help with interstellar design? (for a story)

[toric5]

i am currently in the process of writing a hard si-fi story (not planning on publishing it, I'm not the best writer) and i would like some input on my preliminary notes. if someone would be so kind as to help out with fine tuning the design of the craft some more, that would be appreciated. (also the name is a bit unoriginal, if anyone has a better name, pleas suggest it)

also, not sure if this is the right forum to post this on, but it IS mostly about a spacecraft. if i messed up, please move this.

Design of the Voyager generationalcolony spacecraft

• 
  
• named after the first man made object to exit the solar system.
  
• dual centrifuge rings that counterbalance each other, looks like like 2001 space odessy station, but they spin in opposite directions.
  
• Long central spine, engines at the front pulling rest of ship
  
• engines angled several degrees outward to keep exhaust from frying the rest of the ship.
  
• Some sort of 0g observatory near the back? (possibly 0g recreation somewhere as well?)
  
• pressurized tunnel in the truss, allowing access to much of the ship.
  
• Could some of engine waste heat be used as life support heat? (as design or in survival?)
  
• relatively simple propulsion, not antimatter, fusion maybe?
  
• Closed cycle life support, algae farms, yeast, made into palatable dishes?
  
• Communication system with earth? (not superluminal!!!)
  
• repairs and maintenance done by remote control repair bots. (not fully autonomous)
  
• cargo for setting up colony on extrasolar planet
  
• way to angle floors slightly due to offset gravity during thrusting??? (would that be necessary?)(i do want rings, more space for population that way)
  

story plot outline

• 
  
• going to star with possibly habitable exoplanet, explored by probe some 90 years before voyager departed.
  
• ship breaks in half when one of the centrifuge rings jams. (get something to make it jam?)
  
• torque from the jam snaps spine, craft now in 2 pieces.
  
• this occurs about 2 generations into the journey? (during the coast phase)
  
• Main character is in the observation deck, relaxing, when break occurs
  
• possible end of story is in getting the spine repaired, and the survivors repair the ship to functionality again? (you want an emotional ending? How bout an epilogue where the main character, now an old man, (children?) witnessing the crafts arrival at the system?)
  anyway, thats my outline, what do you think?
  

[Bill Phil]

Sounds vague enough to convince people... It's good not to get into specifics for a hard-sci fi story. The only criticisms I can come up with are naming and the rings. I mean, wouldn't it be smarter to use a cylinder? You have less surface area exposed to the velocity vector, and so less shielding is needed. But you could still make it so that the craft splits into two, maybe some seperators for the truss fire prematurely?

I think it should be either the Wernher von Braun or the Isaac Newton.

(Come to think of it, when I get around to writing my hard sci fi stories I'll take advantage of the Oneill Cylinder Starship...)

[crubs]

Maybe instead of simply having the centrifuge jam, it could be an act of terrorism by some nutty anti-human fundamentalist or something...

Otherwise, about how many people do you plan to have on this ship? I'm assuming it's hundreds at least.

[Bill Phil]

Cylinders also have, for this velocity vector, a more efficient shape volume wise is a cylinder.

[Newt]

Look at some of the entries to the contest at :http://settlement.arc.nasa.gov/Contest/

That is a highschool student competition from NASA Ames, for the design of hypothetical large (10.000 or more inhabitants generally) orbital space colonies, doubtlessly inspried by O'Neil's work. Most of the grandprize winning space colony designs are on the site, and some of the information you can get from it certainly could be relevant to you spacecraft. One of the 2012 winners (Kon Tiki) seems especially similar, in that it has to deal with interstellar travel.

[Nibb31]

Counter rotating rings are a recipe for disaster in terms of reliability and maintenance. As you demonstrate, it's more likely to break. It's much easier and cheaper to simply rotate the entire spacecraft.

[toric5]

i was thinking 200 to 400 population.

i know its quite vague in the description, but thats kinda why I'm here. I frankly have no idea what sort of mass this thing would have. i would ideally like to go from this to a pretty good idea of mass, propellant fraction, etc.

as for the cylinder design, that could work, but how much faster would it have to spin? would there be any problems with coriolis effects? also, it would possibly still have to deal with the gravity vector changing while the ship is under acceleration. (however slight.) On the kon tiki, they solved this with the beaded torus design. (could be adapted for less than 1g of acceleration) is there any way you could do this with a cylindrical section?

yes, i could make it so the whole spacecraft rotates, but that kinda destroys the plot. (still want to think of an excuse. star tracking navigation system that has to be steady?)

[Nibb31]

as for the cylinder design, that could work, but how much faster would it have to spin? would there be any problems with coriolis effects? also, it would possibly still have to deal with the gravity vector changing while the ship is under acceleration. (however slight.) On the kon tiki, they solved this with the beaded torus design. (could be adapted for less than 1g of acceleration) is there any way you could do this with a cylindrical section?

I don't see what the problem is. There is no difference in terms of Coriolis effects between a ring and a cylinder. The spacecraft only accelerates during a period at the beginning and end of the journey. I'm assuming that the action takes place during cruise mode, so there shouldn't be any propulsion events.

Another design assumes a steady 1g acceleration during half of the flight, and a steady 1g deceleration for the other half. This gives you natural 1g gravity for the entire flight without rotation. It does require massive amounts of power though.

yes, i could make it so the whole spacecraft rotates, but that kinda destroys the plot.

In that case, just do like most science fiction writers do, and wave away reality when it bothers the plot. But if you want your spaceship to be a realistic design, no mechanical engineer would ever build one with ANY moving/rotating/unfolding parts that aren't absolutely necessary. They would design the ship specifically to minimize mechanical failures.

(still want to think of an excuse. star tracking navigation system that has to be steady?)

In that case, you would have a rotating ship and an unpressurized rotating mast with the startracker sensors, it would be much easier than having a massive pressurized rotating joint that is bound to wear out after years of constant friction. But even then, a civilization that can build generation-ships surely has the means to use multiple sensors around the circumference of the ship to produce non-rotating imagery with computers.

Edited January 11, 2015 by Nibb31

[Newt]

Acceleration from rotationally generated psuedogravity can be found from the equation a=v^2/r. Here the Coriolis force comes into play because of the difference between gravity change over a persons length, mostly, but is circumvented by the idea of continuous 1g acceleration by the vehicle itself. However, that does sound like not what you are going for.

Some things are easier with a non rotating component (think docking) not impossible, but made far simpler.

What exactly needs to be on the separate nonrotating section? It would be simpler to think of an excuse if that were known. Also it would be simpler to decide mass if roughly the power source, engine type, and other things were. Have you looked at the Atomic Rockets page? This has a mass of good information for this sort of project, and I suggest you browse some of the stuff there if you have not already. As it is most of what you have for the ship seems pretty up in the air, and it would be helped by stating some starting points. Many of these could be justified by things that we do not understand today, really, efficiency of the specific machines, or how long the technology has been being developed, that sort of thing.

[Mazon Del]

Possible reason why the ship is designed with the two contra-rotating rings: The project is financed by some billionaire as a philanthropic gesture (she/he is either part of the original crew, or was never part of the crew) and they liked the design and said "Screw the complexity, my money, my design.". I've heard rumors that within SpaceX a few 'sound engineering decisions' were thrown out in favor of 'cooler' ones because Musk said so. Again, just rumors, no idea if true.

One thing that may hurt or help is that chances are, the ship wouldn't have a cargo bay filled with colony building equipment. It would be far more efficient to show up with cargo holds filled with raw materials, and 3D printers capable of constructing what was needed. Most efficient would be to have nearly empty cargo holds that get filled with some asteroid mining once in system. Most intelligent would be to have some level of base materials with them, but mostly empty holds. This could end up being one point of contention where they say "We could just fire up the printers and use them to make whatever we need to fix the ship." but the problem ends up being that they don't have enough raw material (perhaps one of the cargo bays spills out into space during the accident, so they need to rip up bits of the ship and toss them into the recyclers. If you want social issues on board, you could pull something like what Ascension does (upper deckers (towards the front) having nice almost cruise-ship like existence, and the lower deckers, basically living in rooms next to the engines and stuff, crude by necessity) and have both sides arguing about where the base material should come from. "We need silicon for new control systems! Let's take it from the fake beach that the upper deckers have!" countered by "No! Just crunch up the glassware you guys use for cups and such."

[Voyager55]

Counter rotating rings are a recipe for disaster in terms of reliability and maintenance. As you demonstrate, it's more likely to break. It's much easier and cheaper to simply rotate the entire spacecraft.

Rotating the whole craft has it's own problems, every time there's a need for a course correction the rotation must stop to allow proficient maneuverability. Which means people will have to be welcome to the idea of strapping down everything everywhere regularly.

And while Voyager is cliche and unoriginal name, it's very fitting. How about you give names to the separate modules of the craft? Voyager could be the name of just one of them, or the whole as an assembly.

[Nicholander]

Apologies if this you have already been to this website, but the Atomic Rockets website helped me out A LOT when trying to design realistic spacecraft, so you it's here: http://www.projectrho.com/public_html/rocket/index.php

[AndrewBCrisp]

Some thoughts and questions:

What is the purpose of this voyage? You've mentioned that the destination is an exoplanet mapped some ninety years prior to the launch of your generation ship. Ninety years is a long time, in human terms: roughly 3 generations, or 22 election cycles in most democratic systems. Plus, starships - whether they are multi-generation slowboats or near-c speedsters - cost a lot of resources. So what's so important that humanity (Earth? Some planetary colony, or perhaps a solar-system wide culture?) invests the time and money to build, crew, and launch this ship towards a planet that is known only in the history books - and which nobody who boards the ship will ever see?

Possibilities include:

Scientific inquiry - human exploration of the exoplanet may not have been possible at the time of the robot surveys, but it is now (or at least, barely possible).

Exporting of dissidents - In the past, a number of cultures solved internal clashes by sending their malcontents somewhere else to live as they please. The ancient Greeks used this strategy to settle all around the Mediterranean, Europe exported a number of religious and cultural groups to North America, and so on. It was a win-win situation; both the dominant culture/polity and the malcontents got what they wanted. Future cultures might try this again to keep ethnic/cultural/religious conflict at home to a minimum. Such a political motive might also explain the sudden interest in a planet 90 years after it was first surveyed. (However, if you go this route, you'll need to take into account why these dissidents are being sent to another star, rather than to homestead somewhere else in the Solar System).

Desperate survival - Something Very Bad is about to overtake the human race or some fraction therefrom. There might be a plan A in action to deal with it, but the generation ship is plan B .

Some or All of the Above - Each of the above reasons is enough to launch a starship, but multiple reasons certainly increase the odds that a ship will be built, and built well.

I mention this because purpose will affect how your ship is designed. A scientific expedition might have a minimum population needed for genetic stability (probably fortified with a gene bank), but a migration program or survival contingency will probably aim for a much larger starting population (10,000 minimum).

A smaller population might keep itself together in a single habitat rather than subdivide into smaller groups. After all, if you're near minimum needed population, losing a good-sized fraction to an accident is pretty much the end of the entire mission right there.

A larger population however, would see advantages in dividing into two or more smaller habitats as part of the starship. For one, redundancy. If your 10,000 are divided into, say, two groups 5,000 strong, and something happens to wipe one of them out, the remaining group can still go on to found the colony with a minimum of fuss. A second reason is to promote cultural diversity and cultural interchange. Generation ships have a major problem in that they are, in essence, island cultures and if you have only 1 culture present, it can decay quickly. A bad idea that gets a hold in the culture can spread far and fast, wreaking damage that will be felt for generations, if it doesn't destroy the culture outright. Contact with Earth, via radio or message laser, would be too far away for any kind of cultural support. Having a second culture interacting in real time with the first provides some insulation against bad ideas taking hold, and can provide stimulation and inspiration as the two cultures share concepts during the voyage.

Such a division could give you the rationale to having two separate habitats, be they rings or cylinders. One might even have two (or more) separate ships - one per culture - flying together as a fleet, each ship within a few kilometers of the other. Or the ships could be connected by tethers during cruise mode (thus providing an easy means of transport between ships - and a plausible something which could Break with Disastrous Consequences for the plot).

[Impaler]

Much simpler to have a single wider Torus with two adjacent counter-rotating wheels inside it, the wheels are in contact with rails, motors and brake-shoes between each other to spin up and spin down. The rotating wheels are not pressure vessels, just thin walled habitat space that can be opened up to get access to the space between the wheels and the space between the wheels and the outer torus shaped pressure vessel. To access the no rotating parts of the ship or move between the who wheels when they are rotating you get into a small elevator car which is 'on top' (aka closer to the center of rotation) of one of the wheels, it is on rails and accelerates opposite the wheel it is on to an opposite be equal speed bringing it to rest with respect to the hub (your now weightless). An identical car doing the same thing on the other wheel comes alongside and you go though the aligned open doors like an elevator. Or if your going to the hub a sleeve extends down around the car and it locks into an vertical railing that allows it to ascend an elevator shaft to the hub, the same sleeve then deposits it back onto the rails when going back. All the rails and wheels are like thouse on roller coasters, double wheels that grip the rail and can't be knocked off, they must be made to 'let go' by active command likely some kind of electromagnet.

[GreeningGalaxy]

Picking apart your design a little bit:

dual centrifuge rings that counterbalance each other, looks like like 2001 space odessy station, but they spin in opposite directions.

--Looks like this has been addressed by others. counter-rotating rings would work okay for a ship that has to maneuver a lot, assuming you had some way of making rotor bearings that never wear out, but for a generation star-hopper, I wouldn't think you'd need that. The main benefits of that setup would be no gyroscopic forces (the rotating rings cancel each other out) and having a zero-G segment at the center, so if you don't actually need any of that, I wouldn't recommend bothering.

Long central spine, engines at the front pulling rest of ship

--I really would not do that. For a ship designed to go between stars, you're going to need a LOT of engine power / exhaust velocity (like fusion, as you suggested). That's going to produce some very intense neutron radiation, which you're going to want nowhere near your habitat.

engines angled several degrees outward to keep exhaust from frying the rest of the ship.

--Yeah, even this probably wouldn't be enough. The radiation from the exhaust won't be directed in a pencil-thin, coherent beam, it's going to be shooting off all over the place. See Project Rho's items on shadow shields; that's probably more what you'd need.

Some sort of 0g observatory near the back? (possibly 0g recreation somewhere as well?)

--My fun-loving side says "do it do it do it!" It might be slightly sub-practical (I guess ideally you'd have the crew confined to the rings and save the inner column for all things starship-related), but Kim Stanley Robinson's Red Mars features just such a thing (with the whole ship spinning) so I say go for it. It'll make your readers wish they were there.

pressurized tunnel in the truss, allowing access to much of the ship.

--I suppose this ship will need to be rather big, much bigger than you'd really be able to expect for something totally EVA-accessible. The central column will probably be packed with machinery, but I don't suppose putting a small maintenance tunnel system inside is a bad idea. Making it pressurized might be very challenging if you want it to be able to access everything important, but then you'd have to crawl through a tunnel full of sharp-edged, catch-point-ridden machinery in a spacesuit. Kevlar compression-style space suits, maybe?

Whatever you do, don't make the tunnels that go close to the reactor/drive commonly used. If you do choose to include them, remember that anyone you send down there is probably not ever coming back up.

Could some of engine waste heat be used as life support heat? (as design or in survival?)

--Sure, that would probably be pretty trivial. Assuming the heat travels from the cold side of the generator to the radiators in a fluid of some sort, just have a few of those pipes go to the HVAC system and back on their way to the radiators. That'd be as efficient as anything in terms of heating systems.

relatively simple propulsion, not antimatter, fusion maybe?

--It'll be a while before fusion becomes "simple" in any sense, but it still looks rather attainable in the long term even so. You probably won't get the delta-V you need from just an inertial fusion drive with onboard fuel/propellant, though - maybe something like a Bussard ramjet? Those are tricky, because you have to do proton-proton fusion (which is hard), and also because it's very hard to go faster than your exhaust velocity, since the drag from all that hydrogen you're collecting slows you down. Still, for a generation ship, 12% of light speed isn't terrible, and collecting your propellant from the interstellar medium means you won't have to worry about gigantic propellant tanks.

There are lots of different possibilities for very strong deep-space propulsion systems. I recommend poking around near the bottom of the drive table on Project Rho - there's some very cool stuff to be found there.

You could also do what lots of sci-fi authors do, and just say that the ship is powered by a "stardrive" being some kind of extremely powerful rocket driven by unspecified processes. Personally, I love to go into the technical details about stuff in my writing, but glossing over is often recommended for this kind of thing, because it gives you more wiggle room and doesn't get you caught between the rock of your plot and the hard place of scientific realism.

Closed cycle life support, algae farms, yeast, made into palatable dishes?

--According to my mother (a biology professor), this could actually be substantially harder to do than a sufficiently powerful propulsion system. Sure, it can be done, it's just that there are a lot of variables to account for, and a lot of really tiny things that need to be kept alive. It's possible to do this, but you'll need to make sure that your crew is very careful not to destabilize anything, and that you have lots of buffering options to keep stuff stable as inevitable changes in environmental conditions occur. This isn't as simple as just having enough plants to make oxygen for your crew, you also have to take into account all the microbes that plants, humans, and whatever other critters your bring along are going to depend on. The biosphere on Earth is very interdependent and adapted for exactly the conditions on Earth, so getting it to thrive in a spaceship is going to be a very big obstacle. Make sure you have lots of room for storing oxygen, CO2, and trace nutrients, some chemical reactor systems to stand in for various biological processes in case of emergency (ie a CO2-to-oxygen recycler if all your plants die), and lots of space to store emergency food. You'll also probably want some frozen microbe cultures too, in case someone has a gut flora problem or one of your artificial biomes loses its microbe population.

Communication system with earth? (not superluminal!!!)

--This is relatively easy - just stick a great big laser on your ship, on a turret that can track Earth as your ship spins. You can decide what wavelength you want; I hear that microwave lasers are relatively easy to make powerful (and go through gases with minimal loss), but you can also try to go for visible light or even shorter to maximize your range. Wikipedia has an okay article on this.

Mostly, your most essential thing will be transmitting your signals as a thin, directed beam instead of omnidirectionally. What exactly this is depends on how far you're going - if you're headed to Alpha Centauri, you might just need a large radio dish, but if you're headed halfway across the galaxy, you'll need a laser that can still be seen through all the interstellar gas in the way, which will need to be very powerful indeed.

repairs and maintenance done by remote control repair bots. (not fully autonomous)

--Sounds good. For a generation ship, self-repairing systems are kind of a must. The need for human input does seem rather inevitable, but for the most part, a society advanced enough to build a Bussard ramjet and maintain a stable ecosystem inside a spacecraft shouldn't have much trouble making a near-totally-autonomous repair system.

cargo for setting up colony on extrasolar planet

--Sure, just disassemble the habitable modules of your ship and fly them down one by one. If you pick a planet that already has life, good luck - even putting aside the possibility that someone sentient lives there, the interaction between Terrestrial life and alien life is going to be very unpredictable and probably dangerous (don't go around eating whatever you see, and be mindful that breathing near something is probably a good way to kill it, or you - the the microbes in your body might be extremely infectious to local species, and vice versa). Maybe you could go for something where the planet's temperature is perfect and it has liquid water, but its atmosphere isn't breathable and it has no life? Then a human could walk around with just a breathing mask. Of course, you'd probably start terraforming the place right away whether you wanted to or not, because you'd track microbes all over the place and leave behind tasty chemicals, and eventually you'd introduce something that likes the local environment, and, bam, proto-biosphere.

If you want to be really clever and wildly speculative about colonizing a planet containing life, you might do some large-scale preparation in advance: send a sterilized uncrewed probe to the surface, take samples of the local flora and fauna, and then either grow yourself some humans that can exist in that environment, or else find a way to convert a living human to be compatible. You'll then need to grow enough specimens of local wildlife to use in your shipboard biosphere, and then maintaining that gets a hundred times harder than it would be if you used Earth life.

All of this has some pretty serious ethical and logistical ramifications, obviously, but could easily make for a very interesting story. Just watch your step.

way to angle floors slightly due to offset gravity during thrusting??? (would that be necessary?)(i do want rings, more space for population that way)

--You could have variable-angle floors (probably by having the rings made of a number of cylindrical modules, and then just rotating those). Easier than that, though, would be to design the ship for an always-running propulsion system. You'd get where you're going a lot faster if you have some kind of torch drive that burns constantly throughout the trip - accelerate constantly up to the halfway point, then flip over and decelerate the rest of the way, which will bring you to a stop at the destination. This has another added bonus: if you were to fire the engine hard enough to accelerate at a constant 1 gravity throughout the trip, you could do away with spinning entirely, and have the gravity come from the drive alone. A Bussard ramjet could probably attain this kind of power, but in all honesty you're probably better off with an unspecified handwavium stardrive (or antimatter, but you said you didn't want that) if you go for this.

Regardless, You'll probably want to plan for having the engine on more often than not, because doing a short burn at the beginning of your trip and then trying to coast the rest of the way will probably not get you there in a short enough time to be meaningful (try millions of years).

If you really want your ship to hold up to lots of varied thrust, variable-angle modules would work well enough, but like the counter-rotating rings idea, it just doesn't really seem necessary for the kinds of things your ship will be doing.

This sounds like a cool story! I hope you at least let us all read it when you're done.

[Ambulatory Cortex]

For a thruster, how about a dusty-plasma fission-fragment rocket? Per ProjectRHO, it has 15 million ISP, 344N thrust, and weighs 9 tonnes. It'd use a brachistochrone instead of hohmann trajectory. When talking multi-year journeys, it's pretty hard to beat.

[Armchair Rocket Scientist]

[*]dual centrifuge rings that counterbalance each other, looks like like 2001 space odessy station, but they spin in opposite directions.

As mentioned, this is only helpful if the ship needs to manuever a LOT. This is not the case: it would most likely thrust for a while on departure, spin up after the burn stopped, turn itself around at some point, and then do another long burn to slow down. There would only be a couple of direction changes during the whole mission.

Long central spine, engines at the front pulling rest of ship. engines angled several degrees outward to keep exhaust from frying the rest of the ship.

I'm curious, has there ever been a serious proposal with this design? Sure, a "tractor" configuration is structurally efficient, but just angling the engines will not protect you.

I'm assuming your ship will have a top speed of at least 0.1c, which means its exhaust will be moving at relativistic speeds. This requires exhaust temperatures in the millions or billions of degrees. The exhaust will emit an immense amount of thermal radiation, possibly frying the passenger section all on its own. Not only that, but a lot of this radiation will be in the form of x-rays or gamma rays.

Some sort of 0g observatory near the back? (possibly 0g recreation somewhere as well?)

Just spin the whole thing, and you'll get basically 0 g at the center as long as the engines aren't burning.

Could some of engine waste heat be used as life support heat? (as design or in survival?)

Easily. AFAIK the only time a manned spacecraft has had to worry about not having enough heat is Apollo 13. Even the ISS has big heat radiators, because it has more waste heat than it can use. With any interstellar spacecraft, a much bigger concern is that the engines will produce enough waste heat to vaporize the entire ship if it isn't dealt with.

Closed cycle life support, algae farms, yeast, made into palatable dishes?

If you're spending hundreds of years on this thing, there's no other option.

Communication system with earth? (not superluminal!!!)

Probably, but in all likelihood it would only be used to send very basic messages, like "year fifty-five, the ship has not blown up." I'm assuming the destination is tens, if not hundreds of light-years away, so except during the first decade or so of the trip the communication delays are going to be years long. There's no point in individual passengers "phoning home."

repairs and maintenance done by remote control repair bots. (not fully autonomous)

[*]cargo for setting up colony on extrasolar planet

way to angle floors slightly due to offset gravity during thrusting??? (would that be necessary?)(do want rings, more space for population that way)

Hmm... if you design the ship to thrust at 0.1 g for a year or two, the floors would probably have to be angled. However, the ship may end up being designed with an acceleration such that it burns forward for half the journey, then immediately starts slowing down. In such a case, the accelerations would be low enough that there might not be a real point in tilting the floors, since it would add a lot of mechanical complexity.

[Bill Phil]

Maybe a high-temperature fusion reactor with Liquid Hydrogen running around it a la NERVA.

[qeveren]

I'm curious, has there ever been a serious proposal with this design? Sure, a "tractor" configuration is structurally efficient, but just angling the engines will not protect you.

I've always been fond of Project Valkyrie myself...

[GreeningGalaxy]

Maybe a high-temperature fusion reactor with Liquid Hydrogen running around it a la NERVA.

That's certainly a possible and effective propulsion system, but not nearly strong enough for interstellar travel, since you lose a lot of energy in the conversion to heat, and most of it shows up as thrust instead of Isp. Fusion thermal rockets could easily have the thrust to launch massive payloads off the surface of Earth, but they'd only have an Isp of a few thousand seconds - plenty for mucking about in the solar system to your heart's content, but not even close to what you'd need for an interstellar spacecraft. Ambulatory Cortex's suggestion for a dusty-plasma fission fragment rocket is good (although I may note that it doesn't have "15 million (seconds) Isp," that's actually 15 million m/s exhaust velocity, which equates to an Isp of about 1.5 million seconds), as would be an intertial confinement fusion system (in which lasers are used to zap the deuterium/tritium fuel and make it fuse). Both systems have very high Isps and low thrusts, which is what you'd need for an interstellar trip.

Probably, but in all likelihood it would only be used to send very basic messages, like "year fifty-five, the ship has not blown up." I'm assuming the destination is tens, if not hundreds of light-years away, so except during the first decade or so of the trip the communication delays are going to be years long. There's no point in individual passengers "phoning home."

Communication delays are indeed going to be long, but I see no reason to believe messages will necessarily have to be short or basic. With such long timeframes involved, there's no need for a particularly high rate of information transmission (you can spend a few hours transmitting a few gigabytes and it's no harm to anyone, meaning that Earth gets lots of time to make out the dim pulses from all those light years away), so you could reasonably imagine that the crew members could all record their own little video files to send back home, if so they chose, which would then be sent along with the daily "mail" transmission from the laser array along with telemetry and such.

Of course, I'm no psychologist, but I might also point out that the best crew for a generation ship would be people who don't have any loved ones or particular investments to leave behind on Earth - it's not like they're coming back. The capacity for sending mail back home could certainly be there, but if your crew is always using it to contact people they miss, that could be a sign of a serious morale issue.

[KSK]

Following on from AndrewBCrisp's post, having multiple populations in different habitats gives you a fairly easy route into all sorts of politics, ideological mismatches and inter-faction conflicts of greater or lesser destructiveness. All useful plot-fodder and good for possible reasons why the ship broke apart in the first place.