Re: Obliterating the Rocket Equation with a Torusail


pgarrone@xxxxxxxxxxx wrote:
> 1) Allowing fusion

> > Even if all of ITER's goals are acheived by 2017, that's
> > a scientific demonstration rather than an actual power
> > plant. Any future reactor developed from it would be in
> > the mid-term, or long-term.

> Though it is difficult technology, I think is is
> scientifically valid to contemplate it's use in inter-stellar
> transportation systems.

Yes, I agree--just not in the near term.

> 2) Using a rocket

> > With ICF, it's hard to beat simply directing fusion
> > products rearward with a magnetic field.

> I suppose I have to justify using electric conversion
> rather than a rocket. With a rocket, the obvious choice
> of fuel would be D + 3He. This produces
[...]
> This is quite good Ve. But 3He is hard to come by in
> the solar system. There is some on the moon. It is
> distributed uniformly, so would be hard
> to find concentrated in ores.

If we're talking about a future where fusion power
is used, I'd think the best way to get large amounts
of 3He would be to manufacture it. I'm not sure what
the most economical way would be. One possibility
is to simply use the "waste" products of lithium
deuteride fusion. About half the lithium gets converted
to tritium which instantly fuses with deuterium (producing
energy). The other half turns into helium-3, which
mostly ends up as "waste".

Lithium deuteride fusion power could actually be
done today, with underground cave detonations
powering something similar to geothermal.
Political opposition to this form of nuclear
power in today's world would be...hmm...significant.

> I choose the later because there is no thermodynamic
> limit to the efficiency of photoelectric conversion.

Photovoltaics are subject to the same laws of
thermodynamics as everything else. The
theoretical thermodynamic limit to photovoltaic
conversion is the same as a traditional heat engine.
However, other forms of heat engine plausibly
have better performance.

Obviously, today's photovoltaics don't come
anywhere near the thermodynamic limit, which
limits their use compared to more efficient
heat engines. Things could be really awesome
if efficient photovoltaics were developed. You
could have cars where the engine is little more
than a flame surrounded by PV walls!

> 5) Losses

> The conduction losses as a fraction of the whole
> are insignificant, though as amounts in themselves
> they are significant. Hydrogen gas is a very good
> insulator.

Among all gases, hydrogen gas is the WORST
insulator. At a given temperature, hydrogen
molecules move faster than any other molecules.
Of course, gases in general transfer heat mainly
through convection. In order to turn a gas into
an "insulator", you need to constrain it with some
solid stuff to prevent convection.

> The system essentially uses a thermalizer to convert nuclear
> energy to heat, and beam out that energy using bremsstralung
> radiation. This is the normal nuclear light-bulb concept.
> It has no thermodynamic efficiency limits.

I'm not sure why you think some things are not subject
to thermodynamic efficiency limits. Certainly, the nuclear
light-bulb concept is a heat engine. What you're describing
is obviously a heat engine.

> 6) Propulsion system

> Do you see any fixed scientific efficiency limits
> on electric propulsion?

Not in principle. In principle, electric energy can be 100%
converted into (non-thermal) kinetic energy.

> > I'm talking about the sort of unfriendliness that vaporizes
> > metal so it's lost forever into your exhaust. The lost
> > metal eats into your specific impulse, no matter how
> > much maintenance you put in.

> For a small rocket certainly. But with a mission of this
> scale many tonnes can be lost and not dent the efficiency figure.

Unfortunately, the amount of metal lost is proportional to
the amount of propellant through the thruster(s). If you
scale up the amount of propellant involved, you also
scale up the amount of metal lost. The loss in specific
impulse remains the same.

> 7) Your design suggestions.

> > The photo-voltaics need to be
> > actively cooled at 300K, maybe with liquid droplet
> > radiators. Your reactor's performance is going to be
> > limited by the effectiveness of you heat rejection
> > system.

> I'm thinking of the photo-voltaics as long pointy things
> covered with super-peltier-affect units pumping out heat.

Peltier's have low efficiency, and this isn't going to
change, because of thermodynamics. There's just
no free lunch in pumping heat in the opposite direction
it wants to go.

> The mass flow rate of any projected coolant is enormous.
> I think it would be easier to keep the PV cool then to
> design them to work at 3000 degK.

Then don't use PV. Use a different sort of heat engine
which CAN be designed to work at 3000K. The
theoretical thermodynamic efficiency limit is the same.
Difference being--more traditional heat engines
actually come close to that theoretical limit even with
today's technology.

> 8) Comparison with the torussail concept.

> I will outline direct subjective comparisons
> for my projected mission. Assume for the purposes
> of comparison that fusion-light-bulb-electric-propulsion
> (FLBEP) is possible, and that it
> would also be possible to make reasonably efficient
> bomblets for the TLSBT. (torus-light-sail-bomb-track)

> - With FLBEP system,
> the pellet chemicals and cases can be recycled,
> while with TLSBT they have to be all made
> and placed beforehand. Of course this could
> be cheap matter for acceleration, but for
> a deceleration phase, it would have to be
> carried aboard ship, adding mass to acceleration.
> Obviously this means that the
> FLBEP has to have chemical plants aboard.

For a journey this long, magbrake is easily enough
to provide practically all of the braking. Roughly,
a magbrake reduces ship's speed by one order
of magnitude per decade. For this long journey,
you could plausibly use the magbrake for three
decades, bringing a cruise velocity of .45c down
to .00045c.

> - Its not possible to fully utilize Deuterium with
> TLSBT. With the enclosed system, all the matter
> can be recycled to get a better Ve.

Which is still a net loss, because with a rocket
you are limited to a speed not too much greater
than your Ve. With torusail propulsion, you can
economically acheive speeds much greater than
Ve.

For this long journey, what you really care
about is your ship's velocity, not the velocity
of some exhaust gas.

> - With the TLSBT, it has to be on-time.
> The fuel is going to arrive, ready or not.
> All the deceleration fuel has to be released
> while still accelerating. The FLBEP is more flexible.

Huh? With torusail propulsion, the deceleration
runway can be sent before, with, or after the
starship. It might be deployed during the
acceleration phase, but more pragmatically it
would be deployed during the cruise phase.
It's highly flexible with many options. A rocket
is inflexible in comparison--the fuel needs to be
on board during the acceleration phase. Period.

But as I note above, for this long range mission
no deceleration runway or rocket fuel is
required. You can expend a few decades
mag-braking away your velocity. It's for short
range missions to nearby stars where you
can't afford to spend decades mag-braking.

> - With the TLSBT, there's a bit of flash
> and shock with all the nukes as well.
> It is less practical to have lots of
> mini-nukes, like with the FLBEP.

Actually, mini-nukes are much easier to do
with torusail propulsion. You just need to
collide the mini-nuke with some material
stored on the starship. The immense relative
velocity between the pellet track runway
and the starship provides kinetic energy
to initiate fusion. This can be very efficient
compared to ICF.

But if you're stuck on ICF, you can still use
it for torusail propulsion. In this case,
you can either:

1. Use a mirror to concentrate a long range
laser pulse onto the pellet.

or

2. Use an on-board laser and power it with
MHD generator. This MHD generator
would be fed high velocity plasma from
the "exhaust" stream.

The latter could make sense if you need the
laser anyway for rocket propulsion at the
destination system. In particular, such a
laser could be a convenient way to power
shuttle rockets.

> - With the FLBEP the ship still has a rocket at the
> target system to jet around and gather
> building materials. This would be extra weight
> for the TLSBT.

While true, the desireable parameters for an
interplanetary drive and an interstellar drive are
immensely different. In either case, you wouldn't
want to haul around your massive starship
around the destination system, but rather
multiple "shuttles".

> Conversely, the photo-electric convertors, besides being
> heavy, would be expensive to construct,
> so this could be a compelling reason to lean to TLSBT,
> if they couldnt get them to work.

Well, I think you'd use a more traditional heat engine.
Plausibly more efficient, certainly not too expensive;
doesn't really require new technology.

> However with the more expensive engine the FLBEP
> attains a higher Ve, allowing it to use
> cheaper fuel more effectively.

I see it the opposite way. Torusail propulsion doesn't
REQUIRE a high Ve, which allows it to use cheaper
fuel. And to get up to faster speeds. Without ghastly
mass ratios.

> Unfortunately the choice depends on the exact technological
> possibilities of the time, rather that a scientific analysis,
> so it is not possible presently to make an informed decision.

True enough.

Isaac Kuo

.

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