For newcomers, here is my five part strategy for squeezing the most performance out of Phoenix’s new torsion springs:

(1)  Pre-stretch the nylon spring cord to within an inch of it’s life.  Less hyperbolically, that means:  perform a vigorous vertical stretch of an anchored bundle of spring cord with the shop crane.   The spring cord should be in a bundle maybe eight feet long for this stretch.  The idea is to cause all the cord to elongate about 10% (maybe more) and stay under tension this way for quite a while, probably about 24 hours.   –Must research again  what’s ideal.–   Or do by feel.  “Feel” usually trumps calculation when dealing with something as amorphous as fresh spring cord.  The dynanometer can measure the amount of force being applied for this linear stretch.  Adjustments can be made throughout the duration of this pre-stretching period to keep the pressure on.

(See how useful blogs are.  I didn’t remember that long bundle, dynanometer monitored,  stretch thing, until just now when the writing took me there.)

(2)  Reform the pre-stretched spring cord into the final bundles and install in the field frames, along with the limbs. Make sure all areas that contact the springs are as smooth and pleasing and as soft and cuddly as possible. This means: nice fat radii anyplace the spring cord actually touches.  If deemed helpful, padding and pillows would be acceptable to help prevent chaffing.  Strictly the red carpet treatment for the little dears — perhaps breakfast in bed, hourly massages, that sort of thing.   In short, lull the springs into a state of blase complacency by how nice you are treating them.

(3)  After installing the springs in the field frames, and with all due ceremony and respect, drive shallow angled wedges along the top of the flat crossbars to really stretch out all of the spring’s fibers with many hundred of pounds of force.  How many hundreds?  Well that all depends …. how big is a hammer?   In any event, drive the wedges in deep. Make the springs beg for mercy, all the while utilizing the wedges to balance their power output.  When they get through gripeing at all this fiddling around, do a final round of wedge smacking and low power shooting to confirm the balance.  Then get ready to twist baby!

….. And the springs thought they were stressed out enduring all that linear stretching and power balancing from the wedges.  What comes next will really make them howl! …..

(4)  With great solemnity,  rotate all the washers equally.  (Probably about 180 degrees.  Haven’t quite decided yet.)   Now, if all goes as planned, the balance of power between the two sets of springs should transfer intact right through the twisting operation.  I have every hope that it will because Firefly herself has shown she can stay balanced through 75 degrees of universal washer rotation.  Also, it is expected that the stress will become more evenly disposed throughout the spring. Every tiny bubble of slackness will be wrung out.  All the fibers will do their part, no slackers allowed.  The springs should feel rock hard to the touch.

(5) Fire for effect.  ?????????   Here lies the great unknown in this lengthy wedge machine experiment. Hopefully it is the point at which the games can really begin.  Probably there will  be several repetitions of steps 3 and 4.  The springs need to understand that there is no escape from our training regimen.  They are going to have to work for a living.

But not to worry.  If they are evenly strained, and the chaffing trolls can be kept at bay, rope torsion springs can take a tremendous amount of stress before they fail.

Anyway, that’s the basic plan and what we will be working towards over the next couple of months.





The above photo is of one of Firefly’s torsion springs that has been fully developed into a powerful source of propulsion.  If you take a closer look at it you can sense the strain that is contained within it.  (Click to enlarge.)    Note how all the cords have a smooth and even appearance.

Firefly was not fired in two years, and her springs held this high level of strain while sitting in the uncocked position for all that time.  When she was taken out of mothballs and fired again, Firefly’s velocity was exactly the same as when she had been put into storage, 316 fps with a 521 gram bolt.  And I do mean exactly the same, 316 feet per second, which astonished me to no end.  Such is the consistency of a properly developed rope torsion spring made from 3-strand nylon.  My experiments with braided nylon were short lived because it quickly became apparent that the braided material was essentially dead as a spring.  The 3-strand twisted rope was far superior to the braided.  I figure all those intersecting crossover points in the braided material just killed its performance.

Of course, Nylon ain’t Sinew.  Dismissing physical experimentation on reconstructions of ancient torsion engines because they are not powered by authentic materials neglects the fact that there is more going on in the design and development of these devices than the springs that power them.  Given firsthand accounts of ancient ballista performance (e.g. the Lightning as described by Anonymous)  it is no great stretch to conclude that however they were made, sinew springs were reliable power generators.  Our modern nylon analogs can only seek to emulate this.  If an authentic Dura bolt fired from a modern reconstruction can be cast 800 yards or more, the springs that performed the act must bear some measure of equivalency to the originals.

It is this high level of consistent power generation that has helped Firefly shoot with such amazing precision over the years.  It is no great trick to take rope torsion springs to this level, it’s just more fiddling around than most novice catapult makers can conceive of.  Which is probably a good thing.  This is potentially a very dangerous game we are playing.

Leave a Reply