Evolution vs. Revolution: The 1970s Battle for NASA's Future

Sunlight glints off NASA Marshall Space Flight Center's proposed Power Module in this artist concept by Junior Miranda.

According to historians Andrew Dunar and Stephen Waring, writing in their 1999 NASA-funded history Power to Explore: A History of Marshall Space Flight Center, in the 1970s two lines of thought emerged within NASA concerning manned spaceflight's course after the Space Shuttle became operational. On the one hand, there was the "revolutionary" line taken by Johnson Space Center (JSC) in Houston, Texas. On the other was the "evolutionary" line of NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama.

At JSC, many managers assumed that, as soon as the Shuttle became operational, NASA would get a green light to assemble a large, new-design, multipurpose Space Station in low-Earth orbit (LEO). They envisioned that a 1980s President would make a speech much like President John F. Kennedy's 25 May 1961 “moon speech.” Visionary goal thus proclaimed, the funding floodgates would open.

At MSFC, by contrast, many managers expected that NASA budgets would remain tight for the foreseeable future, so that any space technology development that took place would need to be incremental; that is, it would have to begin with existing space hardware and occur in small steps. MSFC's work on the Skylab Orbital Workshop, a temporary LEO space station launched in May 1973 on the last Saturn V rocket to fly, probably helped to shape their outlook.

The 169,950-pound Skylab "cluster," which comprised the Multiple Docking Adapter, the Apollo Telescope Mount (ATM), and the Orbital Workshop, had been conceived originally as an element of the Apollo Applications Program (AAP). As its name implies, AAP had been meant to apply hardware developed for the Apollo lunar program to new tasks. The Skylab Orbital Workshop was a converted Saturn S-IVB stage outfitted with experiment apparatus, crew quarters, and supplies for visiting three-man crews. Three crews were launched to Skylab in 1973-1974; the last orbited the Earth for 84 days.

The Skylab Orbital Workshop floats serenely over the Earth, but this image bears evidence of its nearly disastrous launch and the heroic efforts that saved it. Skylab's reflective meteoroid shield deployed during ascent and peeled away, tangling one of its wing-like solar arrays in debris and loosening the other. A stage separation rocket motor then blasted away the loose array and the tangled array refused to open. Skylab was starved for electricity while temperatures inside it soared, threatening to spoil food, film, and medicines. The first Skylab crew (Charles Conrad, Paul Weitz, and Joseph Kerwin) deployed a sun shield and forced open the jammed array. Skylab went on to host astronauts for a total of 171 days. Image credit: NASA.

NASA built most of a second Skylab, but was unable to secure funding to complete it, launch it into orbit, and launch crews to it. The first Skylab was a success, so MSFC might have expected on that basis to have "earned" funding for the second. The Huntsville Center had, however, learned during the 1960s not to equate success with rewards. It had been responsible for the Saturn V moon rocket, the largest and most powerful launcher ever built. Even as MSFC succeeded in making the mighty Saturn V work, however, it began to suffer funding and staff cuts that by the time Skylab flew would make it a shadow of its former self.

When MSFC engineers looked at the Space Transportation System (STS), as NASA called the Space Shuttle and its stable of expendable upper stages and European-built Spacelab components, they saw not the promise of a big new space station, but rather a system which, once operational, could benefit from evolutionary development. In particular, they noted that Spacelab, which MSFC was assigned to integrate with the Shuttle, could not reach its potential as an orbiting laboratory while the Shuttle Orbiter's planned maximum time in space was only seven days. The Orbiter and its payloads would rely for electricity on the former's fuel cells, which meant that the quantity of fuel-cell reactants the Orbiter could carry would determine their endurance.

The Space Shuttle Orbiter with its Payload Bay doors open to space. A drum-shaped, European-built Spacelab module is shown as a cutaway.  Curved panels raised above the front half of the doors are radiators. The Spacelab module is located near the rear of the Payload Bay to ensure that the Orbiter's center of gravity is placed properly for maneuvers and landing. Image credit: NASA.

In early 1977, with the first STS flight test officially planned for March 1979, MSFC proposed "the first step beyond the baseline STS" — a Power Module (PM) capable of supplying 25 kilowatts of electricity continuously. The PM was partly inspired by joint Department of Energy/NASA Solar Power Satellite studies of the 1970s.

The solar-powered PM was meant to be deployed into LEO from a Shuttle Orbiter payload bay and left in space for up to five years. A succession of Orbiters bearing Spacelab modules and pallets in their payload bays would dock with the PM and use its electricity to remain in orbit for up to 30 days at a stretch.

Alternately, a Shuttle Orbiter could attach a "freeflyer" payload to the orbiting PM and leave it to operate on its own. This appealed to materials scientists, who worried that astronauts' movements on board the Shuttle Orbiter and Spacelab would rattle and ruin their microgravity experiments. Orbiters would periodically dock with the materials science freeflyer/PM combination to retrieve experiment products — for example, large flawless crystals — and replenish raw materials.

In addition to electricity, the PM "building block" would provide thermal and attitude control. The latter would permit a docked Orbiter to conserve its Reaction Control System propellants. Freeflyer payloads meant to be docked with the PM could be built without thermal and attitude control systems, reducing their cost.

Image credit: NASA.

MSFC engineers planned at first to base the PM on the Skylab ATM design. They quickly found, however, that modifying the ATM to meet stringent Orbiter payload bay safety requirements would cost more than a new design. They retained the ATM's octagonal cross-section, however, because they found that it made efficient use of the Orbiter's cylindrical payload bay volume while providing flat surfaces upon which to mount subsystems.

Although it nixed the ATM-based design, MSFC still aimed to lower the PM's cost by using subsystems developed for Skylab, Spacelab, Shuttle, and other programs. These included three Skylab Control Moment Gyros for attitude control and four curved Shuttle Payload Bay door radiators for thermal control. MSFC planned to update and improve Skylab systems used in the PM based on Skylab flight experience. All major PM subsystems would be redesigned for easy replacement by spacewalking astronauts.

The 31,000-pound PM would measure 55 feet long from the framework holding its aft- and side-facing international docking ports to the forward ends of its stowed twin solar arrays. This would leave room in the Shuttle Orbiter's 15-by-60-foot Payload Bay only for a docking tunnel with an international docking port. The tunnel would be bolted to the forward wall of the bay over the hatch linking the bay to the Shuttle Crew Compartment.

This NASA artwork shows a Space Shuttle Orbiter bearing a Spacelab module in its Payload Bay docked with a separately launched Power Module which extends forward over the Orbiter Crew Compartment.

Upon arrival in LEO, the astronauts would open the Shuttle Orbiter's Payload Bay doors and release the five pins that secured the PM in the bay. They would then use the Orbiter's robot arm to lift the PM from the bay and berth its side-facing docking port on the Orbiter docking port. This would position the module so that it extended out over the Crew Compartment.

The astronauts would next extend the PM's twin solar arrays. Fully extended, each wing-like array would measure 131 feet long by 30 feet wide. They would together span a little more than 276 feet. MSFC sized the arrays to generate a total of 59 kilowatts of electricity; that is, 34 kilowatts more than the PM would supply to Spacelab-carrying Orbiters and freeflyers. A portion of this excess would power PM systems, but the majority would charge batteries in the PM so that it could supply a constant 25 kilowatts throughout its roughly 90-minute orbital day-night cycle.

Close-up of Power Module showing international docking ports and curved radiator panels. Image credit: Junior Miranda.

MSFC acknowledged that the big solar arrays would degrade over time; its engineers estimated that over five years they would lose 5% of their generating capacity. Similarly, the PM's batteries would gradually lose their ability to charge and discharge. After five years, a Shuttle Orbiter might be sent up to recover the PM and return it to Earth for refurbishment. Another Orbiter would then launch it back to LEO to continue its duties.

MSFC engineers presented the PM concept to scientists at an MSFC-sponsored solar-terrestrial physics workshop in October 1977. They found broad support for the new capabilities the PM would give to the baseline STS.

Lots of living space: Skylab, Power Module, Spacelab-based add-on supply module, Shuttle Orbiter, and Payload Bay-mounted Spacelab module. Image credit: Junior Miranda.

This view emphasizes the solar arrays on the Power Module and Skylab. The 276-foot span of the Power Module arrays dwarfs the Shuttle and Skylab. The Skylab "wing" array lost during launch in May 1973 is conspicuous by its absence; also notable are two Apollo Telescope Mount "windmill" solar arrays stowed to make way for the Power Module and Orbiter. Image credit: Junior Miranda.

They also proposed that the PM become part of NASA plans to reuse Skylab. MSFC contractor McDonnell Douglas had "interrogated" the abandoned Orbital Workshop's data handling system and found that, nearly four years after its last crew had returned to Earth, reactivation remained feasible. The first step toward Skylab reuse would be for a Space Shuttle to rendezvous with it late in 1979 and boost it to a longer-lived orbit.

The PM would be a late addition to the revitalized Skylab cluster; MSFC did not expect that the new STS element would reach LEO for the first time until 1983, by which time several Shuttle Orbiters would already have visited Skylab. Once added to Skylab, however, the PM would enable Skylab to support as many as six astronauts without a Shuttle Orbiter present. They would perform experiments with large-scale space construction and early space industrialization.

MSFC engineers hoped that the PM might also contribute toward NASA's quest for Skylab's successor. They envisioned that PMs attached to Shuttle Orbiters, free-flyers, and Skylab might lead to PMs attached to Spacelab-derived habitat and laboratory modules during the 1980s: in other words, a new NASA Space Station.

In 1978, the Huntsville center contracted with Lockheed Missiles and Space Company to study PM evolution. MSFC expected that PM development might lead to simultaneous operation of several small specialized "space platforms," each with at least one PM attached. The platforms would not need to be staffed continuously. MSFC argued that several small platforms would best serve scientific and engineering disciplines with conflicting needs, and might cost less than a single large station besides.

In early 1979, NASA Headquarters authorized MSFC to spend $90 million on PM hardware development. The Huntsville center created a PM Project Office in March 1979. At about the same time, however, the space agency abandoned plans to reuse Skylab because the Space Shuttle would not be ready in time to prevent its uncontrolled reentry. Skylab reentered Earth's atmosphere over Australia on 11 July 1979.

JSC, meanwhile, pitched a new-design Space Operations Center (SOC). The space station would include hangars for reusable auxiliary spacecraft and satellite repair, robot arms, habitat and laboratory modules, and truss-mounted solar arrays spanning more than 400 feet. It was conceived primarily as a "space shipyard," a role inspired partly by JSC's 1970s enthusiasm for Solar Power Satellites.

Artist concept of the module cluster of the Space Operations Center (SOC). Most modules are a little less than 60 feet long by 15 feet wide (the length and width of the Space Shuttle Payload Bay). At lower left is a "false Payload Bay" for satellite servicing and spacecraft assembly. Had the SOC been built, this would have included robot arms. A Service Module partly covered with gold thermal blankets is located at upper right and a hexagonal hangar is located below it. The artist has included a Spacelab-derived module near a Shuttle docking port at left. Image credit: NASA.

STS-1, the maiden flight of Columbia, the first Space Shuttle Orbiter, took place in April 1981. James Beggs, President Ronald Reagan's choice for NASA Administrator, was confirmed two months later. Beggs soon sought presidential approval for a Space Station. This move seemed to favor JSC's revolutionary vision. At the same time, however, Beggs informed MSFC that he wanted to buy the new station "by the yard" – that is, as money became available. This approach seemed more in line with MSFC thinking.

In November 1981, NASA Headquarters halted PM, SOC, and other station-related work at MSFC and JSC. According to Dunar and Waring, it did this to take charge of station development and to end MSFC-JSC rivalry. Following Reagan's January 1984 State of the Union Address, in which he called upon NASA to build a Space Station by 1994, JSC's revolutionary vision seemed to win out. JSC was designated "lead center" for Space Station in early February 1984.

Although Reagan authorized NASA to spend only the $8 billion Beggs had told him the Space Station would cost and had specifically called for a space laboratory in his State of the Union Address, the agency's first baseline station design, the "Dual Keel," was an elaborate combination of lab, Earth/space observatory, and shipyard measuring more than 500 feet wide. Like the SOC, it included a small fleet of freeflyers and auxiliary vehicles. It also included a pair of solar-dynamic power systems — a NASA Lewis Research Center innovation — for generating large amounts of electricity.

The Dual-Keel Space Station design unveiled shortly after the January 1986 Challenger accident was dead on arrival, though NASA sought to ensure a future for the design until 1990. Image credit: NASA.

The Dual Keel's complex multipurpose design immediately came in for criticism. Materials scientists, for example, complained that space construction, the comings and goings of auxiliary spacecraft, the whirling turbines of solar-dynamic power systems, the presence of a large crew, and atmospheric drag on such a large structure were bound to spoil the station's microgravity research environment. Congress, meanwhile, accused NASA of low-balling its cost estimate to gain the project's approval.

Congressional cost containment, combined with the 28 January 1986 Challenger accident, concern over the number of assembly and maintenance spacewalks the station would need, and a rapidly expanding U.S.-Russian space partnership (one which would have been unthinkable when Reagan delivered his January 1984 speech), led to a decade-long series of station redesigns. The Space Station shrank and lost many of its proposed capabilities. This untidy evolution yielded the International Space Station (ISS), a U.S.-Russian hybrid with Japanese and European labs and Canadian robotics.

Early days of the International Space Station: from upper left to lower right are visible a Progress freighter, the Service Module with docking node, the FGB, and U.S. Node 1. Image credit: NASA.

Ironically, the first ISS element launched into space amounted to a Power Module. The Russian-built, Russian-launched, U.S.-funded FGB provided the second ISS element to reach space, U.S. Node 1, with electricity and attitude control from December 1998 to July 2000, when they were joined by a mini-space station – the Russian-built, Russian-launched Service Module, which had originally been intended as the "base block" of the Soviet Union's Mir-2 station. At that point, ISS became capable of supporting long-duration crews.

Sources

Guntersville Workshop on Solar-Terrestrial Studies, NASA Conference Publication 2037, "summary papers from a University of Alabama in Huntsville/NASA Workshop conducted 13-17 October 1977, at Lake Guntersville State Park Convention Center, Guntersville, Alabama," NASA George C. Marshall Space Flight Center, 1978.

"The 25 kW Power Module – First step beyond the baseline STS," G, Mordan; paper presented at the American Institute of Aeronautics and Astronautics Conference on Large Space Platforms: Future Needs and Capabilities, held in Los Angeles, California, September 1978.

25 kW Power Module Updated Baseline System, NASA TM-78212, NASA George C. Marshall Space Flight Center, Huntsville, Alabama, December 1978.

Power to Explore: a History of Marshall Space Flight Center, 1960-1990, NASA-SP-4313, Andrew J. Dunar and Stephen P. Waring, NASA History Office, 1999.

More Information

What Shuttle Should Have Been: NASA October 1977 Space Shuttle Flight Manifest

McDonnell Douglas Phase B Space Station (1970)

Where to Launch and Land the Space Shuttle? (1971-1972)

What If an Apollo Lunar Module Ran Low on Fuel and Aborted its Moon Landing? (1966)

"The Eagle has wings!" The Apollo 11 Lunar Module Eagle shortly after separating from Apollo 11 Command and Service Module Columbia in lunar orbit, 20 July 1969. Image credit: NASA.

At 3:08 p.m. U.S. Eastern Daylight Time (EDT) on 20 July 1969, out of contact with Earth over the Farside hemisphere of the Moon, the computer that guided the Apollo 11 Lunar Module (LM) Eagle opened valves in its descent propulsion system, causing nitrogen tetroxide oxidizer and aerozine 50 fuel to come together in its Descent Stage rocket engine. The propellants were hypergolic, meaning that they ignited on contact with each other.

The descent engine fired for a little more than 12 minutes. At the beginning of the burn, Eagle, Apollo 11 Commander Neil Armstrong, and Lunar Module Pilot Edwin Aldrin were in a 54-by-66-nautical-mile lunar orbit. At the end of the burn, the 16.5-ton, 23-foot-tall lunar lander and its occupants were in an elliptical orbit with an apoapsis (low point) 50,000 feet above the Moon's Earth-facing Nearside hemisphere.

Apollo 11's target landing site was known officially as Site 2. Selected because it was flat and equatorial, Site 2 was a 10-mile-long east-west-trending ellipse on the Moon's Sea of Tranquility centered at 0° 42' 50" north latitude, 23° 42' 28" east longitude. Eagle descended to 50,000 feet about 260 nautical miles and 12 minutes of flight time east of Site 2, at which time its computer ignited its descent engine again to begin braking and final descent.

As the LM dropped below 7000 feet, its computer fired attitude control thrusters to tip it slowly upright so that it pointed its descent engine and footpads at the Moon. This maneuver also aimed Eagle's twin triangular windows forward so Armstrong and Aldrin could see Site 2 up close for the first time.

The astronauts immediately realized that they had a problem. They should have been above the eastern edge of the Site 2 ellipse, about five miles from their target landing point at the center of the ellipse. In fact, they had already flown past the center of their target ellipse and were descending toward its northwestern edge.

Apollo 11's flight plan called for Armstrong to let the computer do the flying until Eagle was about 500 feet above the Moon and 2000 feet east of the target touchdown point. He would then take manual control and lower Eagle almost vertically to the surface. He quickly realized, however, that Eagle's computer was steering it toward a boulder-strewn impact crater the size of an American football field. This was later identified as West Crater.

His heart rate leaping from 77 to 156 beats per minute, Armstrong assumed manual control early. Gripping his hand controller, he leveled Eagle's descent, then scooted the LM almost horizontally across the black lunar sky at an altitude of several hundred feet.

While Aldrin read off descent and translation rates, the LM computer flashed erroneous alarms and Capcom Charles Duke in Houston warned that Eagle was running low on propellants. Armstrong flew past West Crater and an adjacent smaller crater, then lowered to a safe touchdown just inside the Site 2 ellipse. At 4:18 p.m. EDT, he radioed his immortal words to hundreds of millions of people: "Houston, Tranquility Base here — the Eagle has landed."

The Apollo 11 Lunar Module Eagle on the Moon at Tranquility Base. Note lunar dirt stirred up by astronaut activities on the surface. Image credit: NASA.

Armstrong and Aldrin landed at 0° 41' 15" north, 23° 26' east, roughly four miles west and about three-quarters of a mile north of their planned touchdown point. Mission Control estimated that Eagle's Descent Stage tanks contained only enough propellants for about 25 seconds of flight when the descent engine was shut off at Tranquility Base. After the flight, more detailed analysis yielded an estimate of 45 seconds, demonstrating that the system for measuring available propellants in real time left much to be desired.

Mission rules called for an abort if propellants for fewer than 20 seconds of flight remained in the descent stage propellant tanks. What if, as Armstrong anxiously sought a safe place to land, flight controllers on Earth had mistakenly estimated an even slimmer propellant margin? They might then have done as mission rules dictated and called on Armstrong to abort the Apollo 11 lunar landing.

In June 1966, Charles Teixeira, with the Engineering and Development Directorate at the Manned Spacecraft Center in Houston, completed an Apollo Program Working Paper on the hazards of a landing abort during the 45-second period spanning from 65 to 20 seconds before planned touchdown. He assumed that the LM would be no more than 338 feet above the Moon 65 seconds before planned touchdown and about 100 feet high 20 seconds before planned touchdown.

As soon as an abort was initiated, the LM's Descent Stage engine would shut down. Nearly simultaneously, four explosive bolts linking the descent stage with the Ascent Stage would fire. A fifth pyrotechnic device would drive a guillotine that would cut the wiring umbilical linking the two stages. The Ascent Stage engine would then ignite to propel the astronauts toward lunar orbit. The abandoned Descent Stage, meanwhile, would fall to the lunar surface.

From abort initiation to Ascent Stage ignition, the abort procedure — which, apart from occurring at altitude, would duplicate the normal LM Ascent Stage launch procedure — would last from two to four seconds. During that time, the Ascent Stage would follow the same path as the Descent Stage; that is, it would fall toward the Moon.

Teixeira assumed that, following an abort during the 45-second period from 65 seconds to 20 seconds before planned touchdown, the four-legged Descent Stage would strike the Moon with enough force to rupture its propellant tanks. An abort within 20 seconds of planned touchdown — when the Descent Stage was at or below 100 feet — would leave the tanks intact.

If the tanks ruptured, either of two things might occur. The nitrogen tetroxide and aerozine 50 they spilled might boil and evaporate rapidly in the lunar vacuum. Evaporation would cool, then freeze, the propellants, so they would remain safely separated. Alternately, the propellants would come together. This might occur, Teixeira wrote, if after impact enough of the Descent Stage structure remained intact around the ruptured tanks to contain the propellants as they boiled.

Propellant mixing would cause an explosion that would drive gases and fragments of the Descent Stage outward at several thousand feet per second. Teixeira estimated that gases and debris would envelope the LM Ascent Stage less than one-tenth of a second after the explosion.

The extent of the damage this might cause would depend mainly on how long the abort procedure lasted; that is, on how quickly the ascent engine could ignite. The faster the ascent engine ignited, the farther away the astronauts would be when the Descent Stage impacted and exploded.

For a two-second abort procedure, gas pressure from the explosion would damage the Ascent Stage if the abort began between 32.6 and 20 seconds before planned touchdown. If the two-second abort began between 44 and 20 seconds before planned touchdown, then the Ascent Stage stood a greater than 20% chance of being struck by a Descent Stage fragment.

For a four-second abort procedure, gas pressure from the explosion would damage the Ascent Stage if the abort began between 53.7 and 20 seconds before planned touchdown. The Ascent Stage stood a greater than 20% chance of being struck by a Descent Stage fragment if the four-second abort began between 65 and 20 seconds before planned touchdown; that is, throughout the period Teixeira considered.

After the Landing: The Ascent Stage of the Apollo 11 Lunar Module Eagle as viewed from the Apollo 11 Command and Service Module Columbia during rendezvous in lunar orbit. Image credit: NASA.

Teixeira called the "critical time spans" during which damage would be likely to occur "rather short." He acknowledged that the risk of a Descent Stage explosion during a near-surface abort might not be great enough to justify "elaborate remedial action" — for example, a major redesign of the LM Descent Stage.

He recommended, however, that a Descent Stage propellant dump "at as high a rate as safely possible" become a part of the standard LM landing abort procedure. After due consideration, NASA elected not to follow his advice. Had Armstrong and Aldrin been forced to abort the Apollo 11 landing while above 100 feet of altitude, Teixeira's recommendation might have come back to haunt the U.S. civilian space agency.

Sources

Hazards Associated with a LEM Abort Near the Lunar Surface, NASA Program Apollo Working Paper No. 1203, NASA Manned Spacecraft Center, 24 June 1966.

Apollo 11 Mission Report, NASA SP-238, Mission Evaluation Team, NASA Manned Spacecraft Center, 1971.

Chariots for Apollo: A History of Manned Lunar Spacecraft, NASA SP-4205, The NASA History Series, C. Brooks, J. Grimwood, and L. Swenson, NASA, 1979, pp. 343-344.

More Information

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

Jimmy Carter's Space Shuttle

Space Shuttle Mission-62: Discovery awaits the imminent arrival of its crew, August 1994. Image credit: NASA.

In January 1978, President James Carter announced a surprise decision: NASA's Space Shuttle, then under development but plagued by delays, cost overruns, and technical snags, would be redesigned to launch and land without a crew on board. A spacecraft based on the tried-and-true Apollo Command and Service Module would launch astronauts to the Orbiter in space. They would enter the Orbiter through a docking unit in the Orbiter Payload Bay and use it as a mini-space station for scientific experiments and satellite servicing. Mission completed, the astronauts would return to Earth in the expendable Apollo, then the Orbiter would return to Earth for refurbishment and reuse. Carter justified his decision by pointing to the Shuttle's lack of credible crew escape systems and abort modes.

Carter's 1978 decision piqued the ire of spaceflight purists in a way that even the Sortie Lab decision of 1972 had not, for it turned the rationale for the Shuttle completely upside-down. The Shuttle had been conceived originally as crew rotation and resupply vehicle for a Saturn V-launched core Space Station. After President Richard Nixon refused to fund the core Station and scrapped the Saturn V, NASA studied a Shuttle-launched Station until it became clear that no Station would receive Nixon's blessing.

Deprived of its true purpose, the Shuttle Orbiter became a piloted spacecraft meant to replace all existing expendable space launch vehicles. It would, NASA promised, dramatically reduce the cost of spaceflight, ushering in a new age of space development. It would also reduce the cost of satellites by servicing them in orbit, serve as a short-term space laboratory by carrying in its Payload Bay a can-shaped Sortie Lab module, and make space readily accessible to non-astronauts.

The 1978 decision to turn the Shuttle into a robot spacecraft ceased to be controversial on the morning of 28 January 1986, when the Orbiter Challenger was destroyed a little more than a minute into Space Shuttle Mission (SSM) 25. Had astronauts been on board, they would have been unaware of the Solid Rocket Booster malfunction that was the root cause of Challenger's destruction. Had they somehow learned of the malfunction, they would have been unable to intervene and would have been trapped at least until the Shuttle stack's twin Solid Rocket Boosters had spent their propellant and detached. That would have been too long, for Challenger was destroyed as its Solid Rocket Boosters still burned.

As it was, Challenger's five-person crew for SSM-25 watched as the automated spacecraft they had been meant to board in orbit for a two-week stay was torn apart by aerodynamic forces and tumbled in fragments into the Atlantic. The Solid Rocket Boosters emerged still firing from the fireball created when the Shuttle's large, fragile External Tank broke up, spilling its liquid hydrogen and liquid oxygen propellants. The Solid Rocket Boosters each painted a twisting smoke-trail across the blue Florida sky until a Range Safety Officer sent the radio command that destroyed them.

As Challenger disintegrated, the Astronaut Transport Spacecraft (ATS) meant to launch the SSM-25 crew into orbit the following day stood atop a Saturn II expendable rocket on nearby Pad 39B. The ATS was an Apollo Command and Service Module spacecraft redesigned to carry five astronauts. The Saturn II rocket comprised the top two stages of the Saturn V - that is, the 33-foot-diameter S-II and 22-foot-diameter S-IVB. It included six uprated J-2 engines - five in its first stage and one in its second - and six small solid-rocket boosters evenly spaced around its base. Without an ATS on top, the Saturn II could launch a 20-ton payload.

After Challenger, some called for an end to unmanned Orbiter flights. They pointed out that the ATS/Saturn II combination included a sizable cargo volume in the tapered shroud that linked the base of the ATS with the top of the Saturn II S-IVB. They referred to early 1970s NASA and contractor studies that showed that increasing the number of solid-rocket boosters to 10 would permit the Saturn II to launch both the ATS and up to 20 tons of cargo.

President Carter, since his election in November 1984 the first President since Grover Cleveland to serve non-consecutive terms, surprised many by declaring his continued support for the Shuttle. This should perhaps not have come as a surprise, given that it had been Carter who made the 1978 decision to launch and land the Orbiter without a crew. The "come-back President" rightfully pointed to the Challenger accident as the vindication of his 1978 decision, and called for continued unmanned Orbiter flights on the grounds that upgrading the Saturn II would not replace all Shuttle capabilities. It is widely assumed that he also sought to continue the unmanned Orbiter flights to preserve the thousands of jobs the Shuttle Program had created.

In August 1986, Carter signed off on NASA's post-Challenger plan to redesign the SRBs and begin construction of two new Orbiters. This would increase the total number of Orbiters in the Shuttle fleet to four, enabling more downtime for inspections and upgrades between flights. To pay for the new Orbiters, Carter reduced the number of annual Orbiter flights to three from the six planned before Challenger was destroyed. As each new Orbiter came online, one additional flight per year would be added, so the four-orbiter fleet would eventually fly five missions per year.

In the meantime, the Hubble Space Telescope reached orbit in May 1986 atop a Saturn II without an ATS. Repairing its flawed optics became a goal for one of the first post-Challenger Shuttle missions. A Saturn II/Centaur launched the third Radio Relay and Tracking Satellite to geostationary orbit in July 1986, enabling for the first time continuous contact between orbiting spacecraft and flight controllers and researchers on the ground.

A Department of Defense-sponsored ATS solo mission designated SSM-X5 launched in December 1986 with a three-person crew to test polar-orbiting missions. (SSM-X1 through X4 had been Orbiter and ATS test missions in the 1980-1981 period.) Shortly after its return to Earth, new NASA Administrator Sally Ride announced that the Defense Department had opted to forego future Orbiter/ATS flights in favor of ATS solo flights.

The Shuttle Orbiter Enterprise soared into space in September 1987 to start the SSM-26 "Return-To-Flight" mission. Its five-person crew arrived in the SSM-26 ATS two days later. The astronauts spent three weeks on board Enterprise.

Columbia reached orbit in November 1987 to begin SSM-27; after its crew docked their ATS and boarded, they piloted the Orbiter to a rendezvous with the Hubble Space Telescope. Through a series of ambitious spacewalks, the astronauts corrected its faulty optics. They returned to Earth after 10 days in orbit. Columbia landed two days later.

Enterprise reached orbit the next time in May 1988 for SSM-29, but returned to Earth early after the Saturn II rocket bearing the SSM-29 ATS malfunctioned shortly after clearing Pad 39A's lightning mast. The ATS's Launch Escape System activated and pulled its Command Module free of the disintegrating Saturn II rocket. The five astronauts on board were uninjured. They would reach Enterprise to carry out SSM-29R in May 1989. The ATS/Saturn II combination had a flight record going back to the first Apollo Saturn V flight in November 1967, so troubleshooting the J-2 engine malfunction that destroyed the SSM-29 Saturn II and returning the system to flight needed only a few months.

The new Shuttle Orbiter Discovery flew an uncrewed orbital test mission (SSM-X6) in December 1989. In October 1991, the new Orbiter Endurance performed a nearly identical test mission (SSM-X7).

Endurance was the first Orbiter upgraded to permit a 12-week orbital stay and docking with two ATSs at one time. It carried out its first long-duration mission (SSM-60) and received two ATSs between mid-April 1994 and mid-July 1994.

Shortly after Columbia's retirement to the National Air and Space Museum in mid-1995, the new long-duration Orbiter Adventure joined the fleet. It would be the last Orbiter constructed and the last retired; its final mission was SSM-90 in February 2003.

By then, the U.S.-Russian-Chinese-European-Japanese-Brazilian International Space Station had become operational, and NASA and Europe had begun flight tests of the jointly developed Hermes shuttle, which became operational in June 2009. NASA retired the ATS in July 2011, ending 43 years of Apollo and Apollo-derived spacecraft missions.

A note on the Presidents: In this alternate timeline Ronald Reagan defeats James Carter in November 1980, but falls to an assassin's bullet (as he very nearly did) in April 1981. His Vice President, George H. W. Bush, finishes Reagan's term, but Carter narrowly defeats him in November 1984 after Bush's Vice President, Alexander Haig, announces a third-party candidacy that draws votes away from the Republican incumbent. Carter declines the nomination in 1988, in part because of Constitutional questions, and Republican James Thompson of Illinois defeats Carter's second Vice President, New Jersey Democrat Bill Bradley, to win the White House. Thompson's two terms (1989-1997) see the collapse of the Soviet Union and the start of an International Space Base with a crew of 25 people.

Sources

The Unmanned Shuttle Decision: Prudence and the Presidency, John Logsdon, NASA, January 1999, pp. 36-49, 53, 111.

SSM-25 Press Kit, NASA, December 1986.

SSM-27 Press Kit, NASA, November 1987.

Enterprise, Discovery, Endurance, Adventure: NASA's Orbiter Fleet, NASA Facts, December 1996.

Chronology of Space Shuttle/Astronaut Transport Spacecraft Missions, 1980-2011, David S. F. Portree, NASA, 2012, pp. 20-22, 26-28, 33-34, 37-40, 45-55, 61-63, 88-91, A-13.

What If an Apollo Saturn Rocket Exploded on the Launch Pad? (1965)

Abort Mode One-Alpha. Image credit: NASA.

No member of the Saturn rocket family ever killed an astronaut. Two Saturn rocket designs were rated as safe enough to launch humans into space: the two-stage Saturn IB, which flew nine times between February 1966 and July 1975, and the giant Saturn V, which flew 12 times with three stages between November 1967 and December 1972, and once with two stages in May 1973. The 200-foot-tall Saturn IB flew five times with astronauts on board (Apollo 7, Skylab missions 2, 3, and 4, and the Apollo-Soyuz Test Project), while the 363-foot-tall Saturn V launched astronauts 10 times (Apollo missions 8 through 17).

Although man-rated, Saturn V rockets experienced four close calls. The first occurred on 4 April 1968, during the unmanned Apollo 6 test flight, when instability in the rocket’s fiery exhaust produced violent fore-and-aft shaking known as "pogo." Two of the five J-2 engines in the rocket’s S-II second stage shut down and pieces broke away from the streamlined shroud linking the Apollo Command and Service Module (CSM) to its S-IVB third stage. The CSM comprised the conical Command Module (CM), which carried the crew, and the Service Module (SM) which included electricity-generating fuel cells and the CSM's main engine, the Service Propulsion System (SPS). The Apollo 6 S-IVB's single J-2 engine under-performed, placing the stage and CSM into a lopsided orbit, then refused to restart.

Had the Apollo 6 CSM carried astronauts, pogo might have injured them; even if they had reached orbit unscathed, the S-IVB engine failure would have scrubbed their moon mission. As it was, flight controllers separated the unmanned CSM from the crippled S-IVB stage and used its SPS as a backup engine for completing the mission's Earth-atmosphere reentry test.

Apollo 12 experienced an even more perilous ascent. Following launch in a rainstorm on 14 November 1969, lightning struck its Saturn V 36.5 seconds and 52 seconds after liftoff. The lightning strikes knocked the Apollo 12 CSM Yankee Clipper's three electricity-generating fuel cells offline, along with its Apollo Guidance Computer and most other electrical systems.

The Saturn V's IBM-built Instrument Unit — its ring-shaped electronic brain, located atop its S-IVB third stage — soldiered on without a hiccup, however, safely guiding the giant rocket into Earth parking orbit. The Apollo 12 crew of Charles Conrad, Alan Bean, and Richard Gordon carried out a successful lunar landing mission and returned to Earth on 24 November. During the mission, Conrad reported seeing dark discoloration on the umbilical housing linking the CM and SM, but it remains uncertain whether this was a scorch mark left by lightning since discoloration has been noted on at least one other CSM umbilical housing (Apollo 15).

NASA would rename the "Uprated Saturn I" (right) depicted in this 1966 illustration the Saturn IB. Image credit: NASA.

Image credit: NASA.

The third Saturn V close call saw the unexpected return of pogo. During ascent to orbit on 11 April 1970, the middle engine of the Apollo 13 S-II stage began to rapidly oscillate fore and aft, then shut down two minutes early. The four remaining J-2 engines burned for longer than planned to compensate. Apollo 13 astronauts Jim Lovell, Fred Haise, and Jack Swigert subsequently left Earth orbit for the moon, but an oxygen tank explosion in their CSM, the Odyssey, scrubbed their moon landing. They used their Lunar Module (LM) moon lander, the Aquarius, as a lifeboat and returned safely to Earth on 17 April.

The final Saturn V to fly, intended originally for Apollo 20 but launched unmanned with the Skylab Orbital Workshop (OWS) on top in place of an S-IVB stage and the Apollo CSM and LM spacecraft, survived a close call on 14 May 1973. A design flaw caused Skylab's meteoroid shield to tear loose 63 seconds into the flight. As the disintegrating shield tumbled down the length of the accelerating rocket, it tore at least one hole in the interstage adapter that linked the OWS to the S-II second stage and apparently damaged the system for separating the ring-shaped interstage adapter that linked the S-II with the S-IC first stage. This meant that the 18-foot-long adapter did not separate from the S-II three minutes and 11 seconds into the flight as planned. The S-II stage had excess capacity, however, so dutifully hauled its unplanned five-ton cargo into Earth orbit.

Apollo 12 might easily have ended in a Launch Escape System (LES) abort. The image at the top of this post shows the LES in action during Pad Abort Test-2 on 29 June 1965. The LES was a 33-foot-tall tower containing three solid-fueled rocket motors. The largest was the Launch Escape Motor, which had four exhaust nozzles. The tower stood atop the Boost Protective Cover (BPC), a conical shell that covered the CM.

There were four successive abort modes during Saturn V ascent to Earth orbit. As the Saturn V climbed toward space, the aerodynamic environment around it changed - the air grew thinner, the rocket moved faster, and increasingly it tilted so that it flew parallel to Earth's surface. As the environment changed, the abort modes changed to compensate.

Abort Mode One was in effect on the launch pad, during S-IC first-stage operation, and during the 30 seconds following S-IC separation, by which time the Saturn V would have reached an altitude of about 56 miles. Had it occurred, the Apollo 12 abort would have taken place during the first part of Abort Mode One. Known as Abort Mode One-Alpha, it took effect 45 minutes before scheduled launch and continued until about 42 seconds after liftoff, by which time the rocket would have climbed nearly vertically to an altitude of 3000 meters (9800 feet).

In the event of a catastrophic Saturn V failure while Abort Mode One-Alpha was in effect, the 155,000-pound-thrust Launch Escape Motor would have pulled the BPC and CM free of the SM, which would have remained mounted on the doomed rocket. Meanwhile, the small side-mounted solid-propellant rocket motor near the LES's nose, the Pitch Control Motor, would have ignited to push the LES-BPC-CM combination eastward, toward the Atlantic and well clear of the Saturn V. The CM would then have dropped free of the BPC and deployed its three large parachutes to descend gently into the Atlantic within sight of Kennedy Space Center.

The Apollo 8 Saturn V rocket — the first Saturn V to carry a precious human cargo — stands on Launch Pad 39A at Kennedy Space Center, Florida. A Saturn V explosion before or during liftoff would have destroyed most of the structures visible in this image. Image credit: NASA.

27 April 1972: The Apollo 16 CM descends to a splashdown in the Pacific Ocean after an 11-day voyage to the moon. A CM descending into the Atlantic after an LES abort would have appeared very similar. Image credit: NASA.

In August 1965, R. High and R. Fletcher, engineers at NASA's Manned Spacecraft Center in Houston, Texas, calculated the characteristics of Saturn IB and Saturn V launch pad explosions to aid LES development. Of particular concern, they explained, was the damage an explosion fireball's heat might do to the CM's nylon main parachutes. In their report they did not, however, reach specific conclusions about parachute heat damage.

High and Fletcher found that calculating the characteristics of launch pad failures was not an exact science, in large part because there were so many variables to be taken into account, and also because no rocket as large as the Saturn V had ever exploded. They explained that "many of the [fireball] parameters may defy an accurate theoretical treatment."

For their analysis, they assumed that all propellants in the exploding rocket would contribute to forming a fireball. This would occur, they explained, because "large overpressures from detonations and the intense heat from both detonations and burning would cause failure of any propellant tanks not initially involved." If a Saturn V exploded on the pad at launch, 5,492,000 pounds of RP-1 refined kerosene, liquid oxygen (LOX), and liquid hydrogen would contribute to its fireball. For a Saturn IB pad explosion, 1,110,000 pounds of RP-1, LOX, and liquid hydrogen would fuel its fireball.

High and Fletcher wrote that the fireball from a Saturn rocket launch pad failure would expand in a "nearly fixed location." For the Saturn V, the fireball would expand to a diameter of 1408 feet. The Saturn IB fireball would expand to 844 feet. The fireballs would thus completely engulf the Saturn launch pads. For both rockets, fireball surface temperature would attain 2500° Fahrenheit, and heat would be felt up to a mile from the launch pad.

A fireball would begin to rise when it reached its maximum diameter. Fireball ascent would commence about 20 seconds after a Saturn V launch pad explosion and about 10 seconds after a Saturn IB explosion, High and Fletcher calculated. The Saturn V fireball would reach an altitude of about 300 feet in 15 seconds, while the Saturn IB fireball would climb 300 feet in 11 seconds. The Saturn V fireball would persist at its maximum diameter for 34 seconds, while the Saturn IB fireball would last for 20 seconds. The fireball would then begin to cool and dissipate.

Though they assumed for their calculations that all propellants in an exploding Saturn rocket would contribute to its fireball, High and Fletcher wrote that some would likely be "spilled on the ground, creating residual pools which [would] burn for relatively long periods of time." This was, they judged, especially likely if a launch pad failure began with the rupture of the fuel tank in the Saturn V's S-IC first stage. The ruptured tank would spill RP-1 onto the pad, then the oxidizer tank located above it would rupture and mix liquid oxygen with the burning fuel, triggering an explosion. They added that "the residual fire and extreme heat of the fireball [would] prevent approach to the ground area enveloped by the fireball for an unknown period."

Sources

Estimation of Fireball from Saturn Vehicles Following Failure on Launch Pad, NASA Program Apollo Working Paper No. 1181, R. High and R. Fletcher, NASA Manned Spacecraft Center, Houston, Texas, 3 August 1965.

Skylab 1 Investigation Report, Hearing Before the Subcommittee on Manned Space Flight of the Committee on Science and Astronautics, U.S. House of Representatives, Ninety-Third Congress, First Session, 1 August 1973, U.S. Government Printing Office, 1973.

Apollo Experience Report - Launch Escape Propulsion Subsystem, NASA Technical Note D-7083, N. Townsend, NASA, March 1973, pp. 1-7.

Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions, W. David Compton, NASA SP-4214, 1989, pp. 177-178.

How Apollo Flew to the Moon, W. David Woods, Springer-Praxis, 2008, pp. 69-73.

More Information

A Forgotten Rocket: The Saturn IB

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

Clyde Tombaugh's Vision of Mars (1959)

Elysium and Tombaugh crater are located near the center of this image from India's Mars Orbiter spacecraft. Image credit: Indian Space Research Organization.

Clyde William Tombaugh (1906-1997) was born in Streator, Illinois, and grew up in Burdett, Kansas, where he built his first telescopes. In 1929, Tombaugh joined the staff of Lowell Observatory in Flagstaff, Arizona, to hunt for Planet X, a world which Boston businessman Percival Lowell had predicted should exist beyond Neptune. On 18 February 1930, 24-year-old Tombaugh discovered Pluto.

A little over two weeks ago, on 14 July 2015, the New Horizons spacecraft raced through the Pluto system. The piano-sized probe bore some of Tombaugh's ashes. The New Horizons scientists applied the unofficial name Tombaugh Regio to a prominent surface feature on Pluto.

It is possible that Tombaugh Regio will not win through the formal planetary feature naming process, in part because a feature named for Tombaugh already exists: a crater on Mars, the world to which he devoted far more professional attention than he did Pluto. Tombaugh crater is located at 162° East, just north of the martian equator in Elysium Planitia, a region that Tombaugh believed was important for understanding the surface structure of the Red Planet.

Clyde Tombaugh with a telescope of his own making. Image credit: Wikipedia.

Although Pluto became Lowell Observatory's greatest claim to fame, Percival Lowell had founded his observatory in 1894 to find proof of intelligent life on Mars. He had theorized that the aging planet was slowly losing its water, and that the dark lines some astronomers glimpsed on its ochre disk were canals its inhabitants had excavated to distribute melt water from the polar ice caps and stave off encroaching deserts.

Lowell believed that spots strung like beads along the lines were oases, and that irregular dark-colored areas (maria) scattered over the surface were desiccated sea beds. Though rejected by most astronomers (including Tombaugh), Lowell's romantic vision helped to inspire the H. G. Wells novel The War of the Worlds (1898) and the "Barsoom" books of Edgar Rice Burroughs. These tales in turn inspired generations of rocketeers and skywatchers.

In the January 1959 issue of Astronautics, the journal of the American Rocket Society, Tombaugh summarized the prevailing view of Mars surface conditions on the eve of its exploration by spacecraft. He first described three areas where improved data had undermined Lowell's romantic vision.

The first was temperature. Depending on its position in its elliptical orbit around the Sun, Mars receives between 53% and 36% as much solar energy as Earth. Astronomers using telescopes equipped with thermocouples had determined that the temperature on the surface at noon normally barely surpassed the freezing point of water, though it could reach 70° Fahrenheit (F) at noon in the southern hemisphere in summer. Tombaugh added that the temperature regularly swings 200° F from frigid midnight to chilly noon over much of the planet.

Low atmospheric pressure also created problems for Lowell's Mars. Evidence was mounting, Tombaugh wrote, that at its surface Mars had an atmospheric pressure only 10% as great as Earth's sea-level pressure. Enough carbon dioxide was known to exist in the martian atmosphere to give the planet an atmospheric pressure about 1% of Earth's. Many planetary astronomers, Tombaugh added, believed that nitrogen made up the remaining nine-tenths of the martian atmosphere, though none had been detected.

Finally, Mars's surface was likely to be subjected to unhealthy levels of radiation. Planetary astronomers had found no evidence of oxygen in the martian atmosphere, Tombaugh reported. Whatever oxygen Mars had was probably locked up chemically in its crust, giving the planet its characteristic rusty color. Lack of free oxygen meant that Mars would also lack atmospheric ozone, which on Earth creates a shield against solar ultraviolet (UV) radiation. This meant that sterilizing UV radiation from the Sun would reach Mars's surface largely unfiltered.

Tombaugh argued that the dark maria could not be sea bottoms; they would be salt-encrusted if they were, so would appear bright white, not dark. Mars, he added, showed no signs of "a visible dendritic [branching] drainage system" akin to Earth's rivers, so was probably extremely arid. He noted seasonal changes in the maria's color which he attributed to plant life. As the polar cap evaporated in springtime, he wrote, atmospheric moisture would move toward the equator. The martian vegetation in the spring/summer hemisphere would absorb the moisture and change hue.

Tombaugh contended that martian plants had evolved novel ways of withstanding the planet's harsh environment. He recounted telescopic observations of Mars he made during its close approach to Earth in 1954.

Normally the southern maria range from green to blue in color. The long dark sash, Sabaeus Sinus, running east to west only a few degrees south of the equator, is habitually bluish-green. Amazingly. . .this marking. . .altogether some 2000 miles long. . .suddenly turned to bright lavender or perhaps magenta! The other maria did not. Why? Can vegetation inhabiting this area shield itself by changing pigment to reflect away a sudden influx of lethal radiation?

Sometimes, Tombaugh reported, the planet's cruel conditions could spell catastrophe for even the toughest martian vegetation. He wrote that Syrtis Major,

the principal dark marking on Mars, undergoes some very strange metamorphoses in color. The north half is habitually of a deep blue color, while the southern half is grey-green to blue-green or sometimes a vivid green. I remember. . .when the whole marking became intensely black — totally devoid of color! In the absence of oxygen, dead vegetal matter would not yield to oxidation and decay. Were we seeing dead vegetal matter when the Syrtis turned black?

Tombaugh assured Astronautics readers that he did not believe in Lowell's intelligent Martians, though he hastened to add that he had "seen over 100 of the controversial canals too well, with telescopes of great effective power" to be able to "dismiss them as unreal." He offered an explanation for the planet's linear features that was first advanced by former Lowell collaborator (later Lowell rival) William Pickering in 1904.

Over the ages, Mars must have been hit by many asteroids. Such dreadful collisions must have produced some visible marks. . .Collisions with asteroids a few miles in diameter going at velocities of the order of 15 [miles per second] might well fracture a planet to the bottom of the crust and to radial distances of hundreds or even a few thousand miles. . .Where a fracture line met the surface, a long narrow strip of shattered rock would be produced, and would offer some haven to a hardy form of vegetation . . . [The plants growing in the fracture strip would] make a dark contrast against a light. . .terrain.

Tombaugh theorized that the dark spots Lowell thought were oases are actually asteroid impact craters. The canals, he asserted, divided the planet's entire crust into a "tetrahedron" pattern. As Mars cooled internally and shrank, some faces of the martian tetrahedron slumped. Tombaugh differed from the majority opinion of his time when he argued that other faces had risen to form high plateaus. Many of his contemporaries confidently asserted that Mars lacked any raised landforms. The northern-hemisphere region of Elysium, Tombaugh added, was probably the highest land on the planet. He explained that it

is sharply pentagonal in shape, [and] bounded by five long canals. . .The corners of the pentagon extend 600 geographical miles from the center. During most of the Martian year, Elysium appears much the same as the surrounding desert. By midsummer of the northern hemisphere, this area becomes white with frost except around noon. . .the whitening develops over the entire area, but always stops abruptly at the edges of the pentagon. One is forced to conclude that the five sides represent enormous vertical escarpments — and just where we should expect to see them — along the canals.

After five decades of Mars exploration by robot spacecraft — the first was Mariner IV, which flew past the planet on 14 July 1965, 50 years to the day before the New Horizons Pluto flyby — we know that Elysium is indeed an uplifted region, though not the highest on Mars. That honor goes to the massive Tharsis Plateau, upon which stand the planet's great shield volcanoes. The tallest of these, Olympus Mons, stands some 27 kilometers above the base datum, the martian equivalent of Earth's sea level.

Topographic map of eastern Elysium Planitia showing Tombaugh crater. The image is based on data from  the Mars Orbiter Laser Altimeter (MOLA) carried on the Mars Global Surveyor spacecraft. Low areas are blue and high areas are yellow and red. Image credit: MOLA Science Team/NASA. 

We know today that, when Tombaugh's contemporaries detected martian atmospheric carbon dioxide, they had found not a minor atmospheric constituent, but instead virtually the planet's entire atmosphere. We have learned that branching channels are common on Mars, though at a scale invisible to Earth-based telescopes, and that Lowell's canals were products of eyestrain, the human mind's tendency to impose patterns on random arrangements of objects, and wishful thinking.

We know also that the dark areas on Mars are mostly sand made from weathering and erosion of volcanic rocks, and that seasonal changes in their color and extent result from obscuring dust storms. We have found cracks in the martian crust, though those associated with asteroid impact craters are only local in extent. The best-known crustal fracture, the 3000-mile-long Valles Marineris canyon system, probably formed through internal stresses associated with the uplift of Tharsis. We know that the planet's overall shape has a pattern, though not one as intricate as Tombaugh's tetrahedron. Rather, Mars has southern highlands and northern lowlands (the latter at least partly underlain by ice, lending credence to the theory that it was once an ocean bottom).

In spite of our improved knowledge, key questions about Mars remain unanswered. We do not know, for example, whether it hosts living organisms. The pitch for piloted Mars exploration which concluded Tombaugh's paper thus remains relevant today.

[W]hy should we be interested in making a trip to Mars?. . .A manned landing on Mars would be a momentous achievement for the human race. It would be a field day for the geologist, biologist, astrophysicist, and meteorologist. They would glean knowledge on the consequences of a set of physical conditions foreign to us. . .Most important, to see at first hand what Nature has done with a world so marginal for life would be of the greatest philosophic and religious value, in helping us to understand our place and our purpose in the Universe.

Astronauts seek signs of past life on Mars. Image credit: NASA/Pat Rawlings.

Sources

“Mars - A World for Exploration,” Clyde W. Tombaugh, Astronautics, January 1959, pp. 30-31, 86-93.

Mars and Its Canals, Percival Lowell, The MacMillan Company, 1906.

More Information

Pluto, Doorway to the Stars (1962)

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 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 on board the Lunar Roving Vehicle (LRV) — a four-wheeled electric car — Apollo astronauts could not stray beyond a "walk-back 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 their PLSS backpacks.

The walk-back 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 decreased, but then so did the distance they would need to hike if the LRV broke down.

The limited endurance of the Apollo LM and PLSS, combined with the walk-back 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 expeditions 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

[t]he 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, 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, Arizona, in 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 basic design principles. The astronaut would fly standing, stabilized as he flew by his hold 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 astronaut upper torso in flight positions. Image 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 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 midpoint 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 walk-back limit. At the mid-term briefing, NASA directed the contractors to redesign their LFUs so that they would 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 (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 expected that during each LFU sortie the amount of 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 landing 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 up to 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, though it did have some experience. 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. Spherical auxiliary propellant tanks could be added to boost range.

The 1964 North American Aviation LFU stressed compactness over range. Image credit: North American Aviation/NASA.

Perhaps because it was starting with a relatively blank slate, NAR's 1969 LFU was very different from either its 1964 design or that of its 1969 competitor. NAR rejected LFU designs that had the astronaut standing, having found such configurations 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 as well as 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 weight. 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, or 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 swing-arm-mounted control panel and the foot-rest, 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 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 walk-back 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 be used to 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 all four LRV wheels rest on the dusty Moon, they unfold seats and other appendages, such as antennas, by hand. 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 walk-back 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 lunar surface 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.

Sources

"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

The Spacewalks That Never Were: Gemini Extravehicular Planning Group (1965)

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

"A Continuing Aspect of Human Endeavor": Bellcomm's January 1968 Lunar Exploration Program

Robot Rendezvous at Hadley Rille (1968)

Dreaming a Different Apollo: Part One