27 September 2017

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 will detach and vent leftover propellants to nudge its course so that it will 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 based on a 1966 NASA concept. 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 televise 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 Mars's 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 Mars's 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 while 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 redeployed 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 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 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.

Titus's FLEM concept influenced NASA piloted flyby studies that took place 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, including the Mars Surface Sample Return concept that FLEM apparently inspired, 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 conceptua1960s 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 Mercury's orbit. Closest approach to the Sun would occur in 1979, during a period of maximum solar activity. Because of this, we rejected both the 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)

09 September 2017

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 Mars's 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 Earth. 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 reenter 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

A Forgotten Pioneer of Mars Resource Utilization (1962-1963)

Two For The Price of One: 1980s Piloted Missions With Stopovers at Mars and Venus (1969)

Think Big: A 1970 Flight Plan for NASA's 1969 Integrated Program Plan

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)