The Ballista

The object of the exercise, for me at least, is to scratch an old and delicious itch.  Growing up in a place called Warfield Park, in Berkshire, during the fifties and sixties,   I sensed the presence of  all those ancient Roman spirits that swirled through the woodlands where we lived.  Of course at the time they were all jabbering on in ancient Latin, so I didn’t have the slightest clue what they were talking about.    At age ten, they did manage to inspire me to fumble around in the back yard and come up with something I thought was a smelting forge.  When I proudly showed Miss Peachy a discoloured lump of tin solder, with a partially melted piece of copper pipe sticking out the top, she and some of the other teachers instantly took an interest and asked how I had done it.  We agreed on the spot that I had indeed made  “bronze”.  I was lucky to have those kind and open people show me the way.

The innocence of those days is long gone now,  but the vitality of those childhood imaginings still burns strongly.  Every time I figure out some aspect of the design on Firefly, it feels like I’m entering the mind of one of those ancient Roman engineers.   They must have sweated through the same details I am grappling with now.   There is method to all this madness.  The people that gave us  roads,  sanitation,  infrastructure, education, security, math,  law, philosophy, surgery and political unity,*   are gone only in the meanest and most literal sense.    The essence of who they were can still be deciphered by carefully treading along the same path they must have taken so many generations ago.

It takes an inquisitive nature to value history for it own sake, no matter what the lesson.    Ideological certainty usually comes from the fear of being wrong about the things we think we believe.  If catapult building has taught me one thing, it is that the closed mind seems strong only to its occupant.  The great trick is to learn balance.  Open!, always open!;  just not so open that when you lean forward your brain falls out on the pavement.

*  Thanks for the list Murray.

The first shot today was an attempt to create an impact crater in masonry.   Stats for this shot are:  draw weight 5000 lbs,  draw length 45″,  bolt length 31″,  bolt weight 7376 grains,  velocity 307.1 fps,  energy 1544 fpe (2094 J).   In the following video we see the above numbers collide headlong with a 10″ concrete building pier.   20100313143734(1)

Reassembling the chunks showed an initial spalling type crater 4 inches in  diameter and about 3/4  inch deep, followed by a narrow penetrating wound  about four inches deep into the heart of the block.  The hardened bolt head suffered zero damage from the exercise.

Not so in the photo below when the same head had a run in with some hard Okanogan granite.  Damage to the rock was limited to a closely defined crater 2 inches across by 1/2 inch deep.  Stats on this shot are:  draw weight 5,000 lbs, draw length 45″,  bolt length 29″, bolt weight 7037 grains,  bolt velocity 309.4 fps,  energy 1495 fpe  (2027 J).

The rock used in the above test is especially hard.   I will try and find something that is in between the concrete block and the granite.

I spent more time with the Audacity software this afternoon and have uncovered an interesting interpretation of the acoustic data re: that 401.8 foot per second shot I was warbling on about the other day.  You will recall that the problem was, I had a hard time believing that the particular shot in question (as represented by this video,  20100307132756(2)), had ever actually made it up to the 401.8 fps claimed by the chronograph.  Before chucking in the towel on this “record breaking” shot, I had attempted to analyze the sound of the shot by looking at a graph generated by the aforementioned software.  As it turned out,  I needed to boost the bass and change the tempo with out changing the pitch.  That done, the following graph emerged.   It helps to magnify this photo to see what I’m going on about.

If we look at the area shaded in the darker blue, it seems this represents the time the bolt was actually in flight.  For all intents and purposes the bolt leaves the machine at the first initiation of the sound pulse.  The thump from the catapult going off,  and the string vibration,  and the whirring from the bolt,  all smooge together and  show up as the area of decreased frequency between the .015 mark and the .041 mark.  It all culminates with the heavier bass tones of impact,  witnessed by the denser lines visible on the graph from the .041 mark onwards to the end.   I believe that the bolt from the video was in flight from the .006 mark to the .041 mark.    In other words it took .35 seconds to zip downrange and punch a hole in the archery backstop.

The other piece of information we need to confirm the velocity of this shot,  is the range to the target.  I had previously misreported that as being 50 yards.  More careful measurements with a tape measure now show it to actually be 47 yards from the face of the target to the tip of the bolt as it rested on the machine in the cocked position.

Armed with this new information,  47 yards in .35 seconds works out to be 402.8 feet per second.  This is very close to the 401.8 feet per second reported by the chronograph.

It appears that I may have psyched myself out and allowed morbid incredulity to defeat what my eyes were seeing on the chronograph.   At least for now, that’s my story and I’m sticking to it.

For one brief shining moment, it looks like Firefly jacked a 7,000 grain bolt up to 401.8 feet per second, and generated 2507 foot pounds of energy.  Who knows?, maybe that was what caused that  strand to break.  In any event, the above video and its sound signature,  along with the positively measured 47 yard range,  is  evidence anyone else can analyze if they want.  I present it in this posting to hold witness to my belief that this level of performance was momentarily achieved by Firefly.

Good on you old girl.  Now we have to fix you up so you can do it again.

In the photo below the pink arrow indicates the  terminal effects of a 115 grain, full metal jacket,  9mm  round from a Browning Hi-power.   It left the barest of indentations in the 1/4″ plate, before turning itself into a splash of lead.  Range was the same 53 yards as used with the ballista shots.

Now of course,  a 9 mm is only a measly pistol round,  but this disparity in penetration between a 2,000 year old technology and that of a modern cartridge,  one that is generally considered not to be a perfect slouch in the penetration dept.,  does give some perspective on what the Romans were able to achieve with their ancient handiwork.   Not exactly stone knives and bearskins.

…….I intend to make mine comfy.  Six feet of 3/4 inch by 1  1/2  inch 4140,  will make for peace of mind in a kamarion that must sustain 5,000 lbs of draw weight.  Accordingly,  I have ordered said material.  It should show up in a week or so.

If we know where all the icebergs are, perhaps my plans to make the old girl more seaworthy will please the you know whos.

A brother minion of the CG’s, has suggested that the free software called Audacity, would be a splendid way to analyze the sound signature mentioned yesterday.   Goofing with it for a while, I plundered my limited mental resources trying to figure it out.

Not much luck there.

This is all a bit worrisome because I am noticing a drop in my ability to do math, at the same time my blogging seems to be a little more fluid.  I wonder if  over-pressuring the brain with blood flow in one area, can starve another.  End result:  selective brain damage.

No fool,  you are just getting old.

In any event, the Audacity of Hope has flickered a bit when it comes to utilizing this software.  Back to something I’m good at.

Ironwork.

The indecipherable squigglies from that “401.8” bollocks.

……….before the bolt takes a munch out of my trailer, we see this speeding torpedo appear.

Here is the video from yesterday that this snapshot was taken from.    20100309124009(1) If you play this vid, and stare at the spot indicated by the pink arrow, you’ll see what I mean.

Because the sad truth is:  the broken strand discovered yesterday may spell the end of any primo performance from Firefly for a while;  for now at least, I will need to make the most of the data that we have.  I find myself wondering if there is any good way to measure the velocity by analyzing the video.  It would be joyous to confirm that the above shot was the 307.7 fps claimed by the chronograph.  It is not that I necessarily distrust this particular reading, but it would be marvy to know for sure.

I will take the rest of the morning to be all CSI and go out to the crime scene and measure the distance from the point in space that the bolt appears,  to it’s final resting place in the side of the utility trailer.  (see below).

Report back later…..

………….It’s later.   My friend and neighbor Richard Rough stopped by to help with the field work.   I looked everywhere,  but couldn’t find him a badge.

The tennis ball above Richard’s head is a very close approximation of where the tail of the bolt first appeared in the video.  Distance from the ball to the impact point on the angle iron is 279 inches.  The length of the bolt is 33 1/2″, and so the tip of the bolt had to transverse 245.5″ from the time it appears on the video to the sound of its impact.

A long pause ensues while I digest all this nonsense………………..

Well that is a fairly stupid way of going about it.  We have the sound of the catapult going off and the sound of impact.  Range is 53 yards.  Measure the time between the two sounds and we pretty well have it.  (Thanks Gareth.  I hadn’t actually forgotten your suggestion.  Just got a little lost trying to plonk down all the data before moving the trailer.)

Although the above data is accurate, this exercise was probably a waste of time.  Ah well!, Richard and I had fun playing CSI.   I never promised this blog would be anything other than a blow by blow account of all our antics here at the little catapult factory.  Nothing like warty exploits to spice up the day.

Speaking of,  “little catapult factory”, we now have a sign and everything.

Cheer up people hoping for Lejre.  The Catapult Gods haven’t given up on us yet.

After coming back from lunch and  seeing what greeted me on the starboard bundle,  I made the firm decision to remain a cabbage for the rest of the day.  Funny, this morning I felt like a King.

This lonely little strand, dangling from the group like an insipid twit,  spells doom for the longevity of these springs.  Xnay the 1/4″,  three strand nylon rope.  Bummer.

As if that wasn’t bad enough,  it looks as if what had been some minor bending in the kamarion (arched strut to all us tourists),  is progressing into something that needs treatment.

The rods clamped to the field frames, in the above photo, indicate the amount of bend that has occurred since stepping up to 5,000 lbs of draw weight.  No worries.  Easy to fix.  Now if this had been wood…….

A new kamarion made from spring grade 4140, is what is needed.  Also the curved stanchion on the starboard field frame is suffering from a case of fatigue   At least it’s posture isn’t very good these days.  Heating the curved part of the stanchion to a salmon red should make straightening it back up pretty easy.  In the photo below we see how the straight stanchion has also taken a bit of a bow.  Also easy to fix.

It appears that the insufficiently strong kamarion,  allowed the twisting moment  from the washers to spring the mild steel field frame into a permanent crank.  I loathe bad posture in my field frames.  It should be possible to upgrade the curved stanchion  by milling away half the outside part of the curve and replacing it with some hard welding rod.  Mistress TIG should assist in that one.

Vanity, vanity, for proclaiming this vessel seaworthy.  “Unassailably robust”, we boasted.  The Gods did not agree.

The bright side to all of this is that some new insights into the artifacts Firefly is based on, may have been gained.  We know, at least with the  limbs and string we are currently using, that it takes around 5000 lbs of draw weight to obtain the kind of performance visible in those videos from yesterday.  It would seem to me that punching holes in 1/4″ plate would be the useful minimum a machine of this size would need to gain any respect on the ancient battlefield.  Anything less would hardly be worth the bother of packing around and maintaining so much kit.  A value judgment, I know, but one has to have standards.

Because it looks like some kind of  spring steel upgrade for the kamarion is mandatory,  then it follows that if the original Kamarion is also made of spring steel,  we might be able to make some useful assumptions about the actual power of the original machine.  ……. This is the point where some heroic scholar needs to jump in and figure out what the metallurgical specs are on the original kamarion.  This may not even be possible.  I hear that it is a pretty rusty fellow.  I bet that redhead guy on CSI could figure it out……..  In any event, if the original kamarion is spring steel, odds are that the Romans were pumping out some serious foot pounds with their machine.

The original kamarion.   (Photo by D. Baatz)

As for that broken strand…… well we were going to try nylon thread anyway.  It was just a matter of time.

20100309093824(1) This video shows an attempt to hit a concrete paving block.   Stats are:   draw length 45″, draw weight 5000 lbs, bolt weight 6985 grains,  velocity 314.8 fps, energy 1537 foot lbs.

The chronograph was checked with 3 shots from my .25 cal.  Kodiak air rifle and all the readings were in the normal range for that particular piece of artillery.  Today’s velocities seem well founded.  All of which leads me to conclude that the 401 fps reported the other day is a highly improbable number.   Grovel, grovel….retract, retract,….. sorry about that.

I have not sighted Firefly in yet.  There is not much point in doing that until she has several more shots under her belt and the bundles settle down.  In the meantime we can have some fun blowing up bolts on the 1/4″ steel plate.  Who knows, I may get lucky and actually hit one of the paving blocks.

Here are three more videos of some admittedly wild shooting.  Below them are photos of the damage done to the target.  All the bolts are very close to 7000 grains.  Range is 53 yards.

20100309103750(1) 312.8 fps,  1520 fpe.   Note the blue striations from heat build up on the edges of the hole.

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20100309104353(1) 280.5 fps,  1223 fpe.  Probably slower because the fins were oversize on this one.   It looks a bit  wimpy because of the way the bolt bounced out,  but check out what it left behind in the 1/4″ plate.  Penetration is 3″.

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20100309124009(1) 307.7 fps,  1471 fpe.    Ouch!  my trusty old utility trailer didn’t like that.  This piece of angle iron is 3/16″ thick.

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Clearly I need to start working on sighting in Firefly.  No luck on hitting one of those pesky concrete blocks yet.  Tomorrow Moriary!  tomorrow! ……

I awoke this morning with a healthy dose of skepticism brewing about that 401.1 fps reading from the chronograph.  To make matters worse, the Canadians had decided to grace us with one of their arctic blasts and the snow was coming in sideways for a couple of hours.  By mid-morning the sun made a begrudging appearance and the snow stopped.   It looked like a good time to try another shot and see if yesterday’s “record” velocity was even plausible.  Unfortunately the serving on the string was in disrepair  and after the shot we observed the situation seen below.

A few days ago ago I had one of my imaginary  Roman busy-bodies sitting on my shoulder warning me to take care of the string serving.  But did I listen…..

Probably this blow out of the serving happened during the power stroke because the velocity was down to 270.9  fps.  I am fairly certain this number from the chronograph was accurate because I had made a point of shooting my Kodiak air rifle through the screens several times before making the ballista shot.  The readings for the air rifle were a consistent 648 to 654 fps.  This is perfectly normal for this particular .25 cal air gun and gave me confidence the chronograph was working accurately.  The 270.0 fps on Firefly was disappointing of course, but at the moment I am considering it the result of the blown out serving.  One correction from yesterday  is for certain,  the draw weight for that supposed 401 fps shot was actually 5000 lbs,  not 4500 lbs as reported.  Apparently the zeroing dial on the dynanometer had been inadvertently rotated because it was reading 500 lbs when it should have been set at zero.

The target seen in the following video is in shadow,  and it is hard to make out all the action.  What we are seeing is today’s 270.9 fps shot, with its slow poke 7000 grain bolt and 1140 foot lbs, take on the 1/4″ mild steel plate that is installed on the rear of my sand trap.  Range is 53 yards.    20100308134735(1)

If you play this video on slow it seems as if the bolt is moving through the air at an angle with the tip down.  I am not sure what might be causing this.  Is it because the 2870 grain tip is too heavy for this length bolt and the whole projectile trundles along nose down?  Perhaps it is because the bowstring gives the tail of the bolt an upward flick at the end of the power stroke.   Maybe it is some perturbation coming from the damaged serving.  At the moment I have no explanation.

This quadrobate tip is made from hardened 4140 (about 40 RC), and penetrated the 1/4″ steel plate to a depth of 2 1/2″.  The tip also went through the piece of 3/4″ plywood behind the plate and stuck out the other side  1  1/4″  into the sand.  The photo below shows how the bolt tipped upwards at the moment of impact.  Not surprising because any slight downward  angle of the bolt would probably be magnified into a full blown spinning upset as the rear of bolt was forced upwards by the impact.

The one way trip experienced by this bolt must have been excruciating.  Note how the explosive deceleration caused the fins to slip their grooves and migrate forward.  I hope to be able to try another shot tomorrow morning when the light is coming in from the East.  (Assuming of course we can get the Canadians to cooperate.)  With a new serving on the bowstring,  we should be able to answer whether that 401 fps shot was for real, or just a figment of my chronograph’s imagination.  I suspect the later.