[Editor's Note] In this timeless article crafted specifically for GeoPrac, Ed Nowatzki, PE, PhD of NCS Consultants, LLC recounts his experiences working on the geotechnical engineering aspects of the Apollo 11 Lunar Excursion Module (LM or LEM) while working at Grumman in the late 1960's. As most people know, Apollo 11 was the NASA mission where man first set foot on the Moon. But before they set foot on it, they had to set the landing pads of the LM there! How did Ed and his associate, Dr. Leslie Karafiath, come up with geotechnical soil parameters for the design of the LM's footpads? How did they determine a bearing capacity of lunar soil? Click through for this facinating article that I hope you will share with your colleagues. A PDF version of this article is available at NCS' website. (NASA Image) [/Editor's Note]

The Ultimate Geotechnical Engineering Challenge

By
Edward A. Nowatzki, PE, PhD
Principal Engineer, NCS Consultants, LLC (www.ncsconsultants.com) 

Introduction

After I completed my Ph.D. in July 1966, I took my first “real job” with the Grumman Aircraft Engineering Company in Bethpage, Long Island, New York.  Grumman went through a few name changes since that time before becoming part of Northrop Grumman, one of today’s largest defense contractors.  Why would a guy with a Ph.D. in civil engineering and a specialization in geotechnical engineering take a job with Grumman, a defense contractor and one of the US Navy’s major contractors for making sophisticated aircraft such as the A-6 “Intruder” and the F-14 “Tomcat”?  Well, Grumman was also the National Aeronautics and Space Administration’s (NASA’s) prime contractor for the design and construction of the lunar excursion module (LEM), better known as the lunar module or “LM.”

July 20, 2007 will be the thirty-eighth anniversary of the first time men set foot on the Moon, an event many people believe to be one of the most significant accomplishments of the millennium, despite the fact that some people still believe the whole Apollo program including the lunar landing was staged.  I consider myself very fortunate to have been a part of that historic event while I worked for Grumman’s Research Department.  This essay contains some recollections of my work on the LM project at Grumman, specifically the geotechnical engineering problems associated with the design of the first lunar lander.

Background

NASA’s plan for manned lunar exploration consisted of multiple phases, starting with the Ranger Program, followed by the Lunar Orbiter and Surveyor programs and culminating with the Apollo Program, which was the name given to the manned lunar landing program.  In 1963 President Kennedy, for political reasons, committed the United States to sending men to the Moon and returning them safely to Earth before the end of the decade (1960s).  By doing so he inadvertently compressed NASA’s well conceived plan for lunar exploration into a time frame that diminished the usefulness of much of the data collected on the unmanned projects for the design of the LM.

NASA’s first task was to determine suitable landing sites for the Apollo Program missions since there was a controversy ranging at the time about the thickness of the lunar regolith (surface or “dust” layer). The Ranger Program (1964-65) provided more than 17,000 photographs of the lunar surface at resolutions not previously obtained by earth-based telescopes, but since the spacecraft disintegrated upon impact, those missions provided virtually no information on the thickness of the “dust” that was believed to cover the Moon’s surface.  However, photos obtained from the Ranger missions were used to define areas where high resolution photography could be taken from lunar orbiters. The Lunar Orbiter program (1966-67) provided high resolution photography of selected areas of the lunar surface.  The goal was to identify candidate sites for further exploration with the intent of selecting sites for the Apollo Program’s manned lunar landings.  After candidate landing areas were selected from the orbiter photographs, the Surveyor Program (1966-1968) was launched to obtain scientific and engineering data through the use of unmanned landers that were instrumented to perform in situ tests on the lunar soil.  According to NASA’s original plan the data collected from the Surveyor Program would be used directly in the design of the LM.  Unfortunately that was not to be the case because of the compressed schedule.

Before going into the geotechnical part of this essay, I believe it would be helpful to present a little background on the US manned space flight program.  The Mercury and Gemini programs were going on concurrently with the unmanned programs described above.  The main objectives of the six manned flights of the Mercury Program (1961 - 1963) were very specific: (1) to orbit a manned spacecraft around Earth, (2) to investigate man's ability to function in space and (3) to recover both man and spacecraft safely.  The main objectives of the ten missions of the Gemini Program (1965 - 1966) were to learn how to "fly" a spacecraft: (1) by maneuvering it in orbit, (2) by rendezvousing with other vehicles and (3) by docking with other vehicles.  In general, the main goals of these pre-cursor programs were to evaluate the performance of astronauts in a zero-gravity environment and to develop and test the systems that would be needed for the eventual manned lunar landing program.

The Apollo Program (1966 - 1972) was the manned lunar landing program.  The Apollo spacecraft consisted of three components or “modules,” all of which were integrated into one vehicle for the long journey between the Earth and the Moon, but each of which performed separate functions once the spacecraft went into lunar orbit.  The three modules were: (1) the command module (CM), which would stay in orbit around the Moon during the lunar landing and carry the three astronauts back to Earth after completion of the lunar excursion, (2) the lunar module (LM), which, as stated previously, was the module that actually landed on the Moon, and (3) the service module (SM), which was the upper part of the LM until it was time for the two astronauts who had landed on the Moon to return to the CM for the trip back to Earth.  The SM separated itself from the LM upon launch from the lunar surface and rendezvoused with the CM orbiting the Moon.  The SM was jettisoned after the astronauts, transferred themselves and their cargo of lunar soil and rocks to the CM.  The Apollo Program consisted of 11 missions, not including Apollo 1, which resulted in the death of the primary crew in a command module fire during a practice session.  The first flight was Apollo 7.  Its mission was to test the command and service modules (CSM) in an Earth orbit.  It was followed by Apollo 8 whose mission was to test the CSM in a lunar orbit.  Then came Apollo 9 and Apollo 10 to test the LM in Earth and lunar orbits, respectively.  Apollo 11 was the first attempted and successful lunar landing mission.  It was followed by the five other successful landing missions and one attempted but aborted landing mission, Apollo 13.  My work at Grumman was primarily in support of the Apollo 11 mission.  The following summarizes the main statistics of the Apollo 11 mission:

• Launched: July 16, 1969
• Landed on the Moon: July 20, 1969
• Landing Site: Sea of Tranquility
• Returned to Earth: July 24, 1969
• Command Module: Columbia
• Lunar Module: Eagle
• Crew
• Neil A. Armstrong, commander
• Edwin E. Aldrin, Jr., lunar module pilot
• Michael Collins, command module pilot

The Geotechnical Challenge

The LM was a unique vehicle because of the environment in which it had to operate.  Its design involved many first-time engineering challenges.  Because of severe weight limitations, design changes required considerable time and could not be made easily.  As noted earlier, President Kennedy’s inadvertent compression of NASA’s original plan for lunar exploration diminished the usefulness of much of the data collected on the unmanned projects for the design of the LM.  For example, the LM’s footpads had to be designed before an unmanned Surveyor landed on the Moon.  Think of the LM as a structure with its four footpads as its structural foundation (Refer to Figure 1).  To complicate matters, the lower portion of the LM had to serve as the launch pad for the top portion (Service Module) to blast off the surface of the Moon and return to the Command Module that was orbiting the Moon.  In short, the footpads had to be designed to meet two criteria: (1) the LM could not settle more than a specified amount upon landing since the greater the settlement the closer the exhaust nozzle of the LM rockets got to the lunar surface (the exhaust cone is clearly visible in Figure 1 directly beneath the LM superstructure just beyond the astronaut), and (2) differential settlement between any two footpads could not result in an angular rotation greater than a certain critical value.  The LM had to provide a stable, near-horizontal platform for the Service Module to be launched into an orbital trajectory so that it could rendezvous with the Command Module that was in a lunar orbit all during the lunar excursion.

The danger of not meeting Criterion 1 was that if the exhaust nozzle of the LM rocket engines got too close to the lunar surface, the exhaust would kick up so much dust that the engines could choke before liftoff.  The danger of not meeting Criterion 2 was that if the orientation of the vertical axis of the LM exceeded an angle of about 15 degrees with respect to the local true vertical, the LM could go ballistic after launch and crash into the surface of the Moon instead of going into a lunar orbital trajectory for eventual rendezvous with the Command Module.  If either of these two criteria was not met, the mission would end disastrously.  Figure 1 shows the LM on the surface of the Moon with one of the astronauts unloading lunar experiments from the cargo bay.

Figure 1
The LM on the surface of the Moon – astronaut unloading scientific instruments (NASA photo)

What a challenging geotechnical problem - no test data to design the foundations for a critical structure that would take two men to the Moon and carry them back to the Command Module for their eventual return to Earth!  This mission carried with it the hopes of an entire nation; and it would be conducted in full view of the entire world!  This challenge was the main reason why I took the job with Grumman, but don’t think for a moment that I faced these problems all by myself.  There were many teams of scientists and engineers all over the country working on the unique problems associated with this project.

Geotechnical studies were being conducted by NASA scientists and engineers (e.g., N.C. Costas at the Marshall Space Flight Center, Huntsville, AL and W.D. Carrier at the Manned Spacecraft Center, Houston, TX) and by faculty at some of the finest universities in the country through NASA-funded research grants (e.g., J.K. Mitchell at UC Berkeley and R.F. Scott at Cal Tech). I was part of the team in Grumman’s Research Department.  My colleague, Dr. Leslie Karafiath, and I were the only two geotechnical engineers on the Research Department’s team.  Although most of the other members of the Research Department’s team were scientists, many of the members of Grumman’s Apollo Project design team were engineers.  Incidentally, Les had over twenty years of geotechnical engineering consulting experience at that time compared to my none.  To say the least, working with Les on this project was a great learning experience for me.

Some of the Problems and Their Solutions

It would take pages to go into any detail about solutions to the problems Les Karafiath and I had to address, but here are a few of the geotechnical challenges we faced and how we managed to resolve them. A question and answer format is used here for this purpose.

Q.  How do you determine the near-surface geologic profile of the Moon, in particular, what is the thickness of the “dust” on the lunar surface?

A.  Fortunately, the research section that Les and I were in was called the “Geo-Astrophysics Section.”  As indicated previously, most of the other members of the group were scientists.  Some of them were planetary scientists, so they gave us some clues as to the geology of the Moon, but nobody knew for sure.  One of the most controversial topics at the time was the composition and thickness of the lunar regolith, i.e., the surface layer or “dust” layer.  The controversy stemmed from the fact that the weathering processes on the Moon are very different from those on earth. For one thing there, all of the scientific evidence at the time suggested that there was no free or adsorbed water on the Moon.  So the products of chemical weathering of parent materials on the Moon as we know them here on Earth would not exist.  Consequently we could dismiss the presence of clay minerals on the Moon, but not necessarily clay-size particles.  In addition, there is no atmosphere on the Moon, so the surface of the Moon gets hit by billions of meteorites and micrometeorites each year.  That is not the case on Earth where most meteorites burn up in the earth’s atmosphere before the reach the surface.  Thus physical weathering by comminution due to meteorite impact plays a major role in the physical weathering of rocks on the Moon, whereas it is virtually no factor here on Earth.

Other factors such as electrostatic agitation of fine dust by charged-particle bombardment from the solar wind are analogous to mass wasting here on Earth, especially in view of the Moon’s gravity being 1/6 of that on Earth 

These and other factors led to much speculation on the part of scientists as to the thickness of the lunar regolith.  I can recall attending a conference in 1966 where Dr. Thomas Gold, a renowned but rather eccentric astrophysicist from Cornell University, suggested that the lunar surface was coated with a deep layer of fine rock powder and warned that astronauts and landers would sink out of sight.  Others suggested that the lunar dust was only a few inches thick.  These wide variations of opinion were disconcerting to me and Les since we were engaged in an engineering design and did not have the luxury of academic theorizing.  The Lunar Orbiter program (1966-67) provided high resolution photography that could be used to obtain good estimates of the thickness of the dust at specific locations on the Moon, but not necessarily at the proposed LM landing sites.  Scientists used photos from the Orbiter program to estimate the thickness of the dust layer from the depth of tracks left by boulders that had rolled down crater walls, such as the one shown in Figure 2.  Les and I could be sure that at least at those sites the dust was not so thick that the boulder disappeared.  By the time the Surveyor Program was in full swing (1966-1968) to verify the thickness of the lunar dust at prospective LM landing sites, Les and I were well on our way to providing geotechnical input to LM project engineers for the design of the LM footpads.  It was satisfying to us that much of the data received from the soft-landing Surveyor spacecraft confirmed our initial estimates of pertinent soil properties.

Q.  How did you identify the type of soil at the surface and estimate its engineering properties such as gradation, density, and shear strength parameters without physically testing the soil?  No guesses allowed – the stakes are too high.

A.  The planetary scientists were convinced that the surface and near-surface geology consisted mostly of basaltic igneous rocks.  Therefore, because of the type of weathering processes on the Moon, the lunar soil was most likely a basalt powder. Before the results of the Surveyor program were made public, the grain size distribution of the lunar soil could only be surmised from comminution theory.  There were some excellent scientists at Grumman who used comminution theory to estimate the grain size distribution that Les and I could expect.  Their estimate turned out to be remarkably close to the distribution obtained from tests on actual lunar soil samples returned to Earth by the Apollo 11 astronauts.  Based on their input Les and I comminuted basalt rocks to prepare a material with the grain size distribution of a silty fine sand.  We used this material in all of our experiments as a simulated lunar soil.  We hoped to perform experiments on the simulated lunar soil in a simulated lunar environment in order to determine values for the parameters needed to perform geotechnical analyses and design.  For example, we needed to estimate the shear strength parameters of the lunar soil (c and φ) in order to perform bearing capacity analyses of the LM footpads and to estimate settlements.  We also needed to make estimates of the in situ density (ρ) or unit weight (γ)?

Figure 2

Our laboratory experimental program was unique, but, as we found out after the fact, the results were not very reliable.  For starters, the Moon has virtually no atmosphere, i.e., the atmospheric pressure is equivalent to a “hard vacuum” i.e., atmospheric pressure <10^-12 mm of mercury.  Therefore, if we wanted to determine the effect of a hard vacuum on the engineering properties of the simulated soil, we would have to construct a vacuum chamber that could be pumped down to that pressure.  With the technology at that time it was virtually impossible to construct a vacuum chamber here on earth that could be pumped down to 10^-12 mm of mercury (1 mm of mercury = 1 torr).  Going from 10^-9 to 10^-10 torr required an entirely new technology that even the most sophisticated laboratories in the country did not possess.  After a few months of their own research, the outstanding technicians at Grumman developed the necessary technology, only to discover that at about 10^-10 torr the walls of our stainless steel vacuum chamber began to out-gas to the extent that the chamber pressure became virtually constant, i.e., the gas molecules naturally existing in the materials from which the vacuum chamber was made began to be sucked out of those materials and into the chamber due to the pressure gradient.  In fact, at about 10^-10 torr we really didn’t know whether we were actually measuring that level of vacuum since the ceramic tip on the pressure sensor was probably out-gassing also.  Figure 3 is a picture of me in the Soils Laboratory at Grumman looking rather puzzled at some pressure measurements. 

Figure 3

A photo of the author in the Soils Laboratory at the Grumman Aerospace Corporation, Bethpage, Long Island, New York (circa 1967).

Strength Testing

Figure 4 is a picture of the stainless steel, quadruple-mold specimen holder that Les and I used to pre-stress simulated lunar soils for exposure to an ultrahigh vacuum, i.e., pressure < 10^-9 torr.  Unfortunately, we could not test the specimens in the ultrahigh vacuum chamber for a number of reasons, but mainly because the necessary feed-throughs invariably leaked.  Therefore we exposed pre-compressed specimens to an ultrahigh vacuum for a period of at least 24 hours and then tested them in unconfined compression after their removal from the pressure chamber.  The observed increase in unconfined compressive strength of the specimens subjected to the ultrahigh vacuum as compared to pre-compressed specimens in air suggested that bonding between molecularly clean surfaces (i.e., no water or gas molecules on the surfaces) had occurred as a result of a complex interaction of thermal and mechanical stresses in the ultrahigh vacuum environment. Unfortunately, we could not extrapolate the results of those tests to lunar conditions with any degree of confidence.  Therefore we had to seek another way to estimate the shear strength properties of the lunar soil.  That is discussed as part of the next question.

Unit Weight

There was a relative wide range of estimates of the mass density of the lunar dust available in the scientific literature.  I did not feel comfortable with any of the published values mainly because I didn’t entirely understand the science behind the ways they were determined.  I turned to Walter Egan, one of my colleagues in the Research Department for help.  Walt’s background as a physicist was in optical sciences with years of experience in photometric and polarimetric measurements, of which I knew nothing except what I had read about in his internal Grumman reports.  Walt was interested in our problem and the two of us conducted research on the use of spectral photometry and polarimetry to establish a relationship between surface porosity and polarization for the simulated lunar soils Les and I were using in our strength experiments.

Figure 4

Quadruple mold, stainless steel specimen holder used for testing simulated lunar soils subjected to an ultrahigh vacuum environment 

We used the relationship derived from these experiments to determine a range of porosities for the lunar surface material based on available polarimetric signatures of the Moon.  By calculating void ratios from porosity and by assuming the specific gravity of the lunar soil particles to be 2.7, we were able to calculate equivalent unit weights for lunar soils and adjust them for the reduced gravity.  The range of our computed values fell within the range of values found in the scientific literature.  Although I felt more comfortable now, there was still no sure way of knowing how good (or bad) our estimates were until the results of some of the Surveyor experiments had been analyzed.  Incidentally, the results of experiments performed on some of the lunar material returned by the Apollo 11 astronauts showed the specific gravity of the solids to be 3.1.  At least we erred on the side of conservatism when we assumed 2.7.

Some of the Problems and Their Solutions, Continued

Q. How were the design shear strength parameters and other soil properties estimated based on available pre-Surveyor information?

A. This was perhaps our biggest challenge.  As indicated previously, the Moon has virtually no atmosphere (atmospheric pressure <10^-12 mm of mercury).  Therefore, the surface of the Moon is being constantly bombarded by meteorites and micrometeorites.  These two conditions were thought to have a major effect on the lunar soil’s shear strength parameters.  Comminution of geologic materials by meteorite bombardment in an ultrahigh vacuum results in molecularly clean fracture surface, i.e., there are no water or gas molecules to adhere to the freshly cleaved surfaces.  The implication of that type of mechanical weathering is that soil particles can bond molecularly with each other (cohere) and with other materials (adhere).  The strength of that bond provides an earth-like effective cohesion or adhesion.  Les and I observed something analogous to that phenomenon in our laboratory studies.  The problem was to estimate the magnitude of such cohesion.  To this end we reverted back to the Lunar Orbiter photos that showed boulder tracks, e.g., Figure 2.  By enhancing the photo images and with a knowledge of variables such as geometric scale and the sun angle, Grumman’s air photo interpreters were able to estimate the size of the boulders and, even more importantly, the width and depth of the tracks that they had left in the soil.

In the meantime, Les and I obtained a narrow range of values for the friction angle of the simulated lunar soil (a basaltic silty sand) from the results of direct shear tests performed in Grumman’s Soil Laboratory under terrestrial conditions.  We assumed that the lunar environment had little or no effect on friction angle based on our observations of crater slopes in the Lunar Orbiter photos.  Les and I then used all of this information in analyses based on the theory of plasticity to back-calculate the cohesion required for a boulder of given size (and weight) to create the corresponding impression as observed in the photo.  As expected, the value of cohesion was greater for the lunar soil than for a terrestrial soil with the same gradation.  Once the shear strength parameters had been determined in this way, a bearing capacity analyses could be performed for the proposed size and shape of candidate LM footpads.  Although the analyses suggested that the anticipated static loads on the footpads would not result in a bearing capacity failure, the LM designers introduced a sophisticated shock-absorber system in each of the four the legs of the LM consisting of crushable honeycombed infills that would prevent impact loads from reaching the footpads.  We also performed settlement analyses based on a plasticity model that we had developed.  Although I don’t recall the exact values, I remember that the results of these analyses showed that the anticipated settlements were well below the limit required to satisfy Criterion I.  Figure 5 shows the extent of settlement of the LM footpad and the apparent cohesion of the lunar soil as demonstrated by the vertical-sanding footprints. 

Figure 5

Buzz Aldrin standing by one of the Eagle's foil-wrapped footpads. (A tiny image of Neil Armstrong taking the photograph can be seen on Aldrin’s reflective faceplate.)  The slightly arms-out stance derives from the pressurized suit.  A plaque on the landing stage, which is still on the Moon, is engraved: "Here men from the planet Earth first set foot upon the Moon, July 1969, A.D.  We came in peace for all mankind."  (NASA photo)

Q.  What are the effects of the Moon’s reduced gravity on solutions to conventional geotechnical engineering problems here on earth?

A.  The effect of the Moon’s reduced gravity (1/6 of that on the Earth) on soil-structure interaction was relatively straightforward to evaluate and was easily accounted for by modifying gravity-dependent terms in any structural and/or geotechnical equations that are used conventionally to solve problems on earth, e.g., the γ-term in the bearing capacity equation.

Q.  What are the effects of the Moon’s unique environmental conditions on lunar solutions to other conventional engineering problems here on earth?

A.  Because there is virtually no atmosphere on the Moon, there is nothing to retain ambient “air” temperatures.  Therefore, objects on the surface of the Moon experience virtually instantaneous temperature changes of up to 356º C (640º F) over a very short distance, i.e., between the sunlit and shaded areas on any surface.   Scientists and engineers had to evaluate this phenomenon on the integrity of items manufactured on earth, such the LM itself, the astronauts’ space suits, etc.  Another unanticipated problem was adhesion of the lunar dust to materials manufactured on earth.  The Apollo 11 astronauts reported that every time they returned to the LM, much of the exterior surface of their spacesuits was covered by dust.  The same with tools that they were using outside of the LM.  This phenomenon was probably due to molecular bonding between the earth-manufactured objects and the molecularly clean surfaces of the lunar soil particles as described previously. 

Q.  How do you determine the magnitude of differential settlements that could occur between any two of the four legs of the LM and assure that such differential settlements do not cause the vertical axis of the LM to be more than 15 degrees from true vertical?

A.  You don’t.  But there were some very good reasons why we were not overly concerned with this potential problem.  First of all, the Apollo 11 landing site was chosen to be in the area of the Moon known as the Sea of Tranquility.  Figure 6 shows the area to be relatively smooth and free of large craters and their associated debris.  Second, although all of the complex maneuvers related to this journey (rocket burns, midcourse corrections, etc.) were computer controlled, even those directly related to the descent itself (the LM was “launched” from the orbiting command module with the landing trajectory computer-controlled), the astronauts, all of whom were veteran military pilots, insisted that the actual landing be controlled by the crew.  They felt, and rightly so, that they could make a last minute decision, if necessary, to avoid a potential hazard, such as a large boulder or the edge of a crater.  Since this aspect of the lunar landing was still not fully resolved up to the time of liftoff, I think it is appropriate to go into a little more detail here on the landing itself because the answer thus far seems rather flippant for a question whose answer has such serious consequences.  The following paragraphs are excerpted from:

http://www.fukuoka-edu.ac.jp/~kanamitu/study/solar/solar/apo11.htm#descent.

Figure 6

The Apollo 11 landing site at an altitude of approximately 9 miles, one orbit before descent was begun. Tranquility Base is near the shadow line, a little to the right of center.  (NASA photo)

Each of the astronauts had a small, double-paned, triangular window in front of him. On the inner surface of each pane in Armstrong's window, there was a long vertical scale marked in degrees and, at right angles to it, a similar but shorter horizontal scale.  At pitchover, Armstrong positioned himself so that the vertical scales were aligned; and Aldrin read a computer output to him that indicated just where he should look on the scale to see the computer's intended landing point.  In principle, if he didn't like the spot, he could pulse the pistol-grip hand controller forward or back or to either side and thereby tell the computer to move the target a small amount in the indicated direction.  According to plan, Aldrin was to give Armstrong an "angle" every few seconds until, at an altitude of about 500 feet, the window targeting scheme lost its usefulness and Armstrong took over complete manual control for the final descent.

However, once Aldrin had given him an initial target angle, Armstrong realized that the computer controls were taking the LM into a field of boulders on the northeast shoulder of a crater the size of a football field.  Although the site selection team had picked a smooth patch of ground, the state of the art of spacecraft guidance at the time of Apollo 11 wasn't nearly as refined as it would be for the later missions.  Nowhere on the Moon are craters of that size more than a few kilometers apart and, for this first landing, the NASA flight engineers were not yet ready to fine-tune the approach trajectory to much better than about eight kilometers east or west of the target point and about two kilometers north or south.  The Apollo 11 "landing ellipse" contained dozens of craters a hundred meters across or more, and the important point is that the LM had plenty of range so that Armstrong could avoid even the largest of them.

There was no doubt in Armstrong's mind about not landing in the boulder field if he could avoid it.  It wasn't essential that he land the LM perfectly upright.  A tilt of up to 15 degrees would cause no particular problem with the launch back to the SM.  However, if the exhaust nozzle or one of the landing struts hit a large boulder, there would be a good chance of sustaining structural damage.  Two minutes after pitchover and about two minutes prior to the landing, Armstrong took action.  He decided to overfly the crater and land well to the west of it.  There was clearly not enough time to give the computer an update via the hand controller since the Landing Point Designator (LPD) was designed for fine tuning and what Armstrong needed was a big change.  So he switched to manual control, pitched the LM forward, and began to fly the vehicle like a helicopter.  Within seconds, he had slowed his rate of descent from about twenty feet per second down to about three and flew the LM about 1100 feet west beyond the craters and the boulders

While Armstrong flew the LM toward a good landing spot, his attention was totally focused on the job at hand.  Aldrin did virtually all the talking; and he, too, was all business.  He read the computer output to Armstrong, giving him their altitude, their rate of descent and their forward speed.  Back at mission control in Houston it was obvious that the landing was taking longer than planned.  Indeed, with each passing second there was mounting concern about how much fuel remained.  Because of uncertainties in both the gauges in the tanks and the estimates that could be made from telemetry data on the engine firing, the amount of time remaining until the fuel ran out was uncertain by about 20 seconds.  If they got too low, mission control in Houston would have to order an abort.

Drama was the last thing that any one wanted for the first landing.  The event itself was exciting enough. Finally, Armstrong found a place that he liked and he began to reduce his forward velocity and let the LM ease down toward the surface.  As they came down through 75 feet, mission control radioed that they had sixty seconds of fuel left.  In the cabin, Aldrin had already seen a warning light telling him the same thing.  But they were close now and it was just a matter of easing themselves down.  Armstrong had reduced almost all of their forward velocity by now.  As they began to kick up dust with the engine exhaust, Armstrong asked Aldrin to confirm that they were still moving forward a little.  He wanted to land on the surface that he could see in front of them, rather than on ground he couldn't see behind them.  Aldrin gave him the confirmation that he wanted and, eight seconds later, they saw the blue contact light on the control panel. The ten-foot-long probes that dangled from the landing gear had touched the Moon.  A second or two later the engine shut off and they were down on the surface of the Moon.

In hindsight, it gives me a feeling of great accomplishment when I look at Figures 1 and 5 and see the astronauts and their footprints on the lunar surface.  It is not hard to imagine how proud and yet humbled Les Karafiath and I felt when we realized that due in part to our efforts the LM did not sink into the lunar dust more than the few inches we had predicted, that the attitude of the LM’s vertical axis was much less than the critical value of 15 degrees, and that the lip of the SM’s exhaust nozzle was well above the lunar surface.

Prologue

I have very fond memories of those days and the excitement that surrounded one of mankind’s greatest achievements.  To this day I cherish the commemorative pins shown in Figure 7 that Grumman’s management gave to their employees.  I left Grumman in 1972, about the time funding for the Apollo and Post-Apollo Programs was being substantially cut.  To make a bad pun, it did not take a rocket scientist to figure out that, with NASA’s lunar landing and exploration programs being phased out, the days of a geotechnical engineer in the employment of an aircraft manufacturer were numbered.  Fortunately, I was able to obtain a position with a geotechnical engineering consultant in New Jersey and get back to earth so-to-speak.  That position was the beginning of a career that has involved entirely different types of geotechnical engineering challenges almost every day, but none as great as the one described above.  It was the ultimate challenge, period!

Figure 7

Commemorative pins distributed to employees of Grumman Aerospace Corporation
Left – Lapel pin given to personnel of Grumman’s Apollo Project.
Right – Pin signifying the lunar contact light on the LM – given to all Grumman employees.

Acknowledgement

The author wishes to acknowledge Dr. Naresh C. Samtani, PE, PhD, President of NCS Consultants, LLC, for his effort in reviewing this article and providing comments.

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