There are two basic kinds of propellants for missiles and rockets: liquid and solid propellants. They each have their advantages and disadvantages. Liquid propellants require complicated piping and pumping equipment to feed their engines. They can provide greater propulsive thrust and throttle their power, but take time to build up this thrust when first ignited. Solids do not require complicated engines or plumbing, but rely on sophisticated chemistry and strong casings to withstand the intense pressures that they generate. They can fire much faster, and accelerate more quickly at liftoff, but cannot be throttled in flight.

The very first rockets, built by the Chinese at some unknown period in the first millennium, used solid propellants, a variant of black powder used in early guns. The pace of technological progress in solid propellants—and explosives in general—was extremely slow. In fact, the rockets that Francis Scott Key observed during the War of 1812, called Congreve rockets after their English inventor William Congreve, and which are mentioned in the U.S. national anthem, were virtually identical to those fired by the Chinese eight centuries before. It was not until the middle of the 20th century that solid propellants for rockets made a sudden and dramatic leap forward.

Early propellants, despite their sparks and bluster, were relatively weak. Rockets were primarily sloppy bombardment weapons, far less effective than a cannonball. Their poor performance prompted rocketry pioneers like Robert Goddard to develop liquid propellants, starting first with gasoline, then moving to kerosene and alcohol. Solid propellant rockets still had utility, but most rocket engineers during the 1930s and 1940s focused upon liquid fuels and oxidizers. Solids were used for many military applications, such as short-range rockets, but they were not used for any long-range applications, and certainly not for spaceflight due to their comparative lack of power.

Despite this, solid propellants were extremely attractive for military missile use primarily because they were storable. The first liquid-fueled ballistic missiles like the German V-2, the American Atlas, Thor and Jupiter, and the Russian R-7, all had to be filled with fuel and oxidizer before they could be fired, a process that could take many hours and could be extremely hazardous. Liquid fuels also required complex and expensive ground handling equipment, meaning that any missile base would be expensive and “soft”—i.e., easily damaged in an attack. Solid fuels were a possible solution.

During the 1940s, researchers at the Jet Propulsion Laboratory in Pasadena, California, began working on “castable” solid propellants, which got their name from the fact that they could be cast into molds. John Parsons developed asphalt as a fuel and binder (the substance that holds all the chemicals together) together with potassium perchlorate as an oxidizer. By the 1950s, synthetic polymers replaced the asphalt. But a major improvement came when the rocket designers and chemists added aluminum powder to the mix, which increased the performance of the propellant substantially. In addition to the propellant chemistry, another major development has been lightweight, very strong metal and composite (including fiberglass) material casings to withstand the intense pressures of the burning propellant.

During the 1950s, the U.S. Air Force and the U.S. Navy cooperated with each other to a surprising degree to develop these more powerful solid propellants. The Navy wanted solid propellant missiles for use aboard its submarines, where sloshing liquid fuels were a major safety risk. U.S. Air Force leaders wanted them for a mobile intercontinental ballistic missile (ICBM) that would be Shuttled around the countryside on railroad cars.

The first successful solid propellant ballistic missile was the U.S. Navy's Polaris A1 submarine-launched missile, which became operational in 1960. It had a range of 1,200 nautical miles (3,704 kilometers) and carried a nuclear warhead. It could actually be fired from underwater, which decreased the vulnerability of the submarine to attack. The Polaris A1 was soon followed by upgraded versions with increased range and was eventually replaced in the 1970s by the Poseidon missile, itself replaced by the Trident, which is still in use today in an upgraded form.

The first Air Force solid propellant ballistic missile was the Minuteman, so named because it could be ready to fire on a minute's notice and did not need to be fueled before launch. Plans for launching the Minuteman from railroad cars were canceled in the early 1960s and the missiles were instead based in underground holes called silos. Because these missiles did not have to be pumped full of fuel and oxidizer before flight, very little equipment was required to support them, and they could be encased in very thick concrete to protect them from the force of a nearby nuclear blast. The Minuteman silos were subsequently much cheaper to build than those required for the Atlas ICBM that Minuteman replaced. Later versions of the Minuteman still serve today, along with an even larger American ICBM called the Peacekeeper.

Solid rockets were also adapted for space launch missions. The first American solid propellant rocket was the Scout, which launched relatively small payloads into orbit. Today, the air-launched Pegasus and the ground-launched Taurus also use solid propellants, and Italy is developing its own solid fuel rocket. Generally solid fueled rockets are used for smaller satellites.

Solid rockets also became a way of boosting another rocket's performance. The Air Force added small solid boosters to its Thor rocket to increase its lifting capacity. NASA soon adopted this for the Delta rocket, a variant of the Thor. A major development in solid propellant rocketry came in the early 1960s, when United Technology Corporation (UTC) developed the segmented booster (aerospace companies Aerojet and Thiokol also developed segmented boosters). The motor was built in segments that could be transported separately to the launch site and then assembled. UTC built segmented Solid Rocket Motors (SRMs) with a 156-inch diameter for the Titan III space launch vehicle. Later, Morton Thiokol built even larger segmented boosters for the Space Shuttle (where they are called Solid Rocket Boosters, or SRBs). However, the segments could allow hot gasses to escape from the propellant casing, which is what happened in 1986 when the Space Shuttle Challenger exploded. The National Aeronautics and Space Administration (NASA) and the Air Force explored even larger SRBs during the 1960s, but their size would have made them difficult to transport and handle, and they were abandoned.

Although the Soviet Union had gained an important early advantage over the United States in the development of globe-spanning ICBMs, it quickly lost its advantage. The liquid-fueled R-7 that stunned the world by launching Sputnik in October 1957 and later launched Yuri Gagarin into orbit proved to be an atrocious weapon. Only a handful of them ever became operational as ICBMs. Soviet rocket designers produced better liquid propellants than their early ones that could be stored inside the missile for long periods of time, but they lagged far behind the United States in solid propellant technology.

The Soviet Union followed the American development of solids by about a decade, fielding their first practical solid ICBMs, known as the RT-2, in December 1971. The Soviets were slow to eliminate liquid-propellant ICBMs from their arsenal, probably because of the unreliability and poor performance of their solid propellants, and did not deploy large numbers of solid-propellant ICBMs until the 1980s. Amazingly, they even continued using liquids on their submarines well into the 1980s, despite the fact that these were dangerous and caused several fatal submarine sinkings. The Soviet Union also never adopted solid rocket boosters for their space rockets like the Americans, preferring liquid boosters instead.

Other countries have also adopted solid propellants for their missile and space programs. France developed the SSBS (Sol-Sol-Ballistique-Stratégique) medium-range ballistic missile using solid propellants. Later the European company Arianespace adopted solid boosters for the Ariane 3 and 4 space rockets and developed large SRBs for the Ariane 5 heavy lift rocket. China continued to use liquid-fueled ICBMs into the 21st century, but by the late 1990s had started tests of a new solid-fueled ICBM. Large solid propellant rockets remain a very difficult technology, which explains why countries developing ballistic missile and space capabilities still use liquid fuels. India, Iraq, Iran and North Korea all use liquid-fueled designs for their budding missile and rocket programs.

-Dwayne Day

Sources and further reading:

Baker, David. The Rocket. London: New Cavendish Books, 1978.

Emme, Eugene M., ed. The History of Rocket Technology. Detroit, Mich.: Wayne State University Press, 1964.

Heppenheimer, T.A. Countdown. New York: John Wiley & Sons, 1997.

Hunley, J.D. “The Evolution of Large Solid Propellant Rocketry in the United States” Quest, Spring 1998, 22-38.

_________. “The History of Solid-Propellant Rocketry: What We Do and Do Not Know.” American Institute of Aeronautics and Astronautics, at http://www.dfrc.nasa.gov/DTRS/1999/PDF/H-2330.pdf

____________. “Minuteman and the Development of Solid-Rocket Launch Technology,” in Jenkins, Dennis. The Space Shuttle. The History of the National Space Transportation System, the First 100 Missions. North Branch, Minn.: Specialty Press, 2001

____________ and Launius, Roger, eds. The Development of Launch Vehicles in the United States. Lexington: University Press of Kentucky, 2002.

Von Braun, Wernher, and Ordway, Frederick. History of Rocketry and Space Travel. Third Revised Edition. New York: Crowell, 1975.

Zaloga, Steven. The Kremlin's Nuclear Sword. Washington, D.C.: Smithsonian Institution Press, 2002.