Propulsion units and propellants

Propulsion for a ballistic vehicle usually relies upon a rocket system which is defined as propulsion by ejection of matter, all of which is originally carried within the vehicle being propelled. A rocket engine produces an unbalanced force on the vehicle and in thus able to move it. The propulsive action of the rocket engine arises from the reaction to the acceleration, relative to the rocket engine, of a mass of propellant originally carried within the rocket-propelled vehicle. In the rocket engines so far employed in ballistic missiles, acceleration of the propellant mass is brought about by:

(a) releasing heat energy by a chemical reaction of propellants within a combustion chamber, and

(b) the use of an expansion nozzle to produce a supersonic exhaust stream by expanding the evolved gas from combustion-chamber pressure to ambient pressure.

The combustion chamber and expansion nozzle are together known as the thrust chamber.

The thrust chamber has an injector plate. The design of the thrust chamber is governed by the propellants used, the thrust required, the permissible pressure within the chamber, the altitudes at which the thrust chamber must operate, the combustion temperature, and the method of cooling. After the dimensions of the thrust chamber are determined the throat area, expansion ratio, and flow rate are established. If a long period of combustion is required, regenerative cooling is nearly always used.

Rocket thrust chambers used in ballistic missiles have been fabricated from nickel-alloy tubes through which the coolant flows. The manufacturing process consisted of assembling the tubes in the configuration of the combustion chamber and expansion nozzle and brazing or welding them together to from the shaped thrust chamber. Originally steel bands were welded around them to give the necessary hoop strength. A considerable weight reduction was later made in thrust chambers by eliminating the steel bands and using-untwisted glass filament tape wound around the cylinder.

The thrust vector control is achieved by gamboling or swiveling the thrust chamber itself as a whole or by deflecting the jet be jet vanes or paddles, or by swiveling the nozzle.

The control system of the ballistic missile rocket engine has to ensure that the engine can be started and shut down at the correct times. It has also to ensure that the thrust is maintained at a predetermined level and that propellants are fed to the combustion chamber at the required pressures and at the correct mixture rations. The control systems must sense any malfunctions and incorrect operations of the start and stop sequences and must shut down the engine if abnormal and dangerous conditions develop. After the shut down of the engine, the control system must arrange for the venting of unused propellants.

Propellants which are used in rocket vehicles can be stored in eight solid or liquid form, and the associated engines are known as solid or liquid-propellant rocket engines respectively.

A rocket engine a basically eats as a chamber containing a high-pressure gas whish is continuously replenished as some of the gas escapes through an orifice into the region of lower pressure outside the chamber. The simplest rocket engine consists of a chamber – the combustion chamber – in which fuel can be burned in an oxidizer. The oxidizer can be carried either separately as in liquid propellants or mixed with the fuel as with solid propellants. The fuel and oxidizer are known as propellants. In a liquid-propellant engine they are injected into the combustion chamber. In a solid-propellant motor, the propellant storage tank and the combustion chamber are one and the same. The propellant mixture is triggered electrically, thermally, or chemically, to produce a heat-releasing chemical reaction, and the molecules of the gas produced by the combustion process large amounts of energy, moving rapidly in all directions within the chamber. Individually these molecules each have a high kinetic energy but their motions are randomly directed. To achieve a propulsive effect, this random motion has to be directed as uniformly as possible in a direction away from the rocket thrust chamber.

In issuing through the expansion nozzle, the random motion of the gas particles is changed to a more unidirectional motion. The gas emerges as a high-velocity stream from the thrust chamber.

Fuels for a rocket engine can consist of many substances ranging from solid such as asphalt, synthetic rubber, beryllium, to the well-known commercial liquid fuels like gasoline, alcohol, jet fuels, and even liquefied gas such as hydrogen. Oxidizers are more limited. Principal substances used are liquid oxygen, nitric acid, and concentrated hydrogen peroxide. Atlas, Titan, Thor, and Jupiter used liquid oxygen as the oxidizer.

When the propellants are solid, they are already in the combustion chamber. Liquid propellants, on the other hand, have to be displaced from storage tanks into the combustion chamber. In small rockets units, this in done by compressed inert gas acting on the propellants in their storage tanks. For large engines of the type used in ballistic missiles, a pump feed is used which centrifugal pumps are driven by a gas turbine.

Ignition of a ballistic missile rocket engine is the initiation of combustion. During the ignition phase a supporting flame is maintained and a low flow rate of primary propellants is begun. As soon as the propellants are burning properly either an automatic or a manual changeover to maximum performance burning takes place.