Rocket Propulsion and Electric Motors
Why do space ships have rockets on them and not
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.
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.
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.
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.
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Update: June 2012