The Prophesy Chronicles - Discussion Thread

[TheBigDog]

The Prophesy Chronicles

 

I was going to name my first colony ship The Promise, but I really liked the name The Prophesy, so I am sticking with it for the 2nd generation ship.

 

I had a great deal of fun for the time that we were living out our lives on that ship, and I have often intended to revisit it. Instead I am starting this new story about the ship after the Prophesy. She is Earth's first colony ship, and will carry her crew on a centuries long voyage to settle a new world. There are many stories to be told, and many lessons to be learned as we learn with the crew the hard lessons of life aboard a spaceship going to a place you will never live to see.

 

On this thread I welcome all discussion about the mission, the characters, the ship, the story, etc. I will be posting the story itself on another thread which I will keep closed. The story is one I am telling, the discussion belongs to all of us. If anyone REALLY wants to contribute, I am surprisingly easy to get along with, and welcome all submissions. I will discuss edits and when we have both agreed that it is ready, I will add your input to the story line crediting you.

 

Our first ship had 48 living sections, each arced 15 degrees to make a circle from 12 sections. Four circles made the living section. There was a large center section as well for engines, storage, fuel, laboratory, etc. It also had a Nuclear-Pulse External Combustion Engine (NECE[i just made that up]) and was built for a crew of 100 to explore the solar system outside of the Venus orbit for 5 years without resupply.

 

The Prophesy II is much larger. She has 480 living sections that sit side-by-side in rings of 120. She has five linking rings that cap the living rings at each end. Each of these is made from arched sections that span 3 segments for a total of 40 in each ring; but they appear as continuous long hallways. The rings are about 2.5 kilometers in circumference. There are just over 96 hectares of soil space and almost 2 billion liters of water.

 

The living sections are almost entirely for agriculture. They literally are the life of the ship. Each is internally 100 meters long and 20 meters wide. Internally the ceilings are 15 meters above the floor with the lowest 5 meters dedicated to the water system and soil. The connecting rings are long hallways with access to the support systems for the living spaces.

 

The crew will be as many people as the renewing resources will support. Look for a number between 300 and 400.

 

The center of the ship contains the nuclear power plant, the Ion Engines and fuel, the zero G lab, storage areas for non-renewable supplies, and all of the equipment that will be needed to land on and settle a planet. The ship itself will never land of course.

 

Some of our technology from the Prophesy is coming back, but there is a new golden rule with this ship: NO MOVING PARTS. Moving parts wear our and need maintenance. Those are luxuries that are not in high supply after a couple centuries in space. What you will find is that life on board the Prophesy II is quite, well, primitive. Almost like living with the Amish, wrapped up in a collection of cylinders that comprise a space ship.

 

Almost everything needs to be renewable for the sustaining power of the ship to be harnessed. Think of all the things you use in your daily life that you take for granted. Some are disposable like paper products, others last longer but eventually need replacement like a tooth brush or a razor, and others last for a long time but less than a lifetime like a phone or a carpet. All of these things that wear out and need replacement either need to be very very durable, or replaceable from the living products of the ship's ecosystem. There is a huge storage capability, but 400-500 years is a long long time to plan for, and a ton of dead weight if you bring the wrong stuff.

 

So with all of that said, I am off to bed to dream of life on board the Prophesy II, and to begin my work on the first of her stories.

 

Bill

[TheBigDog]

Some more technical information. The workhorse of the assembly process is the Hercules Rocket. Pure brute force capable of putting 20,000,000 kg into low orbit. All of the launches are done from the Archipelago de las Perlas off the coast of Panama over a ten year span from 2012 to 2022. The first launch actually took place on the first day after the last day of the Maya calendar, marking the beginning of the next world.

 

Ten years, 991 launches, 19.82 billion kilograms of mass are hurled to a speed of 25000 kilometers/hour and assembled 340 kilometers above the earth into the ship known as the Prophesy II.

 

Almost two launches per week for ten years, with assembly of the rockets happening around the globe and supplied by ship from the far corners of the world. Then the gateway between the Atlantic and Pacific became the gateway to the Universe. But I don't need to tell any of you this, you all saw it with me.

 

Bill

[TheBigDog]

OK, I am going to need some help with some math. This is a fictional story, but I want to walk the line of plausibility as much as I can. The ship weighs in at a staggering 19.82 billion kilos. I have granted the Hercules Rocket with the ability to lift the pieces, now we need to get it out of earth's orbit and on with the mission.

 

For propulsion the ship has an advanced Ion Thruster that produces 500N of power. It uses Hydrogen as the fuel. I did some questionable math and found that the ship would need to run the engine for 1261.7 days to reach a velocity of 0.1C. It would presumable need to make a similar maneuver to decelerate upon reaching the target system, which would expend virtually all of its fuel. If all of my math has been optimistic to this point, this is perhaps the most outlandish optimism of all to think that there would be enough fuel on the ship to fun the engine for the equivalent of seven years without interruption. Maybe, but it needs to amount to less than about 6 billion kilos of hydrogen unless we want to have absolutely no equipment for settling once we get there.

 

So the alternative to motoring is to use some form of passive thrust. I think that a solar sail works nicely, and it would be fun to write about, but how much thrust would it actually provide? If I have the ship going to a star system that is 30 light years away, and I want the story line to be in the neighborhood of 400 years, then we need to get close to a speed of 0.1C and then back down again.

 

My question: How much thrust would be provided by a solar sail of 50 square km? How much delta V? The closer the sail can get the ship to 0.1C, the less fuel is needed for the Ion Thruster.

 

Bill

[freeztar]

I'm confused. Are you wanting to use the hydrogen as a propellant in the ion thruster? Or are you talking about two separate engines. The reason I ask is because I'm not aware of any designs that use hydrogen.

 

Ion thruster - Wikipedia, the free encyclopedia

 

Solar sails only work inside a heliosphere, I believe. Actually, scratch that. You can also use a laser. The bigger problems are an extremely high ratio of deployed size to payload, power requirements, and its slow speed.

 

What about a giant VSIMR with nuclear power?

[Donk]

You could save a lot of fuel at the start by using a gravity slingshot.

 

Wiki: Gravity Slingshot

 

It would mean spending twenty years or more within the solar system, but that does have some advantages. Planetary survey. A shakedown cruise. Smaller spacecraft would be able to match velocity for resupply, new devices, personnel changes. Easy communication with home, for technology and news updates.

[TheBigDog]

I'm confused. Are you wanting to use the hydrogen as a propellant in the ion thruster? Or are you talking about two separate engines. The reason I ask is because I'm not aware of any designs that use hydrogen.

 

Ion thruster - Wikipedia, the free encyclopedia

 

Solar sails only work inside a heliosphere, I believe. Actually, scratch that. You can also use a laser. The bigger problems are an extremely high ratio of deployed size to payload, power requirements, and its slow speed.

 

What about a giant VSIMR with nuclear power?

VASIMR is what I was targeting. I was making the leap that it would have the ability to move from Argon to Hydrogen over the next years. In the Wiki article they mention some very promising Hydrogen drives that are still in the experimental phase but I could find no details.

 

I was asking about the sail as a means to accellerate us out of the solar system so we could then drift and use the Ion drive for the transit to the other system. Then use the sail to decelerate into the other star system before using the Ion drive for fine tuning out orbits. I know that there is a decrease in efficiency of a solar sail at you move away from the power source, I am just curious about the potential of that technology to preserve fuel.

 

One of my goals is the have enough plausibility in the science that readers say to themselves "We can do that! Why aren't we already doing this?" as opposed to using science fiction devices that are the equivalent to magic.

 

Thanks for the help Freezy!

 

Bill

[TheBigDog]

You could save a lot of fuel at the start by using a gravity slingshot.

 

Wiki: Gravity Slingshot

 

It would mean spending twenty years or more within the solar system, but that does have some advantages. Planetary survey. A shakedown cruise. Smaller spacecraft would be able to match velocity for resupply, new devices, personnel changes. Easy communication with home, for technology and news updates.

 

Absolutely! I am planning on sling-shotting to help build velocity. I was also thinking along the same lines as you about resupply, etc, but the fact is that it takes as much or more energy to resupply than it does to just stack on more fuel from the start. Also, this is a one way mission, there is no crew changing. Communications will last for a long time, but what is onboard is what they have to work with.

 

Thanks for the input Donk

 

Bill

[CraigD]

Oh boy – this feels like the beginning of … Space Voyage Two :) – the next great hypography collaborative fiction! :hyper:

My question: How much thrust would be provided by a solar sail of 50 square km? How much delta V? The closer the sail can get the ship to 0.1C, the less fuel is needed for the Ion Thruster.

The fundamental physics of light sails is straightforward – conservation of momentum, where the momentum of a photon is [math]p = \frac{E}{c}[/math], where [math]E = h v[/math] where [math]v[/math] is the photon’s frequency. Though the details of working out a practical engineering equation for this are ... detailed ... the result is a simple constant that can be multiplied by the total stellar radiation power at any point and the area of a reflector (the less perfect the reflector, the less thrust it produces, down to one half that of a perfect reflector for a completely black surface) – in short, the thrust force for a particular light sail is

[math]F = 2 K P[/math],

where [math]P[/math] is received power,

and [math]K \dot= 7.3 \times 10^{-8} \,\mbox{pa/W}[/math]

 

[math]P[/math] varies with the inverse square of distance from the radiation source (the Sun, in this case) and the area of the reflector (sail), so, given [math]\frac{P}{A} = 1370 \,\mbox{W/m}^2[/math] for the Earth from the Sun, for a [math]5 \times 10^{7} \,\mbox{m}^2[/math] sail, the thrust would be about 1000 N.

 

Given a total ship mass of [math]2 \times 10^{10} \,\mbox{kg}[/math], this would give an acceleration of [math]5 \times 10^{-8} \,\mbox{m/s/s}[/math], a rate at which – assuming you could contrive the Sun to follow you around at a distance of 1 AU, would require about 19 million years to attain a speed of 0.1 c. :(

 

I think this rules out a light sail, unless it’s much, much bigger, or powered artificially, via a huge laser, as has been proposed and described in some hard science fiction.

 

More promising, I think, is Donk’s proposal

You could save a lot of fuel at the start by using a gravity slingshot.

 

Wiki: Gravity Slingshot

 

It would mean spending twenty years or more within the solar system, but that does have some advantages. Planetary survey. A shakedown cruise. Smaller spacecraft would be able to match velocity for resupply, new devices, personnel changes. Easy communication with home, for technology and news updates.

Since the [math]\Delta v[/math] gained from gravity assists is nearly independent of the ships mass, if you can manage to insert your ship into just the right planet-grazing orbit, you can get performance at least similar to the small spacecraft we’ve already ejected from the Solar System, such as Voyager 1, which, after about 32 years, is currently nearing 109 AUs from the Sun at a speed of about 0.00004 c.

 

The trick, it seems, is to plot a much more powerful multiple gravity assist course than the Voyagers. I see at least these few major enhancement possibilities:

1. Plot a different course. The Voyagers were designed to go interesting places – to planets – and dwell there as long as practical. If your primary interest isn’t sightseeing science, you should be able to get more from a series of gravity assists.
   
2. Use more than just gravity. As the name suggests, gravity assists involve acceleration due to the gravitational force of a large body – a planet or moon – on a spacecraft. This isn’t, however, the only force field many large bodies have – most have powerful electromagnetic fields, also. With some novel engineering, it may be feasible to use electromagnetics to gain a dramatically greater change in momentum from each slingshot pass
   
3. Get a lot of found fuel. Even if most of its [math]\Delta v[/math] comes from gravity – no, let’s call them planet assists, a 20 billion ton ship will need huge amounts or reaction mass simply for course corrections. Rather than carrying all of it, the ship can be a “scoopship”, collecting matter from the upper atmospheres of planets, expecially the gas giants
   

There are a devilish heap of details to be worked out for these and other speculations, but that’s rather the point of the thread, eh? :)

 

From a story plot perspective, that the ship would be a long time – likely at least 1, maybe several human generations – within reach of Earth by small spacecraft, is a major twist, going against the old space fiction cliché of short goodbyes and the ship as an isolated worlds unto itself. Such a generation ship might not be more than partially crewed for its first few generations. Its “shakedown” could be the work of several lifetimes. And – perhaps most dramatic and hard to imagine and plan for – its technology – the parts that are small and replaceable, such as its computers – will likely be generations more advanced than its originals.

 

Challenging stuff to write, as the key characters are likely to be a succession of great-great-etc. grandchildren – challenging to stir identification and empathy for – though immortal characters, especially artificial, computer-based ones, are reasonable.

[TheBigDog]

Thanks Craig.

 

I guess all the the work I did last night figuring out how the rigging of the sails might be made practical was another of Edison's light bulbs that doesn't stay lit.

 

That brings me to a question about Gravity Slingshot. If we use slingshots inside the solar system to maximize our velocity before heading into deep space, can we use another star to accelerate even more?

 

Or perhaps we should pick a target that is closer, so we don't need as much speed to get there in 400 years. List of nearest stars - Wikipedia, the free encyclopedia

 

Cool stuff!

 

Bill

[TheBigDog]

Of course we could search by the list of extra solar planets as well...

 

List of extrasolar planets - Wikipedia, the free encyclopedia

 

And the other alternative is to simply go purely fictional with the choice of star system. Let me eloborate on this some more...

 

This mission presumes that the Prophesy mission of launching God's Eye was a success. This was a large array telescope placed in the Lagrangian Orbit L2 of Jupiter; forever shadowed from the sun. It was a collection of 24 Hubble equivalent devices that work on a much wider range of frequencies. It was being used to find and study potential settlement planets. In the story we have all the information from God's Eye to understand what we are going to find when we get there. This does not mean we know exactly what is there, but it means we can play some percentages.

 

Our ship should be set on a course such that if it arrives at a star system and upon closer reflection finds that it is not a candidate, that it will use that star to help in a course correction to move on to the next best candidate.

 

The ship itself is equipped with advanced observation equipment so the study of the system becomes more detailed with each passing year as they approach, so they may make the decision that they are going to bypass a system generations before they get there. This will play out during the story telling. Such a decision has long term impact on this tiny civilization and cannot be made lightly.

 

I am also planning on using elements of the discussions here in the debates among the characters in the story, so all questions and criticisms are welcome. It gives flavor to the soup.

 

Bill

[TheBigDog]

The workhorse of the Prophesy Space Program is the Hercules Rocket. It a one stage to orbit Goliath with the ability to hurl 20 million kilos into orbit at a punch. I have two numbers as my starting point here: 20 billion kilos as the payload, and 20 billion kilos as the dry weight of the rocket system.

 

As a Hydrogen/Oxygen rocket, I estimated a requirement of 200 billion kilos of fuel to complete a maximum payload launch. To carry that much Oxygen and Hydrogen in a tank similar in shape to the Space Shuttle main fuel tank, the dimensions would be something like 40 meters in diameter and 180 meters high. Inside that tank would be two cylinders with rounded ends holding the two liquids; a large one for Oxygen and a smaller one for Hydrogen. Throw in another 20 meters at the bottom for the rocket motors and you have an empty rocket that stands 200 meters tall. Some of the payloads will be 100 meters tall, so at launch you will be sending essentially a 300 meter tall building into space. And repeating this process twice per week for ten years.

 

Sounds simple enough! The industry to keep this supply chain flowing would be in itself one of the largest efforts in human history.

 

Bill

[Donk]

I am planning on sling-shotting to help build velocity. I was also thinking along the same lines as you about resupply, etc, but the fact is that it takes as much or more energy to resupply than it does to just stack on more fuel from the start. Also, this is a one way mission, there is no crew changing. Communications will last for a long time, but what is onboard is what they have to work with.

As you say, fuel or food resupply would not be viable. I was thinking of small items - computer chips, nanotechnology items, new seed varieties, germ plasm. Plus any items that come under the generic heading of "ohmigod, we forgot to bring a can opener!" :)

 

It would take a lot of fuel to travel from Earth orbit to the gas giants, matching speeds with Prophesy at the end of the trip. Carrying enough fuel to reverse the process would increase the costs massively. An unmanned craft could take its time going back, using the same slingshot technique to work its way slowly into Earth orbit. Manned, it would have to get there much more quickly, and would need to carry all that heavy life support stuff. So I suspect the trip would be mostly robotic, and possibly one-way only.

 

If a special individual is needed on the expedition, no doubt it could be done, at horrendous expense - which could make an interesting plot point along the way.

 

The reverse could be even more interesting. The chief designer/engineer is persuaded to stay aboard to deal with the many last-minute problems. S/he is given an absolute guarantee that a way will be found to get him/her off after no more than five years. Unfortunately...

[CraigD]

That brings me to a question about Gravity Slingshot. If we use slingshots inside the solar system to maximize our velocity before heading into deep space, can we use another star to accelerate even more?

Yes, absolutely. Orbital mechanics is orbital mechanics, depending little for the scale and composition of the bodies, or time scales, involved.

 

:) It gets even more fun when you consider the possibility that there may be more giant planet-to-small star-mass objects between visible stars than we can currently see – brown dwarfs, or, being a bit more speculative, more exotic bodies, like quiet neutron stars with anomalous velocities, or perhaps the ultimate gravity assist bodies, fast-moving black hole with little or no accretion disks.

 

Detecting and plotting gravity assist maneuvers with one or more of these could yield some big delta-V – and works as well with a humongous spacecraft as with a tiny one.

 

We’ll have to do some real, albeit approximate, physics to see what sort of speeds can be had achieved via gravity assists from various bodies – at this point, I can’t hazard a guess to within a factor of a thousand. For me, this’ll require some serious sit-down time – and give me an excuse to dust of and resume work on my beloved simulators. :P

[TheBigDog]

As you say, fuel or food resupply would not be viable. I was thinking of small items - computer chips, nanotechnology items, new seed varieties, germ plasm. Plus any items that come under the generic heading of "ohmigod, we forgot to bring a can opener!" :)

 

It would take a lot of fuel to travel from Earth orbit to the gas giants, matching speeds with Prophesy at the end of the trip. Carrying enough fuel to reverse the process would increase the costs massively. An unmanned craft could take its time going back, using the same slingshot technique to work its way slowly into Earth orbit. Manned, it would have to get there much more quickly, and would need to carry all that heavy life support stuff. So I suspect the trip would be mostly robotic, and possibly one-way only.

 

If a special individual is needed on the expedition, no doubt it could be done, at horrendous expense - which could make an interesting plot point along the way.

 

The reverse could be even more interesting. The chief designer/engineer is persuaded to stay aboard to deal with the many last-minute problems. S/he is given an absolute guarantee that a way will be found to get him/her off after no more than five years. Unfortunately...

I like the way you think. the thing to remember is this is a real lumbering beast. It is going to move away from earth very very slowly. Remember in the Movie "The Jerk" when Steve Martin left home and started hitch-hiking? His family is sitting around the dinner table that night and wondering how he is doing, so one of his brother's opens the window and shouts the question to him; he is still by the front gate with his thumb out. This ship is going to make that type of exit. It is possible that it takes years or more simply to exit earth's orbit, unless a temporary chemical rocket system is employed to push it away in one stupendous shove. I am not sure what the delta V is for going from LOE to escape velocity. But that is a good question...

 

How much energy is needed to accelerate 19.82 billion kg from orbital velocity past escape velocity? Subtlety is welcomed, but brute force is not out of the question.

 

Bill

[TheBigDog]

From a story plot perspective, that the ship would be a long time – likely at least 1, maybe several human generations – within reach of Earth by small spacecraft, is a major twist, going against the old space fiction cliché of short goodbyes and the ship as an isolated worlds unto itself. Such a generation ship might not be more than partially crewed for its first few generations. Its “shakedown” could be the work of several lifetimes. And – perhaps most dramatic and hard to imagine and plan for – its technology – the parts that are small and replaceable, such as its computers – will likely be generations more advanced than its originals.

 

Challenging stuff to write, as the key characters are likely to be a succession of great-great-etc. grandchildren – challenging to stir identification and empathy for – though immortal characters, especially artificial, computer-based ones, are reasonable.

One of the things I was considering is the ship is hundreds of years from earth when they finally arrive at the planet and find that it has already been settled by a ship that was built 200 years later and arrived 100 years earlier. But that is a long way off.

[Donk]

One of the things I was considering is the ship is hundreds of years from earth when they finally arrive at the planet and find that it has already been settled by a ship that was built 200 years later and arrived 100 years earlier. But that is a long way off.

You might want to check out A.E. van Vogt "Far Centaurus" (1944), in Destination Universe. Four astronauts reach Centaurus after 500 years spent mostly in suspended animation. When they get there they find a thriving human civilisation, with ships that can do the trip in a half hour :D

 

In the same book there's another possibly-relevant short story, "The Sound". A massive spacecraft which will take decades to complete. The people building it have the right technical skills, but they also have children who explore the growing ship and are allowed - encouraged - to go just about anywhere. The crew will be formed from those children. Larry Niven used a similar plot point in Oath of Fealty. The only people who don't feel lost and intimidated by the giant building they live in are the guy who designed it and the children who are growing up in it.

[CraigD]

How much energy is needed to accelerate 19.82 billion kg from orbital velocity past escape velocity? Subtlety is welcomed, but brute force is not out of the question.

Energy calculation for rockets can be less useful than one might expect.

 

This question, for example, has an easy answer, if one assumes 100%-ish efficiency in whatever does the accelerating: it’s equal to negative of the ship’s GPE,

[math]E_{\mbox{GP}} = -G \frac{M_{\oplus} m}{r}[/math]

Assuming the ship starts in a low Earth orbit of 350 km altitude (about [math]r=6728000 \,\mbox{m}[/math]), this gives about [math]1.176 \times 10^{18} \,\mbox{J}[/math] – about the same as the electric energy used at present by the US in one year.

 

However, most near-Earth spaceflight isn’t 100% energy efficient, but closer to 50%. For example, you could get a [math]1.982 \times 10^{10} \,\mbox{kg}[/math] ship from LEO to escape with the 1.2 EJs calculated above by, say, tying it to a 46,746 km (10,960 km above geostationary orbit) space elevator, then run it up to the elevator’s “Earth escape terminus” (the energy would be in the form of whatever ran the various winches, elevator motors, and elevator prevent-from-getting-pulled-out-of-whack systems needed) – but first, you have to build a giant space elevator. ;) Using something like VASIMIR engines ([math]v_e \dot=350000 \,\mbox{m/s}[/math]) and Earth escape speed ([math]v \dot=11000[/math]), efficiency will be around 60% (

[math]n_p = \frac{2\frac{v}{v_e}}{1 +\left( \frac{v}{v_e} \right)^2} = \frac{2\frac{11000}{350000}}{1 +\left( \frac{11000}{350000} \right)^2}[/math]

), and will require about 0.22 billion kg of reaction mass (

[math]M_e = \frac{m \cdot \Delta V }{I_{sp} g} \dot= \frac{2 \times 10^{10} \cdot 3300}{30000 \cdot 10}[/math]

) – not too daunting, expecially given the cost of lifting the 20 billion kg ship into LEO, though this is just the first leg of the voyage.

 

With rocket-propulsion, there’s an inverse relationship between energy efficiency, and the amount of reaction mass need to accelerate a payload, so any rocket design for a ship like the Prophesy 2 will almost certainly seek to have even higher [math]I_{sp}[/math] – and thus exhaust speed – that a VASIMIR, in order to minimize reaction mass requires, at the expense of energy efficiency. In this case, in an inversion of ordinary Earth engineering, energy’s cheap, inert mass expensive.

 

(sources: wikipedia articles ”propulsion efficiency”, “specific impulse”)

 

PS: if you value an approximation of accuracy, check my calculation! ;)