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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Wed, 25 Jun 2003 22:50:16 GMT
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Viewed:
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669 times
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In lugnet.space, Leonard Hoffman wrote:
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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.
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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 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.
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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.
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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.
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this is why only light (ie, mass-less photon) travels at light speed.
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Unfortunately, light doesn’t seem to exert any force, so it has zero thrust
potential.
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Message has 2 Replies:
 | | Re: Hypothetical design question
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| (...) No, if you use simple conservation of momentum, at subrelatavistic speeds, you can still get a modest boost with a propellant speed of 2 mph. Your speed increase is propellant weight x 2 mph / remaining vehicle weight. since total momentem has (...) (21 years ago, 26-Jun-03, to lugnet.space, FTX)
| | | Re: Hypothetical design question
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| (...) 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 (...) (21 years ago, 26-Jun-03, to lugnet.space)
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Message is in Reply To:
| | Re: Hypothetical design question
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| (...) Ask your dad about that again. My comment was based on readings from Stephen Hawking's "Brief History of Time" and "Universe in a Nutshell", as well as various online physics FAQ. Granted, in space you don't have to worry about air friction or (...) (21 years ago, 25-Jun-03, to lugnet.space, FTX)
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