Re: Obliterating the Rocket Equation with a Torusail


pgarrone@xxxxxxxxxxx wrote:
> You imply that we can travel round the solar system comfortably now,
> but that the only conceivable method of interstellar propulsion is
> bombtrack.
> This is simply incorrect.

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).

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

> > Specific energy density. High exhaust velocity doesn't really
> > gain you anything if your fuel doesn't have the energy density
> > to support it. For example, fission fuel has sufficient energy
> > density to theoretically accelerate itself to around 3.7% of c.

> So use deuterium with fusion, giving much higher Ve, 0.1C
> and higher. If you wish to make a point that nuclear light
> bulb/photovoltaic conversion/ion drive
> is worthless for interstellar travel, you must assume the
> best possible such drive and destroy
> it. Not describe the least likely implementation.

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.

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).

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.

> > if you attach a nuclear-light-bulb fission reactor to an ion
> > drive which has an exhaust velocity greater than 3.7%c, then
> > you'll be consuming more fuel than the amount of propellant
> > you can exhaust. In short, you'll be dumping some spent fuel
> > overboard while accelerating the rest of the spent fuel through
> > the ion drive. This turns out to give you less thrust and less
> > delta-v than accelerating all of the spent fuel to 3.7%c.

> Nothing in what I wrote implied that I didn't understand that the Ve of

> an ion drive must match the reactor power output.

> Even with your own fuel and numbers,
> given an exhaust of 0.037C and a mass ratio
> of say 500, this would give a delta V of loge(500)*0.037C = 0.22C,
> which allows a cruise of 0.11C, which allows a 10 ton probe with
> 5000 tons of fuel to get to alpha-centauri 3.7 L.Y times 1/0.11 = 33.6
> years,
> which is a finite time, hence interstellar travel is possible,
> so the idea is not "worthless", even on your own incorrect assumptions.

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!!!

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.

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.

Isaac Kuo

.

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