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Re: starship-design: Interstellar Probes

Kyle wrote:

>Depends on what kind of propulsion you use. To get it there fast is
>virtually impossible with current technology. 
>To give you an example of how bad it really is, this is how much it
>would take to send a school bus sized payload past Alpha Centauri in 900
>Propellant:       Specific Imp.:            Fuel mass:
>Chemical           500                      10^137kg (not enough mass in
>Nuclear fission    5000                     10^17kg (a billion
>Nuclear fusion     10000                    10^11kg (a thousand
>Antimatter         50000                    10^5kg (ten railway tankers)
>If you want to get there faster that 900 years, it gets worse. If you
>want to actually stop at the destination, it gets even worse. That's why
>I don't believe we will make it with any of these propulsion systems. It
>would be better for a 2 kg probe, but not good.

I'm not so sure whether the numbers in the table are correct, however the
table does depict an important characteristics: (Kyle, next time you do
calculations please include some rough guide of how you got the numbers.)
- A low specific impulse (Isp) is really bad from a total energy (and thus
mass) point of view.
- A relative small increasing in the Isp will yield spectacular decreases
in energy consumption and thus mass of the fuel.

I'd like to remark that of the 10^5 kg in the last case, only VERY LITTLE
(140 gram) has to be anti-matter, because the amount of energy needed to
give 10^5 kg a velocity of about 5E5 m/s is "relatively" small.

Increasing the Isp to more than 3E5 seconds (exhaust velocities of more
than 1% of the speed of light) will be necessary before any serious probe
will be sent.
>From an energy point of view this doesn't need anti-matter, fusion can
provide enough energy in a relative small package as well. The question is
whether that energy can deliver what we need: few but fast particles.