of a rocket motor. Until the late 19th century, the only available propellant was black powder. It had many disadvantages as a propellant; it has relatively low power, requiring large amounts of powder to fire projectiles, and created thick clouds of white smoke that would obscure the targets, betray the positions of guns, and make aiming impossible. In 1846, nitrocellulose (also known as guncotton) was discovered, and the high explosive nitroglycerin was discovered at nearly the same time. Nitrocellulose was significantly more powerful than black powder, and was smokeless. Early guncotton was unstable, however, and burned very fast and hot, leading to greatly increased barrel wear. Widespread introduction of smokeless powder would wait until the advent of the double-base powders, which combine nitrocellulose and nitroglycerin to produce powerful, smokeless, stable propellant. Many other formulations were developed in the following decades, generally trying to find the optimum characteristics of a good artillery propellant – low temperature, high energy, non-corrosive, highly stable, cheap, and easy to manufacture in large quantities. Modern gun propellants are broadly divided into three classes: single-base propellants that are mainly or entirely nitrocellulose based, double-base propellants consisting of a combination of nitrocellulose and nitroglycerin, and triple base composed of a combination of nitrocellulose and nitroglycerin and nitroguanidine. Artillery shells fired from a barrel can be assisted to greater range in three ways: Rocket-assisted projectiles enhance and sustain the projectile's velocity by providing additional 'push' from a small rocket motor that is part of the projectile's base. Base bleed uses a small pyrotechnic charge at the base of the projectile to introduce sufficient combustion products into the low-pressure region behind the base of the projectile responsible for a large proportion of the drag. Ramjet-assisted, similar to rocket-assisted, but using a ramjet instead of a rocket motor; it is anticipated that a ramjet-assisted 120-mm mortar shell could reach a range of . Propelling charges for barrel artillery can be provided either as cartridge bags or in metal cartridge cases. Generally, anti-aircraft artillery and smaller-caliber (up to 3" or 76.2 mm) guns use metal cartridge cases that include the round and propellant, similar to a modern rifle cartridge. This simplifies loading and is necessary for very high rates of fire. Bagged propellant allows the amount of powder to be raised or lowered, depending on the range to the target. It also makes handling of larger shells easier. Cases and bags require