27 July 2015

Rocket Belts and Rocket Chairs: Lunar Flying Units

This poor Lunar Flying Unit pilot forgot his PLSS backpack. Image credit: Bell Aerosystems/NASA
Apollo lunar surface exploration was a race against time. The Lunar Module (LM) lander carried only so much cooling water for its avionics, only so much breathing oxygen and carbon dioxide-absorbing lithium hydroxide for its crew, and could coax only so much electricity from its batteries. The Portable Life Support System (PLSS) backpack each Apollo astronaut carried while outside the LM could be recharged, but could contain only so much breathing air and cooling water at one time.

The longest Apollo lunar-surface stay and the longest period astronauts spent in their space suits on the lunar surface occurred during the advanced J-class Apollo 17 mission (7-19 December 1972), the last manned moon voyage. During the second of three traverses they conducted during their three-day, three-hour visit to the Taurus-Littrow landing site, astronauts Eugene Cernan and Harrison Schmitt remained outside their LM, the Challenger, for seven hours and 37 minutes.

Operational constraints and conservative mission rules further limited what Apollo moonwalkers could do with the limited resources at their command; for example, during their travels in the Lunar Roving Vehicle (LRV), the four-wheeled electric car designed to expand the area they could explore and the mass of lunar samples and tools they could transport, Apollo astronauts could not stray beyond a "walkback limit." As the term implies, this was the distance beyond which they could not return on foot to the LM before they expended the life support consumables in the PLSS.

The walkback limit meant that Apollo lunar surface crews drove to their planned greatest distance from the safe haven of the LM at the start of each LRV traverse, then worked their way back to the LM through a series of pre-planned traverse stops. As they drew nearer to their base camp, the quantity of expendables available in their PLSS backpacks would decrease, but then so would the distance they would need to hike if the LRV broke down.

The limited endurance of the Apollo LM and PLSS, combined with the walkback limit, helped to dictate the list of landing sites Apollo astronauts could explore. During the mid-1960s, proposed Apollo landing sites with scientifically interesting surface features spaced too far apart for "early Apollo" exploration were transferred to lists of candidate targets for more advanced follow-on expeditions. These would, it was assumed, be carried out in the mid-to-late 1970s within the Apollo Applications Program (AAP).

On 31 July 1967, four years to the day before Apollo 15 (26 July-7 August 1971), the first J-class mission, touched down on the moon with the first LRV on board, lunar scientists had gathered in Santa Cruz, California, "to arrive at a scientific consensus as to what the future lunar manned and unmanned exploration should be." Soon after their two-week conference, they released recommendations. In their hefty 398-page report, they declared that
The most important recommendation of the conference relates to lunar surface mobility. To increase the scientific return. . .after the first few Apollo landings, the most important need is for increased operating range on the Moon. On the early Apollo missions it is expected that an astronaut will have an operating radius on foot of approximately 500 meters. It is imperative that this radius be increased to more than 10 kilometers as soon as possible.
With this in mind, participants in the Santa Cruz conference recommended "that a Lunar Flying Unit [LFU] be developed immediately to be used in AAP and, if possible, on late Apollo flights to increase the astronaut's mobility range." The workshop participants expected that the LFU would have a range of from five to 10 kilometers, which they stated was "a considerable improvement over the present capability, but not nearly enough."

As space scientists met in Santa Cruz, however, Congress in Washington debated deep cuts in NASA programs. In part as "punishment" for the Apollo 1 fire (27 January 1967), on 16 August 1967 AAP's Fiscal Year (FY) 1968 budget was slashed from the $455 million President Lyndon Baines Johnson had requested in January to just $122 million. The President, faced with an unpopular war in Indochina, unrest in U.S. cities, and an increasing budget deficit, begrudgingly acquiesced to the cuts.

In his preface to the Santa Cruz conference report, NASA Associate Administrator for Space Science Homer Newell explained that its recommendations had been "prepared under guidelines. . .developed prior to the 1968 Appropriations Hearings by the Congress." Because of this, they were "optimistic in outlook" and "exceed[ed] the capability of the agency to execute." Newell stressed more than once that the report was "NOT an approved NASA program for lunar exploration."

The ambitious Santa Cruz blueprint for lunar exploration died as it was born, yet the LFU concept it touted remained alive. In January 1969, NASA's Manned Spacecraft Center (MSC) in Houston, Texas, issued a pair of seven-month LFU study contracts. In June 1969 - a month before Apollo 11 (16-24 July 1969) carried out the first manned moon landing - the two competing contractors, Bell Aerosystems (BA) and North American Rockwell (NAR), presented their final briefings to MSC and NASA Headquarters officials.

A Bell test pilot prepares to demonstrate the "rocket belt" at Hopi Buttes, 1966. Image credit: U.S. Geological Survey
BA had studied a "rocket belt" - in reality, a rocket backpack - under contract to the U.S. Army in the late 1950s. The rocket belt used a catalyst bed to decompose hydrogen peroxide into high-temperature steam which it vented through a pair of exhaust nozzles to generate thrust. In 1966, the company demonstrated the rocket belt for U.S. Geological Survey (USGS) lunar scientists among the rugged Hopi Buttes east of Flagstaff, Arizona. Eugene Shoemaker, founding chief of the USGS Branch of Astrogeology, witnessed the demonstration. The following year he co-chaired the Geology Working Group at Santa Cruz, from which emanated the conference's mobility and LFU recommendations.

The BA LFU (image at top of post) was a platform with splayed legs and small (7.5-inch-wide) footpads, not a backpack, but it applied many of the rocket belt's design principles. The astronaut would fly standing, stabilized as he flew by his grip on a pair of handlebar-type hand grips linked mechanically to twin side-mounted rocket nozzles. The grips were based on the Apollo LM hand-controller design. Though safety belts would restrict side-to-side motion, the astronaut would be able to flex his knees, allowing him to absorb the pressure of acceleration and the jolt of touchdown. The BA LFU's simple metal landing legs included no shock absorbers.

Front view of the BA LFU showing the astronaut upper torso in flight positions. Imag credit: Bell Aerosystems/NASA
BA envisioned that its LFUs would always travel to the moon in pairs. The company proposed that one 235-pound LFU and Apollo astronaut should stand by at the LM, ready to mount a rescue, while the other LFU and astronaut flew to an exploration target from 10 to 15 miles away from the LM.

Until the mid-point of the LFU study, NASA had asked BA and NAR to design their LFUs to carry 370 pounds of payload. This would enable them to rescue a 370-pound space-suited astronaut stranded beyond the walkback limit. At the mid-term briefing, NASA directed the contractors to redesign their LFUs so that they could carry at most 100 pounds of payload. BA noted that, if payload were indeed restricted to 100 pounds in the final LFU design, then the second LFU and astronaut could still serve a life-saving function: they could deliver water and oxygen to refill the grounded LFU pilot's PLSS as he hiked back to the LM.

In keeping with NASA ground rules for the study, BA designed its LFU to burn leftover propellants scavenged from the LM descent stage tanks. Grumman, the LM prime contractor, had estimated that from 300 to 1500 pounds of hypergolic (that is, ignite-on-contact) propellants would remain in the descent stage after the LM alighted on the moon. The astronauts would use three 20-foot-long hoses - one for nitrogen tetroxide oxidizer, one for hydrazine fuel, and one for helium pressurant - to fill tanks in the BA LFUs. The hoses and helium would form part of a 90-pound LFU "support equipment" payload in the LM descent stage.

A BA LFU would carry up to 300 pounds of propellants in its twin tanks, bringing its total mass with a space-suited astronaut and a 100-pound payload to about 1000 pounds. Helium would drive the propellants into the twin throttleable rocket engines, which would each produce from 50 to 300 pounds of thrust. Thrust chamber temperature would peak at about 2200° Fahrenheit. BA assumed that during each LFU sortie time spent in flight would total about 30 minutes. The BA LFU would fly at up to 100 feet per second (about 70 miles per hour).

The company assumed that NASA would fly a total of 10 Apollo lunar landing missions through the end of 1973. It envisioned a staged LFU flight program. An early hydrogen-peroxide-fueled LFU would draw on experience gained from the BA rocket belt, which, the company stated, had flown more than 3000 times on Earth. This would permit short-range test-flights on the moon with minimum development risk beginning in 1971, during the fifth Apollo lunar mission.

During early hypergolic-propellant flights - in BA's plan they would commence in mid-1972 - the LFU pilot would fly relatively short distances and climb no higher than 75 feet above the moon. His flight path would conform to the lunar terrain; BA saw this as a means of avoiding any disorientation exotic lunar flight conditions might cause. Later missions might see high-flying, propellant-saving ballistic trajectories that would extend the LFU's range beyond 15 miles.

BA had other big plans for its LFU. It wrote that, with a special 500-pound propellant package attached, its LFU could climb to lunar orbit. During Apollo missions that lasted longer than the three days planned for J-class missions, its LFU might fly as many as 30 times. It might also be flown by remote control or, with engine uprating, eventually propel astronauts through the skies of Mars.

NAR, the other 1969 LFU study contractor, was a relative newcomer to the world of rocket-powered personnel flyers. In 1964, the company - then known as North American Aviation (NAA) - had proposed a compact, foldable LFU somewhat similar to the BA design; that is, the astronaut would stand upright on a small platform and grip control handles. The 1964 NAA LFU also featured an "auxiliary payload/rescue tray" for transporting equipment or a recumbent astronaut. Add-on spherical auxiliary propellant tanks could be added for increased range.

The 1964 North American Aviation LFU stressed compactness over range. Image credit: North American Aviation/NASA
Perhaps because NAR was starting with a relatively blank slate, its 1969 LFU was very different from either its 1964 design or that of its 1969 competitor. NAR rejected an LFU which had the astronaut stand, having found such a configuration to be unstable in flight and likely to tip during landings. It proposed instead a design which had the astronaut sit at the LFU's center of gravity, much like the recumbent astronaut in its 1964 design, in a seat tipped forward slightly to enhance visibility. He would fly strapped in with his feet on a foot rest that would hinge out of the way to allow easy access to the seat. To attenuate landing shocks, the NAR LFU would rely on shock absorbers in its landing legs.

The NAR LFU would use a cross-shaped cluster of four throttleable rocket engines, each with a maximum thrust of 105 pounds, centered directly under the astronaut. This would, the company argued, offer enhanced in-flight stability and redundancy in the event that a single engine failed. The BA design was not flyable if one engine failed; if the NAR LFU lost an engine, the pilot would shut off its opposite number to maintain stability and fly directly back to the LM using the two remaining engines.

Engine redundancy, a seat, and shock absorbers contributed to the NAR LFU's greater mass. The company estimated that it would total 304 pounds without propellants and about 1075 pounds loaded with a space-suited astronaut, a 100-pound payload, and 300 pounds of propellants scavenged from the LM.

NAR's choice of engine position added to its LFU's operational complexity. The low-mounted engines would tend to blast debris from the lunar surface in all directions during LFU landing and takeoff. Dust and rocks thrown out from the LFU might damage the LM, the astronaut's suit and PLSS, and the LFU itself. Because of this, the NAR LFU would take off and land no nearer than 40 feet from the LM. As added assurance against damage, it would take off from and land on a fabric launch pad/landing target laid out on the lunar surface.

Unpacking the NAR LFU from the side of the Apollo Lunar Module. At right is a discarded "thermal cover" for protecting the LFU during flight to the moon. Image credit: North American Rockwell/NASA

An astronaut drags the NAR LFU to its fabric launch pad/landing target. Note hoses for pumping residual LM propellants into the LFU's tanks after it is positioned on the pad/target. Image credit: North American Rockwell/NASA
Boarding the NAR LFU would necessarily have been more difficult than boarding the BA LFU. Though NAR's illustrations show preparing its LFU for flight to be a one-man job, it probably would have needed both astronauts. Image credit: North American Rockwell/NASA
Following deployment from a compartment in the LM's side, the astronauts would drag the NAR LFU to the center of the fabric target, then use 40-foot hoses to fill its twin modified 20-inch-diameter Gemini spacecraft propellant tanks with scavenged LM propellants. NAR estimated that, on average, it could rely on the LM to contain 805 pounds of left-over propellants; that is, enough to fill its LFU's tanks nearly three times. Helium from an Apollo reaction control system tank roughly the size of a basketball mounted atop one of the two Gemini propellant tanks would push the hypergolic propellants into the four engines.

After loading the LFU's two payload racks with equipment, an astronaut would back into the LFU seat, position the foot rest and swing-arm-mounted control panel, and fasten his seat belt and shoulder straps. After a pair of half-mile-long, 200-foot-high test hops that would familiarize the astronaut with LFU flight characteristics under lunar conditions, he would fly the LFU at an altitude of up to 2000 feet to a science target up to 4.6 nautical miles from the LM.

That distance was, of course, much less than the 10-to-15-mile operational radius BA promised for its LFU; this was, however, just as well, since NAR expected to fly only one LFU per Apollo mission. Because of this, its pilot would not be immune to the walkback limit. The company calculated that adding 100 pounds of propellants would increase to 7.8 nautical miles the distance its LFU could fly; it also noted that the LFU could reach science sites high up on the slopes of mountains otherwise inaccessible to Apollo explorers.

The NAR LFU in flight. Note position of control panel (center right) and replaceable helium pressurant tank (upper left). Image credit: North American Rockwell/NASA
NAR LFU swing-arm-mounted control panel. Image credit: North American Rockwell/NASA
During sorties away from the LM, the LFU would land on unprepared lunar ground. This raised the specter of possible damage from engine-tossed debris. To avoid this, NAR proposed turning off the engines some unspecified distance above the surface. This would, the company explained, also decrease the likelihood of tipping; the LFU would land firmly on its four shock-absorbing legs, not slide or skip during touchdown.  It acknowledged, however, that accurately judging height above the surface before switching off the engines might be problematic.

After completing work at his science target, the astronaut would unfold a fabric launch pad and drag the LFU onto it before igniting its engines for return to the LM. Between flights, the crew would refill the LFU's propellant tanks, but not the empty helium pressurant tank; they would instead replace it with a spare stored in the LM descent stage.

Though the NAR LFU would reappear briefly in a 1971 NAR lunar base study, the 1969 studies were the LFU concept's last hurrah. In May 1969, as the BA and NAR study teams completed their final reports, NASA Headquarters announced that the Marshall Space Flight Center (MSFC) in Huntsville, Alabama, would direct industry development of the Apollo LRV. MSFC issued a Request for Proposal in July 1969, about a month after NAR and BA engineers briefed MSC and NASA Headquarters officials on their LFU designs. On 28 October 1969, NASA formally opted for wheels over rocket belts by selecting Boeing as the prime contractor for the LRV.

In the images below, a pair of astronauts release the tightly folded LRV from a compartment built into the side of the LM. They pull lanyards in sequence to unfold and lower it; then, after the LRV sits on four wheels on the dusty moon, they unfold by hand seats and other appendages, such as antennas. Fully deployed, the LRV measured 10 feet long and 7.5 feet wide. Although its mass was just 463 pounds, it could carry a payload (including two space-suited astronauts) of about 1080 pounds on the moon.

During Apollo 15, astronauts David Scott and James Irwin drove their LRV a straight-line distance of five kilometers from their LM, the Falcon. Apollo 16 (16-27 April 1972) saw astronauts John Young and Charles Duke drive 4.5 kilometers from the LM Orion. For Apollo 17, the walkback limit rule was relaxed slightly, so Cernan and Schmitt were able to reach a point 7.6 kilometers from Challenger.

The three LRVs now rest on the moon where their Apollo astronaut drivers parked them. In a program chock-full of remarkable machines, the LRVs stand out from the rest. Had they not extended the exploration range of the Apollo 15, 16, and 17 crews, we would know far less about the moon than we do today.

Had the LFU flown, however, it seems likely that astronauts could have ranged widely over landing sites more complex and extensive than any Apollo explored. After an Earth-moon voyage of a quarter-million miles, the LFU could have added a crucial few miles to surface traverses and enabled astronauts to soar up mountainsides and rugged crater rims. What then might we have discovered?

Unpacking the Apollo Lunar Roving Vehicle, steps 1 through 4. Deploying the rover is a two-man task. Image credit: NASA
Unpacking the Apollo Lunar Roving Vehicle, steps 5 through 8. In the bottom right image, astronauts unfold seats and the rover's small control console. Image credit: NASA

"Lunar Surface Exploration Gear Analyzed," Aviation Week & Space Technology, 16 November 1964, pp. 69-71

One Man-Lunar Flying Vehicle Study Contract: Summary Briefing, Space Division, North American Rockwell, July 1969

Study of One Man Lunar Flying Vehicle: Summary Report, Report No. 7335-950012, Bell Aerosystems Company, July 1969

1967 Summer Study of Lunar Science and Exploration, NASA SP-157, NASA Headquarters Office of Technology Utilization, 1967

More Information

"Assuming that Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

Dreaming a Different Apollo: Part One

Earth-Approaching Asteroids as Targets for Exploration (1978)


  1. This post has outstanding illustrations! David, I hope you'll use them in your upcoming book... And while I understand your comments against nostalgia and its intellectual laziness, it is hard to not feel it while reading this... It is so dreamful to imagine astronauts leaving Earth on top of Saturn 5 only to go flying above the moon in this tiny rocket vehicle... And to consider all other Apollo rocket-powered stages and modules as nothing more than intermediate stages to this one...

  2. These are really nifty vehicles. NASA spent a lot of time looking into lunar flyers, but ultimately they were judged to be too heavy and too unsafe. In the future I expect that this concept will be revived and brought to fruition - if not on the moon, then certainly on Mars.



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