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Rocket Propulsion Fundamentals

By Tom Logsdon

White hot combustion by-products blasted rearward with blinding speed generate the rocket's propulsive force that that hurls a rocket skyward. Pressure inside the rocket combustion chamber pushes in all directions to form balanced pairs of opposing forces which nullify one another, except where the hole for the exhaust nozzle is placed. Here the pressure escapes, causing an unbalanced force at the opposite side of the combustion chamber that pushes the rocket up towards its orbital destination.

Both rockets and jets are based on the same principle that causes a toy balloon, carelessly released, to swing in kamikaze spirals around the dining room. A jet sucks its oxygen from the surrounding air, but a rocket carries its own supply of oxidizer on board. This oxidizer can be stored in a separate tank, mixed with the fuel, or chemically embedded in oxygen-rich compounds. A rocket usually has two separate tanks, one containing the fuel, the other containing the oxidizer. The two fluids are pumped or pushed under pressure into a small combustion chamber above the exhaust nozzle, where burning takes place to create a thrust. A solid rocket rocket is like a slender tube filled with gunpowder; the fuel and oxidizer are mixed together in a rubbery cylindrical slug called the grain. Solid propellants are not pumped into a separate combustion chamber. Instead, burning takes place along the entire length of the cylinder. Consequently, the tank walls must be built strong enough to withstand the combustion pressur e.

Rocket design decisions are dominated by the desire to produce the maximum possible velocity when the propellants are burned. A rocket's velocity can be increased in two principal ways: by using propellants with a high efficiency and by making the rocket casing and its engines as light as design constraints permit.

Unfortunately, efficient propellants tend to have some rather undesirable physical and chemical properties. Liquid oxygen is a good oxidizer, but it will freeze all lubricants and crack most seals. Hydrogen is a good fuel but it can spark devastating explosions. Fluorine is even better but it is so reactive it can even cause metals to burn.

Miniaturized components, special fabrication techniques and high strength alloys can all be used to shave excess weight. But there are limits beyond which further weight reductions are impractical. The solution is to use staging techniques whereby a series of progressively smaller rockets are stacked one atop the other. Such a multistage rocket cuts down its own weight as it flies along by discarding empty tanks and heavy engines. However, orbiting even a small payload with a multistage rocket requires an enormous booster. The Saturn moon rocket, for example, outweighed the Apollo capsule it carried into space by a factor of 60 to 1.

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