The Ballista

During World War One, this French grenade throwing catapult was taken very seriously by its operators,  especially after the fuse was lit.

What the Romans wouldn’t have given for the magic powder in those little iron spheres.   Given the scope of the Roman empire and their evident thirst for all things military, it is surprising some Ancient somewhere didn’t discover the explosive power latent in saltpeter, sulfur and charcoal.  Back when the Catapult Gods reigned supreme they probably kept a tight lid on something as disruptive as chemistry.  There is just no telling what their little monkeys would end up doing with it.

After bumbling through half a dozen different designs for the new limb irons, this one seems the best of the bunch.  According to the computer model, the steel in it will have a volume of 12.55 cubic inches.  This should give it a weight of 3.55 lbs, which is 3/4 of a pound lighter than the limb irons that just failed on the old limbs.  All the steel used will be heat treated car spring material.

The round biscuit that can be seen on the underside of the top strap is 1/4″ thick and puts a lot of section directly where it is needed in the area of maximum stress.  A matching groove will be cut in the limb to accommodate it. The raised area on the surface of the top strap is where impact will occur against the curved pocket on the field frame.    That smaller separate piece of steel on the bottom will be pegged and then bound with an epoxy soaked Dacron whipping  onto the underside of the limb.  It will serve to both protect  the wood from the bite of the inboard bundle,  while its hook will help to keep the bundle from slipping off the heel of the limb.  This latter is not as much of a problem as it is with the outboard bundle because the twisting of this half of the spring serves to move the bundle up toward the limb tip;  but even so, it is good to have something to help control it when the limb goes into its bouncing routine at the end of the power stroke.

It is time to go and rest my head bone.   The old brain needs to sleep on all this awhile before we lunge ahead with anything new.  Making changes on a full scale catapult takes time.  It doesn’t exactly turn on a dime.

In the mock-up below, clay and cardboard make the limb irons visible to judge.  This model  is meant to represent a hardened and tempered, single, unified piece of 4140.  With these dimensions it is still about 3/4 lb lighter than the previous design.

The hook on the end of the high tensile steel strapping is essential to prevent the whole limb being cast bodily out of the machine.    This same force will also serve to wedge the limb deeper into the tapered bottom face of the rectangular socket.   Our actual socket will probably need more taper than is shown here.   Also, it might be a good idea to put some strategically located lightning holes in the socket.

The  lame looking mock-up seen below,  shows the limb tip and a the way I intend to capture the bowstring to stop it flying off the machine whenever it is fired.  The white electrical tape is looped around the bowstring and is meant to represent a stout piece of webbing that will be lashed to the end of the limb.

When the designer is ready, the design will appear.

That clunker of a limb on the starboard side, looks like some kind of a dinosaur fossil compared to the sleek little rapier on the port side.

The port side limb is just a rough mock up so I can judge the strength of the radical new tapers.  Current calculations from this mock up are based on volume and weight and indicate that, including the high tensile steel strapping, the new limb will weigh in at around 5 1/2 lbs.    The old limb is 9 1/8 lbs.   That’s a 3 5/8 lb saving.   Apart from the 1 lb saved by going to a new and much stronger steel strapping design at the base of the limb,  a great deal of blubber has been shed as we near the limb tips.  Also, it appears that the new interface between the limb and the spring bundle is such that the torsion spring looks to gain perhaps an inch or so in length.

Goody, goody sez me.

How many mathematical models are there in an infinite universe?  If Mr. Greene ever develops a mathematical model that proves it possible for there to be a mirror universe that contains the ability to negate any mathematical proof existing in the non-mirror side,  a universe of counter proofs as it were,   …. well,  that’ll be a good day for storytellers everywhere,  I should think.

A good sentry has a disciplined imagination.

…..so anyway,  that was exciting.

Unfortunately I didn’t have video rolling.  The winch worked fine.  However my operation of it left quite a bit to be desired.  I decided to proceed without using the dynanometer.   When the winching became harder I automatically assumed the new winch was not up to the task.    A pop, a ping,  followed by a final rending Kraackk!,  and in an instant, the port limb was a goner. The winch was working fine and I had forgotten that the tensioning on the bundles was already fairly extreme from when I had worked on her last.

I believe the heavy winching actually indicated that the springs had hit a wall and the pressure was spiking in  a way I had never seen before.  It would be easy to clobber myself endlessly for not monitoring the draw weight more closely, but  I have a better idea.

There is no reason to suppose that breaking  one of these limbs is actually a bad thing.   On a superficial level it may be true that limb breakage is never good, but there are factors that actually make it a worthwhile experience.  The whole system: the field frames, the kamarion, the trigger assembly, the stock, the winch, all of it,  sustained what in essence was a proof test load with flying colors.  Even the precious springs looked fine after I unwrapped them for inspection.   This means, that apart from the limbs,  the rest of the machine seems robust and well balanced in terms of overall strength.  I am guessing that my draw weight actually topped 7,000 lbs.  It doesn’t take long for poundages to spike after hitting the wall.   It may be that the springs need to be a couple of inches longer.   That would confer less of an exponential curve on the draw weight.   However, there is no need to go down that road yet.   Some new limbs and a solemn oath to never again venture into the unknown without my trusty dynanometer, will do for now.

Besides, I’m glad they broke.  They always were flawed little piggies.   Destruction provides manure for new growth.  We just hold our nose and smile.  It’s all good.

Skipping is for sissies.

And the Catapult Gods roar with laughter.

This puts a crimp in the tea and crumpets.  No field trip for Bonzo this month.  Bit of a crackerdoodle actually.  Note the clean cleavage line traveling down the center of the limb and emanating from that hole in the wood.   This through-bolt design is really going to have to be replaced with some kind of ferrule.

While it may never be possible for me to declare Firefly finished,   the photo below does show her completed form for the first time on this blog.

The levers are long, my strength limited.  This should really be a young man’s game.

“Boo-hoo”,  say the legionaries on my shoulder.  “What are you? A man, or a mouse?”

……squeak, squeak.

For the purposes of my own sanity while voyaging this vast adventure that I somehow embarked myself upon a couple of years ago (i.e.  page one of this blog),  it will be psychologically helpful for me to disclose my most immediate masterplan to a wider audience.

Step (1)  The Hop.

There are a few hours left to get the winch cranking a load for real.  I have a confident conviction that it will be Groovy.  If it ain’t, it’s just a thing.  Things can be Fixed.

Step (2)  The Skip.

When we are back, we will know it because the shooting is good.  This will involve lots of fine adjustments to balance the torsion springs and get the bolts flying with as little wobble as possible. Tune to Learn.  Insight here will not be possible without the patience to pursue perfection.  (Yes, I know.  That sounds like one of those God-awful slogans corporations use to motivate their proletariat.  Bear with me.  It has a purpose.)

Step (3)  The Jump.

From that Learning, make a better set of limbs.    To include, but not limited to, more radical tapers ,  much lighter tips, and less steel at their base.  There is every indication that this will yield a significant boost to performance.  If we can take our current power levels that hover around 1700 ft lbs  (about the same as a 45-70) and increase them to around 2500 ft lbs  (about the the same as a 30-06), then we can engage in some flat shooting catapultery on a level the Romans themselves would no doubt appreciate.

At that point, we will be contemplating how best to tackle step (4), The Springboard.   (Access to springboard metaphor currently restricted due to insufficient data. )

Yes, what you always suspected, is true.   Around here, bs is not just bullshit alone.

The retaining pin that I slip in place at the beginning of this next video,  has a rather unique way of locking into position.  Very childproof and all that.  It requires a twisting motion rather than just a straight push/pull .  I will leave it to my sharp- eyed readers to figure out how it works.

Click these numbers for vid,   20110320180840