of the initial DPS burn, the craft gradually drift, requiring course corrections. Again, without a means of checking their readings against reference stars, the crew relied on the day/night terminator on Earth. The first course correction took place 105 hours into the flight and used the DPS, once again, Lovell put Apollo 13 right back on the ball.
The second came 32 hours later as Apollo 13 approached Earth, and used the LM’s RCS. 30 minutes after this, the CM / LM combination separated from the service module, and the crew got its first look at the extent of the damage to the latter. An entire 4.6 m long section of the SM had been destroyed, blowing out a surface panel, completely wrecking the mounting rack for the power cells, disrupting the oxygen tanks, with damage to both the high-gain communications antenna at the back of the SM and the SPS engine bell – possibly justifying Kranz’s concerns about using the SPS in a direct abort scenario.
Three final issues now remained: charging the CM’s batteries, powering up the CM, and separating the CM from the LM ahead of atmospheric entry.
It was vital for the CM’s batteries to be at full charge as once separated from the SM, they would be its only source of power. Fortunately, NASA had a procedure for catering for this, using the LM’s batteries to trickle-charge the CM’s batteries. The remaining problems were more complicated.
The fact was that the CM’s systems were not designed to be restarted once fully powered down. So a team led by flight controller John Aaron along with Ken Mattingly had to develop, test and document an entirely new start-up procedure using the most minimal amounts of power to avoid over-taxing the CM’s batteries, and in less than three days by.
Separating the command module from the lunar module also required an entirely new procedure. Under normal circumstances, this would be done by releasing the LM from the CM’s docking clamp and then backing the CSM away from the LM using the service module’s RCS. However, Apollo 13’s return to Earth required the SM to be jettisoned first, so its RCS would not be available. Canadian scientist Bernard Etkin instead calculated the amount of air pressure within the docking tunnel needed to push LM and CM apart if the atmosphere in the tunnel were vented.
This is Apollo Control, Houston and 141 hours 31 minutes into the flight. We’ve had lunar module jettison. Apollo 13, the age of Aquarius ended at 141 hours 30 minutes, ground elapsed time.
– NASA Manned Spsceflight Centre, after the Apollo 13
CM Odyssey had separated from the LM Aquarius
Both of these procedures worked perfectly. The first saw Odyssey come fully back to life, and the second allowed Aquarius to precede the CM into the upper reaches of the Earth’s atmosphere and fall away to burn up.
Atmospheric entry was the final hurdle Apollo 13 had to face. Such was the shallow angle at which it entered the atmosphere, there were fears it might simply “skip” off again and be lost in deep space. The shallow angle also meant the heat shield would be exposed to high temperatures far longer than intended, and so might fail. As it was, the period of communications blackout extended well beyond the anticipated time period, fears grew that the latter had happened.
Finally, after more than six minutes of blackout, the Odyssey responded to calls from mission control splashed down safely in the South Pacific Ocean at 18:7:41 UTC on April 17th 1970 , south-east of American Samoa and 6.5 km from the recovery ship, USS Iwo Jima. Dehydration and Haise’s infection notwithstanding, all three men were in good spirits and within 48 hours were in Hawaii where they were met by President Richard Nixon, who awarded each the Presidential Medal of Freedom, America’s highest civilian honour.
A post-flight investigation revealed the most likely root cause of the Apollo 13 incident lay within a number of factors related to SM oxygen tank 2. This tank had been built for use in the Apollo 10 mission, and was equipped (like all the tanks) with two thermostatic switches. Under NASA’s original requirements, these switches were designed to operate under a 28-volt DC power supply. However, in 1965, the power requirement was updated 65 volts – but these particular switches were never tested at the new voltage.
Prior to Apollo 13’s launch, the SM’s oxygen tanks were part of a launch pad countdown demonstration test, with both filled with liquid oxygen. After the test, engineers were unable to properly drain the tank, prompting them to turn on the heaters inside it in the hope of converting the oxygen to gas which could then be vented.
While this worked, it is believed the two thermostatic switches failed due to running at 65 volts. This in turn prevented a proper shut down of the tank’s heaters which continued to heat the empty tank to 540°C (which went undetected as the tank’s temperature gauge was not designed to read higher than 29°C), most likely damaging the Teflon insulation intended to protect and isolate the electrical wiring in the tank.
Tests conducted as a part of the investigation revealed that a similar set of circumstances, coupled with the repeated use of the fans in a tank (as required during a mission) could cause a complete failure of the Teflon insulation. This in turn would allow an electrical short to occur, igniting the oxygen and an eventual explosion of the type experienced by Apollo 13.
The post-flight investigation also revealed that the S-II centre motor shut down during launch as a direct result of “pogoing”, the term used for vibrations from the thrust of a rocket travelling up and down the vehicle’s longitudinal axis. Given their tremendous thrust, the first and second stages of the Saturn V were particularly prone to “pogo” events. However, with Apollo 13, it harmonised with natural cavitation in the engine’s turbopump to cause a unique set of stress vibrations that, had they not been detected by engine sensors that triggered the shut-down, would have resulted in a catastrophic engine failure just 16 second later, culminating in the loss of vehicle and crew.
The results of these findings led to a redesign of the SM oxygen tank systems and changes already planned for the remaining Saturn V boosters were implemented to dramatically reduce pogo vibrations during orbital ascents. Together, these changes meant the next Apollo flight would not occur until early 1971.
Apollo 13 was marked by a series of unfortunate events – but it was a mission that ultimately ended in success thanks to a combination of forethought, training, skill and luck. With just three notable exceptions, NASA had had the foresight to consider most of the scenarios encountered during them mission and had developed procedures to deal with them – the agency just never imagined so many would all be required at the same time. Both the training and the skill of the three astronauts also played a significant role in ensuring their safe return to Earth. But luck also played a hand because had the initial explosion occurred almost anywhere else in Apollo 13’s journey either to or from the Moon, the crew would have certainly been lost.
The mission was also remarkable for the way it united planet Earth. For four brief days, differences of religion, politics race or gender were set aside by the peoples of the world with access to the news, as they joined in hope and prayer for three men so far from Earth.
Of the crew, Jim Lovell marked Apollo 13 as his final flight into space, having previously flown on Gemini 7 and Gemini 12 and Apollo 8, the first Apollo flight around the Moon. He and his wife Marilyn both had cameo roles in the 1995 film Apollo 13, and he is currently the only living person to have travelled to the Moon twice without once setting foot on it.
Fred Haise never flew in space again, but remained with NASA transitioning over to the space shuttle programme, and in the late 1970s commanded the shuttle test vehicle Enterprise in its three verification glide flights. He had been slated to command the second shuttle flight into orbit, but delays to that programme meant he departed the agency before the first orbital launch of the shuttle in 1981.
Jack Swigert also never flew in space again, having become embroiled in the Apollo 15 postal covers incident. He left NASA in 1977 to enter politics. In February 1982 developed a malignant tumour in his right nasal passage, and later that year was diagnosed with bone marrow cancer shortly before winning Colorado’s 6th congressional district as a Republican. He died at the age of 51 from the disease on December 27th, 1982, seven days before the beginning of his congressional term.
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