The Ballista

This will be our fourth field trip in search of the performance  envelope of the elusive, Lightning class,  ballista.  The cast of characters in our afternoon adventure, were as follows:  The Rebecca, Brian & Kachi, an impishly good-natured Angela, the noddypoll doing the shooting, and, of course, our faithful reconstruction of the Orsova artifacts, Little-Miss Firefly.  In good “Lightning” fashion, we find Firefly thumping out her ancient war dance, on a flat plain a few miles West of town, early on a Sunday afternoon.   Click for Vid.    20121028141638 .   In that video, we heard Kachi’s reaction to the first shot of the day, which was performed with a 521 gram bolt from my matched set of extra heavy  bolts.  And now, for the discerning eye,  that same shot rendered in a better quality mpeg.  20121028141638(2)

In this next photo, that tiny diagonal fleck on your computer screen is one of our “heavies” rocketing skyward.  Click to enlarge.

And here is the video of the event that this still was clipped from.  Click for Vid.   20121028145328(3).    The camera location here is about 100 yards off of Firefly’s port side, and maybe 50 yards downrange of her firing line.   It is just possible to glimpse the flight of the bolt in the first two seconds, before it exits the field of view and climbs up the slope of  it’s nearly 1/2 mile parabola.   Although it can’t be seen in the video, the bolt  landed 50 yards short of the end of that grassy plain we are shooting down, just a sliver in front of where the sage meets the grass at the end of the field.

When they were standing on the rocky outcropping, off to the side,  my companions were able to confirm that there was a distinct whistle coming from all three of the Dura bolts that we fired.    In this next video, that whistle is clearly audible,  but unfortunately cut short by the bellowed exclamations of that big twit in the steel helmet. Witnessing such a stable launch with his first Dura Europos bolt, left the poor fellow unhinged with joy.  Click for vid.   20121028143301(1)

And finally for the videos, we have this launch moment of a “heavy”contrasted nicely against the overcast sky.    Click for Vid.   20121028142650(2)

……So anyway, that was the best of what we were able to do in the filming department.  Yeah! I know, it’s all a bit too elusive and hard to follow.   But who knows?  Maybe there is someone out there who will value the humble reality of it all…..

And now we move on to the data.

I suppose I’d better cop to it right from the git-go.  My intention to break 1,000 yards with the new Dura bolts was never given a fair chance to succeed because it found itself countered by another one of my intentions.  This latter involving a wooden wedge.

Long story short — I implemented this bright idea to tighten the lashing that binds Firefly’s tail to the counterstay with an oaken wedge driven between those two parts.   Clearly a “senior  moment” was having it’s way with me.   The effects of this little brush with senility ended up  changing the elevation of the machine; dropping her angle from an intended 44 degrees, down to 38 degrees. Naturally this caused all of the day’s shooting to fall short of it’s true potential.   Something I didn’t realize until I slapped the inclinometer on the deck for a quick check after the shooting  was finished.  Not my finest moment, and all that.

So whatever this field test is about, it is not about maximum distance shooting any more.   This scale chart shows an average range of 728 yards for the heavies, when we had something like 790 yards on  the previous trip.

Click to enlarge.

And when we include the firing line,  the chart looks like this when everything is to scale.

Despite the fact that these groups  are a good sixty yards short of the range achieved with our previous field tests (thanks to that pesky wedge),  the size of the groups is much improved over our previous attempts.  The three Dura bolts that we fired showed a perfectly stable flight pattern to the naked eye, and perhaps that is why they all fell inside a 30 foot circle at an average range of 854 yards.   There was virtually no wind for this field test, and this must also be why the groupings were so tight at this great a distance.  As for the heavies,  nine out of twelve shots fell inside a 60 foot circle at an average range of 728 yards.   Given the larger sample size of this particular test, this is a good indication of what these machines would have been capable of  in terms of indirect fire at maximum range.  More than enough precision to lob heavy pointy things into the ranks of  any close ordered enemy, threaten access to key thoroughfares and meeting places, or even dispense fire arrows into shipping at long range.  Just the kind of extended military capability a shrinking Empire is greedy for, as it topples towards it’s final destiny.

We also note, with some delight, that the case of wandering zero experienced in the previous field trip, seems to have resolved itself.   I am not even going to attempt a detailed explanation of why that might be so,  (probably the bundles finally settled in after the new MK X limbs were installed);   just thanking the CG’s for their beneficence.

The velocity for the 276 gram Dura bolts averaged 360 feet per second, while the 521  gram heavies were doing right around 300 fps.   It should be noted that in terms of tuning on the spring bundles, although we have monkeyed around with the rotation of the washers to improve the limb balance,  it has been done in a way that did not add any new torque into the system.  If one half of a bundle was tightened, then the other half was always loosened the same amount.  And it has been this way since the beginning of summer and for all the field tests we have done.

It may be time to up the ante, a bit.

Fog swaddled, the field we have been using for our 1/2 mile distance shooting, is not viable for testing today.  Visibilty is down to 200 yards maximum, and launching lethal projectiles without having their landing zone plainly in view,  just ain’t my thing.

Before we take to the skies again, I’d like to fix this little tuning issue.

Those wooden dowels clamped to the limbs are telling me that the starboard limb is sitting twisted in the bundle, while the port limb appears just about right.  In our last round of shooting we did notice that the starboard limb was rising above it’s ideal lateral plane as it approached full draw, perhaps due to how it was originally oriented when it’s “lock down” in the tightening bundle occurred.  Despite this discrepancy, during shooting, the bolt still managed a nice balanced lift-off from the machine, so there was no impulse to correct the condition at the time.

Now, however, these dowels indicate  more asymmetry in the sweep of the limbs than my highly balanced, catapult of a mind, can bear.

Corrections must be made.

Part of the hypothesis we will be testing with this adjustment, is whether or not the vice-like grip of the bundle is sufficient to lock-in the axial position of the limb.    We will loosen the top of the starboard bundle and rotate  the  limb in a counter-clockwise direction — then  those indexing dowels will more closely match one another and appear to be parallel to the bowstring.   Past experience suggests the limb will hold it’s new axial position just fine, once the bundle squeezes down on it during the washer tightening procedure.   But, we’ll see exactly how well that premise holds up when shooting commences.

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A few taps on my ancient, red handled pipe wrench caused the limb to twist into a more favorable position, and now the dowel indicators line up nicely.

After the bundle was tightened back to it’s original position,  the limbs still appear even.

Now for a few shots to see if they hold position.

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By the time we got to it, the light had fallen and it was not possible to see the flight of the bolt any more.  However, the next best thing was to cock Firefly and just take a gander at where those Mk. X limbs were tracking.

The answer was about what I’d expected.  Not good:

Viewed from the dangerous end, the starboard limb appears markedly higher than the port one.   It has been this way for all the shooting we’ve done these last few months.  If performance was adversely effected, it certainly wasn’t evident in the stability with which the bolt was launched.  Even with this much discrepancy, the actual straight and narrow flight of all the shots we’ve made with the machine in this condition, has been pretty near perfect.

Our axial reorientations, while noble in their own right,  clearly had little effect on the lateral sweep of the starboard limb.  Sterner measures are in order.

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This next photo shows the effect of adding 15 degrees of rotation to the the top half of the starboard bundle, while slackening off the lower half of that bundle by the same amount.  The photo is taken at full draw, directly after the adjustment was made.

The improvement in the lateral alignment of the limbs is clearly better than in the previous photo.  By adding 15 degrees to the top, and taking away 15 degrees on the bottom, no net tension has been added to the system.  In the past we have noticed that once the balance between the two bundles has been achieved, then that balance is maintained, as long as the rotational moves of the washers are done as equal mirrors of one another.

The next step will be to do a bit of live fire, and see if that starboard limb has truly submitted to our training.

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Okay, it is now the future, and we’ve performed field test number four, in which 15 shots were discharged.  The answer to whether or not some kind of stable lock down occurs when the bundles are adjusted down on a carefully positioned limb, is:  Indeed yes!   Axial orientation and limb sweep are holding steady.

“Top sails!”  Mr. Gallant, “and let her run with the wind!  My pretty Penny brightens everyday!”

Really?  You made it into the blog this far?  

Well, Sunday went bust in the midst of a heating stove emergency.  However, the word is my old friend Brian Kern, and his wife Kachi, will be knocking at our door later this week;  we’ll save our last field trip of the season pending their arrival.  It’s always good to share one’s catapult kinetics with friends,  that’s the only sure relief I get from the eye-rolls of certain locals.   The charms of ancient Roman torsion artillery do not appear to have penetrated the consciousness of the Okanogan, yet.  Usually I’m asked if that “thing” could kill a deer.  To which I respond with eye rolls of my own.

It all becomes very ocular intensive when we drive through town with Firefly in the back of the  truck, looking like one of those “technicals”,  straight out of Mogadishu or someplace.  It’s good for ’em, we figure.  Necks need exercise too.

Resplendent in their ugly, orange paint job,  our somewhat expedient versions of a famous third century Roman ballista bolt are just about ready for their trial run.

The diminutive size of these full scale replicas of the Dura-Europos artifact are contrasted with the larger style of bolt we’ve been testing this summer.  While finding ballista bolts that are sticking out of the ground after they’ve traveled more than half a mile downrange is not as difficult as some might imagine, this high visibility color will certainly make the job easier.  These little darts only weigh 276 grams each, while that behemoth of a converted broom handle tips in at 521 grams.    This latter is the same type of monster bolt that made it out to 821 yards several weeks ago.  So,  if they fly straight, it’s anyone’s guess how far our little soldiers of orange will go when they get kicked out of Firefly at 400+ feet per second.

I’m thinking Sunday, if the weather agrees.

By adding clay to the end of our ash, Dura shaft we can get a good idea of how heavy the steel head should be to give us an ideal Forward Of Center (FOC) balance point for optimum flight characteristics.

From previous testing with our 521 gram bolts, and by general consensus from the crossbow community, it seems fair to say  that a 12.5 %  FOC  is just about ideal.  In the photo above, the pencil mark on the right indicates that balance point as it is shifted forward from the dimensional middle of the bolt by the weight of the head.   Figuring as best we can from Dr. Nankov’s drawing, the original Dura-Europos bolt was 21 5/8″ long, which means that by utilizing that 12.5 % FOC number the “ideal”  balance point is moved 2.7″ down the shaft from it’s mid-point.

The upshot of all this is to suggest that the steel bolt head needs to weigh about 135 grams, which, coincidentally, is the same weight as the wooden shaft seen here.  By the time we add the fins, the finished bolt weight will probably hover around 275 grams.

So, to review our maximum ranges:

This summer, Firefly propelled a 521 gram bolt 821 yards.

A 415 gram bolt made it out to 892 yards.

And now we’ll see what a 275 gram Dura bolt, with skimpy little fins and a bulbous rear end will do.  It may become unstable and just waggle it’s way downrange for a few hundred yards.   Or maybe, just maybe,  it will work as advertised and break that mythical 1,000 yard mark.

Dr.  Emil Nankov has provided us with a good pictorial rendering of the Dura-Europos artifact, in his fascinating paper,  “The Fortifications of the Early Hellenistic City of Seuthopolis:  Breaking the Mold.”

I am using the excellent scale reference on this drawing to dimension the three bolts we are making.  However, I am not sure I agree with the idea that the Dura-bolt only had two fins.  If we look at this next photo of the actual artifact,  the alignment of the flattened tail of the shaft to the single remaining fin seems to indicate that it must be the top fin of a regular three fin fletching plan.

Above photo from Duncan Campbell’s book, “Greek and Roman Artillery 399 BC – AD 363”.

I suppose we could say that our version of the Dura bolt will be a combination of what we can glean from both these photos. Figuring the weight of that iron tip,  seems a good place to start.  However, for that we will need to make up a tapered shaft so we can model the best balance point.

At this stage, the great wondering is whether our version of the Dura bolt will top 1,000 yards or not.   If it leaves the machine cleanly, with little or no sideways tail kick, it just might make it.

As does my appetite for action.  There are 12 matched bolts waiting to investigate the “wandering zero issue”.  But, because this may be the last field test before the snows hit, I think to set my sights a little higher, as well.

Three exact-ish copies of the Dura-Europos, bolt artifact, are underway.  More later…

The first things that look remotely like rain clouds, since last July.

And not yet a drop.  Not a drop.  Although, promises have been made for Sunday.