Chrysler's Transportation and Work Station Capsule (1965)

Chrysler-built Saturn IB first stages in the final phase of assembly at NASA's Michoud Assembly Facility, November 1967. The clustered Redstone rocket bodies are most obvious on the stage at far left. Image credit: NASA.

In the 1960s, the Chrysler Corporation, an automobile company founded in 1925, manufactured the first stage of the Saturn IB, the first and last member of the Saturn rocket family to launch astronauts into space. After four successful unmanned test flights beginning in February 1966, Saturn IBs launched the Apollo 7 Command and Service Module (CSM), the first Apollo spacecraft to carry a crew (11-22 October 1968); the three CSMs that ferried crews to and from Skylab, the first U.S. space station (May 1973-February 1974); and the Apollo-Soyuz Test Project (ASTP) CSM and the Docking Module its crew used to link up with the Soviet Soyuz 19 spacecraft in Earth orbit (15-24 July 1975). The ASTP CSM was the last Apollo spacecraft to fly.

That Chrysler built the Saturn IB first stage should not be surprising. The company's Missile Division built intermediate-range Redstone missiles for the U.S. Army starting in 1950; the first flew in 1953. An upgraded Redstone, the Jupiter, served both as a missile and a space launcher. A modified Jupiter launched Explorer 1, the first U.S. Earth satellite, on 31 January 1958. Safety-enhanced Redstones launched suborbital Mercury spacecraft containing Alan Shepard, the first American in space (5 May 1961), and Virgil Grissom (21 July 1961).

The first Saturn I rocket lifts off, 27 October 1961. For this suborbital test of the Chrysler-built S-I first stage, the rocket included a dummy S-IV second stage. Image credit: NASA.

Redstone and its derivatives prepared Chrysler for its contributions to the 162-foot-tall Saturn I, which can reasonably be described as NASA's rocket for learning how to fly big rockets. Saturn I flew 10 times between October 1961 and July 1965. Chrysler's Saturn I first stage included eight Redstone rocket bodies clustered around a central Jupiter rocket body. The Jupiter and four Redstones were filled with liquid oxygen; the remaining four Redstones held RP-1 aviation fuel. Chrysler's Saturn IB first stage followed an identical pattern.

The last three Saturn I rockets each launched a Pegasus meteoroid-detection satellite, the first active payloads launched on a Saturn rocket. Pegasus 1 reached orbit in February 1965, Pegasus 2 in May 1965, and Pegasus 3 in July 1965. The Pegasus series was crucial for understanding the threat micrometeoroids posed to spacecraft and astronauts.

Shortly after Pegasus 1 reached space, at the Second Space Congress in Cocoa Beach, Florida, Chrysler engineers R. Dutzmann and E. Dunford described a small free-flying capsule for performing work outside spacecraft and space stations. They presented their paper in April 1965 during the six-week period that separated humankind's first spacewalk by Alexei Leonov (Voskhod 2, 18 March 1965) from the first U.S. spacewalk by Ed White (Gemini IV, 3 May 1965).

Dutzmann and Dunford's capsule design arose from a perceived need for new methods of protecting spacewalking astronauts — methods that would enhance protection but not compromise the ability to perform work. The Chrysler engineers explained, for example, that planned nylon fabric space suits might shield an astronaut from meteoroids if a coverall made of woven aluminum wire were added; the coverall would, however, make astronaut movement difficult.

Meteoroids were only one possible hazard of walking in space. Dutzmann and Dunford noted that objects in space have no weight, but retain their mass. They feared that a massive object — for example, a rocket stage — inadvertently set in motion might catch an astronaut unawares and crush him against another massive object.

The Chrysler engineers wrote that NASA planned to use a pure oxygen atmosphere at a pressure of 3.5 pounds per square inch (psi) inside its spacecraft and space suits. Experiments had shown that, should a space suit develop a leak, the astronaut would experience oxygen starvation if the pressure fell by just 0.8 psi. Increasing the flow of oxygen into the suit might keep pressure above the 2.7-psi critical level long enough for him to reach safety, but only if the suit perforation were small.

In addition, they noted that a space-suited astronaut would have no place to keep his tools. Attaching tools to the astronaut would impede movement.

The Chrysler engineers offered a brief assessment of past proposals for heavy (up to 2.5 ton) "taxis" that would include "comfortized" pressurized cockpits and mechanical manipulator arms. Such "luxuries," they wrote, could not realistically play a role in space operations before the 1970s. The technological leap required to move from a basic fabric space suit to a complex work vehicle was too great; also, tests had shown that existing mechanical manipulators were up to four times less efficient for doing work than astronaut arms and hands.

Side view of the interior of Chrysler's Transportation and Work Station Capsule with astronaut positions indicated. A = outline of astronaut position with capsule doors closed; B = outline of astronaut position at work site with capsule doors open. Work site attachment arms are shown in black outline in closed-door (retracted) position and in red in open-door (extended) position. Image credit: Chrysler Corporation/David S. F. Portree.

Top view of the interior of Chrysler's Transportation and Work Station Capsule. A1 = left sliding door/window; A2 = right sliding door/window; B = hand controller for guiding capsule; C1 = left oxygen hose; C2 = right oxygen hose; D1 = left sticky pads at ends of arms for holding capsule at work site (arms shown retracted in closed-door position); D2 = right sticky pads at ends of arms for holding capsule at work site (arms retracted in closed-door position); E = tool storage; F = hydrogen peroxide propellant tank; G = pressure seal curtain deployment channel; H = Whipple Bumper hull. Image credit: Chrysler Corporation/David S. F. Portree.

Dutzmann and Dunford thus proposed an intermediate developmental step in spacewalk technology: a three-foot-diameter, eight-foot-tall "Transportation and Work Station Capsule," normally kept unpressurized, that would provide the space-suited astronaut with an extra layer of protection from injury, a shelter in case of suit damage, improved mobility, worksite lighting, and places inside to store up to 30 pounds of tools. With a 230-pound space-suited astronaut inside and a full load of 30 pounds of hydrogen peroxide propellant, the cylindrical capsule would weigh just 550 pounds.

The capsule hull would comprise two layers of aluminum, each a fraction of an inch thick, separated by an empty space a little less than an inch wide. Meteoroids would strike the outer layer, break apart and partly vaporize, then strike the inner layer. Testing showed that this design, based on the Whipple Bumper concept, could provide meteoroid protection equivalent to that offered by a solid aluminum hull four times as thick. The Whipple Bumper was named for its inventor, comet astronomer Fred Whipple.

Dutzmann and Dunford suggested that the empty space between the aluminum layers be filled with aluminum honeycomb to improve structural strength. The lightweight Whipple Bumper hull meant that the Chrysler capsule's structure would weigh just 88 pounds.

The capsule's dome-shaped ends would each contain a thruster group. A total of 12 catalyst-bed thrusters, not too different from the Mercury spacecraft attitude-control thrusters, would draw hydrogen peroxide from a tank located at the capsule center of gravity. Upon contact with the catalyst, the hydrogen peroxide would turn to high-temperature steam and vent from the thruster nozzle. Each thruster could produce up to 10 pounds of thrust.

The astronaut, who would stand within the capsule, would open a pair of sliding doors with windows and lean out through the open doorway to perform tasks using tools gripped in gloved hands. Two pairs of telescoping arms with sticky pads at the ends, arranged one above the other, would extend through the 27-inch-by-78-inch door opening on either side of the astronaut's hips and thighs to hold the capsule in place at the work site.

A pair of Chrysler Transportation and Work Station Capsules in action in lunar orbit. Image credit: Chrysler Corporation.

A unique feature of Chrysler's capsule was its "pressure seal curtain" system. In the event of a suit puncture or tear, a transparent plastic sheet sleeve would rise up a deployment "channel" from a donut-shaped storage area in the capsule floor to surround and enclose the astronaut.

Dutzmann and Dunford offered a timeline for pressure seal curtain activation. They assumed that the astronaut's fabric space suit would most likely become damaged while the capsule was attached to a work site since at all other times the astronaut would remain inside the capsule with the doors closed.

They expected that a "pressurization emergency" would be detected 10 seconds after suit damage occurred. Five seconds post-detection, the capsule would automatically detach from the four arms holding it at its work site. This would clear the way for its sliding doors to shut 10 seconds after detection. Fifteen seconds after detection, the seal curtain would rise up and attach itself to the capsule "ceiling." Simultaneously, a backup oxygen supply mounted behind the astronaut's shoulders would activate, increasing flow into the astronaut's damaged suit. Air leaking from the suit would begin to fill the seal curtain volume 30 seconds after leak detection.

Much like the fabric space suit, the seal curtain would vent oxygen overboard to prevent buildup of exhaled carbon dioxide. Dutzmann and Dunford assumed that, to avoid oxygen depletion and carbon dioxide buildup within his helmet, the astronaut would open his visor soon after pressure within the seal curtain exceeded 2.7 psi.

The astronaut would then pilot the capsule back to its docking structure on the home spacecraft. If the capsule remained within 1000 feet of the docking structure, as Dutzmann and Dunford recommended, the trip would last less than 10 minutes.

The three Saturn I-launched Pegasus satellites would reveal that the threat from meteoroids in space was less severe than expected, but other dangers lay in wait for 1960s spacewalkers. The Soviet Union would for years claim that Alexei Leonov's spacewalk was a complete success, when in fact he could not control his movements, overheated, and became stuck sideways in Voskhod 2's inflatable airlock.

Ed White's excursion outside the Gemini IV spacecraft, less than a month after the Chrysler engineers presented their capsule design, was nearly as successful as Leonov's was claimed to have been. More careful analysis would, however, have pointed to potential problems — White's suit expanded during his spacewalk, and he exceeded the cooling capacity of his air-cooled space suit while struggling to squeeze into his narrow seat and close his balky spacecraft hatch.

Not until humankind's perilous third spacewalk on 5 June 1966 would the inadequacies of air-cooled space suits become obvious. During an ambitious attempt to fly free of the Gemini IX spacecraft using a 168-pound hydrogen peroxide-fueled Astronaut Maneuvering Unit (AMU) backpack, pilot Eugene Cernan tore his suit's outer layers, overheated, and became blinded by perspiration as he struggled against his suit's internal pressure. Cernan's AMU free flight was called off and NASA was forced to descope its planned series of complex Gemini Program spacewalks (see "More Information" below).

Sources

"Design Considerations for a Free Space Transportation and Work Station Capsule," R. Dutzmann and E. Dunford, Proceedings of the 2nd Space Congress, April 1965, pp. 403-430.

Chrysler's Ballistic Missile and Space Activities: First 20 Years, Chrysler Corporation, 1972.

Walking to Olympus: An EVA Chronology, Monographs in Aerospace History #7, David S. F. Portree and Robert C. Trevino, NASA History Office, October 1997, pp. 1-5, 11 (https://history.nasa.gov/monograph7.pdf — accessed 12 November 2017).

More Information

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

Rocket Belts and Rocket Chairs: Lunar Flying Units

Mission to the Mantle: Michael Duke's Moonrise (1999-2009)

This NASA image of the gibbous Moon by photographer Lauren Harnett includes an intruder — the International Space Station (ISS) (lower right). The Moon, last visited by humans in December 1972, is about 384,400 kilometers away; ISS, permanently occupied since November 2000, is about 1000 times nearer Earth.

A casual glance at the Moon's disk reveals signs of ancient violence. Nearside, the lunar hemisphere we can see from Earth, is marked by gray areas set against white. Some are noticeably circular. The Apollo expeditions revealed that these relatively smooth basalt plains are scars left by large impactors — comets or asteroids — that pummeled the Moon more than 3.5 billion years ago. These gray areas cover about 20% of the lunar surface. They are concentrated on the nearside, the lunar hemisphere that faces the Earth.

An Earth-based observer cannot view the largest and oldest giant impact basin because it is out of view on the Moon's hidden farside. South Pole-Aitken (SPA) Basin is about 2500 kilometers wide, making it perhaps the largest impact scar in the Solar System. Lunar Orbiter data revealed its existence in the 1960s, though little was known of it until the 1990s, when the U.S. Clementine and Lunar Prospector polar orbiters mapped surface chemistry over the entire Moon. Their data showed that the basin floor probably includes material excavated from the Moon's lower crust and upper mantle. In the first decades of the 21st century, laser altimeters on the U.S. Lunar Reconnaissance Orbiter (LRO) and Japanese Kaguya spacecraft confirmed that SPA includes the lowest places on the Moon.

Lunar hemispheres centered on the Moon's highest point (left) and lowest point (right). Both occur in the Moon's Farside hemisphere and are believed to be associated with the excavation of the South Pole-Aitken Basin perhaps 4 billion years ago. On this U.S. Geologic Survey topographic map, blue indicates low areas and gray and black indicate high areas. 

South Pole-Aitken (SPA) Basin with major features labeled. The 140-kilometer-wide crater Antoniadi includes a 12-kilometer-wide unnamed crater, the floor of which is more than nine kilometers below the mean lunar radius (the lunar equivalent of Earth's sea level). It is the lowest point on the Moon. Image credit: NASA/DSFPortree.

Michael Duke, a retired NASA Johnson Space Center geologist with the Colorado School of Mines, participated in both Apollo and 1990s lunar explorations. In 1999, Duke was Principal Investigator (PI) leading a team that proposed a robotic SPA sample-return mission in NASA's low-cost Discovery Program. To fit under Discovery's mission cost cap of $150 million (in Fiscal Year 1992 dollars), Duke's team proposed "the simplest-possible mission" — a single lander with no sample-collecting rover, a lunar-surface stay-time of just 24 hours, and a low-capability lunar-orbiting radio-relay satellite (needed because farside is not in line-of-sight radio contact with Earth). Believing that these limitations added up to a high risk of mission failure, NASA rejected the 1999 Discovery proposal.

In 2002, however, the National Research Council's planetary science Decadal Survey declared SPA sample return to be a high scientific priority and, at the same time, proposed a new class of competitively selected medium-cost missions. The latter marked the genesis of NASA's New Frontiers Program, which originally had a cost cap per mission of $700 million.

The New Horizons Kuiper Belt Object (KBO) flyby mission was already under development when NASA created the New Frontiers Program. NASA gave New Frontiers a highly visible first mission by adopting New Horizons into the program. Selection of the KBO mission came to be regarded as the first New Frontiers proposal cycle, though it included no competition. NASA had taken a similar approach when it made Mars Pathfinder its first Discovery Program mission in 1992.

Geologist Michael Duke in 2004. Image credit: NASA.

Duke's team immediately began to upgrade its SPA proposal for the second New Frontiers proposal cycle. In October 2002, Duke described the new SPA mission design at the 53rd International Astronautical Federation Congress (the Second World Space Congress) in Houston, Texas. To avoid tipping off competing New Frontiers proposers, his paper provided only limited technical details.

Duke argued that the SPA sample-return mission could collect ancient deep crust and mantle rocks without a costly rover. Clementine and Lunar Prospector had shown that at least half of the surface material in the central part of SPA was native to the basin, so stood a good chance of having originated deep within the Moon.

Furthermore, Apollo demonstrated that any lunar site is likely to yield a wide assortment of samples because the Moon's low gravity and surface vacuum enable asteroid impacts to widely scatter rock fragments. The Apollo 11 mission to Mare Tranquillitatis, for example, found and returned to Earth rocks blasted from the Moon's light-hued Highlands. Duke proposed that the SPA sample-return lander sift about 100 kilograms of lunar dirt to gather a one-kilogram sample consisting of thousands of small rock fragments. These would have many origins, but a large percentage would be likely to have originated in the Moon's deep crust and mantle.

A SPA sample-return lander sifts lunar dust in quest of small fragments of lower crust and upper mantle material. The gray dome mounted sideways on the right side of the lander, above the sample arm attachment point, is the sample-return capsule for carrying a one-kilogram sample through Earth's atmosphere. Image credit: NASA.

NASA rejected the Discovery SPA mission in part out of concern for lander safety. Duke noted that, with the New Frontiers Program's $700-million cost cap, the SPA sample-return mission could include two landers. This would provide a backup in case one crashed. He pointed out, however, that automated Surveyor spacecraft of the 1960s had found the Moon to be a relatively easy place on which to land even without the benefits of 21st-century hazard-avoidance technology. Two landers would also increase the already good chance that the mission could collect samples representative of the basin's earliest history.

A $700-million budget would also enable a relay satellite "more competent" than its bare-bones Discovery predecessor. It might be placed in a halo orbit around the Earth-Moon L2 point, 64,500 kilometers behind the Moon as viewed from Earth. From that position, the satellite would permit continuous radio contact between Earth and the landers. A satellite in lunar orbit could remain in line-of-sight contact with both the landers and Earth for only brief periods.

NASA had argued that a single day on the Moon provided too little time to modify the SPA Discovery mission if it suffered difficulties. The SPA New Frontiers mission would, therefore, remain on the Moon for longer. Duke noted, however, that stay-time would probably be limited to the length of the lunar daylight period (14 Earth days) because designing the twin landers to withstand the frigid lunar night would boost their cost.

In February 2004, Duke's mission — christened Moonrise — became one of two SPA sample-return missions proposed in the second New Frontiers proposal cycle. In July 2004, NASA awarded Moonrise and a Jupiter polar orbiter called Juno $1.2 million each for additional study. In May 2005, the space agency selected Juno for full development.

Juno's selection did not end proposals for SPA Basin sample-return, though it did mark the beginning of the end of Duke's involvement. In the third New Frontiers proposal cycle, which began in 2009, a Jet Propulsion Laboratory/Lockheed Martin/Washington University in St. Louis team led by Brad Jolliff, Duke's deputy PI in the 2003-2004 cycle, proposed a SPA Basin mission called MoonRise. In 2011, the SPA sample-return mission was again selected as a New Frontiers finalist, but it lost out in the final selection to the OSIRIS-Rex asteroid sample-return mission. MoonRise was not selected as a finalist in the 2017 New Frontiers cycle.

Sources

"Sample Return from the Lunar South Pole-Aitken Basin," M. Duke, Advances in Space Research, Volume 31, Number 11, June 2003, pp. 2347-2352.

"NASA Selects Two 'New Frontiers' Mission Concepts for Further Study," D. Savage, NASA Press Release 04-222, NASA Headquarters, 16 July 2004.

NASA Facts: MoonRise - A Sample-Return Mission From the Moon's South Pole-Aitken Basin, NASA Facts, JPL 400-1408, June 2010.

"MoonRise: Sample Return from the South Pole-Aitken Basin," L. Akalai, B. Jolliff, and D. Papanastassiou; presentation to the International Planetary Probe Workshop, Barcelona, Spain, 17 June 2010.

Personal communication, B. Jolliff to D. Portree, 3 March 2018.

More Information

Peeling Away the Layers of Mars (1966)

An Apollo Landing Near the Great Ray Crater Tycho (1969)

Catching Some Comet Dust: Giotto II (1985)

Lunar GAS (1987)

A New Step in Spaceflight Evolution: To Mars by Flyby-Landing Excursion Mode (1966)

The 12 images and captions that accompany this post are a post-within-a-post. R. R. Titus, author of the study discussed in the main post text, did not describe his Flyby-Landing Excursion Mode (FLEM) spacecraft, so these images constitute a plausible guess. They draw inspiration from the 1966 NASA Planetary JAG piloted Mars/Venus flyby design; the SIM Bays used to explore the Moon from lunar orbit during Apollos 15, 16, and 17; Orbital Workshop concepts; the informed imagination of their creator, artist William Black; and sundry inputs from the post author. In the image above, a stack assembled in Earth orbit from three modified Saturn S-IVB rocket stages and the FLEM spacecraft undergoes final checks before beginning Earth-departure maneuvers.

This dramatic image is 21st-century artist William Black's tribute to a very similar hand-painted NASA image of the 1960s. Earth-orbit departure would require three S-IVB stage burns at perigee over about two days. Because of this, the first and second S-IVBs (center left and lower left) would not be in view when the third S-IVB ignited its J-2 engine. After engine shutdown, the third S-IVB would detach and vent leftover propellants to nudge its course so that it would not follow the FLEM spacecraft to Mars.

During its first dozen years, NASA piloted spaceflight followed an evolutionary course, with simple missions and spacecraft leading to more complex and capable ones. Single-man Mercury suborbital missions led to Mercury orbital missions of increasing duration; then, in 1965-1966, two-man Gemini missions progressively added maneuverability, the ability to rendezvous and dock, spacewalk capability, and flight durations of up to 14 days.

Next came Apollo. NASA conducted four piloted preparatory missions in 1968-1969 ahead of the first lunar landing attempt. Apollo 7 (October 1968), launched on a two-stage Saturn IB rocket, tested the Command and Service Module (CSM) in Earth orbit. The CSM comprised a drum-shaped Service Module (SM) and the conical Command Module (CM) bearing its three-man crew.

As in biological evolution, contingency played a role in spaceflight evolution: for example, Apollo 8, intended originally as a Saturn V rocket-launched high-Earth-orbit test of the CSM and the bug-like Lunar Module (LM) moon lander, became a CSM-only lunar-orbital mission after the LM was delayed and the Soviet Union appeared close to launching a cosmonaut around the moon.

The Apollo 8 astronauts reached lunar orbit on Christmas Eve 1968. In addition to forestalling Soviet attempts to upstage the first Apollo lunar landing mission, Apollo 8's 10 lunar orbits tested upgrades made to the Manned Space Flight Network, NASA's world-wide radio communications and tracking system, and gave console operators in Mission Control early experience in supporting a piloted lunar mission. The Apollo 8 CSM left lunar orbit on Christmas Day 1968, and splashed down in the Pacific Ocean on 27 December 1968.

Apollo 9 (March 1969) saw the first Earth-orbital test of the LM and CSM together. Apollo 10 (May 1969) was a dress-rehearsal in low-lunar orbit for Apollo 11 (July 1969), the first piloted lunar landing.

Apollo 11 is perhaps best understood as an engineering mission; it was a cautious end-to-end test of the Apollo system with a single two-and-a-half-hour moonwalk and only limited science objectives. Apollo 12 (November 1969) demonstrated the pin-point landing capability required for pre-mission geologic traverse planning by setting down near a known point on the moon: specifically, the Surveyor III automated soft-lander, which had landed in April 1967. It also saw a pair of moonwalks lasting nearly four hours each and deployment of the first Apollo Lunar Scientific Experiment Package (ALSEP).

Apollo 13 (April 1970), the first science-focused mission, suffered a crippling explosion midway to the Moon, scrubbing its lunar landing, but its crew's safe return to Earth demonstrated the Apollo system's maturity and the Apollo team's experience. Apollo 14 (January-February 1971) included two moonwalks, each lasting more than four-and-a-half hours. They included a strenuous 1.3-kilometer trek through the hummocky ejecta blanket surrounding 300-meter-wide Cone Crater, a natural drill hole in the scientifically important Fra Mauro Formation.

Apollo 15 (July-August 1971), Apollo 16 (April 1972), and Apollo 17 (December 1972), designated "J" missions, featured a host of evolutionary improvements. Beefed-up LMs permitted surface stay times of up to three days at complex and challenging landing sites, heavier returned lunar samples, and more complex ALSEPs. Space suit improvements and Boeing's Lunar Roving Vehicle enabled geologic traverses ranging over kilometers of the lunar surface. Each "J" mission CSM included a suite of sensors which its pilot could turn toward the moon. Apollo 15 visited the Hadley Rille/Apennine Mountains area; Apollo 16 the central Nearside Lunar Highlands; and Apollo 17, Taurus-Littrow, on the edge of Mare Serenitatis.

Conceptual FLEM spacecraft. A black-and-white band midway along its hull marks where it will split in two as it nears Mars. To the left of the band is the Mars Orbiter (R. R. Titus called it the "excursion module"); to the right, the Flyby Spacecraft (the "parent" spacecraft). The Orbiter mission module has a single deck with living and working space for two astronauts; the Flyby Spacecraft, two decks with room for four crew.

As early as 1962, engineers foresaw two evolutionary paths for Saturn rockets and Apollo spacecraft hardware after they accomplished President John F. Kennedy's goal of a man on the Moon by 1970. The engineers were guided by President Lyndon Baines Johnson's 1964 decision that, to contain cost, NASA's piloted space program after the Moon landing should be based on hardware developed for Apollo. This marked the advent of the Apollo Applications Program (AAP).

One path would see Moon missions continue more or less indefinitely, growing ever more capable and culminating in a permanent lunar base in the 1980s. Alternately, NASA might repurpose Apollo hardware to build, launch, and maintain an evolutionary series of space stations in Earth orbit.

The space station path appeared pedestrian compared to the lunar base path, yet it offered great potential for long-term future exploration. This was because it promised to prepare astronauts and spacecraft for long-duration missions in interplanetary space. In 1965-1966, NASA advance planners envisioned a series of Earth-orbiting space workshops based on the Apollo LM and the Saturn IB rocket S-IVB stage. Apollo CSMs would ferry up to six astronauts at a time to the workshops for progressively longer stays.

Some planners thought that NASA should jump straight from the early space workshops to piloted Mars landing missions using nuclear-thermal propulsion, but others called for a conservative continuation of the evolutionary approach. If the latter had won the day, the mid-1970s might have seen a new-design space station climb to Earth orbit atop an improved Saturn V rocket. Derived from Apollo hardware and new technology tested on board the orbiting workshops, the station would in fact have constituted a prototype interplanetary Mission Module. A crew might have lived on board without resupply or visitors for almost two years to help prepare NASA for its first piloted Mars voyage.

In keeping with the evolutionary approach, the first piloted voyage beyond the Moon might have been a Mars flyby with no piloted Mars landing. The piloted Mars flyby spacecraft, which would have carried a cargo of robotic Mars probes, would have been built around the Mission Module tested in Earth orbit. The mission might have commenced as early as late 1975, when an opportunity to launch a minimum-energy Mars flyby was due to occur.

As they raced past Mars in early 1976, the four flyby astronauts would have released automated probes and turned a suite of sensors mounted on their spacecraft toward Mars and its irregularly shaped moons Phobos and Deimos. They would have reached their greatest distance from the Sun in the Asteroid Belt, so asteroid encounters would have been a possibility. As their Sun-centered elliptical orbit brought them back to Earth's vicinity in 1977, they would have separated in an Apollo CM-derived Earth-return capsule and reentered Earth's atmosphere.

In addition to observing Mars close up, the astronauts would have continued the effort, begun in earnest during Gemini flights and continued on board the Earth-orbiting workshops and prototype interplanetary Mission Module, to determine whether extended piloted missions were medically feasible. The flyby crew might have confirmed, for example, that artificial gravity is a must during years-long interplanetary voyages. Their results would have shaped the next interplanetary mission, which might have taken the form of a piloted Mars orbiter in the spirit of Apollo 8 and Apollo 10, or, if the space agency felt sufficiently confident in its abilities, a Mars orbital mission with a short piloted excursion to the martian surface in the spirit of Apollo 11.

Sixty days from Mars: two of the FLEM mission's astronauts transfer to the Orbiter. After exhaustive systems checks, they fire explosive bolts to cast off the two-part "spacer" linking their spacecraft and the drum-shaped Flyby Spacecraft. A docking mechanism retracts, then small thrusters push the two spacecraft apart.

The Orbiter backs away from the Flyby Spacecraft. The two astronauts on the Flyby Spacecraft inspect the Orbiter's exterior and transmit television of its departure to Mission Control on Earth.

Using its Apollo-type thruster quads, the Orbiter turns away from the Flyby Spacecraft, positioning itself for the separation burn that will cause it to reach Mars 16 days ahead of the Flyby Spacecraft. Visible on both are engines based on the Apollo Lunar Module descent engine design. The four engines on the Flyby Spacecraft are part of the Earth Return Module (ERM) braking stage. The ERM, an Apollo CM with seating for four astronauts, is linked by its nose-mounted docking unit to the aft deck of the Flyby Spacecraft Mission Module. Life support gas and liquid tanks supplying the Mission Module surround the CM; these act as radiation shielding, allowing the CM to serve double-duty as a solar storm shelter.

In January 1966, United Aircraft Research Laboratories engineer R. R. Titus unveiled a proposal for a new intermediate step in spaceflight evolution. He dubbed it FLEM, which stood for "Flyby-Landing Excursion Mode." FLEM missions would, Titus wrote, have a natural place in the evolutionary sequence between piloted Mars flybys and piloted Mars orbiters. A FLEM mission might even have become the basis for an early, very brief, piloted Mars landing.

Titus explained that, in the "standard stopover mode," a label that encompassed Mars orbital and landing missions, all major maneuvers would involve the entire Mars spacecraft. This meant that the Mars spacecraft would need a large mass of propellants, which in turn meant that many expensive heavy-lift rockets would be required to launch the spacecraft, its propellants, and its Earth-orbit departure stages into Earth orbit for assembly.

Propellant mass required would vary greatly from one Earth-Mars transfer opportunity to the next over a roughly 15-year cycle because Mars has a decidedly elliptical orbit. Because of this, the Mars spacecraft and the sequence of launches needed to boost its components and propellants into Earth orbit would have to be redesigned for each standard stopover mode Mars mission.

The United Aircraft engineer added that errors or malfunctions during standard stopover mode "high-risk" Mars capture and escape maneuvers could result in "complete mission failure" because the entire spacecraft would be involved. Because the Mars spacecraft would be very massive already, it would be difficult and costly to include extra propellants that would enable a mission abort that could save the crew.

Titus noted that required propellant mass might be reduced and made to vary less over multiple Earth-Mars transfer opportunities if the spacecraft skimmed through the martian upper atmosphere to slow down and capture into Mars orbit (that is, if it performed aerocapture). If, however, artificial gravity were found to be necessary for crew health, then stowing a spinning artificial-gravity system of sufficient radius behind an aerocapture heat shield would probably prove infeasible. The mission would then have to rely entirely on propulsive braking.

Titus explained that his FLEM concept, in addition to forming a natural evolutionary extension of piloted Mars flybys, would address many inherent problems of the standard stopover mode. One part of the FLEM spacecraft, the "parent" spacecraft, would not capture into Mars orbit. It could include a spinning artificial-gravity system. The other part, the "excursion module," would capture into Mars orbit using chemical rockets or, perhaps, by skimming through the martian atmosphere behind an aerocapture heat shield.

Orbiter alone: immediately following the separation burn, the two astronauts aboard the Orbiter deploy and orient its solar array and high-gain dish antenna; they are identical to those on the Flyby Spacecraft. The array points at the Sun, out of view beyond the upper left corner of the image, while the antenna aims at the bright blue-white dot that is Earth. Just above the Orbiter, half-lit Mars is visible. Also in view are the Orbiter's four science pallets (covered by white rectangular panels), gray airlock hatch, and yellow spacewalk handholds. The airlock, partly surrounded by propellant tanks, serves as the Orbiter's solar storm shelter.

He noted that Earth-Mars transfer opportunities that need less propellant for Earth departure tend to arrive at Mars moving quickly, while opportunities that require more propellant for Earth departure arrive at Mars moving slowly. In the former instance, the excursion module would need a large quantity of propellants to slow down enough for martian gravity to capture it into orbit, so would be the most massive of the two FLEM modules. Because of this, the lower-mass parent spacecraft would ignite its rocket motors to slow down so that the excursion module could reach Mars first.

In the latter case, the excursion module would not need a large mass of propellants to capture into Mars orbit, making it the least massive of the two FLEM spacecraft. It would thus fire rockets to speed up and reach Mars ahead of the parent spacecraft.

Titus calculated that separation 60 days ahead of the Mars flyby would enable the excursion module to reach the planet 16 days ahead of the parent spacecraft; separation 30 days before flyby would enable it to reach Mars when the parent spacecraft was nine days behind it. While it awaited the arrival of its parent, the excursion module might remain in Mars orbit or all or part of it might land on Mars for a stay of several days.

In Mars orbit: during the Mars orbit capture burn, the Orbiter crew stowed the solar array and high-gain antenna to protect them from deceleration damage. Soon after achieving Mars orbit, they re-deployed both, then ejected the panels covering the Orbiter's instrument pallets. The two forward pallets include identical suites of high-resolution film cameras; the aft pair, identical suites of spectrometers. Identical pallets provide redundancy if instruments fail. As the Orbiter circles Mars, passing in and out of daylight, its solar array can temporarily lose lock on the Sun; in the image above, this has occurred, so the array reflects a crater under scrutiny by the Orbiter's instruments.

The Orbiter crew is not inactive as the Mars-facing instrument pallets record images and other data. In addition to maintaining Orbiter systems, they use handheld and porthole-mounted cameras to capture images of "variable phenomena." These include clouds, dust storms, morning fogs and frosts, and the retreating or expanding edges of the seasonally variable polar ice caps. The Orbiter's single-deck Mission Module has eight portholes.

FLEM, Titus noted, offered a "partial success capability" which, he opined, "may be very attractive." If the excursion module were lost, then the astronauts remaining on board the parent spacecraft could still return safely to Earth. If the excursion module were found during pre-separation checkout to be incapable of accomplishing its mission, it would not undock from the parent spacecraft, and the FLEM mission could still achieve some of its goals while settling for a Mars flyby.

Assuming that the mission took place as planned, the excursion module would ignite its rocket motors as the parent spacecraft passed Mars to depart Mars orbit and catch up with it. Following rendezvous, docking, and crew transfer, the excursion module would be cast off.

Sixteen days after Orbiter capture into Mars orbit, the Flyby Spacecraft raced past Mars. The Orbiter crew ejected their spacecraft's solar array and ignited its engines to catch up with their ride home. In this image, the two astronauts on board the Flyby Spacecraft (left) have turned on rendezvous lights. The Orbiter engines glow red after the "chase" maneuver. Its antenna is positioned so it can serve as a handrail should an emergency spacewalk to the Flyby Spacecraft be needed. The Orbiter halts near the Flyby Spacecraft, then its crew steps outside to collect film cassettes from the four pallets. The Flyby Spacecraft crew records the spacewalk and inspects the Orbiter exterior for damage that might interfere with docking. The Orbiter astronauts stow the recovered cassettes inside their spacecraft, then dock with the Flyby Spacecraft. The reunited FLEM astronauts then transfer film cassettes to the Flyby Spacecraft and discard the Orbiter.

To squeeze even more benefit from FLEM, Titus proposed a variant of the standard ballistic flyby (that is, one in which the only major propulsive maneuver would occur at the start of the mission, when the spacecraft departed Earth orbit). His "powered flyby" would include an optional propulsive maneuver near Mars that would dramatically reduce FLEM spacecraft mass during unfavorable Earth-Mars transfer opportunities, limit the wide swings in propellant mass required from one Earth-Mars transfer opportunity to the next, and slash total trip time.

The maneuver would be optional in the sense that, if it could not occur, the FLEM spacecraft's Sun-centered orbit would return it to Earth, though after a longer than expected trip. During return to Earth after a powered flyby, the FLEM spacecraft would pass as close to the Sun as orbits the planet Mercury.

Titus determined that a powered-flyby maneuver in 1971 would have almost no effect on spacecraft mass at Earth-orbit departure — both the standard ballistic and powered-flyby FLEM spacecraft would have a mass of about 400,000 pounds — but would slash trip time from 510 to 430 days. The most dramatic improvement would occur in 1978, when the ballistic-flyby FLEM spacecraft's mass would total nearly two million pounds and its mission would last 540 days. The powered-flyby FLEM spacecraft would have a mass of just 800,000 pounds at the start of Earth-orbit departure and its mission would last only 455 days.

The FLEM concept apparently influenced NASA piloted flyby studies that occurred under the auspices of the Planetary Joint Action Group (JAG). The NASA Headquarters-led Planetary JAG, which met between 1965 and 1968, included representatives from NASA Marshall Space Flight Center, NASA Kennedy Space Center, and the NASA Manned Spacecraft Center, as well as Washington, DC-based planning contractor Bellcomm. The Planetary JAG's work will be described in detail in subsequent posts.

FLEM was a mission mode without a spacecraft for accomplishing it — but no longer. For this post, artist William Black worked with the author to create a plausible FLEM spacecraft based on flown and conceptual 1960s space hardware. In keeping with the post's overriding theme, we put FLEM into an evolutionary program. We assumed at first that NASA would launch a piloted Mars flyby in 1975 and follow it in 1978 with a FLEM mission that would include a powered Mars flyby. As explained in the post text, a powered flyby would reduce mission duration and, in 1978, cut by more than half the quantity of propellants needed to reach Mars. It would, however, also cause the FLEM Flyby Spacecraft to pass as near the Sun as the orbit of Mercury. Closest approach to the Sun would occur in 1979, during a period of maximum solar activity. Because of this, we rejected both 1978 flyby opportunity and the powered flyby. We chose instead a Mars flyby launched in 1981 for our FLEM mission, when solar activity would be in decline. Earth-return speed would be high (about 50,000 feet per second), so we decided to employ a maneuver pioneered during the 1975 flyby: a braking burn during Earth approach to reduce reentry speed to Apollo lunar-return speed (36,000 feet per second).

The two images above show the Apollo CM-based Earth-Return Module (ERM) and its braking stage during final Earth approach. The ERM/braking stage would have backed out of its "hangar" in the Flyby Spacecraft two days before the events portrayed here. In the top image, the braking stage engines have just shut down after the braking burn; their engine bells still glow red. In the bottom image, the spent braking stage has been cast off. Below the ERM, city lights outline the Indian subcontinent; in the background, over the western Pacific, dawn glows. The ERM will soon streak through the atmosphere over China and Japan, deploy four parachutes, and descend to a mid-morning splashdown southwest of Hawaii, completing the first FLEM voyage and a new evolutionary step toward humans on Mars.

All images in this post are Copyright © 2017 by William Black (http://william-black.deviantart.com/) and are used by special arrangement with the artist.

Sources

Manned Mars and.or Venus Flyby Vehicle Systems Study — Final Briefing Brochure, SID 65-761-6, North American Aviation, 18 June 1965.

"FLEM - Flyby-Landing Excursion Mode," AIAA Paper 66-36, R. R. Titus; paper presented at the 3rd AIAA Aerospace Sciences Meeting in New York, New York, 24-26 January 1966.

Planetary Exploration Utilizing a Manned Flight System, Planetary Joint Action Group, NASA Office of Manned Space Flight, 3 October 1966.

"Manned Expeditions to Mars and Venus," E. Z. Gray and F. Dixon, Voyage to the Planets, Proceedings of the Fifth Goddard Memorial Symposium, 14-15 March 1967, pp. 107-135.

Wonderful Life: The Burgess Shale and the Nature of History, Stephen Jay Gould, W. W. Norton & Co., 1990.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests

To Mars by Way of Eros (1966)

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980 (1967)

NASA Glenn Research Center's 2001 Plan to Land Humans on Mars Three Years Ago

August 2014. Image credit: NASA.

In October 2001, at the 52nd International Astronautical Congress in the European aerospace center of Toulouse, France, nuclear propulsion engineers at NASA's Glenn Research Center (GRC) in Cleveland, Ohio, led by Stanley K. Borowski, Advanced Concepts Manager in GRC's Space Transportation Project Office, described a variant of NASA's 1998 Mars Design Reference Mission (DRM) based on Bimodal Nuclear-Thermal Rocket (BNTR) propulsion. The BNTR DRM concept, first described publicly in July 1998, evolved from nuclear-thermal rocket mission designs Borowski and his colleagues had developed during President George H. W. Bush's abortive Space Exploration Initiative (SEI), which got its start with a July 1989 presidential speech commemorating the 20th anniversary of Apollo 11, the first piloted moon landing mission.

This post contains more than its share of significant acronyms. As an aid to the reader, these are grouped alphabetically and defined at the bottom of the post, just ahead of the list of sources.

NASA's first Mars DRM, designated DRM 1.0 in 1997, was developed by a NASA-wide team during the 1992-1993 period. It was based on Martin Marietta's 1990 Mars Direct mission plan. SEI's demise temporarily halted NASA Mars DRM work in 1994.

The civilian space agency resumed its Mars DRM studies after the announcement in August 1996 of the discovery of possible microfossils in martian meteorite ALH 84001. This enabled NASA planners to release their baseline chemical-propulsion DRM 3.0 in 1998. There was no official DRM 2.0, though a "scrubbed" (that is, mass-reduced) version of DRM 1.0 bears that designation in at least one NASA document.

Shortly thereafter, NASA's Johnson Space Center (JSC) in Houston, Texas, which led the DRM study effort, was diverted from DRM work by the in-house COMBO lander study (more on this below). Left largely to its own devices, NASA GRC developed a pair of DRM 3.0 variants: a solar-electric propulsion (SEP) DRM 3.0 and the BNTR DRM 3.0 discussed here.

In BNTR DRM 3.0, two unpiloted spacecraft would leave Earth for Mars during the 2011 low-energy Mars-Earth transfer opportunity, and a third, bearing the crew, would depart for Mars during the corresponding opportunity in 2014. Components for the three spacecraft would reach Earth orbit on six Shuttle-Derived Heavy-Lift Vehicles (SDHLVs), each capable of launching 80 tons into 220-mile-high assembly orbit, and in the payload bay of a winged, reusable Space Shuttle Orbiter, which would also deliver the Mars crew.

The SDHLV, often designated "Magnum," was a NASA Marshall Space Flight Center conceptual design. The Magnum booster would burn liquid hydrogen (LH2)/liquid oxygen (LOX) chemical propellants in its core stages and solid propellant in its side-mounted boosters. Magnum drew upon existing Space Shuttle hardware: its core stages were derived from the Space Shuttle External Tank and its twin solid-propellant rocket boosters were based on the Shuttle's twin Solid-Rocket Boosters.

The mighty Magnum was the conceptual ancestor of the equally conceptual Ares V and the Space Launch System, now under development. Image credit: NASA.

SDHLV 1 would launch BNTR stage 1 with 47 tons of LH2 propellant on board. Each BNTR DRM mission would need three 28-meter-long, 7.4-meter-diameter BNTR stages. The BNTR stages would each include three 15,000-pound-thrust BNTR engines developed as part of a joint U.S./Russian research project in 1992-1993.

SDHLV 2 would boost an unpiloted 62.2-ton cargo lander into assembly orbit. The cargo lander would include a bullet-shaped Mars aerobrake and entry heat shield (this would double as the cargo lander's Earth launch shroud), parachutes for landing, a descent stage, a 25.8-ton Mars surface payload including an in-situ resource utilization (ISRU) propellant factory, four tons of "seed" LH2 to begin the process of manufacturing propellants on Mars, and a partly fueled Mars Ascent Vehicle (MAV) made up of a conical Earth Crew Return Vehicle (ECRV) capsule and an ascent stage. The cargo and habitat lander engines would burn liquid methane fuel and LOX.

SDHLV launch 3, identical to SDHLV launch 1, would place into assembly orbit BNTR stage 2 containing 46 tons of LH2 propellant. SDHLV launch 4 would place the unpiloted 60.5-ton habitat lander into assembly orbit. The habitat lander would include a Mars aerobrake & entry shield/launch shroud identical to that of the cargo lander, parachutes, a descent stage, and a 32.7-ton payload including the crew's Mars surface living quarters.

The BNTR stage forward section would include chemical thrusters. These would provide maneuvering capability so that the stages could dock with the habitat and cargo landers in assembly orbit. During flight to Mars, the thrusters would provide each stage/lander combination with attitude control.

2011: the unmanned BNTR 1 stage/cargo lander and BNTR 2 stage/habitat lander spacecraft orbit the Earth prior to departure for Mars. Image credit: NASA.

The BNTR 1/cargo lander combination would have a mass of 133.7 tons, while the BNTR 2/habitat lander combination would have a mass of 131 tons. Both combinations would measure 57.5 meters long. As the 2011 launch window for Mars opened, the BNTR stages would fire their engines to depart assembly orbit for Mars.

Each BNTR engine would include a nuclear reactor. When moderator elements were removed from its nuclear fuel elements, the reactor would heat up. To cool the reactor so that it would not melt, turbopumps would drive LH2 propellant through it. The reactor would transfer heat to the propellant, which would become an expanding very hot gas and vent through an LH2-cooled nozzle. This would propel the spacecraft through space.

Following completion of Earth-orbit departure, the BNTR engine reactors would switch to electricity-generation mode. In this mode, they would operate at a lower temperature than in propulsion mode, but would still be capable of heating a working fluid that would drive three turbine generators. Together the generators would make 50 kilowatts of electricity. Fifteen kilowatts would power a refrigeration system in the BNTR stage that would prevent the LH2 it contained from boiling and escaping.

Much like the LH2 propellant in BNTR propulsion mode, the working fluid would cool the reactor; unlike the LH2, however, it would not be vented into space. After leaving the turbine generators, it would pass through a labyrinth of tubes in radiators mounted on the BNTR stage to discard leftover heat, then would cycle through the reactors again. The cycle would repeat continuously throughout the journey to Mars.

2012: Cargo lander/Mars Ascent Vehicle Landing. Image credit: NASA.

As Mars loomed large ahead, the turbine generators would charge the lander batteries. The BNTR stages would then separate and fire their engines to miss Mars and enter a safe disposal orbit around the Sun. The landers, meanwhile, would aerobrake in the martian upper atmosphere. The habitat lander would capture into Mars orbit and extend twin solar arrays to generate electricity. The cargo lander would capture into orbit, then fire six engines to deorbit and enter the atmosphere a second time.

After casting off its heat shield, it would deploy three parachutes. The engines would fire again, then landing legs would deploy just before touchdown. The GRC engineers opted for a horizontal landing configuration; this would, they explained, prevent tipping and provide the astronauts with easy access to the lander's cargo.

As illustrated in the cargo lander image above and the MAV launch image below, the four MAV engines would serve double-duty as cargo lander engines. In addition to saving mass by eliminating redundant engines, this would test-fire the engines before the crew used them as MAV ascent engines.

2012: Automated propellant manufacture for MAV ascent begins. Image credit: NASA.

The cargo lander would touch down on Mars with virtually empty tanks. After touchdown, a teleoperated cart bearing a nuclear power source would lower to the ground and trundle away trailing a power cable. Controllers on Earth would attempt to position it so that the radiation it emitted would not harm the astronauts (for example, behind a sand dune or boulder pile). The reactor's first job would be to power the lander's ISRU propellant plant, which over several months would react the seed hydrogen brought from Earth with martian atmospheric carbon dioxide in the presence of a catalyst to produce 39.5 tons of liquid methane fuel and LOX oxidizer for the MAV ascent engines.

SDHLV launch 5, identical to SDHLV launches 1 and 3, would mark the start of launches for the 2014 Earth-Mars transfer opportunity. It would place BNTR stage 3 into assembly orbit with about 48 tons of LH2 on board. Because it would propel a piloted spacecraft, its BNTR engines would require a new design feature: each would include a 3.24-ton shield to protect the crew from the radiation it produced while in operation. The shields each would create a conical radiation "shadow"; the radiation shadows would overlap to create a safe zone in which the crew would remain while they were inside or close to their spacecraft.

2013: the BNTR 3 stage and the first Crew Transfer Vehicle components dock automatically in Earth orbit. Image credit: NASA.

Thirty days after SDHLV launch 5, SDHLV launch 6 would place into assembly orbit a 5.1-ton spare Earth Crew Return Vehicle (ECRV) attached to the front of an 11.6-ton truss. A 17-meter-long tank with 43 tons of LH2 and a two-meter-long drum-shaped logistics module containing 6.9 tons of contingency supplies would nest along the truss's length. BNTR stage 3 and the truss assembly would rendezvous and dock, then propellant lines would automatically link the truss tank to BNTR stage 3.

A Shuttle Orbiter carrying the Mars crew and a 20.5-ton deflated Transhab module would rendezvous with the BNTR stage 3/truss combination one week before the crew's planned departure for Mars. Following rendezvous, the spare ECRV would undock from the truss and fly automatically to a docking port in the Space Shuttle payload bay. Astronauts would then use the Orbiter's robot arm to hoist the Transhab from the payload bay and dock it to the front of the truss in the spare ECRV's place.

2014: Crew and a deflated Transhab arrive on board a Space Shuttle Orbiter to complete Crew Transfer Vehicle assembly. Image credit: NASA.

The Mars astronauts would enter the spare ECRV and pilot it to a docking at a port on the Transhab's front, then enter the cylindrical Transhab's solid core and inflate its fabric-walled outer volume. The inflated Transhab would measure 9.4 meters in diameter. Unstowing floor panels and furnishings from the core and installing them in the inflated volume would complete assembly. Transhab, truss, and BNTR stage 3 would make up the 64.2-meter-long, 166.4-ton Crew Transfer Vehicle (CTV).

The CTV's truss-mounted tank and BNTR stage 3 would hold 90.8 tons of LH2 at the start of CTV Earth-orbit departure on 21 January 2014. The truss tank would provide 70% of the propellant needed for departure. In the most demanding departure scenario, the BNTR engines would fire twice for 22.7 minutes each time to push the CTV out of Earth orbit toward Mars.

2014: Crew Transfer Vehicle departs Earth orbit. Image credit: NASA.

Transhab cutaway (weightless design). Floor and ceiling would be reversed in the NASA Glenn artificial-gravity design. "Down" would thus be toward the top of this image, where the airlock and Earth Crew Return Vehicle capsule would be located. Image credit: NASA.

Following Earth-orbit departure, the crew would jettison the empty truss tank and use small chemical-propellant thrusters to start the CTV rotating end over end at a rate of 3.7 rotations per minute. This would create acceleration equal to one Mars gravity (38% of Earth gravity) in the Transhab module. Artificial gravity was a late addition to BNTR DRM 3.0; it made its first appearance in a June 1999 paper, not in the original July 1998 paper describing BNTR DRM 3.0.

In artificial-gravity mode, "down" would be toward the spare ECRV on the CTV's nose; this would make the Transhab's forward half its lower deck. Halfway to Mars, about 105 days out from Earth, the astronauts would stop rotation and perform a course-correction burn using the attitude-control thrusters. They would then resume rotation for the remainder of the trans-Mars trip.

The CTV would arrive in Mars orbit on 19 August 2014. The crew would halt rotation, then three BNTR engines would fire for 12.3 minutes to slow the spacecraft for Mars orbit capture. In its loosely bound elliptical Mars orbit, the spacecraft would circle the planet once per 24.6-hour martian day.

2014: Crew Transfer Vehicle arrival in Mars orbit. Image credit: NASA.

The crew would pilot the CTV to rendezvous with the habitat lander waiting in Mars orbit, taking care to place it in the CTV's radiation shadow. If the cargo lander on the surface or the habitat lander in Mars orbit malfunctioned while awaiting the crew's arrival, then the crew would remain in the CTV in Mars orbit until Mars and Earth aligned for the flight home (a wait time of 502 days). They would survive by drawing upon contingency supplies in the drum-shaped logistics module attached to the truss.

If the orbiting habitat lander and landed cargo lander checked out as healthy, however, then the crew would fly the spare ECRV to a docking port on the habitat lander's side. After discarding the spare ECRV and the habitat solar arrays, they would fire the habitat lander's engines, enter the martian atmosphere, and land near the cargo lander.

The habitat lander's horizontal configuration would provide the astronauts with ready access to the martian surface. After the historic first footsteps on Mars, the astronauts would inflate a Transhab-type habitat attached to the side of the habitat lander, run a cable from the habitat lander to the nuclear power source cart, unload at least one unpressurized crew rover, and commence a program of Mars surface exploration that would, if all went as planned, last for nearly 17 months.

In case of hardware failure or other emergency, the crew could retreat to the MAV and return early to the orbiting CTV. They would, however, have to wait in Mars orbit until Mars and Earth aligned to permit a minimum-energy Mars-Earth transfer (that is, until the originally planned end of their stay at Mars).

2014-2015: The first Mars campsite. In the foreground is the habitat lander with inflated Transhab surface habitat; in the background, the nuclear power source cart and the cargo lander with Mars Ascent Vehicle. Image credit: NASA.

2014-2015: Exploring Mars with a crew rover and two teleoperated robot rovers, one small and one large. Image credit: NASA.

2014-2015: Drilling for water, geologic history, and, just possibly, life. Image credit: NASA.

2015: Mars Ascent Vehicle liftoff. Image credit: NASA.

Near the end of the surface mission, the unmanned CTV would briefly fire its nuclear engines to trim its orbit for the crew's return. The MAV bearing the crew and about 90 kilograms of Mars samples would then lift off. Taking care to remain within the the radiation shadows of the CTV's BNTR engines, it would dock at the front of the Transhab, then the astronauts would transfer to the CTV. They would cast off the spent MAV ascent stage, but would retain the MAV ECRV for Earth atmosphere reentry.

The CTV would leave Mars orbit on 3 January 2016. Prior to Mars orbit departure, the astronauts would abandon the contingency supply module on the truss to reduce their spacecraft's mass so that the propellant remaining in BNTR stage 3 would be sufficient to launch them home to Earth. They would then fire the BNTR engines for 2.9 minutes to change the CTV's orbital plane, then again for 5.2 minutes to escape Mars and place themselves on course for Earth.

Soon after completion of the second burn, the crew would fire attitude-control thrusters to spin the CTV end-over-end to create acceleration equal to one Mars gravity in the Transhab. About halfway home they would stop rotation, perform a course correction, then resume rotation. Flight home to Earth would last 190 days.

2016: Return to the Earth-Moon system. Image credit: NASA.

Near Earth, the crew would stop CTV rotation for the final time, enter the MAV ECRV with their Mars samples, and undock from the CTV, again taking care to remain in the BNTR engine radiation shadows as they moved away. The abandoned CTV would fly past Earth and enter solar orbit. The MAV ECRV, meanwhile, would re-enter Earth's atmosphere on 11 July 2016.

The authors compared their Mars plan with the baseline chemical-propulsion DRM 3.0 and with the NASA GRC SEP DRM 3.0. They found that their plan would need eight vehicle elements, of which four would have designs unique to BNTR DRM 3.0. The baseline DRM 3.0, by contrast, would need 14 vehicle elements, 10 of which would be unique, and SEP DRM 3.0 would need 13.5 vehicle elements, 9.5 of which would be unique. BNTR DRM 3.0 would require that 431 tons of hardware and propellants be placed into Earth orbit; the baseline DRM 3.0 would need 657 tons and SEP DRM 3.0, 478 tons. Borowski and his colleagues argued that fewer vehicle designs and reduced mass would mean reduced cost and mission complexity.

The BNTR DRM 3.0 variant became the basis for DRM 4.0, which was developed during NASA-wide studies in 2001-2002 (though NASA documents occasionally back-date DRM 4.0 to 1998, when BNTR DRM 3.0 was first proposed). DRM 4.0 differed from BNTR DRM 3.0 mainly in that it adopted a "Dual Lander" design concept developed as part of JSC's 1998-1999 COMBO lander study. COMBO was the brainchild of William Schneider, NASA JSC Engineering Directorate boss.

Dual Lander concept. The lander in the foreground is the habitat; the background lander is the Mars Descent/Ascent Vehicle. Image credit: NASA.

The Dual Lander concept grew from COMBO's main design guideline, which was to develop a low-mass "Apollo-style" piloted Mars landing mission. A major change from past Mars DRMs was no reliance on ISRU. As in BNTR DRM 3.0, two cargo missions would leave Earth one minimum-energy Earth-Mars transfer opportunity ahead of the crew; in DRM 4.0, however, these would take the form of a Mars lander that would also include an ascent vehicle for returning the crew to the CTV in Mars orbit and a cargo lander with an inflatable donut-shaped habitat. The former could by itself support a short-stay (~30-day) Mars surface mission; the latter would enable a Mars surface stay of more than 400 days.

In 2008, a decade after BNTR DRM 3.0 first was made public, NASA released a version of DRM 4.0 modified to use planned Constellation Program hardware (for example, the Ares V heavy-lift rocket in place of the Magnum and the Orion Multi-Purpose Crew Vehicle in place of the ECRVs). The space agency dubbed the new DRM Design Reference Architecture (DRA) 5.0.

The DRA 5.0 Mars plan acknowledged that, largely as a result of the 1 February 2003 Columbia accident, the Space Shuttle would be retired after the remaining Orbiters — Endeavour, Discovery, and Atlantis — completed their part of the task of building the International Space Station. The last Space Shuttle mission, STS-135, took place in July 2011.

DRA 5.0 also saw the return of ISRU. A Descent/Ascent Vehicle (DAV) and a Surface Habitat (SHAB) would capture into Mars orbit in the first minimum-energy Earth-Mars transfer opportunity. The DAV would descend, land, and begin making propellants for its ascent stage. The SHAB would loiter in orbit awaiting arrival of a crew on board a Mars Transfer Vehicle (MTV) launched from Earth during the second Earth-Mars transfer opportunity of the mission. The crew would transfer to the SHAB in an Orion/service module and land on Mars near the DAV. After a stay on Mars lasting more than 400 days, they would lift off in the DAV ascent stage, dock with the waiting MTV, and return to Earth.

Though DRA 5.0 exerts influence on current NASA planning, the precise form a piloted Mars mission will eventually take remains unclear at this writing. NASA increasingly has shifted its attention toward finding low-cost stepping stones that could lead to a piloted Mars landing in 2033. A crew-tended — that is, not permanently staffed — Deep Space Gateway space station in cislunar space, for example, could be established by 2026 through a series of Orion missions launched using the Space Launch System (SLS) heavy-lift rocket (SLS replaced Ares V in 2010). Other possible interim steps toward Mars include an SLS-launched robotic Mars sample return mission in the mid-2020s and a piloted mission to Mars orbit in 2030 using a Deep Space Transport based partly on Deep Space Gateway hardware.

Acronyms

BNTR = Bimodal Nuclear Thermal Rocket
CTV = Crew Transfer Vehicle
DAV = Descent/Ascent Vehicle
DRA = Design Reference Architecture
DRM = Design Reference Mission
ECRV = Earth Crew Return Vehicle
GRC = Glenn Research Center
ISRU = In-Situ Resource Utilization
JSC = Johnson Space Center
LH2 = liquid hydrogen
LOX = liquid oxygen
MAV = Mars Ascent Vehicle
MTV = Mars Transfer Vehicle
SDHLV = Shuttle-Derived Heavy-Lift Vehicle
SEI = Space Exploration Initiative
SEP = Solar-Electric Propulsion
SHAB = Surface Habitat
SLS = Space Launch System

Sources

"Bimodal Nuclear Thermal Rocket (NTR) Propulsion for Power-Rich, Artificial Gravity Human Exploration Missions to Mars," IAA-01-IAA.13.3.05, Stanley K. Borowski, Leonard A. Dudzinski, and Melissa L. McGuire; paper presented at the 52nd International Astronautical Congress in Toulouse, France, 1-5 October 2001.

"Vehicle and Mission Design Options for the Human Exploration of Mars/Phobos Using 'Bimodal' NTR and LANTR Propulsion," AIAA-98-3883, Stanley K. Borowski, Leonard A. Dudzinski, and Melissa L. McGuire; paper presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit in Cleveland, Ohio, 13-15 July 1998.

"Artificial Gravity Vehicle Design Option for NASA's Human Mars Mission Using 'Bimodal' NTR Propulsion," AIAA-99-2545, Stanley K. Borowski, Leonard A. Dudzinski, and Melissa L. McGuire; paper presented at the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit in Los Angeles, California, 20-24 June 1999.

NASA Exploration Team (NEXT) Design Reference Missions Summary, NASA, 12 July 2002 [draft].

"Enabling Human Deep Space Exploration with the Deep Space Gateway," Tim Cichan, Bill Pratt, and Kerry Timmons, Lockheed Martin; presentation to the Future In-Space Operations telecon, 30 August 2017.

More Information

Humans on Mars in 1995! (1980-1981)

Bridging the Gap Between Space Station and Mars: The IMUSE Strategy (1985)

The Collins Task Force Says Aim for Mars (1987)

Sally Ride's Mission to Mars (1987)

Footsteps to Mars (1993)