Nice articles here on the problems that led to the cancellation of the
VentureStar/X-33, a single-stage to orbit vehicle:

Lockheed Martin X-33.http://en.wikipedia.org/wiki/Lockheed_Martin_X-33

X-33/VentureStar - What really happened.http://www.nasaspaceflight.com/content/?id=4180

Interestingly the main problem was making the liquid hydrogen tanks
light enough, certainly not a high tech problem. I wonder if
lightweight storage could be achieved by storing the hydrogen in very
many micron-scale hollow fibers. See the table of tensile strengths
listed here:

Tensile strength.http://en.wikipedia.org/wiki/Tensile_strength

The solutions investigated for the hydrogen tanks for VentureStar
included using high strength aluminum alloys or composite fiber tanks.
The composite tanks were lighter but had a problem of debonding under
high pressure. Note in the table of tensile strengths carbon fiber has
a better strength to weight ratio than the aluminum alloy listed by a
factor of 19 to 1. And the high strength glass fibers known as S-glass
is better than the aluminum alloy by 10 to 1. There is also a special
steel fiber known as scifer steel not listed in the table that has a
tensile strength of 5500 MPa at a density of 7.8 g/cc. That is better
than aluminum alloy by a factor of 4 to 1. It might even be for the
carbon fibers and the S-glass fibers their strength to weight ratios
are so high you wouldn't need to store the hydrogen in liquid form.
You could store it as high density gas. That would eliminate the
weight of the cryogenic systems for the hydrogen.
However, a key question here is whether this strength will be
maintained in the radial direction. All the strengths listed for the
fibers are for pulling along their lengths, i.e, their longitudinal
tensile strength. But to use the fibers as thin hollow pressure tubes
will require their strength to hold in the radial direction. After
investigating this question before for hydrogen storage, I know that S-
glass and scifer steel fibers do retain that strength in the radial
directions. I'm not sure if this is true for the carbon fiber. (BTW,
the high strength polymer fibers listed in the table such as Kevlar,
Dyneema, or Spectra are unsuitable because their strength only holds
in the longitudinal direction, not radially.)
Another key problem for using high strength fibers as hollow tubes is
that they are only about 10 microns wide. So millions to billions of
them would be needed to form sizable storage tanks. You would need a
method of opening and closing these microscopically thin tubes at the
same time for a throttleable engine. Perhaps one solution would be to
have only a small portion of them being used at any one time and
letting those completely empty out, then open another portion, and so
on until all the fuel is used up. This would be an easier solution
than having so many precisely controlled valves at the micro-scale
that operated all in unison.

Bob Clark

Did they look at using aerogel to fill in the honeycombs instead of
the heavier foam insulation?

I like to think that the air force and DOD/CIA took all the parts and
made something anyway, something that's flying right now, probably
unmanned.

Nice articles here on the problems that led to the cancellation of the
VentureStar/X-33, a single-stage to orbit vehicle:

Lockheed Martin X-33.http://en.wikipedia.org/wiki/Lockheed_Martin_X-33

X-33/VentureStar - What really happened.http://www.nasaspaceflight.com/content/?id=4180

Interestingly the main problem was making the liquid hydrogen tanks
light enough, certainly not a high tech problem. I wonder if
lightweight storage could be achieved by storing the hydrogen in very
many micron-scale hollow fibers. See the table of tensile strengths
listed here:

Tensile strength.http://en.wikipedia.org/wiki/Tensile_strength

The solutions investigated for the hydrogen tanks for VentureStar
included using high strength aluminum alloys or composite fiber tanks.
The composite tanks were lighter but had a problem of debonding under
high pressure. Note in the table of tensile strengths carbon fiber has
a better strength to weight ratio than the aluminum alloy listed by a
factor of 19 to 1. And the high strength glass fibers known as S-glass
is better than the aluminum alloy by 10 to 1. There is also a special
steel fiber known as scifer steel not listed in the table that has a
tensile strength of 5500 MPa at a density of 7.8 g/cc. That is better
than aluminum alloy by a factor of 4 to 1. It might even be for the
carbon fibers and the S-glass fibers their strength to weight ratios
are so high you wouldn't need to store the hydrogen in liquid form.
You could store it as high density gas. That would eliminate the
weight of the cryogenic systems for the hydrogen.
However, a key question here is whether this strength will be
maintained in the radial direction. All the strengths listed for the
fibers are for pulling along their lengths, i.e, their longitudinal
tensile strength. But to use the fibers as thin hollow pressure tubes
will require their strength to hold in the radial direction. After
investigating this question before for hydrogen storage, I know that S-
glass and scifer steel fibers do retain that strength in the radial
directions. I'm not sure if this is true for the carbon fiber. (BTW,
the high strength polymer fibers listed in the table such as Kevlar,
Dyneema, or Spectra are unsuitable because their strength only holds
in the longitudinal direction, not radially.)
Another key problem for using high strength fibers as hollow tubes is
that they are only about 10 microns wide. So millions to billions of
them would be needed to form sizable storage tanks. You would need a
method of opening and closing these microscopically thin tubes at the
same time for a throttleable engine. Perhaps one solution would be to
have only a small portion of them being used at any one time and
letting those completely empty out, then open another portion, and so
on until all the fuel is used up. This would be an easier solution
than having so many precisely controlled valves at the micro-scale
that operated all in unison.

Bob Clark

Hoop stresses of a tube under pressure increase with the diameter but
volume increases with the square of diameter. �High volume/weight
pressurised gas storage would, therefore, favor a bigger cylinder.

Container weight scales with area times thickness, and
thickness scales with hoop stress,