Writing: Input Wanted Building a Generation Ship: The SFV Exodus

[N1206]

One other big problem: Constant acceleration.
I think a gigatonne is probably on the low side for a total mass, but you may have solar sails, or boost, or directed coronal mass ejections or some other fanciness on the go, but consider the energy requirements in the first second of the flight at a constant 10 m/s:
K_E = 1/2 mv²
m being a terakilo, and v being ~10 m/s means that you got to have an engine that can produce ~50 TW of usable power. 24 hrs a day, 365 days a year, that's 438000 TWh. In 2019 humanity as a whole was using around 170,000 TWh (https://ourworldindata.org/energy-production-consumption)

So, your accelerations are unlikely to be constant, and your trip time will be different than you first proposed. McGuffin up your power supply and have it putting out constant power. Your acceleration will ramp up as you chew up fuel and reaction mass, but 10 m/s off the start is going to break your science, unless you McGuffin up direct mass-to-K_E conversion reactionless drive technology.

And Newton still applies: for every action there is a an equal and opposite reaction. To get a gigatonne going one direction, something has to get accelerated the opposite direction. I think maximum velocity is ~2.5 the speed of the thrust matter. So you are looking at trying to whistle up stuff to around 0.25 c. In how short a distance could you do that? SLAC is about 3.2 km long. Without mucking about with relativity, you would be looking to apply 50 TW of energy to a reaction mass such that it whistles up to 0.25c. That calculation doesn't look too impossible in terms of firing reaction mass out the back, but every ounce of mass needed to get things going is 100 ounces more of fuel and reaction mass.

 

[John Strickland]

The source I had on fusion engines indicated that they put out a massive amount of heat and a small amount of thrust. My engine set put out 14,000 TW, a staggering amount of energy, but only about 400,000 tons of thrust. IF an anti-energy use person saw this they would freak out. So if you had 1 million tons of thrust, your starting acceleration would be 1/1000 of a G, so mine provided only 0.0004 G ! Yes that acceleration lasted for many decades, but the engines could be shut down and serviced if needed at any time. The habitation area is rotating to provide gravity, so the inhabitants would not really notice the lack of ship acceleration. They might notice the deceleration toward the end of the trip. I allow a cruise period as a buffer for service periods, so the cruise period can be shorter.

 

[DaveC426913]

What is the reason for a linear crew cylinder? Air pressure? Why not slightly curved due to the structural load from the rotation? I assume, that at least the internal floors or levels are made to be perpendicular to the local "gravity" vector.

Apologies to FL for missing this, and apologies to OP for going back to it.

Pretty sure KSR says he took the design directly from a bona fide modern design that uses standardized empty fuel tanks perhaps even from the shuttle. But it was for an interplanetary trip so, so KSR simply scaled it up to interstellar.

The cylinders aren't decked; they're open land, with villages. The land can be quite rugged, even mountainous, so local normal gravity is a matter of standing on a plain or a hillside. In fact, the land surface is so divorced from the cylinder floor that water flows and pools just where they see fit.

Of course, any structures will be plumb.

The reason I did up these sketches in the first place is because the geometry of the ship, as described by the author didn't make sense (15 degrees at each end of the joining bridges. So I asked on an 'Ask a Question About This Novel' forum-y thing. Pretty sure it was the author himself that cleared it up.

 

[John Strickland]

Cylindrical tanks with hemispherical ends are currently a standard industrial structure. Since we have yet to start fabricating (in space) the components of tori for rotating space settlements, curved sections of tori, like a bent tank, are not a standard structure. However, by the time we start fabricating an interstellar vessel, we had better be fabricating tori for multiple uses, including rotating space habitats of varying sizes. Space fabrication is fairly mandatory since launching huge, curved sections of a torus from the ground would be rather difficult!
A typical torus that is 500 meters across would have a tube cross section 50 meters across, with the top "deck" being at the widest point as measured along the radius from the torus's hub. This would give a sky ceiling of 25 meters or about 80 feet, and about 8 levels below decks. The major vs minor diameter ratio (here 10 to 1) is not absolute and some designs use much fatter tori, so that the hub of the structure is much smaller, merging almost into a disk in some designs.

My concepts use a large structure called a jig factory to build a torus continuously until about 90 % of it is done. with the missing 10 % finished externally and then inserted. This system is like a 3-dimensional assembly line, with most of the work being done by robot arms very similar to existing assembly line units. The same factory unit can turn out torus after torus of the same size and shape. A set of identical tori could then be incorporated into a slowboat under construction.

 

[N1206]

Ah, the novel with the exploded moon was:
Ice Prophet by William R. Forstchen. (1983)

 

[John Strickland]

Forstchen is the one who, more recently, wrote the "One Second After" trilogy, showing what would happen if an enemy detonated nuclear bombs IN ORBIT over the USA. The result would be the loss of our power grid, communications, most of our civilian vehicles and within a few months, most of our population due to starvation. To my knowledge, the Feds still have done little to no work on dealing with this problem.

One Second After is almost a "bible" for those of us who keep warning of this danger. The risk is still unmitigated by a lack of attention by both the Administration and the Congress over the last decade or two.

Exploding Moons can make a good story, but most of them rely on "magic" physics to do the exploding. A 500 km wide asteroid hitting the moon would work but it would have to come from outside our solar system. The energy that would need to be released to actually "blow up" the moon is beyond most people's comprehension.

 

[jedishrfu]

This thread is getting rather long and is starting to veer in other directions. Since it has likely answered the many questions from the OP, I think its a good time to close it and thank everyone who has posted here.

Jedi

PS: @John Strickland if you want to continue expanding on your ideas about a generation ship, you are encouraged to start a separate thread on the subject.