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starship-design: Re: Solar sail breaking
Nick Tosh wrote:
> Hello everybody.
> From what I've gathered from reading past mailings, stopping a starship
> powered by a Sol based particle/EM rad beam is a major problem.
Rex <dotarSojat@aol.com> has analysed this problem and a solution
has been found. While Rex claims it's not very efficient, he does
prove that it is theoretically sound, and violates no physical laws.
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.
This has been shown to yeild more momentum from the "engine", than the
ship would absorb from the sail.
Mass ratios and exhaust velocities, depend heavily upon conversion
(converting recieved microwave energy to kinetic energy in the exhaust)
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)
Now the problem here is that current linear accelerators are not
very efficient, according to Rex. He states that at best, they are
only about 1% effiecient.
In a letter I sent to Tim, I said that I doubted current lineac
efficiencies could be used to model practical maximums.
Consider the following senario:
---------------begin included text------------------------------
Junior designer comes into directors office and says "Hey Boss, I just
figured out how we can boost the efficiency of our main accelerator to
Senior designer <with smirk> "Really, how much electricity will this
Junior desinger <proudly> "5000 Megawatt-hours per year."
Senior designer <deadpan> "And how much will these improvements cost?"
Junior designer <smugly> "Only 25 million dollars, well within our
Senior designer <figuring in head> "Since we pay about 5 cents per
kilowatt hour, your improvements will save 250,000 dollars a year,
but it will take 25 million to implement it. Thus it will take 100
years to pay for the improvement."
Junior designer <dejectedly> "Oh, I never looked at it from that point
Senior designer <laughing> "That's why I'm the boss"
Of course, the junior designer, if he was any good would have figured
the payback time himself, and saved himself the embarassment of this
of exchange. So I'm not sure that accelerator efficiency here on earth
that good of a gauge of maximum possible.
-----------------end included text------------------------------
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!
While it is true that we will need a spinning section while we are in
the target system, we should not need one during the flight.
So again, I propose the following ship configuration:
Consider the in-flight section to be like a soda can, and the habitat
section to be like a larger soup can. While accelerating toward TC,
the crew resides in the soda can, at the turn-around point, the soda
can is extracted from the soup can, turned around, and put back in.
Upon reaching TC, the crew moves into the soup can, which is spun up
to provide the required gravity. All of the exploration equipment
is stored in the soup can. The soda can is now free to be rehabed,
and the soup can acts like a dry dock. When the time comes to leave,
the soda can is what returns. the soup can stays behind as a base,
and if for some strange reason, we decide to leave a permanent force
behind, then we have a place for them to live. if not, then everyone
gets back into soda can and heads for earth. The return Module uses
a microwave Sail to accelerate away from TC, and uses Earth's Masers
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
In contrast, the only other viable alternatives (from a physics
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
ship to build.
Kevin "Tex" Houston http://umn.edu/~hous0042/index.html