Propulsion Systems Help

By — McGraw-Hill Professional
Updated on Sep 18, 2011

Propulsion Systems—The Chemical Rocket

All space vehicles require some means of attaining high speed. The most obvious schemes work according to the action-reaction principle. When matter is ejected from the rear of a space ship, the ship moves forward. The spacecraft can be rotated 180 degrees, and the matter thereby ejected from the front when it is necessary to decelerate.

The Chemical Rocket

During World War II, the Germans demonstrated that rockets could send small payloads rapidly from one point to another on Earth’s surface. This technology led to larger and larger rockets, culminating in the American Saturn booster and the Space Shuttle and in the Russian Soyuz booster. These rockets can put people and satellites into Earth orbit and can even hurl them beyond the influence of the Earth’s gravitation.

Chemical rockets are still used by all spacecraft today. They literally burn materials to produce thrust. If you’ve had any experience with model rockets, you know the basics of how a rocket works. Some rockets use solid fuel (model rockets, for example), whereas others employ liquid fuel. The larger the rocket, and the more fuel it carries, the farther it can send a payload.

It is tempting to think that in order to freely roam among the stars, we need only build a huge rocket. Unfortunately, things aren’t this simple. The rocket necessary to send a manned mission to one of the Sun’s nearby neighbor stars and back within the span of a human lifetime would require many times more fuel than a practical rocket could ever carry. If we are to become an interstellar species, we’ll have to come up with something a lot more efficient than the chemical rocket engine.

Put your mind in “futurist mode.” Think about chemical rockets for awhile. Soon you’ll begin to see that chemical combustion is a ridiculous means by which to attempt to explore the vast tracts of outer space. It’s even sillier than burning the remains of eons-old plants and animals to propel vehicles over the Earth’s surface and through the lower atmosphere. Someday people will look back on these technologies and wonder why humanity stuck with them for so long.

The Ion Engine

Hot gases produced by the combustion of flammable fuels are not the only way to produce thrust. Another method that has been suggested as a means to get spacecraft starbound makes use of powerful linear particle accelerators. Instead of using the high-speed subatomic particles to smash atoms, however, the particles are ejected out the rear of the device, resulting in a forward impulse.

Figure 19-2 is a simplified functional diagram showing how an ion engine can work. The source ejects large quantities of ionized gas. Hydrogen is a logical choice; it is plentiful in the Universe. It could be “scooped up” from interstellar space to supply the fuel for the engine. The positive ions of hydrogen are protons. They can be accelerated by negatively charged anodes , through which the particles pass. As the protons go through each succeeding anode, they gain more speed until finally, when they pass through the last anode, they are moving so fast that the reaction force pushes the spacecraft forward.

Traveling and Living in Space Why Venture
Into Space? The Ion Engine

Figure 19-2. Conceptual diagram of an engine that produces thrust by accelerating atomic nuclei (ions) to high speed.

Ion engines do not produce much thrust, but they are efficient. They can keep operating for a long time. Once a spacecraft has gone beyond the gravitational influence of the Earth, an ion engine eventually could bring the ship up to enough speed to make an interstellar journey within a human lifespan.

Unfortunately, linear particle accelerators require enormous amounts of power. The only known system that can provide enough power is a nuclear reactor. If a hydrogen fusion reactor is ever developed, it could be used to power the ion engine. But then the fusion reaction itself would be a better source of thrust than accelerated ions.

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