# Apollo Lunar Module Landing

How the Apollo Lunar Module landed on the airless Moon with one throttled rocket engine, radar, a small computer and a pilot in the loop

By Ham Sandwich

Topic: Space

[Open the interactive Jig](https://ask.jig.com/j/kEidleTmwzoh)

## Guide

### 1. Landing where there is no air

Wings and parachutes work by pushing on air, and the Moon has none\. So the Apollo Lunar Module had one way to slow down: point its rocket engine the way it was going and burn\. Drag anywhere to look around\.

Apollo 11's lander, Eagle, touched down on July 20, 1969\.

### 2. One\-sixth gravity

Near touchdown Eagle had a mass of about 7 tonnes\. On the Moon that weighs only about 11 kN, a quarter of what its descent engine could push, so it could hover gently\. Switch to Earth: there it would weigh 69 kN, far more than the engine's 45 kN\.

The descent engine could be throttled down to about a tenth of full thrust, a first for a crewed spacecraft\.

### 3. Braking from orbit, feet first

The landing burn began about 15 km up, while Eagle still moved at about 1\.7 km/s\. For most of the 12\-minute burn it flew almost on its back, engine pointing forward, mainly cancelling speed rather than holding itself up\. That is the sideways part of the approach\.

The guidance computer flew this braking phase\.

### 4. Pitch\-over: seeing the site

About 2 km up the computer swung Eagle nearly upright, so the crew could see ahead through the triangular windows\. A scale etched on the commander's window, read against an angle shown by the computer, marked where they were heading\.

With clicks of his hand controller the commander could move the target\.

### 5. Judging height with radar

Eyes are poor at judging height over ground with no trees or buildings for scale\. A landing radar under the descent stage bounced four beams off the surface: one measured height, three measured speed\. The computer blended them with its gyroscopes\.

The computer had only 2,048 words of working memory\. Its 1202 alarms meant it was overloaded and dropping low\-priority jobs\.

### 6. Armstrong takes the controls

The computer was steering into a crater ringed with large boulders\. Armstrong switched to a semi\-manual mode: the computer held the descent rate he chose while he tilted the lander to fly on\. To move sideways, the whole craft tilts its engine\.

Mission Control called 60 and then 30 seconds of fuel left before the land\-or\-abort point\.

### 7. Contact light

Probes 1\.7 m long hang below three of the footpads\. When one touched, a Contact light lit and the engine was shut down\. Eagle settled the last moment in gentle lunar gravity onto legs with crushable honeycomb shock absorbers\.

The ladder leg had no probe, keeping the crew's path down the ladder clear\.

### 8. Why it worked

The Moon made rocket landing gentler than on Earth: one\-sixth gravity, no wind or reentry heat, and about 1\.7 km/s to shed instead of 7\.8\. Apollo combined a throttleable engine, radar, a small clever computer and a pilot in the loop\. Six crews landed this way\.

Apollo 11, 12, 14, 15, 16 and 17 landed between 1969 and 1972\.