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To see why the exhaust speed is important, let's do a calculation. Let's start with a rocket of mass $m$ going at speed $u$. (We measure all speeds with respect to some inertial reference frame.) Now, suppose it exhausts a tiny amount of propellant of mass $\delta m$ and the propellant is traveling at speed $u_P$. After it exhausts that fuel, the rocket ...


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Imagine the rocket before and after throwing a small ("infinitessimal") amount of fuel out its exhaust. You apply the momentum conservation notion by equating the increase in the rocket's forwards momentum with the momentum of the fuel thrown backwards. The easiest inertial frame to do one's analysis in is that of rocket immediately before the increment ...


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If I am reading your question correctly, I believe you are asking whether or not you can reach outer space by simply ascending to a certain distance rather than reaching a specific velocity--that is, if by climbing in a hot air balloon you can reach a point where the gravitational potential energy is zero. If this is what you're asking, then there is a ...


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What you're describing is called an air launch.See also the Wikipedia article on air launch to orbit. As far as I know balloons are not used because they can't lift heavy enough loads. The launch is generally done using a large airliner. There is some related discussion in the question Why do space crafts take off with rockets instead of just ascending like ...


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This scheme would be terribly inefficient thermodynamically. Firstly, where is the energy needed to generate the electrical pulses coming from? Secondly, neither the process of electrolysis of seawater or the combustion of the resulting hydrogen and oxygen is 100% efficient. Finally can this system really generate enough thrust to propel a boat? If the ...


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The rocket equation is a cruel tyrant. By far the hardest challenge is just finding a propulsion system that can achieve the necessary velocities. Project Daedalus proposed to use an inertial confinement nuclear fusion system, but we really don't know if that can be made to work. The best existing inertial confinement technology is the National Ignition ...


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The energy required to split the water into oxygen and hydrogen is quite a bit less than the energy you would get back when you burn it (commercial electrolysis units run at about 70% efficiency). I think that makes this an inefficient method of propulsion. If you just (electrically) heated an amount of water until it became steam you would reach the same ...



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