What Powers Missiles? Liquid Fuel, Solid Fuel, and How They Work

News coverage of the war with Iran has highlighted the fact that its large missile arsenal includes both solid-fueled and liquid-fueled missiles. These two forms of propulsion have been part of modern missile technology almost since the field emerged about a century ago. This offers a good opportunity to examine how they work, what sets them apart, the strengths and weaknesses of each propulsion method, and what other uses they have.

A Matter of Pressure

Unlike shells, bombs, or bullets, missiles and rockets carry their own fuel and continue generating thrust after launch. All missiles currently in operational use rely on chemical propulsion: they burn fuel and move forward by expelling gases produced by combustion. Rocket flight driven by the forceful emission of gases is an application of Isaac Newton’s third law of motion, which states that for every action there is an equal and opposite reaction. In the past, some believed that rocket propulsion could not work in space, reasoning that the gases would need to push against something, such as air, and that in the vacuum of space there would be nothing to push against and nothing to push back. But this view is mistaken. It is the very expulsion of the gases that pushes the rocket in the opposite direction, even in a vacuum. In 1969, while Apollo 11 was on its way to the first crewed landing on the Moon, The New York Times published an apology to the American rocket pioneer Robert Goddard, after mocking in a 1920 article his claim that rockets could operate in space. “The Times regrets the error,” the newspaper wrote a quarter-century after Goddard’s death.

The chemical reaction that causes these gases to be released is, of course, the burning of fuel. Combustion is a chemical process in which a fuel reacts with an oxidizer. The process releases energy in the form of light and heat, and its products are emitted as gases. For example, the combustion reaction of propane gas—one of the gases commonly used for cooking—is:

C3H8 + 5O2 → 3CO2 + 4H2O

That is, each propane molecule reacts with five oxygen molecules to produce three molecules of carbon dioxide and four molecules of water. Because the reaction also generates heat, the water is emitted as vapor—that is, in the gaseous state—and the high temperature increases the gas pressure, thereby increasing the thrust produced by the engine.

Rocket developers today prefer fuels that produce an especially large volume of gas. One of the main components in such fuels is hydrazine, N2H4, or its derivatives. Breaking the bond between the nitrogen atoms releases a great deal of energy, as it does in explosives, and oxidizing hydrazine produces a substantial amount of nitrogen gas (N2) and water vapor, providing strong thrust.

Combustion powers most types of engines. In an internal combustion engine, such as a car engine, gas pressure drives pistons, and a crankshaft converts that motion into the rotation of wheels or propellers. But whereas in internal combustion engines – and even in jet engines – the oxygen needed for combustion comes from the outside air, in rocket propulsion it is carried inside the missile as part of the propellant, sometimes as pure oxygen and sometimes in oxygen-containing compounds. Rocket engines also have very few moving parts. In a liquid-fueled missile, most of the motion comes from the pumps that feed the fuel and oxidizer into the combustion chamber. The exhaust gases are expelled through a relatively narrow tube called a nozzle, which increases the speed of the gas flow in accordance with Bernoulli’s principle.

To compare the performance of rocket engines and different fuels, engineers use a measure called specific impulse, which reflects the change in momentum per unit mass – in other words, the velocity the engine can impart to the missile. It is measured in units of velocity, though it is sometimes normalized using Earth’s gravitational acceleration, so that specific impulse is expressed in units of time alone.

Liquid Fuel

Missiles powered by liquid fuel usually have two propellant tanks—one for the fuel itself and one for the oxygen or other oxidizing agent, such as hydrogen peroxide (H2O2). Pumps feed the fuel and oxidizer into the combustion chamber, and an ignition system provides the spark needed to ignite it. There are also hypergolic fuels, which ignite spontaneously on contact with the oxidizer and therefore do not require a separate ignition system.

Today, a wide range of fuels is used for rocket propulsion. Hydrogen (H2) is an excellent fuel, providing high specific impulse because of its low mass, and its oxidation produces only water vapor. The SLS rocket, which is intended to launch American astronauts to the Moon as part of the  Artemis program, is powered by liquid hydrogen and liquid oxygen. Condensing a gas into liquid form makes it possible to store more fuel in a tank, but it also requires cooling to extremely low temperatures and lengthy fueling procedures—conditions that are not well suited, for example, to military missiles. In addition, hydrogen is difficult to handle because of its tiny molecules, and even NASA has struggled with hydrogen leaks.

Other liquid fuels include methane (CH4); kerosene, a mixture of carbon chains usually 12 to 15 atoms long that is also used as jet fuel; hydrazine and its derivatives, such as unsymmetrical dimethylhydrazine (UDMH), with the formula H2NN(CH3)2; nitric acid (HNO3); dinitrogen tetroxide (N2O4), which can also serve as an oxidizer; and various mixtures of these and other substances in different ratios, depending on the needs and intended purpose of a particular launch system.

Most liquid fuels cannot be stored inside a missile for long periods. Some are corrosive and can damage the missile’s metal casing. Others require cooling, and if they are stored under conditions that allow them to warm up and expand, they may deform the missile’s structure. Fuels in the hydrazine family are better suited for long-term storage, but they are toxic and carcinogenic, which poses serious safety concerns for the personnel who maintain such missiles.

For military use, this means that liquid-fueled missiles have to be fueled shortly before launch. This is a time-consuming operation carried out while the missile remains vulnerable on the ground, and it may also create an additional intelligence signature—for example, the movement of fuel tankers to the launch site.

Designing liquid-fueled missiles also involves additional challenges, including the movement of the fuel itself inside the tank, especially when it is not full. This can shift the missile’s mass distribution and potentially reduce its performance. To address this, tanks are often fitted with rings or other internal protrusions designed to limit the movement of liquid inside them. In addition, because most of a missile’s mass at launch is fuel, its center of mass changes as the fuel burns. Advanced propulsion systems, however, take these changes into account and compensate for them.

Solid Fuel

The use of liquid fuel began about a century ago, in 1926, with the pioneering work of Robert Goddard. All rockets launched before that—since their invention, probably in ancient China in the 12th or 13th century—used solid fuel.  Essentially, they were tubes packed with gunpowder, with an opening at one end that allowed the gases to escape. Gunpowder contains both the fuel and the oxygen source in a single solid mixture, much like the material in the heads of matches. Indeed, wrapping several matches in a solid cylindrical casing produces a miniature demonstration of rocket propulsion. Even small household fireworks, such as those used on birthday cakes, contain both solid fuel and an oxidizer—and can burn even underwater.

Modern solid-fuel missiles are not fundamentally different from those ancient rockets. Gunpowder, which produces low specific impulse and poses safety risks, has been replaced by materials that generate a larger volume of gas, but they are based on the same principle and contain both a fuel and an oxidizer.

Solid fuels can include compounds of zinc and sulfur, as well as materials such as nitrocellulose. Today, however, most solid-fuel missiles use metal-based fuels such as magnesium or aluminum, together with substances that serve as oxidizers and, in some cases, as fuels themselves, such as ammonium nitrate (NH4NO3) or ammonium perchlorate (NH4ClO4). These materials are also widely used for other purposes, including agricultural fertilizers, which makes their use difficult to monitor. In some cases, the mixture is supplemented with small crystals of advanced explosive materials such as RDX or HMX, which increase the specific impulse but also raise the risk of accidents or missile failure.

To function efficiently, the fuel and oxidizer must be cast inside the missile as a single mixed mass, so that the combustion reaction can proceed properly. The mixture has to be packed to exactly the right density. If its texture is too powdery, meaning it has too much surface area, it may burn too quickly, generating excessive gas pressure and causing the missile to explode. Air bubbles trapped inside the fuel can create a similar problem. On the other hand, if the material is packed too densely, combustion will proceed too slowly, and the gas pressure needed for efficient propulsion will not be produced.

In solid-fuel missiles, the fuel-oxidizer mixture is cast in a cylindrical layer around the inside of the missile, leaving a hollow core at the center. This cavity serves as the combustion chamber, and the gas pressure generated inside it is released through the engine nozzle, producing the thrust that propels the missile.

The use of solid fuel for ballistic missiles received a major boost during the Cold War between the United States and the Soviet Union. Each superpower had to be prepared for a nuclear strike by the other and needed to be able to respond immediately with a strike of its own. When missiles have to be launched within minutes, there is no time for fueling, so both sides developed missiles that could remain on the launch pad for long periods while still being ready for ignition and launch at very short notice.

Which Fuel Is Better?

The fact that both forms of propulsion are still in use suggests that each has its own advantages and disadvantages. The main advantage of liquid fuel is its higher specific impulse. This means that a warhead of a given mass can be launched by a smaller missile, which is easier to transport and less exposed to the enemy. The larger and longer-range the missile, the more the advantage tends to shift toward liquid fuel, because a solid-fuel missile with comparable performance would have to be enormous. That means it must either be stored in a fixed location – which may be known to the enemy – or transported in a highly complex way, undermining its main advantage: rapid launch.

On the other hand, as noted, liquid-fueled missiles must be fueled before launch, a process that takes valuable time and exposes the launchers. Such missiles also require more maintenance, and some rely on toxic fuels, which makes them more difficult to handle safely. As a result, most smaller, shorter-range missiles are powered by solid fuel, whereas longer-range missiles are more often powered by liquid fuel.

Another drawback of solid fuel is that once it is ignited, control is very limited: the rate of combustion cannot easily be regulated, and the engine cannot simply be shut down. With liquid fuel, by contrast, the pumps can be controlled remotely, the fuel flow can be increased or reduced, and the missile’s speed and flight profile can be adjusted accordingly. These capabilities are less important for military missiles, however, and far more relevant for rockets that carry payloads or people into space. In recent years, moreover, solid fuels have been developed that allow some degree of control over combustion, and even shutdown and restart, by dividing the fuel into small segments and controlling their ignition electrically. There are also hybrid rockets, which combine solid fuel with liquid oxygen, but they are rarely used in military applications.

Civilian space launch vehicles also use both kinds of fuels and propulsion systems. Here, too, large rockets are generally powered by liquid fuel, usually cryogenic propellants—that is, fuels condensed into liquid form at very low temperatures. The SLS rocket is powered by liquid hydrogen and liquid oxygen. SpaceX’s giant Starship uses liquid methane (CH4) and liquid oxygen, in part because these substances could be produced on Mars and used there to refuel the spacecraft. Falcon 9, SpaceX’s main workhorse, is powered by liquid kerosene and liquid oxygen. The first stage of the giant Saturn V rocket, which launched the Apollo astronauts to the Moon, also used liquid kerosene and liquid oxygen, while its second and third stages, designed to operate mainly in space, used liquid hydrogen and liquid oxygen.

In space itself, there are propulsion systems that are not practical on Earth, such as ion propulsion, which accelerates electrically charged atoms to high speed and then ejects them into space. Such engines produce very little thrust, making them unsuitable for atmospheric flight, but they can operate for very long periods and have very high specific impulse, because the particles are expelled at high speed over extended periods. Over time, in the vacuum of space, they can gradually accelerate a spacecraft to impressive velocities, and are therefore used mainly today for orbital corrections of satellites and spacecraft, which can be carried out gradually over time.

Translated with the assistance of ChatGPT. Edited, revised, and reviewed by the editorial staff of the Davidson Institute of Science Education.