Which backup provides the strongest redundancy against a power failure?

A second system with a separately protected power source. Exactly. A backup must avoid the failure that disabled the primary path.

Why is counting spare parts insufficient?

The parts must survive the fault and provide the required function. Exactly. Availability and compatibility determine whether extra capacity is usable.

Why did Apollo 13 need a canister adapter?

Usable command module canisters did not fit the lunar module system. Exactly. The capacity existed, but the two systems were physically incompatible without adaptation.

From the archive · September 15, 2026

The spare that can actually help

When does having more than one system make you safer?

01 · Word

Redundancy

Pronounced rih-DUN-dun-see

noun

Extra capacity or an additional route that can preserve a function when something fails

Examples

A second power source provided redundancy only if the same fault could not disable both.
The spare pump added little redundancy because it depended on the broken controller.

Origin

In ordinary language, redundancy can mean something unnecessary or repetitive. Engineering gives the apparent excess a purpose: capacity that seems idle during normal operation may become essential after a failure. The distinction is between extra parts and an available extra function.

02 · Idea

Count independent ways to keep working

Two systems can look safer than one while sharing the same point of failure. A pair of computers connected to one failed power supply cannot cover for each other. Useful redundancy therefore asks a more demanding question than how many components are installed: which functions remain available after a specific fault?

Apollo 13 made that question concrete. The lunar module carried resources for a different phase of the mission. After the service module accident, it could provide a refuge. Yet spare supplies did not automatically fit the new job. The crew and ground teams had to connect available capacity to urgent needs.

A spare is useful only if it survives and fits.

Test the backup against the failure you actually need it to cover.

Limits

Extra systems add weight, cost, and complexity. They can introduce new failure paths, and independence is always relative to a particular threat. Redundancy helps preserve a function; it cannot guarantee that the original mission remains achievable.

03 · Moment

A lunar lander becomes a lifeboat

Between Earth and the Moon, April 1970; Apollo 13 abandons its landing

An oxygen tank failure in Apollo 13's service module forced the crew to abandon the planned Moon landing. The astronauts moved into the lunar module and used its supplies to stay alive while preserving command module resources for reentry. Equipment intended to support a landing now supported a return.

Mission: Apollo 13
Accident: April 13, 1970
Refuge: Lunar module Aquarius

The caveat

The lunar module was not an unlimited, fully interchangeable duplicate. Water, power, and other resources required careful management. The successful return depended on adaptation and operations as well as hardware already aboard.

Removing carbon dioxide created a stubborn compatibility problem. The command module had usable lithium hydroxide canisters, but their shape did not fit the lunar module's system. Ground teams devised an adapter from available materials and passed instructions to the crew. Having the chemical capacity was only half the solution; air had to flow through it.

The connection

Extra capacity improves resilience only when it survives the failure and can perform the function that is now missing.