Eagle's Descent — Apollo Lunar Module Landing Simulator

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.

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.

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

The pilot window during the P64 approach

1 2 3

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.

1

LANDING 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.

2

CRATERS — dark bowls with bright rims. The shadowed near wall shows their depth. Avoid them when choosing a site.

3

TARGET 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.

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

P63

BRAKING PHASEIN THE SIM — ignition to high gate near 7,000 ft, throttle and steering by the computer. Eight minutes for Eagle.

P64

APPROACH 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.

P65

AUTOMATIC LANDINGNOT IN THE SIM — the computer’s own final descent to the pads. No crew ever let it fly this phase.

P66

RATE 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.

P67

MANUAL LANDINGIN THE SIM — attitude and throttle by hand, the computer only displays. COMMANDER mode here.

P70
P71

ABORTSP71 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

V06

DISPLAY 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.

V16

MONITOR DECIMALNOT IN THE SIM — like V06 but refreshed about once a second. Aldrin kept V16 N68 up through the braking burn.

V37

CHANGE PROGRAMNOT IN THE SIM — V37, ENTER, then the program number. The way to call up any program by hand, P68 after touchdown for instance.

V57

PERMIT RADAR UPDATESNOT IN THE SIM — let the landing radar correct the computer’s altitude and velocity once it locked onto the surface.

V99

ENGINE 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.

V05
N09

SHOW ALARM CODESIN THE SIM — the display the computer jumps to on a program alarm. Register 1 holds the latest code.

NOUNS — WHICH DATA

N63

VELOCITY, ALT RATE, ALTITUDEIN THE SIM — the P63 display, in ft/s, ft/s and ft.

N64

REDESIGNATION 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.

N60

HORIZONTAL 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.

N68

RANGE TO SITE, TIME TO GO, VELOCITYNOT IN THE SIM — the braking-phase monitor. Range in tenths of a nautical mile.

N09

ALARM CODESIN THE SIM — the three most recent program alarms.

PROGRAM ALARMS

1202

EXECUTIVE 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.

1201

NO 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.

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/s	gear fails above 10
Sideways speed	≤ 4 ft/s	faster tips the craft
Tilt plus ground slope	≤ 12°	land level, on level ground
Ground under the pads	clear	no boulders, no crater rim
Fuel at contact	> 0	a 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

PILOT

The computer holds attitude and works the throttle. You steer and click the descent rate — as the LM was actually flown.

COMMANDER HARD

P67 instead of P66. No descent-rate hold — you fly the throttle by hand (W/S) inside the engine’s 10–60% band.

FIRST FLIGHT

GUIDED DESCENT ON

Five 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 VIEW

REAL LROC GROUND ALONG YOUR TRACK

COMPUTER FLYING · P63 P64

BY HAND · P66

TOUCHDOWN