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Part one Apollo 13 was launched 50 years ago today. Now regarded as a "successful failure," the story of the aborted Moon landing began years earlier, with the design of mankind's then most advanced spacecraft.

The post-launch explosion that to led what was arguably NASA's finest hour began, according to former director of Flight Operations Christopher Kraft in his book Flight , not with a mishandling of a liquid oxygen tank (although that could well have been a contributing factor) but with a simple decision not to follow up on a change order.

Kraft recalled that Apollo pad technicians sometimes couldn't completely flush the tanks after testing, and so turned on the spacecraft's low power internal heater to boil off the oxygen, a process that could take eight hours.

For the agency's Block 2 spacecraft, those designed to get to the Moon rather than orbit Earth, the pad power supply was upped from 28 to 65 volts, which required a special harness. Manufacturers of the Apollo spacecraft, North American Aviation, and the Apollo Spacecraft Program Office ordered the Beech Aircraft Company, which made the tank, to fit a 65-volt thermostat switch in all the Block 2 tanks, [1]wrote Kraft.

"Beech didn't do it," Kraft added, "and the program office didn't follow up. Apollo 13, and the earlier missions, too, flew with a 28 volt switch. A unique set of circumstances brought on disaster."

When emptying a rocket tank, the liquid oxygen was usually forced out using pressurized nitrogen. After a Countdown Demonstration Test, engineers were unable to subsequently empty Oxygen Tank 2. "The tube in the tank wouldn't function properly," wrote Kraft, "it was misaligned, and liquid oxygen leaked back into the tank instead of draining."

NASA's classic worm logo returns for first all-American trip to ISS in years: Are you a meatball or a squiggly fan? [2]READ MORE

That tube may have been misaligned in 1968 when the tank suffered a drop of approximately two inches at a North American Aviation factory: while the chance of damage was low, the fill line assembly may have suffered. The tank was tested – which didn't require filling it with liquid oxygen – and, once given the all-clear, installed that year into the service module for Apollo 13.

Fast forward to March 1970, and Apollo 13 is on the launchpad. Faced with a tank that needed draining completely before filling with liquid oxygen, the engineers at the Cape used the heaters to boil off the tank's contents. It was the first time the process had been used and "nobody," said Kraft, "recognized the damage that could occur."

There was a thermostat for the heater, remember? It was designed to shut off when the temperature hit 80 degrees Fahrenheit. However, it was also designed for 28-volts not the 65-volts applied to it. "Its contacts literally melted," said Kraft, "and closed the switch."

Continuous power then ran through the Teflon-insulated wires for the next eight hours. The temperature of those wires rose past 800 degrees Fahrenheit, with some estimates putting it nearer 1,000 degrees, melting the Teflon and leaving the wires bare.

The instruments available to the ground technicians, according to Kraft, only went to 100 degrees: "They didn't see anything happen." There was no outward sign of the failure of the switch, nor a chance to inspect the wiring.

The tank had, after all, passed its ground inspection.

The launch of Apollo 13

Having demonstrated with Apollo 12 that a near pin-point landing was possible, Apollo 13 was to intended to kick-off lunar exploration properly despite waning public interest and the cancellation of Apollo 20. Apollo 18 and 19 were also looking doubtful, and would eventually be cancelled by 1971.

Commanded by Gemini and Apollo veteran Jim Lovell, with Jack Swigert as Command Module Pilot and Fred Haise as Lunar Module Pilot, Apollo 13 had already suffered a few issues. Alan Shepard was supposed to lead the mission, though Lovell's crew, slated for Apollo 14, were swapped in when it was decided Shepard could use a little more training time. Swigert was also a late addition to the crew, added days before launch after Ken Mattingly, intended to be the lunar pilot, was exposed to German measles, having never previously contracted it, and removed from the trip.

The launch of Apollo 13, which was supposed to send the Command Module Odyssey and Lunar Module Aquarius to the Moon, occurred at 1913 UTC on April 11. The first-stage flight was uneventful, or at least as uneventful as a Saturn V gets.

The second stage did not go quite so well, as the [3]Flight Evaluation Report [PDF] on vehicle AS-508 explained: "High amplitude oscillations in the 14 to 16 hertz range during the S-II mainstage were sufficiently severe to cause the center engine to shut down 132 seconds early." While this effect was an "inherent characteristic," the high amplitude was unexpected. No matter, though, mission controllers swiftly calculated how long the remaining four engines would need to run for and passed the information to the crew.

After the second stage burn was complete, the S-IVB ignited and placed the spacecraft into orbit. Following the usual orbital check-out of the spacecraft's systems and trajectory parameters, the go for translunar injection burn was given.

"We heaved sighs of relief," [4]recalled Flight Director Gene Kranz in his book Failure Is Not An Option , "thinking we had gotten through what would probably be the one major glitch in the mission."

NASA's [5]history office remarked: "At 46 hours 43 minutes Joe Kerwin, the CapCom on duty, said, 'The spacecraft is in real good shape as far as we are concerned. We're bored to tears down here.' It was the last time anyone would mention boredom for a long time."

The story continues in part two – though, here are some additional tomes and materials to peruse if you're so inclined (spoiler alert)... ®

Further reading

As well as Flight: My Life in Mission Control by the late Chris Kraft and Failure Is Not an Option by Gene Kranz, Normal Accidents by Charles Perrow was also [6]useful in researching this article, as was Apollo 13 [7]by James Lovell and Jeffrey Kluger. Thirteen: The Apollo Flight That Failed by Henry S. F. Cooper is also well worth a [8]read .

NASA's own history archives proved a delightful click-hole into which to tumble.

Finally, season two of Kevin Fong's superb 13 Minutes To The Moon [9]podcast is highly recommended – as is season one, which retells the Apollo 11 landing. And also some movie starring Tom Hanks. ®

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[1] https://www.goodreads.com/book/show/141498.Flight

[2] https://www.theregister.co.uk/2020/04/03/nasa_worm_logo_spacex_iss/

[3] https://ia600304.us.archive.org/30/items/nasa_techdoc_19900066486/19900066486.pdf

[4] https://www.goodreads.com/book/show/141499.Failure_is_Not_an_Option

[5] https://science.ksc.nasa.gov/history/apollo/apollo-13/apollo-13.html

[6] https://press.princeton.edu/books/paperback/9780691004129/normal-accidents

[7] https://www.barnesandnoble.com/w/apollo-13-james-lovell/1103016470

[8] https://www.goodreads.com/book/show/1391538.XIII

[9] https://www.bbc.co.uk/programmes/w13xttx2/episodes/downloads

[10] https://go.theregister.co.uk/tl/1936/-8579/choosing-a-low-code-vendor?td=wptl1936