[extropy-chat] FWD [forteana] Re: Are dwarfs better forlongduration spaceflight?]

[Mike Lorrey]

--- Adrian Tymes <wingcat at pacbell.net> wrote:

> --- spike <spike66 at comcast.net> wrote:
> > > Actually, considering how small a fraction makes up the crew and
> > crew
> > > support - the "payload" - of any manned space vehicle to
> > date...yes, we
> > > are talking about shaving close to the margin...
> > 
> > Ja, ok I see now where is the disconnect.  Assume the
> > surface-landing manufacturing machinery is launched
> > separately, sent on ahead.  It can go with slow, highly
> > efficient ion drives, all the tricky stuff, since it
> > has no consumables.  The hab module is in a hurry, so
> > it will likely use chemical rockets on a Hohmann 
> > transfer orbit.  
> 
> Doesn't help enough.  Even if we're just talking a one-person
> spacecraft to Mars and back, the constant thrust of a nuclear rocket
> would beat out the higher ISP but lower total thrust of chemical
> rockets.

You are backwards here. Chem doesn't have higher Isp, it has lower.
LH2/LOX is typically 400-450 sec, nuke is typically 600-800 sec,
ion/plasma ranges from 1500 to 10000 sec.

Nor would a nuclear rocket have constant thrust the whole trip, they'd
thrust for the first couple weeks or less of the trip, then thrust
again at orbital insertion. Only with Ion would constant thrusting be
used.

> 
> > The design exercise is now just the human 
> > hab module.  In that scenario, I imagine the
> > vehicle as a spherical shell about 4 times her height.
> > The propulsion system scales with the mass of that shell,
> > which scales as the cube of the diameter.
> 
> The scaling factor of different propulsion systems trumps the
> difference in crew dimensions.  For example: calculate how much
> rocket you would need to make the trip in a few months (one way) on
> hydrazine.
> Then calculate how much you would need for nuclear propulsion.

And how long each takes will tell you how much supplies, oxygen, etc
you need. You trade off hab space for fuel bulk or vice versa one way
or the other.

> 
> But you're still missing the non-physics objection: the economics and
> practicality of finding and training that one perfect (by your
> guidelines) person, not to mention the cost of specializing habitat
> manufacture (which would ordinarily be partially borne by other
> missions hoping to send their own people up, but could not because
> your
> habitat rules out the people they wish to send), would seem to impose
> costs that at first would grossly outweigh the savings you'd gain by
> slightly reducing the mass.  Physics is not the only determiner of
> economics, but economics is quite often the determiner of whether you
> can or can not pull off a project of this nature.

Astronaut selection and training is ALREADY this way: they seek perfect
physical specimins with the requisite education and training. Spike is
looking for a specific type of *imperfect* specimin.

Mike Lorrey
Vice-Chair, 2nd District, Libertarian Party of NH
Founder, Constitution Park Foundation:
http://constitutionpark.blogspot.com
Personal/political blog: http://intlib.blogspot.com


	
		
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