NASA saved Apollo 13 with a duct-tape CO2 adapter

After an oxygen tank exploded in the Apollo 13 Service Module on April 13, 1970, the crew abandoned the crippled Command Module and crowded into the Lunar Module as a makeshift lifeboat for the return trip to Earth. The Lunar Module's life-support system was designed to scrub carbon dioxide from the air for two astronauts on a short moon landing, not three astronauts for the multi-day trip home, and its square lithium hydroxide canisters — the chemical filters that actually absorb CO2 — were running out fast.

The Command Module carried plenty of spare lithium hydroxide canisters, more than enough to last the trip, but they were the wrong shape for the Lunar Module's receptacle: square cartridges built for a socket that only accepted round ones. There was no way to manufacture a correctly shaped canister and get it to the crew, and CO2 levels inside the cabin were climbing toward levels that would eventually impair judgment and then kill the astronauts, on a fixed timeline no amount of urgency could slow down.

Engineers jury-rigged a CO2 scrubber adapter from onboard scraps—bags, manual, tape—to fit square canisters into round sockets, saving Apollo 13.

The adapter used only materials confirmed onboard, so the fix was buildable without delivery.

Ground testing on identical materials let engineers perfect the design before instructing the crew.

The adapter solved the shape mismatch, enabling the square canisters to work in the round socket.

Clear step-by-step instructions allowed stressed astronauts to assemble it correctly under pressure.

When a fix can't be sourced, design it from what's verifiably on hand, not what you wish you had.

The astronauts assembled the mailbox exactly as instructed, and carbon dioxide levels in the cabin fell to safe levels within minutes of it going into use, holding steady for the remainder of the trip home. All three astronauts returned safely on April 17, 1970, and the improvised device is credited as one of the direct reasons the crew survived the failure.