Re: Obliterating the Rocket Equation with a Torusail


pgarrone@xxxxxxxxxxx wrote:
>raphfrk@xxxxxxxxxxxx wrote:

> > The difference between a rocket and the bomb track is probably not
> > massive due to the fact that orbital velocity is only around double the
> > exhaust velocity, so the exponential doesn't really start to kick in.

> I concur. Planning my own mission, with V/Ve of 3 or so, I found that
> the bombtrack equation offered twice the advantage of the rocket
> equation. But carrying two tonnes of fuel onboard is much preferable to
> capturing one tonne of fuel, accelerating it to ship speed, and then
> using the energy of capture to post-accelerate the products out the
> back again, which would be necessary for perfect "bomb-track"
> operation.

You're right about the preferability of the rocket. It sounds like
your mission would have involved some sort of ramjet, since a
ramjet slows down the incoming medium to nearly ship speed.
(By "slows down", I mean in the reference frame of the ship.)

Torusail propulsion is more akin to the scramjet concept, where
the medium isn't slowed down at all anywhere in the process.

> Obviously there is a ratio of V/Ve where the bombtrack becomes
> acceptable. The thread is about a ratio of 10, so of course the numbers
> favour bombtrack in that situation. But they dont consider bombtracks
> for interplanetary missions. Instead they consider rockets with higher
> Ve, ion drives etc.

Yes, indeed. If you can get the Ve, then the operational
simplicity of a rocket is compelling.

> So the question becomes which is easier; to develop the bombot
> technology or to develop more efficient rockets with better Ve.

For fast interplanetary propulsion, we already have good enough
Ve. For fast interstellar propulsion, we probably will never have
good enough Ve. A theoretically perfect fission rocket is too slow.
A decent fusion rocket might be good enough, but it's an open
question whether it can be done at all. A decent anti-matter drive
can almost certainly do it, but anti-matter is plausibly always
going to be very expensive to produce.

> 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.

Isaac Kuo

.

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