The Ballista

Each morning we awake to smoke.  It hangs in the air as a gauzy reminder of how vulnerable this land is to the slightest spark of ignition.  The field where we have been testing Firefly is covered with cheatgrass.  This insidious weed, apart from having barbed seeds that worm their way into clothing and cause festering wounds when they penetrate the skin, has the mildly apocalyptic habit of drying out into the most finely textured tinder imaginable.  Horrible and dangerous stuff, but with the saving grace of making an excellent beer; although this latter is hardly enough to mitigate its destructive effects on the landscape.

Fire is a constant danger in late summer, and the fear of it has curtailed our field trials until the first rain.  So,  just in case you were wondering,  we may be taking a snooze, but the dreamin’ ain’t over yet.

Now that I’ve had a moment to think about it, it seems logical to break up Monday’s test group into 3 smaller groupings.  (Click to enlarge.)

Those three circles in the chart each have a radius of 24 feet.  The fact that the shots inside these circles are all consecutive in the order in which they were fired, is very significant.

Here is what I observe about these shots:

Shot # 1 came out of the gate pretty darn fast, as shot #1 very often does.  We discount it as an enthusiastic anomaly.

Shots # 2, 3 & 4 plunked down conveniently inside a 24 foot radius.

Shots # 5, 6 & 8 also fall inside a neat 24 foot radius. (My field notes confirm that shot #7 was performed during a period of gusting winds.  Actually, it is that same shot we provided a pull back video of yesterday.  Wind was quartering over the starboard side as I remember.  This probably explains why it is so far out to the left of the middle circle.  And so, we discount it too.)

Shots #9, 10, 11 & 12 all hit inside another 24 foot radius.

It is clear, that when viewed as consecutive shots, these groupings indicate a classic  case of  “wandering zero”.

Cases of wandering zero are tricky little buggers.  I have a British Jungle Carbine that suffers from this same condition.  Take it out on Monday and it’ll shoot a nice tight group right in the center of the bullseye.  Take it out Tuesday and it’ll still shoot a nice group, it’s just that the group has strayed several inches out of the bullseye.  Another day, another movement of the group.  This is wandering zero.  The Brits never did cure their jungle carbine of this annoying tendency.  But, guns ain’t catapults, and I’m still optimistic about our chances here.  In Firefly’s case it could easily be that we just need to shoot her some more until the bundles fully stabilize after the installation of the new limbs.

If the wandering zero issue can be resolved it is likely that Firefly can put a substantial majority of her 12 matched bolts into a single 24 foot radius at 800 yards.   This is going to be interesting!

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The all time king of “wandering zero”,  my Dad’s .303, Mk.V, Lee Enfield, “Jungle Carbine”.

That big ol’ allen bolt trapped in the curved stanchion of the field frame is just a positioning spacer to see how our new Mk.X limbs are going to look on Little Miss Firefly.  Important stuff seeing as the Fall Fashion Season is almost upon us.

Well, I think they’re gonna look kinda cool.  And who’s to say they ain’t authentic?  Nobody really knows what those pesky old Romans got up to when designing the limbs for their radically innovative, iron frame inswingers.   After all, they were Italians right?

The nocks on the end of the limb, and the compression rings in the center, are formed by glue soaked wrappings of a high tensile string. I am confident the binding technique seen here, could be credibly rendered in flax/sinew and natural resins/glues.

Exploiting the characteristics of any particular design requires a granular understanding of the details.   Surely the Romans were big-time granularists.

When their epoxy soaked bindings finally cure, these Mk. X  limbs will need their bandages removed and a few minutes of sanding to tidy them up a bit.

To form them into an oval shape,  the  bindings are constricted by some temporary wrappings  of plastic film. Laid over that is even more tightly wound electrical tape.  With these low stretch Dacron bindings, the spreaders that are installed under the tension bands should hold their position nicely.

The high tensile wood screws seen projecting from the butt of the limb are screwed into pilot holes, with a smear of epoxy added for good measure.  The screws will be cut flush and they should guard against any of the minor end grain splitting we witnessed in the Mk. IX’s.

With any luck we should be able to risk a shot this coming Tuesday.  The weatherman promises a 3 mph wind from the north.

An upward tug with the chain hoist quickly finds the limit of all the easy stretch in our “low stretch”, Dacron tension band.

Some equally inauthentic Nylon serves to form a pair of  spreaders that will redirect the force vectors acting on the limb.

These spreaders could probably be made from hard white oak, the grain crosswise to the axis of the limb to help counter splitting. However, our purpose here is only to test the principle of  a vectored tension band, and at this stage I don’t want even the remotest chance the spreaders will self destruct.  Progress is aided by the careful excision of potential hassles.

Also, just visible in that last photo are shallow scallops cut into the tops of the spreaders.  These depressions will receive sturdy bindings that will anchor the  spreaders and tension bands down onto the ash limbs.  It is imagined that this arrangement will sufficiently stiffen the limb that the 1/4 lb. weight invested in each spreader assembly will be paid back several times over by allowing the limb to explore higher poundage draw weights in greater security.

It is more important to test the principles that are at stake here than to get lost in making sure all the materials used are strictly authentic.  Once we get her ticking,  it is likely we can reverse engineer the better designs back into period correct materials.   This is all part of the process.

Shortly before it’s rupture, after being inadvertently dry-fired during our last field trip, the port limb  (seen on the right in the photo below) demonstrates a tad more curvature at full draw than it’s companion on the other side.

It’s likely that slight  differences in the grain structure of the two pieces of ash effected the relative stiffness of each limb.  Tillering the limbs, as is common in regular bowmaking, is not really a practical venture in ballista making.  It would  undoubtedly be better to devise a scheme that simply raised the stiffness of both the limbs.  This is not so easy if you are trying to keep the weight down, while still presenting a historically authentic plan.  Clearly, the Dacron material in the tension backings presented here is far from authentic, but as an analog for sinew or flax in a composite type construction, it is not completely without merit.  Similarly, we might regard these Dacron backings as prototypes for ones made from thin strips of spring steel; a material that would most certainly pass the authenticity test.

A plan comes to mind where a wooden spacer could be inserted under the middle of the tension band to raise it away from the surface of the limb.  This would redirect some of the forces back into the center of the limb and stiffen it with a minimum weight penalty.  This would be similar to the design  concept of the ancient cable-backed bow.

The better route that we seek is not always a neatly disposed, straight line.   Meandering baby steps can sometimes be turned into real leaps forward.  At least, so we tell ourselves halfway out of that hole we stumbled into last week.

The Mk.X replacement limbs are also made from ash, just like the previous set, and have been fitted into the limb irons ready to have their tension straps installed.  Firefly  will be up and running by the end of the week and we’ll be back to fretting over the wind again.   In the meantime, my training regimen begins:

First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin. First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin.  First the bolt, then the pin. First the bolt, then the pin.  ………

Etcetera.

It would be charitable to say, “they” were not amused.  Melted the wax from my wings and everything, “they” did.  The Catapult Gods are an unforgiving lot.  But we’ll get to all that.  There are the first two shots to talk about before that boom gets lowered.

The weather promised to make Friday afternoon quite the excellent little shooting session, what with the wind down below two knots, the sun at our backs, and the sky a bright, bright blue.   It was the sort of day I’ve been dreaming about these last few weeks. We even had our good friend and neighbor, Richard Rough, along for company.  It sure looked like it was going to be a productive field test.

Shot number one was a real Jim-Dandy.  It left the machine without the slightest trace of tail waggle and burrowed into that perfect sky with one of the cleanest launches I’ve seen.  The naked eye can only track Firefly’s flying bolts out to maybe 200 yards, not even halfway to the apex of their parabola.  This heavier bolt, with it’s greater length and forward  shifted center of gravity, was performing brilliantly.  Click for vid:  20120907145206(1) .     It clocked in at 330 feet per second, and I suspect that is  largely because I had just changed the serving on the string.  This means that this 521 gram projectile developed 1944 foot pounds of muzzle energy, which is about 175 pounds higher than we were getting before with the 400 gram bolt.  For a machine of this type, that’s a hefty chunk of horse power under the hood. When we got our laser readings on the distance they showed that this shot had traveled a confirmed 837 yards.   With a bolt this heavy, that is at the higher end of the range envelope I was hoping for.  Also the bolt penetrated to a depth of twelve inches into the packed dirt, rather than the eleven inches it had on our last trip out to this particular field.  I figured the extra 121 grams of projectile weight were at work here.  Things were going great!

….Too bad I was about to fuck it all up.

Things started to go south with shot number two.   We decided that someone should run the engine in the truck while I cocked Firefly with the electric winch.  This would allow us to charge the winch battery and shoot indefinitely.  Or at least until we ran out of gas, and that wasn’t likely as I’d just filled up the dual tanks.

(Good thinking little catapult maker.  It sure sounds like you’ve thought of everything!)

As it was, the action of having someone run the truck engine was enough of a disturbance to my fragile powers of concentration that I neglected to put the winch into drive mode, and experienced an embarrassing  slipping of it’s clutch for way longer than I should have.   Such is the sensitivity of my nature for making a bungle of things, I remember wincing at the thought that the cameras were recording all this slovenliness.   I mention this oversight only by way of backdrop for what happened next.

After finally getting the winch into gear and completing the firing procedure, shot number two showed a mildly disappointing velocity of 312 fps.   Click for vid:   20120907150230.    Why so large a difference in the two speeds?  ……My fussy little target-master was getting his anal on.

This is not what the doctor ordered.  Not what he ordered at all.  God knows what this was going to do to our precious group size.  (Later, shot number two was found 40 yards short of shot number one, although in a nice straight line with it.) Clearly, this was not the sort of gilt-edged precision I was hoping for.  Shooting off my twelve matched bolts into a nice tight cluster at almost half a mile just wasn’t going to be that easy.   Not easy at all, by the look of it!   (Can you hear the mania creeping in?)  And then in ways worse than imagined,  I managed to implement  that prediction with astonishing efficiency.  “Easy” was about to be a doomed option.

Somehow during the final stage of the firing procedure for shot number three I broke the usual sequence of first loading the bolt and then pulling the safety pin, in favor of first pulling the pin, and then abruptly having the machine dry fire itself before I could correct the situation.

Firefly was quick to tell me, she didn’t like that.

Clearly, the contact angles on the catch and trigger need to be adjusted.  But no need to place the blame there.  Whatever the reasons, I managed to perform a dry fire of my precious baby.  A very big No-No.  Many thousands of demerits will be accrued over this one.  Without the payload of that heavy bolt to dampen the snap of the bowstring at the end of the power stroke, it was more than the port limb could bear.  As limb breaks go, this one was fairly boring.  More of a rupture really, than a break .  The tension strap worked admirably, and, as if to add some measure of order to this minor catastrophe, the splinters dropped neatly into a waste paper bin I’d been using to hold the spare bolts.   Still, it was all very stupid.  So stupid!

However,   I learned a long time ago that if you fall off the catapult, you just dust yourself off and climb right back on. The Mk. X limbs await.  As do a reconfigured trigger lever and rigorous new training regimen.

Ain’t aggression wonderful?  Often it’s just what’s needed to climb out of that hole you very carefully dug, and then fell into.

Seeing as we are getting so picky in our old age,  I decided to invest in this little honey to brighten-up the proceedings.  If the Weather Gods cannot be placated, they can at least be measured.

It should show up on Monday, the 10th.  Perhaps by then we can do better than the six knots the weatherman is advertising for Saturday.  If not, then knots be dammed, we’ll settle for five or six of ’em, and pray for lulls.

A knotty problem to be sure. (Wince)

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And later that same day:

In our pre-flight testing,  at 55 yards we managed to plop the first two shots right through the same hole before the deep cycle battery chugged to an end.   The battery has enough juice to cock about a dozen shots before it needs to be recharged, and it looks like I used up ten of those just testing the winch.  A few hours on the charger and we’ll be ready for action again.  (Note to self:  purchase longer jumper cables for field testing.  Then, with the motor running, the truck can power our little darlin’ all afternoon.)

Note: in the above photo the radial slices in the cardboard are where the fins cut through.  It is interesting that the rotation of the bolt induced by the spiral in the placement of the fins does not yield a more consistent result at this short range.  I would have thought that if the second shot had the good grace to go through the same hole as the first, it might have also had the fins rotate it into the same position.  Evidently, not.  The same bolt, with the same speed, at the same range, hitting the same mark, does not necessarily clock it’s rotation to a fixed point.  Just another factoid in the great puzzle that is this thing we are doing.

Both of these shots were done with the same 521 gram, blunt tipped bolt, from our recently made matched set.  Velocity was 316 fps for each shot.   That means our new set-up is producing 1782 foot pounds of muzzle energy as opposed to 1771 foot pounds back when we were shooting the lighter  399 gram bolt at 360 fps.  That lighter bolt is the one that made it out to 892 yards on our last field trip.

…not to put too fine a point on it or anything

Every preparation that can be made for our extreme range accuracy tests, has been made.  Now we must wait for a calm day. Without that there is not much point in even going out.  One or two knots is all I ask (although even that is a bit much for our purposes).  The forecast shows that everything in the near future is twelve knots and above.  One fellow has suggested that a bit of wind won’t hurt the accuracy of my one pound bolts at all.  Which, of course, is all proper nonsense.  Some folks just don’t understand the shooting sports, or, apparently, the exact nature of what we are trying to accomplish at half a mile with a chunk of ancient torsion tech.

Although, in fairness, it has been observed before on our little fifty yard firing range behind the shop, that a difference in ten knots or so doesn’t seem to move the group placement at all.   At that short range the shots kept plopping through the same hole in the target with monotonous regularity.  Which, of course,  is exactly what we’re after.  However, at 800 yards, the effects of air movement on the bolt will almost certainly be enough to degrade the group size.  Especially if it is gusting.

And so we wait.

Wait and wait and wait…..