is near light speed achievable?


by sr241
Tags: lightspeed, rocket engine
DaveC426913
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#19
Jan10-11, 08:53 AM
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Quote Quote by russ_watters View Post
The Bussard Ramjet is still just theoretical/science fiction.
Anything that is not yet built is theoretical/science fiction. That does not change the answer to the question 'what would happen if you could apply constant thrust'.

I know where you're coming from; I can smell the crackdom too. It's just that my philosophy is to facilitate much as possible (peppered with caveats and warnings) until they violate physics. I don't see the good in shutting people down on principle.

Anyway, that's two takes on the same question. Good for the OP to see differing viewpoints.
russ_watters
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Jan10-11, 05:26 PM
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Quote Quote by DaveC426913 View Post
There is no fundamental law. We just don't have any viable way of doing it.
Dave, I'm not sure if you took jardenjames up on his pm offer to view sr241's "invention", but it was, as I expected, a reactionless propulsion PMM. It violates conservation of momentum.
DaveC426913
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Jan10-11, 09:28 PM
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Quote Quote by russ_watters View Post
Dave, I'm not sure if you took jardenjames up on his pm offer to view sr241's "invention", but it was, as I expected, a reactionless propulsion PMM. It violates conservation of momentum.
I did not. There was no need.

Still, IMO, these kinds need our help more than others.
sr241
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Jan11-11, 09:24 AM
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here is the link https://sites.google.com/site/exhaus...earpropulsion/

be specific on why it will not work

I am already half convinced it wont work
JaredJames
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Jan11-11, 09:33 AM
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You are ignoring conservation of momentum.

It doesn't matter how slowly you stop the ball, it imparts an equal reaction force towards the rear as it initially did towards the front. They cancel out.

As per my email, all of the kinetic energy of the ball is converted to heat whether you stop it instantaneously on the back wall of the space craft or through your "friction tube". This is independent of momentum. You can't convert the momentum to heat.

The problem with your understanding is that you believe only a quantity of the kinetic energy becomes heat and the rest goes towards a counter-acting force. Thus, the reaction acting against the forward motion is less than the initial forward acting motion, giving a net forward motion. This is incorrect and a flawed understanding of the physics of the system. You have combined two different concepts and drawn an incorrect conclusion.

I recommend reading up on Conservation of Momentum and Conservation of Energy.
DaveC426913
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Jan11-11, 10:46 AM
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Quote Quote by sr241 View Post
here is the link https://sites.google.com/site/exhaus...earpropulsion/

be specific on why it will not work

I am already half convinced it wont work
No matter how you slow the ball from moving forward, its movement will cause the spaceship to recoil, leaving its centre of mass stationary.

Yes, energy is lost due to friction. But it is not lost from the kinetic enrgy of the craft, it is lost from the device that pushes the ball forward (whatever device or mechanism that might be, it doesn't matter). That mechanism will have to work harder against friction. i.e. it will have to put out more energy (energy lost to heat friction) while the ball will simply move the same amount.

So, ultimately, your device uses a mechanism that works hard to create friction which creates waste heat. None of that has anything to do with the device moving.

In principle: there is absolutely nothing you can do inside the craft to move the craft's centre of mass.
jambaugh
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Jan11-11, 08:08 PM
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Quote Quote by jarednjames View Post
From just that rather short wiki entry alone, it doesn't seem particularly viable to me.
That short wiki entry is not the sum total of information you can find on the topic. I linked to it as an unambiguous definition. You do your own homework.
DaveC426913
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Jan11-11, 08:13 PM
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Quote Quote by jambaugh View Post
That short wiki entry is not the sum total of information you can find on the topic. I linked to it as an unambiguous definition. You do your own homework.
That seems a rather aggresively-defensive reaction.

"I propose X as viable. Here's a link."
"Your link is not convincing."
"You do your own homework."
JaredJames
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Jan11-11, 08:20 PM
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Quote Quote by jambaugh View Post
That short wiki entry is not the sum total of information you can find on the topic. I linked to it as an unambiguous definition. You do your own homework.
I have, and from what I've read so far it is rather unrealistic.

This image here shows it requires a lens "30,000 miles across" and "100 billion light sensitive processors".

I also refer you to his own paper, equation 35, page 4 where he shows the lens required for a 1km sail is 50,000km with an estimated mass of 50,000 tons. Along with said system requiring "excessively large" amounts of power.

Paper available here: http://www.boomslanger.com/images/starwisp.pdf

Your exact words were: "This idea seems to me to be the most viable means for us to send probes to the nearest stars.".

Based on the above, I'd say that is not so.

I'm not exactly sure what I did to deserve that reaction, but I'm pretty sure the rules are that you back up your claims.
jambaugh
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Jan11-11, 09:30 PM
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Quote Quote by DaveC426913 View Post
That seems a rather aggresively-defensive reaction.

"I propose X as viable. Here's a link."
"Your link is not convincing."
"You do your own homework."
OK, granted. Pardon but I was a bit miffed. I erased my lengthier reply without sending and opted for a terse reply to match the terse dismissal.

Here's the technical paper: http://www.boomslanger.com/images/starwisp.pdf

I wasn't making a claim other than my opinion of comparative viability. The starwisp doesn't need invented technology such as cracking fusion, only some reasonable engineering and hardware design. We could begin tomorrow if we had the will.

The infrastructure could be recycled to home power production from orbit and pay for itself over time. (That may be a stretch but if not the infrastructure could power space based industry and knowledge ALWAYS pays eventually) We could start smaller using wisps to explore e.g. the Oort cloud and probe past the heliopause.

Oh, and when the alien invaders come calling we can roast them in their ships with our microwave weapons battery!
JaredJames
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Jan11-11, 09:37 PM
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Getting 50,000 tons into space and then the construction of said materials for a 50,000km emitter lens is considered "reasonable engineering"?

What was the cost per kg to get stuff into space?
jambaugh
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Jan11-11, 09:58 PM
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Quote Quote by jarednjames View Post
Your exact words were: "This idea seems to me to be the most viable means for us to send probes to the nearest stars."
Yes I said "most viable", as in better than alternatives (barring certain breakthroughs).

BTW the fresnel lens is not the only option. One can phase correlate the array of transmitters.
(I think... I'll have to do some calculations....but not tonight, classes start tomorrow, delayed by the ice storm and I've got to teach at 8:00am.)

Of course one is talking about a vast engineering project, far grander than anything we've ever done before. But it is feasible within current technology and reasonable extrapolation of engineering.
JaredJames
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Jan11-11, 10:03 PM
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Quote Quote by NhocCuteGirl View Post
so we can't really imagine a realistic machine that could send a macroscopic object to such speeds
See above discussion concerning the "starwisp".

I may not like it, but it's certainly possible for getting a 1km array up to near relativistic speeds.
jambaugh
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Jan11-11, 10:07 PM
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Quote Quote by jarednjames View Post
Getting 50,000 tons into space and then the construction of said materials for a 50,000km emitter lens is considered "reasonable engineering"?

What was the cost per kg to get stuff into space?
Depends on where you find it. If I were lord and master of the Earth and determined to build the thing, my first stage would be to establish a base on the moon and set up materials production. The "lens" is a wire mesh, not a big hunk of plastic. Getting 50,000 tons into space is a matter of lifting 5000 10ton spools.

But I challenge you to suggest a cheaper way to get a probe to the nearest star. Again I said "most" viable, not "super-easy-cheap-why-didn't-Columbus-just-bypass-America-and-go-to-Alpha-Proxima" viable.
jambaugh
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Jan12-11, 05:58 AM
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Here's some math for the curious...

Figure the sustained acceleration you need to reach a given % of c (including over 100%) in simple Newtonian physics then take the hyperbolic tangent. That's the relativistic % of c.

Example you boost to what would be c in Newton's universe, that's about 8500 g-hours (almost a year accelerating at 1 g) and you will get to tanh(1)= (about) 0.76 or 76% c as seen by the people watching from your starting frame.

To figure the energy you need, you would take the hyperbolic cosine minus 1 times the mass (the minus 1 subtracts out the initial mass-energy). E.g. cosh(1) = 1.543 so you need about 54% of the payload mass times c^2 in energy to get up to 76% lightspeed.
That's about 48,900 TeraJoules per kg or 48,900 Tera-watt seconds per kg payload...
or 13.6 TW hours per kg or about 19 GW months.

Figure the output of a few industrial scale power plants sustained over a month to accelerate at 10 or 12 gees per kg payload.

That's delivered to the payload. With a beamed source (light or microwaves or whatever) one must figure one or two factors of 10 for losses and beam spreading.

It's not unthinkable but certainly would require we keep the payload size under say 20kilograms and a major industrial presence in space. And .76c may be too ambitious, more like .01c or .005c. Something we could imagine happening in the year 2200. I mentioned we wouldn't need to assume new tech (like cracking fusion) but we'd probably need orbital fusion power plants to get that kind of power output or some other energy production improvement.
sr241
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#34
Jan12-11, 06:50 AM
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Quote Quote by NhocCuteGirl View Post
Hi sr241,
First note that according to relativity, no matter how fast the rocket goes, the people inside always see light going at lightspeed (the constant c). This is elementary relativity.
_
c is speed of light in vacuum; that means light slows in a medium like air and water. so in air and water speed of light is going to be less than c, how can you correlate this with the said principle (speed of light is constant).
JaredJames
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#35
Jan12-11, 09:25 AM
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Well jambaugh, I concede the point to you.

As un-realistic the prospect of the starwisp appears to me, it really is one of the most viable methods of getting to the nearest stars.

I didn't realise just how 'bad' current propulsion systems were. They're really not up to much when it comes to interstellar travel.

I suppose the scale of the starwisp is a good demonstration of what it takes to get to the nearest star. Even if it is only in a very basic way.
DaveC426913
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Jan12-11, 10:10 AM
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Quote Quote by sr241 View Post
c is speed of light in vacuum; that means light slows in a medium like air and water. so in air and water speed of light is going to be less than c, how can you correlate this with the said principle (speed of light is constant).
As he said, this is elementary.

Speed of light in a vacuum is a constant, regardless of your frame of reference. If you want to get into propogation of light in a medium, that's a whole different kettle of fish that's got nothing really to do with this topic.


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