Subject:
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Re: Hypothetical design question
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Newsgroups:
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lugnet.space
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Date:
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Thu, 26 Jun 2003 17:17:28 GMT
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Viewed:
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654 times
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In lugnet.space, David Laswell wrote:
> In lugnet.space, Leonard Hoffman wrote:
> > Granted, in space you don't have to worry about air friction or
> > gravitational pull (relatively), but burning fuel won't accelerate you to
> > just any speed you desire.
> >
> > using some of newton's theories, it takes a certain amount of energy to move
> > an object a certain distance. or to say it in vacuum terms, it takes a
> > certain amount of energy to accelerate an object to a certain speed. NASA's
> > rockets will accelerate you up to a given point, allowed they have enough
> > fuel to burn. this is the amount of energy that the oxygen-hydrogen engines
> > produce.
>
> I hadn't really thought about this problem before, but I can see how it would
> be a problem. If your propellent has an exit velocity of 2mph, it shouldn't
> ever be able to make you go faster than 2mph. Once you've hit that point,
> the propellent wouldn't even be moving when it exits the thruster, so it
> shouldn't be exerting any force at all on the vessel as it crawls along,
> abandoning its propellent as it goes.
I think you may need to look into relativistic reference frames... the exit
velocity is w.r.t. the vehicle, not where it is traveling or what its current
velocity is... the upper limit on vehicle velocity achievable with a fuel of a
given specific impulse is due to mass fraction limits, not exhaust velocity at
the nozzle.
> > However, when accelerating towards light speed, the amount of energy
> > required to accelerate increases. getting close to light speed, the amount
> > of energy approaches infinity. and the more mass you have (like extra fuel)
> > will increase the amount of energy needed to accelerate.
>
> Hence also the reason why people have actually come up with the insane idea
> of building a huge pan on the back of your ship and occassionally toss a nuke
> on the fire to give you a boost.
That's not actually why it was proposed, IIRC. Orion drive ships have great mass
fraction characteristics at low speeds and huge potential payload capabilities.
> > this is why only light (ie, mass-less photon) travels at light speed.
>
> Unfortunately, light doesn't seem to exert any force, so it has zero thrust
> potential.
You might want to look into the physics behind light sails, I think. light does
indeed exert force. Here's a ref I found by googling, I am sure there are others
http://www.etext.org/Zines/Quanta/sailing.html
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Message is in Reply To:
 | | Re: Hypothetical design question
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| (...) I hadn't really thought about this problem before, but I can see how it would be a problem. If your propellent has an exit velocity of 2mph, it shouldn't ever be able to make you go faster than 2mph. Once you've hit that point, the propellent (...) (21 years ago, 25-Jun-03, to lugnet.space, FTX)
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