Eagle's Descent — Apollo Lunar Module Landing Simulator

EAGLE’S DESCENT
APOLLO 11 · MARE TRANQUILLITATIS  ·  P63 — BRAKING
PILOT MODE  ·  ESC PAUSE  ·  SOUND ON  ·  ⏏ QUIT

DSKY

COMPACTYPROG66VERB06NOUN60
+00000
-00000
+00000

CABIN COMMSTIME FROM P63 · NEWEST FIRST

01020304050LPD 42

MASTERALARM

LUNAR CONTACT

FUELLOW

HORIZ VELOCITY

FWDLAT2020

+6.6
FWD FT/SEC

+2.0
LAT FT/SEC

ALT

500
FT

ALT RATE

−16.0
FT/SEC
ROD SET −16

ATTITUDE

THRUST

1006010

31%
AUTO · ROD

FUEL

100500

2:04
EST TO DRY
100% LEFT

ABORT STAGE

PRESS A

WSDESCENT RATE
↑↓PITCH
←→ROLL
QEYAW
SPACE↑↓←→MOVE SITE (P64)
PTAKE CONTROL
XENG STOP AT CONTACT
AABORT STAGE
RRETRY
MSETUP

APOLLO LUNAR MODULE LANDING SIMULATOR
EAGLE’S DESCENT

Step into the Apollo Lunar Module (LM) and into its commander’s shoes.
You have separated from the Command Service Module, and the Apollo guidance computer has flown the LM down from orbit. Now the surface is rising to meet you, riddled with craters and boulders.
Take the (simplified) controls during the final descent — read the terrain, control the descent rate, manage fuel, and put the vehicle down soft to recreate the historic landings.

THIS SIMULATOR NEEDS A PHYSICAL KEYBOARD — OPEN IT ON A LAPTOP OR DESKTOP

LEARN
FLY
ABOUT

BY SANDEEP SHARMA

ABOUT

CREATED BY
Sandeep Sharma
gosandeep.com

I have always had a deep fascination for Apollo spacecraft, especially the Lunar Module (LM). The technical ingenuity of its design, reliability during the missions, and the quirky form make it my favorite machine of all time. I have often dreamed about what the Apollo astronauts (particularly Armstrong and Aldrin) experienced, as they guided the vehicle towards the unknown lunar surface.
This is my attempt to simulate the last three stages of the descent sequence: Braking (P63), Approach (P64) and Final Descent (P66, Rate of Descent). The last one requires manual attitude and rate of descent control to achieve a soft touchdown. I’ve tried to maintain a balance between historical accuracy and simplification, so that an average person like me can manage to land safely with a bit of training.
The flight model is based on NASA’s Apollo Lunar Module documentation. Lunar sites are modeled using elevation data from NASA’s Lunar Reconnaissance Orbiter Camera (LROC).
I hope fellow space enthusiasts will enjoy the nuances, challenges & thrill of achieving a soft lunar landing.

Note

This is a simulator, not an emulator.
The simulator requires a keyboard, and is not mobile friendly (yet).

REFERENCES

Apollo 11 guidance computer sourceLuminary 099, the flight software the Lunar Module actually carried. Used as reference for the LPD look angle (LOOKANGL, in the landing guidance equations), the half-degree and two-degree redesignation steps (REDESMON), and the register formats behind Nouns 60, 63 and 64. The descent guidance itself is a simplified stand-in, not Luminary’s own.
Virtual AGC

Lunar Reconnaissance Orbiter Camera (LROC) NAC digital terrain modelsNASA / GSFC / Arizona State University. I have used elevations with heights measured from stereo pairs shot in lunar orbit. The approx 1.5 km square you fly over is this data around the mission’s real touchdown point, resampled to 3 m spacing.
LROC NAC imageryThe orbital photographs of each site on the pre-flight screen. The ground you actually fly over is not made from these.
Procedural, not measured: the surface texture, every boulder and the horizon past the square.

Apollo Operations Handbook, Lunar ModuleLMA790-3-LM-11, 1971. Reference material for the engine performance, descent programs and the crew procedures.
Apollo Experience Report: Spacecraft Structural WindowsNASA TN D-7439, 1973. The commander’s forward pane is drawn from the dimensions in this report.

Apollo Lunar Surface JournalThe landings as they happened, with the crew transcripts annotated by the people who flew them.
Apollo Flight JournalThe same treatment for everything between launch and the surface.

BUILT WITH
three.js (for the out-the-window view), DSEG for the seven-segment face on the DSKY.
SIL Open Font License 1.1.

CLOSE

MISSION BRIEFING

PILOT WINDOW
INSTRUMENT PANEL
DSKY
KEYBOARD
TIPS

OUT THE COMMANDER’S WINDOW — P64 APPROACH

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The LM featured two small, triangular forward windows for the crew. Installed obliquely, the windows gave a downward view of about 65 degrees and an outboard view of 80 degrees. The commander’s window featured etched grid markings (the Landing Point Designator or LPD) that let them align their descent trajectory with pre-planned targets on the lunar surface.

1LANDING POINT DESIGNATOR (LPD) — this consisted of two etched lines on the window glass, used to fix the landing point of the LM based on the current trajectory. The guidance computer showed a value on the DSKY (Noun 64), which the pilot read out to the commander, who then looked out through the window and used that reading against the graduated LPD scale to see the exact point the computer was steering to. If that patch looked bad, he would use the hand controller to steer away, and the DSKY would relay an updated reading. In this simulation, you will additionally see an amber pointer and ring where the value read against the scale crosses the centre line, marking the target landing point.
2CRATERS — dark bowls with bright rims. The shadowed near wall shows their depth. Avoid them when choosing a site.
3TARGET RING — the computer’s target landing point, projected on the ground. Hold SPACE and tap the arrow keys to move it. This is visible during P64 only, and is intended to be a teaching aid. The Apollo astronauts actually used only the LPD scale.
CLOSER TO THE GROUND LOOK OUT FOR
SHADOW — the LM’s shadow, thrown ahead by the low sun behind you. It slides toward you as you descend and meets the craft at touchdown.
DUST — blown up by the engine below about 100 ft. It may obscure the view, so fly the last seconds using the instruments.

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The real panel was a wall of switches, gauges and circuit breakers split between the two crew stations. Everything needed to fly the descent sat in front of the commander, where it could be read in the second or two he could spare from the window.

This column carries those flight instruments at cockpit scale. Switch banks, circuit breakers and the second crew station are not modelled. The time to dry under the fuel gauge is an addition, because on the real flights Houston worked that number out from the propellant gauges and called it up.

1MASTER ALARM — the master warning light. It lights on fuel exhaustion, on a tilt beyond 30° in P66, and on a program alarm. Press the lamp or ENTER to clear it. In PILOT mode a program alarm clears itself, as Houston’s “go” call did.
2LUNAR CONTACT — lights blue when the 5.6 ft probes below the footpads touch the surface. Count one second, then stop the engine (X). Cut at the instant of the light and the craft drops onto the pads.
3FUEL LOW — lights amber when less than 5.6% of descent propellant remains.
4HORIZ VELOCITY — velocity across the surface (FPS). The vertical needle is left/right drift, the horizontal is forward/back. Land with both near zero.
5ALT — vertical distance of the craft from the lunar surface, in feet (FT).
6ALT RATE — rate of change of altitude in ft/s (FPS), also known as rate of descent (ROD). Negative values indicate descent. ROD SET is the rate the pilot sets with the W and S keys.
7ATTITUDE — a sphere fixed to the Moon, seen through a window that turns with the craft. The horizon shows pitch by its distance from the centre and roll by its tilt, and the numbered meridians show yaw, the direction you are facing.
8THRUST — descent engine power as % of maximum. Only 10–60% is usable.
9FUEL — descent propellant remaining (%). Below it is an estimate of burn time (time to dry) left at the current throttle (min:sec). The real LM cockpit had no such timer. Houston worked the time left out from the propellant gauges and CapCom relayed the calls, sixty seconds and thirty seconds.
10ABORT STAGE — jettisons the descent stage and fires the ascent engine back to orbit, ending the mission (A). Conditions for abort included descent engine failure, critical system failures late in the descent, or an unsafe trajectory and imminent crash.

THE DISPLAY AND KEYBOARD — P64 AT 2,000 FT

The DSKY (Display and Keyboard) was the primary interface for the Apollo Guidance Computer, allowing astronauts to communicate with the onboard navigation systems. It featured a physical numerical keypad alongside rows of status indicator lights and glowing digital display registers. Operation relied on entering two-digit VERB (action) and NOUN (data) codes to execute tasks like calculation requests or engine burns.

Here the three registers show the numbers, PROG is the running program and COMP ACTY flickers while the computer works. A program alarm lights MASTER ALARM and the display jumps to V05 N09. The display follows the real unit with the four legends gathered into one row, the warning-lamp block and keypad are not modelled, and ENTER stands in for PRO.

PROGRAMS
P63BRAKING PHASEIN THE SIM — ignition to high gate near 7,000 ft, throttle and steering by the computer. Eight minutes for Eagle.
P64APPROACH PHASEIN THE SIM — high gate to low gate. The LM pitches up, the site comes into the window and the LPD goes live. SPACE and the arrows redesignate.
P65AUTOMATIC LANDINGNOT IN THE SIM — the computer’s own final descent to the pads. No crew ever let it fly this phase.
P66RATE OF DESCENTIN THE SIM — attitude by hand, descent rate in 1 ft/s clicks, throttle by the computer. Every landing ended this way. PILOT mode.
P67MANUAL LANDINGIN THE SIM — attitude and throttle by hand, the computer only displays. COMMANDER mode here.
P70P71ABORTSP71 IN THE SIM — P70 on the descent engine, P71 stages and fires the ascent engine. The ABORT STAGE button here is P71.

VERBS — WHAT TO DO
V06DISPLAY DECIMALIN THE SIM — show the noun’s three values in decimal. The landing programs put this up on their own, V06 N63 in P63, V06 N64 in P64, V06 N60 in P66.
V16MONITOR DECIMALNOT IN THE SIM — like V06 but refreshed about once a second. Aldrin kept V16 N68 up through the braking burn.
V37CHANGE PROGRAMNOT IN THE SIM — V37, ENTER, then the program number. The way to call up any program by hand, P68 after touchdown for instance.
V57PERMIT RADAR UPDATESNOT IN THE SIM — let the landing radar correct the computer’s altitude and velocity once it locked onto the surface.
V99ENGINE ON ENABLENOT IN THE SIM — flashed just before ignition. The crew pressed PRO to let the descent engine light. The sim starts with the engine already burning.
V05N09SHOW ALARM CODESIN THE SIM — the display the computer jumps to on a program alarm. Register 1 holds the latest code.
NOUNS — WHICH DATA
N63VELOCITY, ALT RATE, ALTITUDEIN THE SIM — the P63 display, in ft/s, ft/s and ft.
N64REDESIGNATION TIME + LPD, ALT RATE, ALTITUDEIN THE SIM — the P64 display. Register 1 holds two numbers with a blank digit between them, the seconds left to redesignate on the left and the LPD angle on the right. The angle is what Aldrin read out to Armstrong.
N60HORIZONTAL VELOCITY, ALT RATE, ALTITUDEIN THE SIM — the P66 and P67 display. Register 1 is the total speed across the ground. The forward and lateral parts, with their signs, are on the X-pointer readouts.
N68RANGE TO SITE, TIME TO GO, VELOCITYNOT IN THE SIM — the braking-phase monitor. Range in tenths of a nautical mile.
N09ALARM CODESIN THE SIM — the three most recent program alarms.
PROGRAM ALARMS
1202EXECUTIVE OVERFLOWTRANQUILITY BASE ONLY — the computer ran out of job slots. It restarted, dropped the low-priority work and kept flying. Houston called “go” within seconds. The rendezvous radar, left in a mode that flooded the computer with interrupts, was the cause.
1201NO VAC AREASNOT IN THE SIM — the same overload reported by a different pool of memory. Eagle got one 1201 and four 1202s on the way down.

KEYBOARD CONTROLS
A key held down keeps tilting the craft. Tap the opposite key to level off. Only P66 (P67 in COMMANDER mode) accepts flight inputs unless you take control early with P.

↑ ↓Pitch. Tilt forward to speed up, tilt back to brake.P66
← →Roll. Slide left or right.P66
Q EYaw. Turn the craft about its vertical axis.P66
W SDescent rate. Slower or faster, in 1 ft/s clicks. Shown as ROD SET.P66 · PILOT
SHIFT W SThrottle up or down by hand, within the 10–60% band.P67 · COMMANDER
SPACE ↑↓←→Hold SPACE and tap an arrow to move the target ring one LPD click: ½° up or down, 2° left or right.P64
ENTERSkip the P63 braking phase. Later, acknowledge a program alarm (the DSKY PRO key).P63 · P64
PTake manual control early.P64
XEngine stop. Press about a second after the blue LUNAR CONTACT light.P66
AAbort stage. Fires the ascent engine and ends the mission.ANY
RRetry the same site.ANY
MBack to mission setup.ANY
ESCPause.ANY

HOW TO LAND SOFT

1Read the site during P64. Dark bowls with bright rims are craters. Speckled ground is a boulder field. Pick smooth grey ground and move the target ring onto it early, while the range is long and a click still moves the point a long way. The LPD number on the DSKY and in the comms console beside the window is what Aldrin read to Armstrong. Find it on the scale and look through it.
2Kill the drift first. At the P66 hand-off at 500 ft, null both HORIZ VELOCITY needles to under 4 ft/s before you work the descent rate. A small tilt held for a few seconds is enough. Level off with the opposite key.
3Ride the rate down. Use W and S to set the descent rate. A good profile is about 3 ft/s at 100 ft and 1 to 2 ft/s in the last 20 ft. Watch your shadow slide toward you. When it reaches the craft you are seconds from contact.
4Count one at the light, then stop the engine. The blue LUNAR CONTACT light means a probe 5.6 ft below the pads has touched. Armstrong called it, counted one, then pressed engine stop, so the craft settled the last few feet on a dying engine. Cut at the instant of the light and she drops. Cut too late and she bounces. Run the tank dry and it falls. Keep an eye on TIME TO DRY.

TOUCHDOWN LIMITS

Vertical speed≤ 7 ft/sgear fails above 10
Sideways speed≤ 4 ft/sfaster tips the craft
Tilt plus ground slope≤ 12°land level, on level ground
Ground under the padsclearno boulders, no crater rim
Fuel at contact> 0a dry tank is a crash

THE THREE PROGRAMS
P63 and P64 fly themselves. The computer brakes, then steers toward the target ring while you judge the ground. Your control begins with P66 (P67 in COMMANDER mode, throttle by hand), the final descent phase, which Armstrong selected by hand near 500 ft. See the INSTRUMENT PANEL tab for the PROG display and each program in detail.

WHAT ARMSTRONG DID
Eagle’s computer was steering into a boulder field beside West crater. Armstrong took manual control at about 500 ft, flew past it, and set down with under a minute of fuel left. The same choice is yours on every descent.

CLOSE

← HOME

PRE-FLIGHT
DESCENT CONFIGURATION

CONTROL MODE
PILOTThe computer holds attitude and works the throttle. You steer and click the descent rate — as the LM was actually flown.
COMMANDER HARDP67 instead of P66. No descent-rate hold — you fly the throttle by hand (SHIFT+W/S) inside the engine’s 10–60% band.
FIRST FLIGHT
GUIDED DESCENT ONFive short lessons on the way down: pick the site, take control, null the drift, set the rate, cut at the light. The two flying lessons watch your numbers and clear only when you have them in hand. Switches itself off after a safe landing. Click to toggle.

LANDING REGION

APOLLO 11 · MARE TRANQUILLITATIS The historic site. Smooth mare plains — but Eagle had to overfly a boulder field to get down.
APOLLO 12 · OCEANUS PROCELLARUM Crater-chained plains. Pete Conrad’s pinpoint landing, a walk from Surveyor 3.
APOLLO 15 · HADLEY–APENNINE A narrow shelf between the Apennine peaks and the winding Hadley Rille.
APOLLO 17 · TAURUS–LITTROW A boxed valley strewn with boulder fields. The final Apollo landing.

REAL GROUND: A 1.5 KM SQUARE OF LROC ELEVATION DATA AROUND EACH TOUCHDOWN POINT. BOULDERS ARE SIMULATED.SITE IMAGERY AND ELEVATION: LUNAR RECONNAISSANCE ORBITER, NASA/GSFC/ARIZONA STATE UNIVERSITY.
APOLLO 11  ·  PILOT MODE
BEGIN DESCENT

PAUSED
ESC TO RESUME
QUIT TO HOME

POST-FLIGHT DEBRIEF

FLY AGAIN · R
SETUP · M
HOME

FLIGHT PATH
VELOCITIES

GROUND TRACK · TOP VIEW

DESCENT PROFILE · SIDE VIEWREAL LROC GROUND ALONG YOUR TRACK

COMPUTER FLYING · P63 P64
BY HAND · P66
TOUCHDOWN

VELOCITIES · TIMEFT/S · RED DASHED LINES ARE THE TOUCHDOWN LIMITS

COMPUTER FLYING · P63 P64
BY HAND · P66
TOUCHDOWN LIMIT

This document details an Apollo Lunar Module Landing Simulator designed to replicate the final three stages of the descent sequence: Braking (P63), Approach (P64), and Final Descent (P66). The simulator aims to immerse the user in the experience of a lunar landing by integrating historical operational data and physical modeling, striving for a balance between historical accuracy and manageable training for the average user. The simulator relies on NASA's Apollo Lunar Module documentation, the Apollo 11 guidance computer source material, and elevation data derived from the Lunar Reconnaissance Orbiter Camera to model the lunar surface and terrain, including craters and boulder fields.

The simulation is driven by a virtual display and keyboard system that mimics the Apollo Guidance Computer's Display and Keyboard (DSKY). The DSKY interface featured numerical registers and command inputs, where the flight path was managed through specific programs corresponding to different phases of descent, such as P63 for braking and P64 for approach. The DSKY utilized specific data registers, including N63 for velocity and altitude rate, N64 for redesignation time and Landing Point Designator (LPD) information, and N60 for horizontal velocity, all of which were read out to the simulated commander. The interface also incorporated status indicators like MASTER ALARM and FUEL LOW to reflect critical system status.

The simulation distinguishes between two control modes: PILOT mode, where the guidance computer manages attitude and throttle while the user controls the descent rate, mirroring the actual LM flight, and COMMANDER mode, where the user takes manual control over attitude and throttle, simulating the commander’s role. The interaction is executed through a keyboard, where specific keys control pitching, rolling, yawing, and descent rate, allowing direct manipulation of the spacecraft’s trajectory. The simulator models visual cues such as the LM’s shadow, the appearance of dust, and the visual location of craters and terrain features which the pilot must utilize to navigate.

Achieving a soft landing requires adherence to specific procedural steps derived from the simulated environment. Pilots must first identify the terrain, distinguishing between smooth plains and boulder fields, and accurately determine the target landing point using the LPD markings and the DSKY data. A critical aspect of the final descent is neutralizing horizontal velocity before setting the final descent rate, often involving a controlled tilt to level off. The rate of descent must be managed carefully, aiming for slower vertical speeds near touchdown, such as approximately three feet per second at one hundred feet, transitioning to one or two feet per second in the final twenty feet. The process culminates in observing the blue LUNAR CONTACT light, which signifies that a probe has touched the pads, necessitating an immediate engine stop.

Landing limitations are defined by strict physical constraints that the simulator enforces, ensuring realistic outcomes. These limits include a maximum vertical speed of seven feet per second, constraints on sideways speed, limitations on tilt relative to ground slope, and the crucial requirement that there must be some fuel remaining upon contact. Failure to adhere to these limits results in a crash, underscoring the necessity of precise control. The simulator reinforces that the final descent phase is a judgment call, exemplified by historical events where precise manual maneuvering, such as Armstrong’s decision to fly past a boulder field, determined the success of the landing.