I have crash-landed on the Moon. It was my own fault. I got too - TopicsExpress



          

I have crash-landed on the Moon. It was my own fault. I got too cocky as I neared Tranquility Base, turned the burner off to coast a little in the lunar gravity, found myself descending faster than expected, put the burner back on too energetically, shot back upwards, and by the time the Moons gravity had pulled me back to the same height, Id blown all my fuel, and the rest was ignominy and the destruction of a multi-million dollar Lunar Excursion Module. And it wasnt a computer game or some virtual reality simulation. Ah no, this is from the time when, over forty years ago, the height of computational sophistication available to the ordinary person was a pocket calculator. I have had a few over the years, but I could never afford the pinnacle of pocket calculator desire, the scientific calculators made by Hewlett-Packard, which back then cost about the same price as a small caravan and a Jeep to tow it. Such was the immense reputation of HP scientific calculators that they were taken on board the Apollo spacecraft missions so that astronauts could make on-the-fly adjustments to navigation and flight control in the event of computer failure. And they were used in exactly that way. I remember buying my first calculator, made by Royal, who used to make typewriters. It cost approximately £40 then, which was about a weeks gross salary. It didnt even have buttons, it had a stylus pen connected to a wire which you used to tap metalised number squared on the front panel. It was what was called a basic four-banger (+,-,x,/), and I asked the salesman if it had a memory. He smiled wisely, clearly anticipating the question, pulled from his top pocket a sharpened pencil, and said, Theres your memory, no extra cost. Thats what £40 bought you back then. At these prices, buying an HP calculator would have been like paying for a divorce settlement. In fact, that’s exactly what buying one would have led to. So all I could do was have occasional day-dreams, and make do with the basic model of Casio or Sharp or Rockwell or Commodore or whatever it was I could afford. One day, a friend who had been working in Switzerland came home for a spell, and he had with him the Supreme Object of Desire : the Hewlett-Packard HP25 scientific calculator. It had everything. A heap of scientific and engineering functions, a dazzling array of buttons with multiple functions, and three thick spiral-bound guidebooks. Unlike ordinary calculators, it used a strange but powerful system of keying in data, called Reverse Polish Notation. Unlike ordinary calculators, there was no ‘=’ key and no parenthesis keys, and unlike ordinary calculators, where you entered a number followed by an operation, like + or x, then the next number, with HP calculators, you entered the two numbers first, then the operator key. So whereas ordinary calculators would compute a simple addition as : 2 + 3 = an HP calculator would compute this as : 2 3 + And where an ordinary calculator keystroke sequence might be something like : (3 + 4) x (7 - 3) = an HP calculator’s keystroke sequence would be : 3 4 + 7 3 - x Bizarre? It looks it. But once you understand what’s going on, it is simply brilliant, and reflects the way people actually do calculations on paper, instead of how they read them in textbooks. And it was programmable. You could store keystrokes, and have the HP25 make repetitive calculations automatically, and even have it take different actions depending on what the previously calculated results had been. Ken lent me the calculator for about a month, along with the guidebooks. Unfortunately, the books were of little use to me in explaining how the calculator’s RPN logic system worked, as they were in German, and my entire German vocabulary came from throwaway lines in films. Trying to explain Reverse Polish logic in German was no more effective than explaining it in Greek. Still, I managed to work it out, and was able to key in many of the programs listed in the program book, which did exotic though not necessarily useful things, like calculating the probability that in any sample of new army recruits, more than 3% would suffer from hangnail. (That’s an actual example. Must be a peculiarity to the US Army.) By the time Ken was ready to return to Zurich, and take his HP25 back with him, I’d become pretty adept at using it, and my favourite program was a recreational one called Lunar Lander. You keyed in the program, and when it began to run, it simulated the physics of a spacecraft in descent to the lunar surface from a height of 500 feet (that’s these unmetricated NASA chaps for you) and a downward velocity of 50 ft/s. You have just 60 lb of fuel, and can burn up to 10 lb in one burn. The aim is to descend to the surface with a final velocity of less than 5 ft/s. No sophisticated animated display, not a single moving pixel, and no rocket sounds. Just two numbers on the red LED display, your height, your velocity, and your imagination for the pictures ; and you burned fuel in lots until you’d either coasted gently down, or had crash-landed. It was very easy to burn too much fuel and reverse your slow descent into an upward rocketing away from the Moon, which cost fuel and left you with the prospect of not having enough to see you through the remaining drop to the lunar surface. Yet it was insanely addictive, and I was sorry to have to hand it back when the time came. That was around thirty-five years ago. Not very long ago, about a year or so ago, I bought a new calculator. I didn’t need one, as I’ve a collection of fairly powerful models that are more like handheld computers than calculators (because I like to plot graphs : there, I’ve said it publicly), but this one was special, for it was a new HP scientific calculator, and what’s more, it had, unlike every other calculator brand on the market, the fantastic RPN calculation system. And unlike thirty-five years ago, I could afford it. So I bought it. The HP35s. All the power of the HP25, and then many times more. They’re made in China now, so they don’t have quite the I-could-run-this-over-with-a-snow-plough-and-it-would-still-work feel about them, but they’re still solid and reassuring. (That snow-plough story isn’t hyperbole. It actually happened in the US about forty years ago, when a surveyor dropped his HP calculator and a snow plough crunched over it. It still worked.) Alas, no more the once-characteristic comprehensive guide books, just a PDF file on a disk, and no more the library of mathematics and engineering programs that used to come with HP calculators, but there are plenty of other sources for those. I had hoped there’d be a Lunar Lander program, to relive those hairy moments aboard the LEM, and see if four decades of experience could get me safely down to the surface again. Sadly, no Lunar Lander in the PDF guide book. I’d either have to write one, or find out if someone had converted the old HP25 version to run on the HP35. I knew I wouldn’t be the only calculator gadgeteer with a nostalgic hankering for the game, and sure enough, another HP35s user had done just that. I found the ported version, and I loaded it. Exciting! Within a few minutes, I was aboard the Lunar Excursion Module again, calculator in hand, firing the retro rockets with bursts of 5 and 8 and 9 lb of fuel (we NASA types are still not metricated), descending jerkily towards the Moon and occasionally blowing the spacecraft back upwards, waiting with baited breath till the weak gravity brought me back down to a height of 172 ft above Tranquility Base, only to discover I had just 2 lb of fuel left, and the realisation, with horror, that re-entry parachutes don’t work on an airless satellite. I have crash-landed on the Moon. Again. Neil Armstrong, your reputation is undiminished, you will always be The Man on the Moon, who got it right first time when it counted. Meanwhile, I shall commandeer another spacecraft, and blow NASA’s Apollo budget until I get it right.
Posted on: Tue, 10 Jun 2014 18:13:54 +0000

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