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Old 29th July 2017, 03:54 PM   #201
Roboramma
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Originally Posted by GlennB View Post
1970's ... fusion technology in space ...

Are you sure you want to quote this as a plausible mission? Or just a fun SF concept maybe? Are we discussing actual science here or is there an implicit SF sub-forum lurking behind the scenes?
It was your article.

But the implication you seem to be making is that modern technology has reached the limit of the possible. Perhaps the particular version of fusion technology in space that was mentioned in the article isn't feasible, but would you suggest that fusion power in space is impossible in principle? Or that it will always be economically infeasible?

We have never even attempted an interstellar mission, and yet you think it's reasonable to base your understanding of the limits of such missions on what happen to be the fastest interplanetary probes we've developed? It doesn't make sense to take a 747 to go down the street to pick up a bottle of milk, but if you are trying to cross the pacific the tradeoffs change.

If you want to give the limits on ion drives, give a theoretical analysis of what's possible, not simply what has been done. If you want to give the limits on any type of interstellar travel you need some sort of theoretical analysis, not just an argument from incredulity.
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Old 30th July 2017, 01:55 AM   #202
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Is interstellar travel hard/impossible due to limitations of tech (can be overcome), or of nature (hard limits)? To find out, did some google fu, and found some handy labcoats who did all the work for me :

Quote:
Ignoring the rocket equation, in kinetic energy alone, to accelerate 1000 tons at 1G for 1 *month*, you would need about the same amount of energy as the human race consumed through all sources in 2013 - or around 400 terawatts of power. That gets you to about 0.1C. Somewhere around this point, interstellar gas starts to become dangerous ionizing radiation, and interstellar dust molecules become bullets.
Mega ouch!
Quote:
The difficulty of relativistic travel tends to be underestimated. Look at it this way. To get to the speed where time travels half as fast for the ship, you also end up doubling the mass of the spacecraft. That extra mass has to be paid for somehow. It's paid for by the energy you impart to the ship through acceleration. If you accelerated that ship by imparting energy from the outside with a perfect, 100% efficient system, you'd have to turn a kilo of matter into pure energy with no losses for every kilo you increased the ships mass.
And it's much worse with a self contained rocket. You also need to accelerate the fuel that you haven't used yet.
So, it comes down to, to accelerate to a speed where time for the ship is 10% that measured by someone standing still and slow down again once you reach your destination, you'd need the most perfect imaginable system (with today's physics) and spend 99.99% of your initial mass to do it.
Anything better will need more than technology. It will take different physics than we understand now.
Sounds like both new physics and a way to make space a true vacuum (nope) are needed for cruising around the galaxy.
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Old 30th July 2017, 08:51 AM   #203
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Originally Posted by Roboramma View Post
But the implication you seem to be making is that modern technology has reached the limit of the possible.
No, as that would be plain silly.

Originally Posted by Roboramma View Post
Perhaps the particular version of fusion technology in space that was mentioned in the article isn't feasible, but would you suggest that fusion power in space is impossible in principle? Or that it will always be economically infeasible?
I'm not saying it's impossible. I've been saying all along that "technically possible" doesn't equate to "will happen" just because it's amusing to discuss futuristic projects. "Possible but insanely time-consuming and expensive" can equate to "impossible in practice".
Meanwhile it would be encouraging if terrestrial fusion reactors could at least turn out a few joules of useful power. After several decades of r+d they still don't.[/quote]

Originally Posted by Roboramma View Post
We have never even attempted an interstellar mission, and yet you think it's reasonable to base your understanding of the limits of such missions on what happen to be the fastest interplanetary probes we've developed?
70% efficiency is hard to top when 100% is the theoretical max.

Originally Posted by Roboramma View Post
If you want to give the limits on ion drives, give a theoretical analysis of what's possible, not simply what has been done. If you want to give the limits on any type of interstellar travel you need some sort of theoretical analysis, not just an argument from incredulity.
See above. How about you propose a drive that can get us a few 10's of light years (minimum) in less than 50k years that has a significant basis in known or developing technology?
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Old 30th July 2017, 08:51 AM   #204
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Huge distances between the tiny number of interesting bits I think is enough explanation. Our reality is boring, there are no warp drives, there is no hyperspace just boring reality. Over the years the more I have thought about this has lead me to conclude that if we do continue to exist we will spend more and more time in created virtual realities. There is simply no reason for us to live in our boring reality. We will find much more enjoyment in realities in which warp drives can exist, in which we can meet aliens, and witness spectacular super novas close up.
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Old 30th July 2017, 09:12 AM   #205
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Originally Posted by Darat View Post
Huge distances between the tiny number of interesting bits I think is enough explanation. Our reality is boring, there are no warp drives, there is no hyperspace just boring reality. Over the years the more I have thought about this has lead me to conclude that if we do continue to exist we will spend more and more time in created virtual realities. There is simply no reason for us to live in our boring reality. We will find much more enjoyment in realities in which warp drives can exist, in which we can meet aliens, and witness spectacular super novas close up.
Think I'll shamelessly plug my AI Couch Potato Cosmos at this juncture.
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Old 30th July 2017, 09:50 AM   #206
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Originally Posted by GlennB View Post
I'm not saying it's impossible. I've been saying all along that "technically possible" doesn't equate to "will happen" just because it's amusing to discuss futuristic projects. "Possible but insanely time-consuming and expensive" can equate to "impossible in practice".
Sure, and myriad has made that argument in this thread in a way that while I disagree with him I also find him to be quite reasonable. I don't think you've actually made that argument, and it certainly wasn't the argument that I was responding to in your post.

Quote:
Meanwhile it would be encouraging if terrestrial fusion reactors could at least turn out a few joules of useful power. After several decades of r+d they still don't.
So, are you suggesting that fusion reactors are impossible, and this explains the lack of useful power, or that they are possible but "insanely time-consuming and expensive"? You certainly don't seem to be suggesting that they simply require more R&D, as that would be meaningless with regards to this discussion.


Quote:
70% efficiency is hard to top when 100% is the theoretical max.
I wasn't suggesting better efficiency. You can put more mass on your ship. Yes, you are subject to the rocket equation, but that doesn't mean that ships bigger than the ones we've built won't make a difference when it comes to changing the mission parameters from interplanetary to interstellar.



Quote:
See above. How about you propose a drive that can get us a few 10's of light years (minimum) in less than 50k years that has a significant basis in known or developing technology?
Project Orion?

But again, why am I limited to technology we already have. I agree with the limitation that the technology be in line with the laws of physics, certainly.
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Old 30th July 2017, 10:11 AM   #207
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Originally Posted by Roboramma View Post
So, are you suggesting that fusion reactors are impossible, and this explains the lack of useful power, or that they are possible but "insanely time-consuming and expensive"? You certainly don't seem to be suggesting that they simply require more R&D, as that would be meaningless with regards to this discussion.
Fusion reactors are certainly possible, as a lot of researchers are demonstrating. Slow in coming, but possible.
Fusion drives as a way of crossing interstellar distances are a different kettle of fish.

Originally Posted by Roboramma View Post
But again, why am I limited to technology we already have. I agree with the limitation that the technology be in line with the laws of physics, certainly.
If we're not limited to technology that can at least be described and considered plausible then we're engaging in magical thinking - "science will take care of that problem" kind of thinking.

But I come back again to the point I've made a number of times - technical plausibility doesn't equate to a project that could actually happen. Money, time, politics and even ethics come into the equation, and I think they add up to The Great Filter.

Some time ago in another thread I examined Elon Musk's idea of synthesising methane on the surface of Mars as a way of refuelling one of his ships (the one carrying 50 [?] multi-billionaires, if his most recent presentation is anything to go by ). Turned out it would take some thousands of solar panels running for a year just to cover the energy of enthalpy of the reaction, even at 100% efficiency, and totally ignoring the energy cost of assembling the plant, collecting the reagents and producing the liquid oxygen. That ain't gonna happen, even though it's technically possible, as the cost of landing and assembling that much kit would be prohibitive.

Theory and practice can't always meet.
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Old 30th July 2017, 10:50 PM   #208
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Originally Posted by Hlafordlaes View Post
Is interstellar travel hard/impossible due to limitations of tech (can be overcome), or of nature (hard limits)? To find out, did some google fu, and found some handy labcoats who did all the work for me :

Mega ouch!
If gas becomes ionizing radiation by speeding it up, this guy solved the principle problem of obtaining enough energy for such a journey. Speed up until matter around you becomes energy and harvest that energy.

Either that or his physics is way out of reality. I suspect the latter.

Quote:
Sounds like both new physics and a way to make space a true vacuum (nope) are needed for cruising around the galaxy.
Not really, someone's physics is about out of sync with reality. Chemical rockets always were a non-starter.

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Old 31st July 2017, 11:49 AM   #209
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Originally Posted by McHrozni View Post
If gas becomes ionizing radiation by speeding it up, this guy solved the principle problem of obtaining enough energy for such a journey. Speed up until matter around you becomes energy and harvest that energy.
So to get the energy to travel at relativistic speed, all you have to do is travel at relativistic speeds then harvest the energy? You may want to think on that a little longer.

And yes, the interstellar medium would be high energy particle radiation if you were traveling through it at high enough speed, the same as if you were still and it was moving high speed.

Last edited by phunk; 31st July 2017 at 11:50 AM.
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Old 31st July 2017, 10:02 PM   #210
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Originally Posted by phunk View Post
So to get the energy to travel at relativistic speed, all you have to do is travel at relativistic speeds then harvest the energy? You may want to think on that a little longer.
I did say there is an alternative explanation, didn't I?

Quote:
And yes, the interstellar medium would be high energy particle radiation if you were traveling through it at high enough speed, the same as if you were still and it was moving high speed.
Okay, particle radiation is an appropriate way to describe it. What is the penetration of such large particles?

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Old 2nd August 2017, 04:40 AM   #211
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Originally Posted by phunk View Post
So to get the energy to travel at relativistic speed, all you have to do is travel at relativistic speeds then harvest the energy? You may want to think on that a little longer.
All you need is the energy to get up to cruising speed, and then you can maintain cruising speed indefinitely. This drastically reduces the mass requirements at launch, for longer range missions.

Scramjet prototypes typically use a booster rocket to get up to the speed where the jet can operate. It's much more efficient for some use cases than rockets alone.

And really, humans have been thinking about this kind of thing for a long time. Portuguese traders didn't sail from Lisbon to Macau with all the water and food they needed already aboard. They replenished their stores along the way.

This signature is intended to irradiate people.

Last edited by theprestige; 2nd August 2017 at 04:42 AM.
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Old 2nd August 2017, 01:06 PM   #212
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Originally Posted by McHrozni View Post
I did say there is an alternative explanation, didn't I?
You presented a false dichotomy between a non sequitur and the author being wrong. The third option is that the author is right and you misinterpreted things.

Let's make an analogy to make it more clear. You said:

Quote:
If gas becomes ionizing radiation by speeding it up, this guy solved the principle problem of obtaining enough energy for such a journey. Speed up until matter around you becomes energy and harvest that energy.
The analogy:
Quote:
If still air becomes wind by speeding it up, this guy solved the problem of obtaining energy for sailboats on days without wind. Speed up until the air around you becomes wind, then open your sails!
Silly, ain't it? Colliding with the relatively stationary interstellar medium at high speed does make it behave like high energy particle radiation. But the energy in those collisions is your own momentum. You can't harvest your own momentum to gain more momentum.

Quote:
Okay, particle radiation is an appropriate way to describe it. What is the penetration of such large particles?

McHrozni
Not much penetration of the particles themselves, but the byproducts of the collisions are showers of smaller, better penetrating particles. Shielding is important flying through space at high speed, even before you take into account larger things you may collide with.
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Old 2nd August 2017, 01:22 PM   #213
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Originally Posted by theprestige View Post
All you need is the energy to get up to cruising speed, and then you can maintain cruising speed indefinitely.
Yup, inertia, doesn't cost anything to cruise indefinitely.
Quote:
This drastically reduces the mass requirements at launch, for longer range missions.

Scramjet prototypes typically use a booster rocket to get up to the speed where the jet can operate. It's much more efficient for some use cases than rockets alone.

And really, humans have been thinking about this kind of thing for a long time. Portuguese traders didn't sail from Lisbon to Macau with all the water and food they needed already aboard. They replenished their stores along the way.

This signature is intended to irradiate people.
Your examples are about gathering fuel/supplies, perfectly valid.

The post I was responding to was about gathering energy from colliding with the interstellar medium at high enough speed that it behaves like radiation. That energy is your own momentum. You aren't gaining anything, in fact you'd be losing some.
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Old 2nd August 2017, 02:21 PM   #214
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phunk,
You would think that! but actually there are sails made to sail directly into the wind and your silly analogy is indeed possible.

Quote:
Fundamental laws of physics tell us that it is impossible to build a vehicle that propels itself with the power it extracts from its own apparent wind and/or from the rotation of its own wheels. But the same laws also tell us that it is possible to build a vehicle that propels itself with the power it extracts from the kinetic energy potential that exists between its apparent wind speed and its ground speed. Such a potential exists if and only only if there is external wind. It should be noted that the magnitude of this potential depends on the true wind speed on the ground, independently from the speed and direction of the vehicle. In order to tap into this potential, the energy converting device must simultaneously interact with the air and the ground. When mounted on a moving vehicle, it can interact with the air through a set of rotating airfoils (propeller / turbine), and with the ground through a transmission and the wheels of the vehicle.
https://www.sites.google.com/site/oi...wered-vehicles

I am not 100% sure if the proposed starship could do the same or not. But I wouldn't automatically discount it.
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Old 2nd August 2017, 03:11 PM   #215
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Originally Posted by Red Baron Farms View Post
phunk,
You would think that! but actually there are sails made to sail directly into the wind and your silly analogy is indeed possible.



https://www.sites.google.com/site/oi...wered-vehicles

I am not 100% sure if the proposed starship could do the same or not. But I wouldn't automatically discount it.
Sailing directly into the wind requires relative motion between the air and the surface the craft is riding on. My analogy called for still air. And of course, in space, there is no surface.
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Old 2nd August 2017, 11:48 PM   #216
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Originally Posted by phunk View Post
Silly, ain't it? Colliding with the relatively stationary interstellar medium at high speed does make it behave like high energy particle radiation. But the energy in those collisions is your own momentum. You can't harvest your own momentum to gain more momentum.
Of course you can't harvest momentum, but there are other ways to harvest the energy from radiation. Plants do it all the time.

Quote:
Not much penetration of the particles themselves, but the byproducts of the collisions are showers of smaller, better penetrating particles. Shielding is important flying through space at high speed, even before you take into account larger things you may collide with.
No argument there, shielding it vital. If the spaceship has a diameter of 25 meters and a cilindrical shape and travels at 0.1c, it collides with a volume of about 150 billion cubic meters of interstellar space every second. Average density of our universe is about 5 hydrogen atoms, so it collides with about 750 billion particles every second.

One hydrogen atom has a mass of 1.67x10-24 kg, at 0.1 c that amounts to the energy of about 1x10-9 J or about 10 GeV. This is about five times the energy released in fission of Uranium or Plutonim isotopes.

240Pu produces about 400,000 fissions every second per kilogram, so the amount of radiation is broadly comparable to having 50,000 tons of Plutonium stuck to the front of the hull.

Yeah, shielding is going to be important too
That, or a slower travelling speed, whichever is more fesiable.

McHrozni

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Old 3rd August 2017, 12:37 AM   #217
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One concept is to encase the front of any ship travelling at relativistic speeds in a thick layer of ice to absorb interstellar dust and radiation.
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Old 3rd August 2017, 01:00 AM   #218
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Originally Posted by The Great Zaganza View Post
One concept is to encase the front of any ship travelling at relativistic speeds in a thick layer of ice to absorb interstellar dust and radiation.
That's one option, all you need is a comet.

Making the ship as slender as possible is also a good option. If the cross-section is halved the amount of shielding needed drops by a factor of 4.

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Old 3rd August 2017, 02:46 AM   #219
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Originally Posted by The Great Zaganza View Post
One concept is to encase the front of any ship travelling at relativistic speeds in a thick layer of ice to absorb interstellar dust and radiation.

Maybe stopping at some long forgotten colony to build up the depleted ice-shield, perhaps have a quick affair with a native, and then bugger off again?
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Old 3rd August 2017, 02:48 AM   #220
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Originally Posted by phunk View Post
Sailing directly into the wind requires relative motion between the air and the surface the craft is riding on. My analogy called for still air. And of course, in space, there is no surface.

This. Generating any usable energy normally requires some sort of differential in heat, pressure or motion or the like.
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Old 3rd August 2017, 08:13 AM   #221
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Originally Posted by McHrozni View Post
Of course you can't harvest momentum, but there are other ways to harvest the energy from radiation. Plants do it all the time.
That radiation only exists because of your momentum is being lost in collisions with the particles in the interstellar medium. It's not a power source. It's drag on the ship. At best, you're converting some of your own momentum into heat, slowing the ship, maybe generating some energy, but you can't use that energy to accelerate enough to compensate for the fact that you're slowing down. Doing that would violate conservation laws.

Now, if you wanted to gather some of the interstellar medium with a ramscoop or something, use it as fusion fuel, that's a different story. That energy is coming from fusion and not from your momentum. If you can get enough energy from that, and it's enough to get your exhaust velocity higher than your ship's speed, then you can accelerate.
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Old 3rd August 2017, 11:18 AM   #222
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Originally Posted by phunk View Post
... and it's enough to get your exhaust velocity higher than your ship's speed, then you can accelerate.
Anything pushed out the back at any speed will accelerate the rest of the ship forward. It doesn't matter how fast the ship is already going. It's all relative.
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Old 3rd August 2017, 01:48 PM   #223
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Originally Posted by jrhowell View Post
Anything pushed out the back at any speed will accelerate the rest of the ship forward. It doesn't matter how fast the ship is already going. It's all relative.
If you're carrying your own fuel, sure.

If you are scooping up the interstellar medium to be your fuel, then no. You'd have to spit it out fast enough that it is moving in the opposite direction of your ship relative to the original interstellar medium, not relative to your ship, or you can't accelerate. In simple terms, you lost more speed picking it up than you gained throwing it out.

It's conservation of momentum. You gain momentum X, it needs to lose momentum X. It needs to be pushed backward relative to its starting speed before you picked it up, not relative to your ship.
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Old 3rd August 2017, 03:25 PM   #224
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Originally Posted by phunk View Post
If you're carrying your own fuel, sure.

If you are scooping up the interstellar medium to be your fuel, then no.
OK, I see that.
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Old 3rd August 2017, 04:37 PM   #225
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Originally Posted by phunk View Post
If you're carrying your own fuel, sure.

If you are scooping up the interstellar medium to be your fuel, then no. You'd have to spit it out fast enough that it is moving in the opposite direction of your ship relative to the original interstellar medium, not relative to your ship, or you can't accelerate. In simple terms, you lost more speed picking it up than you gained throwing it out.

It's conservation of momentum. You gain momentum X, it needs to lose momentum X. It needs to be pushed backward relative to its starting speed before you picked it up, not relative to your ship.
As I understand the physics, a vessel that scoops up the interstellar medium as fuel (i.e. a Bussard ramjet) is limited to its exhaust velocity.

When the vessel reaches that velocity, there is no net gain in momentum from throwing the fuel it catches.

Now if it could somehow push on that medium as it goes past, instead of catching it ...
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Old 4th August 2017, 12:35 AM   #226
McHrozni
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Originally Posted by phunk View Post
That radiation only exists because of your momentum is being lost in collisions with the particles in the interstellar medium. It's not a power source. It's drag on the ship. At best, you're converting some of your own momentum into heat, slowing the ship, maybe generating some energy, but you can't use that energy to accelerate enough to compensate for the fact that you're slowing down. Doing that would violate conservation laws.

Now, if you wanted to gather some of the interstellar medium with a ramscoop or something, use it as fusion fuel, that's a different story. That energy is coming from fusion and not from your momentum. If you can get enough energy from that, and it's enough to get your exhaust velocity higher than your ship's speed, then you can accelerate.
No, you misunderstood what I meant. I wouldn't use it as the sole power source, but to back up whatever power source I had. I'm losing the energy in the collisions anyway and if some of that energy can be recovered I will need to carry less fuel. Ideally the system that would do that could also be used for some other purpose, e.g. to harvest energy from the star at destination to provide power for the early colony.

You still need other power sources of course, this is only a means to reduce the fuel requirements somewhat and possibly brake when you're getting close to your destination.

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Old 4th August 2017, 12:39 AM   #227
McHrozni
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Originally Posted by Humots View Post
Now if it could somehow push on that medium as it goes past, instead of catching it ...
You're probably better off having a particle accelerator inside and accelerate the said medium to relativistic speeds in relation to you to gain momentum. This would be the propulsion system anyway, all this harvesting does then is to reduce the mass of the fuel you need to carry somewhat (again, you lose the momentum from friction anyway).

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Old 4th August 2017, 04:11 AM   #228
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Originally Posted by 3point14 View Post
Maybe stopping at some long forgotten colony to build up the depleted ice-shield, perhaps have a quick affair with a native, and then bugger off again?
I never really did understand the point of that book.

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