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Name: Ian
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Question:
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?



Replies:
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.

Vince Calder


Ian,

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.

Matt Voss


Hi Ian

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.

Bob Froehlich


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.

Richard Barrans


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.

Steve Ross


Hi, Lan.

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.


Ian -

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.

Larry Krengel


Ian,

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 direction.

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
Physics Instructor
Illinois Central College



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