The Ballista

The snooze continues.

…….. Wait a minute though.  Something just woke me up.

…… Porcus Maximus.   All dressed up, and only one place to go.

I am thinking that if Myth Busters can get away with using a mini-gun to machine gun a filleted salmon in a barrel of water (the, “shooting a fish in a barrel, ” hypothesis),  then clearly the Little Catapult Factory can study the terminal ballistics of a 6,000 grain Roman ballista bolt, when it is zipping along at 330  feet per second,  and collides with helmeted Porcus here.  It really is science.   I promise.

Tomorrow I’ll switch to sharps.  Gotta get ’em dialed in for this.

The deepest thanks to Clodius Secundus  LEGIO lll CYRENAICA .   The helmet  is just the ticket to spice up our research  a tad.   Randi Richert will most certainly be Mentioned in Dispatches.

My tryptophan induced lethargy is starting to lift.   Through a veil of turkey breast, mashed potatoes and spectacular brussel sprouts,  I see an ancient ballista bolt hissing with clarity.

It doesn’t matter what your passion is.

Make it.  Experience it.  Enjoy it.  Understand it.

Also, be thankful and do no harm.

Most everything else is just advertising.

Click for dessert:   20111022124922(2)

Today I tried a lighter weight bolt.  It tipped the scales at 5, 825 grains (377.5 grams), and was clocked going at 345 feet per second.  This yields a muzzle energy of 1540 foot pounds.  Compare this to that recent series of shots , where we had a heavier 6893 grain bolt clocked at an average velocity of 300 fps,  for 1377  foot pounds of energy,  and  it does appear that Firefly prefers these lighter bolts for delivering a punch.

More speed, more energy, a flatter trajectory:  what’s not to like?    So far I have only been able to record this fairly boring video of our new found velocity gains.  There was no particular target I was trying for, it’s  just a flight test into the back stop.  Click for vid.   20111118133451(1) .    The snowflakes give it an animated, almost Ridley Scott kind of look.  However,  everything is exactly as the camera captured it.   Firefly sounds much quieter now that those bronze hard stops have been banished to a desk drawer somewhere.

It’s not all roses though.  From my perspective as the shooter there was a slight off-axis spiral in the tail of this bolt as it flew through the air, perhaps an inch or so out of true.     I believe I’ve figured out what is causing it and how to eliminate these occasional waggle tails once and for all.    More later.

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This just in:

Last shot of the day.  Bolt weight 5,825 grains, velocity 358 fps, energy 1657 foot pounds.   It looks like that “parity” to my old Gallwey machine is starting  to disappear as Firefly’s tuning intensifies.  Also, zero waggle tail.

In the photo below, the relative size difference in the torsion springs between these two machines is a real eye opener when you appreciate that they are, quite by chance, almost identical in power.  (1450 + foot pounds of muzzle energy for Firefly the inswinger on the left, and 1400 + foot pounds for the wood framed,  Gallwey outswinger  on the right.)    This parity may change substantially when the Mk. IX limbs are installed on Firefly.  Work on these new, lighter weight limbs is underway.

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Now that those brutish hard-stoppy things have been been excised, there is an air gap between the limb and the stanchion.

In this next photo we see the gap on the port side .

And this next is the starboard side gap.

I have discovered that if I tighten the washer by the minimum amount (7 1/2 degrees) , that translates into 3/16″ of movement in the lateral stopping point of the limb.   In the above photos, the air gap on the port side is slightly larger than the air gap on the starboard side.   This small disparity in the gaps does not seem to hurt performance at all;  the condition occurred when I  induced an additional 7 1/2 degrees of twist into the starboard bundle.  Before adding that 7 1/2 degrees, the situation of the gaps had been exactly reversed;  more gap on the starboard side, and less gap on the port.  This is an indication of the effect of our primary tuning variable:  the seven and a half degree balancing act between the torque in the two bundles.  The gaps seen here have stayed constant for the last two days and through 14 shots that averaged 300 fps.  If the gaps remain constant like this,  it is a positive visual indication that the machine is staying in tune.  The sheer convenience of this cannot be overstated.

The 7 1/2 degrees of rotational discrimination in the washers is something explained in an excellent little book called “Engineering in the Ancient World”, by J.G. Landels.  Mr. Landels was Senior Lecturer in Classics at the University of Reading.  (Cheers! to good old Reading!  I grew up there back in the fifties and sixties.)

This next photo can be enlarged by clicking on it.

In this diagram Mr. Landels shows us the locking geometry for the washers on an ancient catapult that was discovered in the remains of an arsenal in Ampurias, Spain.  The combination of the 22 1/2 degree hole pattern and the 15 degree hole pattern,  yields a Vernier type relationship between the two hole patterns.  This, in turn, allows a minimum rotational discrimination in the washers of 7 1/2 degrees.  In other words, it controls the amount of twist put into the bundles by 7 1/2 degree increments.  Interestingly,  Mr. Landels goes on to suggest that this is perhaps the first recorded incident of a Vernier type system showing up in the historical record.

See you were right all along.  You just knew there had to be a reason it was seven and a half.

The bronze hard stops have been removed.   They used to be lashed into the pockets of the curved stanchions.    My experiments over the last week have focused on trying to balance the effect of hard stops that provide a fixed stopping point for the limbs, with the needs of a bowstring that must always feel tight and snappy in the at rest position.   This balancing strategy had become a maintenance nightmare of epic proportions.   Finally,  the time had come to ditch all those beautiful preconceptions I had nurtured for so long about the necessity of hard stops to protect the bowstring.

Our previous attempts at making the bowstring act like a limb brake had all met with failure when the bowstrings kept tearing in two.   The dacron yacht braid they were made from back then was of questionable provenance.   This latest stuff is  5/8″ Samson XLS and is rated for 13,000 lbs.  It seems to be just the ticket.   Now the smack of the power stroke is arrested entirely by the bowstring.

With the hard stops removed there is now a gap between the limb and the stanchion,  and there is no question that the bowstring is always taut in the at rest position.  Velocities are now averaging 300 feet per second with a 6900 grain bolt.  Vive! La difference!

While persistence makes change inevitable, it can also cause long delays when false assumptions are not thoroughly rooted out.  So much for compromise.

Here is our righteous bowstring doing the wapata-wapata  dance.  The limb bounce seems to be way down too.

Click for 4 x slow motion Vid.     20111116132702(1)

I realize that I haven’t posted any full length photos of Firefly in quite awhile.  Here are a couple of pics so that newcomers to our humble blogiment can see what this Firefly creature looks like.

The long bayonet extension for mounting the chronograph was developed in response to those still photos from the other day.  It looked like the string was flicking out far enough to interfere with the Doppler reading, (i.e. generate an error reading).

Added 7  1/2 degrees of twist into the top half of the port bundle.  Found that the shot did not move right far enough to have perfect agreement with the line of the bolt groove.

Added 7 1/2 degrees of twist into the bottom half of the port bundle.  This caused the second shot to move too far to the right.    And so again,  the bolt’s flight did  not line up with the bolt groove that is carved into the flight deck.

Added 7 1/2 degrees of twist into the top half of the starboard bundle.

Bingo!   With the third shot, we have the lateral line of the bolt’s flight  (i.e. the plan view of its parabola), coinciding neatly with the line of the bolt groove.

After six shots today, the string held a steady position just skimming the flight deck with a slight downward pressure.  So slight is the down pressure that this Wilkinsian axiom of a neutrally positioned bowstring seems possible after all.  (Sorry RR.  I have to report ’em as I see ’em.)  More testing will be needed to see how this change effects performance.

I would like to return again to the subject of the Dura Europos bolt.

Photo by, Yale University Art Gallery; and copied from, Greek and Roman Artillery 399BC-AD 363, by Duncan Campbell.

I quote myself from a previous posting:

“This artifact is the only intact specimen of a Roman ballista bolt ever discovered.  It was excavated in Dura Europos, Syria, and is likely the type of projectile used by the Orsova ballista. Bolts of this type must have been made in the tens of thousands by the Romans. The wooden shaft is made from ash and the quadrobate iron tip is a four sided square, designed to punch through shields, armour, clothing, etc.  The thinner foreshaft of the Dura Europos bolt was designed to aid penetration, it also raised the ballistic coefficient to aid in a long flat trajectory. The swelling at the rear of the bolt not only helped absorb the powerful thrust of the bowstring, but also acted as a kind of aerodynamic counter weight to help keep it steady in flight. This inherent stability was further aided by the short stubby fins.  These fins were made from maple and glued into grooves cut in the ash body.   Overall length is 18″, thickness at the rear end is 1  3/16″ and it tapers down to a bit over 1/2″ in the front.  It was reported to be a good deal shorter than the type of bolt that came before it.  Late style ballistas like the Orsova model did not have any kind of narrow opening around the bolt groove that the projectile would have to pass through.  (See earlier postings of Gallwey ballista.)  The arched strut on these later iron framed machines meant they were better suited to firing the shorter style bolts.  The danger with a short bolt is that if something goes wrong, and the bolt turns sideways during the power stroke, it will likely smash into the supports in the middle of the box frame.  With an arched frame the whole mess will be cast out the front, avoiding unpleasant ricochets and perforated catapultiers.”

Since writing that two and a half years ago,  I have experimented with several of these style bolts.  In my experience they all have one thing in common — without exception they all snapped in two a few inches back from the head on their first shot.   This feature, while no doubt appreciated by their originators as a simple precaution to prevent these projectiles from being fired back at them,  was not appreciated by yours truly who was trying to fire a long series of shots to build up data.  Consequently, I swore off Dura style bolts for general testing purposes, and resolved only to look at them again when the machine was finished.

I mention all of this because it is very nearly time to again pick up the trail of these elusive and fragile projectiles.   Because they seem to break no matter how carefully arranged the backstop is,  we might as well get some data out of them by experimenting with their penetrative capability.

The patient spent the night and morning convalescing as the PC-7,  high compression, epoxy paste set up around her walnut stock and iron saddle.  (That, I’ll warrant, is a sentence never before uttered on the planet.)

The stock and its integral flight deck and bolt groove,  have been shifted in relation to their previous position by one degree in the XZ axis.  This effectively raises the nose of the flight deck by about 3/8″, so that now the string rests on the deck with a gentle pressure,  (i.e. the at rest string deflection suggests that the center line of the limbs is set at about  1/4″ below the deck).

Here is a photo showing the altered saddle.

I welded up the top edge of the saddle and milled it into the one degree angle seen here.   Now the edge margin on the forward slot is greater than the rear slot.

By afternoon,  she was ready for camera and action.   This is the exact same bolt used in the previous test when that pesky 3/8″ string gap caused the tail to lift up.  Click for 4X slow motion video:   20111107135003(1)(1)

And broken down into stills:

Our personal state of the art in video and such like,  makes these ghostly images the best that we can manage right now.  I’m not so sure that they aren’t telling me exactly what I had hoped to see.  To my eye the bolt seems a little less down pitched than the one in the stills from the previous posting.

Also, the flight of the bolt looked very clean to the naked eye.  No porpoising or waggle evident.   At 50 meters  it struck a foot to the left of its previous sight setting.   This shot vector is way off from what the center line of the bolt groove would indicate as ideal.  The divergence of the shot to the left suggests that another 7 1/2 degrees of washer rotation on the top half of the port bundle is in order.  It should be easy to correct tomorrow.

The important thing here is that the bolt showed no signs of porpoising.  Porpoising is when the bolt can be seen to be traveling nose up or nose down for a fraction of a second before the fins straighten it out.  Porpoising is a lot of wasted energy.

After the shot,  the line traced by the bolt going through the air should appear clean and minimal in your memory.  There shouldn’t be any fuss going on.

As promised, I managed to get some video of a bolt with a shaft diameter sized head on it (i.e. no up-pitch created by a larger than shaft sized head on it), as it is being fired.  We are testing to see if that excessive gap between the string and the deck is causing the bolt to pitch downwards coming out of the machine.  This is important because I do not want to be restricted to only shooting bolts with oversize heads.

Here is the 4X slow motion video,  20111104142657

And breaking down the action into stills, we see the following:

The second photo in this sequence clearly shows the bolt with its tail raised up higher than its head relative to the flight deck.   This will never do if we are to test something already as  naturally down-pitched as a Dura style bolt.

It looks like Firefly will need to go into surgery for a few days as I correct the angle of the stock and flight deck relative to the vertical axis of the field frames.