All ballistic missiles are made up of three essential elements: a propulsion system, which provides the energy necessary to reach the target; a guidance system, which contains steering of the missile during powered flight and ensures the correct initial conditions for the ballistic trajectory; and the payload, which destroys the target.
Rocket propulsion involves combining fuel and an oxidizer in a combustion chamber, in which chemical reactions produce a high-pressure, high temperature gas. Exhausting that gas produces thrust that propels the missile.
Ballistic missiles can use solid or liquid propellant rocket propulsion systems. In general, liquid systems are somewhat more energetic than solid systems because liquid fuels and oxidizers that yield more energy can be chosen. The advantage of solid propellant systems is that they are rugged, easily stored, transportable, and have no moving parts. The trend in modern missile systems has been toward the use of solid propellants because of their simplicity of operation and reduced logistical requirements; however, some countries have greater access to liquid propellant technology and, therefore, continue to develop new liquid propellant missiles.
The accuracy of a ballistic missile depends on its ability to achieve an exact velocity and location in space at the end of its powered flight. Ensuring that this velocity and location are precisely attained is the job of the guidance and control system. Throughout the powered phase of flight, the instruments in the inertial navigation system (INS) must continually sense all the components of the missile's acceleration. The guidance computer uses these sensed accelerations to determine the missile's "state" (velocity, location, and orientation) and sends corrective messages to the missile's steering system to eliminate deviations from the required flight profile.
The function of the ballistic missile payload subsystem is to ensure that the weapon reaches the target and detonates at the correct time and place. Ballistic missile payloads can be nuclear, conventional, or chemical/biological. Chemical and biological weapons have also been incorporated into payload systems for short-range ballistic missiles. The effectiveness of these designs, and their ability to be scaled to longer rangers, is speculative.