Re: Obliterating the Rocket Equation with a Torusail


pgarrone@xxxxxxxxxxx wrote:

>>For fast interplanetary propulsion, we already have good
>>enough Ve.

>Hardly. It takes years to get to the outer planets.

If you want to get there much faster than that, it costs too
much, IMO. Current electric rockets look to get somewhere
in the range of 5,000-10,000sec Isp, which is a reasonable
compromise between speed and energy cost.

>>>The limiting factor with better Ve is the shear power of the rocket
>>>required, since the ratio of momentum to power is inversely
>>>proportional to Ve. I am pinning my hopes on nuclear-light-bulb with
>>>hyper-efficient photovoltaic conversion and ion drives.

>>Good for fast interplanetary travel. Worthless for fast interstellar
>>travel.

>Why?

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.
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.

Anyway, with the rocket equation as it is, an exhaust velocity
of 3.7%c is not going to give you a fast interstellar cruise
velocity with a reasonable mass ratio.

Isaac Kuo

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