The Sword and Laser discussion
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Leviathan Wakes
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LW: why is the Nauvoo a generation ship?
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Ian wrote: "With 1G, wouldn't you hit the speed of light pretty quick? It might take a while to get somewhere according to a clock on Earth, but according to the shipboard clock you get anywhere you want at sp..."
that chart seems fishy. We would have ventured to the stars by now had it been that simple. Yet we havent even landed a person on mars. there has to be more to it than just that chart and relativistic science. I think that LW avoids light speed for a reason, it is confusing and gets in the way of things to easily. I believe the intent for the Nauvoo was to have it fly around a speed of 1g most of the time. because even though they may have tons of fuel there is no way they have enough to accelerate indefinitely. they would get to one speed, then descend to free fall (no engine) then accelerate again later.
that chart seems fishy. We would have ventured to the stars by now had it been that simple. Yet we havent even landed a person on mars. there has to be more to it than just that chart and relativistic science. I think that LW avoids light speed for a reason, it is confusing and gets in the way of things to easily. I believe the intent for the Nauvoo was to have it fly around a speed of 1g most of the time. because even though they may have tons of fuel there is no way they have enough to accelerate indefinitely. they would get to one speed, then descend to free fall (no engine) then accelerate again later.
I believe the intent for the Nauvoo was to have it fly around a speed of 1g most of the time. Sean, can you back up that intent with quotes? Since I don't remember reading that. (Doesn't mean it's not there though.)
or was Nauvoo flying at 0.3g or something? I guess that might make the difference.
Anyways, there's nothing 'that simple' about sustained 1G acceleration! But yes that's really 'all' that is needed for interstellar flight, speed-wise. And it appears to be something that was a solved problem in LW. You can even ignore the on-ship time dilation and it still wouldn't need a generation ship (though I don't know why you would... I loved how Hyperion and others have incorporated time dilation into the plot).
Well, part of the issue is that as you accelerate your mass increases due to relativistic effects. The thrust you need to reach 1g from rest is a hell of a lot larger once you're at 50% of C.This thread from 2003 has a good deconstruction of that. Poster "wolf_meister" pretty much nails it.
quoting astroboy from the linked thread:"The energy requirements to accelerate a ship the size of the Enterprise to only half the speed of light is roughly 2000 times the total annual energy used in the world today (2011). Source "The Cosmic Perspective" by Bennett"
As long as our propulsion systems require carrying reaction-mass, we will not be able to sustain a constant acceleration. We have to boost up to a given speed, then coast most of the way.
Sean wrote: "I believe the intent for the Nauvoo was to have it fly around a speed of 1g most of the time. because even though they may have tons of fuel there is no way they have enough to accelerate indefinitely. they would get to one speed, then descend to free fall (no engine) then accelerate again later."What would be the point? Better to do all your acceleration at the beginning so that you're traveling at your top speed for the whole trip instead of doing it a little at a time.
Putting aside the issue of reaction mass, there's another huge issue here as well: travel at large fractions of light speed will expose you to immense amounts of radiation.Even hard vacuum contains a small number of (usually hydrogen) atoms. The kinetic energy as those atoms hit a ship traveling at even 0.5c is in the particle accelerator realm. I hear that's bad for you.
The Nauvoo is heading to Tau Ceti (12 light years away) and the book talks about it being a 100 year trip, so allowing for acceleration/deceleration time I'd say they're looking for a cruising speed of around 0.1c to 0.15c. I'm not sure how feasible it is for our form of life to go faster than that.
Regarding reaction mass, there's a bit of hand waving done in LW. Whatever the Epstein drive eats, it seems to consume a lot less than a "torch". IIRC the developer of the drive is still heading out-system under thrust, and it's been quite a long time relative to the narrative that that first drive has been running.The ship was not designed to coast, by the way. The description of the superstructure references interior spaces that were designed as nature preserves or parks, and there's no mention of a wheel. I can't see a lake doing to well when the thrust gets shut off.
The david darling link doesn't account for deceleration time. Yes, if we could accelerate at a constant 1g the ship would get to Tau Ceti (their stated destination), pretty quickly from the point of view of ship time... and we'd fly right past it at close to light speed. Remember, if the goal is to go from Here to There and enter orbit there, you need to accelerate, then turn the ship around and DEcelerate at (roughly) the midway point in order to stop at your destination.
In the book it's also mentioned that they don't know for sure whether there's a habitable planet there, so it would be a bit foolish to engineer a ship that works fine if there is and Bang You're Dead if there's not. I think they are over engineering on the assumption that it will take longer than a straight-line acceleration would be and that they might need to move on from Tau Ceti because of a lack of any planet to live on there.
Plus, well... it's fiction and very possible that they did this just because they wanted it to be a generation ship.
Rickg wrote: "In the book it's also mentioned that they don't know for sure whether there's a habitable planet there, "Well, in Chapter 42 they do mention efforts to terraform Mars. If they get to Tau Ceti, and find a planet that wouldn't support life, a generation ship could buy them some time for terraforming their new home.
David wrote: "Rickg wrote: "In the book it's also mentioned that they don't know for sure whether there's a habitable planet there, "Well, in Chapter 42 they do mention efforts to terraform Mars which are bein..."
SHH!!! Not Done yet!! :)
Yes, good point, if they find a terra formable planet and have the tech with them they could do that while living on the ship. Or if there's no decent planet there, they could reaccelerate and try another star. An over engineered ship gives you options...
Rickg wrote: "SHH!!! Not Done yet!! :)"Gah! Sorry. Edited.
No worries. Minor spoilers don't bother me.
Just looked it up, Tau Ceti is only 12 light years away, why wouldn't they have sent some unmanned probes, to there and other nearby systems? After the events of LW it's easy to see why someone would be in a big hurry to get out of the Sol system fast, but prior to LW I don't see what the big rush would be.Sort of sounds like they just didn't really do the math on this... since yea I was also under the impression that Nauvoo was going to be kept under constant acceleration.
Ok heres a chart that shows travel time to various nearby stars both from our perspective and thw ships..TT is only 14 years away at 1 g, 5 in ahip time.http://www.astro.wisc.edu/~dolan/gurp...
Hmmmmm... so on average, they are planning to accelerate very, very slowly. As in, they aren't trying to get too close to the speed of light, just keep up a constant acceleration for ease of living. Napkin math time! Assume an average constant acceleration over the course of the journey. Okay, well, it'll average to zero (total change in velocity is 0), so let's calculate to the midpoint. Let T be the time planned for the entire trip, so T/2 to the midpoint. (12 light years)/2 = 299792458(speed of light in m/s) * 6 years * 31556926 (seconds/year) = 5.67631705 × 10^16 metersD = A*(T/2)^2 + V1*(T/2) + D1 (only calculating half the trip, and where V1 is initial speed and D1 is initial displacement... both 0, so...)
5.67631705 × 10^16 = A*(T/2)^2
A =2.27052682 × 10^17/T^2
Let's see if that works... say T = 100 years = 3155692600 seconds. Then A = 0.0228001263 m/s^2 = ~G/430. Pretty darn slow. Also, probably accurate.
At that acceleration, they would accelerate to approximately the midpoint of a 12 light year trip in 50 years and then begin decelerating at the same rate until they reached their destination.
As a point of reference, a ship accelerating at 0.5G will reach nearly the speed of light in about a year over a distance of about half a light year. The ship will get asymptotically close to the speed of light as it continues to accelerate (undergoing time dilation), but never reach it. At least, that's what physics tells us today.
By the way, actually reaching the speed of light would be horrible, because if special relativity is correct (your GPS says it is), then
tells us that any amount of time spent at the speed of light is equivalent to infinite time from the perspective of an observer at rest. Like the rest of the universe, for example.We're assuming that they don't want to get close to the speed of light, or at least, not stay there for very long, if they can help it. This makes sense if they need to navigate around objects, and if they are trying not to tax their engines by hitting drag (gravity wells, solar wind, background radiation, etc). I'll say that this is plausible, although it does seem like they're being remarkably unambitious by not just plotting an ideal course and taking the trip in bursts at around 1G acceleration/deceleration... but I don't know enough about Mormons to draw a deeper point here.
You did your math backwards, you started with the T. I didn't get the impression at all that they would be going at 1/430 G from the book.I mean in general that seems like a reasonable speed when under constant acceleration. But with the Epstein drive?
What my equation is telling you is that, assuming you never reach near-light speeds, in order to determine the average constant acceleration you must first choose the desired time for the trip. Conversely, you could pick an average constant acceleration and calculate the travel time, but then I'd have to do more than napkin math to factor in the speed of light as a hard cap on your velocity. My general assumption here is that human life is relatively cheaper than, say, maintenance on the engines. Which would explain why they are willing to travel slowly in the first place.
I do agree with you, however. If they have the capacity to make a constant acceleration trip, there is no reason for them to take so long. Even at half a G, a 15 light year voyage should only take about 16 years.
Sky, precisely. Even a 0.1g acceleration is 21 years. You have to move acceleration all the way down to something like 0.01g to even approach a century.
I'm not as accomplished in math as many of my previous distinguished posters, but to answer the threads title question.I found this quote from the book P. 325 (Nook version)
Generations would live and die in it, and if they were mind bendingly lucky enough to find a planet worth living on the end of the journey, the people who came out of it would never have known Earth or Mars or the Belt.
well yep, that's what makes it a generation ship. Is there a quote implying what G it will run at, or if it run at microgravity, or will essentially be weightlessly coasting most of the time?
Darren wrote: "I thought it was because it was a Mormon ship."I'm not sure I see the connection you're making here. Would you care to elaborate?
I would think the main limiting factor is fuel. The technology in the book is basically some sort of fusion drive, which means helium-3 as fuel. There's only so much of that they can carry with them, and they can't stop in interstellar space to mine more.
@Darren: generation ship refers to a interstellar ship where generations live their entire life because the travel time is 100+ years. It's not a ship where people research their ancestors. :Dit's such a well-established trope that it has a wikipedia page:
http://en.wikipedia.org/wiki/Generati...
@Simmy yea I wish they just spent a sentence to define the longterm limits of the drive they used...
Darren wrote: "David(LA,CA) wrote:"I'm not sure I see the connection you're making here. Would you care to elaborate?"Mormons are all about genealogy. It's taken very seriously."
So, you though they had invested in a massive starship for the purpose of researching their ancestry?
Darren wrote: "No one gets my humour."I got it.
Maybe they're specifically going for the "forty years in the desert" effect from the Old Testament, and trying to get a generation that "knew not Egypt."
I thought it was a 100 year ship because they couldn't count on finding anything at the first star they stopped at, and because they didn't have enough room for the fuel needed to go really fast, what with how many people they wanted to load onto the ship.
I didn't try to follow all the math, but remember that the idea isn't to go flying past the finish line at near light speed, it's to stop there. So you need to spend as long slowing down as you do speeding up. Plus I think that the assumption is even if you find an earthlike planet, without a Genesis device, you're going to spend a good long while terraforming. And if you don't find an earthlike planet it's a matter of fueling up and trying again.
The latest podcast resolves this one with a comment from Ty Franck. The ship has a giant drum section for gravity during cruise. It can't keep accelerating because it can't carry that sort of fuel.
Ok... argh! That seems at odds with everything I read in the book, I don't recall that at all. Plus, unless they are planning on pumping all the free standing water (the text states that there were areas designed as nature preserves, including having small lakes) they would need slop walls (like on Rama) to stop all that water from behaving inconveniently. Cripes, I feel like I'm obsessing over warp field dynamics, or specific shield frequencies, but the solution feels slapdash.
Yea it doesn't make sense that it's possible for a yacht-like ship to be under 1G for months at a time, or a ship loaded down with military systems to go at 1G for weeks at a time, but years at a time is out of the question due to fuel issues. There certainly are plenty of challenges with going at high speeds. But with the way things work in LW it
doesn't make sense that fuel would be one of them!
Additionally it doesn't make sense that the Nauvoo was going to fly blind. Unmanned probes should be mapping the local galactic neighborhood no problem. Basically an unmanned probe would be the fusion engine + fuel and the late 21st equivalent of a Raspberry PI tiny computer. I don't see why it wouldn't have the fuel to reach incredible speeds using the LW tech.
I mean I could understand that the society of LW is too self-involved to reach for the stars with the exception of the Mormons looking for Kolob (the planet God lives on). Mormons aren't known for their poor planning, why aren't they sending probes to the local star systems if no one else can be bothered to?
Something random I found on google, but the table there matches what I've seen elsewhere:
http://www.daviddarling.info/encyclop...
So it'd only be a generation ship in that a newborn at the start might be a teenage mother at the destination.
I'm guessing folks with more physics knowledge and those armed with ctrl-f in their Kindle can help clarify. It's possible the book mentioned why Nauvoo was a generation ship when they have such great drive technology, I just don't remember.