Solid Fuel vs Liquid Fuel

All rockets operate the same way. Something is ejected from one end to make the rocket go forward in accordance with Newton's third law of motion. There is, of course, more than one way to do this. For several hundred years, rockets burned solid fuel. This created a jet of hot gas that propelled the rocket forward. Solid-fuel rockets have a number of advantages. They are extremely simple, powerful, and easy to manufacture and store. However, they cannot be easily controlled. Once a solid-fuel rocket starts to burn, it is virtually impossible to turn it off. It is also impossible to control the rate of the burning. The rocket cannot be slowed down or sped up.


combustion chamber nozzle outer casing

combustion chamber fuel nozzle nozzle

The invention of the liquid-fuel rocket changed all that. Instead of mixing its oxidizer and fuel into a solid mass, the two remain separate. They are also liquids, which can be stored in tanks. For example, most liquid-fuel rockets use liquid oxygen as their oxidizer. Almost anything that burns, such as kerosene or gasoline, can be used as a fuel.

Powerful pumps force these liquids into a part of the engine called the combust chamber. This is where they mix and are ignited. The speed of the pumps can be controlled or turned off entirely. Therefore, the rocket engine's speed can be controlled and the engine can also be started and stopped at will.

Liquid-fuel rockets are much more complex than solid-fuel rockets (since they require special fuel tanks, pumps, a specific type of motor, and so forth). But their advantages far outweigh their disadvantages, especially since liquid fuels are much more powerful-pound for pound—than solid fuels.

Facing page: The solid-fuel rocket (right) is extremely simple. A cylinder is packed with propellant. When the propellant is burned, the resulting gases shoot from a nozzle at one end of the rocket. Solid-fuel rockets are inexpensive and relatively simple to make, but they have several problems. The first is that solid fuels do not produce as much energy as liquid fuels. A second problem is that once it is started, a solid-fuel rocket cannot be throttled or turned off.

A liquid-fuel rocket (left) solves these problems — though at the expense of being much more complicated and difficult to build. Because the fuel and oxidizer are kept separate and fed into the motor by pumps or a pressurized gas, the liquid-fuel motor can be easily controlled and even turned off and restarted.

Hermann Oberth was born in in Nagyszeben in the Austro-Hungarian Empire—the modern-day small town of Sibi that time, the inhabitants of N many of its neighboring towns c(

1894 in Nagyszeben in the Austro-Hungarian Empire—the modern-day small town of Sibiu, Romania. At that time, the inhabitants of Nagyszeben and many of its neighboring towns considered themselves German. Because of this German heritage, Hermann's father, a physician, decided that his son would attend the great German university at Heidelberg. There he would train to become a doctor too.

Young Hermann, however, had other ideas. He had long been fascinated by the space travel stories of Jules Verne and Kurd Lasswitz, a German science fiction writer. Instead of pursuing his medical courses, Hermann combined his interest in space travel with his interest in mathematics and physics. He started to work on a mathematical theory of space travel.

By 1922, having completely abandoned the study of medicine, Oberth was almost ready to publish his doctoral dissertation. In it, he discussed in more detail than anyone had ever done before the possibilities of rocket flight into space. He proved mathematically that a liquid-fuel, multistage rocket could work. He described xjk

The rocket-powered vehicles (bottom) of Max Valier helped popularize rocketry in the 1920s. Wernher von Braun (top and center) was the genius behind the German rocket program of the late 1930s that resulted in the V-2.

Inspired by reading about Jules Verne's space adventures, Hermann Oberth (left) devoted his career to proving mathematically that space travel was a real possibility. His books were important influences in the early development of liquid-fuel rockets and multistage rockets. They also inspired many of the engineers and scientists who eventually made spaceflight a reality.

how such a rocket could be built, steered, and navigated through space. He explained how it could reenter Earth's atmosphere and be recovered. Oberth also discussed the hazards spaceflight held for humans and how people could survive in space. He even described space stations and space telescopes.

That spring, however, Oberth was shocked to read a newspaper article about Robert Goddard's booklet. He had no idea that anyone else in the world had been thinking about the same subject. He couldn't find the booklet in Heidelberg and finally wrote to Goddard, who promptly sent him a copy.

When the booklet arrived, Oberth read it eagerly. He was amazed by Goddard's ideas and calculations. But he also realized that Goddard had not gone nearly as far in his thinking as he had. Goddard had only suggested that an unmanned, solid-fuel rocket might carry an explosive charge to the Moon large enough that its flash would be visible from Earth. Oberth, on the other hand, had speculated beyond this.

Oberth realized that liquid fuels—such as gasoline and liquid oxygen—could provide twice the exhaust velocity of any powder. It

was only by means of liquid-fuel rockets that the conquest of space could be accomplished. By the time Oberth read Goddard's 1919 booklet, Goddard had not only come to the same conclusion but had begun work on constructing a liquid-fuel rocket. Of course, Oberth didn't know this, especially since Goddard kept his work secret.

In 1923 Oberth published his own dissertation (which had been rejected by his college as "utopian," or impossibly idealistic). It was a ninety-two-page book called Die Rakete zu den Planetenraumen (The Rocket into Planetary Space). It did not sell well, mostly because Oberth's mathematics were very complex and his writing style quite technical. In fact, the publisher was so reluctant to publish the book that Oberth had to pay most of the printing costs.

A writer of popular science articles named Max Valier, however, recognized the importance of Oberth's claims. Valier published a book explaining the ideas in simpler terms. The twenty-nine-year-old Oberth was amazed to discover that he had become an overnight sensation. His ideas quickly spread throughout Germany and, finally, the rest of the world. They proved that a liquid-fuel rocket was possible and that it was the key to successful spaceflight. The publisher of the original book was astonished when he had to reprint the book in an expanded edition.

Max Valier was one of the most energetic popularizers of rocketry and space travel in the years preceding World War II (1939-1945). Valier was one of the first martyrs to the dream of spaceflight. He died in 1930 when a liquid-fuel rocket motor he was working on exploded.

But when Oberth was first writing, no one, as far as he knew, had ever built a liquid-fuel rocket. The only rockets familiar to anyone were ordinary fireworks rockets. Even the few rockets still used by the military were not much different. But a big difference existed between those rockets and the kind of rockets that Oberth proposed. For one thing, a skyrocket never moves very quickly. Almost immediately after its supply of fuel runs out, the rocket will fall back to the ground.

A liquid-fuel rocket, on the other hand, would be capable of achieving a tremendous speed. So much so that when its fuel runs out, it would keep going. It could go perhaps many times farther than it had traveled while powered by the fuel. Historian Willy Ley described the effect: When a liquid-fuel rocket is powered, say, for the first mile of its flight, it is as though it were a projectile in a gun 1 mile (1.6 km) long. The bullet from a gun will keep going for a long distance after it leaves the muzzle of the gun. In the same way, the rocket will also keep going for a long distance after its fuel is exhausted.

Oberth knew that this meant rockets were capable of reaching much greater distances than anyone had previously suspected. But Oberth went even further than proving the mathematical possibility of the liquid-fuel rocket. In the second part of his 1923 book, he outlined the characteristics of just what a high-altitude, instrument-carrying rocket would be like. In the third part, he described a manned spaceship in such detail that his readers thought they could go out and build one just by following his directions.

The Rocket Society

On June 5, 1927, a dozen German rocket enthusiasts—led by Max Valier—met in the backroom of a small restaurant in Breslau in what would become Poland. All were fascinated by the possibility of using rockets to explore space. The club they founded that day was called the Verein für Raumshiffarht (VfR)—Society for Space Travel. It soon attracted some very high-profile members, although many of them joined by mail and never attended meetings.

Hermann Oberth became one of the first members. Dr. Walter Hohmann, who had written one of the first books about space travel after Oberth's, also joined. Willy Ley, who had recently written a popular book about space travel for the general public, was one of the group's founders and leaders.

Some other early members included the Russian professor Nikolai Rynin. Rynin had just published the first volume of a nine-volume history of rocketry and ideas about spaceflight. Robert Esnault-Pelterie, a French airplane manufacturer who had been giving lectures about spaceflight, also joined. Another prominent member was Guido von Pirquet, an Austrian scientist who had earlier founded the Austrian Society for Rocket Technology. Among these prestigious men were many lesser-known ones, including, by 1930, an enthusiastic teenager named Wernher von Braun.

The fledgling society immediately ran into a stumbling block. When the members tried to register the society with the authorities, they were told that they could not.

The German word raumshiffarht,

which means "spaceship travel,"

Wernher von Braun had an overwhelming interest in rockets and could not be found in any diction-space travel. ary. Therefore, it did not officially

exist. They would have to change the name of their society if they wanted to register it legally. But Johannes Winkler (another founder of the VfR) argued the name couldn't be changed since it described the society exactly. Finally, it was agreed that if a definition of raumshiffarht were added as a footnote to the permit, the society could be registered.

German rocket experimenters gather for a meeting in 1930 (Werner von Braun stands on the far right).

German rocket experimenters gather for a meeting in 1930 (Werner von Braun stands on the far right).

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