Square Peg, Round Hole – The Story of Apollo 13
Artist's impression of Apollo 13 Command Module. Credit: NASA

Square Peg, Round Hole – The Story of Apollo 13

12/04/2017Written by Robbie Bosley

Nearly 50 years ago, the crew of Apollo 13 raced the clock to return home safely. Here’s how to replicate one of their more ingenious solutions to a crisis.

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Artist's impression of Apollo 13. Credit: NASA

Nearly 50 years ago this week the crew of Apollo 13 managed to deal with one of the most dangerous faults in spaceflight history. It’s an incredible story of tenacity, ingenuity and survival.

The resourcefulness of the crew and ground engineers is no more apparent than in their solution to the problem of carbon dioxide poisoning, perhaps best described as ‘fitting a square peg into a round hole.’

At the end of this post, follow the instructions to see if you can recreate the ‘mailbox’ device that was invented for this very task.

Damaged Apollo 13 Service Module. Credit: NASA
Jack Swigert constructs the CO2 filter. Credit: NASA
The jury-rigged CO2 filter on Apollo 13. Credit: NASA

In the days leading up to the Apollo 13 launch on 11 April 1970, no one seemed to care that astronauts Jim Lovell, Fred Haise, and Jack Swigert would soon be arriving on the moon. This had been done twice before, and there were more pressing matters to focus on – Paul McCartney had recently left the Beatles.

But on 13 April, 56 hours after launch, a piece of faulty wiring in one of the oxygen tanks ignited a spark, and the tank exploded catastrophically. The crew lost two of their three fuel cells in the explosion, and their only remaining oxygen tank began leaking into space. With this accident, the world’s media suddenly began widespread coverage, with millions worldwide tuning in to coverage of the event.

Fortunately, Apollo 13 came with a backup. The part of spacecraft that would have been used for landing on the Moon now became a lifeboat, complete with its own oxygen supply. The astronauts climbed inside the Lunar Module (LM) to wait it out until re-entry. The old plans for landing on the Moon were abandoned – the new mission objective was survival.

The crew still faced huge problems. They had almost no power, so they couldn’t afford to heat their module. The crew began to freeze, as did their supplies of food and water. The lack of power also meant no navigation computer, so Apollo 13 was effectively flying blind.

One of the biggest challenges was the amount of carbon dioxide, a gas that humans breathe out, but which is toxic in high concentrations. The crew had plenty of filters on board, thanks to extras in the Command Module (CM). However, in a twist of fate, the CM filters were square, while the LM filters, and thus the hole they fitted in, were round. Suddenly, the survival of the Apollo 13 crew depended on their ability to fit square ‘pegs’ into a round hole.

The ground team worked against the clock. They had only the items on board the spacecraft to work with. With barely minutes to spare, the ground team completed an ingenious device to fix the carbon dioxide problem, nicknamed the ‘mailbox,’ because of its appearance. It was made with the most basic of equipment, but it saved the crew members’ lives.

With some small adjustments, you can have a go at making your own.

How to

Instructions for making a ‘mailbox’ device, i.e. fitting a square peg into a round hole:

1. You will need a lithium-hydroxide canister, an A4 piece of card, some duct tape, a small thin plastic bag, a piece of hose, and a sock. If you don’t keep spare lithium-hydroxide canisters around, you can buy one for about £1900 (!), or improvise by taking a small box, cutting two circular holes in two opposite sides, and fitting an old toilet roll or paper towel tube through both holes, creating a tube through the middle, before sealing it up with duct tape.

2. Call one side of the canister with a hole in the ‘top’ and the other the ‘bottom’. Take a long piece of duct tape and wrap it around the sides of the canister sticky side out, very tight, next to the top of the canister. Repeat this process for the bottom of the canister, so you have two pieces of duct tape, wrapped around the sides, both touching the top and bottom respectively.

3. Fix the duct tape in place by taking some more duct tape and wrapping it around the canister sticky side down so that it runs up one side, across the top, down another side, and across the bottom. These bits of tape should be off-centre, so you don’t block the holes in your canister. Use four bits of tape, so that by the end the top and bottom of the canister should resemble a noughts-and-crosses board.

4. Take the piece of card and cut it so that it is the same width as the canister. Then fold it over the top of the canister, creating an arch. Use the sticky outside of the duct tape already along the top to stick it down, then stick a piece of duct tape over the top to fix the card in place.

5. Place the plastic bag over the top of the arch and canister, sticking it down to the outward-facing tape already at the bottom edge. Use more duct tape around the top and bottom edges of the canister to stick the bag down.

6. Cut a small hole in the bag at an open end of the arch, and push the hose through the hole, getting it as close to the top hole in the canister as possible. Seal the hose in with duct tape.

7. Push the sock into the hole in the bottom of the canister

8. Place many small strips of duct tape across this hole to make sure the sock doesn’t fall out.

Voilà! The free end of the hose can be attached to the hole for the round canister. For more detail, and a transcript of the conversation between Apollo 13’s flight crew and ground crew as the ‘mailbox’ was being made, follow this link.

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Find out more Visit 4th deck of our Rocket Tower for more Space Race stories, and our Space Oddities gallery for Apollo-era curiosities.
Apollo 13 crew returned to Earth. Credit: NASA

Apollo 13’s near-fatal accident fuelled worldwide interest, with millions of people tuning in to watch the brave descent of Lovell, Haise, and Swigert. The crew did make it back to Earth alive, but it was a near thing.

Since then, books have been written (including one by Lovell himself), documentaries have been made, and even a Hollywood blockbuster film, all depicting the struggle involved in getting the crew of Apollo 13 home.

About the Author: Robbie Bosley is a physics student at the University of Leicester and works at the National Space Centre as a Science Interpreter.