So generation ships. It’s been a few years, but I’ve brought them up before. My basic thought for an interstellar settlement effort is this: don’t send a completed city or ship, send a small seed ship along with the tools and raw materials to bootstrap itself into a thriving city that arrives at its destination just as it’s running out of space and materials for growth.
Something that might look like this in it’s various stages:
The grey sphere represents the ore-pile sent with the original ship, and the four ships show the growing colony as it expands from 200 people to 500 to 1000 to 10,000 just in time to arrive at one of the nearest stars. The ore pile doubles as micro-meteorite shielding in the direction of travel, which for a habitat traveling at percentage levels of light-speed is somewhat more critical than might otherwise be the case.
The hemisphere shows the parachute for a medusa style nuclear pulse rocket used to brake the ship at its destination, and the little blue sphere shows the final colony collapsed for deceleration, into a form where it can serve as the initial hab in a bolo-style station that will be the start of a much larger city in it’s new solar system.
The nice thing about this general idea for an interstellar spaceship is that aside from the engine itself, and the impossibility of immigration or emigration, the basic model of running and growing the city itself is identical to that of how most of the stations I draw would be built and expanded over time. So, ideally, by the time someone decides to take their city to another solar system humanity will already have experience on building, expanding, maintaining, and upgrading multi-decade or even multi-century habitat projects.
The other thing to consider is the 200-10,000 population range and the size of the ore pile is built around a 2-400 year trip using nuclear pulse propulsion with a large enough reserve of uranium at the far end to actually decelerate the thing. That’s probably somewhere between realistic to optimistic for the initial trip. Barring fundamental changes in physics, the rocket equation is cruel. On the other hand, once the initial trip has been made you can build the infrastructure to use laser-pumped solar sails for both the start and the stop parts of the trip and cut the mass of the ship (and hopefully the travel time) way down.
Is travel to the stars even possible? (science documentary: video).
Is travel to the stars even possible? (science documentary: video).
Alberto Caballero joins John Michael Godier to discuss his concept for a new interstellar spaceship called Solar One.
Solar One would be the first crewed interstellar spaceship and use a combination of lasers, solar sails, and other means of propulsion to get close to 30 percent of the speed of light.
Which planets and stars would be the best to go to first? The Trappist system? Alpha Centauri?…
Is travel to the stars even possible? (science documentary: video).
Is travel to the stars even possible? (science documentary: video).
Alberto Caballero joins John Michael Godier to discuss his concept for a new interstellar spaceship called Solar One.
Solar One would be the first crewed interstellar spaceship and use a combination of lasers, solar sails, and other means of propulsion to get close to 30 percent of the speed of light.
Which planets and stars would be the best to go to first? The Trappist system? Alpha Centauri?…
Ok, back again. And it's time to do a very little bit of math to get a few more details for the interstellar nuclear-bomb powered spaceship.
And to start, we'll look at our bomb. The bomb I've been using for my calculations so far is a purely theoretical bomb with a yield of 35 megatons that then-secretary McNamara suggested could be strapped to a Titan II rocket in an interview with Time Magazine in the 60's.
Now, a warhead meant to be strapped to a Titan II rocket is going to have to be pretty similar in both mass and geometry to the actual warheads used on Titan II rockets.
And that gives us the information we need to plug in to the equations in this paper, describing the Medusa nuclear pulse rocket.
First up is a basic safety check. This equation lets us know what the bare minimum size the parachute catching the nuclear explosion needs to be to keep the explosion from melting the parachute:
And altogether, that gives us a minimum radius of roughly 5 2/3 km.
Or, this when you put it next to the 100 meter radius asteroid the craft is going to start off as:
And, to round out the week, from our mass and acceleration estimates before we know that there are 4.25 megatons of bombs acting as reaction mass and that the burn time is going to be just over 113 hours. With a mass per bomb of roughly 3700 kg, that works out to just over 2.5 bombs exploding per second to keep this thing moving.
So, considering everything in the last three parts, we’re looking at something like a 450 year trip from our solar system to Proxima Centauri. The goal is to get there with a thriving and active community of 10,000 people who have been growing and building their community all along, and are ready to get to work expanding into their new home.
That gets us a basis to use for how large the community needs to be at its destination, and we can use that knowledge to make a few rough and dirty estimates. Here’s a possible shape for a stable habitat for 10,000 people:
The radius is roughly 225 meters, the rotation rate is 2 rpm, and assuming radiation shielding of 5 tons per square meter all round, the mass is on the order of 4.25 million tons. Because the propellant for our ship to slow down won’t be expended until the final five days of the trip, it can be used for radiation shielding, and adds nothing to the mass of the ship.
The same isn’t true for the propellant to speed up. To achieve a 450 year travel time, we need half of the initial mass of our ship to be nuclear bombs. Which means we have a total mass of 9.5 megatons, 6.375 of which are (or in the case of the slowdown propellant, will be) nuclear weapons.
If we were get all of that mass out of a spherical asteroid of reasonably typical density, the asteroid we’d mine would have a radius of roughly 100 meters, like this:
And if the devices used for propulsion had roughly the same ratio of uranium to total weight as the Little Boy device, the uranium needed would mass 95 kilotons, or slightly higher than 1.5 times as much uranium as was mined in the entire world in 2015. And since U-235, which is what we’d actually need, makes up 0.72% of natural uranium and 80% of the uranium that went into Little Boy, we’d only need 167 years of mining at 2015 productivity levels to gather what we need. More modern nuclear devices, like the ones we’d actually want to use, make far better use of their uranium and would presumably require less of it. But there’s a limit to how much time I want to spend trying to find specifications for nuclear devices on the internet, particularly from my own computer and I’ve already surpassed it on this project.
This is going to be unfortunately short. I spent about an hour developing a model to let me calculate the benefits, or lack thereof, of trying to leverage the Oberth effect for an interstellar spacecraft. If you didn’t know, the Oberth effect is just the name given to the fact that since kinetic energy is proportional to the square of velocity and a given burn from a rocket results in a fixed increase in velocity, it’s always more energy efficient to burn your rockets when you’re already travelling at a high velocity.
In this case, the idea is that for an interstellar craft using fission pulse propulsion the limit to how hard the craft can accelerate is how much acceleration the craft and the passengers can handle. So, conceivably, our craft could burn half of its mass to accelerate to almost 1% of the speed of light in under five days. And at that speed, it could do most of it’s acceleration while it was still close to the Sun to take advantage of the Oberth Effect.
The case I tested was for craft starting out in an orbit similar to Jupiter, and then using a gravity assist from Jupiter to drop it into an orbit that passes to within half the distance between Mercury and the Sun before lighting up its engines. It was an obnoxious little spreadsheet model that had to account for the changing mass of the spacecraft, the resulting change in acceleration, and the effects of the continually changing potential energy of the craft relative to the Sun as it traveled. And for all of that, I found that a craft taking that path could shave just over ten years off of its travel time to the nearest star... out of 455 years if it just left directly from its original orbit.
So the title is a bit of a lie, it’s a 2% reduction in travel time, even counting the extra couple years spent falling down towards the Sun. And it’s better than using Jupiter to speed up the craft instead, which saves less than two years.
Dropping from Saturn instead of Jupiter results in a net loss of time, thanks to the relatively small increase in final velocity and the extra three years of falling time. And dropping closer to the Sun helps, but passing twice as close to the Sun only cuts an additional six years off the travel time. So the maneuver would be worth doing, and in general the closer you can pass to the Sun the more benefit you get, but even if you scrape the Sun’s surface you aren’t shaving more than 30 years off your trip and at a certain point the extra damage from approaching the Sun will make getting closer self-defeating.
Last time, I picked a propulsion system for my generation ship. I’ll be using a variant of the Orion nuclear pulse propulsion system known as Medusa. Using the best nuclear weapons we could reasonably manufacture using current technology in terms of yield to weight ratio, the tyranny of the rocket equation gives us this chart:
We’re talking about launching a modest-sized town from our solar system to it’s nearest neighbor, using what might be the most powerful propulsion system we could actually build using essentially current technology. And to reach the closest star in a single human lifetime would mean expending a pile of nuclear bombs massing twenty times as much as the town itself. Slowing the trip from one to three hundred years would reduce that pile of nukes to only twice the size of the town. Cutting that pile of nukes in half adds another 150 years to the trip, and another 300 years the next time you do it, and another 650 years beyond that the next time.
Of course, that chart doesn’t include stopping. Which is kind of important. And it makes things a little bit worse:
It would be nice to use something like a magnetic sail for braking instead, and hopefully see a reduction in travel times, but honestly the travel speeds we’re talking about here aren’t large enough to make good use of a magnetic sail for that purpose. And that’s without getting into any of the potential issues with using magnetic sails in the first place.
