A ballistic missile is a type of large and powerful missile designed to deliver a warhead across large distances to a predetermined target. Ballistic missiles follow suborbital trajectories, reaching space (100 km+) altitudes and exiting the Earth’s atmosphere, in some cases traveling as high as 1,200 km over the surface for intercontinental ballistic missiles. Such missiles are called “ballistic” because after an initial boost phase, the rest of the course is usually determined by ballistics. A smooth parabolic line.
Ballistic missiles come in many shapes and sizes. In the United States, ballistic missiles are divided into four range classes:
- intercontinental ballistic missile (ICBM) – over 5500 kilometers
- intermediate-range ballistic missile (IRBM) – 3000 to 5500 kilometers
- medium-range ballistic missile (MRBM) 1000 to 3000 kilometers
- short-range ballistic missile (SRBM) up to 1000 kilometers
For ranges less than 350 km, the ballistic missile never leaves the Earth’s atmosphere. Note that the only three ballistic missiles ever actually used in battle were only in the short-range category, and contained conventional explosives. Most ballistic missiles in existence today are meant to carry nuclear warheads, though none of these have been used in war yet.
Ballistic missiles use either a solid or liquid fuel. The older missiles, such as the V2 rocket used by Nazi Germany during WWII and the first ballistic missiles built by the US all used liquid fuel. In many cases, the fuel in a liquid propellant ballistic missile is liquid hydrogen whereas the oxidizer is liquid oxygen. The two must be kept at cryogenic temperatures or they revert to a gas phase. During launch, the two gases are rapidly pumped out of storage chambers in the presence of a spark, which ignites the mixture and propels the rocket forward. The byproduct of the burning fuel is water vapor.
The liquid phases of these hydrogen and oxygen are desirable for rocketry because of their improved energy density over the gas phase. Another upside is that liquid-propelled ballistic missiles can have their engines throttled, turned off, or restarted as desired. A downside is that storage of such missiles is a hassle, as the fuel requires constant refrigeration to be ready for launch.
Another variety of liquid propellant are hypergolic propellants. Hypergolic propellants ignite on contact, requiring no ignition source. This is useful for frequent starting and restarting for space maneuvering applications. The most popular version uses monomethyl hydrazine (MMH) for the fuel and nitrogen tetroxide (N2O4) for the oxidizer.
More modern ballistic missiles use solid fuels, as they are easier to store and maintain. The Space Shuttle, for instance, uses two reusable solid boosters, each filled with 1.1 million pounds (453,600 kg) of propellant. The fuel used in powdered aluminum (16%), with iron powder (0.07%) as a catalyst, and ammonium perchlorate (70%) as the oxidizer.
Most ballistic missiles are designed to reach their target in between 15 and about 30 minutes, even if the target is on the other side of the world. Because they are so essential for national security, they are among the most carefully built machines on the planet.