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Re: starship-design: Genuine STR question
Steve VanDevender wrote:
> Johnny Thunderbird writes:
> > right --- you didn't speak to the issue of decoupling the drive system
> > from the energy production system. I went to some length to stick a
> > battery in the ship with the potential to power the entire journey,
> > just to point up you can make an interstellar journey on stored energy
> > alone. No fusion reaction, no antimatter reaction, no coal, just passive
> > reaction mass which is not fuel, pushed out by stored energy. I don't
> > like a drive without a reaction, but it makes the point a feline can be
> > flayed more than one way, to widen perspective on the way things
> > have to be.
> Just how would you store all that energy? The best way to store
> energy using the least excess mass that I know of is to store it
> as equal parts of matter and antimatter. Energy in a battery
> _is_ mass.
Methinks you do but too corporeally philosophize. Antimatter in a
magnetic bottle, perhaps illuminated with cooling lasers, is a good way
to store fuel, which has very impressive potential energy. It is the
ultimate fuel. It may not be the lowest-mass approach to energy
storage, however. Energy stored in fields does not have mass as
one of its predominant aspects, just a secondary characteristic,
more abstractly called mass equivalent.
Energy stored in fields is just energy, input and output as electric
current, and maintained as magnetism. It is very controllable, and
almost perfectly efficient, assuming use of superconductors. Say
your most basic battery is a superconductor, helically wound around
a torus. The amount of energy stored in this battery is limited only
by the structural yield strength of the materials, and by how well the
specific superconductor rejects magnetic fields. Crucially, the amount
of energy stored has no direct relationship to the mass of the device.
In a system using the potential energy of fuels, there is always a
direct relationship between the mass of the fuel and the total energy
available. By storing energy in fields, we break this relationship.
In starship construction we are free to think big. Instead of making
our coils out of wire, we make our coils from the flight paths of
charged particles, which we have somehow cajoled or persuaded
to fly in exactly the geometry we want. It's cheaper than wire and
a lot lighter, and we're building big. The efficiency of a flight of
charged particles in free space is exactly the same as current in a
superconductor: perfect. If for the moment you will allow me
a sorceror to stand by and direct these particles in their magic
paths, in time I can show you that there are dynamic forces in
equilibrium which will take the sorceror's place.
For now, the sorceror shows the particles they should zoom
around in a toroidal helix. Again we build up a magnetic field,
which stores energy directly with perfect efficiency. This time,
our battery is a mile across instead of a meter across, as in our
lab superconducting magnet. The amount of energy it can store
is greater in proportion; but what does it weigh? It weighs less
than our one meter lab testbed, because it doesn't have any wire
in it. It weighs less than a photograph of our lab superconductive
magnet, maybe. But it's big, and stores a great heap of energy,
and it is very rigid indeed. That's the kind of battery I like.
Don't tease me about my sorceror, unless you want to jump
into serious engineering with charged particle dynamics. Most
folks would rather eat nails. But if it worries you too much
to think about big structures which contain no solids, retreat to
storing energy in superconductive magnets. I mentioned the
energy stored is not proportional to the mass of the coil, as
long as mechanical strength is respected. You may wind
each coil around a torus of Hydrogen ice, which gives fair
mechanical strength at low mass, cheaply. For cooling, shade
works pretty good, in space. The energy you can store in
each coil is really surprising; it takes a hellacious magnetic
field to crush a tube of solid Hydrogen, and that's how much
juice you can feed it. The coil weighs the same, no matter
how much energy it contains.
So saying batteries must be heavy isn't strictly true. The
proportionality between mass, and the amount of energy
available, only holds with fuel systems. Energy storage in
fields doesn't have such a restriction, and its perfect efficiency
is always a big plus. Field storage techniques will hold an
arbitrary amount of energy, which by coincidence just
happens to be exactly the amount of energy we need on
our starship. That's my general answer to your "how "
question, though in particular I like to have more particles
and stuff whizzing around, while there are magnetic lines
of force handy to hang them on, to store yet more energy.
Humans have burned fires for eons, until it has become
practically instinctive with us. Heat engines are the foundation
of our technology. We can predict their performance very
precisely with thermodynamics. We know they are not the
best approach for all technological tasks, particularly those
demanding the ultimate in efficiency. Space transportation
is emphatically an application where efficiency should be
stressed above all else. I say we should de-emphasize the
heat engine and all variants of thermal reactors in starship
design, for all applications in which electromagnetic machinery
can be used instead.
No complex heat engine has ever approached a 40% level
of efficiency, but electrical machinery quite typically achieves
98% efficiency in practice. This tells us fire may not be the
best answer this time, nor fuel our main concern. If we can
store astronomical amounts of energy in cold fields and cold
dynamics, we reduce the need for both fire and fuel.