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Thread: about space ships

  1. #1 about space ships 
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    It might be a silly question; but I am posting it because 'am not aware of many facts in physics, but eager to get it answered. I know moving objects/vehicles ignited by fuel travel in a plane because it applies a force to the plane and the plane applies equal and opposite force (Newton's law) and the body moves in that plane. How does a space ship move in vacuum ; in space where there is no air or particles to give thrust? How does it work once it crosses the Earth's atmosphere ?
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  3. #2  
    Universal Mind John Galt's Avatar
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    This is one of the commonest questions asked when people first start thinking about how rockets work, so it is certainly not a silly question.

    Imagine an explosion inside a container, one strong enough to restrain the explosion. There is pressure exerted on all of the internal surfaces. Now imagine puncturing a hole in the container. The gases can escape through that hole and no pressure is exerted on the container there, yet opposite that hole an unbalanced pressure is still being applied. The container moves in that direction. That's the principle of the rocket.

    Inside the atmosphere it works less efficiently because it has to overcome the air resistance. In a vacuum there is no such problem.


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    And don't confuse rocket engines with turbine engines, which use air to propel themselves.
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    Quote Originally Posted by eldhosepg View Post
    It might be a silly question; but I am posting it because 'am not aware of many facts in physics, but eager to get it answered. I know moving objects/vehicles ignited by fuel travel in a plane because it applies a force to the plane and the plane applies equal and opposite force (Newton's law) and the body moves in that plane. How does a space ship move in vacuum ; in space where there is no air or particles to give thrust? How does it work once it crosses the Earth's atmosphere ?
    Thanks,
    The basic operating prnciple here is conservation of momentum. The gas ejected by the nozzle has momentum. In order for momentum to be conserved the rocket must gain momentum equal and opposite to the momentum ejected by the nozzle. This is a continuous process. The time rate of change of momentum = force. Therefore the exploding gases exerts a force on the rocket. There's got to be a better way to explain that but I'm at a loss right now.
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    Sorry Pyoko but turbine engines work on the same principle , Newtons third law.
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    A rocket interacts with its own exhaust (when it propels itself in vacuum).

    Rocket engines are slightly more efficient in vacuum than in atmosphere. That's because pressure difference between ignition chamber and environment is higher, I believe.

    Keep in mind, that there are two forces that keep a plane in the air: one is thrust and the second one is created by the wing (lift). That's why a plane can very well fly with the engine not capable to lift it vertically. See Thrust-to-weight ratio - Wikipedia, the free encyclopedia

    Also spaceships usually accelerate on only small fractions of their trajectories. Mainly they fly according to the laws of celestial mechanics.

    Currently there are no spaceships capable to fly both in the atmosphere and vacuum of space. Space Shuttle lifts as a rocket and lands as a glider. So it has (had, now shuttles are retired) only one attempt to land. Boeing X-40 is basically the same thing.

    There was one unfinished Soviet spaceplane project (See Buran (spacecraft) - Wikipedia, the free encyclopedia). A system of two jet engines was planned for this spaceship. However, it was not ready for Buran's only flight.

    Here is another concept: Baikal (rocket booster) - Wikipedia, the free encyclopedia.
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    Quote Originally Posted by fizzlooney View Post
    Sorry Pyoko but turbine engines work on the same principle , Newtons third law.
    Of course. I am talking about mechanics, not physics. Turbine engines in a Boeing 747 rely on huge air intake (not just oxygen), which pushes the plane. I was saying rocket engines do not rely on that and rely on more direct oxidation of the rocket fuel, hence it is "easier" to use them in space. That was what I was getting at.

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    Is it possible that we can replicate the sun's power and send out sun flares out to thrust us forward . Just an idea.. a crazy one
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    Quote Originally Posted by IWANTTOKNOWMORE View Post
    Is it possible that we can replicate the sun's power and send out sun flares out to thrust us forward . Just an idea.. a crazy one
    People are currently working on fusion reactors, and this, I believe, will be the future of a lot of technology. If we could figure out how to do it without having the harsh conditions of the Sun, then we will be well on our way. Current technologies are focused on plasma/ magnetic fields (also lasers) that are extremely high power to begin with. If I were to use "magic", I would make Cold Fusion a reality.
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    Quote Originally Posted by IWANTTOKNOWMORE View Post
    Is it possible that we can replicate the sun's power and send out sun flares out to thrust us forward . Just an idea.. a crazy one
    Ion drives and VASIMR are probably the closest thing to this, currently.
    Variable Specific Impulse Magnetoplasma Rocket - Wikipedia, the free encyclopedia
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    I remember my 8th grade physics teacher spent a whole day on explaining this and I forgot the whole thing. But my mind's now refreshed thanks to a few helpful posts here and a Google search.

    I think John Galt's post (post #2) pretty much sums it up best. Pretend you're on a rolly chair on a recently greased, hard surface. Lift your feet and throw a ball in front of you. You will move in the opposite direction. Why? Because you didn't just do work on the ball, the ball did work on you too with equal and opposite force (there's Newton's 3rd Law for ya).

    The same goes for a rocket in a vacuum. The hole where the fuel ejects from offers no resistance to the fuel. But on the solid side, the fuel will push the rocket forward. Again, since there's no push on the other side, the rocket will go forward. Of course, the fuel product will eject much faster because it's less massive than the rocket. But rocket scientists have developed advanced systems so that rockets can achieve much faster speeds.
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    Quote Originally Posted by eldhosepg View Post
    It might be a silly question; but I am posting it because 'am not aware of many facts in physics, but eager to get it answered. I know moving objects/vehicles ignited by fuel travel in a plane because it applies a force to the plane and the plane applies equal and opposite force (Newton's law) and the body moves in that plane. How does a space ship move in vacuum ; in space where there is no air or particles to give thrust? How does it work once it crosses the Earth's atmosphere ?
    Thanks,
    Short answer,
    Point maneuvering thrusters, computer software/programming for yaw, roll, pitch and a pilots controller.
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    http://kerbalspaceprogram.com/

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    f anyone's interested in simulating rocket behaviour. It's very accurate and can be used for personal experiments.
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  15. #14  
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by IWANTTOKNOWMORE View Post
    Is it possible that we can replicate the sun's power and send out sun flares out to thrust us forward . Just an idea.. a crazy one
    Ion drives and VASIMR are probably the closest thing to this, currently.
    Variable Specific Impulse Magnetoplasma Rocket - Wikipedia, the free encyclopedia
    I'm very interested in ion drive technology. It makes the nuclear drives in the Orion project even seem quaint. It's a little beyond my ability to fully comprehend, though. I understand the general idea, but get lost in the details. It's too bad sound waves don't travel through a vacuum...we'll never know if they sound like TIE fighters.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
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