Du Huo Fei Pao (毒火飛砲)

Drawing of a paper fuse (top right), paper sealing tube (middle right), wooden fuse (top bottom), shell casing (top left) and bombard (bottom left), from 'Wu Bei Ji Yao (《武備集要》)'.

Du Huo Fei Pao (毒火飛砲, lit. 'Poisonous fire flying cannon') was a bombard-launched explosive shell designed to be launched from Wan Kou Pao (碗口砲, lit. 'Bowl-muzzle cannon'), an ordinary stone-throwing bronze bombard already in widespread use since the founding of Ming Dynasty.

First appeared in late 15th/early 16th century and entered mass production by 1544 at the latest, Du Huo Fei Pao was a spherical cast iron shell filled with black powder and up to five taels of sulfur, arsenic and other poisonous substance/irritants (hence the name "poisonous fire"), and was equipped with a fuse known as Mu Xin (木信, lit. 'Wooden fuse'), which consisted of burning fuse(s) winded around a screw-threaded wooden rod and sealed inside a thick paper tube to prevent premature contact between the fuse and bursting charge inside the shell. A cleverly designed ignition device, Mu Xin allowed for easy adjustment of time-delay before detonation through increasing or decreasing the number of threads on the wooden rod (and thus the length of the burning fuse).

Despite its ingenuity, Mu Xin did have some drawbacks, namely It was a fairly complicated device assembled from three different components (burning fuse, wooden rod and paper tube), not to mention hand-carving screw threads onto a wooden rod took considerable amount of skill and time. Fortunately, an alternative that offered similar level of consistency whilst being far easier to made was later discovered. Known as Zhi Xin (紙信, lit. 'Paper fuse'), this alternative fuse was simply the motor part of a Chinese rocket being adapted into a fuse.

EXTRA: Bigyeok Jincheonroe (비격진천뢰 or 飛擊震天雷)

Drawing of a large Bigyeok Jincheonroe, from 'Yungwon pilbi (《융원필비》 or 《戎垣必備》)'.

Bigyeok Jincheonroe was a cast iron shrapnel shell invented in Joseon Dynasty Korea by weapon engineer Yi Jangson (이장손 or 李長孫) in 1591, and proved instrumental in the liberation of Gyeongju city from Japanese invaders during Imjin War that broke out one year after its invention.

Named after another Chinese weapon, Bigyeok Jincheonroe bore a striking resemblance to Du Huo Fei Pao even down to design of the fuse based on screw-threaded wooden rod, which suggests that its invention may be inspired by Du Huo Fei Pao. Nevertheless, there were some notable differences between the two as well, chief among them material of the sealing tube—the Koreans used a bamboo tube whereas the Chinese used a paper tube—which likely influenced the design of the fuse hole as well. Specifically, Bigyeok Jincheonroe had a large rectangular fuse hole with a matching iron lid, but this was not found on Du Huo Fei Pao (which had a normal round fuse hole). This was due to the fact that bamboo is relatively rigid and smooth-surfaced, which may cause the fuse to slip out of the hole if it is not pinned in place by the iron lid. In contrast, paper is supple and coarse and serves as natural wadding, allowing the fuse to fit snugly into the hole like a stopper.

Bamboo sealing tube (left), iron lid (middle), and bomb casing of a Bigyeok Jincheonroe with a squarish fuse hole (right).

Interestingly, the Koreans also never appeared to adopt the more convenient Zhi Xin fuse.