NASA’s Lunar Module: Everything You Need to Know

The Lunar Module was an iconic spacecraft which carried two-man crews to and from the Moon’s surface during NASA’s Apollo Program of the 1960s and ‘70s.  Along with the Saturn 5 rocket and the Apollo Command and Service Modules (CSM), the Lunar Module is the third of the trinity of vehicles which made the moonlandings possible.

Image of eagle on the moon

Tranquility Base:Lunar Module Eagle stands in the morning sun on the Moon in July 1969. Note Buzz Aldrin’s legs as he crawls through the hatch. Dim-witted and/or lazy conspiracy theorists claim this hatch was too small to allow a space-suited human to pass through it. Nonsense! (Image Credit:NASA)


Originally there would have been no Lunar Module.The Apollo CSM’s origins go back to the 1950s, and it was intended to be a multi-purpose vehicle for all kinds of missions in Earth and lunar orbit. President John F. Kennedy’s declaration of the United States’ goal to land a manned mission on the moon by 1970 suddenly made it the focus of the project, yet it was not entirely suited to this role.

As originally conceived the entire Apollo spacecraft with a crew of three would have risen from Earth and landed on the Moon, blasting off to return home. By mid 1962 this had been studied and found to be grossly expensive to achieve, requiring the development of gargantuan booster rockets and to be so technologically complex that a landing might not be made until well into the 1970s. Instead a concept called Lunar Orbit Rendezvous was proposed (you can read a transcript of the press conference where this was announced here). This promised to be (relatively) easier and was possible with the Saturn 5 rocket which was being developed at the time.


Image of Lunar Landers

Lunar landers compared in an early NASA artwork depicting a notional LEM (Image credit:NASA)


How would this work? An Apollo CSM would be launched (with three astronauts on board) along with a Lunar Excursion Module (LEM) on a single Saturn 5 rocket. Thrown moonward entirely by the Saturn 5, the CSM and LEM would fly together into lunar orbit; two astronauts would fly the LEM to a gentle touchdown on our satellite. On completion of their explorations the pair would take off in the two-part LEM’s Ascent Stage (leaving behind the Descent Stage with its heavy engine and landing gear) to rejoin their orbiting colleague. The three explorers would discard the LEM before returning to Earth in the CSM. In July 1962 NASA requested LEM design concepts from the US aerospace industry, then at the height of its post-WW2 powers.



Nine designs were put forward, and the winning concept came from Grumman Aerospace, a company famed for its sturdy and successful naval aircraft. In September 1962, Grumman’s engineers set about the task of building the first true spaceship.  Why do I say that? All previous crewed spacecraft (and as of 2011 all subsequent spacecraft) passed through the Earth’s atmosphere during part of their flight. In contrast the LEM would spend its entire working life in the vacuum of space and would make no concessions to aerodynamics (unlike aircraft there would be no series of hundreds of test flights gradually expanding the performance envelope). Right from the start, the lander would clearly look completely unlike the sleek rockets of 1950s pop-culture. A bulbous, spindly-legged vehicle was envisaged, and many in NASA and Grumman nick-named it the ‘Bug’.


Image of LM evolution

Evolution (or in this case intelligent design) of the Lunar Module (Image credit:NASA)

The Bug began weighing 10 tonnes. It featured a spherical Ascent Stage with a docking port on top and a second facing forward (the astronauts would use this to access the Moon’s surface), while the pilot would look for a landing site through large helicopter-style bubble windows. The LEM was to have had three legs, but analysis suggested that three was not enough to guarantee a safe landing on uneven terrain. Five legs would be much better, but heavier. To save weight four legs were eventually used.

By early 1964, the LEM was recognizable as the craft that eventually flew to the Moon. The boxy Descent Stage stood on four splayed-out shock-absorbing legs ending in bowl-shaped pads, on top of it sat the curious-looking Ascent Stage. Grumman’s engineers had sweated blood to reach this point, struggling to prevent the craft’s mass ballooning to an unacceptable weight. The spherical cabin was gone, instead a cylindrical shape was used, the second docking port became a simple hatch and the large and heavy windows were replaced by small triangular panes. The astronauts even lost their seats, instead standing shoulder to shoulder as they controlled the vehicle (an arrangement which proved no inconvenience in lunar gravity).


Astronaut Alan L. Bean, lunar module pilot for the Apollo 12 mission, is about to step off the ladder of the Lunar Module to join astronaut Charles Conrad Jr., mission commander, in extravehicular activity

Alan Bean, Lunar Module Pilot for the Apollo 12 mission, is about to step off the ladder of the Lunar Module Intrepid to join astronaut Charles Conrad on the Ocean of Storms (Image credit: Charles Conrad/NASA)

Such was the need to make the LEM light that Grumman considered equipping the astronauts with a rope ladder or even just a length of knotted rope to climb from the hatch to the surface.However an aluminium ladder was used albeit a ladder too flimsy to support an astronaut’s full weight on the Earth’s surface.  The lunar lander’s shape was not the only thing to change, its name did too: it was redesignated the Lunar Module as “Excursion Module” sounded too frivolous, as though it was intended for taking the astronauts on a picnic.


Image of LM general arrangement

A general arrangement diagram of a J-series Lunar Module  carrying a Lunar Rover Vehicle  (Image credit:NASA)


By January 1968 when the first LM flew in space on Apollo 5 (an unmanned test flight in low Earth orbit) the design was complete. As a creature designed for an alien environment, the Bug had an alien appearance. At either side of the cylindrical cabin was a propellant tank, one for the Aerozine 50 (a mix of hydrazine and unsymmetrical dimethylhydrazine) fuel, the other for the oxidiser (nitrogen tetroxide). These two propellants had very different densities so their tanks were of differing sizes giving the Ascent Stage a lop-sided look.The total mass of the propellant was about 2388 kg. Behind the crew cabin was the Aft Equipment Bay, a box of environmental control systems and electronics including the craft’s Apollo Guidance Computer (the AGC, at first sight laughably primitive to our contemporary eyes  but an ingenious and completely successful piece of engineering ). Four sets of quad RCS thrusters to maneuver the LM through the airless void were spaced evenly around the exterior. Radio communication and radar dishes were placed here and there.


This diagram shows the layout of the fuel (propellant and oxidiser) tankage in both sections of the Lunar Module. (Image credit: NASA)

This diagram shows the layout of the fuel (propellant and oxidiser) tankage in both sections of the Lunar Module. The diagram was prepared before the first landing so it does not quite depict any of the craft that reached the Moon, notably the paddle-like RCS plume deflectors are not present nor is the location of the stowed rover used on Apollos 15-17 shown. (Image credit: NASA)


The Ascent Stage sat on the legged Descent Stage, an octagonal box housing the throttleable descent rocket motor and its propellant tanks (containing a total of 8212 kg of Aerozine 50 and nitrogen tetroxide propellant) , and a modest cargo space for the equipment and instruments to be used on the Moon. (For more technical details see Grumman’s Lunar Module Quick Reference Data .)


The Lunar Module "Spider" ascent stage is photographed from the Command/Service Module on the fifth day of the Apollo 9 earth-orbital mission. The Lunar Module's descent stage had already been jettisoned. (Image credit: NASA/David Scott)

The Apollo 9 LM Spider’s ascent stage is photographed from an unusual respective in an unusual location. This was the view from the CSM on the fifth day of the  Earth-orbital mission. Note the Ascent Engine nozzle. (Image credit: NASA/David Scott)



Many accounts of Apollo refer to the LM with words like ‘flimsy’ and ‘fragile’ but these are not wholly correct. Much of the exterior was covered in protective multi-layer insulation foil, in some locations this was taped or stapled into place. Exhaust gases from the vehicle’s engines and jets could disturb this foil and occasionally rip it to tatters, damage which is clearly visible in some images. However beneath the foil insulation was the craft’s robust pressure hull of metal skin and stringer construction. To avoid riveting or welding the skin and stringers together, Grumman precisely chemically milled the skin panels out of solid aluminium ingots so that the skin and stringers were the same piece of metal. Although its weight eventually rose to almost 15 tonnes, the LM was a fine flying machine, handling like a “nimble, responsive jet fighter”.



James McLoughlin recently generously donated a print of Apollo 9’s LM signed by the crew to the Planetarium.This is currently displayed in our Exhibition Area. (Image credit:Tom Mason, Armagh Planetarium)


Men first flew the LM in March 1969, when Jim McDivitt, David Scott, and Rusty Schweickart successfully tested an LM (Callsign Spider) in Earth orbit during the Apollo 9 mission. Months later Apollo 10 flew to lunar orbit in May. This mission did everything short of landing: astronauts Stafford and Cernan descended to within 15.6 km (9.7 miles) of the Moon’s surface in the LM ‘Snoopy’ and cruised over the Moon’s mountain tops. In July 1969, Armstrong and Aldrin made history by landing Eagle on the Sea of Tranquility.


Apollo 16's LM from the reach. The Aft Equipment Bay is prominent. Just above it is the tiny rectacular "tab" of the sublimation plate used to "vent" heat. (Image credit: NASA)

Apollo 16’s LM Orion from the rear. The Aft Equipment Bay is prominent. Just above it is the tiny angled rectangular “tab” of the sublimation plate used to “vent” heat. (Image credit: NASA)


How did the crew fly the LM? Both crewmen could control the vehicle, but oddly it was the Commander rather than the Lunar Module Pilot who  actually flew it to the Moon. After undocking from the CSM, the LM fired its descent engine for several minutes to drop out of lunar orbit, descended automatically under the control of the AGC (using radar to measure altitude) until it was 500 ft or so above the surface, then the astronauts would take manual control to use the LM’s two hand controllers to adjust the programmed landing site to ensure they were going to land on a flat area and not in a boulder field or crater.


 It is cruciform. Since the Descent Stage had to support the Ascent Stage not only during landing, but also during launch from Earth, it had to be very robust. They built it using heavily-reinforced box structures.

Lunar Module Descent Stage under construction.  As the Descent Stage had to support the Ascent Stage not only during landing, but also during launch from Earth, it had to be quite sturdy. It was made from box structure in a cruciform shape which appeared octagonal with its thermal insulation applied. (Image credit: NASA)




Image of LM under construction

LM Ascent Stage under construction Notice the cylindrical cabin crew compartment and spherical oxidiser tank without their usual skins. (Image credit:NASA)


e we can see the normally hidden aft electronics bay, the white sphere of a fuel tank for the ascent engine, the four nozzles of a set of reaction control thrust - ers and the dish of a communications antenna. (image credit: NASA)

In this image of an LM Ascent Stage under construction we can see the normally hidden aft electronics bay, the white sphere of a fuel tank for the ascent engine, the four nozzles of a set of reaction control thrusters and the dish of a communications antenna. (image credit: NASA)


To leave the LM meant sealing up the spacesuits and venting all the air from its cabin, before crawling feet first through the hatch, down the porch to the ladder. Returning required the opposite procedure. The LM’s crew enjoyed minimal human comforts with no cooking or washing facilities and only a rudimentary lavatory. Hammocks were slung across the tiny cabin to sleep in. Before taking off the crew would dump any surplus items to lighten the Ascent Stage. A little junkpile of discarded life support packs, overshoes and waste bags grew at the foot of the LM’s ladder. Dressed in their spacesuits, the astronauts fired the small, simple but powerful ascent engine under their cabin and their tiny spacecraft blasted off, using the Descent Stage as a launch pad. This take off was a modest affair compared with the earthshaking Saturn 5 launch which began the Moon missions, the Lunar Module Ascent stage didn’t need to attain the 2.4 km/s to escape the Moon’s gravity,  it just had to reach a lunar orbit (orbital speed in the range 1.5-1.7 km/s) to rendezvous with the CSM. Seven minutes after ignition the astronauts would be in lunar orbit awaiting the rendezvous with the CSM. Once the LM crew transferred into the CSM, their LM Ascent stage was abandoned. All three crew returned to Earth in the Command Module.


Image of apollo-16-ascent stage

Apollo 16’s Ascent stage looks a bit the worst for wear during its rendezvous with the CSM, see final picture (Image credit:NASA)


Developing the LM was not easy and took longer than planned but in the end Grumman’s engineering team succeeded brilliantly. The Saturn 5 and the Apollo CSM both suffered failures in their gestations, requiring extensive redesign, but the LM did not. Hoping to capitalize on the development effort, Grumman offered variants of the craft for a space programme which was expected to encompass dozens of Moon missions, including a “Shelter” version to act as a temporary base, served by freighter variants, even a wheeled version which could have landed and then trundled across the dusty moonscape. Another, in an example of ploughshares into swords, would have seen the LM, a vehicle of peaceful exploration, transformed for strange military purposes into the Covert Space Denial Module. This ‘space fighter’ for the USAF would have been able to use a mechanical arm to molest Soviet satellites, or even blast them to pieces with a recoilless gun. However, only one LM variant was actually built, the Extended Stay version. This was essentially a ‘Mark II’ LM, with more fuel to prolonged hovering to allow better selection of the landing site, with more cargo space for experiments and a roving vehicle, and improved life support for a longer stay (68 hours) on the Moon. Externally identical to its predecessors, this redesigned LM was used on Apollos 15 through 17.


Image of LM at NASM

The National Air and Space Museum in Washington DC had a real LM (built but never used for orbital testing) parked outside its McDonald’s restaurant. The varying textures and colours of the insulating covering (carefully selected for optimum thermal management of the components below) are very apparent. From head-on the Ascent Stage looks oddly like a face, with the windows as squinting eyes, the hatch a gaping mouth and cheeks containing electronics and propellent tanks. Some dim-witted and/or lazy conspiracy theorists claim the astronaut mannikins at this exhibit are deliberately smaller than life-size, to make the LM appear larger than it was. Again nonsense! (Image credit:Colin Johnston, Armagh Planetarium)


Apollo 11 and the subsequent lunar landings were successes thanks to the superb design and construction of Grumman’s Bug, and the LM’s flexible design was instrumental in saving the lives of the crew of Apollo 13. Today, four complete unflown LMs are displayed in US museums, the wreckage of six Ascent Stages lie scattered across the Moon’s surface, Apollo 10’s Ascent Stage orbits the Sun, while six LM Descent Stages rest on the lunar wilderness as memorials to the first days of space exploration.


Further reading

Chariots for Apollo: A History of Manned Lunar Spacecraft by Courtney G Brooks, James M. Grimwood, Loyd S. Swenson

Lunar Module Quick Reference Data

(Please note that Armagh Planetarium is not affiliated with NASA or any company which built components of the Lunar Module. We regret that we cannot help you trace, contact or research people who worked on this project.)

(Article by Colin Johnston, Science Education Director)

Image of A16_LM_descent_stage

Apollo 16’s Descent Stage lies derelict on the Moon. It will probably outlast the Pyramids.(Image credit:NASA)