Further measures must be taken to stop missile-wielding aggressors. The main distinction between a ballistic missile and an unguided artillery rocket is the guidance and control system-the most expensive and technologically most advanced subsystem in a ballistic missile. Another distinction is that guided missiles are usually multi-staged, enabling them to attain considerably longer ranges. The figure on page 39 shows, schematically, a two-stage ballistic missile and the major technologies involved in its design. But with these exceptions, solid fuels are currently used almost exclusively.
Solid fuels are generally less energetic than liquid ones but much simpler to handle in the field.
There are two major types of solid fuels. Obviously, rocket fuel production requires much bigger batches of material, which calls for somewhat different logistics. But the difference is a matter of scale rather than technological sophistication. It is more complicated to extrude the propellant for a large rocket and to perform the necessary quality control tests than to do so for a batch of bullets, but the chemistry and mechanics are essentially the same. It has several other advantages, particularly for larger rocket motors, but it is also commonly used in a variety of small rocket motors, including unguided antitank rockets.