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

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

L. Parker wrote:
>On Sunday, November 16, 1997 10:40 PM, Isaac Kuo [SMTP:kuo@bit.csc.lsu.edu] 
>wrote:

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

>What, you think I am trying to go to Mars?

No, I was just trying to get it nearer a realistic value.

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

>No, it was fairly explicit, a crewed ship would be limited by its 
>acceleration to a final cruise velocity of 0.003 c (Matloff & Mallove, 
>JBIS, 1981 & 1983 also Ehricke, JBIS, 1972) and Icarus, with a 1 km sail 
>would attain 0.012 c without any additional boost from beamed power 
>(Eshleman, Science, 1979). Forward actually proposed a combination of the 
>Perihelion maneuver and beamed power to reach the 0.03 c figure.

You mean .3c, of course.  And this _requires_ the use of a high
power laser (yes, a coherent microwave beam is a microwave laser).

Your e-mail suggested that the .3c was "for free" just from the
maneuver around the Sun.  Of course, it isn't.

>Beamed power alone pushed Starwisp to 0.20 c in a few days.

Forward's proposal would take 2 weeks, which I wouldn't call
"a few days".

>Starwisp is designed 
>from the start to withstand hundreds of g's acceleration and these figures 
>take it into account (Forward, Journal of Spacecraft and Rockets, 1985). As 
>for thermal shielding see Ehricke, JBIS, 1972.

Actually, it is proposed to withstand 100 gees.  There is no attempt
by Forward to demonstrate how this could be acheived.  Theoretically,
negligible structural strength is required so long as the laser
pushes absolutely evenly across the entire 1km of the Starwisp.

In practice, even the slightest error (which realistically there
will be) will rip apart the Starwisp as soon as the beam is turned
on.

You must realize that none of these proposals are complete, and
pretty much all of Forward's proposals have huge gaping holes.
At first, he proposed using a ring of microwave beams emitting
in phase in order to acheive the aperture needed using VLBI.
Obviously, he didn't know that it wouldn't work.  Later he came
up with the fresnel lens concept, which would work (but reduce
beam power by about 50%).

He never bothered with the engineering difficulties of making
and stabalizing such a huge fresnel lens, though.

>> 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).

>Umm, you'll just have to take my word for this one, the systems are still 
>classified.

Not all data is classified.  The costs of these these things are
often reported in news reports and AvLeak articles.  For instance,
you've probably heard a gazillion times how much a Tomohawk cruise
missile costs, seeing as they mention it every time they get used.

>> 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
>> foil.

>Again, Forward's design allowed for this. Rightly or wrongly, I don't have 
>the time or the inclination to second guess his engineering.

No it didn't.  He just didn't bother with that part of it.  He
also didn't bother with figuring out how much the Starwisp would
get heated by the laser, which is an even more basic issue.

>> It uses a powerful laser to accelerate,
>> so it doesn't require any on board fuel.

>No, it uses a microwave beam from a power satellite to accelerate, NOT a 
>laser.

It is a coherent microwave laser.

>> 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.

>Okay, so I was stretching that part a little bit <G> that was why I said 
>VIRTUALLY though. BTW, Starwisp doesn't use a fresnel lens either. You are 
>confusing your designs...

Umm...yes it needs a fresnel lens.  It's huge multi-kilometer diameter
fresnel lens made up of consentric rings of reflective material.

>> However achieving this data will require probes like NASA's 1000AU
>> proposal--relatively heavy probes packed with useful sensors.

>Which interestingly enough will probably use fusion thrusters...

No, it uses a fission reactor powering an ion rocket.  This is an
actual _proposal_, which you can see on NASA's web page.  In other
words, something they could actually build and fly, if given the
funding.

>Isaac, I am not defending sails, I prefer fusion or antimatter powered 
>engines actually. But sails do have some usefulness and most of this 
>argument was settled almost 20 years ago.

I'm not attacking sails--however I am pointing out that the additional
boost you could get with a solar flyby for an inter_stellar_ mission
is not ever going to be worth it.  Such a sail will need to be
powered by one of:

1. Laser
2. Particle beam
3. Nuclear bomb track

Whereas the small additional help a solar boost could give is far
outweighed by the disadvantages.

>This is OLD information. If you 
>don't believe me, go to the Advanced Propulsion Concepts conference at JPL 
>next March. I believe there are SEVERAL papers being presented on advanced 
>solar sail concepts.

A solar sail is _extremely_ different from a powered sail.  The thing
about a solar sail, which has great potential for interplanetary
missions, is that they need virtually no structural strength beyond
that needed to keep them from wrinkling up due to vibrations (of
the payload).  They are micro-gee devices that absorb a pretty small
amount of heat due to the Sun.

A powered sail, OTOH, is really only applicable to interstellar
missions (or as anti-vehicular weapons).  For interplanetary
missions, laser powered rockets will vastly outperform laser
powered sails in almost every aspect.  For interstellar missions,
you need maybe 10% c to make it worthwhile.  Because of diffraction
limitations, you are limited to accelerating over several weeks,
so you need very high acceleration levels.  This means the sail
needs to be able to withstand and stay "flat" while under dozens
of gees as well as worry about being heated by the laser.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
 __|_)o(_|__
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi