Apollo Lunar Landing Game

The physics of a lunar landing

The Moon pulls at 1.62 m/s², a sixth of Earth's, and there's no air, so nothing slows you but the engine. The lunar module's descent engine gave about 45,000 newtons. It could be throttled between about 10% and 60%, or run at full thrust, but not in between, because that range wore the nozzle. Here the throttle steps in 5% detents, like a careful pilot nudging it. Burning at a specific impulse of 311 seconds, it used about 14.8 kg a second at full thrust.

To hover, thrust must equal weight. With about 7,800 kg of lander that's close to 28% throttle, which sits between two detents, so you have to keep working it. To move sideways you tilt the whole craft and let the engine push you, then tilt back to stop. That's how the crews flew it. The reaction control thrusters can shove you sideways directly too, but at only about 0.1 m/s², so they're for the last few metres.

Radar, alarms and limits

The landing radar measured height and speed off the surface. Tilt too far and its beams no longer see the ground, so the readouts drop out. During Apollo 11 the guidance computer threw 1202 and then 1201 program alarms: it had more work than time. It shed the low priority jobs and kept flying, and Steve Bales in Mission Control called go. In the game an alarm freezes your displays until you answer it.

The gear was built for about 3 m/s down, 1.2 m/s sideways and a 12° slope, with the descent stage riding not far above the surface, so a big boulder under it is a problem. Out of fuel near the ground, the real crews would have fired the abort stage: the ascent stage separates and climbs away on its own engine. It saves the crew but ends the landing.

The six sites

Six crews landed between July 1969 and December 1972. Apollo 11 came down in the Sea of Tranquillity after Armstrong flew past a boulder field by West crater with about 30 seconds of fuel called. Apollo 12 landed in the Ocean of Storms, close enough to walk to the Surveyor 3 probe. Apollo 14 went to the hilly Fra Mauro highlands, Apollo 15 beside the deep Hadley Rille, Apollo 16 to the rough Descartes highlands and Apollo 17 into the narrow Taurus Littrow valley. Each day brings one of them, with terrain made up in its character.

Scoring

A landing scores for softness, staying level, not sliding and fuel left, then multiplies by precision: ×3 within 30 m of the target, ×2 within 100 m. Commander pays ×1.5 and Armstrong ×2. Abort and you score nothing, but the crew is safe. You can fly today's landing again as often as you like, and your best counts.

I'm Arthur. I'm not a pilot or an engineer, just someone who has watched the landing footage far too many times. If you know better, email me. For more space, try Orbit.

Questions

What was the 1202 alarm on Apollo 11?

A program alarm from the guidance computer meaning its work queue had overflowed, partly because a radar switch setting fed it extra work. The computer dropped low priority jobs and kept flying, and Mission Control called go on it. A 1201 alarm, the same problem in a different queue, followed.

How much fuel did Apollo 11 have left when it landed?

Mission Control called 30 seconds shortly before touchdown. Later analysis found there was more than the gauge showed, because the propellant had been sloshing.

How strong is gravity on the Moon?

About 1.62 metres per second squared, roughly one sixth of Earth's.

How did the lunar module steer?

The descent engine could be throttled between about 10% and 60% or run at full thrust. Sixteen small reaction control thrusters tilted the craft, and tilting the engine moved it sideways. The thrusters could also push it sideways directly, but only gently.

How hard could the lunar module land?

The gear was designed for about 3 m/s down, 1.2 m/s sideways and a 12° slope. The crews aimed for much less.

What does the abort stage button do?

On the real lunar module it separated the ascent stage from the descent stage and fired the ascent engine to climb back to orbit. In the game it saves the crew but ends the landing.

Is this game realistic?

The gravity, thrust, fuel use, gear limits and alarms are real. It's two dimensional, the terrain is made up in the character of each site, and the lander holds its attitude for you, which is simpler than the real controls.