How do ballistic missiles work?
- Ballistic missile are a marvel of modern engineering, but the central phenomenon that allowed for their invention was identified over 300 years ago by Sir Isaac Newton who observed that “for every action force there is an equal and opposite reaction.”
- Known as Newton’s Third Law of Motion, this concept explains the generation of thrust, which is the force that moves the missile. The missile’s fuel, or propellant, is burned in an engine, creating hot exhaust gases, which are funneled through a nozzle at the rear of the missile. This action generates thrust at the missile’s base forcing it upwards/the opposite reaction.
- To achieve liftoff, the thrust must be greater than the missile’s weight. The magnitude of the thrust is determined by the velocity at which exhaust gases are expelled, and depends on the type and quantity of propellant and the engines used.
- The missile will continue to accelerate until it runs out of propellant, at which point it will have reached its maximum speed and continue to move at that speed until another force acts upon it, such as gravity or air resistance. The time at which this occurs is known as the burnout time and the velocity it reaches is the burnout velocity.
- Getting the time and velocity correct when a missile runs out of propellant is critically important to ensuring its payload hits its target. A missile will use up all its propellant during a short period known as the boost phase, after which its trajectory is set and cannot be changed. For an ICBM, this period may last only a few minutes.
- After the boost phase, the missile stops accelerating and is propelled by its own momentum, entering what is known as the midcourse phase. An ICBM may coast through space for up to 20 minutes before re-entering the atmosphere for the terminal phase of its flight path.
- A ballistic missile generally does not have onboard motors to make last minute corrections, meaning engineers must get the burnout time and speed, launch angle and direction correct during the boost phase to ensure they hit their target.
- If the burnout time is a little too long, the missile will accelerate to an incorrect speed and overshoot its target. Likewise, if the burnout time is too short the missile will travel too slowly and undershoot its target. Since missiles travel great distances, even minor deviations will have a significant impact on their ultimate trajectory.
- To see how difficult it is, in practice, to correctly target a missile, try to hit the target in the applet below by experimenting with different speeds and angles.
What are the principal parts of a ballistic missile?
Ballistic missiles have four principal parts: the airframe, engine, propellant, and payload. Below is an annotated model of a ballistic missile.
What types of propellant do ballistic missiles use?
- Missiles use either solid or liquid propellants, and will have different engines depending on which type of propellant is used. Regardless of the type, all propellants require two components to burn: oxidizer and fuel.
- The oxidizer provides an on-board source of oxygen, and is necessary because unlike aircraft and cruise missiles that use oxygen in the atmosphere to burn their fuel, ballistic missiles travel at high altitudes where the air is thin or in the vacuum of space.
- Liquid propellant missiles have separate fuel and oxidizer tanks within the airframe. The fuel and oxidizer are mixed in the proper proportion, vaporized, and then burned to produce a gas at high pressure, which exits the nozzle and accelerates the rocket in the opposite direction.
- Solid propellant is a pre-formed mixture of fuel and oxidizer that is packed inside a cylinder. When ignited, the propellant burns in place, generating hot gases that exit through the nozzle at a high velocity.
- Liquid and solid propellants present advantages and disadvantages, making them more or less suitable for different applications.
- Liquid propellants are generally preferred for space missions because they are more efficient, enabling the higher exhaust velocities needed to lift the often heavier payloads on space launch vehicles into orbit. They also allow for greater control over the rocket. For example, the engine can be throttled, stopped, and restarted. Once solid propellant is ignited, it will keep burning until it is used up, meaning there is no way to control the burn after it starts.
- For military applications, solid propellants are advantageous because missiles don’t need to be fueled immediately prior to launch, and are thus always combat ready. Many liquid propellants are difficult to store and transport safely, making it impossible to keep missiles pre-loaded with fuel. Consequently, liquid propellant missiles require a long fueling process before they can be launched.
- Although better suited for different applications, both solid and liquid propellants are employed in space launch vehicles and ballistic missiles in use today. For example, NASA’s Space Shuttle is outfitted with solid-fueled booster rockets on the side of the main fuel tank. In general, solid propellants are more difficult to manufacture, and thus most countries with ballistic missile programs start out developing liquid fuels.
How can a country extend the range of its ballistic missiles?
How far a ballistic missile can travel is essentially determined by the velocity it achieves when it uses up all of its propellant (burnout velocity). Thus if a country can increase the burnout velocity of its missiles, it can hit targets that are farther away. Several factors impact burnout velocity, all of which can be manipulated to extend the range of a missile.
Propellant Mass and Quality
- Similar to increasing the size of a car’s gas tank, a missile that can carry additional propellant can burn longer, reaching higher velocities, and as a result, traveling further.
- Accommodating more fuel can only go so far towards increasing a missile’s range since at some point the overall weight of the missile would become greater than its thrust, preventing liftoff. To increase thrust while adding more fuel, some countries, such as North Korea, resort to clustering multiple rocket engines into a single missile.
- Alternatively, countries can use a more efficient propellant, which enables a missile to travel further without adding more fuel weight. Such propellants, however, are expensive and difficult to develop.
- Some countries have achieved longer ranges by lengthening existing missiles to accommodate more fuel. For example, Iraq extended the range of its Scuds from 300 km to about 600 – 900 km by taking sections from the fuel tanks of different missiles and splicing them together to form larger ones. 
Payload and Structural Weight
- A missile made out of less dense material or carrying a lighter payload will reach higher speeds, and consequently, longer ranges on the same amount of fuel.
- Several countries, including Egypt, Iraq, and North Korea, have reduced payload size to extend the range of their short-range ballistic missiles (SRBMs), but as range increases, lighter payloads yield incrementally smaller dividends. Reducing payload weight, typically by using fewer nuclear or smaller conventional warheads, also reduces the destructive potential of a missile. Thus countries would prefer to increase range through alternative techniques.
- Using lightweight materials for the missile’s airframe and internal components can also lead to weight savings. However, long-range missiles are considerably larger than short-range missiles and move through the atmosphere at higher speeds, subjecting them to greater forces at liftoff and during flight. Thus, long-range missiles require advanced materials that are lightweight but also very strong. Such materials can be challenging to manufacture.
Dead (Non-Fuel) Weight
- An important factor impacting range is a missile’s ability to discard dead weight during flight.
- To achieve ranges beyond 1,500 km, a missile’s propellant is typically compartmentalized into separate segments, a technique known as staging. When the propellant of one section is exhausted, the motor and tanks are ejected, and a second segment takes over. By discarding dead weight, this second segment can reach a higher speed and deliver its payload farther as a result.
- Intermediate- and intercontinental-range ballistic missiles almost always consist of two or more stages. Medium-range missiles typically consist of one or two stages.
- Staging introduces several design challenges, since, for example, a misfiring of the detaching mechanisms can cause the missile to tumble. As a partial alternative to staging, booster rockets can be attached to the external body of the missile and fired during the initial, boost-phase of a missile launch. Once exhausted they detach and fall away, making the missile lighter so it can travel further. To develop ICBMs, staging is still necessary.
How is a ballistic missile different from a peaceful space launch vehicle?
- They are very similar. Although there are some important differences, several technologies are essential to both, enabling countries to advance their missile capabilities under the cover of peaceful programs, and complicating efforts to control the spread of ballistic missiles.
- There are two key differences between missiles and space launch vehicles: accuracy and the use of re-entry vehicles.
- Satellites typically do not require extremely precise orbits, and thus a space launch vehicle does not need an especially accurate guidance system.
- For missiles, even small inaccuracies during the boost phase will accumulate as the payload is pulled back to earth by gravity. Since longer range missiles have longer boost times, they must incorporate more accurate guidance systems, which can be challenging to develop.
- At ranges over 1,000 or 2,000 km, a missile must house its warheads inside a re-entry vehicle (RV) to protect them from the extreme heat generated upon reentry into the atmosphere.
- Early RVs used a blunt shape to reduce velocity and dissipate reentry heat, but this made them inaccurate and more vulnerable to missile defenses. Modern RVs are more streamlined, and coated in exotic materials that burn up and carry away heat in the process.
- Developing RVs that burn evenly and predictably without veering off course is technically challenging, and requires extensive flight testing.
- The type of propellant used can also indicate, although not definitively, whether a rocket is a missile or space launch vehicle. Missiles tend to use solid propellants, since they allow for launch at a moment’s notice, while space launch vehicles, which don’t need to be kept in a launch-ready state, tend to use liquid propellants.
- A space launch vehicle can be modified to act just like a missile or vice versa. The United States and the Soviet Union converted ballistic missiles into space launch vehicles.
How do cruise missiles work?
- Unlike ballistic missiles, cruise missiles do not follow a ballistic trajectory. Rather, much like an airplane, cruise missiles are equipped with a rocket or jet engine, and wings to propel and guide them to a target.
- The hallmark of modern-day cruise missiles is their incredible accuracy and ability to avoid detection by flying at very low altitudes. Such capabilities are made possible by GPS and several other sophisticated guidance systems, which monitor the missile’s speed and direction and compare radar measurements and images of the terrain below to digital maps and photos stored on-board.
- The slower speed, low altitude, and in-flight maneuverability of cruise missiles makes them ideal for chemical and certain biological payloads, since such agents generally need to be dispersed over a wide area to have a significant impact.
- Several countries have also developed nuclear-armed cruise missiles, such the Russian KH-55 (center), the Pakistani Ra’ad and Babur (left), and the now retired U.S. nuclear variant of the Tomahawk Land-Attack Missile (right), the TLAM-N.
How easily can countries develop missiles that can deliver warheads?
- A developing country pursuing an indigenous ballistic missile capability from scratch would encounter substantial difficulties. According to the U.S. Office of Technology Assessment, such a country would require “a total of 300-600 well-coordinated and experienced engineers, technicians, and manufacturing personnel” to design, develop, and produce even short-range, first-generation missiles. 
- Most countries seeking an indigenous capability have not started from scratch. Many developing countries benefited greatly from the widespread sale of short-range Soviet Scud missiles in the 1970s and 1980s.
- Using this baseline technology, several recipients learned to re-produce and modify Scud missiles through a process called reverse-engineering—disassembling foreign-procured missiles, learning how to manufacture their components, and producing new missiles. Countries may then gradually improve upon their existing designs by introducing and testing new features. (To learn more, see Module 4: North Korea’s Scud Story)
- While such methods may work to expand the range of Scud-like missiles from 300 km to about 600-900 km, moving to intermediate- or even intercontinental-ranges introduces a host of new design challenges mentioned throughout this module, including staging as well as manufacturing larger propulsion systems, very strong yet lightweight materials, better propellants, more accurate guidance systems, and re-entry vehicles (RV).
- To overcome these challenges, a country cannot simply rely on document instruction, but must develop or recruit a large cadre of highly skilled and experienced engineers.
- For many years, developing the navigation and propulsion systems needed to produce cruise missiles with sufficient accuracy and range presented major hurdles for countries other than the United States and the Soviet Union. However, the rapid commercialization of key dual-use technologies (such as GPS) has greatly facilitated cruise missile programs in a growing number of countries. 
- Producing small, lightweight, and highly efficient engines remains the chief technological challenge to developing cruise missiles with supersonic speeds and ranges beyond 1,000 km. However, aspiring countries could turn to less efficient engines, which are more widely available and easier to develop indigenously. 
 “The Proliferation of Delivery Systems,” in U.S. Congress, Office of Technology Assessment, Technologies Underlying Weapons of Mass Destruction, OTA-BP-ISC-115 (Washington, DC: U.S. Government Printing Office, December 1993), p. 220 and p. 226.
 “The Proliferation of Delivery Systems,” in U.S. Congress, Office of Technology Assessment, Technologies Underlying Weapons of Mass Destruction, OTA-BP-ISC-115 (Washington, DC: U.S. Government Printing Office, December 1993), p. 223.
 “The Proliferation of Delivery Systems,” in U.S. Congress, Office of Technology Assessment, Technologies Underlying Weapons of Mass Destruction, OTA-BP-ISC-115 (Washington, DC: U.S. Government Printing Office, December 1993), p. 245; Dennis M. Gormley, Missile Contagion: Cruise Missile Proliferation and the Threat to International Security, (Annapolis, MD: Naval Institute Press, 2008), p. 4.
 Dennis M. Gormley, “Missile Contagion,” Survival: Global Politics and Strategy 50, No. 4 (August – September 2008), p. 139.
Note: Header Graphic Credit: WikiMedia Commons, U.S. Navy Naval Air Warfare Center, Weapons Division (NAWCWD)