Re: Obliterating the Rocket Equation with a Torusail

1) Allowing fusion

> Do you need a reminder of how long we've been spending
> billions of dollars on fusion research and how long
> fusion power has been two decades away?

Do I answer rhetorical questions?

>
> 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.

2) Using a rocket

> You're the one assuming 25th century science/technology;
> I didn't want to presume such a feat impossible.
>
> As it is, you don't need to "contain" the gas to acheive
> rocket thrust. You only need to direct it, and this can
> be done using magnetic fields. You later clarify that
> you're thinking in terms of intertial confinement fusion,
> in which case you're not actually containing the core
> at fusion temperatures at all anyway. For that, you
> can direct the high velocity fusion products directly
> using a torus shaped magnetic field.
>
> >I'm talking about inertial confinement. Laser beams zap
> >the pellets. This is scientifically plausible.
>
> 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

4He with 3.712 Mev gives 0.044C
p with 14.641 Mev gives 0.176C
To give an average Ve of 0.07C.
If the energy was perfectly distributed, each particle
would have 0.088C. So not much is lost.

(Some of the energy is lost in a D-D side reaction
and also bremsstrahlung. I am ignoring these.)

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. Each such mission would
need millions of tons, and I think there would
only be enough on the moon for one big ark mission.
The other source is a floating atmospheric
station in Neptune. I subjectively regard this
as an expensive option.

Deuteurium is much more readily available
in the oceans of Earth and Europa.
So I justify deuterium using fuel availability.
With smaller unmanned missions, D + 3He would be
the fuel of choice using a magnetic rocket
as you suggest.

With deuterium, the products from 4 x 2H are
3He 0.820 0.024C
3H 1.011 0.026C
p 3.022 0.080C
n 2.499 0

Assume the neutrons are unavailable. This gives
a weighted average of 0.029C. So unless deuterium
can be converted to 4He as a whole, it is not comparably
efficient. If it is, it achieves 0.11C.

So it boils down to an arbitrary choice between
D + 3He with heroic 3He atmospheric
floating fuel extractor on Neptune,
or deuterium with heroic photo-electric convertors.
I choose the later because there is no thermodynamic
limit to the efficiency of photoelectric conversion.

I would conceed that if D + 3He rocket engine, then
the bombtrack equation becomes more serious as a contender,
because the fuels are more equal.

3) Assuming high efficiency.

> Wait--you're using the rocket to decelerate, doubling
> your delta-v requirements? In that case, your estimate
> of rocket performance seems too high. I see later on
> that you assume 99% efficiency, which seems rather
> optimistic.

Certainly. This would not work with any inefficiency
because the waste heat would not be able to be disposed
of. Assume 10**4 meters, 1 percent of 1 petawatt
losses, the temperature from stefan-boltzman would be
11000 degrees kelvin. Even this is too high.
So there has to be highly efficient photo-electric conversion.
99.9% would be more practical.

4) Shockwaves.

There would have to be many pellets per second.
The speed of sound means that energy gets to the
wall from the centre in 7 milliseconds. So the
frequecy of pellets should be many times 100 Hz
to avoid resonances. They would be timed
and spaced for maximal cancellation.
If it had a frequency of 100000 per second,
giving 10 milligrams fuel per pellet,
the energy from one at the wall is 100000 joules/square meter.
Most of this would be reflected due to density difference.

I'm optimistic that it isnt enough to rip matter apart.
This means that the whole thing has to have enormous
numbers of lasers and mechanical pellet firers.

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.

For conduction, suppose 1 meter from the wall,
it is 1000 degreesK.
Use thermal conductivity = 180.5e-3 Watts/meter/degreeK
Power = conductivity * 4 * PI * R * R * T / D
= 180.5e-3 * 4 * PI * 30 * 30 * 1000 / 1
= 2.04 MegaWatts = 2.0e-7 percent of total energy

I would regard cooling inflow as able to handle the conduction
problem. It has negligible affect on efficiency.
Shockwave has to be negligible, otherwise it would
blow up.
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 just using fusion which you say is the "unusual" step.

6) Propulsion system

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

> 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.

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.
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.

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.

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

- 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.

- 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.

- 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.

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.

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

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.

.

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