The Landing Gear: Aluminum Honeycomb, Contact Probes, and a One-Way Trip
How Grumman designed legs that could absorb the impact of landing on the Moon, fold into an adapter ring, and never be used again
The Lunar Module’s landing gear had to do three things: absorb the impact of landing at up to 7 feet per second on terrain that might be sloped, rocky, or soft; keep the descent stage level enough for the ascent stage to launch cleanly; and weigh as little as possible while doing both. Grumman Aircraft Engineering Corporation solved this with four legs made of aluminum tubing, crushable aluminum honeycomb cartridges, and a deployment mechanism that used explosive bolts and springs. The gear weighed approximately 450 pounds total—about 2.8% of the LM’s total weight at touchdown. It was used exactly once per mission and left on the Moon forever.
Four Legs and a Geometry Problem
The landing gear consisted of four leg assemblies arranged 90 degrees apart around the descent stage. Each assembly had three main components: a primary strut connecting the footpad to the outrigger fitting on the descent stage, and two secondary struts that braced the primary strut laterally to prevent the leg from folding sideways under load.
The geometry was driven by two competing requirements. The footpads needed to be far enough apart to provide stability on sloped terrain—the farther apart the feet, the steeper the slope the LM could land on without tipping over. But the legs needed to fold compactly enough to fit within the Spacecraft-LM Adapter, the conical fairing that enclosed the LM during launch atop the Saturn V third stage. The adapter was 154 inches in diameter at its base, and the LM with deployed gear spanned about 31 feet.
The compromise was a gear that deployed outward and downward from a folded position against the descent stage. In the stowed configuration, each leg assembly was held flat against the descent stage by explosive-bolt restraints. During deployment—which occurred after the LM separated from the S-IVB stage and was extracted from the adapter by the Command/Service Module—the restraints released and springs pushed the legs outward. Dampers slowed the extension, and over-center locks snapped into place when the legs reached their fully deployed position, preventing them from retracting.
Deployment was one-way and irreversible. The locks had no release mechanism. Once deployed, the legs stayed deployed. If a deployment failed—if a lock didn’t engage, if a strut hung up—there was no way to cycle it. The crew verified deployment by checking talkback indicators on the instrument panel. Four gray indicators meant four locked legs.
Crushable Honeycomb: The Shock Absorber
The primary energy absorption mechanism was elegantly simple: aluminum honeycomb cartridges that crushed progressively under impact loads. The cartridges were cylinders of aluminum honeycomb—a lightweight structure of thin aluminum foil formed into a hexagonal cell pattern—pressed into steel housings within the primary and secondary struts.
When the LM touched down, the vertical and lateral impact loads were transmitted through the footpads into the struts. The struts telescoped as the honeycomb crushed, absorbing kinetic energy by permanently deforming the aluminum cells. The crush force was nearly constant regardless of how far the strut compressed—a property of honeycomb that made it an ideal energy absorber. The deceleration was smooth and predictable rather than the sharp spike that a rigid landing would produce.
The primary struts contained the main vertical-load honeycomb cartridges, sized to absorb the energy of the LM descending at up to 7 feet per second at touchdown (the maximum landing velocity specification). The secondary struts contained smaller cartridges that absorbed lateral loads—energy from horizontal velocity at touchdown or from the LM sliding on a slope.
The total stroke available in the primary struts was approximately 32 inches. A nominal landing at 3 feet per second might compress the struts only 4 to 6 inches. A hard landing at the maximum 7 feet per second used more stroke but stayed within the design margin. The struts were sized so that even the worst-case landing scenario—maximum velocity, maximum slope, one leg hitting a rock—left enough stroke margin to prevent the descent stage structure from contacting the ground.
Post-mission photography of the landed LMs showed varying amounts of strut compression. Apollo 11, which landed relatively softly, showed minimal compression. Apollo 15, which landed on sloping terrain at Hadley Rille with a significant horizontal velocity component, showed asymmetric compression—the uphill legs were shorter than the downhill legs, and the LM sat visibly tilted. But all four legs held, the ascent stage launched successfully from the tilted platform, and the gear performed as designed.
The Footpads: Spreading the Load
Each leg terminated in a circular footpad approximately 37 inches in diameter, made of aluminum. The footpads served two functions: distributing the landing load over a large enough area to prevent the leg from sinking into the lunar regolith, and providing a stable base on uneven terrain.
The lunar surface bearing strength was one of the great unknowns before Apollo 11. The Surveyor robotic landers had measured surface properties at several sites, indicating that the regolith could support the LM’s weight. But the variety of lunar terrain—from flat maria to cratered highlands—meant the footpad had to work on surfaces ranging from hard-packed regolith to loose, powdery soil.
The 37-inch diameter was a compromise between load-spreading ability and the stowed geometry constraint. A larger pad would have been better for soft surfaces but harder to fold against the descent stage. The resulting ground pressure under each pad—about 1.5 psi at the LM’s landing weight in lunar gravity—was well within the measured bearing capacity of the regolith at the Apollo landing sites.
The footpads had a dish shape—slightly concave—that helped keep them from sliding on slopes. Each pad also had a thin lip around its periphery that dug into the surface on contact, adding friction. The pads themselves showed very little sinking on any Apollo mission; the worst case was a few inches of penetration at the softer sites, well within the strut stroke margin.
Contact Probes: 67 Inches of Fiberglass
Extending below three of the four footpads were the lunar surface sensing probes—lightweight fiberglass rods, approximately 67 inches (5 feet 7 inches) long, that hung vertically below the pads. When any probe touched the lunar surface, it bent, completing an electrical circuit that illuminated the CONTACT light on the instrument panel.
Only three of the four legs carried probes. The fourth leg—the one nearest the forward hatch and the ladder—was deliberately left without a probe to eliminate the risk of a probe bending under the footpad and puncturing an astronaut’s suit as they descended the ladder. The absence was a conscious trade: slightly less landing detection capability in exchange for crew safety during the surface EVA.
The probes were retained against the primary strut during flight and deployed along with the landing gear. They were simple, passive devices—no electronics, no mechanisms, just a flexible rod with a switch at its base. When the probe tip touched the surface and the rod bent beyond a threshold angle, the switch closed and the CONTACT signal was sent to the instrument panel.
The contact light was the cue for engine shutdown. The crew procedure was immediate: at the CONTACT call, the commander pressed the ENGINE STOP button, cutting propellant flow to the descent engine. The LM then fell the remaining distance—roughly 5 feet—under lunar gravity alone, touching down at approximately 5 feet per second vertical velocity. This free-fall was necessary because the descent engine nozzle extended below the footpads, and firing the engine at close proximity to the surface could cause damage from exhaust pressure reflecting off the ground.
On Apollo 11, Aldrin called “Contact light” and Armstrong shut down the engine essentially simultaneously. On later missions, some commanders kept the engine running briefly past contact to ensure the LM was committed to the surface before shutdown—a technique born of training experience and test pilot instinct, though the official procedure remained “engine stop at contact.”
The Ladder and the Forward Hatch
The front leg—the +Z leg, in LM coordinate convention—supported the egress ladder, a lightweight aluminum structure attached to the primary strut that allowed the crew to climb from the forward hatch down to the footpad and then to the surface. The ladder had nine rungs and descended from the porch (a small platform outside the forward hatch) to the primary strut’s footpad.
The distance from the bottom ladder rung to the lunar surface depended on how much the front strut compressed at landing. On Apollo 11, the compression was minimal and the last step from the ladder to the footpad—and then from the footpad to the surface—was larger than expected. Armstrong’s famous “one small step” was actually a drop of roughly three feet from the bottom rung to the surface, a fact visible in the television footage.
NASA added a small table—the Modular Equipment Stowage Assembly, or MESA—to the front leg on later missions. The MESA was a fold-down work surface and storage compartment that deployed from the descent stage when a lanyard was pulled. It held the lunar surface camera, sample collection tools, and other equipment the crew needed at the start of the EVA. The MESA deployment was one of the first tasks performed after egress, and the television camera that filmed the first steps was mounted on the MESA.
Left Behind
The landing gear was part of the descent stage, which served as the launch platform for the ascent stage. When the ascent engine fired, the explosive bolts and guillotine cutters that separated the two stages left the descent stage—with its legs, footpads, engine, propellant tanks, and MESA—sitting on the lunar surface. The gear had served its purpose and would sit undisturbed on the Moon essentially forever, in a vacuum with no weather, no corrosion, no biological degradation.
Photographs taken from the Lunar Reconnaissance Orbiter, launched in 2009, show the descent stages of all six landed Apollo LMs still standing at their landing sites, their shadows stretching across the regolith, the footpads still bearing the weight of the empty descent stages. The landing gear that Grumman built in the 1960s is still deployed, still locked, still doing its job—holding the remnants of the vehicles that carried humans to the Moon.