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Re: starship-design: Re: Solar sail breaking

On Mon, 21 Oct 1996, Kelly Starks x7066 MS 10-39 wrote:

> At 12:48 AM 10/21/96, Kevin \"Tex\" Houston wrote:
> >
> >The idea is my old Microwave Augmented Rocket System (MARS).  A ship
> >departs earth with tanks full of Reaction Mass (RM), and accelerates
> >away from earth using a microwave sail.
> >
> >At the turn-around point, (not necessarily the halfway point), the
> >ship converts the energy from the microwaves into electricity, and
> >uses that to accelerate the RM to a hefty fraction of C.
> >
> >Here is what Rex had to say on this subject.
> >
> >--------begin included text-------------------------------
> >The effects of reduced conversion efficiency (eta less
> >than 1) on required exhaust velocity, final sail furl and, most
> >importantly, required mass ratio are given in the table below:
> >
> >  eta   exhaust velocity   final sail furl   mass ratio
> >  1.0        0.883             0.160            9.41
> >  0.9        0.849             0.105           15.44
> >  0.8        0.809             0.060           29.05
> >  0.7        0.760             0.029           67.57
> >
> >Producing high efficiency of conversion from received power to
> >exhaust power may be as challenging (and as crucial to the
> >success of the concept) as constructing the emitter or the sail.
> >----------end included text-------------------------------
> >
> >So even a fairly high efficiency of .7 means a mass ratio of 68.
> >This is not too bad, considering some of the other mass ratios I've
> >seen bandied about.  (114 for really good anti matter ship)
> What are the cruise speeds and flight times that you can get to with this?

There is no cruise per se.  The ship is a constant thrust mission, 
but the thrust steadily decreases throughout the mission. then builds
up again.

top speed is .93 C
flight time is 8 years for the crew, and 15 years for earth.

> Kevin:
> >Another problem with this design has been pointed out by Kelly.
> >I've said that an advantage with this system is that while in flight
> >we done need any spinning sections.  Kelly's point is that this would
> >have an adverse effect on the crew.
> >
> >Because the power beam from earth is constant (to minimize hassle),
> >and the ship is going to approach light-speed (.9331 of C) then
> >something has to give.  That something is the precieved acceleration
> >felt by the crew.  This will fall to ~1/7 earth normal near the
> >turn-around point.  As the ship begins to decelerate, the "gravity"
> >will again climb toward earth normal.
> >
> >Kelly fears that extended time at less than 1 Gee will have an adverse
> >effect on the crew.  I disagree with this because the change will be
> >relatively slow.  Even after Six months at Zero Gee, Shannon Lucid was
> >able to walk out of the shuttle after it landed (much to the dismay of
> >NASA doctors to be sure.)  and this was an abrupt change, a slow build
> >up of "gravity" should be easy to adapt to, especially if a strict
> >exercise program is instituted.  hell, we always wondered what the crew
> >would do with their spare time, now we know.  They'll be exercising!
> Risian studies on MIR have shown exercise doesn't seem to help much as far
> as G induced heath problems.  (And they had their guys exercise 6 hours a
> day!)

Yes, but that is exercise in micro-gravity.  Our crew will have some
gravity at every stage of the flight.  at the start and end, it will
be more earth like;  near the middle, it will be just like living on
the moon.  if we can have permanent colonies on luna, we can do this 

> >I really think this is the best design we've come up with yet.  Aside
> >from the cost and the political will issues, none of this technology
> >is beyond our capability.  We know how to make solar collectors, masers,
> >linear accelerators and closed system ecologies.
> >
> >The question remains, can we build them large enough, precise enough,
> >efficient enough and will they last long enough to make it to TC and
> >back agin.  But then, these are engineering problems, not physics
> >problems.
> By "them", I assume you mean the sol maser array?

Sort of.  I mean, can we build the solar collectors big enough, the 
maser array precise enough, the lineac efficient enough, and will
the closed system ecology last long enough.

> >In contrast, the only other viable alternatives (from a physics
> >standpoint)
> >is a fusion-sail hybrid (Kelly's fuel-sail) an anti-matter rocket, and
> >the argosy concept.  The fuel sail and the anti-matter rocket both
> >require technology that we do not yet posses, and may have trouble with
> >by 2050 or even 2100.  The argosy concept would take centuries to
> >get to target star.
> >
> >The MARS remains the Fastest, lightest, and easiest (relatively
> >speaking)
> >ship to build.
> Hey!  You forgot my Explorer class with laser launched fuel canisters for
> boost phase!  ;)

No I didn't, MARS is both faster and lighter.  we won't even talk about

> Also you skiped over the technical problem of building and operating the
> return launcher array.  Also I cringe when physisists come up with ideas
> that have clean physics, but very dirty engineering, that they don't want
> to worry about.
>  ;)

you mean like the explorer with laser launched fuel canisters?  ;)

> On the other hand if MARS could be made to work, it would have big speed
> advantages over the others we've come up with.  Thou I wounder about its
> range limits.

This is one of the problems that's worried Tim.  Tim fears that the
beam will spread out too much.  I think that the beam can be focused 
into a nearly paralell stream of photons.  thus there will be no 
spreading and diffusion.  _IF_ this can be achieved, then range won't
be a problem at all.  The ship will be able to travel to any star
within a good 100 light years on a 3-4 light year pulse of maser
energy.  Since most of a 100 light-year trip will be spent at insanly
high fractions of C, I expect the one-way trip time to be on the order 
of ten years for the crew.  of course a return mission would arrive
a good 250 years after departure.