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The Limitations of Intergalactic Travel

by eNtRopY
Tags: intergalactic, limitations, travel
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Cosmo Novice
#55
Jun1-11, 04:42 AM
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The ideas of Von Neumann probes are interesting for our cosmic exploratory development. Tehnologically advanced genocidal Von Neumann probes may also be the answer to the Fermi paradox - which I find an amusing idea.

As this thread seems open to speculation I will throw in my two penneth. *IF* we are to ever actualise interstellar travel then it may be entirely necessary for technology to provide an extended degree of control on the physical Universe. Such as the ideas of advanced Alcubierre Drives, contained singularities and post physical evolution. All of which are highly speculative and may be impossible to realise.

I do not think anything we can currently develop or technically create (such as solar sails, ion propulsion etc) or even when these technologies have been refined, that they will provide realistic interstellar travel.
sophiecentaur
#56
Jun1-11, 05:05 AM
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The VN Probe sounds a bit like inventing a new bacterium. This could evolve, all on its own, and decide to put an end to Humanity, on the grounds that we are an absolute shower and a blot on the Galaxy. Shooting ourselves in the foot or what?
Ryan_m_b
#57
Jun1-11, 06:54 AM
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The thing is if we ever invent VN probes we would drastically change the parameters of what we are talking about.

VN probes, by definition are self-replicating machines. If we still wanted to colonise space we could send some to the moons of a gas giant and get them to dismantle all of them before using the mass to build millions of O'Neill cylinders. Into all of these cylinders we put different ecologies and study them to discover which one works best. Using this super-experiment we could crack the problem of building a sustainable environment by observing what works and what doesn't (without danger to human life or Earth's ecosystem)

There's really no need to go interstellar from that point because we can just live in millions of habitats orbiting the sun, the increased surface area allows populations so large we'd have to use standard form.

VNs seem a silly idea for space colonisation because you don't actually get to colonise anywhere, nobody leaves your planet you just make another planet full of humans. Though even if you did manage to build some sort of fantastical universal constructor capable of being packed into a small enough mass to be sent interstellar you would still have to crack the problem of designing an AI to raise the children on the other end. Children do not develop from passive media (i.e TV). They need interaction, specifically human interaction. If we ever overcome the hard problem of consciousness we might begin to see how we could go about making an AI but until then we're stuck where we are.
Zentrails
#58
Jun1-11, 09:25 AM
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Quote Quote by Cosmo Novice View Post
I do not think anything we can currently develop or technically create (such as solar sails, ion propulsion etc) or even when these technologies have been refined, that they will provide realistic interstellar travel.
Impractical might be a better word than unrealistic or maybe economically/politically unfeasible.
Ryan_m_b
#59
Jun1-11, 10:03 AM
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Quote Quote by Zentrails View Post
Impractical might be a better word than unrealistic or maybe economically/politically unfeasible.
Very impractical. Looking at the numbers on the Spacecraft Propulsion article of wikipedia current solar sails produce 9 newtons per thousand square metres at a distance of 1 AU (~300watts per metre). Working off of those numbers to propel a 1 tonne probe at 1g would require a sail 1km2.

As I said a suitable investment in resources might help us send interstellar probes via beamrider but no manned.
Cosmo Novice
#60
Jun1-11, 10:25 AM
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Quote Quote by Zentrails View Post
Impractical might be a better word than unrealistic or maybe economically/politically unfeasible.
Socio and economic feasibility are generally what I was referring to. I stand by unrealistic - the reason I stand by unrealistic as opposed to impractical are that impracticalities automatically assume possibility, I refute that the technological examples I gave (Solar sail, ion propulsion etc) will realise interstellar capability. If interstaller technologies are ever theoretically proven (technologies beyond what currently exists) then I am happy to refute my comment and agree with impracticality.

Although really this is just semantics :)
Ryan_m_b
#61
Jun1-11, 11:14 AM
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Quote Quote by Cosmo Novice View Post
the reason I stand by unrealistic as opposed to impractical are that impracticalities automatically assume possibility
Totally agree. Something can be technologically impractical whilst still being totally unrealistic
Lsos
#62
Jun2-11, 10:25 AM
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Quote Quote by ryan_m_b View Post
It always boggles my mind when people express opinions suggesting that NASA worked out space travel decades ago and that all it would require is some investment and a bit of polishing off and we'll be skipping around the galaxy like true space cadets. Space travel is hard.

Antimatter/matter propulsion has the highest specific impulse that we know of. With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond. That means the ship can thrust at 1g for roughly 10 and a half days reaching a speed of ~10,000,000 mps which is 3.3% of the speed of light. To get to near 100% you would need thirty times this but remember you need to decelerate at the other end, that gives you a 60:1 ratio of fuel to ship if we use Am/M. Now Project Orion proposed using nuclear bombs but these can only match Am/M if the following few hypothetical were met;

The entire mass fissile material is converted to energy
-- It isnt, of all the uranium only ~2% undergoes fission. Of this only a half of a percent is converted to energy. Little boy, the Hiroshima bomb, contained over 60kg of uranium but only a penny's worth converted to energy. This means you need to pump up that ratio from 6:1 to 6,000-60,000:1

The bomb's mass is entirely fissile material
-- It isnt, most of the bomb is casing/primer etc. I can't find the exact figures with a brief google but it would be reasonable to assume that only 1-10% of the bomb is actually fissile. this pushes the ratio further to 60,000-600,000:1

The whole energy of the explosion hits the back of the ship
-- It won't, for a 1,400miles3 ship if we make it a cube that makes a ship ~11 miles on the side with each face 121miles2. If the explosion occurs 30 miles from the ship (about the recommended for Orion) then only 0.4% of the energy will hit the ship (the energy radiates as a sphere, the ship obscures a small part of this). This again pushes the ratio to 1,500,000-15,000,000:1

Aside from the horrendous fuel requirements there's a tendency for people to assume that all the other issues are just minor details when in actual fact all areas of space colonisation are extremely non-trivial. For an interstellar colony ship you need to;

Create a sustainable biosphere for the ship
--We have very little idea how complex ecologies work here on Earth let alone how to recreate one that is immune from ecological disaster.

Create an environment capable of growing food
--Same problem as above yet with the added problem of a ship biosphere being a small closed system. In addition a wide diversity of foods combined with the appropriate bacteria to fill up our guts (which contain 1kg of vital gut flora).

Pack a fully capable industrial system into a colony ship
--Many industrial complexes run over tens of km, add up all the wide variety of industries across the world plus the infrastructure and put it all in one place. In addition you need to redesign all of it to have near 100% recyclable capability (remember that closed system?)

Pack a fully capable work force
--In today's high-tech and diverse society there are literally 10s-100s of thousands of different specialities. Provide enough people in the profession to staff each job plus enough to train the next generation and the total number of people in the workforce? You're looking at a figure measured in the 10s-100s of millions of people

Design a long-term stable socio-economic system
-- Societies on Earth don't exactly have a track record of long term-stability. An interstellar trip could take 100s-1000s of years. The vehicle isn't going to be analogous to a captain and his crew, it's more like rolling up an entire country's population building a wall around it and then sending it off alone. Remember a single failing point and the whole mission is gone

Solve all of those problems without invoking magic wands of super-nanotech, AI and robots and then you can play space cadet.

Sorry for the long rant but it's a pet peeve of mine when people blindly assume that manned space exploration/space colonisation is easy then pretentiously claim that it's only reason X that we can't do it.

I don’t understand how everyone can so easily dismiss the nuclear-pulse propulsion idea, especially in this thread which is obviously open to some off-the-wall concepts.

ryan_m_b, you’re the first source I have ever met that for one reason or another doesn’t accept the Orion Propulsion idea. No offence, but I’m sure you understand that to me, all the other sources are going to be more credible than you.

I am left with simply digesting the above dissertation you made. Again, no offense, but it appears like both an incomplete AND cherry-picked collection of data designed purposely to maul nuclear-pulse propulsion, but not necessarily reflect reality.

For example,

“of all the uranium only ~2% undergoes fission”. This is perhaps true for the Hiroshima bomb…the first ever bomb of its type not only used, but tested. The second bomb used was 10x more efficient, and modern bombs, boosted by fusion, are much more efficient than that.

“only 1-10% of the bomb is actually fissile”. Again this is perhaps true for the very oldest designs, but I’m sure modern ones are much better designed than that. Just a few years after Hiroshima they could make bombs two orders of magnitude lighter with the same yield.

“If the explosion occurs 30 miles from the ship (about the recommended for Orion) then only 0.4% of the energy will hit the ship (the energy radiates as a sphere, the ship obscures a small part of this).” ~30 meters was the recommended for Orion. The bombs were shaped charges which directed almost all the available energy at the pusher plate.

“With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond”. This part I can’t don’t understand because I have no idea how this alleged matter/ antimatter propulsion system is supposed to work. The only thing I can figure out is that it’s incredibly inefficient, as matter/antimatter annihilation produces enough energy which, if fully harnessed, could move a 1:1 ship not much slower than the speed of light. This matter/antimatter propulsion harnesses only a small fraction of the available energy. This leaves many possibilities, including that it’s more inefficient, or perhaps similarly to the Orion concept, and therefore it’s likely that it has the same sources of inefficiencies (if not more) as those outlined above. This would mean that they were calculated twice.

And then, of course, nobody says that the spaceship has to be 1:1 fuel to payload. 1:1 is damn good. Hell, some modern commercial jet liners do that.

I don’t have time to go through all the numbers and see for myself if Orion is feasible, but I hope you can understand how an armchair space cadet such as myself will, for now, continue to take their word for it, and not yours.
Ryan_m_b
#63
Jun2-11, 10:41 AM
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Quote Quote by Lsos View Post
“of all the uranium only ~2% undergoes fission”. This is perhaps true for the Hiroshima bomb…the first ever bomb of its type not only used, but tested. The second bomb used was 10x more efficient, and modern bombs, boosted by fusion, are much more efficient than that.
I hardly cherry picked data, I used what I had. Even if it was 10 times more efficient we still aren't talking on the order of 100% efficiency.

“only 1-10% of the bomb is actually fissile”. Again this is perhaps true for the very oldest designs, but I’m sure modern ones are much better designed than that. Just a few years after Hiroshima they could make bombs two orders of magnitude lighter with the same yield.
Again I am unaware of a nuclear device where the fissile material accounts for the majority of the mass.

~30 meters was the recommended for Orion. The bombs were shaped charges which directed almost all the available energy at the pusher plate.
Are you kidding? A nuclear explosion 30 metres away???? Do you have any links for that? How small would the explosion have to be not to destroy the ship/flood it with radiation and yet provide useful thrust?

“With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond”. This part I can’t don’t understand because I have no idea how this alleged matter/ antimatter propulsion system is supposed to work.
Here's the wiki article on antimatter rockets and here's a NASA article outlining the Isp

The only thing I can figure out is that it’s incredibly inefficient, as matter/antimatter annihilation produces enough energy which, if fully harnessed, could move a 1:1 ship not much slower than the speed of light. This matter/antimatter propulsion harnesses only a small fraction of the available energy. This leaves many possibilities, including that it’s more inefficient, or perhaps similarly to the Orion concept, and therefore it’s likely that it has the same sources of inefficiencies (if not more) as those outlined above. This would mean that they were calculated twice.
Sorry but how did you work any of that out? A moment ago you mentioned not knowing anything about how antimatter propulsion would work yet now you are claiming that the specific impulse of antimatter is somewhere close to 30megaseconds (close to what you would need to get "not much slower than the speed of light"

And then, of course, nobody says that the spaceship has to be 1:1 fuel to payload. 1:1 is damn good. Hell, some modern commercial jet liners do that.
Of course it doesn't have to be 1:1 but that's a good reference to the efficiency of a propulsion system hence why specific impulse assumes it. You may wish to still believe in Orion but you could at least look into it yourself, especially with the things I've discussed that are nothing to do with propulsion.
JaredJames
#64
Jun2-11, 10:45 AM
P: 3,387
Quote Quote by Lsos View Post
I don’t have time to go through all the numbers and see for myself if Orion is feasible, but I hope you can understand how an armchair space cadet such as myself will, for now, continue to take their word for it, and not yours.
Recommend you read through this: http://en.wikipedia.org/wiki/Project_Orion

The numbers just aren't enticing.
Lsos
#65
Jun2-11, 11:36 AM
P: 774
Are you kidding? A nuclear explosion 30 metres away???? Do you have any links for that? How small would the explosion have to be not to destroy the ship/flood it with radiation and yet provide useful thrust?"
A fraction of a kiloton was my understanding. The wiki page provides a range of numbers ranging from 25-60 meters, as well as pretty much every other link I've seen. I'd be interested in where you got the miles figure from...would an atomic bomb do anything more than give you a sunburn from 30 miles away?

Sorry but how did you work any of that out? A moment ago you mentioned not knowing anything about how antimatter propulsion would work yet now you are claiming that the specific impulse of antimatter is somewhere close to 30megaseconds (close to what you would need to get "not much slower than the speed of light"
I just used E=mc^2 and then calculated velocity from the resulting energy. Of course I realize that neutrinos and gamma rays and the whole "action-reaction" thing will make the whole process inefficient. The point I'm making is that the antimatter rocket in question already took these inefficiencies and more into account, perhaps overlapping or completely encompassing the sources of inefficiencies which you went over again with Orion. And perhaps it would have it's own problems that Orion wouldn't. The links you showed don't lead me to believe otherwise.

Of course it doesn't have to be 1:1 but that's a good reference to the efficiency of a propulsion system hence why specific impulse assumes it. You may wish to still believe in Orion but you could at least look into it yourself, especially with the things I've discussed that are nothing to do with propulsion.
Of course. I just brought that up to underline the fact that Orion was never meant to be 1:1.

JaredJames, I have looked at that link as well as many others. For 60s technology using fission, I'm still impressed, and I still haven't seen anything that makes me think it wouldn't work.
Ryan_m_b
#66
Jun2-11, 12:33 PM
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Well I'm happy to change my stance on the basis of the distance. I can't actually find the link I got mine from.

What do you mean by calculating the resultant velocity from E=mc2? Are you trying to go directly from mass -> energy -> momentum? I still fail to see why you think an antimatter rocket would be less efficient than Orion.

The fact still remains that project Orion (and for that matter Daedalus) were both concepts, not fully worked blueprints. They little more bearing as a realisable product as Da Vinci's drawings of a helicopter. Note that I'm not saying that nuclear fission/fusion are not potentially good propulsion technologies, I'm objecting to the notion that we've got it pretty much all figured out.
qraal
#67
Jun3-11, 11:10 PM
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Quote Quote by ryan_m_b View Post
Well I'm happy to change my stance on the basis of the distance. I can't actually find the link I got mine from.

What do you mean by calculating the resultant velocity from E=mc2? Are you trying to go directly from mass -> energy -> momentum? I still fail to see why you think an antimatter rocket would be less efficient than Orion.

The fact still remains that project Orion (and for that matter Daedalus) were both concepts, not fully worked blueprints. They little more bearing as a realisable product as Da Vinci's drawings of a helicopter. Note that I'm not saying that nuclear fission/fusion are not potentially good propulsion technologies, I'm objecting to the notion that we've got it pretty much all figured out.
Orion is described in great detail, sufficient to begin construction. They fully intended to build it, but the Test Ban and Outer Space Treaty ended hope of using it in a civilian role. But the fission version was insufficiently energetic for an interstellar mission. Dyson did a sketch of that in a 1968 paper, but that's as far as that got.

Daedalus was more than a concept. The ignition system is described in great detail in the original reports and the rest of the vehicle was deliberately designed using known or near-term technology. The only "futuristic" parts were the computer system and the need for gas-mining Jupiter via gas-core nuclear rockets.

Alan Bond & Tony Martin, who led the Daedalus study, went on to design World-ships for interstellar colonization. They would've been immense, with cruise speeds of just 0.005c, but propelled by gigaton nuclear pulse units ignited by accelerator driven ignition units. Gargantuan but not inconceivable if O'Neill-style space colonies became the normal habitats of much of humanity. Definitely not "near-term" but not a big techno-stretch.
sophiecentaur
#68
Jun4-11, 01:49 AM
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I think you are taking the term "detailed design" here a bit more literally than can be justified.
The very best one can hope for here would be broad feasibility studies as no enough is known of the practicalities.
Aamof, there are two ways in which 'efficiency' affects design. The amount of energy actually involved in refining 'fuels' and building the unit is highly relevant and should not be dismissed when considering feasibility.
Lsos
#69
Jun4-11, 07:16 AM
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Quote Quote by ryan_m_b View Post
What do you mean by calculating the resultant velocity from E=mc2? Are you trying to go directly from mass -> energy -> momentum? I still fail to see why you think an antimatter rocket would be less efficient than Orion.
Yes, that's what I did....mass-> energy -> velocity....if just to establish a theoretical limit and compare it to the figure given in the article. What I gathered is that the article is being conservative.

Why would an antimatter rocket be less efficient? I'm not saying it would be less efficient, just that it might have similar (or more) sources of inefficiency. The article explained nothing about their antimatter concept, other than that antimatter could be used as a fuel. This leaves us to speculate all we want. First thing that comes to mind is that simply containing the antimatter could be a 1x - ?100000x larger pain in the *** than encasing some uranium. For all we know they're just using the antimatter to heat water and throw it out the back. They really gave us nothing to work with. The article wasn't informative at all and seemed like they shot from the hip to arrive at the specific impulse figure. Not only that, but their very own estimate for antimatter specific impulse was only 100x better than fission. It seemed designed for nothing more than to incite interest, and certainly not as a foundation from which to invalidate Orion.

Note that I'm not saying that nuclear fission/fusion are not potentially good propulsion technologies, I'm objecting to the notion that we've got it pretty much all figured out.
Ok I'll go with that. It seemed like you were dismissing the very concept as hogwash, but maybe it was necessary to balance out my overly optimistic vision.
Ryan_m_b
#70
Jun4-11, 07:23 AM
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Quote Quote by Lsos View Post
Ok I'll go with that. It seemed like you were dismissing the very concept as hogwash
Oh no no no, far from it. In my view nuclear propulsion could be a good way of achieving interplanetary travel, however there are potentially better technologies (e.g VASIMR) than lobbing a radiation inducing, EMP producing nuclear bomb out of the back of your ship.

My peeve is the idea that there are firm blueprints to how to build these things, lot's of work has been done on the idea but its nowhere near the stage where we have a "Project Orion kit, just add money!".

On the subject of colonising off Earth we have the collection of troubles I outlined that are nothing to do with propulsion (i.e establishing a biosphere, industry, society) etc. There is a perception I regularly come across that all we need to live in space is better rocketry, but there's so much more left to do!

but maybe it was necessary to balance out my overly optimistic vision.
Maybe you can counteract my pessimism
Lsos
#71
Jun4-11, 08:03 AM
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Quote Quote by ryan_m_b View Post
On the subject of colonising off Earth we have the collection of troubles I outlined that are nothing to do with propulsion (i.e establishing a biosphere, industry, society) etc. There is a perception I regularly come across that all we need to live in space is better rocketry, but there's so much more left to do!
Yes this is a problem that I didn't even want to touch. We can't seem get along on the whole planet, and have been (are?) not far from destroying the entire thing. A colony spaceship? No...I don't see any solution except figuring out how to kill everyone on board and reviving them at the destination.
sophiecentaur
#72
Jun4-11, 03:29 PM
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I think you should be taking into account just how much water you would need in order to get hold of enough Deuterium / Tritium for all this Fusion Fuel. It may be fun to talk of 'concentrated' fuel for a starship but is it actually available? Have you considered the practicalities???


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