Mechanical Artillery

Mechanical artillery was invented in Syracuse, the principal Greek colony in Sicily, between 399 and 397 B.C. (see The Workshops of Dionysius in GURPS Low-Tech Companion 1). The first such device, the gastraphetes (p. 72), was a one-man weapon amounting to a crude, outsized crossbow. Larger weapons appeared not long after; they were standard military equipment by the time of the Roman Empire, and remained in use through most of the Middle Ages, until gunpowder replaced them. The Chinese Empire used similar engines, probably invented independently. A later Chinese development, the beam sling, traveled west and inspired counterweight artillery in medieval Europe.

Mechanical artillery comes in two basic types: two-armed, ultimately based on the bow, and one-armed, ultimately based on the staff sling. Either design requires a frame to support the working elements and prevent the weapon from discharging until released; this adds to the machine’s weight. Two-armed engines launch either bolts or stones; one-armed devices almost always hurl stones.

Several power sources saw use in mechanical artillery. The earliest designs, known as flexion artillery, were large, heavy bows that stored elastic energy in wood. Later ancient engineers developed torsion artillery, the wooden arms of which were rigid levers moved by more efficient springs made from animal sinew – or, in emergencies, from hair (there are stories of women cutting off their long hair for catapult-makers). Torsion weapons stored more energy for the same weight. The beam sling dispensed with energy storage, getting power from soldiers pulling on ropes attached to a lever; this was traction artillery. Finally, the trebuchet (the large “catapult” of medieval Europe) used the same sort of lever powered by a raised counterweight many times as heavy as the missile it hurled.

Flexion, torsion, and counterweight engines – and many man-portable crossbows (see Bows, Crossbows, and Rated ST, p. 74) – require great strength to cock the bow or raise the weight. An exceptionally strong man, or a beast, may be able to substitute for a crew of ordinary men; e.g., an elephant might be harnessed to a beam sling (see Combining Strength, p. 9). Theoretically, a superhumanly strong man could use artillery without mechanical aid! In general, anyone with enough ST to carry a mounted weapon can also cock it unaided.

Several mechanical aids are available to help average men, or crews of men, exert the necessary force. Many can be used with crossbows with high rated ST.

The device used to draw the bowstring on a gastraphetes (p. 72), giving the weapon its name, which means “belly bow.” A long slider is pushed out the front of the weapon, a claw at the end engages the string, and then the user leans into a curved stock, forcing back the slider. Strength limit: user’s ST +30%. Takes 2 seconds to prepare the slider, 2 seconds to lift, and 2 seconds to cock, plus 2 more seconds to draw and place the bolt. Built into the weapon, and included in its weight and cost.

This machine is used with heavy flexion and torsion engines. A wheel at the side of the engine is turned, usually with the aid of projecting handles that offer a better grip. The wheel pulls a cord that draws the weapon. Takes 2 seconds to ready the weapon, plus a variable time to draw it (see Windlass, below). Built into the weapon, and included in its weight and cost.

An adjunct to the stirrup (see below). Instead of bending, the crossbowman kneels, catches the string on the belt hook, and stands up, drawing the string. These three steps take 1 second, 2 seconds, and 2 seconds – a total of 5 seconds. Strength limit: user’s ST +10-20%. $25, neg.

A standard device for cocking heavy crossbows, used by horsemen. A fork-like lever is braced against the crossbow stock. Two hooks extending out from the fork engage the string. Pulling back the fork’s handle draws the string back. Using it takes 20 seconds. Strength limit: user’s ST +30-40%. $50, 2 lbs.

A single metal stirrup built into the front of a heavy crossbow. The user puts his foot through it, bends over to take the string in two hands, and uses his full body strength to draw the string. For effects, see Bows, Crossbows, and Rated ST (p. 74). Increases the crossbow’s cost and weight: $50, 2 lbs.

Trebuchets often use treadmills as power sources to raise their massive counterweights. A common design has a pair of treadmills at left and right. Weight and cost are included in the trebuchet’s statistics.

A detachable equivalent of the winch, used with heavy crossbows and arbalests. It trades off speed for power, multiplying the operator’s effective ST by a power ratio but multiplying the time required to draw the bow by a larger factor. It takes 5 seconds to attach the windlass to the weapon in the first place. Several models exist, each with its own specific ratios:

A more sophisticated mechanical device for drawing a crossbow, made entirely of metal. A frame holds a toothed shaft that engages the string. Rotating a handle turns a gear whose teeth mesh with the shaft’s, drawing it back. Like the windlass, available in variable power ratios. Costs the same as a windlass with the same ST multiplier, weighs half as much, and takes twice as long to use. Still requires 5 seconds to attach initially.

Mechanical artillery made possible attacks much more powerful than unaided human strength could achieve, involving larger missiles and longer ranges. Such weapons had a variety of uses in war. The heaviest could defend a city or a military encampment against an assault or a siege – or, in a prolonged siege, be set up to overwhelm a city’s defenses. Smaller engines were built into movable siege towers or placed on ships. Field artillery could be carried onto the battlefield and set up in an advantageous position; one man could carry the smaller pieces, while the larger ones could be broken down for several men to carry, or loaded onto carts. The carroballista of the later Roman Empire had its own wheeled carriage and harness for draft animals.

These weapons use the Gunner (Catapult) skill. They’re mostly two-armed, with the ends of the arms pulling a cord that transmits force to a bolt or a stone. Most can be tilted backward on their base to allow indirect fire; such attacks require the Artillery (Catapult) skill.

Torsion-powered weapons typically have a narrow field of view between the springs. Seeing through this to aim is difficult! Before taking an Aim maneuver with the weapon, the operator must take a second and make a Vision roll to acquire the target.

Modifiers: A basic +10; target’s SM; range penalties (p. B550), which can be offset by taking extra time (p. B346).

To achieve faster fire, operators often stand beside the weapon and look around rather than through it. This takes no extra time but doesn’t allow Aim bonuses.

A torsion-powered stone-thrower with two horizontal arms that pull a cord – in effect, a gigantic prodd (p. 73). Large ballistae can hurl bundles of 1-lb. stones as well as single large stones.

Used by the later Roman legions, the carroballista is the oldest known form of field artillery: a stone-thrower mounted on a wheeled carriage. Usually pulled by draft animals, but light enough for soldiers to pull if necessary.

The original gastraphetes (p. 72) was a personal weapon comparable to a heavy crossbow. This was later scaled up to larger, heavier models that could only be drawn with mechanical devices. Two examples are described here: a large double gastraphetes that shoots two long bolts, and a lighter mountain gastraphetes that shoots one shorter bolt. The mountain gastraphetes has an unusual asymmetrical mount that lets it be set up on sloping ground.

Early flexion-powered stone-throwers were called lithoboloi or petroboloi. The table describes two models, one light and one heavy.

The “automatic weapon” of the ancient world, designed for the highest possible rate of fire in order to intimidate enemy infantry. It’s a torsion-powered bolt-thrower with a hopper filled with bolts mounted directly above it. Rotating a handle at the side draws back the bowstring, operates a cam that drops one bolt from the hopper, and then releases the string and launches the bolt. The mechanisms are coupled by a flat chain, like that of a bicycle. This is the oldest known example both of cams and of flat chains. Some ancient generals were reluctant to use the polybolos – they feared that its rapid firing rate would encourage soldiers to waste ammunition!

Ancient ballistae that threw bolts rather than stones were nicknamed “scorpions” for both their sting and appearance. Like stone-throwing ballistae, these are torsion- powered, two-armed weapons with a low arc of fire. Scorpions are rated by the length of the shaft they propel.

A medieval weapon resembling a large crossbow; in fact, crossbows are sometimes called “arbalests.” Here the name is used for a larger weapon. A built-in winch supplies power.

This is a bizarre hybrid design: a one-armed weapon, like a monankon (p. 81) or a trebuchet (p. 81), but hurling a bolt rather than a stone, and as direct rather than indirect fire. The power source is flexion; a thick wooden arm, like half of a bow, is winched back and released, letting it spring forward to strike the end of the bolt and drive it out a hole. Springalds were mainly used to defend fortifications against assaults by besiegers.

These weapons use the Artillery (Catapult) skill. Their construction is one-armed, normally with a sling attached to the end of the arm to hold a stone. They lob projectiles in a high arc, giving them a minimum range; see the table notes. Such engines are almost always used from fixed emplacements, to defend or besiege a fortified site. Changing their line of fire is a slow, difficult process; see Aiming Fixed-Mount Weapons (below). The weapons on the table are just examples – larger models are recorded!

Early artillery weapons were often too massive for their aim to be changed easily. They were built into large vehicles, placed on fixed mounts, or simply anchored to the ground.

In indirect fire, changing targets along the same line of fire can be managed by varying missile weight (for catapults) or powder charge (for bombards). This can be done with an Artillery (Catapult or Cannon) roll. A miss always lands along the line between weapon and target; use Scatter (p. B414), but any odd roll falls short along that line while any even roll overshoots. For bombards, a malfunction (p. B407) when the charge was reduced to shorten range means the shot doesn’t even clear the barrel and must be removed (treat as a stoppage); a malfunction when the charge was increased to lengthen range means the bombard explodes!

Changing lines of fire – or setting up such a weapon in the first place – calls for an Engineer (Combat) roll or an Artillery (Catapult or Cannon) roll at -2. The task takes a base 5 hours, which can be reduced by accepting penalties for haste (p. B346); the maximum time reduction of 90% (-9 to skill) allows changing aim in 30 minutes. Huge weapons impose greater penalties: an extra -1 per full ton of weight. The work crew must be able to move the weapon; its weight can’t exceed 15 x (total BL). The line of fire of a vehicle-mounted weapon can be changed by turning the vehicle. See Vehicle Weapon Mounts (p. B467).

A simpler design than the ballista, the monankon has one arm, one torsion spring, and no string; the sling pouch is fastened directly to the end of the arm. The arm swings from horizontal to vertical, hits a cushion, and stops, releasing the missile. The later Roman version was nicknamed “onager” because of its recoil, which was compared to the kick of a wild ass.

The main heavy weapon of China and the Middle East, the beam sling – sometimes called a mangonel, cheiromanganon, or traction trebuchet – has at its heart a massive lever. The short arm is pulled by men hauling on ropes, while the long arm has a pouch or sack that holds the ammunition. The sizes of beam slings are denoted by the number of men needed to power them. Some Middle Eastern models improved performance by adding weights to the short arm, a step toward the counterweighted trebuchet (below).

The classic medieval “catapult,” designed to smash castle walls with huge rocks. It’s similar to the beam sling (above), but a massive counterweight replaces the men hauling on ropes. The basic trebuchet has the counterweight mounted directly on the short end of the beam. More advanced models attach it with hinges, letting it swing freely through different angles as it pulls down the beam, so that it descends in a path closer to vertical. Such a trebuchet wastes less energy in horizontal motion of the counterweight and consequently has much greater range.

Engines that deal impaling damage shoot bolts. Those that do crushing damage hurl roughly spherical stones, with one flattened surface to keep from rolling. For ballistae only, an extravagant commander may substitute bundles of 1-lb. lead bullets (the largest known size) for bundles of 1-lb. stones; these give +1 to damage and make damage type huge piercing (pi++).

Cost depends on weight:

• Bolts: $33/lb.
• Lead Bullets: $1.60/lb.
• Stones: $1/lb.

In addition to an artillerist or gunner, many weapons need a crew to load them, supply traction, etc. These people don’t need Artillery or Gunner skill, and don’t normally make skill rolls. If the GM requires a roll, use Soldier skill – although crewmen may substitute ST-based Artillery or Gunner, if better.

Terms and notation are as defined on pp. B268-271, with these additions:

Shots: Except for traction artillery, parenthetical loading times mostly cover drawing the bow or lowering the throwing arm. Loading ammunition is a comparatively minor task. For crewed engines (footnote [2]), multiply reload time by total crew size (add one for the gunner) to compare the amount of labor required.

ST: A giant who wishes to wield such an engine as a personal weapon must meet its ST requirement and have enough BL to lift it one- or two-handed, as follows.

Weapons with footnote [4] are normally mounted on a fixed tripod; heroically strong users can lift them off this two-handed and shoot them from the shoulder. Engines with footnote [3] have a roughly rectangular base; a giant must wield such a weapon one-handed, like a clumsy pistol, subject to the rules for using two-handed weapons onehanded (p. B270).

Bulk: Where two scores appear, the worse is for giants using the thing pistol-style!

Notes:

• [1] Minimum range is 25% of maximum range.
• [2] Requires crew. Use the listed number of men for beam slings. See the Crew Size Table for other weapons.
• [3] Rectangular-based weapon; see table introduction for effects.
• [4] Detachable tripod required; see Tripod Mount Table (p. 83).
• [5] Stats include built-in wheeled carriage, harness for two draft animals, and storage space for 100 bolts (weigh 50 lbs.). Total weight is low enough for the crew to move it if necessary, but the weapon is far too awkward for a giant to wield!
• [6] Requires five Ready maneuvers between shots; emptying the 20-round magazine takes 2 minutes.

Ancient writers recorded several sorts of nonstandard artillery projectiles. One option – multiple smaller stones or bullets – is described for the ballista (p. 80). Incendiary attacks using pots of charcoal and pitch become possible at TL2, and vases filled with Greek fire at TL3; see Incendiaries (p. 84). At TL2, bolts can be used as large flaming arrows (see p. B410 and Alternate Arrows, p. 73) – but on a critical failure, apply the burning damage to the catapult itself!

Ingenious artillerists can invent other missiles. For one-armed engines, these can be heavier than normal at the price of decreased range:

The Greeks and Romans also developed ballistae and scorpions light enough to use as personal weapons. Unlike the crossbows under Bows, Slings, and Thrown Weapons (pp. 72-78), which are custom-made (see Bows, Crossbows, and Rated ST, p. 74), these come in standard “calibers,” based on the length of bolt or weight of stone hurled. The spring is a standard diameter suited to the ammunition, and every other part of the weapon is in proportion to the spring. Despite the differences, these weapons still use Crossbow skill. They have the narrow window of visibility of full-scale ballistae, though, and are subject to the limitations on Aim under Direct-Fire Weapons (pp. 79-80). The later Roman Empire brought a more advanced design into use.

This two-armed, torsion-powered bolt-thrower – made from iron rather than wood – is small enough for one man to use. Roman cavalry were trained to cock, load, and fire it from horseback. Drawing the string doesn’t pull the arms out to the side, but in toward the center, while the string takes a sharp “V” shape that aims most of its force forward. The torsion springs are placed out to the sides, and their wide spacing gives a better field of view, allowing an Aim maneuver without a previous Vision roll.

Terms and notation are as defined on pp. B268-271, with one addition:

ST: The first ST listed is that needed to aim and shoot the weapon effectively. The second, in brackets, is the mechanism’s rated ST. If this exceeds 10, only use it when pulling back the string by hand; reduce it to 10 when cranking the weapon.

See Ammunition Costs (p. 81). Lead bullets for ballistae (only) give +1 to damage and make damage type large piercing (pi+) for 0.03-lb. and 0.06-lb. models, or huge piercing (pi++) for larger ones. Gadgeteers might lob other hard spheroids of suitable weight (e.g., chemical-filled glass balls) from ballistae.