Inside the Meter That Landed the Eagle

There is perhaps nothing more captivating than space exploration in the minds of builders, tinkerers, and hardware hackers. The Mars rover missions and the recent Starship test flights are among humanity’s greatest technical achievements to date. But none of these events has quite the allure of the Apollo Moon missions. Not only did these missions put the first people on the Moon, but they did it using primitive hardware that is less powerful than a calculator is today.

In the hierarchy of awesomeness, the massive Saturn V rockets may have been king. But not far behind is the Lunar Module. After all, without these vehicles, people would never have set foot on the Moon. And no one could ever forget the famous words uttered on the first landing of a Lunar Module: “The Eagle has landed.”

For these reasons, the Lunar Module thrust meter deep dive recently done by Mike Stewart is one you don’t want to miss. The particular meter that Stewart has was never on a space mission, but it was prepared for exactly that purpose in 1969, and it is identical to the meters that made their way into a Lunar Module.

Stewart’s examination reveals a fascinating, and surprisingly complex piece of engineering designed for a single critical phase of the mission: the descent to the lunar surface. This device isn't a standard analog gauge. It is a "servometric indicator," utilizing internal feedback loops and servos to provide incredibly fast, accurate readings. This design choice led to a unique quirk: if the meter lost power, the needles would instantly freeze in their last position rather than falling back to zero.

The meter has a dual-needle display in which the left needle indicated actual thrust based on engine pressure, while the right needle showed the commanded throttle percentage. The wiring for the right needle was notoriously complex because throttle commands could come from the astronaut manually or from the Apollo Guidance Computer automatically. To handle the engine's inability to throttle below 10%, the meter used internal bias pins to accurately interpret signals depending on which mode was active.

To bring this device back to life, Stewart created custom connectors to avoid damaging the vintage hardware. When powered up with 28 volts, even after sitting for over 50 years, the meter was found to still be perfectly calibrated to original NASA specifications. Finally, Stewart illuminated the meter’s eerie, high-voltage electroluminescent backlighting, offering a glimpse of exactly what Apollo crews saw during their descent to the Moon. Check out the video below to see it for yourself.