Re: Obliterating the Rocket Equation with a Torusail


> No, I specifically talk about _fast_ interstellar propulsion, and I
> also mention other methods of acheiving fast interstellar propulsion
> (i.e. mag-beam, laser sail).

OK. It's only nuclear-light-bulb/ion-drives you reject.

> But the method you proposed isn't suitable for fast interstellar
> propulsion.

Duh.

> Using deuterium for fusion is currently less efficient than
> using uranium for fission. In fact, deuterium fusion efficiency
> is currently negative--you actually have to pay energy in
> order to fuse deuterium, rather than getting energy from it.

It's unreasonable to assume that nuclear fusion won't be
developed in the near term, and over the next few
centuries. Deuterium is a nuclear fusion fuel.

> However, let's optimistically assume you have worked out
> the technology needed for controlled D-D fusion. Why in
> the world would you use a nuclear lightbulb/photovoltaic
> conversion/ion drive? Plausible methods of acheiving D-D
> fusion involve extremely high temperatures and/or
> pressures. All you need to do to get efficient high
> velocity thrust out of that is let some of the reaction
> products escape the reactor (this probably occurs
> anyway).

To get the maximal energy from D all the way to 4He,
one needs to salvage 3H, 3He, and the D that results from
the neutrons being scattered and absorbed by the hydrogen
gas/plasma in which the reaction takes place. This is only
possible in an enclosed system.

> The nuclear lightbulb concept only makes sense with a
> low pressure/low temperature reactor, like a gas core
> fission reactor. But a fusion reactor? Not plausible.

Please expand. For this idea to work, the gas has to be dense enough
to absorb all the neutrons, and convert all the power to photons
when it gets heated. What is the limitation on temperature/pressure?
Why not fusion?

> I do not consider .11c to be "fast" interstellar travel. For me,
> "fast" interstellar travel starts at around .2c. But even if you
> consider .11c to be fast enough to be called "fast" interstellar
> travel, your idea can't work. A 10 ton probe? You mean, including
> the fuel tanks. And the reactor. And the heat rejection system.
> And the ion drive. Of these, only the fuel tanks could in principle
> fit within the 10 ton mass budget (theoretically, the fuel can be
> in the form of solid uranium rods in a self-supporting structure).
> The worst part is the ion drive--thrust/weight ratios for ion drives
> even at current Isp levels are in the microgee range. For a
> .037c exhaust velocity, the thrust levels would be negligible.
> Even assuming many orders of magnitude improvement in ion
> thruster power/weight ratio, it would take hundreds of years to
> reach .11c--assuming the ion drive has to accelerate only itself!!!

>>From the textbook "spacecraft systems engineering"
by Fortescue and Stark they mention a 10 cm electrostatic
ion drive for satellite positioning
with 10mN thrust, 30 Km/s Ve, 1 amp discharge current,
anode voltage 42 Volts.

Ramp up the voltage so Ve is 0.037C gives a voltage of 5700000 volts.
The thrust would be 3.7 newtons. Scale this up in current by a few
hundred, and a 10 tonne mass could be accelerated to 0.11C
in 5 years or so. So while I agree it is a technological problem,
I do not concede it is inherently unsolvable. I would concede
it is probably outside current technology.

I think it is an unfair method of argument to conceed a
performance figure, then when it is used against you, attack that
figure, which is what you have done with this 0.037C value.

I have been working on an interstellar ark configuration.
The reactor and motor together weigh 50000 tonnes.
The payload is 85000 tonnes.
The power is one petawatt.
The reactor is a sphere of 30 meters radius.
The gas pressure is 10 megapascals.
Top speed is 0.178C.
Travel time is 440 years.
Acceleration time 67 years,
Decelleration time 13 years.
It launches in the 25th century.
Total launch weight is 4.678 megatonnes.
The drives work at 11.87 mega-volts, 84 mega-amps.
Deuterium Fuel consumption 1.7 Kg/sec

I can respond to your taunts more easily if you discuss this
configuration.

> I gave you the benefit of the doubt and assumed "ion drive" was
> a typo. But you've repeated it many times now. In principle,
> a far more powerful electric drive than an ion drive may be possible.
> I like the idea of theta-pinch pulsed plasma thrusters, myself.

I meant to use "ion drive". I meant a generic drive
where he4 is ionised and electromagnetically accelerated.
I did not mean to exclude plasma drives.
Thanks for the idea.

> Note that with torusail propulsion, acheiving .11c only requires a
> mass ratio of 9. That assumes perfect efficiency, but that's okay
> since you assumed perfect efficiency also. With losses and
> overhead, the mass ratio could plausibly be more like 15. But
> why go at only .11c? You can halve the time required and only
> quadruple the mass ratio. With a mass ratio of 60, you'd get
> a cruise velocity of .22c.

You have to present all the facts in one go so that all the assumptions
can be cross-checked. Questions in my mind are what about
decelleration phase, what about fall-off in efficiency as speed
increases,
what about extra shielding due to torus shape, whats the Ve of the
fuel,
how do you arrive at the figure. What about shielding from all the emf.
Its too hard to go back and forwards between
your website and your postings.

Me agreeing to allow you perfectly
efficient bombs on your bombtrack, it you allow me 99 percent
photovoltaic conversion efficiency is possibly a fair trade, but
hardly scientific. Besides that you're still attacking the rest of
my concept so no way. Besides that I like attacks.

I honestly think there is a technological conflict between hi-tech
guidance gear and low-tech propulsion technology with the bombtrack
concept. Perhaps an evil overlord has forbidden the development
of more sophisticated forms of propulsion, but the rebels steal
the guidance gear.

.

Relevant Pages

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