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 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 tedious process that lasted until 3 August 1965 - such designs were relegated to the dust-bin.
Mars, it turned out, had 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 Mars mission planners had assumed. The Philco Aeronutronic Mars Excursion Module (MEM) (image at top of post), for example, a lifting body, 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 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.
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 that the Jet Propulsion Laboratory had published just before his paper went to print.
The "semi-ballistic" Apollo CM shape, Woodcock 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 forward. Perhaps best of all, the Apollo Program would generate a large body of experience with use of the CM shape in Earth's 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) at its widest point and, hidden beneath a protective nose-cone ("separable cap"), a 27.3-ton ascent stage "payload." The ascent stage mass, itself 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.
Woodcock advised against separation from the mothership during approach to Mars prior to orbit capture; though it would reduce the quantity of propellants the mothership 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 atmosphere entry corridor would be very narrow.
The crew would ride in the spherical capsule atop the ascent stage during descent and landing. MEM deceleration would cease with the lander still moving at 0.5 kilometers per second; at that point, the MEM's bowl-shaped heatshield would eject, landing legs would extend, and four landing engines with a mass of 800 kilograms would ignite. Woodcock's MEM design did not include parachutes. At the same time, solid-propellant rockets would blast the separable cap away from the MEM. With the conical cover gone, the MEM pilot would be able to see the ground 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 be 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 in tanks that would form partial toruses with circular cross-sections. The tanks would be 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 Apollo 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.
Much as Apollo engineers envisioned that the basic Apollo Lunar Module design would be modified to give it entirely new capabilities (for example, unmanned delivery to the moon's surface of a long-distance manned moon rover) 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 lander in which cargo and a "camper-type" pressurized rover would replace the MEM ascent stage and the surface operations shelter. The rover crew would arrive separately in a conventional MEM.
Woodcock also offered a design for a one-way nuclear power module MEM that could power a long-term Mars surface base built up from one-way extended-stay shelter MEMs. The former would include a shielded reactor monitored from a Mars surface crew quarters module and a skin-mounted radiator for discarding waste heat. The latter would house five or six astronauts and contain three levels: communications & control on top; living quarters in the middle; and a laboratory on the bottom. The lab would connect to a "sortie room/decontamination airlock" based on the Mars surface crew quarters design. 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.
References:
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.