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Basic Mechanical Artillery: An Introduction to Catapults

Basic Mechanical Artillery

An introduction to catapults

Doughal Stewart


Catapult is a generic term that describes any mechanical devise that hurls projectiles through the air.  This missive will focus on the most common forms of catapults, or mechanical artillery, routinely used by armies of all nations through most of history, a brief description of the machines, their uses, and their drawbacks and superiorities.


There are three main styles of mechanical artillery, named so for the way they impart power to their respective projectiles.  Those styles are: tension, torsion, and gravity.


Tension style would include Bows, Crossbows, the Belly Bow or Gastraphetes, “Slapboard” Ballista, the Bow Onager or Spoonapult, and the Chinese Triple Crossbow.


The first three: bows, crossbows, and the Gastraphetes all operate with the use of some form of hardwood or composite made bow, across the ends of which is stretched a bowstring.  This bowstring imparts power to the usually arrow or bolt style projectile.  These are largely shorter ranged; up to 400 yards, and the technology they employ is some of the oldest.  This however, does not diminish their value on the battle field.  In fact bows and crossbows have been used throughout history and up until today.  The Gastraphetes disappeared from history with the advent of the much larger and stronger torsion style ballista.


The “slap board” onager is just that.  A flexible board was mounted to a static frame at the lower end.  A trough was mounted tangentially near the top of the flexible board, in which a loose arrow or bolt was placed.  The flexible board was drawn back and released so that the top end of the board would strike the knock end of the arrow, thus sending it along a flight path.  This is an inefficient use of the power in the flexible board and would have had an extremely limited range.  Few period examples of this type exist, and only in illustrated form.  A much better use of the board’s potential energy would be in use as a bow.


The bow onager is also an inefficient use of power.  A large composite or hardwood bow is mounted to the subsidiary structure of a frame lying on the ground.  A throwing arm with either a spoon or sling on the top end is hinged in the center of the ground frame so that the bowstring contacts the back of the throwing arm.  As the arm is pulled down and away from the subsidiary structure, the bow imparts power to the throwing arm.  The arm is released, sending a projectile flying.  This design was used to employ the power of the bow, but in such a manner as to hurl round stone projectiles instead of arrows.  Again, not many examples of this type of catapult exist and are mainly illustrations. 


Probably the most successful application of tension power is the Chinese triple crossbow.  A regular crossbow design is augmented by a second reverse facing and a third forward facing bow.  A bowstring is threaded through a system of pulleys in a manner so that when the string is pulled the first bow bends to the rear, the second bow bends toward the front, and the third bow bends to the rear as well.  This in effect imparts the power of three similar sized crossbows into one unit.  It is purported that larger versions of this device could hurl an arrow up to a mile and was used by the Chinese until the advent of gunpowder.


The torsion powered catapults are in two families: the ballistea and the mangonels.  Ballistea include the Euthytone, Palintone, and Hatra, or Cheiroballistra.   


Both the Euthytone and Palintone are constructed in a similar manner.  A large frame is built with two vertical skeins of elastic strands at both sides.  This frame is attached to a projectile trough in much the same manner as a crossbow.  The trough also acts as platform for a winching device to pull back the bow string.  A small throwing arm is placed in the center of each of the skeins.  They skeins are twisted to impart tension on the throwing arms and a bowstring is strung between the two.  The bowstring is drawn back by the winch and; in the case of the Euthytone an arrow is placed in the trough.  The bowstring is released, hurling the arrow at one’s opponents.  In the case of the Palintone, the trough is shaped differently so that a stone can be fired.  The skein frame and skeins are usually larger as well to accommodate the heavier weight of the stones.


The Hatra or Cheiroballistra is similar in shape and design as the ballista above.  However, in this case, the frame is much wider to accommodate throwing arms that are mounted so that when tensioned, point toward the front of the machine instead of the rear.  As the bowstring is pulled back the arms come back and together in the center of the frame, over the projectile trough.  This change in torsion direction allows the arms to be pulled back through a full 90 degrees of arc instead of the 35 to 45 degrees in the other ballistae.  This puts much more power to the string and the projectile.  Ranges on this type of ballista easily exceed 400 yards.  It also means that a lighter, smaller engine can be used in place of one of much larger stature to the same effect.  The only drawback is this style of ballista would more that likely be used in an offensive capability due to the forward thrusting throwing arms.  The other ballistea could be used in a more confined space such as in a fortification and pressed up against an embrasure.  This style was used in both a large scale artillery version and a smaller scale crossbow version.


The Onager, or Scorpion, and the Espringald belong in the Mangonel family of catapults.  The Espringald is similar in design to the Cheiroballistra.  However, instead of being mounted to a freestanding pivoting mount, as in the case of all the ballistea, the Espringald’s torsion bundles are mounted in a large, immobile box type frame.  The arms were drawn back and the unit is loaded in a similar way as the Cheiroballistra.  It is in how the unit was deployed where we see the major difference.  While the Cheiroballistra could be used as a filed piece, the Espringald was used mainly as a breech defense device.  A single or series of Espringalds would be loaded and positioned behind a door or wall on the verge of being overrun.  As the breech was opened, the Espringald would be fired into the onrushing troops.  The force of impact would be so great as to be able to penetrate several ranks of men before becoming inert.  This deployment would leave the Espringald to a largely defensive, one time use type role in battle.


The Onager combines the increased arm rotation of the Cheiroballistra, the mechanical advantage of the sling, and the simplicity of a single skein into one machine.  The onager is a rectangular shaped ground frame with an upright subsidiary structure which acts as an arm stop.  The torsion skein is located between the main ground joists and the throwing arm is placed in its center.  The skein is tensioned so that the arm is forced against the arm stop.  A sling is attached to the top of the throwing arm.  The arm is winched back, a projectile is loaded into the sling, and then the arm is released to send the projectile skyward.  The range is comparable to the dual skein ballistea and is just as accurate; however one has to move the entire machine to aim it.  Another advantage the single skein provides is a more compact design which uses less wood and is simpler to produce than the ballistea.


Gravity is the driving force behind the Trebuchet family of catapults.  These include the Traction Trebuchet, the Fixed Counterweight Trebuchet, and the Hanging Weight Trebuchet.


Originating in the Arab world, most trebuchet built in Medieval Europe were of a similar fashion. Two large “A” frames were built and mounted so that there was a clear space between the two.  In this clear space, from a pivoting axle, was mounted a throwing arm.  The throwing arm was set up so that there was a longer and shorter end.  From the shorter end a weight, some ropes, or both were hung and from the longer, usually a sling, but sometimes a spoon was mounted.


The Traction trebuchet used ropes and the power of people pulling on them to fire projectiles.  While this allowed for quick reloading, the range was limited by the number of people used and the amount they could pull.  Also a level of discipline was involved in getting large numbers of people to pull at the same time.


Trebuchet built in the China, varied in the form of their mountings, but still worked using similar principals of lever action and largely followed the traction style of power.  Chinese trebuchet came on mobile carts, lean-to type stands, but most notably on a single pole.  The Xuanfeng, or “Whirlwind” trebuchet was mounted, sometimes on a mobile platform, to a small cube like base from the center of which was mounted a single vertical pole.  The throwing was then attached to the top of this pole.  While the Whirlwind could cover a given area from a single position with relative ease, it was limited in its overall mobility by its relatively small base.    


A fixed counterweight trebuchet has weight directly attached to the short end of the throwing arm.  This makes for a simpler machine, but causes a recoil action that propagates through the entire frame causing the machine to buck in the direction of fire.  One solution to this problem was to put the trebuchet on wheels.  The wheels allow the entire machine to rock back and forth as the energy from the counterweight dissipates.  This prevents the trebuchet from trying to flip itself over.  Tracks were added later to reduce wear on the wheels.


About this same time someone figured out that hanging the counterweight by ropes and then later a pivoting frame and box was much better.  This adaptation meant that the trebuchet didn’t move from its base.  The machine was safer and could handle much larger counterweights.  At their height, hanging weight trebuchet could support weights of up to two tons.  These incredible counterweights could propel a 250 pound projectile up to 450 yards with devastating effect.  All this weight, though, made the trebuchet largely immobile.  While some modicum of directional control was attained by moving the projectile trough from side to side between the “A” frames, these massive structures which took days to build were relegated to large fortification reduction. 


The munitions that these engines of war could exploit varied as greatly as the engines themselves, but fall into four main categories: bolts, stones, incendiaries, and biologics.


Bolts are long shafted projectiles with some form of sharp point at the front and a form of fletching at the knock.  Bolts are generally shorter and of a thicker diameter than a regular arrow.  The bolt head, as the arrow head, could take on nearly any shape, but one of the most used was the Bodkin head.  The Bodkin was a pointed head formed from a bar of square stock metal.  The head was mounted to the wooden shaft by the tubular shaped rear socket which was bored out in a conical shape to accept the wooden shaft.  The fletchings while feather shaped, were of a lighter weight metal like copper or brass.  And instead of the traditional three, there were usually only two, made from a single piece of material. 


Accurate records for stone projectiles only come to us from the Greeks and Romans because they were the first to implement standardized projectiles.  While stones were used throughout history, during the Dark and Middle Ages they were used mostly on an ad-hoc basis in generically designed one time use only trebuchet.  The ballistea the Romans designed were based on complex mathematical calculations for very specific purposes and maintained as capitol investments for long lived fortifications.  Their artillery were also used as naval and to a lesser extent, field pieces.


The most common used stone sizes were 10, 15, 20, 30, and 50 minea; one mina being approximately one pound.  Larger sizes included the 1, 2, and 2 ½ talent stones, a talent being 60 minea.  The stones were of a largely spherical shape but left rough hewn and not polished smooth.  One might imagine that the Romans figures that the rough exterior acted the same way on the stones as the divots do on a modern golf ball.  But it is probably closer to the truth that it was just quicker and easier to leave them unfinished.  The Romans mass produced and stored large caches of standardized caliber stones.


Incendiary devices took the form of large clay pots.  These pots were filled with a flammable mixture of petroleum and naptha making what would later be known as Greek Fire.  A wick was placed in the lid of the jar and the caustic cocktail was ignited when the pot struck a solid object and shattered on impact.  Another form on incendiary was a tightly bound ball of brush and twigs.  This ball was soaked in Greek Fire, ignited, and then shot in the direction of the enemy.


Biologics, while not limited to the Middle Ages, is where they are best known for being used.  This usually came in the form of a dead or diseased animal carcass thrown into a besieged city to infect and terrify the population.  Human remains, animal and human feces were also used. 


Despite the onslaught of gunpowder into projectile propulsion, mechanical artillery was able to maintain a presence well into the 20th century. 


During the trench warfare of World War One, a need arose for small pieces which could fire existing grenades from your trench to your enemies.  All sides developed some type of grenade throwing devices but the most notable were the Leach Trench gun and the West Spring gun.  Both weapons were developed by the British and used by American and French forces as well.  The Russians also developed a grenade throwing device. 


The Leach Trench gun was a four foot long “Y” shaped device.  It had a large rubber band stretched between the ends of the “Y”.  The band was pulled back, a grenade inserted into the band, and then the band was released to fire the weapon.  This was a very rudimentary weapon and suffered from bands that were not consistent in manufacture and that fatigued with use.  Plus, the grenade would often fall out of the band upon firing.


The West Spring gun can best be described as a treb-u-pult.  It utilized a throwing arm mounted in a similar fashion as a regular trebuchet. But instead of a traction or counterweight, an array of vertically mounted coil springs was used to provide power.  The entire device was about the size of a small sofa.  It was cocked with long levers and a grenade was placed into a cup at the end of the throwing arm.  The French used a version of the gun which had horizontally mounted springs and was on wheel.


The latest use of mechanical artillery in combat was reported in 1969 by Colonel David H. Hackworth.  A unit of Viet Cong used a “catapult not unlike the ones used in medieval times” against an American fire base in the Mekong Delta.