The Ballista

The little experiment I mentioned yesterday involved this rough and ready method of shortening the limbs to see if  different string angles have any effect on performance.  Clearly the potential weight savings from having shorter limbs will not factor into this equation.

The first of six shots can be seen in the video below.

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The first two shots of this string of six was done with the string in its normal position placed in the nocks at the ends of the limbs.  Velocities were 231.9 fps and 238.3 fps.  The next three shots were all done with the string in this new position modeling shorter limbs.  Velocities were 227.8,  225,3  and 220.6 fps respectively.  The draw weight in this new position climbed to 4800 lbs, up 200 lbs  from the 4600 lbs registered with the string in its normal position. While it is true that the new string position gave slower velocities, they were not slower by much, and so it remains an open question as to whether truly shorter limbs with a couple of inches removed would provide a net boost in velocity by virtue  of being lighter in weight, especially out on the ends where unnecessary weight has a more pronounced effect.  More grist for the mill.  Always more grist.

The photo below shows the splats lined up in the order they were performed.

The final sixth shot of the day, seen on the far right, is noticeably more deformed than the first five.  This is because its velocity was boosted up to 270 fps by another major change that was  introduced into the limb set up.  I removed the bronze hardstops from the curved stanchion, and that allowed the limbs to rotate an extra 15 degrees past the usual 90 degree position they have always had relative to the string.  This allowed for a longer string and a longer draw before a 4800lb draw weight was achieved.  That represents a 50 fps increase basically for free.  One may wonder why this 270 fps is remarkable, given that 285 fps was achieved the other day with the limbs in their normal 90 degree at rest position.  Probably the pretensioning in the bundles was much higher the other day and that pretty well overwhelms any efficiencies that might be apparent from other factors.  (Stupid me didn’t record the draw weight from then because the dynanometer was not zeroed properly , so now there’s no way to know for sure)  Today’s testing was done with a deliberate attempt to keep the pretensioning of the bundles consistent throughout the experiment.  Not maxed out,  just consistent.  That being the case, it really does seem that having the limbs rotate past 90 degrees boosts the velocity quite a bit.  This is all something that Cap’t Harpoon advised me of some time ago,  and only now am I  getting to test it.  More on the good Captain later.

Okay, you see that small tree branch out there at 100 yards.   No not that one.  The one to the right of that fat trunk and left of that skinny trunk.    Kinda of in the middle between them and maybe six feet above the hill.  Yeah that’s it.    You’ve got it.    Now watch this……..

Click here for video, 20100628183604

….. Perfect!  Now that’s shootin’.

Seriously though, I have witnessed each time out of four shots now, shooting the glans into the distance against the dark tree line, when the sun is at my back and shining on it for maybe a second, that the glans is NOT in fact tumbling.  It flys sideways in the same orientation it held in the pouch.   It may twist that orientation into a tilt, but I have not yet seen it actually tumble end over end.  One of the  close range tests into the catulpa log the other day had shown evidence that suggested tumbling.  Probably it tumbled all right,  as a function of penetrating rather than flying.  Flying they look good.  Pump up the velocity by another 75 fps to straighten out that rainbow trajectory a bit,  and maybe we’ll really have something.

When viewed directly from behind (a perspective I have not yet captured on video)  the flight of the glans seems remarkably true, not veering off to the left or right by much.  Of the four shots observed this way, only one showed a slight tendency to drift to the left maybe four or five feet at 100 yards.  The other three seemed to plop into position nicely.

Not that I was sighting mind you,  just lookin’.

So, the plan is this:   Do a shot or two every day with the current set of stalward limbs  while  also working on a lighter  set of  Hi-speed,  racehorse limbs.    This “every day” ethic is vital to getting something done.  Back in the day when the Rebecca and Nick were hand making their house they called  this the “plod theory”.  It’s a tortoise and the hare thing.  Anyway, I digress…..

Yesterday I tumbled to an easy way to effectively shorten the current limbs  so we can try a shot with the double string and lead glans.  I will try an experiment this afternoon and report back.    Shortening the limbs is something Captain Harpoon has been trying to get me to do these last few months.  The pace of progress being what it is, only now am I getting to it.

Hmm……Yet again, I digress.  It must be all the recent lead exposure.

New limbs and shoot.  Shoot and new limbs.  Something like that.

Skooting out of the machine at almost 200 miles per hour,  a one pound lead glans crashes into the tree line one hundred yards away.    Click here for video,  20100627140457(1)

Now that Firefly has shown she can take a steady diet of 5000 pound draw weights,  it is time to move on to more serious performance tweaks.  Plans have I laid for a new set of limbs.  They should boost the velocity quite a bit.   More details soon.

Below is a slomo 4X vid of a 6050 grain glans taking a whack at my catulpa log.  Velocity is 270.1 feet per second, energy 971 foot pounds.  Draw length is now at 45″.

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The next video is in real time and the draw length has increased to 47″.  Velocity is 285.4 feet per second, energy 1096 foot pounds.

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The photo below shows that the first of these two shots hit the log sideways and penetrated one inch deep.  The second shot that struck lower down,  hit end on and can be seen buried in the log to a depth of 2″.

Although today’s test only represents two data points, it does appear that ancient lead glandes may well have tumbled end over end in flight.  Perhaps there are some subtle nuances of how the pouch is rigged that might affect this.  Much more testing is needed to know for sure.

Today’s energy levels and velocities would seem to indicate that the Romans may well have adapted their inswinger ballistas to be glans projectors merely by changing the strings.  (A half minute operation if you’ve got the right gear and a few extra men to spare.)   By following the simple logic of attempting to wring as much function as we can out of the basic design given to us by the original Orsova artifacts, it takes, I think , more imagination to disclaim the likelihood of  inswingers being used as  powerful weapons with lead slugs as well as with the usual  sharp bolts.

Maximum function need not always require maximum lethality.  It just depends on the situation.  Unlike a crossbow that always needs to be drawn to full cock, the thing about a ballista is that you can vary its power by how far back you pull its moving catch.  Clearly at  full power a  lead slug packing 1100 foot pound would be profoundly  fatal.  However,  mob control is something the Romans probably had need of.   Set for stun, and the lead glandes may have been just the ticket for intimidating rather than killing.  While bolts are expensive to manufacture and maintain,  glandes are nothing more than lumps of cast lead.   While I doubt a lead glans would ever better the accuracy or armour penetration of a good bolt,  for the cost of a bowstring with a pouch in it and a few lead slugs,  it is  hard to imagine that the Romans would not have found the capabilities of a glans shooting ballista useful for certain situations.

The double string with integral leather pouch,  seen in the photo below, is a quick and dirty affair, made just to test the principle of tossing a lead glans out of Firefly.  I decided to fire this contraption at only 30″ of draw because the red connecting string started to pull out under the catch.  It needs a heavier serving to prevent this.

This first test, seen in the video below, only developed 202 feet per second with its 6000 grain lead glans.  That represents a very weak 544 foot pounds of energy.  However, considering the short 30″ draw, this is to be expected.

Click here for video, 20100625125122(1)

It will be interesting to see how much the power increases when we add another 20″ of draw to this test.  I expect that the glans will always leave the machine with a high arc because it has to bump over the lower string, which would seem to put an upward cast on the projectile.

Although it can’t be seen in the video, I was able to make out the flight of the glans in real time,  and it appeared to be traveling sideways,  just the same way it had been held in the pouch.  If it can be confirmed that the glans is not tumbling in flight, this is very good news for future development.

The old propane cylinder meets its fate with studied indifference.  Range is 50 yards.  Velocity 306 feet per second.  Bolt weight 7200 grains.  Energy 1496 foot pounds.

Click here, 20100624131007(1)

The pinch factor exerted on the shaft after it punctured the first sidewall was enough to decelerate the bolt to the point it only left a healthy dent in the opposite wall.  The hole diameter is one inch.  If this propane cylinder is any kind of rough approximation of an ancient breastplate, it doesn’t seem like the enemies of Rome would stand much of a chance if hit square on.  I wonder how this cylinder will fair if the bolt strikes three or four inches off the center line on a curved surface.

Below: The dent in the far wall is visible above the bolt,  just over the weld bead.  It looks like my nice, hand forged bolt head is now held captive by the enemy.  It will take a cunning plan to free it.

Moral: if someone is shooting ballista bolts at you, never hide inside the propane cylinder.

The propane cylinder seen in the following video is bone empty.  I am curious to see how well Firefly does on an oblique surface similar to an ancient breastplate.

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Six inches to the right and smacked the stick on the target frame .  Yeah that’s pretty oblique buddy.

We will try again tomorrow.  At least you got to hear what an ancient  ballista bolt sounds like,  barreling in from the East at 220 miles per hour.

I tried  5 shots this morning and the velocity was down to 290 fps.   These springs are still a little green and need to be twisted up again.  The following vid shows two inevitable clues for this.  A few of the outer ropes appear to show movement in the “at rest position”.  Also, and perhaps most telling,  application of the wheatie meter shows about 1/4″ of movement,  when it should show zero movement and only minor bruising t0 the meter face.

Click here, 20100623110352

The following video is in 4X slow motion.   It shows that the field frame and kamarion and limbs appear to be handling the strains in good form.  No permanent bending of any of the components appears to be happening.  Thanks CG’s.  We are all deeply thrilled.

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The acute string angle at the 49″ draw can be seen in the following video.  The fins on this bolt probably need a touch more trimming to clear the string.

Click here.   20100622105903o

This particular shot got an error reading from the chronograph.  However, no reason not to think it wasn’t in the same 325 fps range as yesterday.  The kamarion is noticeably less jiggly now that it has gone and got a case of the muscles.     …… The Rebecca informs  I  might consider it a  worthy role model.