The Mars Excursion Module: One More Small Step (Integrated Program Plan, Part IV)

Mars Excursion Module 1967

The Mars Excursion Module, as shown in 1967’s “Definition of Experimental Tests for a Manned Mars Excursion Module, Volume IV”. By 1969 it would evolve to have a truncated rather than rounded nose, and the base section was larger to notionally support storing a rover. This view affords an excellent look at the central ascent vehicle and its tanks, which would leave the rest behind. Public Domain image via NASA. Click for a larger view.

What it was: The last piece of the Integrated Program Plan’s mission to land astronauts on Mars in the 1980s. First proposed in 1966 (though with a similar, smaller craft being advocated by Philip Bono in 1964), in 1969 it was presented by Wernher von Braun to the Space Task Group and adopted as part of NASA’s proposal for the post-Apollo focus of that agency. Though not developed by him, it represented the culmination of his lifelong dream to visit Mars vicariously through the people he would be instrumental in sending.

Details: In a world where NASA’s Integrated Program Plan of 1969 went forward, you might be an astronaut on the first mission to Mars. After getting to orbit on board the Space Shuttle (likely the “DC-3” of Max Faget‘s design, or similar), you’d board a Nuclear Shuttle-driven

mem-schematic

MEM schematic view, 1969, showing the standing pilots and the tunnel to the lab and living area in the lower section. Public domain image from NASA. Click for a larger view.

interplanetary ship gingerly fuelled by personnel in Space Tugs. The journey would be a long one: hundreds of days, and possibly including a flyby of Venus, the exact duration depending on the year when the mission got underway. Eventually you’d get to your destination, though, and assuming all was well your ship would settled into an elongated orbit around the Red Planet. Half of the crew would stay aboard, while the remaining six astronauts (hopefully including you) would get to go down to the surface. To accomplish that, you’d use the Mars Excursion Module (MEM).

NASA studied a variety of craft that might make this final leg of the journey, but the MEM as pictured in 1969 first made an appearance in a study done at the Marshall Space Flight Center in June of 1966. A year previous, Mariner IV had shown that NASA’s previous best estimate of Mars’ atmospheric density, which had been about 25 millibars, or about 2.5% of Earth’s atmosphere, was much lower than expected at just 6 millibars. Previous designs were useless, relying as they did on lifting bodies and parachutes that would get some “bite” from the air on the way down. Marshall’s suggested shape, resembling an oversized Apollo capsule, was the first to deal with the reality of the situation: after entering the Martian atmosphere, even the MEM’s capsule-shaped body would only slow to a terminal velocity of 900 m/s and then carry on at that rate until hitting the ground. As this is just a little over 2000 miles per hour, it would bring the “terminal” to the fore if nothing else were done.

By November 1967 the details had been worked out after the problem was handed off to North American Rockwell. Their MEM was now recognizably the craft pitched by von Braun in 1969, though different in carrying four astronauts. During the descent the crew would be in the command module which took up the point of the MEM’s conical shape, while the lower deck’s laboratory, living quarters, and external airlock would be reached through an internal airlock and tunnel. The capsule was 30 feet in diameter at its base (9.1 meters) and 29 feet tall (8.8 meters). At re-entry time it would weigh 46.1 tonnes. Altogether this made it approximately the same size as the full Apollo CSM if the latter craft’s service module had continued the slope of the capsule on top of it instead of following parallel lines down to the engine. The contents of the extra volume enclosed made for a 14-tonne difference in mass, though, and later iterations of the MEM upped its base to 32 feet, the same diameter as a Saturn V, with a corresponding increase in weight.

The MEM in the context of the larger Mars Expedition craft.

The MEM in the context of the larger Mars Expedition craft. Public Domain image from 1968’s “Integrated Manned Interplanetary Spacecraft Definition, Volume IV”, via NASA.

After departing the Mars Expedition ship, the MEM would use a solid retrorocket to leave orbit and a liquid fuelled one to land, parachutes being all but useless in the thin air. On the plus side NAR’s engineers noted that Mars’ pitiful atmosphere meant that heat shields could be a lot lighter than those needed for re-entry on Earth. The one on the underside would a titanium honeycomb covered with ablating AVCOAT, also used by the Apollo capsule as well as with the Orion MPCV. The ones on the upper slopes of the MEM, made of titanium or L605 cobalt alloy depending on the heat it would endure, could be jettisoned to lose weight partway through the trip down to the ground. Daringly, two of the crew would remain standing to pilot the craft (probably supported by a harness when under heavy deceleration loads), and get the MEM landed on its six-legged landing gear. If they had to, they could hover the MEM above the surface for as much as two minutes.

Once down, the crew of the MEM would begin a 30-day stay on Mars. On the surface the MEM would be powered by two fuel cells in the mission module. An S band microwave link would be used for a TV signal back to Earth, as well as telemetry and voice communications, while a VHF link would be used for communications back to the orbiter as well as linking astronauts on the surface to the capsule.

Various configurations of the MEM as it goes through its mission

The many faces of the NAR MEM, from Mars deorbit, lower left, to ascent and rendezvous with orbiter, lower right. Public domain image via NASA, same source as previous. Click for a larger view.

When the mission was over, the toroidal mission module and the landing gear of the MEM would be discarded. Getting back off Mars was arguably the most difficult part of the mission, as North American Rockwell found that even LOX and LH2 was not powerful enough to do the job. Instead they settled on FLOX (70% liquid oxygen and 30% liquid fluorine) as the oxidizer and methane as the fuel, with careful staging of the ascent module’s tanks to minimize mass during the flight, in order to make it back to orbit. Rather than have to deal with two different sets of propellants, the landing engine would have burned the same. This is a somewhat alarming choice, both because the words “liquid fluorine” are always alarming and because FLOX and methane have never been used together in an operational rocket engine (Atlas engines had tested with FLOX and kerosene at least, in the years prior to 1967). The MEM’s reaction thrusters used an odd combination too, chlorine pentafluoride as an oxidixer and hydrazine.

North American Rockwell declared that they could build the MEM given seven years from 1971 to 1978, including heat shield tests from orbit, two manned test flights, and a 242-day “soak” in LEO vacuum to simulate the transit to Mars, with an actual Mars mission sometime from 1981 onward. Hardware development costs would be in the range of US$3.1 to US$5.0 billion.

What happened to make it fail: We’ve discussed the Mars Expedition as a whole previously, and the answer is still the same. Richard Nixon was uninterested in manned space programs and was only willing to support the Space Shuttle for fear of being remembered as the man who ended the Space Age. It’s easy to paint Nixon as the villain here, but he was a reflection of the reality that was the incoming 1970s: the economy was sputtering, Vietnam was costing a fortune, a majority of the American public didn’t care, and Congress was deeply hostile to a Mars mission. A crewed trip to Mars was pushed off nebulously to the year 2000, safely a minimum of five presidential administrations away. Even slight familiarity with American politics unmasks this as the political equivalent of your mother saying “We’ll see” when you asked her for a dog.

What was necessary for it to succeed: The IPP’s Mars landing was the end point of a large number of complex programs. In rough order these were: a Space Shuttle based on a winged orbiter, a LEO Space Station, small Space Tugs, orbiting propellant depots, Reusable Nuclear Shuttles, a Moon base, and possibly an Orbiting Lunar Station. By 1972 NASA’s future was busted down to the Shuttle, and as of 2016 they’re all the way up to step two.

The world where the Integrated Program Plan was followed is a very different one from ours, so it’s difficult to say what could possibly have brought the MEM to fruition. The best-known attempt is SF author Stephen Baxter’s misanthropic novel Voyage, but his suggested alternate outcome of the Kennedy Assassination isn’t sufficiently different to overcome the economic and social tides that sunk the IPP. Early collapse of the USSR in the late 1960s? Election of Gerard O’Neill as dark-horse, third-party President of the United States in 1976? Wernher von Braun finds a magic monkey paw? Your guess is as good as mine.

Technically, the MEM was sound. It was just about everything else outside its conical shell that went awry.

Other Fun Stuff

A picture of a MEM on the surface of Mars by artist Tom Peters

A neat shot of the heat shield jettison on a variant MEM using a ballute to maintain attitude, also by Tom Peters.

Sources

“An Initial Concept of a Manned Mars Excursion Vehicle for a Tenuous Mars Atmosphere”, Gordon R. Woodcock. NASA, Marshall Spaceflight Center. 1966.

“Definition of Experimental Tests for a Manned Mars Excursion Module. Final Report, Volume IV—Briefing”, Geoffrey S. Canetti. North American Rockwell. 1967.

“Integrated Manned Interplanetary Spacecraft Definition, Volume IV, System Definition”, Anonymous. Boeing. 1968.

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7 thoughts on “The Mars Excursion Module: One More Small Step (Integrated Program Plan, Part IV)

  1. Flourine rocketry was always a big ask; the MEM is a classic design, but was always close to the edge – not least due to the exotic, deadly, chemistry. On a happier note, lovely 3D printed models of the MEM are available on Shapeways – not cheap, but a joy to behold!

    • So too was storing cryogenic propellants for the best part of a year, though at least NAR had the decency to point out that going this way was unprecedented. Not so the fluorine-guzzling engines, except in the lesser sense that they had to be developed like any other new rocket engine.

  2. Fluorine oxidiser does give you a lovely big Isp, but even short-term storage is distinctly non-trivial when weight is important – and then there’s the cold soak and the bumps of travel. Odds of an explosion on landing would seem distressingly high.

    That “disposable tanks wrapped round the ascent stage” design looks very Bono/ROMBUS.

    • The key difference being that it’s a fluorine “spike” to LOX that was being used. I have not been able to find much about it, though I find off-hand references to it being safer than pure liquid fluorine. I’m far from convinced, though, and plan on keeping an eye out for references.

    • Though I’ve just found a secondary source that says the FLOX for the MEM was to be 85% liquid fluorine instead of the typical 70/30 and 30/70 mixes. No no no no no. That cannot be good.

  3. In fact Rockwell look also in other Propellant combination
    basic design was Hydrogen/Oxygen and alternative FLOX/Methane
    All MEM graphics show the Hydrogen/Oxygen Version !
    FLOX/Methane were only three spherical tanks around the Core

    Lockheed made studies for alternate propellant also for MEM
    “Propellant Selection for Spacecraft Propulsion Systems”
    “Volume II: Mission and vehicles”
    “Contract NASW-1644”
    “Lockheed, 30 August 1968”

    consider were
    Hydrogen/Fluorine
    Ammoniac/Fluorine
    Hydrogen/Oxygen
    Methane/FLOX30 (Oxygen with 30% Fluorine)
    Methane/Oxygen difluoride (OF2)
    CIF5/MHF-5

    CIF5 is Chlorine pentafluoride
    MHF-5 is 55% Monomethylhydrazine, 26% Hydrazine and 19% Hydrazinium Nitrate,
    beliefe me very very toxic stuff !
    it was also consider as RCS fuel for MEM
    What about Chemical contamination of landing site ?

    the MEM was so popular that even 17 year later in 1985
    the Marshall Flight Center overwork the NAA Design for Potential Mars mission.
    they replace the proposed FLOX/Methane Fuel and Plug-in engines,
    by conventional hardware and Hydrogen/Oxygen propellant.

    Source (need register to see the Picture )
    http://www.secretprojects.co.uk/forum/index.php/topic,14221.msg271896.html#msg271896

    • My, my, that is a very ugly and toxic looking list. As I was saying to Roger in a previous comment, I have found a secondary source that says the FLOX was to be an 85/15 mix of fluorine to oxygen, and I would like to be very far away from that.

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