The Mobility Subsystem

The largest subsystem of the LRV was the Mobility Subsystem, which consisted of the chassis, and the equipment and controls required to suspend, propel, brake and steer the rover. This included the forward, center and rear chassis, suspension, wheels, drive control electronics, traction drives, brakes, steering linkage, fluid dampers (shock absorbers) and the hand controller used to steer, accelerate and brake the LRV. Boeing brought in General Motors and its A.C. Electronics Defense Research Laboratories (later called Delco Electronics Division) as a subcontractor to engineer these critical systems. To get the compact envelope necessary to fit into the Lunar Module's quadrant to the right of the ladder, the rover would have a front and rear folding chassis, with attached suspensions folding inward on top of the center chassis section. In addition, the suspension system had to be designed to fold 135 degrees toward the centerline of the chassis during the folding of the front and rear sections. Aluminum alloy 2024 and 2219 was used extensively throughout the mobility subsystem.

The LRV's chassis, suspension and traction drives

"I was the engineering manager of the group which came up with the folding concept and eventually the vehicle part of the Lunar Rover,'' Ferenc Pavlics stated in an interview with this author. "The vehicle was over 120 inches long, but the space available was only about sixty inches, so it really had to be folded so that it took up half its length. In addition, it had to fit into a triangular-shaped envelope, which was one quarter of the Lunar Module descent stage. To accomplish that, I came up with the idea of folding the chassis' ends 180 degrees over onto itself. Then the suspension linkage was designed such that when we folded the wheels under the chassis, they

Ferenc Pavlics

The final flight-approved configuration of the Control and Display Console featured a non-glare black panel with white instrument and switch nomenclature. The Alarm Indicator flag on top of the console (shown in the Warning position) would flip up when high-temperature conditions existed for the batteries, the drive motors or gearboxes. Note the wire guards around the select switches to prevent accidental position change, and the reverse inhibit switch on the hand controller. (NASA)

The final flight-approved configuration of the Control and Display Console featured a non-glare black panel with white instrument and switch nomenclature. The Alarm Indicator flag on top of the console (shown in the Warning position) would flip up when high-temperature conditions existed for the batteries, the drive motors or gearboxes. Note the wire guards around the select switches to prevent accidental position change, and the reverse inhibit switch on the hand controller. (NASA)

took up a 45-degree angle. So, when you folded the suspension links under, and then you folded the chassis extensions on both ends, then it would take up this triangular shaped envelope in the LM quadrant with the belly of the vehicle facing out, while at the same time protecting the critical vehicle components such as the drive motors and steering mechanisms."

The forward chassis contained the batteries, drive control and navigation electronics. It also contained mechanical provisions for mounting the Lunar Communications Relay Unit (LCRU), the Ground-Commanded Television Assembly (GCTA) on the right and the S-Band High-Gain Antenna on the left.

The chassis of the LRV underwent load testing in this test fixture at Boeing. A failure during one test resulted in changes to the LRV's chassis construction. (Boeing)

Hinges with torsion springs were used to unfold the forward chassis during deployment. The center chassis contained the Crew Station Subsystem and the Control and Display Subsystem (explained below). The aft chassis was designed to contain crew equipment stowage and lunar sample collection and storage, which included the LRV Aft Pallet Assembly and Lunar Hand Tool Carrier. Hinges with torsion bars were used to unfold the aft chassis during deployment.

The suspension system was made up of upper and lower triangulated control arms, with the widest portion attached to the chassis by upper and lower steel torsion bars and dampers, and the other end attached to the wheel's traction drive suspension attachment fittings. The torsion bars and fluid dampers (as well as the wire mesh wheels) acted as the shock absorbers. The suspension would prove as challenging to engineer as the traction drive was for Romano's team in Santa Barbara.

"We had problems welding up the suspension system,'' Romano said. "We would weld them up and test them in a pull fixture. We had a devil of a time making all the weld joints equally strong. We went to North American Rockwell and used their Tungsten Inert Gas (TIG) welding machine. If we didn't get those welds to pull right to get the strength we needed, the thing would not have been man-qualified and we wouldn't have made it. We were sweating those days, I'll tell you.''

Harmonic Gear Drive Lrv
Each of the four traction drive motors had a rated output of 0.25 horsepower, with a combined output of one horsepower for the LRV. The drives were completely sealed to prevent damage from lunar dust. (NASA/MSFC)

Each traction drive assembly was quite sophisticated and, as history would prove, worked perfectly on the Moon. Each was made up of a %-horsepower 36 VDC series-wound brush-type motor that transmitted power to a harmonic drive with a harmonic wave generator operating through an 80:1 gear reduction flex spline.

"The drive train in the wheel was a very unique system,'' Romano recalled. "The idea came from an Italian engineer, Walter Musser, and he licensed it to United Shoe Machinery in Massachusetts which built them for us. That was a headache, let me tell you. During testing, after several thousand revolutions, the flex spline would fatigue and crack. We worked with United Shoe Machinery and our metallurgists to come up with various methods of annealing the steel spline. The reason we had the flex spline was so the rotating parts could be sealed and would not be subjected to the vacuum of space and the potential intrusion of lunar dust. In a vacuum, if the bearing was lubricated, the lubrication would boil off. Also the DC motor, which had a carbon brush, required moisture in order to survive when it was running on the commutator. So the motors were pressurized with nitrogen, having seven per cent moisture so the brushes wouldn't wear out. [Author note: Additionally, a special fluorinated hydrocarbon lubricant called "Krytox" was used as an internal lubricant in the harmonic drive.] I went back and forth several times to try to get them to toe the line, but they came through and delivered spacecraft-quality units.''

During development of the traction drive, a program was also underway to develop brushless motors for use on the LRV, but in the end Saverio Morea felt more confident in the proven brush-type motor design. Speed control of the motors was the job of Bruce Velasco and was achieved using pulse-width modulation from the movement of the hand controller. Within each traction drive was a magnetic reed switch that activated nine times during each wheel's revolution to determine speed, odometer and navigation readings. The traction drive was fitted with a mechanical brake that was activated via a cable connected to a linkage in the hand controller. Moving the hand controller rearward cut power to the drive and simultaneously activated the mechanical brake. In addition, the traction drive incorporated a freewheeling bearing; in the event of a drive failure, the drive could be decoupled and allowed to free-wheel using this bearing. Tests determined that the rover would be able to operate - that is, get the astronauts back to the LM - with only one traction drive operating and the other three decoupled. To this business end of the traction drive was attached the most visually distinctive component of the Lunar Roving Vehicle - the wire mesh wheel.

A wheel like no other

The design of the Lunar Roving Vehicle's wheels reflected the years of previous development that had taken place at GM's Defense Research Laboratories in Santa Barbara, initially for the JPL/NASA Surveyor Lunar Roving Vehicle Program (SLRV) in 1964 and 1965, and then under the 1966-1967 MSFC Wheel and Drive Program contract. GM also did extensive soil bin testing of many types of metal wheels during these programs, as well as many studies and development tests performed in relation to lunar surface mobility, before settling on the patented wire mesh design. When the RFP for the Lunar Roving Vehicle was issued to contractors for bids, a lot of preliminary work had to be performed with regard to the lunar terrain and lunar soil itself. About a year-and-a-half before the start of the LRV program, a Lunar Surface Engineering Properties/Trafficability Panel was formed at Marshall Space Flight Center in Huntsville. The panel was co-chaired by Dr. Nicholas Costes at MSFC and Mr. Otha H. Vaughn. In 1956, Vaughn had come to Huntsville with von Braun's U.S. Army Ballistic Missile Agency team. By the late 1960s, he was working in the Aero-Astronautics Laboratory at MSFC when he was asked to join the panel to define a lunar surface and mobility model.

"We tried to use as much information as we could find,'' Vaughn stated, "such as radar data, thermal data, photometric function data and surface roughness data, to determine how rough the lunar surface was in terms of mobility. I spent many hours looking at photographs from Surveyor and Lunar Orbiter to come up with what I thought was a good representative lunar scale roughness model. Dr. von Braun would come down and look at a lot of the lunar photographs we had. The Lunar Orbiter photos had one-meter resolution and you could actually see where boulders had come rolling down the hillsides. We went back to look at the work by the U.S. Army Waterways Experimental Station which had done a lot of work on off-road vehicle mobility. That was our starting point. I also worked with the U.S. Geologic Survey Astrogeology Branch in Flagstaff, Arizona to come up with good surface model parameters. From a mobility standpoint, we tried to establish the minimum

The wire mesh used in the LRV's wheels was hand-woven using this special loom. During manufacturing, it was formed in such a way that there was no seam anywhere along its compound curved surface. (NASA/MSFC)

amount of ground clearance we had to have for the rover, how wide it should be to get between and around the craters, and what the bearing pressure of the wheel on the lunar soil should be. Dr. Costes and Dr. W.D. Carrier out at Johnson Space Center came up with a good soil model for the lunar surface. Nicholas and I worked together to try to come up with a composite model of the lunar soil, craters and debris.''

The Lunar Surface Engineering Properties/Trafficability Panel was really just the tip of the iceberg with respect to lunar surface studies and tests. A veritable brains trust was brought to bear on various aspects of the LRV with respect to lunar soil, geology and vehicle mobility. The MSFC Astrionics Laboratory provided test data on the operational characteristics of the LRV traction drive system and on implementing the computer program relating to the LRV's mobility performance and power profile analysis mathematical model. The MSFC Space Sciences Laboratory developed the computer program relating to wheel-lunar soil interaction. Lunar soil simulation studies were conducted at the Geotechnical Research Laboratory of the MSFC Space Sciences Laboratory and at the University of California. The soil mechanics experiments from Surveyor were closely studied, and the soil and rock samples from Apollo 11, 12 and 14 proved invaluable. The U.S. Army Engineer Waterways Experiment Station in Vicksburg, Mississippi performed extensive tests with six versions of the GM-Boeing wire mesh wheel using lunar soil simulants of crushed basalt that were similar to samples collected from Apollo 11 and 12, as specified by Marshall Space Flight Center. They also conducted wheeled mobility tests.

These tests were performed under 1-G conditions, but test results were needed at /6 -G to study not only the behavior of the wheel and suspension geometry, but also wheel interaction with the lunar soil simulant.

In the spring of 1970, an LRV Dust Profile Test Program was begun. These tests were conducted by the Simulation Branch in the Mechanical and Crew Systems Integration Division of the Astronautics Laboratory at MSFC. A test fixture was engineered that would replicate, as much as possible, the conditions that the LRV's wheel and suspension system would encounter on the Moon. The fixture included an electrically-powered, 2.5 m diameter bed with a lunar soil simulant trough that could hold ten inches of soil simulant. A single wheel and suspension assembly with fender was mounted to the fixture's vertical shaft. Over this was a 2.5 m diameter by 1.25 m high hemisphere with an air-tight access door to create a vacuum chamber. Instrumentation, lighting and a film camera were mounted inside and there were three viewing ports on this hemisphere. A vacuum pump evacuated the air from the chamber. There were four primary test objectives: (1) to evaluate the performance of the LRV fender designs and configurations, (2) to develop an understanding of the suspended solids behavior of lunar soil with respect to astronaut-vehicle performance and efficiency, (3) to identify and evaluate the problems generated by wheel-fender interaction, and (4) to develop a better understanding of lunar soil mechanics. The tests also proved that the fenders could not be removed to save weight; they were crucial to dust control.

The tests were conducted in two phases. Phase I would take place on the ground

The finished wheel assembly of the LRV weighed a mere 5.4 kilograms. Despite the often rugged lunar surface conditions during Apollo 15, 16 and 17, the LRV did not experience a single wheel failure. (NASA/MSFC)

to establish the proper function of the test fixture and to learn from preliminary test results. This phase was instrumental to the effective design of the fender; to contain and control the path of lunar soil simulant to prevent it from being disbursed over the rover or the astronauts. These tests revealed the need for fender flaps and extensions, which were employed to prevent soil from being thrown onto the LRV or astronauts. Phase II involved tests to be performed at -G aboard an Air Force KC-135-A aircraft, which the test fixture was designed to fit inside. These flights were conducted by the Astronaut Office and the Flight Crew Support Division of the Manned Spacecraft Center. The crew would fly parabolic flight profiles that would achieve /£ Earth's gravity. During May 1971, sixty-five tests were performed under these conditions. The four key objectives of the test program were achieved to help validate the wheel and fender design, the interaction of the wheel with the lunar soil stimulant, and the effectiveness of the wire mesh wheel and its riveted chevron tread pattern.

The wheel design effectively evolved from NASA's Lunar Wheel and Drive Experimental Test Program during the MOLAB and LSSM programs. These tests were performed under ambient and thermal-vacuum conditions and two designs were initially evaluated. The first was a metal-elastic wheel with a flat metal tread and a complex of interior circular cross-section metal springs. The second design was a wire frame wheel with interior hoops and solid aluminum rim. This design was eventually chosen for the LRV as initially proposed by Boeing.

"The wheel development was instigated by the fact that under the temperature conditions, which in shade goes down to about — 300 degrees Fahrenheit and in the sunlight of day goes up +250 degrees or so, rubber or plastic materials could not be used,'' Pavlics stated. "We had to invent an all-metallic but still flexible wheel. Since this was a manned vehicle going at a reasonable speed over rugged terrain, it had to provide the astronauts with a good ride quality. So, the wheel had to be flexible and have good flotation over the soft lunar terrain. That is how we started developing this all-metallic wheel.

"We tried many different types and different materials, and finally nailed down this configuration which was a flexible wire frame-type of wheel. The behavior of the wheel was like a low-pressure pneumatic tire. It was flexible and it had a good footprint over the soft terrain so it didn't sink into the soil. At the same time, it provided a certain amount of damping because the interwoven wires, as they deformed, had a friction at the joints, so it didn't bounce like a spring would. The other thing we had to be concerned about, because of the low gravity on the Moon, was that the wheel had to be designed to be very soft, to have a deformation under the static load of the vehicle. At the same time, the dynamic forces are, of course, the same on the Moon as everywhere else, so when you run into a rock or an obstacle, the impact force would be the same as it would be on Earth. That is why if you look at the wheel, which is kind of transparent because of the open wire mesh construction, you can see a secondary wheel inside which was made out of titanium spring material - the bump stop hoops. As the flexible part of the wheel deformed when it ran into an obstacle, it would engage this secondary hoop-type wheel and that would absorb the dynamic impact force.

The Control and Display Console of the Qualification Unit was clearly marked "Non-Flight." The inboard hand-holds with light colored grips were vital for properly seating on the Lunar Rover in 1/6 gravity in the astronaut's pressure suits. The left hand-hold also served as a mount for the Low-Gain Antenna, and the right hand-hold served as the mount for the 16mm Data Acquisition Camera (DAC). The Sun Shadow Device is in the stowed position to the right of the Heading Indicator. (NASA)

The Control and Display Console of the Qualification Unit was clearly marked "Non-Flight." The inboard hand-holds with light colored grips were vital for properly seating on the Lunar Rover in 1/6 gravity in the astronaut's pressure suits. The left hand-hold also served as a mount for the Low-Gain Antenna, and the right hand-hold served as the mount for the 16mm Data Acquisition Camera (DAC). The Sun Shadow Device is in the stowed position to the right of the Heading Indicator. (NASA)

"The nominal static load on each wheel was about 67 pounds (147 kg) with the vehicle, astronauts and all the equipment on board,'' Pavlics added. "But when it ran into a rock, at say ten miles per hour (16 kph), the forces would be more than ten times as high, so we had to design the wheel for impact forces like that. We actually tested the wheel up to 1,000 pounds (455 kg) of force, which it was able to take. Because of the secondary wheel inside, it could be deformed all the way to contact with the secondary hoop wheel without any permanent deformation of the primary wire mesh wheel. The wire diameter and material came about from stress analysis which was performed. If we used fewer spring wires, they would have to be thicker to support the same load, but if we had thicker diameter wires, at the same deformation, the stresses would be higher. So we optimized that and came up with a relatively small diameter, high-strength steel spring wire, so that the stress levels would be below the allowable level. This was really defined by fatigue properties. We tested the wheels over the equivalent of 120 kilometers, which was more than twice the expected usage.''

The wheel was manufactured by GM's Defense Research Laboratories using 0.84 mm diameter steel spring wire. The wire was cut to a length of 81.3 cm and then 800 of these wires were hand-woven in a special jig to give a seamless wire mesh that had 64,000 wire intersections. To this were riveted titanium tread strips in a specific chevron pattern, resulting in fifty per cent coverage of the contact patch. This, in turn, was riveted to a spun 2024-T4 aluminum alloy disc and rim. To prevent the collapse of the wheel under impact with lunar rocks, the wheel featured an inner frame, as mentioned by Ferenc Pavlics. This had a circumferential ring and hoop springs made of titanium. The resulting wheel assembly measured 81.3 cm in diameter and was 22.8 cm wide, but weighed only 5.5 kg.

The LRV employed Ackermann geometry steering. The wheels did not steer in parallel, but the innermost wheel in a left- or right-hand turn had a greater steering angle corresponding to its shorter turning radius. The RFP stated that the LRV would employ both front and rear steering as a redundancy in the event of either one failing. If this occurred, the failed steering could be locked in its center position to prevent drifting while the other steering system operated. The two steering systems were electrically powered through separate forward and rear steering motors with speed reducers and servo systems, and were mechanically independent of each other. The original steering rate specification was later changed to a higher rate as a result of feedback from the crews in the trainer. Steering was controlled, as was speed and braking, through the hand controller.

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