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The Descent

So, what's the obsession with lunar lander then? I list the follow facts for justification:

  • the Apollo mission was the single coolest thing ever.
  • I grew up in the magnificent era of arcade games.
  • I was obsessed with space and spaceflight as a kid.
  • I recently got interested in the moon landings again.

I have to say that for the most part my background reading has been more to do with the actual Apollo mission rather than the game itself, however I have read the story about how the game got developed within Atari.

My reading list recently has included We Reach the Moon by John Noble Wilford. Published in July 1969 it's a very unimposing paperback that was produced as a special by the New York Times. When I ordered mine second hand off Amazon for 99 pence plus postage I had no expectation that I would also getting the '64-Page Souvenir Space Album in Full Color' as advertised on the cover. I was right.

The book is interesting from a historical perspective as it was the first published book about the entire Apollo program - being released just 72 hours after the splashdown of Apollo 11.

At the splashdown of Apollo 11, the entire Apollo programme to date was the culmination of eight years of concerted effort by 20,000 industrial contractors and scores of Universities - a total of 400,000 people. The programme cost $24 billion. The forerunners, the Mercury programme cost $392 million and the Gemini programme cost $1.3 billion.

Obviously the focus of any Lunar Lander game or simulation is the LM - pronounced 'lem' - the Lunar Excursion Module. Tom Buckley of the New York times summarised the vehicle:

"An ungainly, underslung octagonal base is supported by four spindley, awkward jointed legs. The base is surmounted by a faintly cranial-looking structure - essentially a stubby cylinder, but one that has been hopelessly deformed by a number of eccentrically shaped and sharply angled protuberances - from which a hatch gapes like an idiots mouth and two triangular windows glare like eyes. Four bathtub handles, the skirts of the small rocket engines that control the LM's attitude in flight, jut from the cylinder's sides, and its top is garnished by a collection of antennas - dish, curled and ball topped - that look like flowers on an old woman's hat. The LM manages to suggest simultaneously a not-very-well-thought-out cubist sculpture, the machine sculpture of Jean Tinguely and the Edsel automobile"

The development of the LM itself cost a cool $1.6 billion. For their money, NASA got a vehicle with the following characteristics:

  • 23 feet high, 31 feet diameter (with legs extended)
  • a million parts, mostly transistors
  • 40 miles of wiring
  • two radios
  • two radar sets
  • six engines
  • a computer
  • a scientific experiment kit

Some of the interesting design features were:

  • the outer skin was the thickness of aluminium foil
  • the ladder the astronauts used to descend to the surface would have buckled under the astronauts weight on Earth due to increased gravity.
  • the rocket engines were built to require no sparking device.
  • it had no hydraulic or pneumatic systems.
  • the legs contained crushable honeycomb in the struts to absorb the impact of landing (clearly a 'one-time' system)
  • the computer had circuits running in triplicate with a 'majority vote' system
  • the whole machine weighed just less than 16 tons

The descent engine generated 10,000 lbs of thrust. It could be throttled down to 10 percent of its power or up to 94 percent. The engine burned a mixture of hydrazine and unsymmetrical dimeythyl-hydrazine with nitrogen tetroxide as an oxidizer. The fuel and oxidizer burned on contact without the need for a spark.

The ascent engine was built by a different company and generated 3,500 ilbs of thrust - it could be started and stopped but not throttled. A bigger engine was not required because the original 32,000 lbs of weight was reduced by 10,000 lbs by leaving the descent stage behind, and because lunar gravity is 1/6 of that of Earth.

The process of descending to the moon went as follows:

Whilst still linked to the command module, and during the 33 minutes that the combined vehicle was out of radio contact with Earth, the astronauts had to fire the main engine to slow the vehicle down so that it could be captured by the lunar gravity.

At 50,000 feet the descent by the LM to the lunar surface was started. The throttlable engine built up thrust gradually firing continously for seven minutes as the LM descended along the steadily steep trajectory. The guidance computer was driving the rocket engine. At 21,000 feet the astronauts were riding upside down. At 7,200 feet with the landing site 5 miles ahead the computer ordered maneuvers to tilt the 'bug' almost upright. The Eagle closed in on the target, dropping about 20 feet a second until it was hovering almost directly over the selected landing spot at an altitude of 500 feet.

At this point Armstrong realised that the computer was guiding the LM to a landing in a 'football sized crater containing boulders and rocks'. The computer maintained control of the rocket firing for 90 seconds whilst Armstrong manouvered to a suitable landing site. A blue light in the cockpit indicated touchdown as the five foot long probes touched the surface.

As the descent engine fired Armstrongs pulse measured 110 beats a min (normal was 76) - at touchdown it was 156!

Next time, the ascent... in the meantime check out the excellent web site We Choose the Moon

LEM Diagram

LEM Photo