What propulsion should we use to reach the stars?
The Pathfinders begins with the organizers of humanity's, first, interstellar colony program. The propulsion they choose determines everything. Travel time, payload, and therefore scope.
I sat down today to plan the chapter where they go over their options. Personally, I'm a fan of light sailing Von Neumann probes. However I didn't want my characters to have it that easy. In the setting, the year 2050 is relatively constrained. Von Neumann research is banned, and AIs are not trusted. With these easy answers removed, what would the characters pick?
In my mind, I already had the answer: antimatter. I picked it because fuel is a core problem of interstellar travel. You're going to need a lot of it. Stopping on arrival, means even more fuel. You'll need fuel just to carry the fuel you'll need, in order to slow down.
What kind of quantities are we looking at? Consider Project Daedalus, a plan by the British Interplanetary Society. It's destination, Bernard's Star, is 6 light years away. Daedalus would have been a nuclear fusion pulse rocket. It would fuse deuterium pellets 250 times a second, for four years. It's payload was 450 metric tons, it's fuel, 50,000 metric tons. It was to do a flyby - it didn't have enough fuel to slow down.
Source: British Interplanetary Society
Both fusion and fission rockets have dreadful efficiency. A fission rocket converts 0.1 percent of its fuel into energy. A fusion rocket could do 0.3 to 0.9 percent. You can see how this leads to staggeringly high fuel loads.
Antimatter though, converts 100% of its mass into energy. You cannot get better than this if you're carrying fuel. Here's an online calculator you can use to see just how much energy is packed into antimatter.
Source: EdwardMuller.com
So antimatter fuel is the best stuff ever, with 100% conversion! Or is it?
Turns out that I was quite wrong. Antimatter will give you 100% conversion to energy - only if it's a positron colliding with an electron. With other particles, other products are released as well. 100% conversion isn't even desirable. What it is, are a mass of high energy x-rays. These can't really be guided out an exhaust. X-rays can be bent, but there is nothing practical about it.
Beamed Core Antimatter Propulsion
A bit of Googling, and I started papering over the gaps in my layman's knowledge. There are a few antimatter engine concepts out there. What stood out to me, was beamed core propulsion.
Beamed core involves colliding protons with anti-protons. The products are not pure energy. Instead, you also get a bunch of pions, both charged and neutral. With beamed core, a powerful magnetic nozzled forces the charged pions out the exhaust.
Source: Keane and Zhang
Hence, the pion rocket.
The requirements for the magnetic nozzled involved seemed beyond our technology. Then in 2012, a high school student and his mentor solved this problem.
They downloaded some simulation software from CERN, and ran some numbers. They then designed a magnetic nozzle that worked with a much weaker magnetic field. The design not only solved this, but raised the exhaust velocity from a predicted 0.3c, to 0.69c.
You can read their original paper here, It's quite layman friendly.
So What Are The Numbers?
Part of the challenge with writing hard science fiction, is that you need to get your Science straight. This is that much harder if you're just a layman, like me.
Now that I had cleared up the propulsion, I needed to flesh out the details. What was the efficiency, for mass converted into thrust? How much fuel would be carried by a pion rocket to Alpha Centauri?
I found one reference to a reference, for 11% efficiency. It's not something I could use, so I have to give up on that front (for now). However, after more trawling, and I found this lovely chart on NextBigFuture.com:
Source: NextBigFuture.com
It shows that a 1 metric ton of Antimatter, gets 1 metric ton of cargo, up to a tenth the speed of light. Those are fantastically easy numbers to work with. Even better, 0.1c is the speed of the colony ships.
Making Antimatter
Even at ton for a ton, a hundred ton probe still needs a hundred tons of antimatter. We produce antimatter routinely, in particle accelerators. However, the amounts are depressingly small. It is estimated that CERN could produce a single gram of antiprotons - in a hundred billion years.
However, this antimatter is produced a byproduct. What if a facility was built, specifically to produce it? Scientist and Scifi writer Robert Forward tackled this question. Forward determined that production of antimatter could be scaled from one part in 60 million to one part in 10,000.
Cribbed from the excellent space travel blog, Centauri Dreams:
"Where will we get the energy to run these magic matter factories? Some of the prototype factories will be built on Earth, but for large scale production we certainly don’t want to power these machines by burning fossil fuels on Earth. There is plenty of energy in space. At the distance of the Earth from the Sun, the Sun delivers over a kilowatt of energy for each square meter of collector, or a gigawatt (1,000,000,000 watts) per square kilometer. A collector array of one hundred kilometers on a side would provide a power input of ten terawatts (10,000,000,000,000), enough to run a number of antimatter factories at full power, producing a gram of antimatter a day." - Forward
Farming Antimatter
Antiprotons have been discovered in space in the Earth's magnetosphere, specifically the Van Allen radiation belt. It's formed by cosmic rays smashing into the belt - and is a self-renewing resource. Jason Bickford writes that we can expect the same at other planets in our solar system, that have magnetospheres. Jupiter stands out here - and here's his paper on it.
Whether it's manufactured in sunward factories or mined from planetary magnetic fields, there will be nothing easy or cheap about obtaining antimatter for deep space missions. Especially if tons and tons of it are needed.
The alternative is of course go fuel-free: solar sails pushed by lasers or microwaves. However, I need things to be a bit challenging for the characters. Antimatter for pion rockets will be what they're after. Let's see how challenging this is going to be for them.
Making Antimatter
Even at ton for a ton, a hundred ton probe still needs a hundred tons of antimatter. We produce antimatter routinely, in particle accelerators. However, the amounts are depressingly small. It is estimated that CERN could produce a single gram of antiprotons - in a hundred billion years.
However, this antimatter is produced a byproduct. What if a facility was built, specifically to produce it? Scientist and Scifi writer Robert Forward tackled this question. Forward determined that production of antimatter could be scaled from one part in 60 million to one part in 10,000.
Cribbed from the excellent space travel blog, Centauri Dreams:
"Where will we get the energy to run these magic matter factories? Some of the prototype factories will be built on Earth, but for large scale production we certainly don’t want to power these machines by burning fossil fuels on Earth. There is plenty of energy in space. At the distance of the Earth from the Sun, the Sun delivers over a kilowatt of energy for each square meter of collector, or a gigawatt (1,000,000,000 watts) per square kilometer. A collector array of one hundred kilometers on a side would provide a power input of ten terawatts (10,000,000,000,000), enough to run a number of antimatter factories at full power, producing a gram of antimatter a day." - Forward
Farming Antimatter
Antiprotons have been discovered in space in the Earth's magnetosphere, specifically the Van Allen radiation belt. It's formed by cosmic rays smashing into the belt - and is a self-renewing resource. Jason Bickford writes that we can expect the same at other planets in our solar system, that have magnetospheres. Jupiter stands out here - and here's his paper on it.
Whether it's manufactured in sunward factories or mined from planetary magnetic fields, there will be nothing easy or cheap about obtaining antimatter for deep space missions. Especially if tons and tons of it are needed.
The alternative is of course go fuel-free: solar sails pushed by lasers or microwaves. However, I need things to be a bit challenging for the characters. Antimatter for pion rockets will be what they're after. Let's see how challenging this is going to be for them.
Source: Madmaxz.com
Have you factored in skimming gravity wells for inertia? The fuel is one thing from point to point, but if you slingshot off Jupiter, you can shave lots of momentum. Also, the chances of giants in the subject system is pretty high now from what we're seeing, so using that could make a difference?
ReplyDeleteHola Good Sir!
ReplyDeleteNo, I had not. It also raises the idea of trying fuel just ahead of the slingshot, if Jovian antiprotons are being harvested.
Gas giants at in the Centauri system would definitely make a big difference. I'm having the colony ships be especially rugged. This is so that their components can be useful for a very time.
On arrival, the power plants detach and remain in the outer solar system. This is to assist visiting / leaving ships with refueling. Having them be gas giant, antiproton harvesters, works very nicely for this.