Gene Cernan’s Troubled EVA, 1966

Star Bound №20: The Future Was Then

Bruce McCandless III
13 min readDec 21, 2022


Chapter One: The Spacewalk From Hell

This is a story about big dreams and tiny circuits. It’s about moonshots, a mild form of mania, and a machine called the Manned Maneuvering Unit. It seems to wind up at a dead end, but that’s not true. There’s a little alley leading off to one side. Take it. You’ll find a metallic fragment of modern magic flickering still, waiting to be unleashed. To fly free. With no tethers.

No doubt the desire to soar is older than recorded history. It comes to us first as myth. Imprisoned on the island of Crete, the inventor Daedalus fashions wings from feathers and wax. He makes another pair for his son, Icarus. This is the way they’ll escape their island confinement, Daedalus explains. But there’s a catch: Fly too close to the sun, Daedalus warns the young man, and the wax in the wings will melt. And so it does. Icarus becomes so intoxicated by flight — the rush of the wind, the star sparks of sun on the water below — that he ventures too high. The wings disintegrate, and he plunges to his death in the Ionian Sea.

Other broken bodies followed. According to legend, the Chinese official Wan Hu, inspired by the principles behind fireworks, tried to go airborne by creating the first rocket-powered chair. It was to be propelled by dozens of gunpower-powered projectiles, attached to the back of the chair and all lit simultaneously. Wan Hu either flew far away or blew himself up, as no trace of him was found after the rocket chair exploded. In the 11th Century, the English monk Eilmer of Malmesbury tried a more conventional means of flight. He made himself a pair of wings and jumped from an abbey tower, gliding for a spell before he intersected with the earth and broke both of his legs. Dozens of others put their lives on the line in the quest for the clouds, both before and after the Wright Brothers managed to achieve mechanized flight in 1903. Balloonists, parachutists, hang gliders, kite surfers and wingsuit daredevils are the spiritual descendants of Icarus, Wan Hu, and Eilmer of Malmesbury. It’s one thing to take to the air in a metal canister. It’s quite another to feel the sky on your skin. It’s fine to ride on a boat, but some of us want to swim.

Only sixty years after Orville Wright soared over the sand at Kitty Hawk — six decades of rapid advancements in flight, rocketry, and, eventually, space travel — human beings on opposite sides of the globe were racing to see who would be first to the moon, 240,000 miles away. There were significant technological detours along the way. As the name suggests, the “Apollo Applications Program” was established by NASA in 1967 to develop ways to use or adapt Apollo hardware for an ambitious range of space-exploration projects — a permanent space station, a mission to Mars — following America’s planned moon landings. In his book Carrying the Fire, Apollo 11 command module pilot Michael Collins describes one of these projects: evaluating competing designs for a method by which an astronaut could leave the safety of his vessel and move about in space. Like Daedalus.

Only in a pressure suit.

Cosmonaut Alexei Leonov was the first spacewalker. He left the comparative safety of his Voshkod spacecraft in March of 1965 and floated beside it for twelve minutes. He had trouble getting back in, as his pressure suit had puffed up so much in the vacuum of space that he couldn’t fit into the spaceship’s inflatable airlock. The Russian had to depressurize the suit to dangerous levels before he could re-enter, at which point, he said, sweat was “sloshing” around inside it with him. The Soviets didn’t mention the difficulties involved. They played up Leonov’s EVA as simple and easy. The spacewalk was a propaganda win, one of many for the Soviets in those days, and NASA felt compelled to respond in kind. Three months later, American astronaut Ed White ventured outside his Gemini capsule with the means not only to float, but to fly.

The means was a gas-firing pistol nowadays referred to as the “hand-held maneuvering unit,” or HHMU. The United States Air Force started experimenting with pistol-like propulsion devices in the in the late fifties. By 1965, NASA had selected a model to test in space. At the time, it was known as the ZIPU, for Zero Gravity Integrated Propulsion Unit. Among the astronauts, it was simply called a “zip gun.” It consisted of two Gemini ejection-seat oxygen bottles pressurized to 4000 psi, a pressure regulator from a Mercury spacecraft environmental control unit, and a control handle with three thrusters attached, two pointing aft and one forward, for expulsion of the gas. As a contemporary newspaper article described it, “the ZIPU is essentially a piece of pipe about two feet long with both ends turned backward at right angles. The ends of the pipe form nozzles that point backward when a man holds the device near his belt. A third nozzle, sticking out at the center of the device, pushes forward.” With the ZIPU, the astronaut could fly one way or the other depending on which nozzles were activated.

In a theme that repeats itself all through the history of maneuvering units, terms of reference for the ZIPU were fluid and imprecise. In one newspaper article from 1965, Buzz Aldrin offers his thoughts on the device, which is described as a “jetpack” — though no part of the ZIPU, including its fuel bottles, was worn on the back. Aldrin, one of the American space program’s most gifted and ambitious stars, explained why some sort of propulsion device was needed for work in space. The lifeline used by cosmonaut Alexei Leonov on his historic first spacewalk, said Aldrin, “could be hazardous,” because it “could tangle and prove to be a noose. The problem with being attached is that if you are going 200 feet from the ship and you give a jerk in the line, you’re going to move toward it at a good rate of speed and you could smash into the capsule.” He compared movement in space to “underwater SCUBA diving” — an apt observation, as SCUBA diving was also a young technology for working and exploring in a hostile environment, and training underwater would eventually become routine for astronauts practicing for EVAs.

Ed White with ZIPU, 1965.

Ed White tested the ZIPU while tethered on his Gemini 4 spacewalk. It seemed to work as intended, though as the gun only had enough propellant for less than a minute of “flight,” it wasn’t much of a trial. Michael Collins tested a nitrogen-fueled version of the device himself, with mixed results, on Gemini 10. He moved (or “translated,” in astronaut-speak) from his Gemini spacecraft to an Agena drone vehicle and back again to retrieve a micrometeoroid collection package. The HHMU was small, light, and easy to grasp, both physically and conceptually. The main problem with the device was aligning the unit’s propulsive force with the astronaut’s center of mass. Even a slight miscalculation in this regard could send the astronaut spinning off target. The bottom line, after a very limited set of tests: The HHMU worked, but it didn’t work well.

Another means of maneuvering proposed at the time was a set of “rocket boots,” featuring propulsive systems built at first into the astronaut’s footwear and then later incorporated in a sort of sled that was controlled by the user’s feet. They were inspired by the Hiller Flying Platform, a device that attracted attention in the Defense Department in the 1950s for its potential to transport an individual infantryman over the countryside. As astronaut Bruce McCandless II recalled:

Back then, it was a lawnmower, or a souped-up lawnmower engine, directly driving a ducted fan that was aimed down with a little bitty platform for an infantryman to stand on top of. And when he’d throttle up high enough, the thrust was sufficient to lift him clear of the ground, and by leaning left or right, or fore or aft, he could cause the thing to be propelled. The jet shoes concept, that figured that you’d put a thruster underneath each crewman’s foot, and that would allow him to maneuver in space in direct analogy to this ducted fan arrangement. However, the problem that the investigators did not appreciate initially was that, in the infantry example, you had the gravity vector to work against, so you could throttle down on the engine and settle back to the earth or use it to equilibrate things. In the zero-gravity environment, once you put in a thrust, you continue inertially until you [do] something to take it out.

The Hiller Flying Platform.

Like a lot of far-fetched contraptions conceived in the fifties, the Flying Platform didn’t pan out. It was emblematic of an era of technological paranoia and enormous possibilities, a time when scientists and engineers struggled to understand nuclear power, intercontinental ballistics, and the path into outer space. After the destruction wrought by Hitler’s V-2 missile and the atomic bombs dropped by the U.S. on Japan during the Second World War, secret weapons were serious business. Everyone knew the Soviets were racing to develop their own. Hollywood goggled over the risks of radiation-enhanced insects eating Baltimore. Military men studied anti-gravity propulsion, jetpacks, flying jeeps (“fleeps”), and revolving, bicycle wheel-like space stations. The Flying Platform looked promising in early trials. But it proved to be more trouble than it was worth for the sake of moving single soldiers, and it never really worked to get an infantryman high enough above the terrain — and the helpful “ground effect” of low-altitude flight — to make him an effective scout. Plans to make a bigger platform also required making the platform heavier, which effectively ruined its ability to fly at all. Nevertheless, there were those who insisted the concept would work in space, where mass became more manageable. McCandless tested what he called the “jet shoes” in the summer of 1969. In a memo to astronaut office chief Deke Slayton, he concluded that they were impractical and unworthy of further investment. While Slayton concurred, jet shoes nevertheless remained a contender for several more years, at least in part because they were a pet project of engineers at NASA’s Langley Space Center.

In McCandless’s opinion, the more promising option was the Astronaut Maneuvering Unit, or AMU. Basically a gas-powered jetpack, the device was seen as exotic even by the astronauts who intended to test it. Mike Collins called it “a little spacecraft in itself” that was “really far out.” Collins spent 22 hours along in the Apollo 11 command module, circling several times over the far side of the moon as his crewmates walked on the surface. He knew from far-out. The AMU was originally developed by the United States Air Force’s Aero-Propulsion Laboratory, which in November of 1963 proposed development of “an individual back-pack experiment which would permit the astronaut to maneuver independently around the Gemini vehicle.” Like the jet shoes, the AMU had antecedents in Defense Department experiments. Then, as now, engineers labored to invent a jetpack that would work in 1 G — i.e., the gravity of Earth. The most famous of these devices was the Bell Aerosystems Flying Belt, which was dramatically demonstrated for President John F. Kennedy in 1961, and later used by James Bond to escape a gang of international ne-er-do-wells in the 1965 movie Thunderball. In the mid-sixties, the “Bell Aerospace Rocket Man” toured the country, appearing at fairs and other gatherings. Described as a “Buck Rogers type who floats rounad using using a jet pack on his back. The jet pack enables him to leap 50 to 60 feet in the air and approximately 200 feet in distance.”

The Bell Aerospace Rocket Belt.

Powered by hydrogen peroxide exhaust, and often piloted by engineer Harold Graham, the Bell jetpack did work — but only for around 21 seconds, with a fairly large fuel requirement and a crew required to service and launch the device. Given these operational constraints, the Army’s interest in the machine dwindled, even if popular fascination continued. In the eighties, a company produced a lightweight version of the flying belt called the RB-2000 that could fly for slightly longer durations. An English inventor, Richard Browning, has successfully demonstrated what he calls the “Daedalus Flight Pack,” which is powered by four turbine engines, two of which as attached to the flyer’s arms. Perhaps spurred by the British breakthrough, the U.S. Department of Advanced Research Projects Agency recently announced the award of grants to several small companies for development of proposals for a new generation of American jetpacks.

It was perhaps no wonder that the Air Force would consider the development of a jetpack device for possible use in space, just as the Army tested the Bell Aerospace Jet Belt as a possible infantry aid. Because there is so much less weight and air resistance to overcome, flying in low-Earth orbit is in some ways easier than flying just above the planet’s surface. The Jet Propulsion Laboratory in California accepted technical responsibility for the AMU’s rocket propulsion system. The device had twelve thruster nozzles that used nitrogen gas to “positively expel” hydrogen peroxide via a bladder in the hydrogen peroxide tank. According to one author, “Nitrogen gas forced the monopropellant into the individual nozzles, where it passed over a silver catalyst, producing high temperature (1300 degrees F) steam.” In fact, the searing heat generated by the H2O2 necessitated development of a pair of metallic trousers to protect the astronaut’s lower extremities from the exhaust. The trousers were a technological wonder themselves, an eleven-layer garment fashioned in part of a chromium-nickel alloy called Chromel-R. Nevertheless, they were sometimes referred to as the “iron pants,” as if they’d been constructed as an instrument of torture by the Spanish Inquisition, or, more frequently, and more accurately, as the “stainless steel pants.”

Three AMUs were produced by the Ling-Temco-Vought (LTV) Aerospace and Defense Company. The device consisted of a pressure suit, a chest-pack life-support system, and a backpack maneuvering unit. Together, the components weighed 168 pounds. Each of the AMU’s thrusters, which were fired in pairs, produced 2.3 pounds of nominal thrust and were divided into a primary and an alternate system. As journalist Keith Wilson described it in “Flying Free,” “the unit was a rectangular aluminum backpack carrying 12 thrusters mounted on the corners of the framework. Hydrogen peroxide was used as propellant and thruster firings were controlled from two sidearm supports. It carried self-contained life-support, communications, telemetry, propulsion and manual and automatic stabilization systems.”

On June 5, 1966, Gene Cernan exited the capsule of Gemini 9, 150 miles above the earth, to perform the first in-flight test of the AMU. His initial task was simply to get to the device, which was mounted in the aft end of the Gemini IX Adapter Section. The attempt did not go well. Cernan, a lean, athletic man, was hindered by the stiffness of his pressurized, heavily insulated space suit, including the stainless-steel pants, and by the fact that the exterior of the Gemini spacecraft had only two handholds. With little to anchor himself to, he continually found himself demonstrating Newton’s Third Law of Motion — namely, that every action creates an equal and opposite reaction. Cernan overexerted himself to the point where condensation created on the inside of his helmet visor by his own breath and body heat rendered him essentially blind. His description of his extravehicular activity (his “EVA,” or spacewalk) sounds like something out of an H. P. Lovecraft tale — “The Elders of Langley,” perhaps, or “In the Moonglow of Madness.” In the darkness of space, Cernan’s umbilical tether became a “snake,” a “worm,” and an “octopus” that left him “slipping in puddles of space oil, with no control over the direction, position, or movement of my body” as “the naked sun, an intense ball of gleaming white fire, stared at me, a tiny interloper in its realm.”

As a result of Cernan’s exhaustion and overheating, the AMU test was called off not long after he managed to get himself situated in the unit. The exercise ended badly. NASA flight controllers worried that Cernan might not even make it back inside the capsule. It turned out that the astronaut lost thirteen pounds of sweat on his brief sojourn outside the spacecraft.

Astronauts Collins and Dick Gordon had similar difficulties with spacewalking on Gemini flights 10 and 11, respectively, though they used different maneuvering methods. Like Cernan, both men were tethered to their Gemini capsule. Collins used a nitrogen-powered HHMU that provided some assistance and was able to complete his assignment, though not without some unplanned and unwelcome acrobatics and worries about fouling of his tether. Because he ran out of nitrogen propellant, though, he was unable to complete a series of test maneuvers using the gun. Gordon’s assignment was to attach a tether to the Agena space vessel his Gemini capsule had docked with, for purposes of an attitude stabilization experiment. He found working in space to be so tiring that at one point he straddled the nose of the Agena space vehicle like Slim Pickens riding the bomb in the final scene of Dr. Strangelove. He did manage to attach the tether. But because Gordon was sweating so much that he couldn’t see straight, Commander Pete Conrad ordered him to re-enter the Gemini capsule. Gordon was never able to test the handheld maneuvering unit, and his spacewalk ended after only 33 minutes of a planned 107. In light of the difficulties working in space experienced by Cernan, Collins, and Gordon, a planned second attempt to test the AMU jetpack on Gemini 12 was canceled — a decision that disappointed Buzz Aldrin, the astronaut who was scheduled to perform the test. There was a bright side to the failures. NASA realized the need to add rails and handholds to its orbital vehicles to aid spacewalking astronauts. The agency also started using neutral buoyancy training in large water tanks as a way to simulate EVAs. But the damage was done. NASA had developed a marked dislike not only for the AMU, but also for spacewalks in general. “Shortened Walk,” blared a headline in The Los Angeles Times, referring to Cernan’s difficulties, “Casts Doubt on Untethered Man in Space.”

Thus, as the Gemini era ended, no reliable means for astronaut maneuvering in space had been found. But two men were still looking. And they had no intention of giving up.

NEXT — Chapter Two: Buck Rogers Battles the Budget Men



Bruce McCandless III

I'm an Austin-based writer trying to figure out space, science, and Texas politics. For more, see: