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Re: starship-design: Re: Re: regarding fuel expenditures

L. Parker wrote:
>On Sunday, November 16, 1997 10:31 AM, Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] 

>> That's not the point.  The point is that the potential extra benefit
>> is a ridiculously small amount compared to a percent of c in delta-v.

>I wouldn't call thirty percent of c without expending ANY onboard fuel 

Umm...and the Oberth maneuver around the Sun acheives this how???

I think you mean 1/30 of a percent of c (100km/s).

>> In fact, the disadvantages are such that they overwhelm any advantage.

>What disadvantages? Unless you are talking about crewed starships, which I 
>conceded upfront were impractical for this maneuver, there aren't any 

The disadvantages are the increased structural strength, and increased
heat rejection capability (to deal with solar heating).

>> Even without the human limitation, strengthening an unmanned probe
>> for 400 g's will increase its mass by at least some small fraction.
>> This will make it require _more_ fuel, not less.

>Solid state electronics are routinely subjected to far higher accelerations 
>and continue to function quite well thank you.  (Instantaneous g forces of 
>over 1,000 g have been successfully withstood.) These were airborne systems 
>designed for aircraft with relatively low payloads.

Huh?  I know of the electronics for Copperhead warheads which sustain
in excess of 10,000gees, but they are artillery shells.  However,
because of this hardenning Copperhead shells cost 100 times as much
per round as Hellfire missiles (which have more range and pack
more punch).

>I don't think there is 
>any problem here. Perhaps you aren't familiar with the design criteria for 
>Starwisp, which is the unmanned probe in question here.

Starwisp would get ripped apart by tidal forces if you swung it
around the sun.  There is a _big_ difference between a tiny
little microchip mounted on relatively thick, sturdy, silicon
substrate, and a kilometer wide wire mesh thinner than aluminum

>> It would not be significantly faster.  And that's the point.

>A running head start at 30 percent of c is significant for an unmanned 
>payload unless you can show me that you can accelerate the same payload to 
>the same speed without exceeding the mass of the sail (you can't, the free 
>fuel will get you every time). YOU must include engines AND fuel.

Sure--look at Starwisp.  It will only function if it's launched
far from the tidal forces of a planet or the Sun, of course (this
should be very obvious).  It uses a powerful laser to accelerate,
so it doesn't require any on board fuel.

>The reason that Starwisp is important is that it is virtually within our 
>reach NOW,

Actually it isn't.  It still requires a huge laser which we can't
build yet, a kilometer wide sail which we can't design yet,
miniaturized electronics which we don't have yet, and a super
huge fresnel lens which we can't even begin to design yet.

>next fifty years if we start soon, and they are small and cheap enough that 
>we could literally mass produce them. Without some sort of data on the 
>interstellar medium, and the potential destinations, we could waste a lot 
>of money and lives on useless trips.

However acheiving this data will require probes like NASA's 1000AU
proposal--relatively heavy probes packed with useful sensors.
Forward's Starwisp would only transmit low resolution images of
the target system as it flew by.  Given our advances in telescope
technology, it's not clear Starwisp would ever be worth it.

>If we can get a free boost to 0.3 c 
>from the sun for a probe, I don't really care what other sort of drives are 

Sure, but where in the world do you get this free .3c figure?
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi