The Ballista

Finally the decks are cleared for a serious push on these Mk. VIII limbs.  I mention this fact only because after the complicated mold I just got finished for a favorite customer,  it is going to take a mighty effort to get my nose settled down in front of the grind stone again.   We start with that most refreshing of miracles,  a  clean work bench.

At times like this it is also helpful to remember that the exact scope of what we are attempting is not all together without merit.  I take the liberty of quoting Mr. Pat B. where he provides the following words of encouragement in a recent email:

“First, some encouragement. You asked me a while back about any other
constructors doing something similar to your project. I was reading
Duncan Campbell’s “Greek and Roman Artillery” recently, and he says
this: “The idea of interior swinging arms is not a new one, but it has
never achieved wide acceptance, no doubt owing to the lack of a
full-size replica to demonstrate the practicalities.” That was
published in 2003, and the info could be out of date, but I suspect it
isn’t. I’d say you are out there on your own with Firefly. So in your
darker moments, when you feel like giving up, you might remember that.
No one will find out if the inswinger is a feasible artillery weapon if
you don’t.

But it’s also worth remembering that with no existing models to go on
(and no real clues from Trajan’s Column either, which feature an
artist’s depiction, not an engineer’s drawing), what you’re likely to
end up with is something very different from the speculative mock-ups
of theorists. It might be better to put those out of your head. Because
if your experiments push you in a certain direction, you can be sure
the Romans went that way too. They had plenty of time, centuries, to
evolve their designs, and you can bet they explored every avenue in
search of the most efficient machine. In other words, whatever design
you find works best for you is also likely to be the most authentically
Roman, however bizarre it looks. ”

After so many set backs and flat out failures,  I find that this project is taking more faith and commitment than any I have ever attempted before.   If  I keep reminding myself that this game we are playing is at a level not likely seen since ancient times,  that fact alone helps to keep me focused on the task at hand.  Of course, this laudatory slathering is nothing but pure conceit,  and while it is always the case that there is nothing new under the Sun,  a vigorous suspension of my disbelief seems essential to move things forward.

And so,  we commit to yet another set of limbs, this time with more experience than I care to admit to.  The Catapult Gods favor the number eight.  Or at least,  so I tell  myself.

Nibble, nibble.

Using clay and packing tape, the mock up below shows the basic form of our newest  set of limbs.   Barely visible is the Kevlar tension strap that will be bound and laminated onto the back of the limb.   This strap is also mechanically anchored to the limb iron.

Because I have a hard time keeping all the screwball limb designs we have tried so far, straight in my head, I’ve decided to designate these as the “Mk. VIII Tension Enhanced Limbs”.  These Mk. VIII limbs feature more,  and better tempered  steel in certain critical areas.    Other less critical areas will have steel removed to save weight, (see crosshatched area).

A word about the use of kevlar:  At this time, Kevlar is the preferred material for fabricating a robust tension strap for the back of our Ash limbs.   It is  easy to work and provides a predictable and reliable approach.    If the concept shows promise, it should be possible to devise  tension straps from more authentic materials,  ( i.e. steel tension band, sinew tension band, silk tension…..etc.)

Our crude model is perched on top of the very lovely 4130 material I will be using to make the new irons out of.   Cutting of same begins in five minutes.  Photos later.

“What’s that thing Daddy?”

“Well daughter, that’s clearly a Mk. VIII  ice skate.”

“How do you know that?”

“Because those holes in the end are obviously there to accommodate a pair of cross pins.”

“So?”

“So how else are you going to attach a double wrap of 1/16″ Kevlar webbing?”

………… “Daddy,  do you have fat ankles?”

“The fattest.”

The Rebecca brings some scale to the size of my most recent score in the ash plank department.  Nice tight grained stuff.

Resilient, shock proof (somewhat), and just generally an agreeable material to work with; ash is also one of the more authentic choices for limb timber.  It is light enough and stiff enough to make an effective ballista limb.

The short, darker post in front of the Ash planks, is made from a  3 1/2″ square cross section of very dense Brazilian Walnut,  commonly known as Ipe.   Scoring something like 3500 on the Janka scale, Ipe is an extremely hard, hardwood.  I will utilize all the cross section of this Ipe post to turn up some new,  3 1/2″ diameter drums for the hand winch.  The first set of drums were made from ash, which scores a lowly 1300 J,  and under intense pressure from the winch rope,  has started to crumble like Weetabix.

While we wait for materials I would like to share some visceral depictions of how a torsion powered projectile must have appeared to anyone on the receiving end.    In this footage from last year we recall the ancient hiss and blur of a fast moving, heavy ballista bolt.  Click for vid.   20100309103750(1)

Note the black fins blowing off at impact.    I believe that a long length of the shattered bamboo shaft  did a  spectacular belly flop against the side of the apple bin.   In the photo below we see the  secondary impact site where some kind of fragment, at least,   swatted the side of the apple bin.     Tangled in the weeds, this is how the apple bin  looks now,  a  year or so later.   Artifacts are where you find them, I am told.

I mention the apple bin only to illustrate the residual force still left in the projectile after it had already nipped a chunk out of a  1/4″ mild steel plate with its hardened, square headed bodkin point.  (See below.)

Anyway,  Ouch! on both accounts.

As I recall, the bolt involved in this escapade weighed 7,000 grains and was clocked at 3o7  fps.

………….   So okay, Mon Capitan, by way of comparison to last week’s attempt:   “….. Exactly that heavy and almost certainly faster.”    I’ll say no more on the matter.

It is clear that my tempering of the car spring material left the steel in our limb irons a tad brittle.

I remain convinced that this design for the limb is basically sound.  It was the execution that was at fault.  The next set will not have that ridiculous cutout in the rear and will be made from some brand new 4140 rather than the salvaged car springs I have been using.  I am also thinking it might be a good idea to send the irons out and have them professionally  heat treated as we have clearly reached the limit of my abilities in this area.

The video in the last posting showing the limb rupture is quite instructive.  With an inswinger style machine all of the heavy fragments are  directed in towards the stock in what is a relatively safe direction.  Not that I will allow this observation to diminish my prudence, but it is heartening to see this worst case scenario prove relatively benign.

For now my mind clings to the memory of that one good shot we managed when the news seemed at its best about this project.  The chronograph was not set up as the light was not right for it to work properly.   What I remember of the event strikes me still as remarkable for its velocity and flat trajectory.  I will not hazard a guess at a numerical value for what I saw.  Suffice it to say, the witnessing of that one uniquely powerful shot has infused new momentum into this quest.

Don’t you just love Mercury Retrograde?

Click for Vid:    20110803142319

After shortening the bowstring, I also did a little rework on the pulley block on the winch and have temporarily removed the hook that was there.  This allows the pulley block to connect directly to the dynanometer and shortens up the whole arrangement so that I can get up to a 53 inch draw and still have draw weight readings available.  The one shot that I tried this afternoon only  registered 3500 lbs. with a 51 inch draw.   I was shooting a 7,000 grain bolt and compared to any of the other shooting I’ve done with this machine, it appeared fast.  Very fast.   Given the relative lightness of the draw weight, this is the best of news.

Subjective impressions aside,   I’m eager to set up the chronograph and try to get some hard velocity readings.  More tomorrow.