07 December 2018

Purple Pigeon: Mars Multi-Rover Mission (1977)

Image credit: JPL/NASA
Planetary scientist Bruce Murray became director of the Jet Propulsion Laboratory (JPL) in April 1976, just three months before Viking 1 was due to land on the northern plains of Mars. Though NASA's Langley Research Center managed Project Viking, JPL included Viking Mission Control. When Viking 1 landed, JPL could expect to play host to hundreds of journalists from all over the Earth.

According to his 1989 memoir Journey into Space: The First Thirty Years of Space Exploration, Murray saw this as an opportunity. He quickly assembled a group of six engineers to propose planetary missions that he could pitch to the journalists and, through them, to U.S. taxpayers.

The missions, which Murray dubbed "Purple Pigeons," were intended to include both "high science content" and "excitement and drama [that would] garner public support." They were called Purple Pigeons to differentiate them from "Gray Mice," unexciting and timid missions which Murray felt would help to ensure that JPL had no future in the space exploration business. By August 1976, the Purple Pigeons flock included a solar sail mission to Halley's Comet, a Mars Surface Sample Return (MSSR), a Venus radar mapper, a Saturn/Titan orbiter/lander, a Ganymede lander, an asteroid tour, and an automated lunar base.

Bruce Murray, JPL director from April 1976 until June 1982. Image creditI JPL/Caltech
The Purple Pigeons effort continued even after Viking 2 landed (3 September 1976) and all the journalists went home. In a February 1977 JPL report, for example, JPL engineers described a Purple Pigeon mission that would explore Mars with up to four rovers simultaneously. The Viking-based multi-rover mission would include a pair of identical 4800-kilogram spacecraft, each consisting of a Viking-type orbiter and a 1578-kilogram Mars lander bearing twin 222.4-kilogram rovers. The rovers would, the report stated, perform traverses to "regions difficult to reach by direct landings." This would, it added, fill the gap between "detailed information" from MSSR missions and "global information" from Mars orbiters.

The image at the top of this post shows a somewhat different (probably later) multi-rover mission design. Its four six-wheel, multi-cab rovers (two of which are operating out of view over the horizon) rely on a single Viking orbiter-type spacecraft to relay radio signals to and from Earth. In principle, however, it is identical to the early multi-rover mission design described in this post.

Most MSSR plans of the 1970s assumed a "grab" sample; that is, that the stationary MSSR lander would return to Earth a sample of whatever rocks and soil happened to be within reach of its robotic sample scoop. The report suggested that the rovers of the multi-rover mission might enhance a follow-on MSSR mission by collecting and storing samples as they roved across the planet. After the MSSR lander arrived on Mars, the rovers would rendezvous with it and hand over their samples for return to Earth. The report contended that its multi-rover/MSSR strategy would be "an enormous advance over even multiple grab samples" collected by MSSR landers at widely scattered sites.

At the time the Purple Pigeons team proposed the multi-rover mission, NASA intended to launch all payloads, including interplanetary spacecraft, on board reusable Space Shuttles. The Shuttle orbiter would be able to climb no higher than about 500 kilometers, so launching payloads to higher Earth orbits or interplanetary destinations would demand an upper stage. The powerful liquid-propellant Centaur upper stage would not be ready in time for the opening of the Mars multi-rover launch window, which spanned from 11 December 1983 to 20 January 1984, so JPL tapped a three-stage solid-propellant Interim Upper Stage (IUS) to push its Purple Pigeon out of Earth orbit toward Mars.

After an Earth-Mars cruise lasting about nine months, the twin multi-rover spacecraft would arrive at Mars a week or two apart between 16 September and 27 October 1984. They would each fire their main engines to slow down so that Mars gravity could capture them into an elliptical orbit with a periapsis (low point) of 500 kilometers, a five-day period, and an inclination of 35° relative to the martian equator.

The multi-rover landers would then separate and each fire a solid-propellant de-orbit rocket at the apoapsis (high point) of its orbit to begin descent to the surface. Landing sites between 50° north latitude and the south pole would in theory be accessible, though the need for a direct Earth-to-rover radio link would in practice prevent landings below 55° south.

The landers would each be encased within an aeroshell with a heatshield for protection during the fiery descent through the martian atmosphere. The aeroshell would have the same 3.5-meter diameter as its Viking predecessor, though its afterbody would be modified to make room for the large cooling vanes of the twin rovers' electricity-producing Radioisotope Thermal Generators (RTGs).

JPL's dual rovers packed inside their modified Viking-type aeroshell. Image credit: JPL
After the landers touched down, the orbiters would maneuver to areosynchronous orbit. In such an orbit, 17,058 kilometers above the martian equator, only minor orbital corrections would enable a spacecraft to "hover" indefinitely over one spot on the equator. Each orbiter would position itself over a spot on the equator near its lander's longitude so that it could relay radio signals between its rovers on Mars and operators on Earth.

The multi-rover lander, which would serve no purpose beyond rover delivery, would constitute a radical departure from the triangular Viking lander design, though it would use Viking technology where possible to save development costs. It would comprise a rectangular frame to which would be attached three uprated Viking-type terminal descent engines, two spherical propellant tanks, and three beefed-up Viking-type landing legs.

Multi-rover lander. Image credit: JPL
The 1.5-meter-long rovers would be mounted on the lander frame with their four 0.5-meter-diameter wire wheels compressed. Releasing a latching mechanism would permit the wheels to expand, raising the rover off four stabilizing "taper pins." The pins and one terminal descent engine would then swing out of the way, ramps would deploy, and the first rover would roll onto the rocky martian surface. The second rover would then ride a motor-driven "dolly" to the first rover's initial position before unlatching and joining its twin on the ground.

JPL envisioned that its four-wheeled rovers would each deploy a one-meter-tall boom holding a still-image camera, a floodlight, a strobe light, a weather station, and a pointable horn-shaped radio antenna. The camera/antenna boom, the tallest part of the rover, would stand about two meters above the surface. Controllers on Earth would then put the rovers through an initial checkout lasting at least two weeks. The checkout would culminate in slow "manual" (Earth-controlled) and faster semi-autonomous (Earth-directed but rover-controlled) traverses.

JPL's nuclear-powered rover viewed from above (top) and from the side. Image credit: JPL 
In semi-autonomous mode, operators would plan traverse routes and science targets using stereo images from the rover camera taken from terrain "high points," then would command the rover to proceed. The rovers might assist each other in traverse planning; for example, "high point" pictures from one might fill in blind spots in the other's field of view. "After the first few kilometers of traverse," the JPL engineers assumed, operators on Earth would "begin to build an intuitive feeling for the Martian geography and its impact on the rover capabilities, allowing them to plan better paths." The rovers would also photograph each other to enhance the mission's "general public appeal."

The rover mobility system would include one electric drive motor per wheel, eight proximity sensors for obstacle detection, inclinometers to monitor rover tilt, motor temperature sensors to judge wheel traction, a gyrocompass/odometer, a laser rangefinder with a 30-meter range, and an "8-bit word, 16k active, 64k bulk, floating point arithmetic and 16-bit accuracy" computer. The JPL engineers judged that their rovers would be capable of moving at up to 50 meters per hour over terrain similar to that seen at the Viking 1 landing site.

Sunset at the Viking 1 landing site in Chryse Planitia. Image credit: NASA
Alpha-scattering X-ray fluorescence and gamma-ray spectrometers would collect data while the rovers were in motion, but all other science, including imaging and sample collection, would occur only while they were parked. Each rover would gather samples using an "articulated arm" with an "electromechanical hand."

In order to avoid "an overabundance of data from a single track," the rovers would travel slightly different routes and rendezvous at the end of each leg of their traverse. They would, however, travel close enough together that each could aid the other in the event of trouble. If one rover became stuck in loose dirt, for example, its companion could use its articulated arm to place rocks under its wheels to improve traction. If one rover of a pair failed, the report maintained, the other would continue to yield "good, solid science."

The rovers would be designed to operate for at least one martian year (about two Earth years) to help ensure that at least one of the four could successfully rendezvous with the follow-on MSSR mission, which would leave Earth in 1986. Estimates of rover traverse distances in 1970s and 1980s studies were typically highly optimistic, and the multi-rover mission was no exception: each of the mission's four rovers was expected to travel up to 1000 kilometers. The JPL engineers concluded their report by calling for new technology development to ensure that adequate power and mobility systems would become available by the time their Purple Pigeon was due to fly.


Journey into Space: The First Thirty Years of Space Exploration, Bruce Murray, W. W. Norton & Co., 1989

Feasibility of a Mars Multi-Rover Mission, JPL 760-160, Jet Propulsion Laboratory, 28 February 1977

More Information

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

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

Making Propellants from Martian Air (1978)

01 December 2018

Exploring Mars from Pole to Pole: MESUR Network (1991)

Pioneer Venus 2 releases its three small Venus atmosphere entry probes. Through artist license, the large probe is visible against the clouds of Venus; it would not in fact have been visible at the time the small probes were released. Image credit: NASA
On 8 August 1978, NASA launched Pioneer Venus 2 (PV2) on an Atlas-Centaur rocket. The 904-kilogram spacecraft, known also as Pioneer Venus Multiprobe, released a 1.5-meter-diameter battery-powered atmosphere entry probe on 16 November and three 76-centimeter-diameter probes on 20 November.

On 9 December 1978, the five parts of PV2 entered the thick, hot Venusian atmosphere. The drum-shaped probe carrier burned up as planned at an altitude of 110 kilometers. Sturdy conical heat shields protected the spherical instrumented probes from aerodynamic heating. As drag slowed it, the large probe deployed a parachute.

Two of the small probes, which did not include parachutes, exceeded all expectations by surviving landing and transmitting data from the hellish Venusian surface. One, the Day Probe, transmitted for 67.5 minutes before succumbing to heat, pressure, and battery failure, setting a new world record for spacecraft endurance on Venus.

PV2 was the last U.S. planetary mission launched until 1989. NASA Ames Research Center (ARC), located near San Francisco, California, managed PV2 and its sister spacecraft, PV1 (the Pioneer Venus Orbiter).

In July 1991, ARC proposed a multiprobe system outwardly not too different from PV2, but intended to create a long-lived network of low-cost science stations on Mars. According to ARC's report on the concept, its network would reflect a design philosophy with "unique characteristics . . . derived from the Pioneer Project corporate memory."

Mars networks were first proposed in the early 1970s. Scientific advisory groups endorsed the network concept repeatedly in the following two decades as the best way to obtain global-scale weather and seismic data. In the late 1980s, at the behest of the NASA Headquarters Solar System Exploration Division (SSED), the Jet Propulsion Laboratory (JPL) Precursor Task Team included a network in its program of precursor robotic missions for paving the way for astronauts on Mars. In common with previous Mars network plans, the 1989 plan invoked spear-shaped penetrators to hard-land stations at low cost.

NASA ARC's Mars Environment Survey (MESUR - pronounced "measure"), on the other hand, invoked cheap rough-landing landers, or "stations," that would deploy protective airbags seconds before landing. MESUR would build up a "pole-to-pole" network of 16 stations during the 1999, 2001, and 2003 minimum-energy Mars launch opportunities.

Each 158.5-kilogram MESUR lander would leave Earth attached toa Mars atmosphere entry deceleration system and a simple cruise stage. Upon arrival at Mars, each would cast off its cruise stage and enter the atmosphere directly from its Earth-Mars trajectory at up to seven kilometers per second. The ARC report compared this with the Viking landers, which entered from Mars orbit at only 4.4 kilometers per second. The lander's heat shield, a two-meter-diameter flattened cone, would be designed to withstand atmosphere entry during planet-wide dust storms, when suspended dust particles might exacerbate shield erosion.

Partial cutaway of a MESUR station on the surface of Mars. Image credit: NASA Ames Research Center
The ARC report acknowledged that the disk-shaped lander might bounce to rest on Mars in either "heads" or "tails" orientation, but rejected as costly and risky a mechanical system for tipping it upright. The ARC engineers opted instead for circular ports that would enable controllers to deploy instruments from either side of the station. Instruments might include imagers, an atmospheric structure experiment, gas analyzers, a weather station, a spectrometer, and a seismometer.

The report explained that solar cells were initially ARC's preferred MESUR power system, but analysis had shown that the number of cells that could be mounted on the lander's small surface would not generate enough electricity to drive its science instruments unless landings were limited to sites within 30° of the martian equator. This limitation was deemed unacceptable by the MESUR Science Definition Team, so engineers opted for a small (nine-kilogram) General Purpose Heat Source (GPHS) Radioisotope Thermal Generator (RTG) "brick" based on Ulysses solar polar orbiter/Galileo Jupiter orbiter RTG technology.

Sixteen MESUR landers would need 16 GPHS bricks over six years. The report noted that the entire MESUR Network would need less than half as much plutonium as the Cassini Saturn orbiter, which would carry two RTGs with 18 GPHS bricks each.

Cutaway of the MESUR Network launch shroud showing four MESUR landers (one is mostly obscured behind the lander support structure) and the solid-propellant Mars transfer orbit injection stage. Image: NASA Ames Research Center
The MESUR mission would begin in 1999 with the launch of a single Delta II 7925 rocket from Cape Canaveral, Florida, with four MESUR landers mounted on a framework within its 9.5-foot-diameter streamlined launch shroud. After a solid-propellant upper stage placed them on course for Mars, the landers would separate from the framework to travel on "independent free-flyer trajectories" that would permit precise Mars landing site targeting. Three side-mounted landers would tumble after separation, but sloshing propellants in their cruise stages would gradually damp their gyrations.

The landers would discard their cruise stages 125 kilometers above Mars. Ten kilometers above the planet, each would deploy a pilot parachute, then cast off its heat shield and open its single main parachute. The landers would image the surface and collect atmospheric structure data during the final eight kilometers of descent.

Just two meters above the landing site, each lander would release its main parachute and inflate its airbags. A small rocket on the parachute would ignite to prevent it from settling over the lander.

The MESUR lander design would permit landings at sites up to six kilometers above the base datum, the martian equivalent of Earth's sea level. The base datum, referenced to the minimum Mars atmospheric pressure required for liquid water to exist on the surface, was established after Mariner 9 mapped the planet from orbit in 1971-1972. (In 2001, a new system referenced to the mean radius of Mars as measured by Mars Global Surveyor's MOLA instrument replaced the base datum.)

Though all 16 MESUR landers would carry the same suite of instruments, their individual landing sites would be selected to cater to different science requirements. The report advised that weather stations should be spaced widely over the planet, while seismic stations should form closely spaced "triads." These conflicting requirements forced a "compromise network design."

MESUR Network Stations 1 and 2 would land near each other on the north rim of Valles Marineris to form a "seismic pair." Station 3, at the foot of Olympus Mons in Tharsis, would also emphasize seismic research. Station 4 would aim to extend the weather record for Chryse Planitia, where Viking 1 accumulated data from 1976 to 1983.

The Tharsis hemisphere of Mars showing proposed positions of MESUR stations. See text for explanation. Image credit: NASA
In 2001, two Delta II 7925s would launch 20 days apart bearing four more MESUR landers and a communications relay orbiter, respectively. The latter payload, based on an existing Earth-orbital comsat design, would serve as radio relay for the expanding network, enabling MESUR stations to return data from sites all over the martian surface.

It would reach Mars in 10 months on a slow "Type II" trajectory to reduce the amount of propellant it would need to slow down so that the planet's gravity could capture it. Launch of the communications orbiter would be delayed until 2001 in order to spread its cost over a longer period.

With the successful arrival of the four 2001 stations, a "minimal network" would be in place on Mars. Station 5, on the Marineris north rim, would create a "seismic triad" with Stations 1 and 2, while Station 6, northwest of Olympus Mons, would create a seismic pair with Station 3. Station 7, east of Solis Planum ("a region of known dust storm activity"), and Station 8, in western Acidalia Planum, would expand martian meteorological coverage.

The final two MESUR Delta II 7925 launches in 2003 would boost four landers each on course for Mars. Stations 9 and 10 would be located near the north and south poles, respectively, while Station 11 would report weather conditions in Aonia Terra, southwest of the great Argyre basin. Stations 12 (northwest Hellas), 13 (Elysium Planitia), and 14 (Deuteronilus Mensae) would further extend martian meteorological coverage.

Station 15 (Sirenum Terra) would form a Tharsis seismic triad with Stations 3 and 6. Station 16, in Syrtis Major on the side of Mars opposite Olympus Mons, would create a seismic pair with Station 13 and, with the Tharsis triad, enable the size of Mars's core to be determined.

The Syrtis Major hemisphere of Mars showing proposed positions of MESUR stations. See text for explanation. Image credit: NASA
The entire 16-station network and its communications orbiter would function for at least a martian year (a little more than two Earth years). This would mean that the 1999 stations would have to endure for three martian years (6.5 Earth years), while the 2001 stations and communications orbiter would need to function for two martian years (4.3 Earth years).

In its 1991 strategic plan, published the same month as ARC's MESUR report, the SSED dubbed MESUR its "baseline plan" for a Mars network mission. In November 1991, NASA elected to move MESUR Phase A development to JPL, where the project was split into two parts.

MESUR Network would be preceded by MESUR Pathfinder, a single-spacecraft mission for technology testing. Pathfinder was built larger than the the planned MESUR landers so that it could deliver to Mars a six-wheeled "microrover." JPL also opted for solar power in place of NASA ARC's RTG bricks and a petal system to permit it to flip itself upright and release the rover instead of small instrument deployment ports.

In 1994, in the wake of the Mars Observer failure, NASA funded the Mars Surveyor Program in place of MESUR Network. Work continued on Pathfinder under the auspices of NASA's low-cost Discovery Program, however, and it landed successfully on Mars on 4 July 1997.

Mars Pathfinder Lander (background) and Sojourner rover. Image credit: NASA

Mars Environmental Survey (MESUR) Science Objectives and Mission Description, NASA Ames Research Center, July 19, 1991

Solar System Exploration Division Strategic Plan: Preparing the Way to the New Frontier of the 21st Century, Special Studies Office, Space Telescope Science Institute, July 1991

More Information

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Martian Adventure (1962)

"Essential Data": A 1963 Pitch to Expand NASA's Robotic Exploration Program

Pioneer Mars Orbiter with Penetrators (1974)

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

Touring Titan by Blimp and Buoy (1983)

18 November 2018

Near-Term and Long-Term Goals: Space Station and Lunar Base (1983-1984)

Homeward bound: an Orbital Transfer Vehicle (OTV) bearing a returning lunar base crew aerobrakes in Earth's atmosphere. After aerobraking it will rendezvous with the NASA space station. Image credit: Pat Rawlings/NASA
In December 1983, the Division of Policy Research and Analysis of the National Science Foundation enlisted Science Applications Incorporated (SAI) of McLean, Virginia, to compare the science and technology research potential of an Earth-orbiting space station with that of a base on the moon. In its report, which was completed on 10 January 1984, SAI cautioned that, because its study was performed "in a very short two-week period," it could offer only "a preliminary indication" of the relative merits of a space station in low-Earth orbit (LEO) and a lunar base. Though SAI did not say so, its study had a short turnaround time because its results were meant to inform the White House ahead of President Ronald Reagan's planned announcement of a NASA space station program during his 25 January 1984 State of the Union Address.

SAI explained that its study used a four-step approach. First, the study team judged which science and technology disciplines could best be served by an LEO space station and which by a lunar base. Next, the team developed a lunar base conceptual design capable of serving the disciplines it identified. It then developed a transportation system concept for deploying and maintaining its base. Finally, the team estimated the cost of its lunar base.

The team identified five science and technology disciplines that would be better served by a base on the moon than by a space station. The first was radio astronomy. Bowl-shaped radio telescopes might be built in bowl-shaped lunar craters, SAI wrote. Radio astronomers might take advantage of the moon's Farside (the hemisphere turned permanently away from Earth), where up to 2160 miles of rock would shield their instruments from terrestrial radio interference. The 238,000-mile separation between lunar and terrestrial radio telescopes would permit Very Long Baseline Interferometry observations, enabling astronomers to map minute details of galaxies far beyond the Milky Way.

A bowl-shaped crater makes an ideal site for a bowl-shaped radio telescope. Visible stars are artist's license; the harsh glare of the Sun in lunar daylight would banish them from view. Image credit: NASA
High-energy astrophysics and physics was SAI's second lunar base discipline. The team noted that, because the moon offers "a large, flat area, a free vacuum, and a local source of refined material for magnets," it might become an economical site for a large particle accelerator.

Lunar geology (which SAI called "selenology") would obviously be better served by a lunar base than by a space station. SAI noted that, despite 13 successful U.S. robotic lunar missions and six successful Apollo landings, the moon had "barely been sampled and explored." Lunar base selenological exploration would focus on "understanding better the early history and internal structure of the Moon" and "exploring for possible ore and volatile deposits." Selenologists would rove far afield from the base to measure heat flow and magnetic properties, drill deep into the surface, deploy seismographs, and collect and analyze rock samples.

SAI's fourth lunar discipline was resource utilization. The study team noted that samples returned to Earth by the Apollo astronauts contain 40% oxygen by weight, along with silicon, titanium, and other useful chemical elements. Lunar oxygen could be used as oxidizer for chemical-propulsion spacecraft traveling between Earth and moon and from LEO to geosynchronous Earth orbit (GEO). Silicon could be used to make solar cells. (SAI pointed out, however, that the two-week lunar night would make reliance on solar arrays for electricity "somewhat difficult.") Raw lunar dirt - known as regolith - could serve as radiation shielding. If water ice were found at the lunar poles - perhaps by the automated lunar polar orbiter SAI advised should precede the lunar base program - then the moon might supply hydrogen rocket fuel as well as oxidizer.

SAI's fifth and final lunar base science discipline was systems development. The team expected that lunar base technology development would be "devoted to improving the efficiency and capabilities of systems that support the base," such as life support, with the goal of "reduced reliance on supplies sent from Earth." Transport system development might include research aimed at developing a linear electromagnetic launcher of the kind first proposed by Arthur C. Clarke in 1950. Such a device - often called a "mass driver" or "rail gun" - might eventually launch bulk cargoes (for example, lunar regolith, liquid oxygen propellant, and refined ores) to sites all around the Earth-moon system.

The SAI team noted that some disciplines might be served equally well by a lunar base or an Earth-orbiting space station. Large (100-meter) telescopes for optical astronomy, for example, might be equally effective on the moon or in Earth orbit. The moon, however, would offer a solid surface that might enable the "pointing stability and optical system coherence" such a telescope would need to perform adequately.

SAI acknowledged that its report proposed "research and development activities. . .too numerous and often too difficult for a first-generation lunar base." It thus divided activities within the five lunar base disciplines into two categories: those suitable for its first-generation base and those that would need a more elaborate second-generation facility. First-generation radio astronomy, for example, would use two small dish antennas on Nearside (the lunar hemisphere facing Earth). In the second generation, a 100-meter-diameter antenna would operate on Farside.

Having defined its lunar base science program, the SAI team moved on to the second and third steps in its study. The team assumed that NASA's Space Shuttle, which at the time they wrote had just completed its ninth flight (STS-9/Spacelab 1, 28 November-8 December 1983), would form part of the lunar base transportation infrastructure, along with an LEO space station. The Shuttle would cheaply and reliably deliver lunar base crews, spacecraft, and cargo to the station, where they would be brought together for flight to the moon. SAI proposed reapplying hardware developed for the LEO station - for example, pressurized modules - to the lunar base program.

An October 1984 paper by study participants Steve Hoffman and John Niehoff for the first Lunar Bases and Space Activities of the 21st Century symposium provided additional details of SAI's Earth-moon transportation system and surface base design. Where details in the October 1984 paper conflict with those in the December 1983 report, the description that follows defaults to information contained only in the latter (mostly).

SAI's lunar transportation system would include three types of spacecraft. The first, the reusable Orbital Transfer Vehicle (OTV), would be a two-stage vehicle permanently based at the LEO station. SAI assumed that NASA would develop OTVs for moving cargoes between the LEO station and higher orbits (for example, GEO) and that this basic OTV design would then be modified for lunar base use. The OTV, which would operate as a piloted spacecraft through addition of a pressurized "personnel pod," would deliver up to 16,950 kilograms of crew and cargo to lunar orbit.

An OTV-derived four-legged lunar lander would form the basis of two vehicles: the Logistics Lander and the Lunar Excursion Module (LEM). The former would include a removable subsystem module for automated lunar landings. The latter would carry a personnel pod for piloted flight. These were listed as the second and third spacecraft in SAI's lunar transportation system, though one might argue that they were actually tricked-up OTVs.

SAI's one-way cargo lunar flight mode. Please click to enlarge. Image credit: Science Applications, Inc.
The three vehicle types would support two basic lunar flight modes. One-way cargo missions would use Direct Descent. The OTV first stage would ignite and burn nearly all of its propellants, then would separate, turn around, and fire its engines to slow down and return to the LEO station for refurbishment. The OTV second stage would then ignite, burn most of its propellants, and separate from the Logistics Lander. The second stage would swing around the moon on a free-return trajectory, fall back to Earth, aerobrake in Earth's atmosphere, and rendezvous with the LEO station. The Logistics Lander, meanwhile, would descend directly to the lunar base site with no stop in lunar orbit.

For two-way crew sorties, the OTV first stage would operate as during a one-way cargo mission. After a three-day flight, the OTV second stage/personnel pod combination would ignite its engines to slow itself so the moon's gravity could capture it into lunar orbit. There it would dock with a waiting LEM carrying lunar base astronauts bound for Earth, who would trade places with the new base crew. In addition to the new crew, 12,750 kilograms of propellants (sufficient for a round trip from lunar orbit to the surface base and back again) and up to 2000 kilograms of cargo would be transferred from the OTV second stage/personnel pod to the LEM.

SAI's roundtrip crew rotation lunar flight mode. Please click to enlarge. Image credit: Science Applications, Inc.
The OTV second stage/personnel pod and the LEM would then separate. The former would fire its engines to depart lunar orbit for Earth, and the latter would descend to a landing at the lunar base. The OTV second stage/personnel pod combination would subsequently aerobrake in Earth's atmosphere and return to the LEO station for refurbishment.

SAI's base buildup sequence would begin with a pair of Site Survey Mission flights. The first would see an unpiloted LEM with empty propellant tanks placed into lunar orbit through a variant of the crew sortie mode. An automated OTV second stage bearing the LEM in place of a personnel pod would enter lunar orbit, undock from the LEM, and return to Earth.

The second Site Survey Mission flight would employ another variant of the Crew Sortie mode. Five astronauts would arrive in lunar orbit on board an OTV second stage/personnel pod and dock with the waiting LEM. The four astronauts of the base site survey team would transfer to the LEM along with propellants and supplies. They would then undock and land at the proposed base site, leaving the OTV pilot alone in lunar orbit. After completing their survey of the site, they would return to the OTV second stage/personnel pod, then would undock from the LEM and return to Earth orbit.

Assuming that the base site checked out as acceptable, Flight 3 would see the start of base deployment. A Logistics Lander would employ Direct Descent mode to deliver to the base site an Interface Module and a Power Plant. The Interface Module, which would be based on LEO space station hardware, would include a cylindrical airlock, a top-mounted observation bubble, and a cylindrical tunnel with ports for attaching other base modules. SAI's proposed Power Plant was a nuclear source capable of generating 100 kilowatts of electricity.

Flight 4 would deliver two "mass mover" rovers, two 2000-kilogram mobile laboratory trailers, and a 1000-kilogram lunar resource utilization pilot plant. The rovers would tow the mobile labs up to 200 kilometers from the base on selenologic excursions lasting up to five days. The mobile labs would carry instruments for microscopic imaging, elemental and mineral analysis, and subsurface ice detection, stereo cameras, and a soil auger or core tube for drilling up to two meters deep. The first-generation lunar resource utilization pilot plant would process 10,000 kilograms of regolith per year to yield oxygen, silicon, iron, aluminum, titanium, magnesium, and calcium.

Flight 5 would deliver the Laboratory Module, the first 14-foot-diameter, 40-foot-long cylindrical base module based on the pressurized module design used to build the LEO station. Flight 6 would deliver the Habitat Module, which would provide living quarters for the seven-person base crew, and Flight 7 would deliver the Resources Module, which would include a pressurized control center and an unpressurized section containing water and oxygen tanks and equipment for life support, power conditioning, and thermal control. The final base deployment flight, a duplicate of Flight 1, would deliver a backup LEM to lunar orbit.

Long-term occupation of the moon would begin with Flight 9, a crew sortie mission that would deliver a four-person construction team. Flight 10 would see three more astronauts join the construction team, bringing the total base population to seven. The OTV pilots for these flights would return to Earth alone after the construction teams undocked and landed at the base in their respective LEMs.

Using the mass mover rovers, the base crew would unload the Logistics Landers and join together the base components. The completed base would provide seven astronauts with 2000 cubic feet of living space per person. They would attach the Lab, Hab, and Resource Modules to the Interface Module, then would link the resource utilization pilot plant to the Lab Module.

The Power Plant would be placed a safe distance away from the base and linked by a cable to the base power conditioning system. The crew would then use hoses to link the Power Plant and base thermal control system to a heat exchanger/heat sink. Finally, after Power Plant activation, the astronauts would use bulldozer scoops on the rovers to cover the pressurized modules with regolith radiation shielding.

Flight 11, the first base crew rotation flight, would see the four-person construction team that arrived on Flight 9 lift off in a LEM and return to lunar orbit, where they would dock with an OTV second stage/personnel pod combination just arrived from Earth. The Flight 9 lunar base team would trade places with them and, following LEM refueling and cargo loading, would descend to a landing at the base. The first construction team and the Flight 11 OTV pilot would then return to the LEO station. On Flight 12, a three-person base team would replace the Flight 10 team.

Lunar base teams of three or four astronauts would rotate every two months. The typical base complement would include a commander/LEM pilot, a LEM pilot/mechanic, a technician/mechanic, a doctor/scientist, a geologist, a chemist, and a biologist/doctor.

Mass mover rover in the field with advanced power cart and deep drill rig. Image credit: NASA
SAI then estimated the cost of its lunar base and three years of operations based on NASA's cost estimates for the Space Shuttle and the LEO station. At the time SAI conducted its study, NASA placed the cost of its proposed LEO station at between $8 billion and $12 billion. This was in fact an underestimation calculated to make the station more politically palatable to the White House and Congress. NASA placed the total cost of LEO station Logistics, Habitat, Laboratory, and Resource Modules and other structures at $7.1 billion, so SAI estimated the total cost of the lunar base Resource, Habitat, Laboratory, and Interface Modules at $5.8 billion.

Although the OTV would find uses in LEO and GEO, SAI charged all of its development and procurement costs (a total of $7.2 billion) to the lunar base. The expendable Logistics Lander and reusable LEM would cost $6.6 billion and $4.8 billion, respectively. The LEM, though structurally beefier and more complex, would cost less because the Logistics Lander would bear the development cost of systems common to both landers.

Based on optimistic NASA pricing, the SAI team assumed that a Shuttle flight would cost $110 million in 1990. The 89 Shuttle flights in the lunar base program would thus cost a total of $9.8 billion. The LEO station, by contrast, would need only 17 Shuttle flights at a cost of $1.9 billion. SAI placed total LEO station cost plus three years of operations at $14.2 billion. Lunar base cost plus three years of operations came to $54.8 billion.

To conclude its report, SAI noted that both the LEO station and the lunar base could be completed in about a decade. The LEO station would, however, serve a broader science user community and would provide an OTV base in LEO for eventual lunar base use. The SAI team argued that the LEO station was a reasonable near-term (10-year) objective, while the lunar base would yield obvious benefits in a long-term (50 years) space program. It added that the
Space Program will function best if it has both near-term objectives and long-range goals. The near-term objectives assure [sic] that we progress with each year that passes. The long-range goals provide direction for our annual progress. The Space Station and Lunar Base appear to serve these respective roles at the present time.

A Manned Lunar Science Base: An Alternative to Space Station Science? A Brief Comparative Assessment, Report No. SAI-84/1502, Science Applications, Inc., 10 January 1984

"Preliminary Design of a Permanently Manned Lunar Surface Research Base," S. Hoffman and J. Niehoff, Science Applications International Corporation; published in Lunar Bases and Space Activities of the 21st Century, "papers from a NASA sponsored, public symposium hosted by the National Academy of Sciences in Washington, D.C., Oct[ober] 29-31, 1984," W. W. Mendell, editor, Lunar and Planetary Institute, 1985, pp. 69-75

More Information

There's a Helluva Good World Next Door

Chronology: Space Station 1.0

As Gemini Was to an Apollo Lunar Landing by 1970, So Apollo Would Be to a Permanent Lunar Base by 1980 (1968)

"A Vision of the Future": Military Uses of the Moon and Asteroids (1983)

09 November 2018

Another Look at Staged Reentry: Janus (1962-1966)

The M2-F1 lifting-body glider (left) and its successor, the M2-F2. Of the experimental lifting bodies NASA built and flew, the Janus spacecraft would have most resembled these pioneering aircraft. Image credit: NASA
In 2013, while spending a gleeful Sunday afternoon searching through old patent applications (don't judge me), I stumbled upon an intriguing design for a piloted spacecraft using "staged reentry." I wrote about it on my old Beyond Apollo blog on the WIRED website.

In 2017, I expanded that post with more context details on the history of lifting body research and better illustrations and posted it on this blog (see the link at the end of this post). At the time, the patent application, filed in January 1964 by TRW engineers C. Cohen, J. Schetzer, and J. Sellars and granted in December 1966, remained my only source of information on the staged reentry concept.

No longer. One benefit of working at a university is that journal articles formerly locked up behind paywalls, out of reach of independent scholars on a budget, are now readily accessible. Last month, while spending a gleeful Sunday afternoon searching through the 1965 volume of The Journal of Spacecraft & Rockets, I stumbled upon a staged reentry design named for Janus, the two-faced Roman god of endings and beginnings. Closer examination confirmed that the Janus spacecraft was indeed the unnamed spacecraft of the 1966 patent.

Janus is an apt name for the proposed spacecraft design, because its most unique features are related to launch and (especially) landing - that is, the beginning and ending of its mission. The name was first used in a confidential May 1962 TRW Space Technology Labs report by I. Spielberg and C. Cohen.

Spielberg, whose name does not appear on the patent application, presented the staged reentry concept at the first conference of the American Institute of Aeronautics and Astronautics in Washington, DC (29 June-2 July 1964) along with Cohen, whose name was the only one to appear on the 1962 report, the 1964 presentation, the 1965 Journal of Spacecraft & Rockets paper based on the presentation, and the 1966 patent. It seems likely, given his continuous involvement, that Cohen originated and championed the Janus staged reentry concept.

Patent applications are not engineering papers; or, perhaps, one may say that lousy is the engineering paper that reads like a patent application. In addition to being more readable, the 1965 Spielberg and Cohen paper provides considerably more detail than the patent application.

The TRW engineers explained the rationale behind the staged reentry concept:
A manned system should provide precision and flexibility in its landing characteristics. It should be capable of routine launch and routine return without a large recovery task force. Moreover, these criteria must be satisfied without curtailing payload volume or weight or reducing the reliability of reentry protection. In general, these requirements conflict, since efficient entry vehicles (e.g., blunt lifting bodies) have poor landing characteristics, whereas vehicles that land well (winged configurations) tend to have low volumetric efficiency and serious reentry design problems. The staged reentry concept. . . circumvents the difficult design compromises that otherwise must be made to ensure good landing qualities, high volumetric efficiency, and desirable reentry characteristics.
The Janus spacecraft comprised two parts that would separate in flight. The largest part was a 26.8-foot-long, 16-foot-wide, 10-foot-deep "pod." Designed to carry three astronauts, it was an 11,660-pound half-cone lifting body with flat aft and top surfaces and a curved, blunt nose.

The TRW engineers described the pod's double-walled structure. Its inner hull, the pressure vessel, would be manufactured from aluminum sheet. The outer hull would be made of aluminum honeycomb with aluminum alloy plates for added strength. Aluminum frames with "I" and "Z" cross-sections would link the two hulls. An ablative heat shield (that is, one that chars and erodes to carry away heat) would cover the aluminum honeycomb, and low-density insulation would fill the space between the inner and outer hulls.

Cutaway view of the Janus spacecraft. Image credit: U.S.Patent Office
The other part of the Janus spacecraft was a 4000-pound delta-wing jet aircraft measuring 21 feet long, 13.3 feet across its wings, and 5.33 feet tall. It would include twin downward facing rudder fins and a belly-mounted air intake feeding a Continental 356-23 turbojet engine. The engine could be started at 18,000 feet of altitude using ambient air or at up to 45,000 feet with supplemental oxygen. Cruise speed at 30,000 feet was about Mach 0.6 (370 knots) and range with a full load of 77 gallons (500 pounds) of jet fuel was 200 nautical miles.

The flat top of the small jet would form the largest part of the top of the lifting body. The jet's underside would form the "ceiling" of the lifting body's 860-cubic-foot pressurized internal volume; that is, the plane's belly, including its air intake, would protrude into the main crew living and working space. Ceiling height, though variable, would measure no less than seven feet.

The jet would ride on three rod-like "pneumatic/explosive actuators" attached to the pod. Latches would link the actuators to holes in the plane's nose and on the underside of its wings. Other latches would anchor the jet's wing leading edges.

Spielberg and Cohen recognized that creating an air-tight seal between jet and pod would pose significant design challenges. They proposed an inflatable or "fluted" (grooved) gasket, presumably made of a rubberized fabric. They admitted that their seal system, though "feasible," was not yet "optimized."

Atop a booster on the launch pad, jet and lifting body would point their noses at the sky. Spielberg and Cohen envisioned that the flat aft surface of the pod would sit atop a launch vehicle adapter that would measure 10 feet in diameter where it linked to the pod. The bottom of the adapter would match the larger diameter of the launch vehicle upper stage.

Just before launch, the astronauts would pass through a hatch in the side of the adapter. Overhead they would see the flat aft surface of the pod, which would include a round hatchway. The hatchway would lead into a cylindrical airlock just large enough to hold one space-suited astronaut. A round hatch in the airlock would in turn lead into the pod. In the near-vacuum of low-Earth orbit, the airlock would permit astronauts to spacewalk without depressurizing the pod.

Forward-facing crew couches would be arranged single-file, one behind the other, in a line beneath the jet fuselage. This would place the astronauts one above the other on the launch pad.

The pod would contain the Janus spacecraft main control console. Intended for use in orbit, it would be mounted on the pod's aft interior wall next to the inner airlock hatch. This would place it out of reach of the reclining astronauts. Critically important controls would be mounted on couch arms.

The patent application said nothing about possible launch vehicles, but in their paper Spielberg and Cohen specified two candidates: Titan III (probably the Titan IIIC variant) and Saturn C-1 (otherwise known as Saturn I). The former could boost 28,000 pounds into the 140-nautical-mile orbit required to forestall orbital decay long enough to carry out a two-week Janus mission; the latter, 20,000 pounds. The total weight of the Janus spacecraft (crew, pod, and jet) was 15,660 pounds, so in theory it could transport 12,340 pounds of unspecified payload if launched on a Titan III and 4340 pounds if launched on a Saturn C-1.

It is worth noting that Janus included no docking mechanism, and that was it not designed to perform significant maneuvers in space (apart from a deorbit burn). This ran against the grain of NASA requirements in the first half of the 1960s, when both Gemini and Apollo were under development. Though it could carry a hefty payload, it could not deliver it anywhere. Presumably, this meant that its payload would always take the form of equipment that would remain inside the pod. It is conceivable, however, that small payloads could be tossed out its airlock and larger ones assembled outside by spacewalkers - Spielberg and Cohen did not, however, suggest these possibilities.

A successful mission would begin with launch from Cape Kennedy on Florida's east coast. The launch vehicle would ascend vertically, then roll toward the southeast on a course that would avoid Caribbean islands and South America. About 10 minutes after liftoff, Janus would reach its operational orbit and separate from the upper stage of its launch vehicle. The crew would then unstrap from their couches and begin work in the pod's large pressurized volume.

They would also work in the jet cockpit. The jet's glass canopy, which would stand higher than the rest of the Janus spacecraft's mostly flat top, would make the cockpit the prime spot for conducting Earth and astronomy observations.

Spielberg and Cohen proposed a novel method for entering and leaving the cockpit. The crew couches would each be mounted on a pair of rails, and the underside of the jet's fuselage would include automatic doors. Operating controls on the couch arms would cause the doors to open and the couch to ride the rails from pod to cockpit and vice versa. The TRW engineers explained that a single set of couches shared between the pod and the jet would save weight, though with the large Janus payload capability this would probably have been a minor concern.

The crew would breathe a 47% oxygen/53% nitrogen air mix at a pressure of 7.5 pounds per square inch. Water for crew needs would come from fuel cells, the primary task of which would be to generate 2.5 kilowatts of continuous electricity by combining liquid hydrogen and liquid oxygen. Fluid circulating in pipes in the pod walls would gather and carry waste heat from the pressurized volume and the fuel cells to a radiator mounted on the pod's aft surface.

For return to Earth, the astronauts would sit in their couches in the pod, turn the Janus spacecraft using small thrusters so that its aft end pointed in its direction of motion, and ignite its 1100-pound solid-propellant retrorocket. After burnout, the retrorocket casing would be cast off and Janus reoriented with its nose aimed forward. Descent toward 400,000-foot reentry altitude would last 14 minutes. At start of reentry, the Janus spacecraft would be moving at about 250 feet per second (fps).

Reentry would be a balancing act. The lifting-body pod would need trim flaps for stability and steering; however, four trim flaps attached in pairs to the bottom edge of its flat aft surface would tend to tip its nose down (that is, give it a negative angle of attack). This would permit hot reentry plasma to course over the pod's top surface, destroying the jet canopy. At the same time, the pod would be tail-heavy, raising its nose and making it aerodynamically unstable.

Spielberg and Cohen proposed a two-part solution: cautiously reshaping the pod's nose and packing its triangular nose volume with heavy subsystems (for example, the fuel cells and their reactants). The former would tend to level its angle of attack and the latter, they calculated, would shift its center of gravity forward to a point 54% of its length (about 11 feet) aft of the pod's nose, yielding a slightly "nose up" angle of attack. The pod's nose would thus bear the brunt of reentry heating, and no reentry plasma would reach the jet canopy.

The Janus spacecraft would reenter at a very shallow angle (just 2°). It would thus shed speed gradually in a low-density atmosphere, preventing maximum deceleration from exceeding 1.9 gravities. An automated attitude control system would operate the trim flaps and small thrusters to maintain stability as the pod descended.

During reentry, the outer hull, safe behind its heat shield, would maintain a temperature below 600° Fahrenheit (F). The inner hull would remain at 70° F throughout the mission. The hot outer hull would tend to expand. If the aluminum frames linking the inner and outer hulls were rigidly attached at both ends, differential expansion would tear them apart. To avoid this, Spielberg and Cohen proposed that the frames be attached to the outer hull by flexible connections and to the inner hull by rigid ones.

A little less than 12 minutes after reentry start, at an altitude of about 120,000 feet, the Janus spacecraft would slow to a velocity of about 50 fps. Deprived of lift, its angle of descent would increase in a little over a minute to about 55°.

At 50,000 feet of altitude, the Janus spacecraft would slow to subsonic speed and begin to lose stability. The mission commander would activate the motors that would raise the three couches into the jet cockpit. Beneath the astronauts' feet, the fuselage doors would close and seal. At 45,000 feet, the spacecraft would slow to Mach 0.9, and jet separation from the pod could occur.

Separation would begin with a command to fire explosive bolts. This would release the latches linking the jet to the pod so that the three rod-like pneumatic actuators could extend, pushing the jet away from the pod with a jolt. The pressure seal would be breached, exposing the pod's interior to the outside environment.

The commander would ignite the jet's engine and fly at a cruise altitude of 30,000 feet to a waiting airfield up to 200 nautical miles away. The jet would land on a nose wheel and skids on the ends of its rudder fins. The pod, meanwhile, would deploy parachutes from its aft surface and descend to a landing on its nose.

In the event of an abort on the launch pad or during first-stage operation, a pair of solid-propellant abort rocket motors mounted on the pod's aft surface outside the adapter linking it to the launch vehicle would ignite to boost the Janus spacecraft up and away. The motors would propel it to an altitude of 6600 feet in 19 seconds. If no first-stage abort took place, the abort motors would eject after second-stage ignition so that the launch vehicle would not need to carry their weight to orbit.

The deorbit rocket motor would play two possible abort roles: in an abort off the launch pad, it could be ignited after the twin abort rocket motors burned out to boost the Janus spacecraft higher and farther downrange, providing more time for successful jet separation; it would also become the primary abort rocket motor after the twin abort motors ejected.

An abort within 200 nautical miles of Cape Kennedy would see the commander separate the jet from the pod as during a normal descent, then fly back to the launch site. The jet could also remain attached to the pod throughout the abort, in which case the entire Janus spacecraft would descend nose down on parachutes to a landing or splashdown at 25 feet per second. Spielberg and Cohen included 1030 pounds of recovery gear in the Janus spacecraft mass budget.

Down-range aborts - for example, during second stage flight - would occur over open ocean, placing land - never mind suitable airports - outside the jet's 200-nautical-mile range. Spielberg and Cohen noted that the lifting body would during second-stage flight be high enough to use its trim flaps and steering thrusters to maneuver closer to land. This would, they judged, permit jet separation within 200 miles of airfields on Caribbean islands or in northeastern South America.

Here is the link to my staged reentry post based only on the Cohen, Schetzer, and Sellars patent of December 1966. In addition to a summary history of lifting body development in the United States, the post contains detailed labeled drawings from the patent application.


"Janus: A Manned Orbital Spacecraft with Staged Re-Entry," I. N. Spielberg and C. B. Cohen, The Journal of Spacecraft & Rockets, Volume 2, Number 4, July-August 1965, pp. 531-536

Patent No. 3,289,974, "Manned Spacecraft With Staged Re-Entry," C. Cohen, J. Schetzer, and J. Sellars, TRW, 6 December 1966

Related Links

X-15: Lessons for Reusable Winged Spaceflight (1966)

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

What if a Shuttle Orbiter Struck a Bird? (1988)

NASA Johnson Space Center's Shuttle II (1988)

24 October 2018

Preparing for Patreon

Apollo 7 launch, 11 October 1968. Image credit: NASA
It has taken me a while, but I now have worked out how I want to treat my Patreon patrons. By that, I mean what incentives to offer and how folks should expect to be charged.

I've learned that I can charge my patrons by new blog post. I like that idea, because I don't post on a regular schedule, and in the past year I've not posted often because of a new job, new home city, and new health problems. I'd hate to automatically charge people on a monthly basis and then miss posting in any given month.

In addition, I am working on two ebooks, publication date TBD. I like the idea of being my own publisher. I've published print works, so this will be something new for me. I expect I can earn more money this way than by publishing through a print publisher. I can also use access to the ebooks as an incentive.

I imagine it would go something like this - lowest-level patrons get the blog. More wealthy and/or generous patrons get a 25% markdown on the price of the ebooks. Even more wealthy and/or generous patrons get a 50% markdown. Insanely wealthy patrons get a 75% markdown. And the MegaPowerBall winner patrons get a 75% markdown, plus the second ebook for free!

I expect I'll also offer extras related to the ebooks. You know the drill: the more you contribute, the more extras you get. I'm not sure yet what the extras will be. I'd be happy to receive suggestions.

There'll be a Wall of Fame page. Not sure of the details yet, but it might go like this: lowest-level patron name is listed. Next up the scale, name with a link. Then name, image - avatar, picture, whatever - and link. Next up, the same but higher up the page. Then, for the top-level patrons, top of the page and perhaps some "advertising" text if they have a blog, podcast, or other project they want to promote. Of course, your image could be your advertising. No rule against that, but your placement would improve if you pledged more.

As for patron levels - my research indicates that most folks are more frugal than I am. By the same token, they pledge to more creators. I have to take into account the "per post" pledge structure, which I've noticed some folks don't understand. I'd hate for someone to think they were pledging $50 per month, then get a $150 charge if I post three posts in a month. I think that argues for lower pledge levels to avoid painful misunderstandings.

So, I think the maximum would be $20 per post, with lower levels of $15, $10, $5, and $1. I'd feel better about telling someone "You should've read the fine print" at those levels than at higher levels.

I seek a $2500 monthly income. At that level of income, I could live comfortably, freeing me to devote all my attention to the blog and ebooks, and add blog improvements (paid original art and posts based on research at costly archives being the first that leap to mind). That doesn't seem impossible, given the number of people who check out this blog. Most seem to be serious, intelligent people, which might imply that $5 per post would not break the bank.

If I posted twice per month, a $5 per pledge patron would get a $10 charge in that month. If I could do that for a year, they would pay me $120. If I could get just 20 people to post at that level, I would hit my goal.

That doesn't mean that $1 pledges would not be welcome. Change only that parameter in the example above, and they would pay in $2 in a month, or $24 in a year. I'd only need 105 people to pledge at $1 per post to reach my goal - and that's far fewer than visit the blog each day when I post frequently.

I invite discussion of these ideas, especially from prospective patrons and experienced Patreon creators. My plan is to start this at the beginning of 2019, which, being the start of the Apollo 11 50th anniversary year, seems to me to be an auspicious time to call on folks to support this blog.

12 September 2018

Keep My Memory Green: Skill Retention During Long-Duration Spaceflight (1968)

All piloted space missions end with Earth-atmosphere reentry. For short-duration missions - for example, an Apollo voyage to the Moon - the period of time between reentry training using simulators on Earth and actual reentry would be short enough that pilot skills retention would be unlikely to become a problem. For longer missions, years might separate simulator training on Earth from actual reentry, almost certainly leading to degradation of critical pilot skills. Image credit: NASA
Serious plans for astronaut space activities take into account human frailties. Long stays in the space environment on board Earth-orbiting space stations have revealed some: for example, loss of calcium in load-bearing bones in microgravity. Other frailties have been part of human experience for many millennia: for example, forgetfulness over time.

In July 1968, when J. R. Birkemeier, with Bellcomm, NASA's advance planning contractor, performed a preliminary assessment of astronaut skills retention during long space missions, the longest human spaceflight had lasted just 13 days, 18 hours, and 35 minutes. During the Gemini VII mission, launched on 4 December 1965, astronauts Frank Borman and James Lovell experienced no obvious degradation of skills as they orbited Earth 206 times. They splashed down just 11.9 kilometers off target in the Atlantic Ocean between Bermuda and the north coast of the Dominican Republic on 18 December 1965.

Astronauts James Lovell (left) and Frank Borman stand on the deck of the aircraft carrier U.S.S. Wasp after a safe splashdown. They orbited Earth for nearly 14 days in the cramped confines of the Gemini VII capsule to demonstrate that humans could survive in space long enough to reach and return from the Moon during the Apollo Program. Their record would not be broken until the Soyuz 9 flight in 1970, which lasted 17 days, 16 hours, and 58 minutes. Image credit: NASA
No Apollo mission was expected to last longer than Gemini VII, so Birkemeier looked beyond Apollo to possible longer-duration missions of the 1970s and 1980s. Bellcomm had since 1962 studied piloted lunar and planetary missions for the NASA Headquarters Office of Manned Space Flight. The studies were useful for Birkemeier's analysis because they included plausible long-duration mission timelines.

Birkemeier pointed to U.S. Navy regulations, which drew the line at three months for pilot skills retention. Navy rules specified that, in the interest of safety, a pilot should be allowed to land a jet on an aircraft carrier only if they had flown high-performance aircraft for five hours in the previous three months. He assumed that critical space mission events - for example, a piloted Mars landing - would all be at least as challenging as landing a jet on a carrier at sea.

The enormous distances between worlds and the limitations of the propulsion systems expected to exist in the 1970s and 1980s meant that, much more often than not, critical space mission events could not occur within three months of a training session on Earth. A typical Mars landing mission, for example, would see astronauts reach Mars about six months after launch from Earth. High-speed Earth-atmosphere reentry at the end of a Venus-Mars-Venus triple-flyby mission would occur 25 months after departure from Earth orbit.

Birkemeier also considered mission activities unlikely to affect safety, but which might determine whether a mission could be considered successful. Mars Surface Sample Return (MSSR) probe operations, for example, had become the centerpiece of piloted Mars flyby mission planning in 1966. The crew would prepare and release the robotic MSSR probe and other probes five months after Earth-orbit departure. The probes would capture into Mars orbit or enter the martian atmosphere a month after that, just before the piloted flyby spacecraft passed Mars.

After the MSSR probe soft-landed on Mars, the flyby crew would remotely examine its landing site via a television camera on the probe and direct operation of its sample collection devices. They would then pack samples into a capsule and initiate MSSR ascent stage launch.

The ascent stage would boost the sealed sample capsule toward the piloted flyby spacecraft. As their spacecraft sped past Mars, the crew would capture the capsule, transfer it to a sealed glove box, open it, and quickly (but carefully) examine the dirt and rocks inside for signs of living organisms - all while attending to other Mars flyby scientific and navigational tasks.

A Mars Surface Sample Return (MSSR) ascent stage (right) bearing a sample of martian dirt and rocks approaches a piloted Mars flyby spacecraft. Image credit: NASA
Birkemeier proposed methods of space mission "skills maintenance." He wrote that "crew members could preserve some degree of proficiency simply by reading instruction manuals, watching training films, studying the controls, and reviewing specific procedures."

More complex tasks - which tended also to be the ones most crucial to mission safety and success - "could not be maintained by bookwork alone," but neither could they be practiced by actual replication of maneuvers. The latter would, for one thing, expend valuable propellants. Birkemeier explained that "an aircraft pilot can make realistic practice landings on cloud banks," but that "no analogous opportunity [existed] for an astronaut wishing to practice Mars landing or an Earth entry while. . .in space."

The obvious solution would be to provide opportunities for inflight mission simulation. Birkemeier suggested that the actual spacecraft control panels could be designed to serve double-duty as simulators, especially if they were also designed to be periodically tested using actual control inputs. The control panels would be temporarily disconnected from the systems they were designed to control and tied to a computer that would simultaneously provide responses to crew actions and monitor control system health.

The Apollo Command Module (CM) simulator at the Manned Spacecraft Center in Houston, Texas in 1966. The CM hatch, with its round window, is visible at the top of the ladder. Light brown cabinets house (among other things) projection equipment that provides a realistic view through the CM windows. At the time Birkemeier wrote his report (July 1968), the simulator used 88,000 words of memory on three computers to simulate six-degree-of-freedom maneuvers in real time. Fifty thousand words on a single UNIVAC 1108 computer was sufficient to simulate three-degree-of-freedom maneuvers. Image credit: NASA
Birkemeier assumed that 1970s and 1980s computers would be at least an order of magnitude more capable than the computers of 1968, and that "simplifying assumptions" might be used to reduce memory requirements. He estimated that a program using 4000 words of memory on a computer with a solution rate of 25 cycles per second could adequately simulate an Apollo Command Module Earth-atmosphere reentry.

Such a simulation would not, however, be capable of generating "out the window" views. Birkemeier urged more study of whether visual cues would in fact be a requirement for adequate in-flight simulation.

Birkemeier estimated that extended Earth-orbital space station missions would need to devote only 4000 words of computer memory to simulations because the only critical task a station crew would need to simulate would be Earth-atmosphere reentry. Extended lunar surface missions would need 4000 words of memory to simulate liftoff from the lunar surface and 4000 words for Earth-atmosphere reentry.

Piloted Mars/Venus flyby missions, which would need to simulate automated probe operations and Earth-atmosphere reentry, would also use 8000 words of computer memory. Planetary landing mission simulations would be memory hogs: they might need as many as 20,000 words of memory.

Birkemeier concluded his report by proposing other ways that computer simulation could be used during long space missions. If a crewmember with critical skills died or became ill, for example, simulators could be used to train a replacement. Similarly, if a substantial change in planned procedures became necessary - for example, if the Apollo Command Module heat shield became damaged so that a new Earth-reentry profile became necessary - then the crew could practice the new procedures ahead of reentry.

Finally, behavioral scientists could monitor simulator performance to obtain information on crew state of health as the mission progressed. Birkemeier wrote that simulation monitoring could be used to assess astronaut psychomotor functions (for example, control of fine and gross physical movements) and cognitive processes (for example, problem solving).


The first part of the post title is a play on a line in Charles Dickens' Christmas ghost story "The Haunted Man and the Ghost's Bargain," published in 1848. 

"Inflight Maintenance of Crew Skills on Long Duration Manned Missions," J. R. Birkemeier, Bellcomm, July 1968

More Information

Astronaut Sally Ride's Mission to Mars (1987)

Starfish and Apollo (1962)

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

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

28 May 2018

Lunar Viking (1970)

NASA's lunar soft-landers: in the background, the Apollo 12 Lunar Module Intrepid; in the foreground with Apollo 12 Commander Charles Conrad, Surveyor 3. Image credit: NASA
In the 1960s, U.S. space assets included two spacecraft designed to soft-land on the Moon. These were automated three-legged Surveyor, of which seven were launched on Atlas-Centaur rockets between June 1966 and January 1968 (five Surveyors landed successfully), and the piloted four-legged Apollo Lunar Module (LM), which landed at six sites between July 1969 and December 1972.

Even as Surveyor 7 successfully soft-landed near the great ray crater Tycho, NASA, science advisory groups, Congress, and President Lyndon Baines Johnson considered plans for a project to soft-land spacecraft on Mars. Originally conceived in late 1967/early 1968 as "Titan Mars 1973," Project Viking, as it became known, received new-start funding in the Fiscal Year (FY) 1969 budget.

NASA's Langley Research Center (LaRC) managed Viking. LaRC, located in Hampton, Virginia, contracted with Martin Marietta in Denver, Colorado, to build two new-design Viking Landers. Meanwhile, the Jet Propulsion Laboratory (JPL) in Pasadena, California, began work on two Viking Orbiters based on its Mariner flyby spacecraft design first flown in 1962. The twin Viking spacecraft would each comprise a Lander and an Orbiter, and each Lander-Orbiter combination would leave Earth atop a Titan rocket with a Centaur upper stage.

NASA at first planned to launch the Vikings in July 1973, when an opportunity for a minimum-energy Earth-Mars transfer would occur. In January 1970, however, tight funding planned for FY 1971 forced a slip to the August-September 1975 minimum-energy Earth-Mars transfer opportunity.

For NASA's piloted space program, 1970 was eventful even though only a single mission took place. The mission, Apollo 13 (11-17 April 1970), was intended to build on the experience gained through the Apollo 11 (16-24 July 1969) and Apollo 12 (14-24 November 1969) landings. The Apollo 11 LM Eagle landed long, but the Apollo 12 LM Intrepid set down close by derelict Surveyor 3 on the Ocean of Storms, demonstrating that the LM could successfully reach a predetermined target.

Landing accuracy was important for planning geologic traverses, the first of which was to have taken place at Fra Mauro during Apollo 13. An explosion in the Service Module of the Apollo 13 Command and Service Module (CSM) Odyssey scrubbed the landing and put off the first lunar geologic traverse to Apollo 14 (31 January-9 February 1971), which also was directed to Fra Mauro.

The Apollo 13 accident and postponement of subsequent missions meant that much of the activity in NASA's piloted program in 1970 concerned planning and budgets. President Richard Nixon saw no cause for a large-scale Apollo-type goal in the 1970s; NASA Administrator Thomas Paine begged to differ. Nixon appointed the Space Task Group (STG) in February 1969 - less than a month after his inauguration - and made his Vice President, Spiro Agnew, its chair. Paine, a Washington neophyte, misjudged Agnew's importance in the Nixon White House, so believed that he had scored big when Agnew declared at the Apollo 11 launch that he believed NASA should put a man on Mars before the end of the 20th century.

Paine took Agnew's statement as an endorsement of the Integrated Program Plan (IPP), NASA's proposal for its future after Apollo. The IPP included a large Earth-orbital "Space Base," nuclear rockets, lunar orbital and surface bases, a piloted Mars landing mission, and Mars orbital and surface bases. At Paine's insistence, the STG's September 1969 report The Post-Apollo Space Program: Directions for the Future offered the White House only the IPP with three different timetables for carrying it out. Nixon's aides, more cognizant of their boss's thoughts on spaceflight, added an introduction outlining a future with no major goals and no target dates.

This NASA Marshall Space Flight Center illustration from 1970 displays Integrated Program Plan hardware elements planned to be operational in the 1990s. 
Paine largely ignored this clear message, instead focusing his efforts on making a permanent Earth-orbiting Space Station NASA's 1970s goal. In addition to a host of Earth-focused uses, the Station would permit astronauts to live and work in space for long periods. This would enable aerospace physicians to certify that humans could remain in space long enough to reach and return from Mars, a voyage that might last three years. A reusable piloted logistics resupply & crew rotation spacecraft - a Space Shuttle - would economically service the Station.

Paine expected that NASA would use a two-stage version of the Saturn V rocket to launch the core Station and other large IPP hardware elements. In January 1970, however, he found himself obliged to announce that Saturn V production would end with the fifteenth rocket in the series. Apollo missions through Apollo 19 would occur at six-month intervals, ending in 1974, and Apollo 20 would be canceled so that its Saturn V, the last of the original Apollo buy, could launch the Skylab Orbital Workshop. Skylab was the last remnant of President Johnson's post-Apollo piloted program, the Apollo Applications Program (AAP), which aimed to apply successful Apollo technology to new space goals; that is, to squeeze the U.S. investment in Apollo for all it was worth.

NASA advance planning developed a split personality in 1970. Some planners assumed that Saturn V rockets would be available indefinitely; others, that the Space Shuttle would launch all IPP hardware.

For example, even as Paine announced the end of Saturn V production, NASA piloted spaceflight planners studied a versatile reusable chemical-propellant Space Tug which could double as a Saturn V fourth stage. As early as 1980, a four-stage Saturn V would launch a Lunar Orbit Space Station (LOSS). The Saturn V S-IVB third stage would boost the LOSS/Space Tug toward the Moon and detach; the Space Tug would then correct the LOSS's course en route to the Moon and slow it so that the Moon's gravity could capture it into lunar orbit.

Subsequent Saturn V missions would build up a propellant farm and fleet of Space Tugs in lunar orbit. Astronauts in Space Tugs with crew cabins and landing legs would then descend from the LOSS to resume piloted lunar surface exploration and build a Lunar Surface Base (LSB).

Space Tug outfitted for piloted lunar landings. Image credit: NASA
In June 1970, five planners with Bellcomm, the NASA Headquarters planning contractor, completed a multi-part memorandum in which they bemoaned the "prolonged gap in the lunar program. . .of at least six years" that NASA's Space Tug/LOSS/LSB plans would create. They argued that the gap would threaten the multidisciplinary community of lunar scientists Apollo and its robotic precursors had created. The gap also meant that Apollo exploration would make discoveries that could not be followed up until at least 1980. Construction of the LSB could not proceed immediately after the LOSS was established; piloted Space Tug missions to check out prospective LSB sites would need to take place first.

The Bellcomm team proposed a novel method of filling the gap after Apollo 19 and hastening construction of the LSB. They sought to repurpose spacecraft designs expected to become available in 1975: namely, the robotic Orbiter and Lander of the Viking Mars exploration program.

At the time they wrote, neither the Viking Orbiter nor Viking Lander designs were final. The Lander, for example, would eventually carry three biology experiments and two scanning cameras, but the Bellcomm team assumed only two biology experiments and one camera. They saw this as an advantage, for it meant that the Mars Viking design was not so far along that it could not to some degree take into account anticipated Lunar Viking needs.

Lunar Viking Lander. Image credit: NASA/Russell Arasmith
The most obvious modification to the Mars Viking design for lunar missions would be replacement of the Lander aeroshell, heat shield, and parachutes with a solid-propellant landing rocket. The Lunar Viking Orbiter would expend liquid propellants to slow itself and the Lunar Viking Lander so that the Moon's gravity could capture the combination into lunar orbit, then would perform maneuvers to adjust its orbit ahead of Lander release. The Lander would then detach and, at the proper time for a landing at its target site, ignite the solid-propellant rocket.

After its propellant was expended, the motor casing would fall away. The Lunar Viking Lander would then complete descent and soft-landing using liquid-propellant vernier rockets.

The Bellcomm team outlined six basic Lunar Viking missions; some included several variants. For example, the first Lunar Viking mission, the Orbital Survey Mission, would have three variants. None would include a Lander and all would use only instruments planned for the Mars Viking Orbiter. All three would complete their main objectives a month after capture into lunar orbit.

The Orbital Survey Mission variant #1 would see a Viking Lunar Orbiter map the entire Moon in visual wavelengths at eight-meter resolution from 460-kilometer-high lunar polar orbit. Variant #2 would map the entire lunar surface in stereo at 12-meter resolution. For variant #3, a Lunar Viking Orbiter would operate in 100-kilometer orbit. This, the Bellcomm planners explained, would enable it to image potential Lunar Viking Lander and Space Tug landing sites at two-meter resolution.

The Mars Viking Orbiter was meant to transmit data at a rate of just 1000 bits per second over a distance ranging from tens of millions to hundreds of millions of kilometers (that is, from Mars to Earth). The Lunar Viking Orbiter, on the other hand, would transmit from only about 380,000 kilometers (that is, from the Moon), so in theory could transmit about 75,000 bits per second. The Viking Orbiter data recorder could, Bellcomm estimated, store up to 100 images. The Lunar Viking Orbiter would use these capabilities to image the Moon while it was out of radio contact over the Farside hemisphere and transmit the Farside images to Earth while it passed over the Nearside hemisphere.

A Titan III-C rocket would be sufficient to place the Lunar Viking Orbiter into a 100-kilometer circular lunar polar orbit with plenty of propellant remaining on board for additional maneuvers. An Atlas-Centaur SLV-3C rocket would suffice if after lunar-orbit capture no other maneuvers were planned.

The second type of Orbiter-only Lunar Viking mission would use a Titan III-C-launched Orbiter outfitted with a scientific instrument suite tailored specifically for lunar investigations. The Bellcomm team modeled their specialized Lunar Viking Orbiter science payload on instruments expected to be mounted in the Service Module of the advanced Apollo 16, Apollo 17, Apollo 18, and Apollo 19 CSMs.

The Bellcomm team's third Lunar Viking mission would establish twin Farside Geophysical Observatories. A Titan III-D/Centaur rocket - the rocket intended in 1970 to launch the 1975 Mars Vikings - could, they calculated, place a stripped-down Lunar Viking Orbiter with two Lunar Viking Landers attached into a 600-kilometer circular equatorial orbit. The twin Landers would then detach and land at two different Farside sites, out of direct radio contact with Earth. The Orbiter would serve as a communications satellite for retransmitting radio signals from the twin Landers. Landing site selection would be based on Orbital Survey Mission images.

The Farside Geophysical Observatory payload on the twin Landers would comprise instruments similar to those in the Apollo Lunar Scientific Experiment Package (ALSEP) the Apollo astronauts first deployed during Apollo 12. This would extend the exclusively Nearside Apollo seismic monitoring network to the Farside hemisphere.

Unfortunately, a Lunar Viking Orbiter in 600-kilometer equatorial orbit could receive signals from each Lunar Viking Lander only about 10% of the time. The Bellcomm planners noted that an Orbiter in a 5000-kilometer circular equatorial orbit could communicate with a Lander at Tsiolkovskii crater (23° south latitude) 26% of the time. Launching on the Titan III-D/Centaur would, they explained, enable the stripped-down Lunar Viking Orbiter to carry enough propellants to capture into 600-kilometer orbit and, after it released the Landers, maneuver to a 5000-kilometer communications orbit for the remainder of the mission.

Bellcomm's fourth Lunar Viking mission, the Farside Geochemical Mission, would see a Lunar Viking Orbiter/augmented Lunar Viking Lander combination leave Earth atop a Titan III-D/Centaur and capture into a 2000-kilometer circular equatorial orbit. The augmented Lunar Viking Lander would detach and ignite its chemical-propellant motors to place itself into a 2000-kilometer-by-100-kilometer elliptical orbit, then would ignite them again to reach a 100-kilometer circular equatorial orbit.

Finally, it would use its solid-propellant motor to deorbit and chemical-propellant verniers to soft-land at a geologically interesting Farside site. The Bellcomm team proposed that it transport to the surface a rover weighing up to 2000 pounds. Neither the augmented Lunar Viking Lander nor the rover was described. The Orbiter, again stripped down to serve mainly as a communications satellite, would remain in its initial 2000-kilometer orbit throughout the mission.

The Polar Mission, fifth on Bellcomm's list, would see the Lunar Viking Orbiter and Lander perform science together much as the Mars Viking Orbiter and Lander were meant to do. The Orbiter would again serve as a relay, but would also carry a suite of scientific instruments. The Lunar Viking Orbiter would capture into a 100-kilometer lunar polar orbit. As it passed over the Moon's poles, it would search permanently shadowed polar craters for ice deposits.

If ice were found, the Orbiter would release the Lander and maneuver to a higher orbit to improve communications. The Lander, meanwhile, would touch down in cold darkness and use an arm-mounted scoop or perhaps a drill to collect surface material for analysis in an on-board automated lab.

The sixth and most complex Lunar Viking mission, the Transient Event Mission, would aim to find and study Transient Lunar Phenomena (TLP). The Bellcomm team, which devoted an entire appendix of their report to TLP studies, noted that TLP had been recorded for decades at many sites on the Moon by telescopic observers. Appearing as bright spots, color changes, and hazes, TLP were generally interpreted as volcanic gas releases tied, perhaps, to the tides Earth raises in the solid crust of the Moon.

According to the Bellcomm planners, about half of all TLP recorded by 1970 had occurred in and around 40-kilometer-wide Aristarchus crater, located just west of Mare Imbrium in one of the most geologically diverse areas of the Moon. The Lunar Viking Orbiter would thus spend as much time as possible within sight of Aristarchus. This requirement would, along with the need for good image resolution, dictate Lunar Viking Orbiter altitude and maneuvers.

Aristarchus is the largest and brightest crater in this Apollo 15 image. Image credit: NASA
In June 1970, the Mars Viking Orbiter was expected to operate during a six-month Earth-Mars cruise and then for at least three months in Mars orbit. This meant that - in theory - the Lunar Viking Orbiter could be expected to seek TLP for nine months in lunar orbit. In practice, the spacecraft would pass in and out of night several times each day as it orbited the Moon from very near the beginning of its mission, placing added stress on its solar arrays, batteries, and temperature-sensitive systems.

The Bellcomm team expected that the Lunar Viking Orbiter might not last for nine months, but that it would last long enough to detect a pattern in the occurrence of TLP events. Based on this pattern, the Lunar Viking Lander would be directed to a site where it would be likely to witness a TLP event up close.

If the Lunar Viking Orbiter could not spot enough TLP events to enable scientists to detect a pattern, the Lander would be dispatched to Aristarchus. There it would seek evidence of past TLP and stand by in the hope that it might witness a TLP event.

The Bellcomm planners lamented an expected six-year gap in U.S. lunar landings. One wonders how they would have greeted the news that NASA would soft-land no spacecraft on the Moon after Apollo 17 in December 1972 - that after almost 50 years, Apollo 17 remains the last U.S. lunar soft-lander. Three automated soft-landers followed Apollo 17: the Soviet Union's Luna 21, which delivered the eight-wheeled Lunokhod 2 rover (1973); Luna 24, which collected and launched to Earth a small sample of lunar surface material (1976); and China's Chang'e 3 lander (2015), which delivered the small Yutu rover.

20 August 1975: Viking 1 launch atop a Titan III-E/Centaur rocket. Image credit: NASA
The Viking 1 and Viking 2 spacecraft exceeded all expectations. Viking 1 reached Mars orbit on 19 June 1976. The Viking 1 Lander separated from its Orbiter and soft-landed on 20 July 1976. Viking 2 reached Mars on 7 August 1976, and its Lander touched down on 3 September 1976. The Viking Landers performed multiple life-detection experiments (with equivocal results). Together, the four spacecraft of Viking 1 and Viking 2 transmitted to Earth more than 100,000 images.

The Viking 2 Orbiter suffered a propulsion system leak and was turned off on 25 July 1978; the Viking 2 Lander suffered battery failure and was switched off on 11 April 1980. The Viking 1 Orbiter depleted its attitude-control gas supply and was turned off on 17 August 1980. Though designed to operate on Mars for 90 martian days (Sols), the Viking 1 Lander transmitted from Mars until 13 November 1982 - a total of 2245 Sols. It might have lasted longer, but a faulty command caused it to break contact with Earth.

NASA and its contractors proposed many Viking-derived missions for the late 1970s and early 1980s. These included rover and dual-rover missions, sample-returners, and landers and rovers for the martian moons Phobos and Deimos. Their planning efforts in some ways resembled those of Apollo planners in AAP and its successor/remnant, the Skylab Program. The Earth-orbiting Skylab Orbital Workshop was staffed three times in 1973-1974. There was, however, no Viking Applications Program; despite Viking's success, its spacecraft designs saw no further application.

Mariner-based Viking Orbiter with attached Viking Lander capsule. Image credit: NASA

Schematic of Viking Lander as it would appear on Mars with all appendages deployed. Image credit: NASA

The Post-Apollo Space Program: Directions for the Future, Space Task Group Report to the President, September 1969

America's Next Decades in Space: A Report for the Space Task Group, NASA, September 1969

Internal Note: Integrated Space Program - 1970-1990, IN-PD-SA-69-4, T. Sharpe & G. von Tiesenhausen, Advanced Systems Analysis Office, Program Development, NASA Marshall Space Flight Center, 10 December 1969

"U. S. Space Pace Slowed Severely," W. Normyle, Aviation Week & Space Technology, 19 January 1970, p. 16

"Presentation Outline [Space Tug]," NASA Manned Spacecraft Center, 20 January 1970

"NASA Budget Hits 7-Year Low," W. Normyle, Aviation Week & Space Technology, 2 February 1970, pp. 16-18

"Viking Spacecraft for Lunar Exploration - Case 340," R. Kostoff, M. Liwshitz, S. Shapiro, W. Sill, and A. Sinclair, Bellcomm, Inc., 30 June 1970

On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, E. Ezell and L. Ezell, NASA, 1984, pp. 128-153, pp. 185-201, pp. 245-284

More Information

"Assuming That Everything Goes Perfectly Well In The Apollo Program. . ." (1967)

The Russians are Roving! The Russians are Roving! A 1970 JPL Plan for a 1979 Mars Rover

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

A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)

Prelude to Mars Sample Return: the Mars 1984 Mission (1977)