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Re: starship-design: Genuine STR question

Steve VanDevender wrote:

> To respond to this in short. the ability of a self-powered
> spacecraft to accelerate to high relativistic velocities (roughly
> meaning > 70% of c) depends on the efficiency of mass-to-energy
> conversion of its fuel as well as the fuel-to-payload ratio.

right --- you didn't speak to the issue of decoupling the drive system
from the energy production system. I went to some length to stick a
battery in the ship with the potential to power the entire journey,
just to point up you can make an interstellar journey on stored energy
alone. No fusion reaction, no antimatter reaction, no coal, just passive
reaction mass which is not fuel, pushed out by stored energy. I don't
like a drive without a reaction, but it makes the point a feline can be
flayed more than one way, to widen perspective on the way things
have to be.

> Efficiency of mass-to-energy conversion happens to correlate well
> with exhaust velocity.

If they are directly coupled, if your energy conversion process is
integral to your drive system. Energy can be transmitted for miles in
nearly lossless fashion, so your energy conversion could be miles
from your drive without adding measurable loss.

> A fuel that reacts with low
> mass-to-energy conversion, like hydrogen fusion, requires
> tremendous fuel-to-payload ratios on the order of 10^6:1 to
> accelerate the payload to high relativistic speeds, while the
> ideal matter-antimatter reaction can achieve something like
> 80-90% mass-to-energy conversion and requires only a 4:1
> fuel-to-payload ratio to get the payload to the same speed.
> When I get time I'll dig up the URL to the archived item where I
> derived these results.  It's true you can reduce the total
> reaction mass you have to carry by accelerating it to high
> exhaust velocities, but if you use an inefficient fuel you end up
> having to carry an amount of fuel that more than makes up for the
> reduction in reaction mass.  And the only way to reduce the fuel
> mass is to increase the efficiency of mass-to-energy conversion
> of the fuel.

The drive is the legs of the ship. The reactor is its belly. Energy
conversion efficiency is how well the belly does. The efficiency
figures for the drive can be a separate figure, measuring how long
the legs are, if your design does not lock these components
inseparably together. The notion I'm advocating at present, is
that it is inefficient for the drive to have a thermal signature. No
matter how hot your fire, none of the exhaust products will have
anything near relativistic speeds. Whatever your fire, and whatever
your fuel, it will pay you to convert the liberated energy to push
in an accelerator, if you can do so with an acceptably low loss.

Johnny Thunderbird