Thursday, November 19, 2015

Antimatter Batteries

One constant of traveling at fractional lightspeeds is that you have to have the energy to do it. And you have to carry the energy around with you. On a one-way trip with no reserves, you need the energy to accelerate and the energy to decelerate. There is so little matter in space that drag won’t do it for you. So you use forward-facing thrust, which takes energy.

Lots of energy is required, and is it a show-stopper to have to carry so much energy around? Looking at a lower bound might be useful to clarify this question. Consider solely the source of energy for speed changes. A ship needs energy for much more than simply speed, but let’s simply look at this to get an idea of whether it makes everything impossible immediately.

The highest known concentration of energy as matter is an equal masss of antimatter and matter. This will serve as a generator for the lower bound on the mass needed for a power source. Suppose a ship has two tanks, one for hydrogen and another, a very special tank, for antihydrogen. Using anti-hydrogen rather than anti-protons means that those pesky problems of electrostatics don’t have to be dealt with. How good is the efficiency for tankage? Some very clever way of storing antihydrogen on a ship made of matter has to be used, and we don’t yet have much of an idea about how to do it. Maybe in a century or two we will be able to make a better estimate, but let’s use the lower bound for the tankage mass. Zero. You can’t get lower than that.

So we need to look at the mass of hydrogen and anti-hydrogen, which, if annihilated, will propel the ship to a fractional lightspeed. Suppose the ratio of the burned mass to the propelled mass is b, and then the fractional lightspeed achieved by turning the mass of the anti-hydrogen and hydrogen into kinetic energy is simply √(1-1/(b+1)2). This formula looks like this:

Interesting speeds range from 0.01 to 0.1 c, which means that the fraction of ship mass which is this hydrogen and antihydrogen that gets burned ranges from about 0.0005 to 0.05, which does seem overwhelming. This is a lower bound, where lower is almost to the point of absurdity, so a factor of 2 to 10 should be multiplied in to keep things reasonable.

At the lower end of the velocity range, 1% c, the mass of burned fuel is not really important in figuring out the total mass of the vessel. And if we double the burned fuel mass so there is enough for both acceleration and deceleration, it still is not that much. But because kinetic energy, at least in the Newtonian range, goes like v squared, increasing the ship’s speed by a factor of ten, the burnable mass that has to be collected goes up by a hundred. So the upper bound of speed makes the ship look like a giant battery, with living space a small fraction of the total. And propulsion fuel has not yet been taken into account.

This means the tradeoff between speed and travel time will be very, very important in figuring out the total energy required and the total mass of the ship. Energy costs for things like the hotel load, meaning everything for the sustenance of life and maintenance of operations, goes inversely proportional to speed, while fuel goes like the square. These calculations can be made and will likely show that the lower end of the speed range is where the ship would have to operate.

There is also a difference between an autonomous ship and one traveling between spaceports on inhabited planets. The autonomous ship has a tremendous amount of baggage to be carried concerned with its operations at the uninhabited destination planet, or asteroid, or satellite. If the ship is going to be parked in orbit and abandoned, there are some costs which can be eliminated, but if it is going to make a second run, somehow the antimatter battery has to be refueled. While the mass of the battery is not so much, making it takes a great amount of energy.

There is no antimatter lurking around in the galaxy that we know about, so no one can expect to harvest it and stick it into the battery. It has to be manufactured, and we don’t know any easy ways to do that yet. We simply know how much energy is the upper bound of what mass of antimatter is stored.

As a matter of fact, there isn’t much energy lurking around at all. There is gravity, and perhaps someday someone will come up with a scheme to extract gravitational energy in a solar system, and maybe aliens all learn this as part of their asymptotic technology, but if there is no such source, it is pretty much fusion. There is fusion you get for free, from star photons, and fusion you have to pay for, in your own fusion machine.

There has been some speculation that it is possible to do fusion in solid media, the so-called ‘cold fusion’ where deuterium fuses at room temperatures. However, the amount of energy required by a starship is so large, that sources with low production rates would simply be too inefficient to use. Large amounts of fusion power are needed to fill the batteries in any reasonable time. So hot fusion, in a star or in a power station, is needed.

Grabbing solar power for a starship is a bit different that getting it on a planet. A habitable planet has to be at a nice comfortable temperature, meaning that the solar photon flux cannot be too high. This translates into very large areas for collecting the photons. But a starship that can tolerate higher fluxes, perhaps by being extremely reflective, or which can send in a shuttle to an area of high flux, near the star, might be the better way to go.

We do not have much of an idea about how to do the other method of fusion. Some experience indicates the facility might have to be very large, but that is based, not on any successful power-producing experiments, but on designs. Whether something more reasonably sized, and capable of high power production, can be built is not known by us. If aliens with asymptotic technology can do it, then they would likely prefer it as it can be done anywhere the resources, both for the power plant and for the input fuel, exist. It may be that only deuterium can be used, and then an autonomous starship would forever be on the hunt for deuterium sources.

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