On July 20, 1969, the whole world gathered around their flickering television sets and watched in awe as astronaut Neil Armstrong who, if not for someone secretly slipping his very late application to the astronaut program into the pile wouldn’t have even been there (more on this in the Bonus Facts later), climbed down the leg of a strange, spidery vehicle, stepped onto the surface of the moon, and spoke the immortal words: “That’s one small step for [a] man, one giant leap for mankind.” …Followed by the much less memorable second words, “I can – I can pick it up loosely with my toe. It does adhere in fine layers like powdered charcoal to the sole and sides of my boots. I only go in a small fraction of an inch, maybe an eighth of an inch, but I can see the footprints of my boots and the treads in the fine, sandy particles.”

But for the first time in history, a human being had set foot on another world. The historic flight of Apollo 11 was the culmination of a massive eight-year effort to realize President John F. Kennedy’s goal of landing a man on the moon and returning him safely to the earth by the end of the decade. But the road from the earth to the moon was far from a smooth one, beset by numerous hurdles and setbacks. For example, the deaths of the Apollo 1 crew in a launch pad fire on January 27, 1967 prompted a complete redesign of the Apollo spacecraft, while ongoing problems with the Saturn V rocket’s massive F-1 rocket engines nearly resulted in the cancellation of the entire Apollo programme. But perhaps the greatest challenge of all was deciding how to land on the moon in the first place. Solving this seemingly trivial question proved far more difficult than expected, requiring years of careful study and the heroic persistence of an obscure but determined engineer. This is the story of how we learned to land on the moon only a little over a half century after humans were still hitching up covered wagons to go places.

By the time President Kennedy announced Project Apollo in May 1961, scientists and engineers at NASA had already been studying methods for manned lunar flight for several years. Initially, the preferred approach was the simplest; known as Direct Ascent, this involved launching one big spacecraft directly to the moon, landing the whole thing on the surface, lifting off again, and returning to earth. This was the approach seen in nearly all science fiction media up to that point, from Jules Verne’s 1865 novel From The Earth to the Moon and its 1902 film adaptation to the 1929 German film Woman in the Moon, the 1950 American film Destination Moon, and the 1954 Tintin comic book Explorers on the Moon. An early concept for Direct Ascent prepared by North American Aviation showed a spacecraft comprising three sections or modules: at the top was the cone-shaped Apollo capsule or Command Module housing the three-man astronaut crew and fitted with a heat shield to allow the spacecraft to reenter the earth’s atmosphere at the end of the mission. Below this was a cylindrical Service Module containing the oxygen tanks, fuel cells, communications gear, and all the other equipment required to keep the crew alive during the mission. And finally at the bottom was a large Descent and Ascent Stage with landing legs and rocket engines to land the whole vehicle on the lunar surface and lift it back off again.

While theoretically straightforward, in practice the Direct Ascent strategy suffered from a host of practical drawbacks – chief among them being that it was extremely heavy. The Command Module needed to be robust enough to survive the heat and stress of atmospheric reentry, while the Service Module needed to carry all the equipment and consumables needed for the entire mission to the moon and back. All this mass needed to be safely soft-landed on the moon and blasted off again, requiring the use of a massive Descent/Ascent stage and large quantities of fuel – so large in fact, that despite the moon’s gravity being 1/6th that of earth, early estimates put the total mass of the spacecraft at a whopping 90 metric tons! Such a gargantuan spacecraft would, in turn, require an equally gargantuan rocket to haul it from the earth to the moon and back. Known as the Nova, this behemoth would have stood nearly 110 metres or 360 feet tall, weighed 4.5 million kilograms or 9.9 million pounds, and had a first stage delivering a total thrust of 61,925 kilonewtons or 13.9 million pounds force. By comparison, the Saturn V rocket that ultimately took men to the moon stood 86 metres or 282 feet tall, weighed 2.8 million kilograms or 6.2 million pounds, and had a first-stage thrust of 34,500 kilonewtons or 7.75 million pounds force. Not only were engineers unsure if the Nova could even be constructed by the end of the decade, but the rocket would have been too powerful to launch from the pads at Cape Canaveral; indeed, one proposal called for the rocket to be launched from hollowed-out cliffs in Hawaii. Another early concept proposal called for Nova to be fitted with nuclear rocket engines, which would have required launching it from an uninhabited island or a giant barge to prevent contaminating populated areas with radioactive fallout.

There were other, equally concerning flaws with the concept. For instance, nobody knew what the surface of the moon was like or whether it was stable enough to launch a giant spacecraft from. Also, in order to better withstand the G-forces of launch and reentry, the crew lay with their backs to the Command Module heat shield, meaning they would somehow have to land on the moon while facing away from the lunar surface.

Wernher von Braun, the former Nazi rocket engineer and director of the Marshall Space Flight Centre in Huntsville, Alabama, favoured an alternative approach known as Earth Orbit Rendezvous or EOR. Instead of being launched all at once with one massive super-rocket, the lunar spacecraft would instead be launched in pieces aboard many smaller Saturn C-5 rockets and assembled in earth orbit before setting off for the moon. Several variations of this scheme were proposed: in one, the spacecraft sections were launched into orbit pre-filled with rocket propellant; while in another they were launched empty and topped up with propellant by another spacecraft just prior to departure. And in yet another, the spacecraft was assembled by astronauts based aboard an earth-orbiting space station built ahead of time.

But while Earth Orbit Rendezvous eliminated the need to develop a giant and potentially troublesome super-rocket, it soon became clear that this approach was just as problematic and risky as Direct Ascent. For one thing, assembling such a spacecraft would require NASA to perfect techniques for orbital rendezvous and docking – the feasibility of which was unknown at the time and would not be demonstrated until the Gemini 6, 7, and 8 missions in 1965 and 1966 – and for more on how the former mission nearly ended in fiery disaster, please check out our previous video That Time NASA Almost Turned Two Astronauts into Roman Candles. Further, spreading the spacecraft components over multiple launches actually increased overall risk, since even a single failed or aborted launch would likely result in a mission being cancelled. Even a delayed launch could have serious consequences, for cryogenic rocket propellants like liquid oxygen and liquid hydrogen could potentially boil away into space by the time the spacecraft was ready for departure. The use of an orbiting space station as an assembly dock and home base would help mitigate some of these risks, but would also likely push the project far beyond President Kennedy’s 1970 deadline. And on top of all of this, once they reached the moon the crew would still face the same problem of safely landing a massive spacecraft on the lunar surface and blasting off again.

It is worth noting here that many of the difficulties NASA faced in selecting a lunar landing profile stemmed from a combination of politics and locked-in design decisions. The basic Apollo Spacecraft design had been conceived in 1960 by Maxine Faget, chief designer at NASA’s Langley Research Centre in Hampton, Virginia, as a more sophisticated, general-purpose successor to his primitive Mercury Capsule, which carried the first American astronauts into space. Faget chose a crew size of three so the spacecraft instruments could be continuously monitored in three eight-hour shifts, while the size of the spacecraft and the volume of oxygen, fuel, and other consumables carried aboard it were chosen based on a 14-day mission – the maximum time anticipated for a trip to the moon and back. These design decisions resulted in a spacecraft weighing around 4 metric tons. However, at the time lunar missions were seen as a far-off goal, and little thought was given to how the Apollo spacecraft would actually land on the moon. But after the Soviet Union leapfrogged the United States with a string of spectacular space “firsts” including the launch of Sputnik 1, the first artificial satellite, on October 4, 1957; and the first manned orbital flight of cosmonaut Yuri Gagarin aboard Vostok 1 on April 12, 1961, the U.S. government scrambled to choose a spaceflight goal that would allow them to beat the Soviets. Earth-orbiting space stations and manned lunar flybys were quickly rejected as the Soviets could likely accomplish these feats using existing hardware; the only mission that would require both superpowers to develop new launch vehicles from scratch – giving the US a chance to pull ahead – was a manned lunar landing. Maxine Faget’s Apollo design was thus pressed into service as America’s lunar spacecraft far ahead of schedule.

Meanwhile, aerospace contractor McDonnell-Douglas had submitted a number of proposals for Direct Ascent mission profiles using their own 2-man Gemini capsule or a simplified, 2-man version of the Apollo spacecraft – missions which could very feasibly be flown by the end of the decade using a great deal of off-the-shelf hardware. However, by the end of 1961 all the major government contracts for Project Apollo had already been handed out, and neither NASA administrator James Webb nor U.S. Vice President Lyndon Johnson – chairman of the National Aeronautics and Space Council – were willing to take the main spacecraft contract away from North American Aviation. Both Wernher von Braun and Jerome Wiesner, science advisor to the President, fought tooth and nail to have McDonnell-Douglas’s proposal accepted, until finally being silenced by the Kennedy administration. Thus, despite the many logistical advantages of the two-man direct-ascent approach, all subsequent lunar landing proposals were locked in around the more sophisticated – but far heavier – 3-man Apollo spacecraft. The political shenanigans behind North American retaining the Apollo contract would later come to light in the wake of the January 27, 1967 Apollo 1 fire, when shoddy workmanship and questionable design choices led to astronauts Gus Grissom, Edward White, and Roger Chaffee perishing in an oxygen fire during a routine dress rehearsal at Cape Canaveral’s Launch Complex 34.

The NASA administration soon split into two camps, each vehemently defending its favoured approach. In many cases preferences were driven by more than simply numbers; for example, Earth Orbit Rendezvous would require the construction of a space station, a lifelong dream of Wernher von Braun’s which would have many scientific applications beyond the moon landing. It would also require the construction of significantly more launch hardware – an attractive proposition for contractors seeking lucrative government contracts.

But as the debate raged on over the merits of Direct Ascent vs. Earth Orbit Rendezvous, a third possibility began quietly circulating among NASA engineers – an approach known as Lunar Orbit Rendezvous or LOR. First proposed by the Chance Vought Company in 1960, LOR challenged the primary assumption at the heart of the other proposed mission profiles: that the entire spacecraft had to land on and take off from the lunar surface. Instead, the Chance-Vought engineers proposed constructing a lightweight landing vehicle which the astronauts would use to descend to the lunar surface, leaving the rest of the spacecraft in lunar orbit. Once the astronauts had completed their mission, they would lift off from the lunar surface in the lander, rendezvous and dock with the orbiting spacecraft, and discard the now-redundant lander before returning home. As the lander didn’t need to withstand the stresses of launch and reentry like the main crew capsule, it could be made extremely lightweight, greatly reducing the size and weight of the rocket needed to launch the entire combination to the moon. In July 1961, NASA Langley engineer James Chamberlin – a Canadian who had previously worked on the ill-fated Avro CF-105 Arrow interceptor project – fleshed out this concept in a proposal based on the 2-man Gemini capsule. Along with the capsule itself, his proposed mission would carry one or two simple, open-cockpit lunar landers or “bugs”. On reaching lunar orbit, one of the astronauts would leave the capsule, spacewalk over to the “bug”, and fly it down to the lunar surface. Once his mission was complete, he would lift off, rendezvous with the Gemini, and spacewalk back to the capsule before returning home. Chamberlin calculated that the Gemini-based mission could be accomplished using a single Saturn C-3 – a one million kilogram or 2.2 million pound launch vehicle originally designed for the Earth Orbit Rendezvous approach – and the 2-man Apollo-based mission using the slightly larger C-5. Yet despite its many advantages, Chamberlin’s LOR concept was immediately rejected by NASA as too risky, too limited, and – for the reasons previously mentioned – politically threatening to North American Aviation’s Apollo spacecraft design.

Yet Langley continued to explore the LOR concept, modifying it to use an enclosed and pressurized Lunar Excursion Module or LEM which could dock with the 3-man Apollo spacecraft, allowing two astronauts to transfer between the two vehicles without having to perform a spacewalk. But once again NASA rejected the proposal as it involved performing a rendezvous and docking in lunar orbit – considered at the time to be far too risky. If the two astronauts aboard the LEM were unable to dock with the main spacecraft, they would be left stranded 384,400 kilometres from home – far away from any possible rescue. For this reason, NASA continued to focus on approaches which kept the entire spacecraft together throughout the entire mission.

Now enter the hero of our story, a NASA Langley engineer named John C. Houbolt. Born in Altoona, Iowa but raised in Joliet, Illinois, he obtained a master’s degree in civil engineering from the University of Illinois in 1942. That same year, he joined the Langley Memorial Aeronautical Laboratory – then operated by the National Advisory Committee on Aeronautics or NACA – as an assistant civil engineer in the Structures Research Division. However, he soon transitioned into aerodynamics, obtaining a doctorate in aerothermodynamics from the Swiss Federal Institute of Technology in Zürich in 1957 before returning to Langley, becoming Associate Chief of the Dynamic Loads Division in 1960 and Chief of the Theoretical Mechanics Division in 1962 – after NACA had become the National Aeronautics and Space Administration or NASA. Prior to his involvement in the Space Program, Houbolt’s main claim to fame were his investigations into the phenomenon of propeller whirl mode flutter, which was involved in the crashes of two Lockheed L-188 Electra airliners in 1959 and 1960.

As the debate over lunar landing profiles began to heat up, Houbolt and his small research group in the Theoretical Mechanics Division, including engineers Clinton E. Brown and William H. Michael Junior, quickly latched onto the Lunar Orbit Rendezvous concept, which they calculated would be the most fuel and hardware-efficient approach and the only one capable of placing a man on the moon by the end of the decade. There is some debate as to who actually originated the concept; as previously mentioned a version of LOR was submitted by James Chamberlin in July 1961, while Clinton Brown conducted various studies on lunar parking orbits. Houbolt, however, later claimed to have independently come up with the same idea.

What is certain, however, is that Houbolt soon became the single most vehement champion within NASA for Lunar Orbit Rendezvous, aggressively championing the concept at every possible opportunity. But so deeply entrenched were the Direct Ascent and Earth Orbit Rendezvous camps that everywhere he went Houbolt faced stiff resistance. At one meeting attended by Maxime Faget, Wernher von Braun, and NASA Associate Administrator Robert Seamans, following Houbolt’s pitch of LOR, Faget suddenly sprang from his seat and angrily declared: “His figures lie! He doesn’t know what he’s talking about!” Meanwhile, Houbolt’s supervisor ordered him to drop the matter, pointing out that selecting lunar landing profiles was well outside his department’s jurisdiction. Yet Houbolt persisted, and in November 1961 he decided to cut through the red tape and write a now-legendary letter directly to Robert Seamans. This move violated every protocol at NASA and placed Houbolt’s career on the line, a fact he plainly acknowledged:

“Dear Dr. Seamans :

Somewhat as a voice in the wilderness, I would like to pass on a few thoughts on matters that have been of deep concern to me over recent months. This concern may be phrased in terms of two questions:

1. If you were told that we can put men on the moon with safe return with a single C-3, its equivalent or something less, would you judge this statement with the critical skepticism that others have?
2. Is the establishment of a sound booster program really so difficult?

I would like to comment on both these questions, and more, would like to forward as attachments condensed versions of plans which embody ideas and suggestions which I believe are so fundamentally sound and important that we cannot afford to overlook them. You will recall I wrote to you on a previous occasion. I fully realize that contacting you in this manner is somewhat unorthodox; but the issues at stake are crucial enough to us all that an unusual course is warranted.

Since we have had only occasional and limited contact, and because you therefore probably do not know me very well, it is conceivable that after reading this you may feel that you are dealing with a crank. Do not be afraid of this. The thoughts expressed here may not be stated in as diplomatic a fashion as they might be, or as I would normally try to do, but this is by choice and at the moment is not important. The important point is that you hear the ideas directly, not after they have filtered through a score or more of other people, with the attendant risk that they may not even reach you.”

But after this rather humble and apologetic introduction, Houbolt got straight to the heart of the matter, arguing that NASA’s approach to selecting a lunar landing profile was flawed, overly restrictive, and riddled with entrenced bias:

“The greatest objection that has been raised about our lunar rendezvous plan is that it does not conform to the “ground rules”. This to me is nonsense; the important question is, “Do we want to get to the moon or not?”, and, if so, why do we have to restrict our thinking along a certain narrow channel. I feel very fortunate that I do not have to confine my thinking to arbitrarily set up ground rules which only serve to constrain and preclude possible equally good or perhaps better approaches.

…Three ground rules in particular are worthy of mention: three men, direct landing, and storable return. These are very restrictive requirements. If two men can do the job, and if the use of only two men allows the job to be done, then why not do it this way? If relaxing the direct requirements allows the job to be done with a C-3, then why not relax it? Further, when a hard objective look is taken at the use of storables, then it is soon realized that perhaps they aren’t so desirable or advantageous after all in comparison with some other fuels.

…Perhaps the substance of this section might be summarized this way. Why is NOVA, with its ponderous ideas, whether in size, manufacturing, erection, site location, etc., simply just accepted, and why is a much less grandiose scheme involving rendezvous ostracized or put on the defensive?”

He then followed this with a thorough mathematical breakdown demonstrating the inherent advantages of LOR, before concluding that:

“Naturally, in discussing matters of the type touched upon herein, one cannot make comments without having them smack somewhat against NOVA. I want to assure you, however, I’m not trying to say NOVA should not be built, i’m simply trying to establish that our scheme deserves a parallel front-line position. As a matter of fact, because the lunar rendezvous approach is easier, quicker, less costly, requires less development, less new sites and facilities, it would appear more appropriate to say that this is the way to go, and that we will use NOVA as a follow on. Give us the go-ahead, and a C-3, and we will put men on the moon in very short order – and we don’t need any Houston empire to do it.

In closing, Dr. Seamans, let me say that should you desire to discuss the points covered in this letter in more detail, I would welcome the opportunity to come up to Headquarters to discuss them with you.

Respectfully yours,

John C. Houbolt”

Unorthodox and insubordinate as it was, the letter succeeded in getting Seaman’s attention, with the former Administrator stating in 2008 that:

“It was rather strident in the way it was written. My first reaction was, ‘I’d like some way to get that son of a gun off my back.’”

Nonetheless, Seamans was swayed by Houbolt’s arguments, and he replied by promising to put LOR into active consideration. With Seaman’s backing, resistance to Houbolt’s plan soon began to crumble. Further tradeoff analyses revealed the risk of rendezvous and docking in lunar orbit to be much lower than had previously been assumed, while in June 1962, Wernher von Braun, long a staunch advocate of Earth Orbit Rendezvous, unexpectedly reversed course and announced he was now backing LOR. One month later on July 11, NASA Administrator James Webb held a press conference in which he officially announced Lunar Orbit Rendezvous as the chosen mission profile for Project Apollo, stating that it was:

“…most desirable from the standpoints of time, cost, and mission accomplishments.”

Suddenly, the once-maligned John Houbolt became a NASA hero; upon learning of the selection, Houbolt’s supervisor shook his hand and declared:

“I can safely say I’m shaking hands with the man who single-handedly saved the government $20 billion.”

In 1963, Houbolt was awarded the NASA Exceptional Scientific Achievement Medal, the citation for which read:

“[For his] foresight, perseverance, and incisive theoretical analysis of the concept of lunar orbit rendezvous, revealing the important engineering and economic advantages that led to its adoption as a central element in the U.S. manned lunar exploration.”

It is worth noting here that in addition to Direct Ascent, Earth Orbit Rendezvous, and Lunar Rendezvous, a fourth lunar landing profile was also proposed, known as Lunar Surface Rendezvous or LSR. This involved landing an unmanned spacecraft loaded with fuel ahead of the astronauts, who would then land nearby and transfer the fuel into their own tanks, allowing them to lift off from the lunar surface.While this approach reduced the payload that had to be carried by any one launch vehicle, it was deemed far too risky and never seriously considered.

But our story is far from over, for while straightforward on paper, actually implementing Lunar Orbit Rendezvous was a whole other matter. On July 25, 1962, NASA sent out invitations to eleven aerospace contractors to bid on the contract for the Lunar Excursion Vehicle or LEM – of which nine submitted detailed proposals. On November 7, the Grumman Aerospace Corporation of Bethpage, New York – which had conducted extensive preliminary studies on lunar landing vehicles – was selected as prime contractor, with development costs estimated at $350 million. The company had its work cut out for it; at the time, the United States had only a few hours of cumulative spaceflight under its belt, and the Grumman engineers were being tasked with building a spacecraft that could land two astronauts on the surface of another world and return them safely to orbit. There were countless unknowns. How easy would it be for two spacecraft to rendezvous and dock in lunar orbit? How would the astronauts guide the LEM down to the lunar surface and make a soft landing – and what would await them when they touched down? No spacecraft – unmanned or otherwise – had yet landed on the moon or even taken high-resolution pictures of its surface; in the early days of the Apollo Programme, it was feared that the lunar surface might be covered by several metres of fine dust or regolith, which would swallow up a landing spacecraft or make it tilt severely to one side, preventing it from safely lifting off again. And could all these tasks be accomplished using a vehicle light enough to be launched to the moon by a single Saturn C5 rocket – by now renamed the Saturn V? With the 1970 deadline uncomfortably close, much of the hardware would have to be designed before any of these questions could be definitely answered; educated guesses were the order of the day.

The design and development of the Lunar Excursion Module – or simply the Lunar Module, as it was later renamed – is a huge subject worthy of its own separate video. However, we will attempt to summarize it here as best we can. Early on, engineers settled on a strange, insect-shaped vehicle comprising two main sections: a lower descent module with legs and a rocket engine to allow the vehicle to touch down on the lunar surface, and an upper ascent stage containing the pressurized crew cabin. The whole vehicle would be stored aboard the Saturn V rocket in a cone-shaped shroud or adapter just behind the Apollo CSM. After lifting off from the earth, the CSM, LM, and SIVB [“S-four-B”] upper rocket stage would enter a parking orbit around the earth, allowing the crew to make last-minute checks before making a Trans Lunar Injection or TLI burn to send them on their way to the moon. Shortly thereafter, the CSM would detach from the rocket stack, turn around, dock with the LM, and extract it from its adapter. Then, upon reaching lunar orbit, two astronauts – dubbed the Commander and the Lunar Module Pilot – would enter the LM through a short tunnel, undock from the CSM, and descend to the lunar surface, leaving the third crew member, the Command Module Pilot, orbiting overhead. On completion of the mission, the crew would fire the ascent stage engine, using the now-spent descent stage as a launch pad to send them back into orbit where they would rendezvous with the Command Module Pilot aboard the CSM. Once the crew were safely aboard the CSM, the LM ascent stage would be discarded and the CSM would fire its engines, sending the crew back towards the earth.

While conceptually simple, in practice this basic configuration spawned hundreds of unforeseen design challenges, mainly relating to that universal enemy of spacecraft designers: weight. To allow the LM and the already overweight Apollo CSM to be launched to the moon by a single Saturn V rocket, the lander had to weigh under 10 tons. However, the estimated mass quickly ballooned to over twice that figure, forcing engineers to make numerous clever design decisions in a bid to slim the vehicle down. For example, the original design had the two astronauts strapped into seats in the middle of the ascent stage, but this configuration required the use of large – and very heavy – windows to give them adequate visibility for landing. But designers soon realized that in 1/6 earth’s gravity, human legs are perfectly adequate as shock absorbers. The seats were thus deleted and the cabin reconfigured to have the astronauts land the LM while standing up, secured in place by a system of pulleys and cables. This placed the astronauts’ heads closer to the windows, allowing them to be made much smaller and lighter while preserving the overall field of view. Another major weight-related problem had to do with the heat shielding needed to protect the LM from the extremes of the lunar environment, which could reach 121 degrees celsius in direct sunlight and -133 degrees in the shade. In this case, the problem was solved using brand-new technology: a lightweight, metal-coated Mylar plastic film developed by DuPont, which gave the finalized LM its distinctive gold foil-wrapped appearance. Further, while early concepts featured smooth, rounded surfaces, the final vehicle was largely constructed from a complex array of flat panels closely faired around the various internal components, with the descent stage going from a cylinder to an octagonal prism. The result was a truly alien-looking vehicle unlike anything which had come before, a true spacecraft designed purely for use in the vacuum of space. In a desperate bid to save weight, Grumman engineers made each body panel only as thick as it absolutely needed to be, with some being only as thick as a few layers of tinfoil. This meant that workers at the Grumman factory had to take special precautions lest a dropped tool puncture the hull. Indeed, to eliminate dust, loose fasteners and other debris that might float out, injure the astronauts, or short out electronics, the LMs were constructed in some of the first industrial “clean rooms” under sterile conditions, with workers wearing full “bunny suits” with hair nets, booties, gloves, and face masks. And just to make sure nothing was missed, the completed vehicles were placed in a special jig and turned upside down to shake out any remaining foreign objects.

Other design decisions concerned the unique challenges of landing on the moon. For example, more landing legs ensured greater stability – especially if one of those legs broke on impact – but increased the vehicle’s overall weight. Extensive drop tests conducted with models revealed that four legs were an adequate compromise, and this was integrated into the final design. To avoid having to use hydraulic shock absorbers in the vacuum of space, the legs were fitted with blocks of rigid plastic foam that would crush on impact, absorbing most of the shock of landing. Originally, the astronauts were to enter and exit the LEM cabin by climbing up and down a simple rope – the assumption being that this would be easy in lunar gravity. However, tests using a full-scale mockup and a counterweight system to simulate reduced gravity proved this assumption wrong, and instead the descent stage was rotated to place one of the four landing legs in line with the ascent stage hatch and its “porch”, and ladder rungs added to the leg strut. The hatch itself was originally round, but was eventually redesigned to be square to fit the astronauts Portable Life Support System or PLSS backpacks.

The finalized Lunar Module, whose design was frozen in April 1963, measured 7 metres tall and 9 metres across with the landing legs extended and, despite Grumman’s best efforts, weighed in at 15 tons – fully half again as much as the original design goal. Thankfully, however, the Saturn V design team led by Wernher von Braun succeeded in squeezing 20% more payload capacity out of the rocket, allowing this weight increase to be accommodated. The descent stage of the LM was powered by a 45,000 Newton thrust, fully throttleable rocket engine manufactured by TRW Inc., which burned a combination of Aerozine 50 and nitrogen tetroxide. These propellants are hypergolic, igniting on contact with one another, meaning no separate igniter was needed and the astronauts only had to open a pair of propellant valves to light the engine – and for more on how these nasty substances contributed to a now-forgotten nuclear disaster, please check out our previous video When Dropping a Wrench Almost Caused Armageddon. In addition to the descent engine and propellant tanks, the descent stage also contained wedge-shaped Scientific Equipment or SEQ Bays for storing tools, scientific instruments like the Apollo Lunar Surface Experiments Package or ALSEP and – on Apollos 15-17, the Lunar Roving Vehicle or LRV – AKA the “Moon Buggy” – and for an exhaustive breakdown of the ALSEP system, please check out the video on Our Own Devices, the personal YouTube channel of this video’s author. The descent stage also housed a continuous wave doppler radar to provide the astronauts with their altitude and rate of descent above the lunar surface.

Meanwhile, the ascent stage was powered by a 16,000 Newton thrust Bell Aerospace rocket engine, also fuelled by Aerozine 50 and Nitrogen Tetroxide. These propellants also fuelled the reaction control system or RCS thruster quads that allowed the LM to be manoeuvred in the vacuum of space. Due to the corrosive nature of these propellants, the ascent engine could only be ignited once before having to be rebuilt, meaning the first time a production engine was fired was when the LM lifted off from the lunar surface. If it failed, the astronauts would be stranded with no hope of rescue. Understandably, a huge amount of effort was devoted to ensuring the ascent engine would fire the first time, every time; failure, as they say, was not an option.

In addition to the ascent engines and thrusters, the LM also contained the pressurized cabin for the Commander and Lunar Module pilot, flight controls, batteries, oxygen tanks for the life-support system, navigation and communications systems, and everything else needed to land on the lunar surface, keep the astronauts alive for the duration of their mission, lift off again, and rendezvous and dock with the orbiting CSM. This included a hatch and docking ring at the top of the module, which allowed the LM to dock with the CSM and the crew to transfer from one vehicle to another through a short tunnel. Originally, a second docking port was integrated into the forward ingress/egress hatch to allow the LM crew to take an active role in docking. However, this was eventually deleted in the name of weight savings and responsibility for docking given to the Command Module Pilot. Early on, it was unknown whether the crew would be easily able to perform the procedure of pulling away from the SIVB rocket stage, turning around, docking with the LM, and extracting it from its adapter. Therefore, various rigid and flexible tether mechanisms were devised to assist this process. However, the ten 2-man Gemini missions flown between 1965 and 1966 demonstrated that orbital manoeuvres – including the dreaded rendezvous and docking – were far easier than expected to perform, so these features were deemed unnecessary. Meanwhile, the Ranger and Surveyor unmanned probes, launched between 1961 and 1968, determining that the lunar surface was mostly solid with only a thin coating of dust. The LM and the astronauts inside would be in no danger of sinking into oblivion.

Without aerodynamic forces to provide resistance and damping, the LM was an extremely difficult and unforgiving machine to fly. As a result, Bell Aerospace was contracted to construct three strange, spider-like aircraft known as the Lunar Landing Training Vehicles or LLTVs. Nicknamed the “flying bedstead” by the astronauts, the LLTVs were powered by a downwards-facing 19 Kilonewton thrust General Electric CF700 jet engine and controlled by hydrogen peroxide thrusters, allowing the unusual handling characteristics of the LM to be replicated on earth. The unusual vehicle proved tricky and even dangerous to fly – so much so that on May 6, 1968, astronaut Neil Armstrong was forced to eject from his LLTV when it suddenly flew out of control, barely escaping with his life. His quick reaction and overall skill at piloting the “flying bedstead” were among the many factors behind Armstrong’s eventual selection as the commander of Apollo 11. Hardly remembered today, the unorthodox-looking LLTV was considered by many astronauts to be the “unsung hero” of Apollo, without which a successful lunar landing would have been impossible.

With most of the major unknowns regarding Lunar Orbit Rendezvous now resolved, NASA was finally ready to test the Apollo hardware in the harsh environment of outer space. The first flight of a production Block 1 Apollo CSM took place during the February 26, 1966 launch of AS-201, the spacecraft being lofted on a suborbital trajectory by a Saturn IB rocket. The test was partially successful, the spacecraft suffering serious – but easily corrected – failures in its engine and guidance system. AS-201 was followed on August 25, 1966 by AS-202, which was entirely successful and convinced NASA that the CSM was ready for manned flight. However, the tragic deaths of the Apollo 1 astronauts in a pad fire on January 27, 1967 revealed a laundry list of faults with the Block 1 spacecraft, requiring it to be completely redesigned. Meanwhile, development work continued on the larger Saturn V rocket and the Lunar Module, with the first unmanned launch of the former, Apollo 4, taking place on November 9, 1967. The flight, which completed 3 orbits of the earth, was a complete success, reassuring NASA that achieving a manned lunar landing by the end of the decade just might be possible.

A pre-production Lunar Module was supposed to have been flown aboard Apollo 4, but when the vehicle arrived at Cape Canaveral, NASA engineers discovered hundreds of design and construction flaws. NASA was unwilling to delay its launch schedule, and so Apollo 4 flew without a Lunar Module. The first unmanned LM test flight instead took place on January 22, 1968 during the Apollo 5 mission, the spacecraft being boosted into orbit by a Saturn IB. The main objectives of the flight were to confirm that the LM’s descent engine could be reliably fired multliple times, to test the vehicle’s control and manoeuvring systems, and conduct a “fire in the hole” test where the ascent engine was fired while still attached to the descent stage – the procedure that would be used to lift off from the moon or during an aborted descent. Due to a fault in the second SIVB stage guidance computer, the spacecraft achieved a lower orbit than expected. However, flight director Gene Kranz – who would later famously direct the Apollo 11 and 13 missions – quickly improvised an alternate test procedure and succeeded in carrying out all the mission objectives. This success led to the cancellation of a second unmanned LM test, greatly advancing the Apollo Programme timetable. However, an incomplete “boilerplate” LM test article was flown aboard Apollo 6, the second and last unmanned test flight of the Saturn V rocket. This flight also carried a Block I CSM with a number of Block II modifications developed in the wake of the Apollo I fire – including a new, easier-to-open crew hatch. Interestingly, Apollo 6 carried a large number of cameras, the footage from which is often used in documentaries to represent the launch of Apollo 11 and other manned missions. However, the footage is easily distinguished by the fact that the CSM is painted white; on manned missions the spacecraft were left their natural silver colour.

The first manned flight of the Block II CSM took place from October 11-22 aboard Apollo 7, crewed by Commander Wally Schirra – a veteran of the Mercury and Gemini programmes – Command Module Pilot Donn Eisle, and Lunar Module Pilot Walter Cunningham. Apollo 7 was supposed to be followed by an orbital test of the Lunar Module, but the political desire to achieve a spaceflight spectacular to intimidate the Soviets led to this flight being reconfigured as a lunar orbital mission. Crewed by Frank Borman, James Lovell, and William Anders, Apollo 8 was flown between December 21 and 27, 1968 and marked the first time in history that humans visited another celestial body and observed the far side of the moon.

The first manned orbital test of the Lunar Module was moved to the Apollo 9 mission, crewed by commander James McDivitt, Command Module Pilot David Scott, and Lunar Module Pilot Rusty Schewickart. Flown between March 3 and 13, 1969, Apollo 9 succeeded in fully proving out the LM design as well as achieving a number of spaceflight firsts, including the first docking and extraction of a LM from its adapter; the first independent flight of a pure spacecraft designed only for use in space; the first flight test of an independent life support system – i.e. the PLSS backpack the astronauts would later use on the lunar surface; the first manned “fire in the hole” test of the LM ascent stage engine; and and the second docking of two manned spacecraft after the Soviet Soyuz 4 and 5 mission on January 14, 1969. The mission also reintroduced a practice that would become standard on all subsequent Apollo missions. Prior to the March 23, 1965 Gemini 3 mission, Commander Gus Grissom – who would later perish in the Apollo 1 fire – dubbed his spacecraft the “Molly Brown” – a cheeky reference to the Broadway musical The Unsinkable Molly Brown and Grissom’s July 21, 1961 Mercury-Redstone 4 mission, during which a malfunctioning hatch led to the capsule sinking and Grissom nearly drowning in the Atlantic ocean – and for more on this forgotten near-disaster, please check out our previous video Forgotten History: NASA and the Sinking Spacecraft. Unfortunately, NASA administrators did not share Grissom’s macabre sense of humour, and banned all future astronauts from naming their spacecraft. However, the Apollo 9 crew pointed out that once separated, the CSM and LM would need different callsigns to differentiate them. The Apollo 9 Command Module arrived at the Vertical Assembly Building at Cape Canaveral wrapped in blue plastic, which reminded the crew of a giant candy; the CSM was thus dubbed “Gumdrop.” Meanwhile, the LM was given the callsign “Spider” for obvious reasons.

Apollo 9 was followed by Apollo 10, the final “dress rehearsal” of the Apollo program. Crewed by Commander Gene Cernan, Lunar Module Pilot Thomas Stafford, and Command Module Pilot John Young and flown between May 18 and 26, 1969, Apollo 10 carried out all the steps of a lunar mission except for the actual descent and landing, with Cernan and Stafford bringing the Lunar Module Snoopy within 15.6 kilometres of the lunar surface before firing the ascent stage engine and reuniting with Young aboard the CSM Charlie Brown. But if the astronauts were tempted to disobey orders and become the first men to land on the moon, it would have been a one-way trip, as Gene Cernan later explained:

“A lot of people thought about the kind of people we were: ‘Don’t give those guys an opportunity to land, ’cause they might!’ So the ascent module, the part we lifted off the lunar surface with, was short-fueled. The fuel tanks weren’t full. So had we literally tried to land on the Moon, we couldn’t have gotten off.”

George Mueller, NASA’s Associate Administrator for Manned Space Flight, further clarified:

“There had been some speculation about whether or not the crew might have landed, having gotten so close. They might have wanted to, but it was impossible for that lunar module to land. It was an early design that was too heavy for a lunar landing, or, to be more precise, too heavy to be able to complete the ascent back to the command module. It was a test module, for the dress rehearsal only, and that was the way it was used.”

But while they did not land on the moon, on their return to earth the Apollo 10 crew did set a record for the highest speed attained by a manned vehicle: 39,987 kilometres an hour – a record which still stands to this day.

With the functionality and reliability of the entire Apollo system now proven, NASA was finally ready to take the next step and attempt a manned lunar landing. Apollo 11, crewed by Commander Neil Armstrong, Lunar Module Pilot Edwin “Buzz” Aldrin, and Command Module Pilot Michael Collins, lifted off from Cape Canaveral on July 16, 1969. Four days later, Armstrong and Aldrin climbed into the Lunar Module Eagle, separated from the CSM Columbia, and fired the Eagle’s descent engine, starting their descent towards the lunar surface. However, five minutes later a pair of computer alarms – 1201 and 1202 – began to sound, putting the mission in jeopardy and proving that the LM still had some kinks to work out. After several tense minutes, software engineers determined that the alarms were caused by the LM’s landing and rendezvous radars both being switched on at the same time, leading to an “executive overflow” condition. The error was quickly corrected, and Armstrong and Aldrin continued the descent. However, the astronauts soon ran into further trouble as they realized that the guidance computer was sending them towards a large boulder field on the edge of the 91 metre-wide West Crater. With only 90 seconds of propellant left, Armstrong took manual control deftly guided the Eagle over the crater and towards a smooth area on the far side. At 8:17 PM UTC on July 20, 1969, Eagle touched down on the lunar surface – with only 25 seconds of propellant to spare. On shutting down the descent engine, Armstrong spoke the historic words:

“Houston, Tranquility Base here. The Eagle has landed.”

To which fellow astronaut Charlie Duke, acting as Capsule Communicator or CapCom, replied, slurring his words in relief:

“Roger, Twan—Tranquility, we copy you on the ground. You got a bunch of guys about to turn blue. We’re breathing again. Thanks a lot.”

Watching from Mission Control in Houston that day was the man whose insight and perseverance had made this historic moment possible: John Houbolt. Despite the accolades he had received from NASA, Houbolt had left the agency in 1963 to work for the consulting firm Aeronautics Research Associates. Nonetheless, in July 1969 he was invited to Mission Control by none other than Wernher von Braun, whose dearly-held Earth Orbit Rendezvous concept he had overturned. According to Houbolt, moments after the Eagle touched down on the lunar surface:

“…a wonderful thing happened. Von Braun turned to me … and says, ‘Thank you, John. It is a good idea.’”

Houbolt returned to NASA Langley in 1976 as Chief Aeronautical Scientist, in which role he published more than 120 technical papers before retiring in 1985 to become a private consultant. He died of Parkinson’s in 2014 at the age of 95.

Aside from the program alarms during descent, Apollo 11 encountered only one other issue with the LM: on re-entering the cabin, one of the astronauts’ PLSS backpacks accidentally snapped off the plunger on an ascent engine circuit breaker. Thankfully, the astronauts were able to close the breaker by simply shoving a pen into the hole, and the liftoff carried on as planned.

Indeed, the strange-looking Grumman Lunar Module proved itself a solid and reliable flying machine, suffering only a handful of relatively minor failures throughout its career. For example, during the descent of Apollo 14’s LM Antares on February 14, 1971, the guidance computer began displaying intermittent abort signals. The cause of the fault was traced to a small ball of solder which had come loose beneath a control panel and drifted into a switch, shorting it out. Commander Alan Shepard and Lunar Module Pilot Edgar Mitchell’s initial solution – tapping the panel with a pen – worked at first, but the faulty signal soon reappeared. If the signal reappeared after the descent engine had fired, it would automatically trigger an abort, firing the ascent stage engine and sending the LM back into lunar orbit. Unfortunately, the guidance computer’s software was literally hard-wired in the form of “rope memory” and could not be altered in flight. Instead, software engineers at NASA and MIT came up with a clever workaround, which in simple terms convinced the computer that it was already in abort mode, preventing it from triggering an actual abort. Lunar Module Pilot Edgar Mitchell entered the fix into the LM’s display keyboard or DSKY with just minutes to spare, and he and Shepard made a successful landing – and for more on this and other heroic software fixes that saved NASA missions, please check out our previous video Where Did the NASA Expression “Steely-Eyed Missile Man” Come From?

The subsequent Apollo mission, Apollo 15, was the first of the so-called “J missions” designed for longer-duration stays on the lunar surface. The LM was thus modified to carry more weight in consumables like oxygen and equipment like the Lunar Roving Vehicle or LRV. Among these modifications was an extended descent engine bell for added thrust. However, this left very little clearance between the bell and the lunar surface – a fact Apollo 15 Commander David Scott would learn the hard way. The astronauts had been trained to shut off the descent engine as soon as the probes on the LM’s landing legs signalled ground contact in order to prevent exhaust and lunar dust from being blown back into the engine and potentially causing an explosion. But when Scott carried out this procedure, the Lunar Module Falcon was already travelling faster than usual and slammed to the lunar surface at 2.1 metres per second. The hardest landing in Apollo history, the impact crumpled the engine bell and tipped the LM over at a 9 degree angle, but neither of these proved critical and the rest of the mission was a success.

But of course, nowhere did the LM prove its worth more than during the ill-fated flight of Apollo 13. On April 13, 1970, while en route to the moon, an oxygen tank aboard the CSM Odyssey exploded, severely crippling the spacecraft and placing astronauts Jim Lovell, Fred Haise, and Jack Swigert in mortal danger. Wishing to preserve whatever power they could for reentry and splashdown, the crew powered down Odyssey and moved into the LM Aquarius, using it as a lifeboat. Though Aquarius required several improvised modifications – including, famously, a MacGyvered adaptor to make the CSM’s square CO2 scrubber canisters fit the LM’s round canister holes – Grumman’s strange “bug” nonetheless performed above and beyond its designed capabilities, keeping the astronauts alive throughout the harrowing four-day journey to the moon and back. So proud was Grumman of this achievement that they cheekily sent North American Aviation, prime contractor for the CSM, a $312,421.24 invoice for “towing” the LM most of the way to the moon and back. The invoice included $400,004 in mileage fees, $536.05 for charging the CSM’s batteries, and an $8 per night lodging fee for an “additional guest in room” – AKA Command Module Pilot Jack Swigert. Amusingly, North American formality refused to pay, arguing that its CSMs had already ferried three Grumman LMs to the moon free of charge.

Even before Apollo 11 touched down on the moon, a division of NASA known as the Apollo Applications Program or AAP was formed to find future scientific uses for Apollo hardware. This included long-duration missions on the lunar surface, and for this purpose a number of advanced versions of the Lunar Module were devised. For example, the Apollo LM Taxi was a standard LM fitted with hydrogen fuel cells and extra oxygen and hydrogen tanks to allow astronauts to remain on the moon for up to 14 days. Meanwhile, the Apollo LM Shelter was a small “moon base” comprising a standard LM with its ascent stage engine and propellant tanks removed and replaced with more consumables storage. Together with the LM Taxi, the LM Shelter would allow astronauts to extend their stay up to three months – or even longer with the help of the Apollo LM Truck, an unmanned, remotely-guided LM designed to deliver food, oxygen, and other supplies to the lunar surface. However, changing politics, flagging public interest in space exploration, and severe budget cuts led to nearly all the proposed Apollo Applications Projects missions being cancelled in 1968. The only one to make it off the drawing board was Skylab, an earth-orbiting space station constructed from a modified Saturn V SIVB rocket stage. Launched into orbit on May 14, 1973, Skylab was visited by three three-man crews between May 25, 1973 and February 8, 1974 before being deorbited and burning up in the atmosphere on July 11, 1979. The Skylab program marked the last operational use of a Grumman Lunar Module, the descent stage of which formed the base of the Apollo Telescope Mount solar observatory. In total, 15 Lunar Modules were manufactured and 10 operationally flown, with the remaining 5 either being used for ground testing or intended for the cancelled Apollo 18-20 missions. Today, three original production vehicles are on display at the National Air & Space Museum in Washington, D.C.; the Cradle of Aviation Museum in Long Island, New York; and the Kennedy Space Centre in Florida. Various test articles are also on display in other museums.

Given the success and cultural impact of the Apollo missions, it is easy to forget that the Race to the Moon was, well, a race – one with one other competitor: the Soviet Union. As was often the case throughout the Space Race, the Soviet approach to landing on the moon was in many ways similar to NASA’s but in other ways very, very different. But that, dear viewers, is a subject for another video.