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#1 2007-10-06 10:45:34
- Terraformer
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- From: The Fortunate Isles
- Registered: 2007-08-27
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Re: PAEs
Has anyone considered using Particle Accelorators for engines. The particles coming out of the back of the craft would be travelling at near the speed of light, which would accelerate the craft to those sorts of speeds.
On a related topic, the laws of physics supposedly break down at light speed. The laws of physics is the only thing preventing FTL.
Use what is abundant and build to last
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#2 2007-10-06 11:02:24
Re: PAEs
I was actually just thinking about this a few hours ago.
You would need vast energies (MW, maybe even GW) to accelerate even milligrams of mass to relativistic speeds. Even if you did manage to accelerate a milligram of mass to .5c, it wouldn't really provide a 10 ton ship much thrust because of the huge mass difference.
m1v1 = m2v2
m1 = 1E-6 kg
m2 = 1E5 kg
v1 = 150E6 m/s
v2 = ?
v2 = m1v1/m2 = 150/10000 = 3/200 = 0.015 m/s
So accelerating 1mg to .5c would give a 10 ton ship 0.015 m/s more speed, while consuming MW or GW or power.
E = 0.5mv2 = 0.5 * 1E-6 * 150E6^2 = 11E9 = 11 GJ
You would need 11 GJ of energy to accelerate that milligram to .5c
P = E/t
t= E/P = 11 GJ / 1 GW = 11 s
A 1GW power plant could fire off 1mg of matter at .5c every 11 seconds.
a = delta-v/t = 0.015 m/s / 11 s = 0.0013636... m/s2 = 0.0001g
With a 1GW power plant working solely on particle acceleration propulsion and at 100% efficiency (theoretical maximum) you would be able to produce one ten thousandth of a g in acceleration.
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#3 2007-10-07 07:00:47
- Terraformer
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Re: PAEs
What about my second question about FTL?
Use what is abundant and build to last
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#4 2007-10-07 07:43:47
Re: PAEs
Well, it's more that *Newtonian* physics specifically break down when approaching light speed, and Einstein's relativity begins to take its place as a ruleset. And that ruleset which takes effect when approaching lightspeed still prevents lightspeed.
Wikipedia has a pretty great page on FTL and the various options to try to get around the limit.
http://en.wikipedia.org/wiki/Faster-than-light
Just because laws "break down" doesn't necessarily mean they break down in our favor... 
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#5 2007-10-07 08:21:41
- Terraformer
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Re: PAEs
But it's kind of impossible to know what it's like at or near light speed because currently we can't get to it.
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#7 2007-10-08 01:37:17
- cIclops
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- Registered: 2005-06-16
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Re: PAEs
I was actually just thinking about this a few hours ago.
You would need vast energies (MW, maybe even GW) to accelerate even milligrams of mass to relativistic speeds. Even if you did manage to accelerate a milligram of mass to .5c, it wouldn't really provide a 10 ton ship much thrust because of the huge mass difference.
m1v1 = m2v2
m1 = 1E-6 kg
m2 = 1E5 kg
samy, 10 t is 1E4 kg.
Yes momentum is what counts, accelerating a milligram mass packet to 0.5c is hard to do without a heavy accelerator and lots of power (even more weight). Far easier to accelerate say 100kg to a corresponding lower speed of 1450 m/sec (.5/1E5) to provide the same momentum exchange. This of course is what chemical rockets do 
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#8 2007-10-08 04:59:30
Re: PAEs
Whoops, sorry about the decimal error.
And yeah, propelling large masses at low speeds is much more energy efficient than small masses at high speeds. Of course, the tradeoff is that you can only burn for a very limited time because even the biggest fuel tank will be exhausted pretty quickly that way. While a small-mass high-speed engine can thrust almost forever because it spends its fuel tanks so slowly, but in return it's VERY energy inefficient and needs vast amounts of power to get anywhere near useful thrusts.
Basically, if we had nearly unlimited energy (even a GW nuclear reactor wouldn't be enough, we're talking orders of magnitude bigger) then it'd be great to waste all that energy on propelling stuff at relativistic speeds and have a very low fuel consumption. We could get to Pluto by spending a kilogram of reaction mass. But until we get FAR beyond GW nuclear reactors, we can only accelerate trace masses to relativistic speeds, and that produces very little useful thrust.
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