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Name: Alex
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Question:
Why do space ships have rockets on them and not electric boosters?



Replies:
Hi, Alex.

I am not sure what you mean by 'electric boosters'. Spaceship propulsion operates on Newton's Third Law - for every action there is an equal and opposite reaction. Basically, in order to create a force on the vehicle to move it in a direction, some sort of mass has to be propelled away from the spaceship going the opposite direction. Rockets work well for this, expelling gases in one direction to propel the ship in the other. If you are referring to electric motors, then the motor would have to have something to turn against to create a force on the ship. It cannot create much of a force turning against empty space.

That said, many satellites use what they call 'electric thrusters' for attitude control. In these, electric power is a big part of the propulsion system. They still expel fuel, though. There are several types. The American Institute of Aeronautics and Astronautics (www.aiaa.org) has much literature on the subject.

The most well known system using electricity as a major propulsion system component is the ion thruster, such as used on the space probe Deep Space 1. The electricity creates a potential which accelerates the fuel particles out of the other end at an extremely high efficiency. These types of engines are very efficient, but have low thrust, so they could not launch a spacecraft into orbit. They are designed for use in space only.

David Brandt, P.E.


Alex,

The key element here is pounds of thrust per pound of fuel and how heavy the generator is. Electrical boosters cannot produce enough energy to lift the space ship off the ground. In addition, they also weigh a lot! The solid rocket fuel that is used is some of the most powerful explosives that we have. The burning is of course controlled, but then as the fuel is used up, the rocket becomes lighter, creating greater and greater speeds. Electrical generators do not get lighter as they produce more and more energy. In addition, electrical generators must be powered by some type of fuel, so if the generator can be removed and the fuel can be used to directly power the ship, then that saves weight and gains efficiency.

Matt Voss


Alex:

Good question. One possible reason is weight. As you know, batteries are very heavy and a rocket powered with electricity might be too heavy to fly.

R. Avakian


The challenge to using electricity as a means of propulsion in space comes from a lack of anything to 'push' against. Airplanes get to move air, cars can push against the ground, and ships can move themselves with propellers in the water. But in Space, about the only thing that can be pushed against is your own fuel, which rockets use by throwing as fast as possible in the opposite direction. (This is Newton's Second law in action)

There is however, a type of electric engine that does work in space. It is called an Ion engine. The principle of throwing your fuel as fast as possible remains, but instead of burning a large amount of fuel, Ion Engines use electricity to throw small amounts of fuel much faster. Right now they are not terribly powerful engines, but they are highly efficient, getting far more thrust from a limited fuel supply. However, because they are not very powerful yet, you will probably not see any manned spacecraft carrying them for a long time.

Ryan Belscamper



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