Colossus and Luminary: The Two Souls of Apollo's Flight Software
How two separate AGC software packages—one for the Command Module, one for the Lunar Module—were built by competing teams under impossible deadlines
• Matt Dennis
Essays
The Digital Autopilot: How the AGC Flew a Spacecraft with Reaction Jets
The software that translated guidance commands into precisely timed thruster firings—managing attitude, translation, and engine gimbal across every phase of flight
• Matt Dennis
The DSKY and PINBALL: How Astronauts Talked to a 1960s Computer
The electroluminescent display, the Verb-Noun grammar, and the 4,000 lines of I/O code that gave Apollo crews a conversation with their guidance computer
• Matt Dennis
Erasable Memory: Managing 2,048 Words of RAM on the Way to the Moon
How the AGC's programmers divided 4 kilobytes of magnetic core RAM among navigation, guidance, autopilot, and display—and why every word was a negotiation
• Matt Dennis
The Executive and the Waitlist: The AGC's Real-Time Operating System
How a 70-pound computer with 2K of RAM ran a priority-based multitasking OS that could shed its own workload mid-landing to keep astronauts alive
• Matt Dennis
P52: The Star Alignment Program Every Apollo Astronaut Had to Master
How astronauts sighted stars through a sextant to align the AGC's inertial platform—the ritual that kept Apollo on course across 240,000 miles
• Matt Dennis
P63, P64, and P66: The Three Programs That Landed on the Moon
The AGC software sequence that guided the Lunar Module from orbital velocity to touchdown—braking, approach, and the final manual phase that put Armstrong in control
• Matt Dennis
The Interpreter: A Virtual Machine Inside the Apollo Guidance Computer
How MIT built a double-precision math engine on top of the AGC's primitive instruction set—a software layer that made lunar navigation possible
• Matt Dennis
P20 and P22: How the AGC Tracked a Target Across Lunar Orbit
The rendezvous navigation programs that let two Apollo spacecraft find each other above the Moon—using radar, optics, and orbital mechanics solved in real time
• Matt Dennis
Restart Protection: The AGC Software That Could Reboot Itself Mid-Flight
How the Apollo Guidance Computer's restart system preserved critical state across software restarts—a fault-tolerance design that kept computing through failures
• Matt Dennis
The Docking Tunnel: 32 Inches Between Two Worlds
The pressurized passage that connected the Command Module to the Lunar Module—where crew transferred between vehicles, equipment was passed, and Apollo 13's survival hinged on a hatch
• Matt Dennis
Apollo's Fuel Cells: Electricity from Hydrogen and Oxygen
How three Pratt & Whitney fuel cells turned cryogenic gases into electrical power and drinking water—the electrochemical plants that ran the CSM for two weeks at a time
• Matt Dennis
The CM Heat Shield: AVCOAT and the 5,000-Degree Reentry
How a hand-applied ablative material protected the Command Module from the plasma inferno of reentry—a heat shield that burned away on purpose to keep the crew alive
• Matt Dennis
The CM Recovery System: Three Parachutes Between the Crew and the Ocean
How drogue chutes, pilot chutes, and three massive ring-sail mains slowed the Command Module from 300 mph to 22 mph—and what happened when one of them didn't open
• Matt Dennis
The Barbecue Roll: Passive Thermal Control in Deep Space
Why the Apollo spacecraft slowly rotated during translunar and transearth coast—and the engineering behind keeping one side from freezing while the other baked
• Matt Dennis
The Probe and Drogue: Apollo's Docking Mechanism
How a retractable probe and a funnel-shaped drogue connected two spacecraft in orbit—a mechanical handshake at closing speeds measured in inches per second
• Matt Dennis
The CSM Reaction Control System: Four Quads and 12 Jets
How the Command and Service Module's thruster system handled attitude control, translation, and ullage—from translunar coast to the final reentry roll
• Matt Dennis
The Service Propulsion System: The Engine That Could Not Fail Twice
How Aerojet built a 20,500-pound-thrust engine with no turbopumps, no igniter, and redundant everything—the engine that inserted Apollo into lunar orbit and sent it home
• Matt Dennis
Transposition, Docking, and Extraction: The Maneuver That Turned Apollo Around
How the Command Module separated from the S-IVB, rotated 180 degrees, docked with the Lunar Module, and pulled it free—all at 25,000 mph on the way to the Moon
• Matt Dennis
Unified S-Band: One Radio System for Everything
How a single microwave communications system handled voice, telemetry, ranging, television, and computer uplinks between the spacecraft and Earth—a quarter million miles of radio link
• Matt Dennis
The Ascent Engine: 3,500 Pounds of Thrust Between the Moon and Home
The simplest, most reliable rocket engine ever built for human spaceflight—because it was the one engine in Apollo that had no backup and no second chance
• Matt Dennis
The LM Crew Station: Standing Room Only on the Moon
How Grumman designed a cockpit with no seats, triangular windows, and an instrument panel for two astronauts flying a spacecraft they could see through
• Matt Dennis
The Descent Propulsion System: The Only Throttleable Engine in Apollo
How TRW built a hypergolic engine that could throttle from 10% to full thrust—the engine that slowed the Lunar Module from orbital velocity to a gentle hover above the Moon
• Matt Dennis
LM Electrical Power: Silver-Zinc Batteries and the Art of the Power Budget
How the Lunar Module ran on batteries instead of fuel cells—and how every watt was rationed across systems that all wanted more than the budget allowed
• Matt Dennis
The LM Environmental Control System: Keeping Astronauts Alive in a Tin Can
How the Lunar Module's life support system managed oxygen, temperature, humidity, and carbon dioxide for two astronauts in a cabin smaller than a bathroom
• Matt Dennis
The Landing Gear: Aluminum Honeycomb, Contact Probes, and a One-Way Trip
How Grumman designed legs that could absorb the impact of landing on the Moon, fold into an adapter ring, and never be used again
• Matt Dennis
Landing Radar and Rendezvous Radar: The LM's Two Eyes
How the Lunar Module used two separate radar systems to solve two different problems—finding the ground during descent and finding the Command Module in orbit
• Matt Dennis
LM Staging: Splitting a Spacecraft in Half on the Lunar Surface
The explosive separation sequence that turned the descent stage into a launch pad and the ascent stage into a rocket—executed in milliseconds, with no margin for partial success
• Matt Dennis
SWIP: How Grumman Shaved a Ton Off the Lunar Module
The Super Weight Improvement Program that stripped 2,500 pounds from the LM through relentless engineering—chemically milling skins, drilling lightening holes, and fighting for every ounce
• Matt Dennis
The PLSS: A Life Support System You Wore on Your Back
How Hamilton Standard packed oxygen, cooling water, CO2 scrubbing, communications, and telemetry into an 84-pound backpack that kept astronauts alive on the lunar surface
• Matt Dennis
The Blowout That Started the Environmental Movement
On January 28, 1969, a Union Oil well ruptured on Platform A in the Santa Barbara Channel. The 100,000 barrels that followed changed American law forever.
• Matt Dennis
The Tomol: Naval Engineering Before the Europeans Showed Up
The Chumash built plank canoes from driftwood and tar, crossed 25 miles of open ocean, and maintained a maritime trade network for 2,000 years. The engineering was not primitive.
• Matt Dennis
Apollo's Abort Modes: The Escape Plans That Ranged from Terrifying to Nonexistent
Inside the Saturn V abort sequences, dead zones with no survivable escape, and the Launch Escape System that could pull 15g
• Matt Dennis
The Alignment Optical Telescope: Finding Stars from an Alien World
How Apollo navigators used a rooftop periscope and a 37-star catalog to align their guidance platform on the lunar surface—connecting maritime tradition to spaceflight
• Matt Dennis
The Entry Monitoring System: A Scroll, a Needle, and the Reentry Corridor
The analog instrument that could guide Apollo astronauts through reentry without a computer—a scrolling chart, a scribing needle, and the thin corridor between skipping off the atmosphere and burning up
• Matt Dennis
Bombing Your Own Engines: How NASA Solved the F-1 Combustion Instability Problem
NASA engineers couldn't stop the F-1 rocket engine from exploding, so they started detonating bombs inside it on purpose
• Matt Dennis
The Landing Point Designator: Analog Crosshairs for a Digital Moon
How a set of lines scribed on a window gave Apollo commanders the power to see where the computer was taking them—and override it
• Matt Dennis
Lunar Dust: The Abrasive, Electrostatic Menace Nobody Was Ready For
How razor-sharp moon dust infiltrated spacesuits, poisoned astronauts, and nearly wrecked Apollo equipment
• Matt Dennis
POGO: The Saturn V Problem That Nearly Shook Apollo Apart
How a terrifying resonance phenomenon almost ended the Apollo program before it reached the Moon
• Matt Dennis
Apollo Pyrotechnics: 200 Controlled Explosions Between You and Home
The explosive bolts, shaped charges, mortar cannons, and guillotine cutters that held Apollo together—and blew it apart on schedule
• Matt Dennis
The Sublimator: Cooling by Boiling Ice in the Void
How Apollo engineers solved the thermal problem of spaceflight with a device that froze water and boiled it away into vacuum—no moving parts required
• Matt Dennis
The Apollo Waste Management Problem: Human Biology vs. Zero Gravity
How NASA struggled to solve the most basic human need in space, and the Apollo 10 incident nobody at NASA wanted to talk about
• Matt Dennis
Command and Service Module: Your Ride Home from the Moon
Inside the spacecraft systems that kept astronauts alive for 8 days and 240,000 miles
• Matt Dennis
Engineering the Lunar Module: The Most Tested Vehicle Never Flown on Earth
How Grumman built a tissue-paper spacecraft with one engine that absolutely could not fail
• Matt Dennis
Blog Posts
The LLM Council: Multi-Agent Orchestration for Technical Planning
Running competing AI agents in parallel and having them review each other's work through a skeptical council — a pattern for better technical plans.
• Matt Dennis
Saturn V: The Machine That Defined an Era
The engineering marvel behind the Apollo program: how Wernher von Braun's team built the largest rocket ever successfully flown, and why nothing like it has been built since.
• Matt Dennis