Propulsion in a Vacuum
If outer space is a vacuum where there is no
resistance how can a vehicle adjust its course as there is
nothing for the boosters to push against?
Newton's laws of mechanics do not require anything "to push
against". That is a common misconception. One way to see how this
works is to view the mechanism as one that obeys the principle that
the center of mass does / must remain "in the same place" before
and after some mass is moved from place to place.
So if we start with a rocket ship in outer space, in a vacuum,
and start throwing out particles in one direction (that is, fire up
the booster rockets) the vehicle has to move in the opposite
direction so that the center of mass remains fixed in the same
place that it started. The more particles we throw out, and the
faster we throw them, the further and quicker the vehicle must move
in the opposite direction to keep the center of mass unchanged.
This is a way of thinking about the propulsion process, and is
an oversimplification, but it is basically true. It is one of the
conservation laws, the conservation of mass. There are other
conservation laws as well: energy, momentum, angular momentum that
must also be obeyed too, but the conservation of the center of mass
is sufficient for your inquiry.
Just because there is little to no resistance in space, does not
mean that there are no forces that can be applied to or by an
object. In this case, a space ship's thrusters still have the
ability to accelerate to ship. Newton's Third Law states that for
every force there is an equal and opposite force. When the space
ships thrusters project a force out the rear of the craft, the equal
and opposite force must propel the craft forward to counteract that
force. To adjust course, the thrusters can be turned to position
the angle of thrust, which allows for steering.
Rocket propulsion does not require something to push against. It works
through a principle called conservation of momentum. In the least
complex terms, it means that the mass times the velocity (the
momentum) of the exhaust gas equals the mass times the velocity of
the rocket. It gets more complicated as the rocket is moving, because
the rocket is increasing velocity and the exhaust gas is decreasing
its velocity (both relative to the earth), and the rocket is reducing
its mass by burning its fuel. Conservation of momentum is the same
kind of effect that accounts for the recoil of a rifle or the force
you feel pushing back on your hand when you use a garden hose. The
fact that there is air around the rifle or hose does not really
contribute much to the forces you feel -- you would feel the same
kind of force if you shot the rifle or watered the lawn in a vacuum!
Hope this helps.
Ah, but there is! True, a spacecraft cannot alter its course by
adjusting it wings like an airplane or a bird (or a fish, if you
substitute the words "fins" or "tail" for "wings"). However, it can
alter its course by firing rocket engines, which spew out gases in
the direction opposite the thrust that needs to be exerted on the
spacecraft. What the spacecraft is pushing against is he gas it ejects.
The boosters do not have to "push against" anything. When the
rocket fuel is still in the tank, the rocket has a certain momentum,
going at a speed in a given direction. This momentum stays the same
for the overall rocket + fuel, as the fuel is shot out the back. So
if the fuel goes out backwards really fast, then the rocket moves
forward, and rocket+fuel overall system stays same.
To change direction of rocket, shoot fuel out one side or the other.
The rocket will move the opposite direction, again keeping the sum
of rocket + fuel's momentum constant.
Once the fuel is shot out, the rocket people will not think about it
much longer, but it has given the rocket a push in the other
direction. Again, this push is required by physics so that the sum
of momentum of rocket + fuel stay constant, before and after fuel shot out.
Rockets operate on Newton's third law, that says for every action,
there is an equal and opposite reaction. The action is the force
produced by the mass of the combustion gases flying away from the
nozzle at high speed. The reaction is a force on the spacecraft.
David Brandt, P.E.
Newton's Third Law of Motion - the law of action and reaction -
works even when there is nothing to push on. If you were floating
in space and threw a ball of your weight, you would go in the
opposite direction of the ball at the same speed as the ball. In
the case of a rocket engine, the particles are smaller, but there
are many more of them. We live here on earth where it is difficult
to appreciate a situation where there is not a material like water
or air around us, but Newton was right and it works in space.
The one thing that a rocket has to push against is its fuel. A thruster
pushes fuel out in one direction. The fuel pushes back in the opposite
Start with an example of two astronauts floating next to each other. One
astronaut pushes the other. Both go flying away from each other. Next,
consider an astronaut with a bag of heavy stones. He starts throwing the
stones forward. His hand pushes the stone and the stone pushes back on his
hand. With each stone, the astronaut speeds up a little bit. After
throwing many stones, the astronaut is moving quite fast. A rocket engine
throwing billions of molecules out of its tail is based on the same
principle. Of course, the rocket engine throws its fuel molecules much
faster than an astronaut throws stones.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012