Saturday, 26 February 2005

Those Magnificent Men and their Flaming Latrines

Today's Cat post. Catapult that is. Well, Trebuchet.

From the Wall Street Journal :
Mr. Kennedy has been studying and writing about ancient engines of war since his days at Sandhurst, Britain's military academy, some 30 years ago. But what spurred him to build one was, as he puts it, ``my nutter cousin'' in Northumberland, who put together a pint-sized trebuchet for a county fair. you do...
The device hurled porcelain toilets soaked in gasoline and set afire. A local paper described the event under the headline "Those Magnificent Men and Their Flaming Latrines."
What's interesting is how they figured out the mass of the projectiles. Medieval artists weren't too concerned about things like drawing relative sizes accurately, and the principles of perspective were still hundreds of years in the future.
Building a full-sized siege engine is a more daunting task. Mr. Kennedy believes that dead horses are the key.
Well, naturally.
That's because engravings usually depict the trebuchet hurling boulders, and there is no way to determine what the rocks weigh, or the counterweight necessary to fling them. But a few drawings show dead horses being loaded onto trebuchets, putrid animals being an early form of biological warfare. Since horses weigh now what they did in the 1300s, the engineering calculations followed easily.

One thing has frustrated Mr. Kennedy and his partner: They haven't found any commercial value to the trebuchet.
Oh, but every home should have at least one! As Avon Lady Repellant. Or a quick method of garbage disposal. In the UK, for Home-Defense, as guns are apparently illegal. There must be a thousand uses.
Finally, there's the prospect of flinging a man into space - a living man, that it. This isn't a new idea, Mr. Kennedy points out: Trebuchets were often used to fling ambassadors and prisoners of war back over castle walls, a sure way to demoralize the opposition.

Some English sports parachutists think they can throw a man in the air *and* bring him down alive. In a series of experiments on Mr. Kennedy's machine, they've thrown several man-sized logs and two quarter-tone dead pigs into the air; one of the pigs parachuted gently back to earth, the other landed rather more forcefully.

Trouble is, an accelerometer carried inside the logs recorded a centrifugal force during the launch of as much as 20 Gs (the actual acceleration was zero to 90 miles per hour in 1.5 seconds). Scientists are divided over whether a man can stand that many Gs for more that a second or two before his blood vessels burst.
20 g over 1 second would end up as 200 metres/second, near enough. 12 km/minute or 720 km/hr. 0 to 90 mph (call it 160 km/h) in 1.5 secs? 3 gees, no more. Aerobatic aircraft routinely pull more over long periods. And US rocket sled experiments had people pulling 20 gees for several seconds with no ill effects, other than nosebleeds, red eyes, and piles (which fighter pilots also suffer from, they routinely do 5 gee and sometimes 9 or more in short, sharp manouvres).

So it's doable.

As for the Inventor of the Trebuchet?
Only one full-sized one exists today, designed and built by Mr. Kennedy, a wealthy landowner, inventor, military historian and - need it be said? - - full-blown eccentric.
Eccentric? Merely unconventional. And a man after my own heart. But I'm worried about anyone he describes as his "Nutter Cousin" though.

Meanwhile, should you wish to own your own trebuchet - and when it comes down to it, who doesn't? - just surf on over to Kits and plans for fully working models, some so small you can put them on your desktop, others capable of breaching walls of neighbourhood castles.

Or of course, you can always try out the Virtual Trebuchet I mentioned in an earlier post.


EvilPundit said...

Are you sure about those g calculations? 3g might be the maximum for straight-line acceleration, but if you addd in the centripetal acceleration as the arm rotates around its hinge-point, it could be a lot more than that.

Zoe Brain said...

It depends on the exact geometry of the Trebuchet. If you assume that there's a sling arrangement, then there is no centripetal acceleration as such - just a varying acceleration normal to the missile, as the missile's frame of reference gets rotated. Of course, that will impart a significant rotation, but not more than 180 degrees/1 sec assuming the acceleration is over 1.5 secs. And that assumes someone lying down, face up, then being rotated 45 degrees so their feet are in the air, then rotated the other way a full 180 degrees, ending at an angle 45 degrees from vertical, facing in the direction they're catapulted.

More importantly, the acceleration curve won't be smooth. There might be a 1g acceleration for 1/2 sec, then a 7g acceleration for 1/2 sec, then a 1g acceleration for 1/2 sec, averaging 3g.

The point is, that the figures are not humungous: either they're high, but of such short duration as to be negligible, or low enough to be sustained over a long period, or most probably a acombination of the two.

Of course, a well-designed acceleration couch is recommended, as otherwise even small accelerations can result in ruptured spleens, broken ribs etc.

Put the trebuchet on wheels, and have a freely rotating bucket as the counterweight, and the acceleration will be more closely linear, as the frame moves backwards, then forwards.

EvilPundit said...

If I ever have to launch myself into space by trebuchet, I want you to be my engineering consultant.