Ships for Space Travel

(dave schneider) writes:

(George William Herbert) commented:
Gordon D. Pusch wrote:
(dave schneider) writes:
(Henry Spencer) wrote:
A factor of 100 improvement would bring it down within reach of reason,
but a 1km mirror is beyond what's reasonably practical in the near future.
Eventually, yes.

Is there any indication of how big a bubble could be blown in
microgravity to create an Al or Au sphere, that could be sectioned to
provide several spherical mirrors (yes, I know, there's another conic
section that is better for focussing; ignore the man behind the
curtain for the moment) ?

For a sufficiently large focal length, you don't even need spherical sections;
the individual sections can be optically _flat_, and still not deviate from
the ideal figure by more than a fraction of a wavelength. (IIRC, a 10 km
focal length is sufficient for this to be true.) The primarily problem
then becomes one of _aligning_ the array of mirrors --- not machining.

There was a story in Analog science-fiction magazine some time
ago about exactly such a project.

Subplots abounded, but the major technical problem was the
structure's vibrational modes...

Well, the idea I had about the bubble was that it would a) take care
of the aligning of sections and b) reduce or eliminate the need for
truss sections.

You will find that it is not possible to blow large bubbles,
perfectly spherical to within a fraction of a wavelength of light,
even in "zero gee"...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

In article ,
dave schneider wrote:
...a 1km mirror is beyond what's reasonably practical in the near future.

Is there any indication of how big a bubble could be blown in
microgravity to create an Al or Au sphere, that could be sectioned to
provide several spherical mirrors...

I don't think bubble-blowing is going to scale up to that size, given that
many other things (e.g. surface tension) won't scale with it.

In any case, an object that size is *going* to be flexible, so it will
have to be backed with a support structure, probably with active control
actuators too. At that point, you might as well make it in segments
rather than trying to handle a huge thin mirror.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Dave Schneider wrote:

DS ...a 1km mirror is beyond what's reasonably
DS practical in the near future.

DF Is there any indication of how big a bubble
DS could be blown in microgravity to create an
DS Al or Au sphere, that could be sectioned to
DS provide several spherical mirrors...

Henry Spencer wrote:

HS I don't think bubble-blowing is going to scale
HS up to that size, given that many other things
HS (e.g. surface tension) won't scale with it.

HS In any case, an object that size is *going* to
HS be flexible, so it will have to be backed with
HS a support structure, probably with active control
HS actuators too. At that point, you might as well
HS make it in segments rather than trying to handle
HS a huge thin mirror.

As a near-term technology, making large mirrors from
small segments is unbeatable. In the long term, however,
technology based on glass blowing looks attractive:
http://www.islandone.org/LEOBiblio/S....HTM#telescope

http://www.astronautix.com/lvfam/orion.htm

http://216.239.41.104/search?q=cache...=en&ie=UT F-8

http://ffden-2.phys.uaf.edu/213.web....ionfusion.html

The URLs above describe a sort of ship that's possible to build using
nuclear fuel. In the 1940s and 1940s nuclear pulse units - miniature
a-bombs - were proposed as a means to propel spacecraft. This
resulted in Project Orion, which was cancelled with the signing of the
Nuclear NonProliferation Treaty in 1963.

Since that time the same technologies that were explored to create
inertial confinement fusion were also explored to create very small
inertial confinement fission - so called, micronukes. Micronukes -
nuclear hand grenades, can be used directly for propulsion, or
indirectly as triggers for relatively clean mini-H-bombs. In either
case, total energy yeilds are such that total containment of the blast
is feasible, and we end up with spaceships the size of ocean liners to
supertankers - capable of flying across the solar system with ease.

Check it out;

http://www.niac.usra.edu/files/studi...f/76McNutt.pdf
http://fusionenergy.lanl.gov/Documen...tfrefs8-99.PDF
http://128.97.43.7/bapsf/papers/Gekelman-laserJGR.pdf


http://hypertextbook.com/physics/mod...on/index.shtml
http://hyperphysics.phy-astr.gsu.edu...e/fission.html

Lithium-6 Deuteride produces 10 kiloton TNT equivalent explosion when
0.156 kg of it are detonated. At 0.82 gram per cc, this means that
190 cc of the stuff are needed for each blast. A sphere 7.1 cm across.

A 2 ton TNT equivalent fission trigger consisting of 100 mg of
Plutonium is made from wire about the size of a paperclip. If made
from the world's existing stockpile of nuclear weapons;

http://www.nrdc.org/nuclear/nudb/datab19.asp

There would be plenty to go around. Also, Deuterium is abundantly
available in the world's water supplies. And, Lithium-6 consists of
7.4% of the world's supply of Lithium. The US imported 3,000,000 kg
last year

http://minerals.usgs.gov/minerals/pu...ium/450301.pdf

A minimum traditional weapon (not the advanced type supposed here)
contains about 5 kg of Plutonium. So, we have about 50,000 kg
available from current weapons stockpile. So;

50,000,000 grams Pu - 0.1 gram -- 500 million triggers
3,000,000,000 grams Li-6/yr - 156 grams -- 19.2 million units/year

Deuterium is relatively unlimited - since its abundantly available in
the world's water supply.

So, we have enough materials to last us 25 years with 20 million
blasts per year.

156 grams expanding with 10 kiloton 41.84e15 joules of energy - has an
average velocity of;

E = 1/2 * m * V^2 -- V = SQRT(2*E/m)
= SQRT(2*41.84E15/0.156)
= 23,160,532 m/sec

So, if our weapon's experts can design a miniature nuclear explosion
that efficiently deposits the bulk of its energy into the reacting
medium, we can obtain exhaust velocities exceeding 20,000 km/sec!

Compare this with the Space Shuttle's 4.5 km/sec exhaust speed !!!

Okay, with this kind of performance its easy to see that we can do
amazing things.

For example, to move 20 million kilometers (2e10 meters) at 1/10th gee
constant (after escaping Earth) - accelerating half the time and
slowing the other half - to land softly on Mars (assuming its 20
million km away at the time) requires

D = 1/2 * a * t^2 and V = a * t -- t = V/a -- D = 1/2 * a *
V^2/a^2

D=V^2/(2a) -- V = SQRT(D*2*a)
= SQRT(2e10*2*0.982)
= 198,191 m/sec
= 198.2 km/sec

To get to the half way point, and the same amount to slow - with
slight variations due to the relative speeds of the planets which
amount to a few 10s of kilometers per second.

So, a spacecraft that could achieve a 500 km/sec final velocity would
be able to execute a constant 1/10th gee flight to Mars and back, when
it was near Earth.

This trip would take; t = 198,191 /0.982 = 201,823 seconds = 56 hours

to each half way point. A round trip wold take 224 hours - LESS THAN
10 days!

The amount of propellant needed to carry on board would be given by;

Vf = Ve * LN(1/(1-u)) --- u = 1 - 1/EXP(Vf/Ve)
= 1 - 1/EXP(500/20,000)
= 0.0247

Less than 2.5% of the spacecraft mass is needed to be the pulse units
described above.

Okay, so 20 million blasts per year of 0.156 kg pellets translate to
3,000 tons again - divide this by 2.5% - obtains 124,800 tons per year
carried to and from mars in this way.

Of course, this is very inefficient. The most efficient way to carry
stuff by rocket is to have the exhaust speed equal the final speed.
So, if we carry sufficient propellant to energize it to match the
final speed - and pack it around the pellets - then, we can compute;

u = 1 - 1/EXP(Vf/Ve)
= 1 - 1/EXP(1)
= 0.6321

But, this 63.21% is energized to 500 km/sec. That's 125 GJ per kg of
propellant. 20 million pellets, each producing 41.83e15 joules of
energy, yeilds 836.6e21 joules per year. This gives 6.7 trillion kg
of propellant. Divide this by 0.6321 and we obtain 10.6 trillion kg
of rockets. Multiply by 0.3679 to obtain 3.9 trillion kg of payload.

So, an energy efficient rocket fleet would have enough fuel to carry
nearly four billion tons of payload to and from mars each year - with
flight times meaasured in Weeks - and do this for 25 years. That's
100 billion tons. Or 15 tons for every man woman and child on the
Earth!

Clearly, we have the capacity to set up the sort of interplanetary
trading between Earth and mars that we now enjoy throughout the
world's oceans.

(Henry Spencer) wrote in message ...
In article ,
dave schneider wrote:
...a 1km mirror is beyond what's reasonably practical in the near future.

Is there any indication of how big a bubble could be blown in
microgravity to create an Al or Au sphere, that could be sectioned to
provide several spherical mirrors...

I don't think bubble-blowing is going to scale up to that size, given that
many other things (e.g. surface tension) won't scale with it.

In any case, an object that size is *going* to be flexible, so it will
have to be backed with a support structure, probably with active control
actuators too. At that point, you might as well make it in segments
rather than trying to handle a huge thin mirror.

Thanks!

/dps

In article ,
Andrew Nowicki wrote:
HS I don't think bubble-blowing is going to scale...
HS ...you might as well
HS make it in segments rather than trying to handle
HS a huge thin mirror.

As a near-term technology, making large mirrors from
small segments is unbeatable. In the long term, however,
technology based on glass blowing looks attractive:
http://www.islandone.org/LEOBiblio/S....HTM#telescope

That page, unfortunately, trips my bogometer repeatedly. (E.g., he does
not seem to understand that at the scale he is talking about, there is no
such thing as a rigid object, and active control of mirror shape by a
supporting structure is utterly mandatory.) I would give this approach
more credence if it were endorsed by someone with expertise in either
astronomical telescope construction or large optics. As far as I know,
*those* folks all say that when the size gets really big, it's just got
to be segmented.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Andrew Nowicki wrote:
AN As a near-term technology, making large mirrors from
AN small segments is unbeatable. In the long term, however,
AN technology based on glass blowing looks attractive:
AN http://www.islandone.org/LEOBiblio/S....HTM#telescope

Henry Spencer wrote:
HS That page, unfortunately, trips my bogometer repeatedly.
HS (E.g., he does not seem to understand that at the scale
HS he is talking about, there is no such thing as a rigid
HS object, and active control of mirror shape by a supporting
HS structure is utterly mandatory.) I would give this approach
HS more credence if it were endorsed by someone with expertise
HS in either astronomical telescope construction or large
HS optics. As far as I know, *those* folks all say that when
HS the size gets really big, it's just got to be segmented.

Why so much venom?

This used to be a moderated newsgroup for open-minded
discussion of new ideas. Unfortunately, there is hardly
any moderation, creativity, or open-mindedness left here.

A terrestrial mirror is subject to gravity which distorts
the mirror when it tilts. Temperature variation may also
distort the mirror. A space mirror is free of these
distortions, so dividing a space mirror into segments
is less urgent than dividing the terrestrial mirror.
Segmentation drives the cost up, so it is only natural
to avoid it. Plastic flow of hot glass seems to be
cheap way to change the shape of large space mirror.
This method is useless on the Earth, because the force
of gravity would ruin the soft glass mirror.

In article ,
Andrew Nowicki wrote:
HS That page, unfortunately, trips my bogometer repeatedly.
HS (E.g., he does not seem to understand that at the scale
HS he is talking about, there is no such thing as a rigid
HS object, and active control of mirror shape by a supporting
HS structure is utterly mandatory.)...

Why so much venom?

That wasn't venom. When I get venomous, it's lots worse than that.

My point is that it's unwise to use a web page, written by someone who's
neither an optics guy nor an astronomer and clearly doesn't know much
about telescope engineering, as a reference for how to build huge space
telescopes. (As another example, no astronomer would put transparent
optical elements in front of the mirror, because you can't make such
elements transparent to a wide enough range of wavelengths -- one of the
big assets of a reflecting telescope in space is being able to work well
into the UV and IR, and that requires that all optical elements be
mirrors, not lenses.)

A terrestrial mirror is subject to gravity which distorts
the mirror when it tilts. Temperature variation may also
distort the mirror. A space mirror is free of these
distortions...

A space mirror still faces differential thermal stresses (heating will
seldom be exactly even over a large structure -- this is a major issue
for the design of things as small as spacecraft antennas) and transient
distortions from pointing accelerations.

so dividing a space mirror into segments
is less urgent than dividing the terrestrial mirror.
Segmentation drives the cost up, so it is only natural
to avoid it.

On the contrary, segmentation generally drives cost down, because segments
are easier to make and easier to handle for maintenance. The only
question is whether you can combine the segments into a mirror of high
optical quality, and the answer to that is now unquestionably yes.

Plastic flow of hot glass seems to be
cheap way to change the shape of large space mirror.

If you can get the plastic flow to go the right way, which is by no means
self-evident. Besides, there's no reason to bother. A big thin mirror is
going to be flexible even at room temperature, no matter what you do.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Andrew Nowicki wrote in message ...

Why so much venom?

This used to be a moderated newsgroup for open-minded
discussion of new ideas. Unfortunately, there is hardly
any moderation, creativity, or open-mindedness left here.

Just an aside:

There is creativity and open-mindedness here. That's why
your suggestions are being examined and discussed, and why
my wild postings get answers, rather than being dismissed
out of hand.

However, sometimes new ideas are incorrect, or are based
on incorrect facts. Do not mistake disagreement or
criticism of your ideas for lack of open mindedness or
suppression of creativity. It's just debate, and people
are just as allowed to disagree with you as you are
allowed to post new ideas.

In this particular case, you used a reference written by
someone Mr. Spencer apparently thought was ill-informed
about telescopes. Mr. Spencer did not say, "He's a moron,"
or, "You're a moron for using that website," which would've
been a crude dismissal. Instead, he highlighted (highlit?)
what he thought to be the errors in the site's logic. By
listing his objections (as opposed to making an unsupported
dismissal of the website), he made clear his position. You
were free to agree or disagree with his logic in further
debate.

In summary: don't mistake detailed but constructive
criticism for suppression of creativity. It's just debate,
and people come to this newsgroup for debate.

Mike Miller, Materials Engineer

(Gordon D. Pusch) wrote in message ...
(dave schneider) writes:

(Henry Spencer) wrote:

A factor of 100 improvement would bring it down within reach of reason,
but a 1km mirror is beyond what's reasonably practical in the near future.
Eventually, yes.

Is there any indication of how big a bubble could be blown in
microgravity to create an Al or Au sphere, that could be sectioned to
provide several spherical mirrors (yes, I know, there's another conic
section that is better for focussing; ignore the man behind the
curtain for the moment) ?

For a sufficiently large focal length, you don't even need spherical sections;
the individual sections can be optically _flat_, and still not deviate from
the ideal figure by more than a fraction of a wavelength. (IIRC, a 10 km
focal length is sufficient for this to be true.) The primarily problem
then becomes one of _aligning_ the array of mirrors --- not machining.

An idea that keeps going through my head is to make a virtual mirror,
similar to a diffraction grating. A series of free-floating panels of
reflective material in orbit could hold their position with
microthrusters to form the surface of the mirror. The virtual mirror
could be thousands of kilometers across, while each piece is only a
few meters in size.