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 15:06:06 GMT
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Viewed:
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705 times
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In lugnet.space, David Laswell wrote:
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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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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 to stay the same after using the propellant.
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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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No, light does exert force. Have you ever seen those little glass bulbs with
black and white squares of paper mounted on arms? Something sets them spinning.
Also, a solar sail uses light propulsion, cf. Attack of the Clones and The Mote
in God’s Eye.
George
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Message has 3 Replies:
 | | Re: Hypothetical design question
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| (...) Yup, but also note that a propellant exhaust speed of 2 mph is the rough equivalant of having the crew throw things out the back hatch. :) James (21 years ago, 26-Jun-03, to lugnet.space, FTX)
| | | Re: Hypothetical design question
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| (...) Correct. (...) Yes, *something* does. It's not light though, since if it was light, they would spin in the black direction (with the black side leading the rotation).... (the momentum imparted to something perfectly reflecting is twice that of (...) (21 years ago, 26-Jun-03, to lugnet.space, FTX)
| | | Re: Hypothetical design question
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| (...) Actually the ship in SW2:AOTC could not have used light for propulsion. As the ship must have used hyperspace speeds to reach Palpatine so quickly we must conclude that whatever was used was far beyond any real science explanation. Just to put (...) (21 years ago, 27-Jun-03, to lugnet.space, FTX)
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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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