Apollo lunar surface exploration was a race against time. The Lunar Module (LM) carried only so much cooling water for its avionics, only so much breathing oxygen and carbon dioxide-absorbing lithium hydroxide for its crew, and only so much electricity in its batteries. The Portable Life Support System (PLSS) backpack each Apollo astronaut carried on his back while outside the LM could be recharged, but could carry only so much breathing air and cooling water at one time.

The longest Apollo lunar surface stay and longest period astronauts spent in their space suits on the lunar surface occurred during the J-class Apollo 17 mission (7-19 December 1972), the last manned moon voyage. During the second of three surface traverses astronauts Eugene Cernan and Harrison Schmitt conducted during their three-day, three-hour sojourn on the lunar surface, the two men 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 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 the 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 expendables in the PLSS.

The walkback limit meant that Apollo lunar surface crews drove to their planned greatest distance from the 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 PLSSs would decrease, but then so would the distance they would need to hike if the LRV broke down.

During Apollo 15 (26 July-7 August 1971), the first J-class mission, astronauts David Scott and James Irwin drove a straight-line distance of five kilometers from their LM, the Falcon. Apollo 16 (16 April-21 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 limited endurance of the Apollo LM and PLSS, combined with the walkback limit, helped to dictate the list of landing sites the Apollo astronauts would 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 expected, 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 touched down on the moon, 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 398-page report they wrote 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 acknowledged 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 a form of "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 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, the recommendations 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 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 Company and North American Rockwell (NAR), presented their final briefings to MSC and NASA Headquarters officials.

Test pilot strapped to Bell Aerospace "rocket belt" backpack among Arizona's Hopi Buttes in 1966. Image: United States Geological Survey Astrogeology Science Center

Bell had studied a "rocket belt" - in reality, a rocket-propelled 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 then vented through a pair of exhaust nozzles to generate thrust. In 1966, Bell demonstrated the rocket belt for U.S. Geological Survey (USGS) lunar scientists among the volcanic Hopi Buttes east of Flagstaff, Arizona. Eugene Shoemaker, chief of the USGS Branch of Astrogeology, witnessed the demonstration. The following year he co-chaired the Santa Cruz conference's Geology Working Group, from which emanated the conference's mobility and LFU recommendations.

The Bell 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 and had, by 1969, been Bell's favored configuration for several years. The astronaut would fly standing, stabilized as he flew by his grip on a pair of handlebar-type control handles linked mechanically to twin side-mounted rocket nozzles. The handles would use the Apollo LM hand controller design. Though safety belts would help to prevent 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 Bell LFU's landing legs would include no shock absorbers.

Bell envisioned that its LFUs would always reach 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 Bell and NAR to assume that the LFU could carry 370 pounds of payload, and thus could rescue a 370-pound space-suited astronaut stranded by LFU failure.

At the mid-term briefing, NASA directed Bell to design its LFU so that it could carry 100 pounds of payload, and Bell complied. The company noted that, if the LFU payload capacity were indeed fixed at 100 pounds, then the second LFU and astronaut could still serve a life-saving, walkback limit-extending function; they could resupply the grounded LFU pilot's PLSS with oxygen and water as he walked back to base.

Drawing of Bell LFU showing astronaut, handlebar, and motor positions. Image: Bell Aerosystems Company/NASA

In keeping with NASA ground rules for the study, Bell designed its LFU to burn leftover propellants scavenged from the LM descent stage. 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 landed on the moon. The astronauts would use three twenty-foot-long hoses - one for nitrogen tetroxide oxidizer, one for hydrazine fuel, and one for helium pressurant - to fill tanks in the LFUs. The hoses and helium would form part of an LFU "support equipment" payload in the LM descent stage with a total mass of 90 pounds.

The Bell 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 (F). Bell assumed that during each LFU sortie actual flight time would total about 30 minutes, with the LFU flying at speeds up to 100 feet per second (about 70 miles per hour).

Bell assumed that NASA would fly a total of 10 Apollo lunar landing missions through the end nof 1973. It envisioned a staged LFU flight program. An early hydrogen peroxide-fueled LFU would draw on experience gained from the Bell 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 Bell'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; Bell 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.

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

Comparison with medium-sized fonts. North American's 1964 LFU outwardly resembled Bell's upright-astronaut designs. Image: North American Aviation

NAR, the other 1969 LFU study contractor, was a relative newcomer to the world of rocket-powered personnel flyers. In 1964, the company had proposed a compact, foldable LFU basically similar to favored Bell designs; that is, the astronaut would stand upright on a small platform and grip control handles. The 1964 NAR LFU also featured an "auxiliary payload/rescue tray" for transporting equipment or a recumbent stranded or injured astronaut and add-on spherical auxiliary propellant tanks for increased range.

Perhaps because NAR was starting with a relatively blank slate, its 1969 LFU was very different from either its 1964 design or its 1969 Bell counterpart. NAR rejected an LFU in which the astronaut stood, having found that configuration to be unstable in flight and likely to tip over during landings. It proposed instead a design in which the astronaut sat on the LFU at its center of gravity, much like the recumbent astronaut in the 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. The NAR LFU would rely on shock absorbers in its landing legs to attenuate landing shocks, not the astronaut's knees.

An Apollo astronaut deploys the NAR LFU from a compartment in the side of the Lunar Module descent stage. A protective thermal cover for the LFU is visible on the ground at right. Image: North American Rockwell/NASA

The 1969 NAR LFU design had 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 case of single-engine failure. The Bell design would become unflyable if one engine failed; if the NAR LFU lost an engine, the pilot would shut off its opposite number to maintain stability and fly back to the LM using the two remaining engines. Engine redundancy, a seat, and shock absorbers contributed to the NAR LFU's greater mass - 304 pounds without propellants and about 1075 pounds with 300 pounds of propellants scavenged from the LM, a space-suited astronaut, and a 100-pound payload.

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 an added assurance that it would cause no damage, it would take off from and land on a fabric target unrolled on the lunar surface.

Following deployment from a compartment in the LM's side, the astronauts would drag the NAR LFU to the center of the 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, mission planners 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 would push the hypergolic propellants from the Gemini tanks into the four engines.

YouTube has begun asking anonymous users to merge in their real names and photos. Photo: YouTube An Apollo astronaut climbs aboard the NAR LFU. The replaceable helium pressurant tank is visible on top of the propellant tank to the left of the astronaut's seat. The LFU rests in the middle of a fabric liftoff/landing pad designed for visibility and to limit the amount of debris kicked up by the LFU's four rocket motors. Image: North American Rockwell/NASA

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, 200-foot-high test hops during which the astronauts would each become familiar with the LFU's flight characteristics under lunar conditions, one astronaut 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.

This distance was, of course, dramatically less than the 10-to-15-mile operational radius claimed for the Bell LFU; this was, however, just as well, since NAR envisioned only one LFU per Apollo mission. Its pilot would thus not be immune from the walkback limit. The company calculated that adding 100 pounds of propellant would increase to 7.8 nautical miles the distance its LFU could fly. NAR also noted that the LFU could make science sites high up on the slopes of mountains accessible to Apollo explorers.

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.

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; instead, they would replace it with a new one stored in the LM descent stage.

NAR LFU in flight. Image: North American Rockwell/NASA

Though the NAR LFU would reappear in a 1971 lunar base study, the 1969 studies were the LFU concept's last hurrah. In May 1969, as the Bell 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 a two-seater Apollo LRV. MSFC put out a Request for Proposals in July 1969, about a month after NAR and Bell 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.

References:

"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.