Jet Propulsion

A method of propulsion by which, in accordance with Isaac Newton's third law of motion 'to every action there is an equal and opposite reaction', an object is propelled forwards by a stream of gas or liquid (the jet) expelled in the opposite direction.

Within the animal kingdom, creatures such as squid or octopuses make use of short bursts of jet propulsion to move quickly. In general, water is taken into a muscular sac and is expelled rapidly through a small opening to provide a rapid acceleration in the opposite direction. The simplest example of jet propulsion in the underwater world is jellyfish. Although not all species of jellyfish use jet propulsion to travel, many species fill the umbrella section with water and then push the water out in a stream of short duration. However, the jellyfish has little control over direction. Squid, on the other hand, are able to control the direction of the jet, allowing them to move in a definite direction. Some squid are able to reach speeds high enough to shoot them out of the water and onto the deck of a ship.

Several shelled animals such as clams or scallops can use a more primitive form of jet propulsion to rapidly escape from enemies. They achieve this by bringing the two halves of their shells together rapidly, although like the jellyfish they have little control over the direction of the expelled liquid, and therefore little control over the direction of their movement.

Outside of the animal kingdom, the most common use of jet propulsion is the widely-known jet engine or turbojet. The turbojet is a kind of gas turbine, in which air passes through a forward-facing intake, is compressed and fed into a combustion chamber. Fuel is sprayed in and ignited, producing a rapidly expanding ball of hot gas which proceeds rearwards, spinning a turbine in the process, which drives the compressor. The hot gas is finally ejected from the rear of the unit, usually a nozzle or some form of tail-pipe, at very high speed. This is the simplest form of gas turbine, used in supersonic aircraft.

The turboprop, used for moderate speeds and altitudes, adds extra stages that absorb most of the energy from the gas stream to drive a propellor shaft. The turbofan is best suited to high subsonic speeds, and features an extra compressor in front. Some of the airflow bypasses the core engine and mixes with the jet exhaust stream, providing a lower temperature and velocity. Compared to the turbojet this modification results in improvements in economy, efficiency and quietness, yet provides a higher speed compared to the turboprop.

The turboshaft is a form of jet engine used to drive the rotors of helicopters, as well as providing propulsion to hovercraft, ships and trains. Essentially a turboprop without a propellor, power from the extra turbine instead being delivered to an output shaft or a reduction gearbox.

The ramjet is used for a variety of missiles. When running at twice the speed of sound (Mach 2), the pressure in the front intake of the ramjet engine is such that no compressor or turbine is required. It is cheap and light, but consumes a high amount of fuel, which is burnt in the widest section of what is essentially an open-ended barrel-shaped tube. The lack of a turbine, however, means that the rocket must be boosted to operational speed before the ramjet engine will function.

Variants of the jet engine include vectored thrust engines, allowing vertical takeoff, reverse thrust, used to slow down a jet plane on landing, and reheat or afterburning, used in military aircraft to provide short-duration increases in thrust.

Cannon

Before the cannon was invented, the most common method of hurling destructive weaponry at the enemy was the catapult. Known as engines, there were three main types: the ballista, the mangonel and the trebuchet. Cannons did not appear until the 14th Century, the first recorded use being in Florence in February of 1326. From this time onwards the cannon was used on an ever-increasing scale throughout the world. By the start of the 1700s the cannon was a common weapon in European armies, with artillery units becoming of equal importance to the cavalry.

Cannons come in a range of types and sizes. Guns, mortars and howitzers are all considered to be smoothbore, which simply means that the interior of the cannon's barrel was not rifled. Rifling was the cutting of a spiral groove along the interior of the barrel, which gave the projectile spin.

Guns shot at a low trajectory and were useful for long or short range battering of fortifications. They could also be aimed with some accuracy and were therefore useful for destroying enemy cannon. Mortars, on the other hand, shot at a high trajectory, and were most suited to firing overwalls. The high trajectory made them an excellent choice for raining bombs onto the enemy, and with careful planning the bomb could be timed to explode over a group of men and thus cause widespread injury. Howitzers shot at a trajectory someway between the gun and the mortar, with the added advantage that they could handle projectiles considerably larger than the other two methods.

A variety of projectiles were available, the three basic types being those that did not explode, those that did and projectiles that scattered small fragments without exploding. The size of the projectile was dictated by the size of the cannon's bore diameter. Measured in inches, the calibersize tended to closely match the weight of the iron shot at smaller calibers. A cannon with a bore of three inches would shoot an iron ball which weighed between three and four pounds. Similarly, a cannon of bore diameter nine inches would manage an iron ball of between seven and ten pounds. Eventually, the cannon came to be referred to in terms of the weight of shot they fired. A four-pounder, for example, would shoot four pound iron shot. Cannons available ranged from one-pounders right up to thirty-two-pounders, suitable for siege work.

Solid shot was any projectile which did not explode, and included cast iron spheres, bar shot and chain shot. Early artillery, such as the ballista, had shown that spherical objects moved through the air much more effectively than oddly-shaped projectiles. Working on this principle, rounded stones were initially a good choice of projectile, later augmented by a thin layer of iron to improve their shape. An obvious development was to cast perfectly round balls, with varying degrees of success. Early shot featured a raised seam mark from the two part moulds used to cast the shot, though the process was quickly perfected.

Bar and chain shot were simply methods of altering the basic solid ball design, such modifications being made with the specific purpose of tearing into wooden structures, whether on land or at sea. Bar shot consisted of the two halves connected together by a short iron bar, whereas chain shot used a length of chain to achieve the same thing. While this new design was particularly good for tearing through structures or, indeed, rigging, they were of little use in maiming larger quantities of humans, and for this purpose other types of shot had to be devised.

Bombs, or explosive shells, were hollow spheres of iron into which gun powder was poured. A fuse would be inserted and then lit. By ensuring the sphere opposite the hole was slightly heavier the bomb could be encouraged to land upright, helping to avoid extinguishing the fuse. Some bombs were constructed with a short iron lip around the fuse so that it could be grasped with a pair of tongs, allowing it to be easily inserted into the cannon. (Yes, this is exactly the sort of beum encountered by Inspector Clouseau on a daily basis...)

Bombs were well suited to killing soldiers, as were scatter shot projectiles. A canister, into which was loaded small objects, would be fired into the midst of the enemy, where it would break apart and cause widespread injury. Small round shot was a favoured packing, as was grape shot, so called because it came packed in a cloth bag, wrapped around a wooden spindle, the appearance being similar to clustered grapes.

The basic cannon has, of course, long been superseded. In its time, however, the destructive capability of the cannon was considerable, and the importance of such weaponry in a wide range of battles is, as a result, undeniable.

Fuse

There are two kinds of fuse: those used in explosives, and those used in electronic devices.

Explosives

In explosives there are a variety of fuses, the simplest of which consists of a rope or cord which has been impregnated with combustible materials. When one end of the fuse is lit the flame will progress along its length at a predetermined and dependable rate. The other end of the fuse is, as anyone will remember from the Road Runner cartoons, attached to the explosive charge. The whole point of the fuse, and the importance of it burning at a constant, dependable rate, becomes obvious here. The person lighting the explosive will require time to achieve a safe distance (or in the aforementioned cartoon, the coyote will require time to ingest or somehow become stuck to the dynamite), and thus a sufficient amount of fuse is left to provide time for anyone in the area to reach safe distance.

The safety fuse, invented in 1831 by a leather merchant called William Bickford, was the first fuse to achieve this dependable timing. It was, compared to its predecessors, astoundingly consistent. Bickford, motivated to reduce the number of accidents in the mines, wrapped a core of black powder in textiles. Later on the cord featured an added waterproof coating, often asphalt, and later on a further outer sleeve of plastic or textile.

Black powder was eventually banned in many mines, and in 1908 the detonating cord made its debut. In many ways this was similar to the safety fuse, but contained high explosive rather than black powder. It was constructed in a rather unnerving manner, in that a large-bore cylinder of lead was filled with molten TNT and allowed to solidify before being passed, repeatedly, through rollers until it had reached the correct diameter. Called, in its native France, cordeau détonant, elsewhere it was merely called cordeau.

In 1936 the American manufacturers of cordeau developed their own version: Primacord. It was based on the French patents and consisted of a core of PETN or RDX surrounded by a variety of textiles, waterproofing and plastic.

Electronics

In electronics, however, a fuse is almost the opposite; rather than causing explosions it is designed to prevent them, being a simple but important safety device. Imagine the bulb from a torch, in which a thin section of wire glows when electricity is passed through it. If too much electricity is passed through the bulb the section of thin wire will melt. The fuse works on a similar principle; a wire or strip of metal, designed to melt when excessive current passes through it, is placed in the circuit. When a potentially dangerous amount of current passes through, the metal in the fuse melts and the circuit is at that point disconnected from the mains. (This is usually a good thing, though occasionally one is led to wonder whether fuses are designed specifically to blow at critical moments during television programmes or when you're just about to finish the penultimate level of Tomb Raider.)

Candlebomb

A candlebomb is a small bubble of glass, filled with water. Placing this in the flame of a candle will cause the water inside to heat, turning it to steam and causing the glass shell to burst. The question is... why? What possible use could such a device be?

Well, not unlike the indoor fireworks which were popular during the eighties, the after dinner amusement device of choice in the eighteenth century was the candlebomb, whereby any number of guests could be held in fascinated thrall by the evanescent bang of an exploding glass sphere.

It's hard to understand why it didn't catch on... naked flames, hot water and splinters of glass on the table are so much more sophisticated than wafer thin mints.