Around 200 AD, Chinese scientists discovered that an explosive mixture could be produced by combining sulfur, charcoal, and saltpeter (potassium nitrate). The explosive mixture, called huoyao, was used by the military in the 900's during the Tang Dynasty. Imagine their enemy's surprise when the Chinese first demonstrated their newest invention. New weapons were rapidly developed, including rockets that were launched from a bamboo tube. The Chinese began experimenting with the gunpowder filled tubes. At some point, they attached bamboo tubes to arrows and launched them with bows. Soon they discovered that these gunpowder tubes could launch themselves just by the power produced from the escaping gas. The true rocket was born. The date reporting the first use of true rockets was in 1232. At this time, the Chinese and the Mongols were at war with each other. During the battle of Kai-Keng, the Chinese repelled the Mongol invaders by a barrage of "arrows of flying fire." These fire arrows were a simple form of a solid propellant rocket. A tube, capped at one end, contained gunpowder. The other end was left open and the tube was attached to a long stick. When the powder was ignited, the rapid burning of the powder produced fire, smoke, and gas that escaped out the open end and produced a thrust. The stick acted as a simple guidance system that kept the rocket headed in one general direction as it flew through the air. It is not clear how effective these arrows of flying fire were as weapons of destruction, but their psychological effects on the Mongols must have been formidable. Gunpowder changed the methods of war forever.