In 1970, Apollo 13 engineers built a life-saving air filter from cardboard, plastic bags, and duct tape while astronauts drifted through space

Apollo 13 was supposed to be just another Moon landing mission for NASA when it launched in April 1970, but fate had something else planned for the space shuttle due to an oxygen tank explosion that occurred about 200,000 miles away from Earth, damaging the spacecraft and making it impossible for the astronauts to continue their mission as originally planned. This situation resulted in one of the most notable engineering crises in contemporary history, according to the NASA History Office – Apollo 13 Mission Overview. The explosion damaged the spacecraft to such a degree that it drastically cut the power supply, oxygen, and life support capacity of the astronauts, who were now counting on the lunar module to act as a life-saving craft until they could come back down to Earth.

The most life-threatening challenge was the increase in carbon dioxide in the capsule. The filtration system of the lunar module had not been engineered to sustain three crew members on their emergency mission back to Earth. Unfortunately, the filters in the command module and the lunar module were of different shapes. While the lunar module was equipped with cylindrical filters to trap the carbon dioxide, the command module had square-shaped lithium hydroxide filters. It was clear that there was sufficient filtration material, but the square-shaped command module filters would not fit into the cylindrical openings of the lunar module.

Engineers created an improvised adapter using whatever materials were available

On the ground, NASA engineers started designing an adapter as quickly as possible to be used for joining the incompatible filtration systems aboard the spaceship. As NASA – Apollo 13 and “The Mailbox” states, engineers managed to develop an actual solution using cardboard, plastic bags, spacesuit hoses, socks, and plenty of duct tape. Later, astronauts assembled the filter manually using the detailed instructions sent from mission control to them. The awkward device received an informal name – “the mailbox,” due to its appearance. This case was important since high concentrations of carbon dioxide become deadly in an enclosed space very quickly. Thus, engineers were forced to overcome a technical difficulty within a tight schedule and use only the materials available aboard Apollo 13.

The uniqueness of the case is defined not only by the difficult task but also by the limitations of the solution development process. Engineers could not deliver any new equipment into space; solutions should have been found using only the materials present aboard Apollo 13. The task proved challenging since astronauts had to assemble the equipment quickly under stress. As Smithsonian Magazine – Apollo 13 Engineering Crisis reports, this case became one of NASA's best-known solutions.

Apollo 13 became a symbol of improvisation and systems engineering

The crew aboard Apollo 13 finally landed safely back on Earth on April 17, 1970, after a few days of suspense turned what could have been a failed Moon mission into NASA’s biggest survival story. As stated by Britannica – Apollo 13, the historic significance of Apollo 13 does not lie in its technical achievement as a successful Moon mission but in the manner in which NASA engineers and astronauts successfully recovered and stabilized the crippled craft.

What stands out about the carbon dioxide filter problem is the seemingly mundane nature of the materials used for the repair against the backdrop of such an ambitious operation. Cardboard, duct tape, plastic bags, and tubes hardly resemble any sort of high-tech equipment. However, this ordinary material suddenly became a vital part of a rescue plan in outer space, where even the smallest mistakes could mean death for those onboard Apollo 13.