Re: How Rockets Differ From Jets

George Evans wrote:
> And there are so many other questions like how does it behave in a composite
> matrix? It's always being compared with steel, but how does it compare with
> 2422 Aluminum or whatever is the strongest alloy?


The comparison you ask for is beyond me. Metals, however, have low
meltpoints. A fatal flaw in a vehicle designed for hypersonic speed.

There is a tungsten alloy that hits an incredible 6000 deg. F. for it's
meltpoint. Warning: tungsten generates enormous quantities of X-rays
when conducting electricity. Tungsten is also very expensive and
difficult to work with. Find a torch that can cut at 6000+ deg. F.!

Tungsten can, today, be worked and machined but it is 'state of the
art'.

The R&D being accomplished across the country, with the University of
Texas at Dallas being one of the primary, has not even begun to answer
all of the question about CNT. They have, however, turned it into a
long strip of fabric, a fabric so thin it looks more like a shadow than
a substance.

They spoke of making it 18 ply with the ply's angled in different
directions.

One problem with CNT is that it may 'soak up' or be 'dissolved' by LH2.
Hence, the titanium liner. The liner would give you both the form and
something solid to laminate.

The insulative properties, as mentioned by Brad Guth, of basalt fabric
may make it an excellent first layer, or perhaps the best laminate for
the LH2 fuel tanks. The 82kpsi rating of basalt fabric is 'eye
popping' as well.


> I don't pay much attention to Guth but I do to Rutan. My dad built a Long EZ
> and we are both working on a Lancair IV, not a Rutan but carbon fiber
> construction. I am curious how much strength improvement nanotube fibers
> provide over regular carbon fiber. I'm assuming, since they are just
> differently arranged carbon atoms, that they would be similar and that they
> could be used the same in composite materials.


I suspect there would be improvement. I have been watching nanotube
research since they were called 'buckytubes' after the 'buckyballs'
that preceeded them. In their nanoscale state they are almost . . .
magical.

If there is a long gap of time before they are in manufacture then
carbon fiber and/or basalt fabric may become desirable alternatives.


> Is that 100 times stronger in tensile strength for an equal cross section?
> And is that 1/6th the density of steel? Remember a pound of feathers isn't
> 1/6th the weight of a pound of lead. So how do those figures stack up to
> carbon fiber?


I have read -- mostly from University of Texas at Dallas -- of figures
of 100 times the strength at 1/6th the weight. Then of 50 times the
strength of steel (this may have been a comparison to a super strong
steel alloy wire).

I know little of carbon fiber except that it was/is used in missile
nose cones under the Corelle ceramic. A lot of the strength of a
composite comes from the binder. Graphite epoxy is expensive, stiff,
and very strong. Note: some binders have proven so strong, etc., that
they have been cast into plates.


I would build an aircraft out of steel.

The graphite epoxy/carbon fiber of Spaceship One would probably be a
superior material, and lighter to boot. But if you really want steel
then use titanium.

Titanium was used in the SR-71 as well as the F-15 and F-14, and has
proven very, very good as long as it's 2000 deg. F. meltpoint isn't
approached too closely. Today, titanium can be worked with easily.
Not true in the 70's. And, it is non-corrosive. It is stronger than
aluminum so can be made a little thinner saving weight, unless you have
need of the extra strength.



> LH2 slush means solid hydrogen in liquid He. This is *very* cold stuff, so
> how does Titanium do at those temperatures, and how would a nanotube/
> graphite epoxy composite? Also it is quite a trick to bond to metal, no
> doubt even more so at the temperature of liquid He. Have you done any
> testing?

Testing of various materials at near absolute zero cold is very
important for two reasons. Handling cryogenic fuels as you suggested.
And, because Outer Space is extremely cold, especially as you go
farther from the Sun, as in a trip to Mars.

I 'seem to remember' seeing something on CNT being very resistant to
the effects of extreme cold. The danger is that a brittle spaceplane
hull might take a meteor hit -- probably will on a long voyage -- and
shatter like glass.

This is why a spaceplane hull has to be 'state of the art'. It has to
be strong, tough, and capable of extreme heat as well as extreme cold.


> I notice you said that CNT is extremely thermally conductive. This doesn't
> seem conducive to keeping things cold.


It is not. An insulator will need to be used. Nomex is a possibility.
Possibly a nomex/kevlar blanket, though that might add unnecessary
weight.

Also, the vacuuming of the space around the fuel tanks will help both
to lighten the spaceplane and provide excellent insulation.


> > The outer hull, however, should be composite only. Titanium, despite it's
> > 2000 deg. F. meltpoint, is just too meltable. RCC might be a good way to get
> > strength and rigidity to prevent ceramic flexing and breaking.

> Here again, the extreme thermal conductivity of CNT is a drawback.

Not really. Extreme thermal conductivity will be an asset on the hull.
The ability to rapidly spread out thermal pockets, especially in the
leading edge areas, will be a blessing. A CNT hull would form a
massive heatsink.

> Everybody
> mocks the TPS tiles on the shuttle but they really are amazing. If they are
> banded to a CNT/graphite epoxy composite structure, those fillers you are
> concerned about probably wouldn't be necessary.

I wonder about the filler's heat resistance. Not to mention the extra
weight. If the filler is 'that good' then why the tiles in the first
place?



tomcat

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