Large ship propulsion. (Orion revisited)

[heretofore]

Colony ship propulsion

Hello. I've identified a huge problem in my novel and would appreciate any help.

Short version:

How much mass can you safely move, given 100-200 years travel time, to a nearby star system using the world's nuclear stockpile for pulse propulsion? This would be from Earth orbit to insertion around a similar planet.

Long version:

I'm on the second manuscript of a sci-fi series, which assumes the premise of a virtual multiverse. As such the systems programmers will offer 21st century humanity limited black-box technology: One aspect of the novel is how to marshal proven and theoretical technology, not wave a magic wand. The planet Gaia (in a parallel universe) is doomed and we have a scant sixty years to build a colony ship/biosphere. One of many problems: How to accelerate a large vessel out of the solar system and to a new planet, say 10-15 ly away within a century or two.

The ship is a modified Stanford torus, with counter-rotating "wheels" and a cylindrical hub from which we can provide our main thrust. The wheels are segmented so each biosphere can pivot during acceleration/deceleration. It will be assembled in orbit: We are giving doomed humanity all the launch capability it needs, via space elevators and whatnot. According to my research, anti-matter is too problematic to even consider for rocket fuel. But since we're not worried about leaving a trail of radiation behind, it seems that the best method is off-the-shelf nuclear pulse propulsion, a la Orion:
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) and http://en.wikipedia.org/wiki/Spacecraft_propulsion

Let's assume we have 1975's world nuclear weapons stockpile to play with (about 25,000 megatons yield). Dyson's figures show 280 kt total yield used to push about 10,000 tons of mass for interplanetary travel. Does this scale up to mean that 25,000 mt of yield will accelerate/decelerate 2.7 trillion tons to a nearby solar system? Naturally we would use the large area of the aft ring to spread a solar sail for start/finish of the trip. Other helps: whatever makes sense for ion thrusters around our rings; gravity assists where possible; launching fuel supplies in advance to be retrieved along the way; scaling the ship down (tugboat instead of a cruise ship).

Finally, if it would be possible to build, the ship I came up with has four rings with a total volume of approx. 1,400 cubic miles. Let's say for calculation purposes it contains 10% water and 90% air (1 atmosphere). I come up with 654 billion standard tons. We give ourselves lightweight nano-materials in the structure, hub, spokes and machinery. Add people, flora, fauna, supplies, fuel, etc., and round up to 1.5 trillion tons (1,360 trillion kg).

Throw in gobs of brilliant engineering, impeccable programming, laser weapons for space junk, load the nukes and light the fuse. What'd I miss?

Thanks in advance.

 

[Vanadium 50]

If you want to go to a significant fraction of the speed of light, you need to convert a significant part of your spacecraft to kinetic energy. If you want to travel at 1/10 the speed of light (and slow down again on the other side), you need to convert 1% of the mass of your spaceship to energy.

In a hydrogen bomb, a much smaller fraction of the mass is converted to energy: about 0.2% of the fuel, which is itself much smaller than than the warhead.

Since 0.2% < 1%, this won't work. (Especially as 0.2% is probably 0.05% or even less)

 

[Ryan_m_b]

IIRC using nuclear explosions in this fashion could give a specific impulse anywhere from a few kiloseconds to a few hundred kiloseconds. The problem is the sheer amount of power you would need, remember the majority of energy from the explosion is lost (it doesn't hit the ship) and every few seconds you are hitting the back of the ship with a hell of a lot of radiation. If you want to go down this road it would be easier to assume that along with their technology of skyhooks and the like this society has access to fusion rockets.

It seems quite unlikely that you would to be able to accelerate over a trillion tonnes to .1C by throwing nuclear bombs out the back of the ship, most of your mass would have to be taken up by nuclear bombs to achieve that. .001-.01 might be more likely (though it gives you a travel time of thousands of years) if you posit gigantic fuel reserves for fusion rockets. The biggest problems I see though arent to do with your propulsion;

- Why is planet Gaia doomed? If you are suggesting that this civilisation can build an interstellar arc in a number of decades that has the ability to maintain and build a habitable ecosystem from scratch then why can't they save their planet? Why do they even have to leave their system?

- In addendum to the first point if its not an ecological problem why can't they move to other planets in their system? If it is an ecological problem but they lack the ability to maintain an ecosystem how are they going to build one at the other end?

- What social and political set up is this arc ship going to have? There are no human societies that have existed unchanged for hundreds or thousands of years (they may have the same names but they are far different cultures). How are you going to keep the society going? What happens when a war/revolution/totalitarian rule is enacted?

- How is the ship going to be maintained? Whilst your proposed vessel is large it will have to on board multiple factories capable of making anything from refined iron to superconductors, from crop seeds to vaccines. How big is the crew going to be to be able to run all this? During the voyage the ship isn't going to have access to raw materials. What will it do when critical components run out

- It takes millions of specialist experts to run a technologically advanced civilisation. How will your ship accommodate millions of people?

I realize this has gone slightly longer than the question about propulsion but interstellar arcs are simple in idea and horrifically complicated in practice.

 

[heretofore]

If you want to go to a significant fraction of the speed of light, you need to convert a significant part of your spacecraft to kinetic energy. If you want to travel at 1/10 the speed of light (and slow down again on the other side), you need to convert 1% of the mass of your spaceship to energy.

In a hydrogen bomb, a much smaller fraction of the mass is converted to energy: about 0.2% of the fuel, which is itself much smaller than than the warhead.

Since 0.2% < 1%, this won't work. (Especially as 0.2% is probably 0.05% or even less)

In my hypothetical example we have a distance of 10-15 ly to reach in 100-200 years. So a 15 ly journey taking 200 years works out to an average velocity of 50 million mph -- far greater than our Voyager 1's 35,000 mph, but only 0.075% of c.

In any case, I'm unfamiliar with the subject of what could conceivably be utilized in modern society to propel a large mass "from here to there" and so I came across the Orion Project. According to Dyson et al, (I think) there would be enough weapons-grade nuclear material on Earth, given the right configuration and pusher plate set-up, to accelerate/decelerate a 1.5 trillion ton spaceship to a nearby star system. I was just wondering if my assumptions from the Orion Project are correct.

 

[heretofore]

IIRC using nuclear explosions in this fashion could give a specific impulse anywhere from a few kiloseconds to a few hundred kiloseconds. The problem is the sheer amount of power you would need, remember the majority of energy from the explosion is lost (it doesn't hit the ship) and every few seconds you are hitting the back of the ship with a hell of a lot of radiation. If you want to go down this road it would be easier to assume that along with their technology of skyhooks and the like this society has access to fusion rockets.

It seems quite unlikely that you would to be able to accelerate over a trillion tonnes to .1C by throwing nuclear bombs out the back of the ship, most of your mass would have to be taken up by nuclear bombs to achieve that. .001-.01 might be more likely (though it gives you a travel time of thousands of years) if you posit gigantic fuel reserves for fusion rockets.

I do anticipate the use of fusion reactors to power the ship. But I didn't know if hypothetical fusion rockets would out-perform fission a la Orion pusher plate. Now I'm obviously not a rocket scientist, so bear with me. I looked at what George Dyson outlined:
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Sizes_of_Orion_vehicles

This shows a 10,000-plus ton ship for "Advanced Interplanetary" travel using 800 bombs of 0.35 kt yield each. That's a total yield of 280 kt. Now since the world's peak nuclear weapons stockpile (1975) had a yield of 25,000 megatons, that appears enough for a 2.7 trillion ton ship. What am I missing? If the Orion figures are correct, and there's no more efficient concentration of power than a nuclear bomb (other than anti-matter), then why fix what ain't broken? Unless, of course, a fusion rocket would out-perform a fission rocket in weight-to-thrust. If so, we do fusion rockets.

The biggest problems I see though arent to do with your propulsion;

- Why is planet Gaia doomed? If you are suggesting that this civilisation can build an interstellar arc in a number of decades that has the ability to maintain and build a habitable ecosystem from scratch then why can't they save their planet? Why do they even have to leave their system?

Because that is one of the subplots of the novel. They are being supplied space-elevator technology from another race, who traveled to Gaia in the first manuscript. In the second part, they are surely doomed. The orbit of Gaia will be greatly perturbed by a rogue planet flyby, elongating it to intersect with that of Mars.

- In addendum to the first point if its not an ecological problem why can't they move to other planets in their system? If it is an ecological problem but they lack the ability to maintain an ecosystem how are they going to build one at the other end?

Theirs is a mirror of our system. Scratch Mars. It shares a co-equal fate. The sub-plot is to get humanity to do whatever it can, with great assistance, to build an ark and save however many thousands can fit on the ark ship.

- What social and political set up is this arc ship going to have? There are no human societies that have existed unchanged for hundreds or thousands of years (they may have the same names but they are far different cultures). How are you going to keep the society going? What happens when a war/revolution/totalitarian rule is enacted?

I'm not even sure the second manuscript will get to that point. It's a bit much to cover six decades. So I have to resolve issues A-R before I can work on issue S. Still, I've thought about the matter. If you're going to build a society (in this case say 10,000 to 50,000 people depending on final spaceship design), you have certain advantages and disadvantages. You can control the makeup. Disadvantage is the enclosed space, which must be addressed in the design.

- How is the ship going to be maintained? Whilst your proposed vessel is large it will have to on board multiple factories capable of making anything from refined iron to superconductors, from crop seeds to vaccines. How big is the crew going to be to be able to run all this? During the voyage the ship isn't going to have access to raw materials. What will it do when critical components run out

This aspect is far easier to envision than things like how to marshal society to build the thing in the first place. The current direction of nano-technology is to build things from the ground up, via artificial proteins. Just like in life, the resulting structure/form/product is programmed to self-assemble, given the proper environment. As far as crew size, that's about the size of the population. The point of a self-contained biosphere is you don't need any more raw materials than what you have on hand.

- It takes millions of specialist experts to run a technologically advanced civilisation. How will your ship accommodate millions of people?

Does it? It might take millions of people to participate in building a colony ship, but it won't take too many to run it. Bear in mind that if we are to use just today's cutting edge technologies, stuff that's only a year or decade away, then much of the ship will be self-diagnosing and self-repairing.

I realize this has gone slightly longer than the question about propulsion but interstellar arcs are simple in idea and horrifically complicated in practice.

True, which makes for interesting approaches by sci-fi writers. The manuscript I'm working on will take place on the ground. Aside from the huge technical issues, I suspect the big problem will be in dealing with our 21st century dysfunctional civilization. I already see a problem with the banking industry ;-)

 

[Ryan_m_b]

Whilst fission bombs convert a healthy percentage of their mass to energy not all that energy is utilised for propulsion. Think of it this way, the explosion expands in a sphere and only a fraction of that sphere is going to interact with the back of the ship. Fusion would be more efficient in this case, it also overcomes the problems with radiation beating the back of the ship like a drum.

If Mars is out of the question why not any of the moons in the system? The point I was trying to drive was how will they set up a colony on the other end? The arc ship would have to have its own self-sufficient ecosystem to support all those people. If they can build SSEs then they don't need an arc ship, they could build self-sufficient bases on moons throughout their mirror system. Does that make sense? (i hope i don't come across too criticising! This topic is a favourite of mine) I'm struggling to see the motivation for interstellar over in-system bases.

The millions of people needed are to build and maintain a technologically advanced civilisation. There are a huge number of specialised industries behind all aspects of modern life (easy example; lighbulbs are simple, ubiquitous gadgets but to make them we need an industry to mine tungsten, silica, iron etc. Industries to refine them, industries to make the parts and assemble them. Industries to distribute and industries to provide the support infrastructure of transport, power etc etc). That number can't be cut down without loosing trained workers. Even if you had all the textbooks and records of neurosurgery that doesn't make it quick or easy to train a new generation!

Now that number is cut down if you propose advanced nanotech that can break down substances to individual atoms and rebuild (or at least provide you with one super-efficient factory that can reconfigure itself to mine/refine/build everything from blocks of metal to oil, synthetic bacteria to cloned animals). It's cut down further if you posit strong AI, however this all raises the problem once again of why they need an interstellar arc? If you have strong AI then you a super-intelligent, super-fast work force in a tin (or a disk). If they could see the rogue object coming the society could just send some robots, a nanofactory and some AI supervisors to a moon of Jupiter and in a couple of months carve out a self-sufficient closed arcology with enough in-system shuttles to evacuate most of the earth. In fact they could make hundreds of in-system ships massing a few kilotonnes (easy for them if they could make a terratonne arc) which are just highly efficient, generalist, self-replicating fablabs with an engine bolted on and some robots clinging to the outside. These probes could land on asteroids/moons etc and replicate over and over. Once they reach certain milestones they build more specialised factories, in the end for the small investment of a single probe an entire self-sufficient arcology is built.

This is the fundamental problem; if their technology is too low they won't be able to do it, if their technology can achieve it then they could achieve far better far easier.

Lastly I know I've harped on about this ecology thing but if they can't build an maintain self-sufficient environments I don't see how they can colonise another world. If that world has an alien ecology then its likely to be extremely harmful to the colonists, not because of infection (highly unlikely) but because of superantigens, different atmosphere/temp etc and land that cannot grow human crops. Either way they have to have the technology to build SSEs and that brings us back to why they can't just build bases in mirror sol!

 

[Vanadium 50]

In my hypothetical example we have a distance of 10-15 ly to reach in 100-200 years. So a 15 ly journey taking 200 years works out to an average velocity of 50 million mph -- far greater than our Voyager 1's 35,000 mph, but only 0.075% of c.

No, (15 ly)/(200 y) = 7.5% of the speed of light, not 0.075%.

This factor of 100 error in speed corresponds to a factor 10,000 error in energy. You don't have the energy available to do this.

 

[heretofore]

No, (15 ly)/(200 y) = 7.5% of the speed of light, not 0.075%.

This factor of 100 error in speed corresponds to a factor 10,000 error in energy. You don't have the energy available to do this.

Thanks for the catch! It is the calculations of Project Orion I'm interested in. The British version, Project Daedalus, was set up to launch a vehicle to Barnard's Star, 6 ly away in 50 years. So some other people far smarter than I have done the calculations. And thus I'm not qualified to defend their math. I accept it at face value.

To reiterate, if the well studied, proof-of-concept complete, Orion project works for intersteller flight on a 10,000 ton vehicle utilizing 280 kt total yield, then cannot this be scaled up? It seems the Orion/Daedalus research had the problem of a small vehicle not a large one, since nuclear blasts have a lower limit.

Now if you are saying that nuclear pulse propulsion doesn't have the energy it says it has, that's one thing. But if it does, and apparently they've done their homework, then all were talking about is scale.

 

[Vanadium 50]

280 kT = 10¹⁵ J.

Assuming all of the energy is converted to spaceship kinetic energy (a dubious prospect indeed), T = 1/2 mv². For m = 10⁷ kg, that's v = 15000 m/s.

That's about the same as the speed of Voyager 2.

 

[sophiecentaur]

I should stick to good old fashioned Warp Drive. They seem to have sorted that out OK for Science fiction.

 

[heretofore]

280 kT = 10¹⁵ J.

Assuming all of the energy is converted to spaceship kinetic energy (a dubious prospect indeed), T = 1/2 mv². For m = 10⁷ kg, that's v = 15000 m/s.

That's about the same as the speed of Voyager 2.

Perhaps then I am mis-stating the Orion Project.

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Interstellar_missions

Freeman Dyson performed the first analysis of what kinds of Orion missions were possible to reach Alpha Centauri, the nearest star system to the Sun [8]. His 1968 paper "Interstellar Transport" (Physics Today, October 1968, p. 41–45) retained the concept of large nuclear explosions but Dyson moved away from the use of fission bombs and considered the use of one megaton deuterium fusion explosions instead. His conclusions were simple: the debris velocity of fusion explosions was probably in the 3000–30,000 km/s range and the reflecting geometry of Orion's hemispherical pusher plate would reduce that range to 750–15,000 km/s

In this reference:

Later studies indicate that the top cruise velocity that can theoretically be achieved by a thermonuclear Orion starship is about 8% to 10% of the speed of light (0.08-0.1c).[2] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by matter-antimatter pulse units would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light.

At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1g or 9.8 m/s2). At 0.1c, an Orion starship would require 100 years to travel 10 light years.

Emphasis mine.

Gentlemen, I'm not proposing new science, or trying to defend the calculations of people far smarter than I. As a writer I'm simply extrapolating from a small vehicle to a large one.

Now unless Ted Taylor at General Atomics, Dyson et al didn't know what they were talking about (and I don't find any research saying this), then it appears that this pulse propulsion, as Sagan quipped, would be a great use of the world's nuclear arsenal.

Thus, if 1950s-1960s technology, aborted due to the nuclear test ban, could do the trick, no doubt science fiction writing can work with that theoretical part and make a plausible propulsion system for a colony ship: "At 0.1c, an Orion starship would require 100 years to travel 10 light years." Now all I need to do is scale up.

 

[heretofore]

I should stick to good old fashioned Warp Drive. They seem to have sorted that out OK for Science fiction.

Warp Drive is science fantasy. Science fiction is based on known technology/theory and extrapolating forward. So if I did use ftl, I'd have to explain it. And that could easily be done in my scenario, which is a simulated universe. Anyone who programs a model can insert a computer cheat. But that's not what I want to do in this case.

I'd rather take advantage of known technology, such as fusion or fission driven engines. I believe it makes for better reading, anyway.

 

[Vanadium 50]

The argument from authority is proving tiresome.

I posted the calculations for you to see. You either have to accept them, change your premise, or argue Newtonian mechanics is wrong. Take your pick.

 

[heretofore]

The argument from authority is proving tiresome.

I posted the calculations for you to see. You either have to accept them, change your premise, or argue Newtonian mechanics is wrong. Take your pick.

Ahem. I said that I probably mis-stated what "authority" proposed in Project Orion, and you countered with what "authority" says about the mechanics of space flight. Now for some reason the science community has not deigned to contradict the conclusions of Project Orion, only its safety. This isn't argumentum ad verecundiam. Using a respected source in his/her field of expertise and who has not been shown wrong is not a fallacious form of argument.

Now unless you wish to show the error of Dyson's and others calculations on the matter, I would have to conclude that the paucity of scientific arguments to the contrary indicate Project Orion remains a viable vehicle for, at the very least, serious science fiction. No?

 

[Vanadium 50]

I have a PhD in physics. I am an authority.

I did show the error in the calculations - or rather, calculated them myself based on the information you provided. You can pretend the calculations give a different answer, but they don't.

 

[heretofore]

I have a PhD in physics. I am an authority.

I did show the error in the calculations - or rather, calculated them myself based on the information you provided. You can pretend the calculations give a different answer, but they don't.

Physics is a field with many fields. I have a degree in writing, which also has many fields.
In researching a matter outside my field, I would not look to a person(s) with a specialty in mechanical physics regarding quantum physics.

When I referenced Project Orion, the subject is one researched by physicists who are highly specialized in the particular area of nuclear physics involved. To wit: http://en.wikipedia.org/wiki/Ted_Taylor_%28physicist%29" ("theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, astronomy and nuclear engineering").

I do not know if your argument is with me or the calculations of nuclear pulse propulsion. Nevertheless, the conversation has been of benefit, as it caused me to look deeper into the matter. We see that even an early design shows that for http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Interstellar_missions", one would use a 3:1 weight ratio of fuel-to-ship. That means, without improvement, a 1 trillion ton vessel would require 3 trillion tons in nuclear weapons. That tells me I have to reduce the size of the ship, improve efficiency, augment thrust, or all three.

And of course the purpose of the hypothetical exercise is not to show how it can't be done, but to show how it can. Peace.

 

[Ryan_m_b]

a 3:1 weight ratio of fuel-to-ship. That means, without improvement, a 1 trillion ton vessel would require 3 trillion tons in nuclear weapons. That tells me I have to reduce the size of the ship, improve efficiency, augment thrust, or all three.

Actually the problem is far worse than that. 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 10% you would need three times this but remember you need to decelerate at the other end, that gives you a 6:1 ratio of fuel to ship if we use Am/M. Now you've 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 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 (remember the expanding sphere?). This again pushes the ratio to 1,500,000-15,000,000:1

This is all back of the envelope but I hope you understand that Orion was based on a lot of hypotheticals, the figures given are the theoretical upper limits. At the end of the day Am/M is the best fuel we could think of and it would still need a 6:1 ratio, this alone should tell you something.

I'd be interested in the answers to the previous set of questions I asked if that was ok? I am genuinely interested in your thoughts and hope that I've helped somewhat

 

[JaredJames]

Warp Drive is science fantasy. Science fiction is based on known technology/theory and extrapolating forward. So if I did use ftl, I'd have to explain it. And that could easily be done in my scenario, which is a simulated universe. Anyone who programs a model can insert a computer cheat. But that's not what I want to do in this case.

I'd rather take advantage of known technology, such as fusion or fission driven engines. I believe it makes for better reading, anyway.

Just to pick up here, the Warp Drive is actually pretty close to the Alcubierre Drive: http://en.wikipedia.org/wiki/Alcubierre_drive

I'd also note that the sci-fi / sci-fy boundary isn't particularly clear and even in sci-fi you are making assumptions about potential future technology that may or may not be possible. If you didn't and wrote purely on the basis of what is possible based on current science, it wouldn't really be fiction from that perspective. It would just be a fiction novel with scientific content.

I still remain curious as to why people feel the need to explain sci-fi. It kills it for me and from what I'm reading here you're trying to go into far too much detail.

Why can't you just say they use the propulsion method you want (nuclear explosions behind the craft) and just let it be? People will see that and understand roughly what is happening. Any more detail and you're just digging a hole - specifically for the reasons brought up in this thread.

You want warheads capable of the speeds you require? It's sci-fi, you've got them.

 

[Ryan_m_b]

Just to pick up here, the Warp Drive is actually pretty close to the Alcubierre Drive: http://en.wikipedia.org/wiki/Alcubierre_drive

I'd also note that the sci-fi / sci-fy boundary isn't particularly clear and even in sci-fi you are making assumptions about potential future technology that may or may not be possible. If you didn't and wrote purely on the basis of what is possible based on current science, it wouldn't really be fiction from that perspective. It would just be a fiction novel with scientific content.

I still remain curious as to why people feel the need to explain sci-fi. It kills it for me and from what I'm reading here you're trying to go into far too much detail.

Why can't you just say they use the propulsion method you want (nuclear explosions behind the craft) and just let it be? People will see that and understand roughly what is happening. Any more detail and you're just digging a hole - specifically for the reasons brought up in this thread.

You want warheads capable of the speeds you require? It's sci-fi, you've got them.

I disagree, literature works on the basis of the readers willingness to suspend their disbelief. The people reading Sci-Fi tend to be people who are quite keen on the science. Little lose threads can destroy a story, its quite annoying when you are reading and a fundamental basis of the story doesn't make sense. If you are reading a book where petajoules of energy are plucked out of nowhere it annoys you

 

[JaredJames]

I disagree, literature works on the basis of the readers willingness to suspend their disbelief. The people reading Sci-Fi tend to be people who are quite keen on the science. Little lose threads can destroy a story, its quite annoying when you are reading and a fundamental basis of the story doesn't make sense. If you are reading a book where petajoules of energy are plucked out of nowhere it annoys you

You might want to check the consensus on that one.

From feedback on this site in past threads, as long as the creator doesn't try to explain the science there isn't that much of a problem (providing things don't get too ridiculous or blatantly obvious errors are present).

So they have bombs capable of the required energy. As long as they don't try to explain the bombs what's the problem? If you must, don't call them nukes. Who's to argue then?

I'd also disagree that the majority are interested in the science (or have knowledge of it). The majority I've met are either simply willing to dream it's possible or can ignore what they know to go with the story. If you can't do that, then 99% of sci-fi out there is no good for you. In fact, the only thing that I've ever heard noted as being accurate is 2001.

 

[Ryan_m_b]

You might want to check the consensus on that one.

From feedback on this site in past threads, as long as the creator doesn't try to explain the science there isn't that much of a problem (providing things don't get too ridiculous or blatantly obvious errors are present).

So they have bombs capable of the required energy. As long as they don't try to explain the bombs what's the problem? If you must, don't call them nukes. Who's to argue then?

I'd also disagree that the majority are interested in the science (or have knowledge of it). The majority I've met are either simply willing to dream it's possible or can ignore what they know to go with the story. If you can't do that, then 99% of sci-fi out there is no good for you. In fact, the only thing that I've ever heard noted as being accurate is 2001.

Perhaps it's my fault but that's not what I meant. It's not a problem to have fantasy and speculation but it is important to maintain logical consistency. For example; the orginal proposal of this thread is that the world will end so an arc ship must be built. Why an arc ship? Why build a huge interstellar ship to save thousands when the same effort could result in fleets of in-system ships and save millions? It's keeping it consistent that I'd advocate, even if it's not realistic

 

[JaredJames]

Perhaps it's my fault but that's not what I meant. It's not a problem to have fantasy and speculation but it is important to maintain logical consistency. For example; the orginal proposal of this thread is that the world will end so an arc ship must be built. Why an arc ship? Why build a huge interstellar ship to save thousands when the same effort could result in fleets of in-system ships and save millions? It's keeping it consistent that I'd advocate, even if it's not realistic

Oh I see what you mean, yes you are certainly correct in that regard.

 

[heretofore]

Actually the problem is far worse than that. 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 10% you would need three times this but remember you need to decelerate at the other end, that gives you a 6:1 ratio of fuel to ship if we use Am/M. ...

This is all back of the envelope but I hope you understand that Orion was based on a lot of hypotheticals, the figures given are the theoretical upper limits. At the end of the day Am/M is the best fuel we could think of and it would still need a 6:1 ratio, this alone should tell you something.

Thanks. This is very helpful as it points out the incorrect assumptions I've made from Project Orion. My hypothetical being sci-fi, I can at least assume the theoretical limit of 3:1 for pusher plate technology, thus 6:1 for deceleration (though I expect less since the fuel weight is far less at the end). This isn't very attractive. From what I've read about antimatter we end up increasing the mass with the containment structure.

Now I can help the propulsion issue in one of several ways:

1. In the novel's scenario we trace back the history of Kapteyn's Star, which is in retrograde motion to the Milky Way. It is about 12.78 ly from Sol, but 11,000 years ago was 7 ly. It is believed to have originated from the star cluster Omega Centauri. In our alternate universe, Kapteyn's Star traverses the same path, but is followed by a rogue planet also ejected millions of years ago from the star cluster. The rogue is what causes the problems for our sister planet Gaia. BUT, if I add a second star system with a habitable planet also traveling retrograde (toward our solar system for a flyby as it were), the issue of distance and acceleration can be greatly simplified.

2. With the rogue planet perturbing Gaia's orbit out to Mars, we have an inherent acceleration. I can augment this in the scenario of the moon, which can be ejected in a three-body interaction (Gaia/Mars/Moon). This will help immensely if we can get the colony ship to hitch a ride behind the moon, which solves the issue of plowing through the Kuiper Belt's and Ort Cloud's debris.

3. I could cheat and provide black-box technology (which I'm already doing for other plot purposes). I'd rather not overdo it.

4. Concede that our colony ship will be multi-generational as it slogs its way through space for a thousand years. Yuck.

I'd be interested in the answers to the previous set of questions I asked if that was ok? I am genuinely interested in your thoughts and hope that I've helped somewhat

Will do. Thanks again.

 

[sophiecentaur]

Perhaps it's my fault but that's not what I meant. It's not a problem to have fantasy and speculation but it is important to maintain logical consistency. For example; the orginal proposal of this thread is that the world will end so an arc ship must be built. Why an arc ship? Why build a huge interstellar ship to save thousands when the same effort could result in fleets of in-system ships and save millions? It's keeping it consistent that I'd advocate, even if it's not realistic

My question would have to be "who gets to go on this lifeboat and, if I didn't get a ticket, would I be prepared to contribute to the project or would I 'join the resistance'?" That would apply to an inter-stellar project or to one within the Solar System. I'm sure there would be a lot who would ask the same question, too.

I guess it would have to involve such a long timescale in development that it would have to be a semi religious thing, involving a selection process that was approved by a lot of pulling of wool over eyes in order to get global approval. A bit like the Iraq war, perhaps?

 

[heretofore]

If Mars is out of the question why not any of the moons in the system? The point I was trying to drive was how will they set up a colony on the other end? The arc ship would have to have its own self-sufficient ecosystem to support all those people. If they can build SSEs then they don't need an arc ship, they could build self-sufficient bases on moons throughout their mirror system. Does that make sense? (i hope i don't come across too criticising! This topic is a favourite of mine) I'm struggling to see the motivation for interstellar over in-system bases.

1G gravity is the main motivation. If we build a rotating biosphere, eventually most of the life forms will want to settle on a nice cozy planet. Now I understand the logic in what you're proposing. It would make sense to stick around and utilize the solar system's resources until wise to leave. But that's not going to work for me, plot-wise. In any case, eventually we'd want to leave the solar system and find a new place we can call home.

The millions of people needed are to build and maintain a technologically advanced civilisation. ... Even if you had all the textbooks and records of neurosurgery that doesn't make it quick or easy to train a new generation!

We just have to make do. I think 50,000 highly trained people can muddle through this rough patch. No sarcasm intended. Obviously the challenges of existing aboard a colony ship are second only to building it.

(snip)
This is the fundamental problem; if their technology is too low they won't be able to do it, if their technology can achieve it then they could achieve far better far easier.

The scenario is our modern society in an alternate reality. Part of the point is a "what if" question. Our hapless sister planet is peacefully orbited by an alien race (refugees) who bring the space elevator tech, etc. The main characters are from Earth, who already are in contact with the "programmers" of the simulated multiverse (many universes). It's complicated. But anyway, the "what if" is: a) If our society faced an extinction-level event, b) if launch capability was no problem, c) if we had access to the tech to build a colony ship to save a fraction of humanity, THEN how would it play out?

Lastly I know I've harped on about this ecology thing but if they can't build an maintain self-sufficient environments I don't see how they can colonise another world. If that world has an alien ecology then its likely to be extremely harmful to the colonists, not because of infection (highly unlikely) but because of superantigens, different atmosphere/temp etc and land that cannot grow human crops.

I doubt the manuscript will even get close to touchdown. Sixty years is a lot of time to account for, so maybe the climatic ending will be Gaia blown to smithereens, everybody else packed aboard the ship in a bitter-sweet moment. Looks like we'll need psychologists to help us deal with it.

 

[heretofore]

My question would have to be "who gets to go on this lifeboat and, if I didn't get a ticket, would I be prepared to contribute to the project or would I 'join the resistance'?" That would apply to an inter-stellar project or to one within the Solar System. I'm sure there would be a lot who would ask the same question, too.

I guess it would have to involve such a long timescale in development that it would have to be a semi religious thing, involving a selection process that was approved by a lot of pulling of wool over eyes in order to get global approval. A bit like the Iraq war, perhaps?

Absolutely. The social element is the most interesting. Presenting a plausible scenario for constructing an escape ship and the tech needed, is the backdrop to the sticky issue of how it would play out in today's society. Who gets a ticket would be one of a number of issues. But the devil's in the details, so things like how to design a rotating biosphere, propulsion, etc., etc., need attention. I've learned on this thread that even theoretical reaction mass isn't sufficient for a large vessel traveling 15 ly within 200 years.

 

[Ryan_m_b]

One thing you might want to look into is the challenges of creating a biosphere. Ecologies rely on literately thousands to millions of interactions between all sorts of species. Crops for example are highly reliant on the thousands of species of soil bacteria, insects, other crops and fungi to change the soil etc etc.

I'm glad to have helped! Perhaps part of the "resistance" so to speak isn't just guerillas with guns but bureaucracy? It stands to my mind that the biggest thing hindering us besides the technology is politics. There would be endless debates (everywhere from the UN straight down through individual parliaments right all the way to two guys in a bar) right to the point Gaia/earth burns.

You could have the pro-arc lobby,
The pro-base lobby (propose bases spread around the system)
The pro-vault lobby (favour deep vaults in Gaia's crust).

Spattered around them you would have the usual suspects; the Capitalists and the Socialists arguing over which economic system would better build the ship, the liberals and the authoritarians arguing over which type of government would better build the ship, the Godbothers pronouncing it divine judgement for the "evil" of the world blah blah blah.

The social side will be hard to write but I think I'd always get stuck with what technology to give these Gaians. It would be hard to work out how giving that technology would change the society and how that would affect the plot, a bit like dropping different inks in a pot and seeing the colour change. If they have the capability to build an arc (less of a ship and more of a giant city in space) what would their society look like? Perhaps they would already have a serious presence in space. If they know how to make self-sufficient environments perhaps they have terraformed (gaiaformed?) their world turning the deserts into forests and the like.

When you publish i'd love to read it, I'm interested in how its going to pan out

 

[heretofore]

One thing you might want to look into is the challenges of creating a biosphere. Ecologies rely on literately thousands to millions of interactions between all sorts of species. Crops for example are highly reliant on the thousands of species of soil bacteria, insects, other crops and fungi to change the soil etc etc.

Yes. A huge engineering problem. I hope to add to the sci-fi literature on how to do it.

I'm glad to have helped! Perhaps part of the "resistance" so to speak isn't just guerillas with guns but bureaucracy? It stands to my mind that the biggest thing hindering us besides the technology is politics. There would be endless debates (everywhere from the UN straight down through individual parliaments right all the way to two guys in a bar) right to the point Gaia/earth burns.

I've already concluded that our current institutions are not only inadequate to the task, but will have to be cleverly co-opted or neutralized lest they become a hindrance. I think it makes for a great tension-point of having to effect such a gargantuan global effort within the context of a civilization so disfunctional it cannot come to grips with global warming.

You could have the pro-arc lobby,
The pro-base lobby (propose bases spread around the system)
The pro-vault lobby (favour deep vaults in Gaia's crust).

Spattered around them you would have the usual suspects; the Capitalists and the Socialists arguing over which economic system would better build the ship, the liberals and the authoritarians arguing over which type of government would better build the ship, the Godbothers pronouncing it divine judgement for the "evil" of the world blah blah blah.

We'll play them all like fiddles. Seriously, the challenge of competing interests/agendas is another opportunity for plot and tension. I have interesting ideas on how to handle the situation.

The social side will be hard to write but I think I'd always get stuck with what technology to give these Gaians. It would be hard to work out how giving that technology would change the society and how that would affect the plot, a bit like dropping different inks in a pot and seeing the colour change. (snip)

Well it's going to be one wildfire after another. I've decided the best strategy for the main characters is to be proactive and not simply react to how society deals with the situation. In this vein we're just going to have to admit that a socio-political Machiavellian approach is required.

When you publish i'd love to read it, I'm interested in how its going to pan out

Thanks. It takes a while from manuscript to book, and the first one is just now being shopped around. Not to spoil anything, but the main plot has to do with our protagonists interacting with the "programmers," who have their own agenda for even bothering about poor Gaia.

 

[Vanadium 50]

It's not true that if you need x times as much fuel as payload to accelerate, you need 2x to accelerate and decelerate. You need x². Most of what you need to accelerate is the fuel you need to slow down.

 

[Gordianus]

I haven't followed the full thread, but I find the discussion about spaceships most interesting.
However, as a sci-fi fan, I'd rather take good old Isaac Asimov advice:" Assume you have what you need (faster than light travel, positronic brains or time travel) and then analyze the possible consequences". I enjoy Asimov's approach. I doesn't matter how travel in time could be achieved, he envisioned possible consequences of that piece of knowledge.

 

[Ryan_m_b]

Assume you have what you need (faster than light travel, positronic brains or time travel) and then analyze the possible consequences

Very true. In that vain a society that can build an interstellar transport most likely already have established many methods of intrastellar transport. That would be interesting in how those colonies (if they are colonies) work, how they established environments, how their society works, economy, relation to Earthbound nations etc etc.