Thursday, April 20, 2023

Aerial Bombing

 Someone asked about bombardment mechanics to hit a ground target from a skyship or a flying creature. Here’s my take. 

Using Mechanical Devices

            The first consideration is whether the device relies on slingshot-style mechanics, like a catapult or a trebuchet. Mortar-like firearms (fire pots and such) should also fit in this category. Projectiles arc upward before falling straight down. The height of their flight curves is equal to the chosen horizontal range, after which the projectiles fall vertically. The maximum damage potential of these weapons comes from the density and hardness of their projectiles.

            Other siege weapons rely on torque to propel a projectile on a flatter trajectory, such as scorpions and ballistae. Cannons (culverins and such) also fit in this category. These aren’t ideal for conventional bombardment from a flying platform, be it a skyship, a levitating castle, or a large flying creature. Their flight curves’ apogees are about half the weapons’ chosen horizontal ranges after which the projectiles arc a bit more below the horizontal line of sight, about half the chosen horizontal range, before falling vertically. The maximum damage potential of these weapons (usually from a large metal dart or a javelin) is highest at or above the direct line of sight. This makes them less effective as bombardment weapons: halve damage unless they can be pointed straight down.


Manual Drop

            Projectiles can otherwise be pushed overboard, dropped from a dedicated bomb bay, or released from the clutches of a flying mount. The projectiles then fall vertically. The advantage of this method is that the point of impact is easier to estimate (see Odds, next). The drawback is that aiming manual drops requires positioning or moving the launch platform itself (skyship, levitating castle, or flying mount), compared with mechanical devices that can be aimed independently.


Odds of Hitting a Target

            Movement: Bombarding from a moving platform (skyship, levitating castle, or flying mount) incurs a –2 penalty to hit. Add a penalty to hit if the target is moving in a different direction: –1 for ever 60 degrees or fraction thereof. For example, if a target flies in the exact opposite direction as the attacker, the penalty to hit would be –3. If the target moves in the same direction and at the same speed, cancel all movement penalties.

            Dive Attack: Dropping a projectile when diving at maximum speed at a target (like from a dragon or wyvern): +2 bonus to hit (ignore the initial –2 movement penalty).

            Manual Drop: +2 bonus to hit (combined dive attack + manual drop = +4 bonus).

            Altitude: –1 penalty to hit above 1,000 ft. (roughly 300 m). Increase the penalty for each 3,000 ft. of extra altitude (thus –2 at 4,000 ft., –3 at 7,000 ft., –4 at 10,000 ft., etc.)

            Wind: Calm-to-light breeze—nil, moderate-to-strong breeze –1, high wind –2, and gale –3. If any wind shear is involved (DM’s call): add another –2 penalty to hit.

            Clouds or Fog: The time to acquire a clear line of sight to target varies accordingly: clear weather 1 round, light cover 1d4+1 rounds, moderate cover 2d4+1 rounds, and heavy cover 3d4+1 rounds. The time available to aim while a clear line of sight has been acquired varies accordingly: clear weather (no limit), light cover 3d4 rounds, moderate 2d4 rounds, and heavy 1d4 rounds. Heavy catapults, trebuchets, and firepots are the slowest to aim and shoot (6 rounds), or 5 rounds for light catapults and culverins, depending on the game system. Ballistae and scorpions only take 2 rounds to arm, aim, and shoot. Therefore, clouds can prevent using heavier siege weapons.


Convenient Magic

            Accuracy can be improved with the use of magic items. An artillerist scope can be fitted to weapons like a ballista or a scorpion, which involve a relatively flat line of fire, or to a bomb bay beneath a skyship. The optical scope grants a +1 bonus to hit, plus any magical bonuses due to enchantment. A level-2 magic-user spell, Norden’s Uncanny Sight, improves one’s aim with siege weapons or when performing manual drops. The spell grants a +1 bonus to hit up to +6 for every 3 experience levels above 3The spell lasts 4 rounds +1 for every 3 experience levels above 3.


Terminal Velocity

            Projectiles reach their highest falling speed after 12 seconds, moving 90m (300 ft.) per second. For example, freefall bombardment from 12,000 ft. altitude takes about 40 seconds. Add another 10 seconds if the projectile was shot horizontally. A B/X-BECMI round lasts 10 seconds, therefore impact should occur 4-5 rounds after releasing the projectile. An AD&D round lasts 1 minute, so time to impact should be measured in 6-second segments, therefore 7-8 segments (or at the end of a round for the sake of simplicity).


Bombs away!

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