Science: Fuels for Space

What fuels do the Russians burn to make their Sputniks fly so fast? Wild rumors last week gave them credit for wonder-working superfuels. Not necessarily. Conventional rocket fuels such as kerosene and liquid oxygen, if skillfully used, could do the job. But superfuels are coming along—in both the U.S. and Russia.

Most people think of fuel as something that gives off heat when the carbon in it combines with oxygen from the air. Chemists have wider horizons. A fuel means any combination of substances that reacts chemically with a release of energy. The ingredient that “burns” may be a metal or a compound containing a metal. The “oxydizer” may be oxygen-rich, or it may have no oxygen at all. The test is the yield of propulsive energy, which scientists measure as “specific impulse.”

When chemists dream their fanciest dreams, they imagine powering a rocket with liquid hydrogen and liquid ozone (03). This pair is tops for energy. Its reaction has a specific impulse of 373. The specific impulse of the traditional kerosene-oxygen combination is only 249.

30 Seconds, $5,000. Liquid hydrogen is bulky, expensive and extremely hard to handle. Ozone is expensive, poisonous and explosive. Another dream oxidant, liquid fluorine, is about as bad. The National Advisory Committee for Aeronautics has been working on liquid fluorine as an oxidant at a cost as high as $5,000 for a 30-second test of a smallish rocket, but no one thinks that fluorine will come into wide use soon.

One step down from the dream fuels are the boron-containing fuels that have already grown big enough to stir up flurries on Wall Street (see BUSINESS). Boron itself gives much energy, and some of its compounds hold a lot of high-energy hydrogen in easy-to-handle form. Modern boron fuels are stable, reliable and have high (classified) specific impulses. One of them is now being manufactured in considerable quantity by Olin Mathieson Chemical Corp. at Niagara Falls. Gallery Chemical Co., near Pittsburgh, is making its HiCal, a boron-carbon-hydrogen combination.

High energy is not the only requirement of a fuel and its oxidant. Also desirable are cheapness and availability. High density is important for keeping down the rocket’s size and frontal area. Ease of ignition is almost a must. The best combinations are “hypergolic,” igniting spontaneously as soon as mixed. Bad qualities to be avoided are toxicity, corrosiveness, heat instability (exploding when hot) and a tendency to evaporate like liquid oxygen. No fuel is perfect. Beryllium compounds might be good if beryllium were not so scarce and so poisonous.

Solids to the Fore. To be used in liquid-fueled rockets, both fuel and oxydizer must be liquid and thin enough to be pumped rapidly. This rules out promising materials, e.g., boron itself and many of its compounds, that are not liquid at ordinary temperatures. One way around this difficulty is to grind them finely and mix them with a liquid carrier to form a paintlike slurry. The most radical way is to burn them as solids with a solid oxydizer. Through this technique, a long list of new high-energy materials can be used.

Most big rockets of both fact and fiction have used liquid fuel, following the fashion set by the German V2. They are full of delicate pumps and valves that often malfunction. Recently scientific thinking has shifted toward solid fuels. They are essentially mixtures of solid combustible materials with other solids, e.g., potassium perchlorate, that contain large amounts of oxygen. Most of the early ones were brittle; they often cracked-when they started to burn. The flame penetrated the cracks, ignited new fuel and sometimes increased the rate of burning so much that the rocket exploded. Another trouble was that the solid fuel, packed into a cylinder, burned from the rear end forward. The walls of the cylinder were exposed to the flame. They got so hot and lost so much of their strength that they had to be made heavy, cutting the rocket’s range.

2,000,000 Lbs. of Thrust. Solid fuels did better when Caltech’s Jet Propulsion Laboratory made them elastic by mixing them with a rubbery compound produced by Thiokol Chemical Corp. This cut the cracking and explosions. Then Caltech’s men packed the fuel into the rocket in such a way that a carefully shaped hole was left running up its center. The fuel burned from the center outward, and the unburned fuel insulated the metal wall of the rocket from the heat of combustion. The metal kept its high cold strength and so could be made thinner and lighter, improving the rocket’s efficiency.

Solid-fuel rockets are blissfully simple, being essentially cylinders filled (except for the central hole) with rubbery fuel. There are no moving parts to go wrong. A trouble spot was the nozzle through which the hot gases escape, but its bad habit of burning out has apparently been licked. Thiokol claims that its rockets are 97.5% reliable, firing perfectly almost every time. Its technical director, Harold W. Ritchie promises that solid-fuel rockets with 2,000,000 Ibs. of thrust can be built. He also says, mysteriously, that important new ingredients can be added to increase the punch of the rubbery fuel.