Friday, January 1, 2016

Power for Starships

A previous post outlined the energy picture in an overview. The bottom line was that alien civilizations with a high standard of living require fusion power of some sort to be affordable. By affordable, it is simply meant that a standard fusion power plant will produce more energy than it consumes, with absolutely everything considered. Everything means finding the fuel and every single material that is used in the construction of the station, its operation, and its disposal, unless relic stations will be allowed to litter up the planetary surface or subterranean levels. It also means all the operational costs will be covered, even the training of the staff to use it, if everything is not done robotically by then. There has to be net energy our of this process, and it cannot be close to zero, or randomness will kill the operation. Maybe a good limit would be that a fusion plant must produce, over its lifetime, twice as much energy as goes into making it. With this ratio, 2, of gross to net, much of the activity of the civilization will be going into power production. If the ratio is more comfortable, like 10, the civilization will have much more surplus energy to play with and amuse themselves with.

Let’s call this ratio nE for convenience. It is certainly affected by the size of the plant, as there are non-linearities all over. For a power plant to produce energy for an alien civilization, something like an optimal size could be used, as the footprint is what matters, and if there is a whole planet to put these things on, that should be no problem. But the behavior of nE as total power output drops down might be fatal, and that smaller plants are simply not economic. In other words, to make a small plant requires too much energy compared to the total energy it will produce over its lifetime. And, as had been noted in that previous post, if the minimum size to get nE over 2 is too large for a starship, it cannot be used. Why would it be too large? Because the alien civilization would find it unaffordable, in the affordability sense that has been elaborated upon in recent posts.

Other forms of fuel that are known to us are chemical. Even the best chemical fuel, which might be a hydrocarbon and oxidizer, does not produce much energy compared to fusion fuels. A short trip might be possible with it. Radioisotopes also are a form of fuel that has been used in long-term probes, but they have very low power to weight ratios. In a different post, antimatter has been discussed. Storage of antimatter might be simple in the atomic form. Sparks needed are what matters. Pouring a strong acid into a strong base, even at cold temperatures, sets off a reaction. It is impossible to store the two in mixed form. Explosives do store energy in mixed form, although some explosives don’t have a long shelf life and have to be mixed shortly before use. Some spark is needed to set them off, perhaps a small explosion. Fusion fuel, say a D and T mixture, just sits there and the spark has to be something that raises the temperature very high. Pressure needs to be high as well in order to have a large reaction. Temperature alone only would produce token amounts of fusion energy output.

We don’t know what spark is needed for an atomic antimatter matter combination to explode, so it might be possible to store atomic antimatter quite simply. At this point in our collection of knowledge, antimatter is a viable possibility for starship power.

This form of energy storage, or any form for that matter, now known or now unknown, still begs the question of the origin of the energy that goes into storage.

If fusion is the only source of energy in sufficient quantities to gather the necessary energy and convert it to antimatter, or whatever form is used in the starship, and fusion plants need to be very large, cruising the galaxy is difficult. The ship cannot stop somewhere and gather up fuel to go onward, except at an outpost of civilization that already has fusion power plants with enough surplus power to fill up the starship’s supply of energy storage. Exploring the galaxy isn’t the scenario that is allowed in this instance. Instead, what can be done is some short length trips to places that can become colonies, and then once they are established, interstellar travel between them would be possible, if there was any justification for it.

One other thing. Right now we have no idea about what an efficient antimatter production facility would be like. We do know there will be some efficiency associated with it. Consider a fusion power plant located right near an antimatter production facility, including the storage activities. This pair of facilities takes in DT fuel, or maybe He3 or lithium or just deuterium, and outputs storage units of antimatter. On the ship there is a converter of some sort, most likely thermal conversion from the annihilation burning of antimatter with matter. What is the ratio of stored energy that comes out of the dual-facility plant to the total gross energy needed to run it? It certainly must be smaller than nE. Perhaps the conversion of power, say electrical, that comes out of the fusion half of the facility, to stored antimatter energy, is only 1%.

This is a shocker, and perhaps a Great Filter. If nE is 10% and na, to give an algebraic label to the conversion efficiency of the antimatter creation and storage half of the facility, is 1%, it means that a thousand times as much power goes into the plant as comes out. Suppose further than the conversion of stored antimatter energy to propulsive power is only 10% as well. This means, that when we calculate using simple kinematics, how much energy is needed to propel a starship to fractional light speed, the alien civilization must pony up ten thousand times that much energy.

Is this a reasonable thing to expect an alien civilization to agree to do? Obviously they can’t be sending out ships hither and yon, on the whim of individuals. When you look at the alien spaceports pictured in science fiction films, just imagine over the horizon the entire planet surface devoted to power production facilities needed to keep these babies running. Not.


  1. I enjoy all your texts and i just wanted to say that you are awesome! Greetings from Belgrade, Serbia and happy holidays.

  2. In the long run, DT or He3 fusion will not be needed. We may be able to have catalyzed proton - proton fusion or at the very least, lithium -proton or other forms of aneutronic fusion. In this case we will be able to reduce the size of the fusion power plant, and have a much larger source of fuel.