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This article was posted to rec.games.frp.gurps on Monday, November 25, 1996. Just like with several other such articles I liked it so much that I decided to ask its author for permission to put it on my page. Apart from correcting some typos [hopefully all of them : ) ], adding the relevant links and converting the decimal values to European system (commas, not points -- sorry, pals), I've left it basically unchanged.

GURPS Mechanical Artillery

By Nana Yaw Ofori (nofori@pop3.utoledo.edu)

All of the following weapons are built at TL3 levels, though their stats remain unchanged whether they are built anywhere from TL1 to TL4. They are constructed with the design rules from GURPS Vehicles 2nd Edition, with Erratta dating to November 20, 1996.

Explanation of Variables

Spring-Powered Stone-Throwers

These are siege engines powered by the elasticity of a piece of wood or metal. According to GURPS Vehicles 2nd Edition, the maximum ST of a spring-powered engine is 50.
ST Malf Type Dam SS Acc 1/2D Max RoF Wt Vol Cost WPS VPS CPS Crew HP
20 Crit Cr 3d+5 25 2 200 250 1/22 100 2 2500 2 0,04 1 2 10
25 Crit Cr 5d+4 25 2 200 250 1/25 160 3,2 2560 2,5 0,05 1,25 2 13
30 Crit Cr 5d+7 25 2 210 260 1/27 225 4,5 2625 3 0,06 1,5 2 17
35 Crit Cr 6d+7 25 2 210 260 1/30 310 6,2 2710 3,5 0,07 1,75 2 21
40 Crit Cr 7d+6 25 2 210 260 1/32 400 8 2800 4 0,08 2 2 24
45 Crit Cr 7d+8 30 2 210 260 1/34 510 10,2 2910 4,5 0,09 2,25 2 30
50 Crit Cr 8d+7 30 2 220 270 1/35 630 12,6 3030 5 0,1 2,5 2 40

Torsion Powered Stone Throwers:

These are siege engines whose stone-slinging capability comes from the energy stored in a tightly-wound skein of fiber. GURPS Vehicles 2nd Edition makes no distinction between single-armed stone throwers like the Onager, and Double-Armed stone throwers like the Lithobolos. Vehicles II also says that the maximum ST of a torsion engine is 500. However two sources I have looked through -- 'Warfare in the Classical World' and 'The World of the Romans' -- both allude to Stone-Throwers that would be stronger than these, ST 1100 in the case of 'World', and ST 1800 in the case of 'Warfare'. Whether or not I'm going to suggest this as Erratta depends on my doing further research on the topic.

The 'Onager' as listed in GURPS Vehicles 2nd Edition Weapons table is an ST 100 stone thrower. Its damage should be listed as shown here.

The 'Onager' as listed in GURPS Vehicles 1st Edition would be, converted in terms of projectile weight, an ST 300 stone thrower.

The 'Heavy Onager' from GURPS Pre-Industrial Architecture (Available at the GURPSNet FTP site) is apparently inconvertible by these rules.
ST Malf Type Dam SS Acc 1/2D Max RoF Wt Vol Cost WPS VPS CPS Crew HP
50 Crit Cr 8d+7 30 2 220 270 1/35 500 10 5800 5 0,1 2,5 2 30
100 Crit Cr 13d+13 30 2 240 300 1/50 2000 40 8800 10 0,2 5 3 75
150 Crit Cr 18d+18 30 2 270 330 1/61 4500 90 13800 15 0,3 7,5 4 125
200 Crit Cr 23d+23 30 2 290 360 1/71 8000 160 20800 20 0,4 10 5 200
250 Crit Cr 28d+28 30 2 320 400 1/79 12500 250 29800 25 0,5 12,5 7 250
300 Crit Cr 33d+33 30 2 340 430 1/87 18000 360 40800 30 0,6 15 8 400
350 Crit Cr 38d+38 30 2 370 460 1/94 24500 490 53800 35 0,7 17,5 9 400
400 Crit Cr 43d+43 30 2 390 490 1/100 32000 640 68800 40 0,8 20 10 500
450 Crit Cr 48d+48 30 2 420 530 1/106 40500 810 85800 45 0,9 22,5 12 600
500 Crit Cr 53d+53 30 2 440 550 1/111 50000 1000 104800 50 1 25 13 600

Counterweight-Powered Siege Engines

Counterweight-Powered seige ingeins are siege engines which do their hurling from the energy stored in a see-saw arrangement, with a huge heavy weighted box at one end, close to the fulcrum, and the ong hurling arm at the other end, far from the fulcrum. They're big, they're heavy, and they have large crews. According to GURPS Vehicles 2nd Ed, they also have no maximum ST. I only constructed ones up to ST 5000 as the crew requirements and costs, in my opinion, were getting abominably large.

ST 500 stone throwers have decent 'Finger of God' potential. St 1000 stone throwers have 'Fist of God' potential. ST 5000 stone throwers have 'Left fist, Right Fist, and Left Knee of God' potential. I can't imagine anybody bothering to build a stone thrower much stronger than ST 5000.

The 'Trebuchet' from GURPS Preindustrial Arcitecture, converted by projectile weight, is an ST 1000 stone thrower.

All Counterweight Engines posess the following statistics:

Malf: Crit; Type: Cr
ST Dam SS Ac 1/2D Max Min RoF Wt Vol Cost WPS VPS CPS Crw HP
500 53d+53 30 1 440 550 140 1/224 31250 625 67300 50 1 25 25 500
600 63d+63 30 1 480 610 150 1/244 45000 900 94800 60 1,2 30 30 600
700 73d+73 30 1 540 680 170 1/266 61000 1220 126800 70 1,4 35 35 800
800 83d+83 30 1 590 740 190 1/283 80000 1600 164800 80 1,6 40 40 1000
900 93d+93 30 1 640 800 200 1/300 101250 2025 207300 90 1,8 45 45 1000
1000 103d+103 30 1 690 860 220 1/316 125000 2500 254000 100 2 50 50 1200
2500 238d+238 30 1 1440 1800 450 1/500 781250 15625 1567300 250 5 125 125 1200
5000 503d+503 30 1 3000 3800 950 1/707 3125000 62500 6254800 500 10 250 250 2000

Torsion-Powered Bolt-Throwers

Torsion-Powered bolt-throwers are, essentially, giant double-armed catapults powered by fiber skeins. I didn't build any spring-powered bolt-throwers, mainly because it looked like the Torsion-powered ones would be superior in every way.

All torsion-powered bolt-throwers are of the double-armed variety.

The Ballista, from GURPS Vehicles 2nd Edition, is an ST 50 bolt-thrower.
ST Malf Type Dam SS Acc 1/2D Max RoF Wt Vol Cost WPS VPS CPS Crew HP
20 Crit imp 2d+3 20 6 400 500 1/22 32 0,64 1600 0,25 0,005 2 2 5
30 Crit imp 4d 20 6 410 510 1/27 72 1,44 3600 0,56 0,011 2 2 8
40 Crit imp 5d+1 20 6 420 530 1/32 128 2,56 5056 1 0,02 2 2 12
50 Crit imp 6d+2 20 6 430 540 1/35 200 4 5200 1,6 0,032 3,2 2 16
75 Crit imp 9d+2 25 6 470 570 1/43 450 9 5700 3,5 0,07 7 2 27
100 Crit imp 12d 25 6 480 600 1/50 800 16 6400 6,3 0,13 12,6 2 40
125 Crit imp 14d+2 25 6 505 630 1/56 1250 25 7300 9,8 0,2 19,6 2 60
150 Crit imp 16d 25 6 530 660 1/61 1800 36 8400 14 0,28 28 2 70

Greek Fire Projector

This is one of the places I find I like the least what GURPS Vehicles 2nd Edition has to say. Anyone familiar with Vehicles 1st Edition will note that these versions of the greek fire projector, in terms of range, well, suck. Doubling the ranges provided here give values that are, though still pretty short, ones I can deal with. For those of you who care, one 'Shot' is assumed to be a half-gallon of liquid, taking up 0,075 cubic feet.

All Greek fire projectiors recieve an Armor Divisor of (5), meaning that unsealed personal or vehicular armor has its DR divided by 5 when hit by a Greek Fire Projector. On the up side, the PD of armor is doubled vs. this damage.

Greek Fire shots do the listed damage every second for 10d seconds to the target, and won't go out unless completely smothered (Just dumping on water won't do it). Thus, they tend to be very likely to kill PC's (as if anything else in this article isn't)

Light, Medium, and Heavy apparently apply only to the range and size of the Greek Fire thrower.
Type Malf Type Dam SS ACC 1/2D Max RoF Wt Vol Cost WPS CPS Crew Shots HP
Light 14 Spcl 1d(5) 10 3 1,5 2,1 1 150 3 750 4,25 0,5 1 20 13
Medium 14 Spcl 1d(5) 10 3 2,5 3,0 1 275 5,5 1375 4,25 0,5 2 20 19
Heavy 14 Spcl 1d(5) 10 3 3,0 4,5 1 525 10,5 2625 4,25 0,5 3 20 30

Notes

As in Vehicles I, all these weapons are assumed to have DR 5.

I'm also toying with a house rule that postulates that 80% of the required loaders for Counterweight Stone-Hurlers do nothing but contribute their ST for loading, in terms of pulling back the armature. As such, they can be replaced by draft animals numbering (Average ST of Draft Animal)/10.

Vehicles 2nd Edition also made no difference between single-armed and double-armed stone- and bolt- throwers. My suggestions (all untested house rules):

  1. Single-Armed stone throwers cannot be mounted inside a vehicle body. They must always be placed in Open mounts. Double-Armed stone throwers can be placed, well, anywhere.
  2. Double-Armed torsion-powered mechanical Artillery uses two skeins of fiber, one for each arm. As such, they need to be 'tuned' from time to time to keep the projectile firing straight forward. For every day that a torsion-powered artillery piece goes unused, its Acc number drops by 1. Once Acc drops to 0, then impose a further to-hit penalty of -1 to a maximum of -5.
'Tuning' the mechanical artillery piece can be done after a shot is fired. Tuning takes an additional 25% of the time to load and fire. It requires a Gunner/TL[Catapult]-2 or Armoury/TL[Siege Engines] roll. A Critical Success returns the artillery piece to full working order. A normal success decreases the Acc or To-Hit penalty by 1, in the reverse order that it was applied. A failure has no benefit. A Critical failure increases the penalty by 1, with no lower limit.

In a particularly hot, cold, or humid climate, double the rate at which the above penalties accumulate.

Greek Fire Bombs

GURPS Vehicles 2nd Edition is very unclear regarding the construction of greek fire bombs. A suggested fix:

Greek fire Bombs are earthenware jugs hurled by stone-throwing mechanical artillery for the purpose of delivering the flammable substance at a far better range than the Greek Fire Projectors are capable of doing.

Start with the Weight per shot (WPS) of the stone-throwing mechanical artillery piece. From that, find the Diameter (D) of the Greek Fire Bomb in millimeters.

D = Cube root (WPS x 75.000)
From that, find the burst radius of said bomb:

Radius = 0.00057 x D x D
Cost = $50 x WPS
VPS = WPS/33
Combat effects: In the mortal but very long-lived words of Forrest Gump, 'That's all I have to say about that.'