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starship-design: What is savest?

Zenon wrote: (But the message is to all)

>>   ;)  Ah but that one would be impossible, or at least an order or two
>> magnitude more expensive.
>?? As follows from all our discussions,
>the biggest problem is the propulsion.
>And the one-way mission has only HALF of this problem.
>So you calim that having half the problem makes 
>it impossible or more expensive?
>I agree that the one-way outpost-building mission
>will need more supplies (including a sort of factory
>to build even more at the target), but I think
>this additional cost will be negligible as compared with
>the cost of engine/fuel needed for the way back.
>The engine for a two-way mission must be made reliable 
>for twice the time and for two start-cruise-stop cycles.

It is almost a fact that every two-way mission needs to get its fuel for the
return-trip from the target system. That is, if you want to decrease cost
(and thus effort).
If a design allows to do without resources from the target system, it means
that fuel supplies square, which usually makes bad numbers worst.
However most of our designs can't do without resources from the target
system anyway, so we have to face that we'll need some form of industry at
the target system. This does not necessarily mean that we need complete
rocket building factories, but instead specialized fuel or other bulk ore

The question is how much effort would a one-way mission save? It likely does
need less bulk resources, but more specialized resources. There's a big
chance that both kinds of mission will cost as much.

There is however a big advantage of staying in the target system, rather
than "floating" through space another 10 year. In the target system you have
all the resources you want (including energy), but in space you've nothing.
Also in the target system you won't need your most critical (and likely most
deteriorating) part of the ship: The engines.
Kelly continously tells us, that to stay at the target system we need to be
selfsufficient. But don't we need to be selfsufficient if we stay from home
20 years (which is the minimum time for a two-way mission)?

In short, I'm not so much wondering what is cheaper, but more about what is
saver. Or to put is less subjective: What has a bigger chance of succeeding?