Rocket And Missile System ~ WORLD BLOG LOG

From liquid to solid fuel

This first generation of missiles was typified by its liquid fuel, which required both a propellant and an oxidizer for ignition as well as a complex (and heavy) system of pumps. The early liquid fuels were quite dangerous, difficult to store, and time-consuming to load. For example, Atlas and Titan used so-called cryogenic (Hypercold) fuels that had to be stored and handled at very low temperatures (−422° F [−252° C] for liquid hydrogen). These propellants had to be stored outside the rocket and pumped aboard just before launch, consuming more than an hour.

As each superpower produced, or was thought to produce, more ICBMs, military commanders became concerned about the relatively slow reaction times of their own ICBMs. The first step toward “rapid reaction” was the rapid loading of liquid fuels. Using improved pumps, the reaction time of the Titan I was reduced from over one hour to less than 20 minutes. Then, with a second generation of storable liquids that could be kept loaded in the missile, reaction time was reduced to approximately one minute. Examples of second-generation storable-liquid missiles were the Soviet SS-7 Saddler and SS-8 Sasin (the latter deployed in 1963) and the U.S. Titan II. The Titan II was the largest ballistic missile ever developed by the United States. This two-stage ICBM was more than 100 feet long and 10 feet in diameter. Weighing more than 325,000 pounds at launch, it delivered its single warhead (with a throw weight of about 8,000 pounds) to a range of 9,000 miles and with a CEP of about one mile.

In about 1964 China began developing a series of liquid-fueled IRBMs given the NATO designation CSS, for Chinese surface-to-surface missile. (The Chinese named the series Dong Feng, meaning “East Wind.”) The CSS-1 carried a 20-kiloton warhead to a range of 600 miles. The CSS-2, entering service in 1970, was fueled by storable liquids; it had a range of 1,500 miles and carried a one- to two-megaton warhead. With the two-stage CSS-3 (active from 1978) and the CSS-4 (active from 1980), the Chinese reached ICBM ranges of over 4,000 and 7,000 miles, respectively. The CSS-4 carried a warhead of four to five megatons.

Because storable liquids did not alleviate the dangers inherent in liquid fuels, and because the flight times of missiles flying between the United States and the Soviet Union shrank to less than 35 minutes from launch to impact, still faster reactions were sought with even safer fuels. This led to a third generation of missiles, powered by solid propellants. Solid propellants were, eventually, easier to make, safer to store, lighter in weight (because they did not require on-board pumps), and more reliable than their liquid predecessors. Here the oxidizer and propellant were mixed into a canister and kept loaded aboard the missile, so that reaction times were reduced to seconds. However, solid fuels were not without their complications. First, while it was possible with liquid fuels to adjust in flight the amount of thrust provided by the engine, rocket engines using solid fuel could not be throttled. Also, some early solid fuels had uneven ignition, producing surges or abrupt velocity changes that could disrupt or severely confound guidance systems.

The first solid-fueled U.S. system was the Minuteman I. This ICBM, conceived originally as a rail-mobile system, was deployed in silos in 1962, became operational the following year, and was phased out by 1973. The first Soviet solid-fueled ICBM was the SS-13 Savage, which became operational in 1969. This missile could carry a 750-kiloton warhead more than 5,000 miles. Because the Soviet Union deployed several other liquid-fueled ICBMs between 1962 and 1969, Western specialists speculated that the Soviets experienced engineering difficulties in producing solid propellants.

The French deployed the first of their solid-fueled S-2 missiles in 1971. These two-stage IRBMs carried a 150-kiloton warhead and had a range of 1,800 miles. The S-3, deployed in 1980, could carry a one-megaton warhead to a range of 2,100 miles.
The first SLBMs

Simultaneous with the early Soviet and U.S. efforts to produce land-based ICBMs, both countries were developing SLBMs. In 1955 the Soviets launched the first SLBM, the one- to two-megaton SS-N-4 Sark. This missile, deployed in 1958 aboard diesel-electric submarines and later aboard nuclear-powered vessels, had to be launched from the surface and had a range of only 350 miles. Partly in response to this deployment, the United States gave priority to its Polaris program, which became operational in 1960. Each Polaris A-1 carried a warhead of one megaton and had a range of 1,400 miles. The Polaris A-2, deployed in 1962, had a range of 1,700 miles and also carried a one-megaton warhead. The U.S. systems were solid-fueled, whereas the Soviets initially used storable liquids. The first Soviet solid-fueled SLBM was the SS-N-17 Snipe, deployed in 1978 with a range of 2,400 miles and a 500-kiloton warhead.

Beginning in 1971, France deployed a series of solid-fueled SLBMs comprising the M-1, M-2 (1974), and M-20 (1977). The M-20, with a range of 1,800 miles, carried a one-megaton warhead. In the 1980s the Chinese fielded the two-stage, solid-fueled CSS-N-3 SLBM, which had a range of 1,700 miles and carried a two-megaton warhead.