Rockets for Rookies

Rockets for Rookies

by Even Steven

Popular terminology makes a distinction between jets and rockets: a jet takes in air from the atmosphere; a rocket needs no air supply, as it carries its own supply of oxygen. Both types of engines operate by expelling a stream of gas at high speed from a nozzle at the after end of the vehicle. Rockets are distinguished by the means used to produce exhaust material. The most common type of rocket engine obtains its high-pressure gases by burning a propellant. This propellant consists of both fuel and oxidizer and may be solid or liquid. The fuels and oxidizers used to power a jet/rocket engine are called propellants. The chemical reaction between fuel and oxidizer in the combustion chamber of the jet engine produces high-pressure, high-temperature gases. These gases, when channeled through an exhaust nozzle, are converted into kinetic energy creating a force acting in a direction opposite to the flow of the exhaust gases from the nozzle. This propulsive force, termed thrust, is a function primarily of the velocity at which the gases leave the exhaust nozzle and the mass flow rate of the gases.

In order to develop a high thrust with a solid propellant, grains or charges of propellant are employed with large burning surfaces so that a high rate of mass flow is developed. The duration of burning of a propellant charge is determined by the web of the grain and the burning rate. Since the combustion chamber has fixed dimensions and capacity for propellant, the thrust may be either great, but of short duration, or low, but of long duration.

Propellants are classified as either solid propellants or liquid propellants. Nearly all of the rocket-powered weapons in use by the United States use solid propellants. Liquid propellants are still used in some of the older ICBMS and will be used in future cruise missiles. Liquid fuels are more powerful than solid fuels; but other than this advantage, a liquid-fuel rocket is not ideally suited as a weapon-propulsion system. Because of their high volatility and corrosive nature, liquid fuels cannot be stored for long periods of time, which usually means the system must be fueled just prior to launch. This negates its ability to be a quick-reaction weapon, which is usually required in combat situations.

The size and type of a missile selected for a particular function are based on the target, the launch vehicle or platform, range and maneuverability requirements, altitude envelope, and storage requirements. Minimum size and weight may not be the most efficient architecture, and it is often best to employ various types of structures for different sections of the missile to obtain certain design or maintenance advantages.

The components of a missile are located in five major sections: the guidance section, warhead section, autopilot section, and control and propulsion sections. The functional systems of the missile are:

The guidance system
The warhead section
The autopilot
The propulsion system
The control system
The Guidance System.

The guidance system for a homing missile consists of an antenna assembly or electro-optical device protected by an optically transparent cover or a radome in the case of the radio frequency system, and electronic components that analyze signals from the target and compute orders for use by the autopilot. The sensor employed is usually a gimbal-mounted automatic tracking sensor (except the interferometer method) that tracks the target line-of-sight (LOS) and sends signals about the target's movement to the guidance electronics.

The Warhead. The warhead consists of the fuze assembly, warhead, safety and arming device, and fuze booster. The fuze assembly usually contains a contact and proximity fuze. The contact fuze is enabled at all times, and the proximity fuze is actuated electronically. Its circuitry works in conjunction with the guidance section to ensure that the target detection device (TDD) remains unarmed until just prior to intercept, minimizing vulnerability to jamming. The safety and arming device prevents arming of the warhead until the missile is a safe distance from the firing platform.

The Autopilot. The autopilot is a set of electronic instruments and electrical devices that control the electric actuators (motors) of aerodynamic control surfaces (fins). In the absence of signals from the guidance computer, the autopilot maintains the correct missile attitude and maintains the missile flight in a straight line. Called-for-acceleration signals from the guidance computer will cause the autopilot to command corresponding changes in flight path, while continuing to stabilize the missile.

The Propulsion System. Any of the methods of propulsion previously described may be used as long as the missile has sufficient speed advantage over the target to intercept it. The propulsion system must accelerate the missile to flying speed rapidly to allow a short minimum range and achieve sufficient velocity to counter target maneuvers. Powered flight may occur for most of the operational range of the weapon or only at the beginning (boost-glide). Boost-glide weapons are limited in their ability to engage at long range targets that have significant altitude difference or perform rapid maneuvers.

The Control System. The steering or control unit may be located forward, in the midsection, or aft on the missile, depending on where the control surfaces are located. Movement of control surfaces may be electrical or hydraulic, with electrical actuation becoming the dominant method. Some weapons are limited in allowable locations for the control actuators because of size limitations or difficulty in passing signals from the autopilot to remote points on the airframe.