Microwave beamed power

Of course the contrail shown has zero lateral or tensile strength.
Which is quite correct for the contrail of fuel patent you cited. Not
so for the lightweight aerogel. A true analysis would look at winds
aloft and determine the tensile strength needed to sustain a good
enough trail of fuel and oxidizer. Clearly (see below) with a 16 kPa
tensile strength, aerogels have sufficient strength to withstand
considerable wind shear for some period of time. Hydrogen and oxygen
will of course diffuse out of the gel over time, which will cause it to
sag as time goes on. Which means the trail must be used minutes after
it is in place. Rise times versus leakage times is an important
ratio...

Aerogel Specifications:
Apparent density: 0.001-0.35 g/cc
Internal surface area: 600-1000m2/g
% solids 0.07-15%
Mean pore diameters ~20 nm
Primary particle diameter 2-5 nm
index of refraction 1-1.05
Thermal tolerance to 500 C
Coefficient of thermal expansion 2-4x10-6
Poisson ratio 0.2
Young's modulus 106-107 N/m2
tensile strength 16 kPa
Fracture toughness 0.8 kPa*m0.5
Dielectric constant 1.1
Sound velocity through medium 100 m/s

William Mook wrote:

Of course the contrail shown has zero lateral or tensile strength.
Which is quite correct for the contrail of fuel patent you cited. Not
so for the lightweight aerogel. A true analysis would look at winds
aloft and determine the tensile strength needed to sustain a good
enough trail of fuel and oxidizer. Clearly (see below) with a 16 kPa
tensile strength, aerogels have sufficient strength to withstand
considerable wind shear for some period of time. Hydrogen and oxygen
will of course diffuse out of the gel over time, which will cause it to
sag as time goes on. Which means the trail must be used minutes after
it is in place. Rise times versus leakage times is an important
ratio...



As the air pressure drops around the ascending aerogel "propellant
stick", the oxygen and hydrogen will leech out of the aerogel's
structure unless it is covered in some sort of impermeable membrane.
I still think there is going to be a problem regarding pre-ignition of
the aerogel propellant stick- one static discharge anywhere within it,
and your propellant supply gets turned into a giant fuel/air bomb.
The other problem is how the oxygen and hydrogen get put into the
aerogel in the first place; if this is to be done on the ground then
it's going to require some sort of filling building several miles in
length that can turn into the wind to release the filled aerogel
cylinder through its roof, or a non traversing building that can only
release in near dead calm conditions.
In either case the two gases are going to stratify within the stick in
fairly short order, with the hydrogen at the top.
Getting the stick properly aligned for the TAV to ascend through is
going to be a problem also; it must be aligned on the correct bearing
for the intended orbit, as well as be floating at the correct angle to
the horizontal for the intended ascent trajectory.
This implies that the stick must be under control of some sort, most
likely by being towed by an aircraft that attaches itself to the stick
after it floats out of its gas loading shed, and then pulls it skyward
to the intended altitude, trajectory, and ascent angle- probably at
quite a low speed to stay within the structural limits of the aerogel. A
helicopter of some sort sounds like a candidate for a tow aircraft,
although this is going to badly limit the total altitude it can achieve.
Pulling the stick through the air at even low speed is going to generate
terrific drag given its length, and you are going to need a huge
helicopter to even have a chance of moving and controlling it- something
along the lines of a Mil-26 "Halo".

Pat

In article . com,
wrote:

Joe Strout wrote:

And here I've been giving my kids helium balloons that last only a week
or two. Where can I get some of this lighter-than-air aerogel?

lighter-than-air aerogel? sounds like a good material to build aircraft
from but it could become a very difficult to clean up form of litter. I
wonder how pieces of it might affect jet engines or how it might feel
on a windy day to have to dodge chunks of the stuff.

I was being sarcastic. No such material exists.

Best,
- Joe

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| Joseph J. Strout Check out the Mac Web Directory: |
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A closed cell aerogel is nothing but an impermeable membrane! Lots and
lots of gas filled cells stuck together with each cell's membrane
impermeable to the gases. No mixing of gases within the stick, no
stratification of gases, no explosion risk while floating.

Detonation occurs when the cells are mechanically burst by the passing
of a vehicle which are mixed and heated by the shock effects.

The building you imagine would be along the ground track of the vehicle
under boost. It would not orient to the wind.

The building wouldn't be a building either. It would be four pipelines
in parallel. Two propellant pipelines, one aerogel precursor pipeline,
and one mixing line - that opened along its length. Propellant and
aerogel would be mixed in the mixing line - and the mixing line would
then be opened. The effects of local winds over the length of the
pipeline would largely cancel due to the immense drag of a miles long
stick of material you already mention.


The stick would take up the desired trajectory above the pipeline by
simply changing the density of the aerogel along the length of the
stick so that it came to rest at the appropriate density altitude. So,
denser parts of the stick would hover lower in the sky than less dense
parts of the stick. The angle of ascent would merely be a fuction of
the difference in density along the sticks length.

No helicpoters or other aircrat are needed to control it. Its immense
size means that it will be little affected by light winds and by
accurately controlling the density along its length - the altitude of
the stick along its length will be accurately controlled - creating a
precisely controlled trajectory for the vehicle powered by this
propelant stick.

On 9 Aug 2005, William Mook wrote:

[snip]
The building wouldn't be a building either. It would be four pipelines
in parallel. Two propellant pipelines, one aerogel precursor pipeline,
and one mixing line - that opened along its length. Propellant and
aerogel would be mixed in the mixing line - and the mixing line would
then be opened. The effects of local winds over the length of the
pipeline would largely cancel due to the immense drag of a miles long
stick of material you already mention.
[snip]

Large Blimps and Zeppelins certainly care about winds...

But you are probably just writng those off as "too small" and "local"...

Ok, What about regional wind patterns. Say a low pressure center driving
a circulation pattern several hundred miles across...

What about coriolis? Since you are talking about making a thing that big,
coriolis forces *are* going to be a problem.

Since you are writing off "local" wind effects, then mesoscale effects
must matter... how about pressure differentials between weather systems of
several millibars... That will certainly mess up your nice smooth
bouyancy curve. And my experience with hot air ballooning suggests that
temperature will matter a *lot*...

The very concept of these free flying bouyant "propellant sticks" that are
supposed to be gobbled up by an ascending spacecraft is so full of obvious
problems as to resemble a large swiss cheese.

Gene P.
Slidell LA

--
Alcore Nilth - The Mad Alchemist of Gevbeck