Via the Marmot’s Hole comes this cool video of a Korean hwacha in action on Mythbusters:

For those interested hwacha.net has everything you could possibly want to know about hwachas posted on it.

For example here is a brief history of the hwacha:

Hwachas were first constructed in 1407, as part of King Sejong the Great’s programme of military buildup:

• 1377: Gunpowder is first mass-produced domestically in Korea (see below)
• Late 1300s: Early ‘juhwa’ rockets, based on Chinese ‘fire arrows’, are produced in Korea
• Later 1300s: Smaller ‘singijeon’ fire arrows are developed
• 1409: Multiple rocket launcher, i.e. a hwacha, is demonstrated to King Sejong the Great
• 1409-1450: King Sejong constructs about 90 hwacha, but it’s unclear whether they participated in his campaigns against Tsushima and Manchuria
• 1451: King Munjeon’s administration begins producing hwacha in bulk and they become an integral part of border defences

You can read more about the hwacha here.