On the 20th of this month will be the 40th anniversary of the first Moon landing, Apollo 11, famously taking Neil Armstrong and Buzz Aldrin for man’s first Moon walk. The Lunar Excursion Module (LEM) was fundamentally a simple machine which was designed to fall to the Moon rather than fly, with one giant rocket motor underneath and some smaller attitude thrusters that allowed the spacecraft to be rotated so that the main engine could potentially point in any direction when it fired. Control of the descent was therefore by means of a number of rocket engine “burns” that could slow the fall of the LEM. This certainly is a “brute force” method of flying; as with Earth flying machines like a helicopter or the Harrier jet, if you have a powerful enough engine it’s possible to bludgeon the laws of physics into submission.
Later on (in the late 1970’s), the idea of landing the LEM inspired a series of popular computer games, probably most famously the “Lunar Lander” arcade game from Atari. I first saw versions of lunar lander in the early 1970s, running on programmable calculators, and specifically the Science of Cambridge MK14 (an early single-board microcomputer), the first computer that I ever programmed. In the computer game, the program modelled the amount of fuel in the craft, the altitude and the speed, and of course the moon’s gravitational pull of 1.6N/kg. By pressing a button you could “burn”, which used fuel and slowed descent. If you studied Physics or Applied Maths at school you would have had the formulae needed to create this program, and you could even do the necessary calculations by hand. Where the computer becomes important is in the dynamic nature of the calculations: as you burn fuel, the mass of the craft decreases, and therefore the force of the engine creates more acceleration as the flight continues.
The real LEM had a dry weight of around 4000kg, with another 11,000kg of fuel at the start of the flight, and the descent started from a height of 15km. Unlike the game of course, the Apollo 11 descent had two men’s lives depending on the outcome, and the flight did not go smoothly. Armstrong famously landed the LEM (codenamed “Eagle”) with only a few seconds of fuel left in the tank, after deciding that the landing site was too rocky and deciding to fly along the surface for a while, looking for a new site. If you’ve ever played “Lunar Lander”, you’ll know that flying along at constant height is a very expensive operation in terms of fuel, so this is a high-risk strategy.
The LEM also experienced some computer problems during the short flight, with the Apollo Guidance Computer giving “program alarm 1202” repeatedly, causing Armstrong to ask Mission Control whether he should abort the landing. In subsequent analysis, the experts from MIT concluded that the computer overloaded because of the data coming from both the rendezvous radar and the ground radar at the same time. The boffins imagined that only the ground radar would be on during the descent (to give accurate height readings), while the rendezvous radar would be used after takeoff. Armstrong, being a test pilot, was planning for possible emergencies, and if the landing should be aborted, he wanted to be able to find “Columbia” (the command/service module) as quickly as possible as they burned away from the Moon’s surface. You might say that the user exercised that software in the way that the programmers had not foreseen; a problem that’s still all too common in software engineering today.
I would like to think that with today’s technology it would be much easier to go to the Moon: we have faster , smaller computers; superior materials like plastics and carbon fibre; more sophisticated fuels and engine technologies. Certainly the one thing that hasn’t changed in the last 40 years is the courage that it would take to land on the Moon, and we have to pay tribute to the 12 men that have done it.
For those interested , check this link out http://wechoosethemoon.org/
It recreates the Apollo 11 launch and moonlanding in a real time interactive website to celebrate the 40th anniversary.