Gumdrops on Mars (1966)

Exploring Mars in the happy days before Mariner IV. Image credit: Philco Aeronutronic

The Mariner IV Mars flyby of 14-15 July 1965, marked a watershed in Mars exploration planning. Prior to Mariner IV, engineers and scientists could legitimately propose lifting-body and winged gliding Mars landers that could set down on the planet using almost no propellants. This was because the prevailing scientific opinion gave Mars an atmosphere roughly 10% as dense as Earth's. After data from doughty 261-kilogram Mariner IV finished trickling back to Earth – a process that lasted until 3 August 1965 – such designs were relegated to the dust-bin.

Mars, it turned out, has an atmosphere less than 1% as dense as Earth's. In such an atmosphere, gliders and lifting bodies might still be used – however, they would reach the martian surface traveling at supersonic speeds, not the easily managed subsonic speeds pre-Mariner IV mission planners had assumed. The Philco Aeronutronic Mars Excursion Module (MEM) pictured at the top of this post, for example, would slow only to Mach 2 (twice the speed of sound) before it reached Mars's surface. At such a speed, parachute deployment would be problematic, forcing reliance on rockets to slow the MEM below the speed of sound. This would in turn demand substantial quantities of propellants, greatly increasing the MEM's mass, which would generate knock-on mass increases throughout the Mars expedition design.

Less than a year after Mariner IV, Gordon Woodcock, a young engineer in the Advanced Systems Office at NASA's Marshall Space Flight Center (MSFC) in Huntsville, Alabama, proposed what would become the new standard design for MEMs. His four-man MEM was based on the squat conical Apollo Command Module (CM) shape. Two and a half years after Woodcock published his paper, the crew of the Apollo 9 mission (3-13 March 1969), which tested the Apollo Lunar Module in Earth orbit, would name their Command and Service Module spacecraft Gumdrop with good reason.

The Command Module Gumdrop is hoisted aboard the U.S.S. Guadalcanal after the 10-day Apollo 9 mission in Earth orbit, 13 March 1969. Image credit: NASA

For his Mars atmosphere entry simulations, Woodcock assumed a martian surface air pressure of 5.69 millibars – that is, a little more than one-half of one percent of Earth sea-level pressure. He noted that his independently developed Mars atmosphere model compared well with two models the Jet Propulsion Laboratory published just before his paper went to print.

The "semi-ballistic" Apollo CM shape, the MSFC engineer wrote, would have several advantages over lifting-body and delta-winged glider designs. It would, for example, have a low center of gravity and a "wide footprint," making tipping unlikely. The squat shape would enable installation of propellant tanks and payloads with very little wasted internal space.

Furthermore, the Apollo CM-shaped MEM would descend through Mars's atmosphere not nose-first, like the lifting bodies and gliders, but rather tail-first, so it would not need to accomplish a problematic 180° turn or "flip" at supersonic speeds in order to point its braking and landing engines toward the ground. Perhaps best of all, the Apollo Program would generate a large body of experience with use of the CM shape in Earth's upper atmosphere, much of which could be applied to developing the CM-shaped MEM.

Woodcock's 56.1-ton MEM would comprise a descent stage roughly 33 feet across (the diameter of a two-stage Saturn V rocket) and, hidden beneath a protective nose-cone ("separable cap"), a 27.3-ton ascent stage. The ascent stage mass, determined largely by the amount of energy needed to climb to Mars orbit, would size the descent stage, he explained. His MEM would separate from its mother ship in Mars orbit at an altitude of 1000 kilometers, then would fire a retrorocket package to slow down and begin its fall toward Mars's atmosphere.

Gordon Woodcock's Mars Excursion Module (MEM) design. Image credit: NASA

Woodcock advised against MEM separation from the mothership prior to Mars orbit capture; though it would relieve the mothership of the MEM's mass, reducing the quantity of propellants it would need to slow itself so that Mars's gravity could capture it into orbit (and thus the overall mass of the expedition), it would also introduce unacceptable risk. He noted that 10,000 simulations run on an IBM 7094 computer had shown that the safe Mars atmosphere entry corridor for the MEM would be very narrow and thus hard to hit during a high-speed entry from an interplanetary trajectory.

The crew would ride in a spherical capsule atop the ascent stage during descent and landing. MEM atmospheric deceleration would cease at a velocity of 0.5 kilometers per second. The MEM's bowl-shaped heat shield would then detach, landing legs would extend, and four landing engines would ignite. Woodcock's MEM design did not include parachutes.

As the landing engines ignited, solid-propellant rockets would blast the separable cap away from the MEM ascent stage. With the conical cover gone, the MEM pilot would be able to see his prospective landing site for the first time. He would then have 100 seconds of maneuvering time to steer the MEM to a safe touchdown. If rugged terrain made this too short a time to find a safe spot or if a malfunction occurred, the pilot could abort the landing by blasting the ascent stage free of the descent stage and returning to Mars orbit.

MEM mass at touchdown would total 40.9 tons. Following a safe touchdown, the crew would exit an airlock adjacent to the ascent stage cabin and transfer to a Mars surface crew quarters module in the descent stage. The latter would take the form of a segment of a torus with a rectangular cross section.

The MEM descent stage engines would burn non-cryogenic storable propellants drawn from tanks positioned within the MEM to offset its center of gravity, enabling the spacecraft to generate a modest amount of lift during descent. A similar approach would enhance Apollo CM lift characteristics during Earth atmosphere reentry. By revolving around its offset center of gravity using small thrusters, the CM could halt its descent and climb before descending again. This technique was used during Apollo missions to reduce the deceleration felt by astronauts during reentry at lunar-return speed (39,000 kilometers per hour).

Following the successful completion of their surface mission, the MEM crew would return to the ascent stage cabin and blast off for Mars orbit. The performance advantages of cryogenic propellants led Woodcock to opt for liquid oxygen oxidizer and liquid methane fuel in his ascent stage. He envisioned a common propellant tank lined with “superinsulation” with a barrier separating the methane and oxygen. Helium stored under pressure in spherical tanks would drive propellants into the three ascent stage engines, any two of which would be sufficient to launch the MEM to Mars orbit.

Logistics MEM. Image credit: NASA

Shelter MEM. Image credit: NASA

Much as Apollo engineers envisioned that the basic Lunar Module design would be modified to give it new capabilities (for example, unmanned delivery of cargo to the lunar surface) as the Apollo Program evolved from initial brief sorties to in-depth lunar exploration, Woodcock envisioned that his MEM would form the basis of a long-term, increasingly capable and complex Mars exploration program. He proposed a design for a one-way logistics MEM in which cargo and a "camper-type" pressurized rover would replace the MEM ascent stage and the surface operations shelter. A crew would arrive separately in a conventional MEM to unpack the cargo and explore widely in the rover.

Woodcock also offered a design for a one-way nuclear-powered MEM that would provide electricity to a long-term Mars surface base built up from one-way shelter MEMs. The nuclear-power MEM would include a shielded reactor, a reactor control room, and a skin-mounted radiator for discarding reactor waste heat.

Each shelter MEM would house five or six astronauts on three levels: communications & control on top; living quarters in the middle; and a laboratory at the bottom. The lab would connect to a "sortie room/decontamination airlock" that would enable access to the surface. Woodcock calculated that 10.6 tons of water, food, and oxygen with a four-ton reserve could sustain a five-man crew in the MEM on Mars for 500 days. Like the logistics MEM, the power and shelter MEMs would land on Mars unmanned.

The Apollo CM-shaped MEM design became closely identified with piloted Mars missions after NASA MSFC director Wernher von Braun, famous for his 1950s Mars glider lander designs, presented a variation on Woodcock's Apollo-shaped lander theme to President Richard Nixon's Space Task Group in early August 1969. Image credit: NASA

Sources

Summary Presentation: Study of a Manned Mars Excursion Module, F. Dixon, Aeronutronic Division, Philco Corporation; paper presented at the Symposium on Manned Planetary Missions, 1963/1964 Status, NASA George C. Marshall Space Flight Center, Huntsville, Alabama, 12 June 1964

An Initial Concept for a Manned Mars Excursion Vehicle for a Tenuous Mars Atmosphere, NASA TM X-53475, G. Woodcock, NASA Marshall Space Flight Center, 7 June 1966

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