The Ballista

Firefly is starting to paw the ground and snort for the race to be on.  Unfortunately her Warn winch needs a replacement shaft coupling, and that won’t be arriving until Wednesday.   No matter, there are always more bolts to build.  Our intrepid race horse of a catapult could also do with a new bowstring.  The current one is not as good as it should be.   I wouldn’t want some Roman busy-body to get all mil-spec on me about it.

An order of  broom handles arrived today.  I have ten of them 60″ long,  1 1/8″ diameter.  That’s enough to make 20 bolts for the low, low price of  $1.22 each.  Even better, they appear to be made from laminated bamboo,  Bamwood I think they call it.  It’s mostly very straight and appears easy to straighten when it is not.  Fooling around bending  it in the vise, showed it to be plenty tough.   This is really good news  ’cause I was kinda sweating what I’d use for shafting.

Also, I dug out a new-in-the-box  remote control set up for model airplanes.  By using it to control a  servo poised over the start button for the remote used with my camcorder,  I’m planning to get some good downrange footage of the terminal ballistics on these Bamwood bolts.  (That’s fancy talk for videoing them smash into things, up close and personal)

I made up 6 bolts today,  26″ long and varying in weight  from 6500  to 8379 grains.  Painting them day-glow orange seems to help visibility for the video.  I only managed one shot before the light turned sour. Here is some footage of that shot, slowed down 4 times.   20100223153553

The slow motion flight of the bolt is visible for 70 yards and a small spurt of white can be seen as it burrows  into a  patch of snow,  just outside the veggie garden.  Also,  this video clearly shows the bolt flying wide to the left.  More torque is needed on the port side bundle.   I will try and get more video as the tuning proceeds,  and  then we can see the shots finally straighten out.  Stats on this shot as follows: weight of bolt 8379 grains;   draw weight 3700 lbs; draw length 49″;  additional 7 1/2 degrees added to the top washer starboard side;  velocity 233 fps;  energy 1010 foot pounds.  I plan on getting the bundles balanced and the shots flying true, before applying the higher levels of torque to the bundles.   Nap now. ………

………Wake now.  Plonking in at 8379 grains,  the little bug in the above photo is going to be the fat Albert in the crowd.

However, it is nothing compared to this original beauty I discovered on ebay last evening.  Makes a chap think of hardwood sabots, it does.

…. and so anyway, we got off one shot today with the great big heavy harpoon of a bolt we made yesterday.    Before it exploded,  yawing through the plywood on the sand trap, it showed us the following:  10054 grain weight,  208 feet per second, 965 foot pounds of energy.   Slow because of its weight, but not wimpy exactly. In gun world, that’s about the same energy as a .44 magnum.  The good thing is that I only increased washer rotation 15 degrees beyond the tepid twist we had in the last round of testing.   In other words, there is a lot more velocity left to harvest as we continue to increase the torque.  We will have to stop when the bundles  max out with unacceptably high draw weights.  Today’s  draw weight was 3500 lbs.   What is an unacceptably high draw weight?   Now, that is a good question.  Also a dangerous one if we get overconfident.  It is time for some new safety protocols.  More on this later.

Below we see the broken bolt from today’s test laid out on the machine.

This Nessie of a projectile yawed into the target sideways because the starboard bundle was torqued a tad tighter than the port one.   This  shows up in bolt flight as the bolt swooping up and to the left,  relative to the bolt groove.   Confirmation of this disparity between the bundles was evidenced by measurements taken with the new Wheatie meter.  This Mk. 2  model is no longer paunch powered, but has its very own yellow strap and ratchet.  The spring balance replaces the calibrated bruising meter I have packed around for years.  In this photo,  200 lbs of pull  is being applied to the starboard limb to get it to lift away from the stanchion.  By selecting the correct size pin gauge to go into the gap that is formed between the limb and the stanchion,  a relative comparison can be made of the torque in each bundle.  For example,  from the above set up we found that when 200 lbs was applied to the starboard limb, it lifted away from the stanchion .240″.  On the port side it lifted .264″.  Hence the starboard side is more highly torqued.  The closer we get to balancing the bundles, the closer we get to making this baby sing.

My dalliance with super heavy bolts will have to wait a bit.   I happen to have a bunch of 1 1/8″ broom handle material.  (Thanks Richard.)   It is nice and straight and should make good shafting for a dozen or so bolts in the 6,000 grain class.    That is tomorrow’s task.

A couple of people have been pestering me to try a much heavier bolt.   Something on the order of 10,ooo grains seems to be the consensus.  While I have been heavily swayed by prior experience with  500 or so shots on the old Gallwey  ballista (see second posting for this blog, way back when), that worthy beast with its 5″ diameter springs seemed happiest tossing out a bolt around 5,000 grains.   Using the same ammo,  Firefly with her 3″ springs and inswinger design, has been putting out muzzle energies that are the ballistic twin of the old Gallwey machine (1100 ft/lbs.)    It may well be,  that with the new magnum springs preloaded to the max,  Firefly will actually prefer chunking out  heavier projectiles;   at least to the point her master doesn’t get all prissy about the rainbow trajectory.

A flat shooting bonanza is still the final goal.  However,  there is no reason not to start tomorrow’s tests by firing off a heavy harpoon  or two.   Who knows what useful traits we might discover?   Perhaps something retrograde for the Japanese whaling fleet.

The decks of have been cleared for some serious testing.   Just wanted to assure any readers, the molecules are starting to align.   There  should be several buckets of data to report very soon.   Me go back to man cave now and work.

Daughter Sarah has just been subjected to portions 0f this blog.   I have been assured that as a soporific, this project has few equals —  although any of Henry Kissinger’s tomes on international relations would probably be a worthy substitute for the truly sleepless.   Ambien aside,  eyelids fluttering closed are never a good sign when one’s audience is trapped by its own politeness.   The trouble with grand obsessions is that they only seem grand to the obsessed.  If you are reading any of this blog it is probably because, on some level, you too have been touched by the Catapult Gods in their endless quest for new converts.  Whether you are hurling pumpkins,  slinging rocks or blasting out bolts, a deep seated need for some form of kinetic recognition has captured your imagination.  Yes, it is fun.  Yes, it is dangerous.  And yes, you are being driven on by forces outside of yourself.  As a long time sufferer of this disease, I ask all my fellow enthusiasts to take a moment and look into the eyes of your loved ones.  If they appear at all like Sarah’s, you will know the Gods are pleased with you.

This is still down the road a bit yet.  The challenge:  design a twin lever hand winch that will quickly pull back the bowstring at the lower weights experienced during the first half of the draw, yet also be able to gear down and tackle the higher weights at full draw.  A possible solution:  make the drum on the winch a cone shape.  The larger part of the cone will pull the rope in quickly, and as the rope winds on the drum the apparent leverage will increase as the drum diameter decreases.    If it can be made to work, this approach would be a lot simpler and more robust than changing gears somehow.  By keeping a careful record of the rate of change in the draw weight as the string is pulled back, it should be possible to figure out the most ideal cone shape for the drum.

Keep thinking Little Noddy.  If your head doesn’t explode first, you may actually finish this project before the barbarians breach the gate.

As the washers and crossbars are tightened, they move in the direction shown by the red arrow.    Their current location is fortuitous because it looks like the final position of the crossbars will max out  right over the top of the stanchions.    This is ideal because the unsupported portions of the end caps will not be subject to the massive pressure coming from the crossbars.    All my earlier springs  had limited  linear tension cranked into them,  so the spiral in the spring was much greater, and that allowed the crossbars to  rotate around far enough to end up midway between the stanchions.   As mentioned in an earlier posting, this caused some minor bending of the end caps (now all straightened out) .    In response to this minor disaster, the vernier plates were  upgraded with some spring grade 4140.   However,  with the new straighter springs, it appears  I could have saved myself the trouble.  (What is a vernier plate or an end cap?  see posting from 12/25/2008,  “The case for the vernier plate”).

With the crossbars looking like they will have their rotation max out so close to the stanchions, it does kind of raise the question:  is this just good luck?   Or are we looking at some fundamental elegance inherent in  the original Roman design?   I am betting that any torsion spring worth it’s salt (nylon, sinew, or horsehair), and installed with a maximum level of  linear tension, will naturally have the crossbars max out pretty close to the stanchions.   Besides,  if they rotated much beyond the stanchions, the springs would begin to lose that  desirable condition of,  “straightness”.  No matter how you look at it,  it does seem like a positive development.   I sense that the original designers of the Orsova machine, were very much aware of these dynamics.

It is probably a good job that we are located out in the boonies and have miles of wilderness to cavort in.  Firefly  managed to chuck the last of my old Gallwey bolts a bare inch over the top of the backstop today.  That worthy projectile shattered itself into pieces as it’s blunt tip ploughed 3 inches deep into a pine tree.   The Becky is dangerously protective of her trees.   Fortunately for me,  the pine in question had sustained heavy damage from a windstorm and was slated to be cut down for firewood anyway.   The chron0graph showed that this 5,000 grain bolt was trundling along at 245.5  fps.  That kind of speed is pretty low.    On the other hand,  I had only increased the washer rotation by 7 1/2 degrees from the 220 fps achieved on Sunday,  for a net increase of 25 fps.   Yesterday’s prediction on where the Wheatie meter would be after another 7 1/2 degrees of  rotation, was off a bit.   I am still able to shift the limbs away from the stanchions by 3/4″.   This is good.   Very good.   It means I haven’t entered the serious power zone for these new springs and yet we are still churning out over 669 foot pounds of energy.  None of my previous testing has shown anywhere near this much promise at this early stage of tuning.

Further testing will have to wait until I make up some more bolts.   Also,  a new synthetic winch cable has been ordered.  The old one made from wire rope jabbed my hand pretty deeply today,  so I will back off more testing until next week when the new cable arrives.

An easy way to tell how the rotational tuning of the bundles is proceeding, is to turn oneself into a human torque meter.  As can be seen in the following vid, it helps not only to have eaten your wheaties, but to have retained a few of them to act as protective padding. 20100207201050

The degree of torque seen in these bundles has reached the point where I am only able to lift the limb off the stanchion by an inch or so.  With another 7 1/2 degrees of rotation, I will barely be able to move it a 1/4″.   Another 7 1/2 degrees after that,  and it will be impossible for me to shift it at all using this technique.  Past experience has shown that at that point, the velocity of the bolt will climb dramatically for every new 7 1/2 degrees introduced into the bundles.   Along with the rise in velocity will be a concomitant rise in the draw weight.    At this time, I have no desire to push Firefly much beyond 4,500 lbs of draw weight.   As soon as I can make up some new bolts, we will see what kind of velocities  can be recorded as she approaches that limit.  Today we managed two shots and were able to get the old 5,000 grain Gallwey bolts up to a sedate 220 fps.  However,  at this stage the bundles have barely started their rotational tightening sequence,  and today’s draw weight was only 2800 lbs.   Compare this to when the bundles were made from 3/8″ double braid.  The average velocity then was a miserable 200 fps,  while the draw weight  hovered around  3, 700 lbs.

These new “magnum”  springs have a long way left to go before they top out.   I am increasingly hopeful that we can work up to 320 fps with the 5,000 grain Gallwey bolts, and hit 400 fps with  the lighter 3,000  grain Dura’s.  That will be more than enough to justify all those Wheaties.